The morphology of active layer plays an important role in determining the power conversion efficiency (PCE) and stability of polymer solar cells (PSCs), which strongly depend on the dynamic drying process of active layer. In this work, an efficient and universal method was developed to let active layer undergo upside-down drying process in a covered glass Petri dish. For the PSCs based on PTB7-Th:PC71BM, the champion PCEs were improved from 8.58% to 9.64% by mixing 3 vol % 1,8-di-iodooctane and further to 10.30% by employing upside-down drying method. The enhanced PCEs of PSCs with active layers undergoing upside-down drying process are mainly attributed to the optimized vertical phase separation, the more ordered and tightly packed π–π stacking of polymer molecules. Meanwhile, PC71BM molecules can be frozen in more ordered and tightly packed π–π stacking polymer network, which lead to the enhanced stability of PSCs. The universality of upside-down drying method can be solidly confirmed from PSCs with PTB7:PC71BM, PffBT4T-2OD:PC71BM, or PBDT-TS1:PC71BM as active layers, respectively. The molecular packing and phase separation of blend films with different solvent additives and drying methods were investigated by grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy.Keywords: drying method; efficiency; polymer solar cells; stability; upside-down;

For the given organic donor and acceptor materials, optimizing molecular aggregation and crystallinity for appropriate phase separation plays a crucial role in achieving high performance solar cells. In this study, the power conversion efficiency (PCE) of DR3TSBDT:PC71BM based small molecule solar cells (SMSCs) was markedly raised from 7.25% to 9.48% for active layers processed with 0.2 vol % 1,8-diiodooctane (DIO), resulting from the improved fill factor (FF) and short circuit current density (JSC). The performance improvement may be attributed to an appropriate DR3TSBDT molecular aggregation and crystallinity, as well as the optimized phase separation. The influence of DIO concentrations on DR3TSBDT molecular aggregation can be confirmed from the red-shifted absorption and photoluminescence peaks of films along with increase of DIO concentrations. Meanwhile, the ratio of hole/electron mobility approached 1.06 in the optimized SMSCs well according with the highest FF of the corresponding SMSCs. The morphology characterizations indicate that DR3TSBDT molecular aggregation and crystallinity could be finely adjusted by doping appropriate DIO additive.Keywords: Additive; Crystallinity; Phase separation; Power conversion efficiency; Small molecule solar cells;

Filterless narrowband response organic photodetectors (OPDs) present a great challenge due to the broad absorption range of organic semiconducting materials. The reported narrowband response OPDs also suffer from low external quantum efficiency (EQE) in the desired response window and low rejection ratio. Here, we report highly narrowband photomultiplication (PM) type OPDs based on P3HT:PC71BM (100:1, wt/wt) as active layer without an optical filter. The full width at half-maximum (fwhm) of the PM-type OPDs can be well retained less than 30 nm under different biases. Meanwhile, the champion EQE and rejection ratio approach 53 500% and 2020 at −60 V bias, respectively. The small fwhm should be attributed to the sharp absorption edge of active layer with small amount of PC71BM. The PM phenomenon is attributed to hole tunneling injection from the external circuit assisted by trapped electron in PC71BM near the Al electrode under light illumination. These highly narrowband PM-type OPDs should have great potential applications in sensitively detecting specific wavelength light and be blind to light outside of the desired response window.Keywords: absorption edge; narrowband response; Organic photodetectors; photogenerated electron distribution; photomultiplication;

Side group of ITIC-like small molecular acceptor (SMA) plays a critical role in crystallization property. In this article, two new SMAs with n-hexylthienyl and n-hexylselenophenyl as side chain, namely ITCPTC-Th and ITCPTC-Se, are designed and synthesized by employing newly developed thiophene-fused ending group (CPTCN). And thiophene and selenophene side group substituted effects of SMA-based fullerene-free polymer solar cells (PSCs) are investigated. A stronger σ-inductive effect between selenophene side group and electron-donating backbone endows ITCPTC-Se with better optical absorption and higher LUMO level, ITCPTC-Th-based PSCs deliver a higher power conversion efficiency of 10.61%. Charge transport and collection, recombination loss mechanism, and morphology of blend films are intensively studied. These results confirm that side group substituted effects of SMAs are multiple and thiophene is a superior option to selenophene as aromatic side group of ITIC-like SMAs.

AbstractCombining bandpass filters with broadband photodetectors is the most common way to detect light within a specific spectral range in many applications that need to detect only the light within desired spectral range. Here, a method is reported to realize narrowband spectral response of perovskite photodetectors by applying hybrid perovskites layers with the same or a slightly larger bandgap as filters, which are integrated into perovskite photodetectors. The narrowband photodetectors have a full width at half maximum of 28 nm due to the sharp absorption edges of perovskite materials, and the light rejection ratio is above 1000. By tuning the bandgaps of perovskite filters and the associated perovskite layers in the detectors, the response bands are continuously tuned in the whole visible light region to satisfy different applications. Superior to the thick single crystal narrowband perovskite photodetectors, the naorrowband peroskite detectors with this design have a very high response speed only limited by the resistance capacitance constant.

AbstractIt is a great challenge to obtain broadband response perovskite photodetectors (PPDs) due to the relatively large bandgaps of the most used methylammonium lead halide perovskites. The response range of the reported PPDs is limited in the ultraviolet–visible range. Here, highly sensitive PPDs are successfully fabricated with low bandgap (≈1.25 eV) (FASnI3)0.6(MAPbI3)0.4 perovskite as active layers, exhibiting a broadband response from 300 to 1000 nm. The performance of the PPDs can be optimized by adjusting the thicknesses of the perovskite and C60 layers. The optimized PPDs with 1000 nm thick perovskite layer and 70 nm thick C60 layer exhibit an almost flat external quantum efficiency (EQE) spectrum from 350 to 900 nm with EQE larger than 65% under −0.2 V bias. Meanwhile, the optimized PPDs also exhibit suppressed dark current of 3.9 nA, high responsivity (R) of over 0.4 A W−1, high specific detectivity (D*) of over 1012 Jones in the near-infrared region under −0.2 V. Such highly sensitive broadband response PPDs, which can work well as self-powered conditions, offer great potential applications in multicolor light detection.

•The synthesis of bisalkylthiobenzo[1,2-b:4,5-b']dithiophene based oligomers.•The electrochemical characterization and molecular simulation of oligomers.•The effect of ending acceptor on bisalkylthio BDT oligomer properties.•The highest PCE of 5.17% for oligomer based OPVs.Three novel small molecules named DR3TDST, DRCN3TDST and DB3TDST containing bisalkylthiothienyl side-chain on benzo[1,2-b:4,5-b']dithiophene (BDT) core with different electron-deficient ending-groups are designed and synthesized. All three molecules have deep HOMO energy level from −5.40 eV to −5.44 eV, indicating introduction of bisalkylthio group to BDT is an effective method to modulate the molecular energy levels. Different ending-groups exhibit small but measurable effects on absorption, energy level, charge transport, morphology, and photovoltaic properties of small molecules. Bulk heterojunction solar cells fabricated with DRCN3TDST and PC71BM deliverer the highest power conversion efficiency (PCE) of 5.17% after annealing and solvent vapor annealing.Benzo[1,2-b:4,5-b']dithiophene (BDT) core-structured small molecules with bisalkylthio side-group lowered HOMO levels showed the highest power conversion efficiency (PCE) of 5.17% in OSCs.Download high-res image (230KB)Download full-size image

•Two isomerized small molecules based on perylene diimide and spirobifluorene were designed and synthesized.•The different substitution positions have a great effect on the electronic distribution and molecular packing.•The SPFPDI24 based device exhibited the highest PCE of 4.56%.•This work would be instructive in the rational molecular design of small molecule acceptor materials.Two isomeric small molecules based on perylene diimide with 4,4′-/2,4′-substituted spirobifluorene as the core were designed and synthesized as acceptor materials for organic solar cells. According to density functional theory calculations, the different sites of the substitution, either the 4-position or the 2-position of the spirobifluorene led to significant distinction in both molecular structure and electronic distribution. The dihedral angles between the two perylene diimide units were calculated as well, indicating the 2-position linkage molecule had a more isotropic spatial arrangement. X-ray diffraction results showed the more ordered molecular packing of the 2-position linked molecule. In consequence, when blended with a donor, these two acceptors exhibited different device performance, especially the short circuit currents. The solar cell based on the 2-position linkage molecule achieved the best power conversition efficiency of 4.56% with 2% 1-chloronaphthalene as an additive. The hole and electron mobilities were also improved greatly after adding 2% 1-chloronaphthalene, which benefited the short circuit currents.Download high-res image (264KB)Download full-size image

The dynamic drying process of the active layer should play a vitally important role in determining the performance of polymer solar cells (PSCs). Donor molecular packing and acceptor redistribution can be optimized by two successive post-treatments on the active layer. The blend films were freshly prepared by spin-coating method and then immediately transferred to a covered glass Petri dish to allow self-assembly of the donor molecules. The films were then treated with methanol or PFN–methanol solution to adjust the acceptor redistribution. In this study, power conversion efficiencies (PCEs) of PSCs with PffBT4T-2OD:PC71BM as the active layer were improved from 6.74% to 8.75% by employing 80 min for self-assembly and 20 s of methanol soaking. The PCE was improved even further to 9.72% by inserting a PFN interfacial layer. The performance improvement was mainly attributed to the optimized PffBT4T-2OD molecular packing during the self-assembly process, ideal vertical phase separation driven by methanol soaking and efficient charge collection by insertion of a PFN interfacial layer. The molecular packing and vertical phase separation were characterized by grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The experimental results solidly supported the effectiveness of the step-by-step optimization strategy.

It is a great challenge to obtain narrowband and photomultiplication (PM) type organic photodetectors (OPDs) without optical filters due to the broad absorption range and large exciton binding energy of organic materials. Narrowband OPDs with the special structure of ITO/PFN-OX/P3HT : PC61BM (100 : 1,wt/wt)/Al were successfully fabricated with different active layer thicknesses, exhibiting a tunable response window and PM phenomenon under bi-directional bias. The OPDs exhibit U-shaped EQE spectra with two narrowband response windows under forward bias and a single narrowband response window under reverse bias. The best EQE of the optimized OPDs with a 4.0 μm thick active layer approaches 7160% or 8180% for 340 nm or 650 nm light illumination at 60 V and 1640% for 665 nm light illumination at −60 V, respectively. The most important features of the optimized OPDs is that the full width at half-maximum (FWHM) of their response windows is less than 30 nm under bi-directional biases, which can be well maintained at large bias. The PM type narrowband OPDs working at bi-directional bias are the first to be reported with a tunable response range, further indicating that the density of the electrons trapped in PC61BM near the hole injection electrode plays the key role in determining the interfacial band bending for hole tunneling injection from the external circuit.

Solution-processed small molecule solar cells (SMSCs) are fabricated based on DRCN5T:PC71BM as active layers, the power conversion efficiency (PCE) is markedly increased from 3.63% to 9.11% for the active layers undergoing up-side-down thermal annealing and solvent vapor annealing post-treatments. The PCE improvement should be attributed to the appropriate phase separation consisting of enhanced crystallinity of donor and purified acceptor domain at nanoscale. The nematic liquid crystal small molecule BTR is selected as the second donor and morphology regulator to prepare ternary SMSCs. The champion PCE of ternary SMSCs was improved to 10.05% by mixing 1.5 wt% BTR, which corresponds to a 10.3% PCE improvement compared with the optimized binary SMSCs. The performance improvement is mainly attributed to the further optimized phase separation and complementary photon harvesting between DRCN5T and BTR, which could be well demonstrated from absorption spectra, two dimensional grazing incidence X-ray diffraction (2D-GIXD) and transmission electron microscopy (TEM).

•The BTR: PC71BM based SMSCs exhibit high PCE of 9.37% compared with previous reports.•The PCE of ternary SMSCs approaches 10.3% by incorporating 6 wt% DIB-SQ in donors.•The PCE of 10.3% is among the highest value for ternary SMSCs up to date.•Photon harvesting and morphology can be co-optimized by incorporating 6 wt% DIB-SQ.The elaborately prepared BTR:PC71BM based small molecule solar cells (SMSCs) exhibit a champion power conversion efficiency (PCE) of 9.37%, which should be among the highest values for previous reports on BTR:PC71BM based SMSCs. Based on the optimized binary SMSCs, small molecule DIB-SQ was selected as the third component to fabricate ternary SMSCs. The champion PCE of 10.3% was achieved for ternary SMSCs with 6 wt% DIB-SQ in donors. An approximate 10% PCE improvement was obtained due to obviously increased short current density (JSC) of 15.44 mA/cm2 and fill factor (FF) of 73.8%. The main contribution of DIB-SQ can be summarized as the enhanced photon harvesting in long wavelength range and the optimized phase separation for better exciton dissociation and charge transport in ternary active layer. The results further demonstrate that ternary strategy should be an efficient and versatile method to improve the performance of SMSCs.Download high-res image (288KB)Download full-size image

Herein, a liquid crystal material, BTR, was elaborately selected as the third component to complement PTB7-Th for the fabrication of highly efficient ternary polymer solar cells (PSCs). Via incorporating 10 wt% BTR in their donors, the champion power conversion efficiency (PCE) of the PSCs increased from 10.08% to 10.83%, resulting from the enhanced short circuit current (JSC) of 19.23 mA cm−2 and fill factor (FF) of 72.21%. The small amount of incorporated BTR may prefer to distribute in the PTB7-Th networks and has good miscibility with PC71BM. The excitons on BTR may dissociate into free charge carriers at the BTR/PC71BM interfaces and also transfer their energy to PTB7-Th through Förster resonance energy transfer, resulting in improved exciton utilization. Moreover, the molecular arrangement and morphology of the active layers could be optimized by incorporating appropriate amount of BTR as a nucleating agent, also leading to enhanced stability of the ternary PSCs. The positive effects of BTR on the performance improvement of PSCs were confirmed from the inverted and conventional structures of the cells.

The power conversion efficiency (PCE) of organic solar cells has been constantly refreshed in the past ten years from 4% up to 11% due to the contribution from the chemists on novel materials and the physicists on device engineering. For practical applications, a single bulk heterojunction structure may be the best candidate due to the cell with a high PCE, easy fabrication and low cost. Recently, ternary solar cells have attracted much attention due to enhanced photon harvesting by using absorption spectral or energy level complementary materials as the second donor or acceptor based on a single bulk heterojunction structure. For better promoting the development of ternary solar cells, we summarize the recent progress of ternary solar cells and try our best to concise out the scientific issues in preparing high performance ternary solar cells.

Photon harvesting and phase separation can be synergistically optimized by using ternary strategy with appropriate DIB-SQ-doped Si-PCPDTBT:PC71BM as active layer. The power conversion efficiency (PCE) of ternary organic photovoltaic cells (OPVs) exhibits an increasing and then decreasing trend dependent on the increase in DIB-SQ doping ratio. The PCE of OPVs was increased from 5.52% to 6.18% by doping 3 wt% DIB-SQ while keeping the donor/acceptor doping weight ratio at 1:1.5. The PCE improvement can be attributed to the better trade-off between photon harvesting and phase separation in the ternary active layers. The degree of phase separation strongly influences energy or charge transfer between donors and charge transport in the ternary active layers. The dynamic process between donors was investigated according to J–V curves of cells without acceptor under light illumination. Experimental results fully confirm that this ternary strategy should have a brilliant future due to about 12% PCE improvement compared with Si-PCPDTBT:PC71BM-based binary OPVs.

A series of solvent additive-free polymer solar cells (PSCs) were fabricated with PBDT-TS1:PC71BM (1:2, wt/wt) as the active layers; the performance of the PSCs can be enhanced by using MeOH:CH2Cl2 or MeOH:CB mixed solvent spin-coating treatment on the active layers. The maximum power conversion efficiency (PCE) of PSCs was increased from 6.69% to 7.21% or 7.76% by spin-coating the mixed solvents onto the active layers, respectively. The PCE improvement may be attributed to the increase in PC71BM elevated toward the top surface of the active layers during the mixed solvent spin-coating treatment. The redistribution of PC71BM in the active layers could also be indirectly confirmed from the variation of photoluminescence spectra and water contact angles of the corresponding films, as well as the J–V curves of electron-only devices.

Four isostructural donor–acceptor alternating polymers of benzodithiophene (BDT)/naphthodifuran (NDF) and benzoselenadiazole (BSe)/benzothiadiazole (BT) have been developed and evaluated for organic photovoltaics. The substitution of one-atom (Se for S) in the accepting units exerts remarkable impact on the optoelectronic properties of polymers. Extended absorption, narrowed bandgap and higher HOMO energy levels were observed for Se-containing polymers in comparison to their S-containing counterparts. Theoretical calculations confirmed the measurable effect on energy levels as found in experimental studies. Bulk-heterojuction solar cells based on the BDT–BSe copolymer and [6,6]-phenyl-C71-butyric acid methyl ester (1:2, w/w) blend films deliver the best PCE of 5.40%. BSe-based polymers showed enhanced photovoltaic performances than BT-based polymers. The device performance is found to be strongly dependent on the processing conditions and morphology of the active layers.

•The champion PCE of ternary solar cells arrives to 7.62% by doping 9 wt% SQ.•PCE improvement is attributed to enhanced photon harvesting and exciton utilization.•Appropriate SQ doping ratio can optimize phase separation in ternary active layers.A smart ternary strategy was designed to simultaneously improve utilization of excitons on PCDTBT and enhance photon harvesting in long wavelength range by doping appropriate SQ into the dominating PCDTBT:PC71BM system. The phase separation can be finely optimized by adjusting SQ doping ratio, resulting in efficient charge carrier transport channels and the enhanced photon harvesting in the ternary active layers. The dopant SQ provides a potential route for further utilizing the excitons on PCDTBT, especially for the excitons unattainable to the PCDTBT/PC71BM interfaces. The excitons on PCDTBT can transfer their energy to SQ through Förster energy transfer, and then be dissociated into free charge carriers at SQ/PC71BM interface. The holes generated on SQ can be effectively transferred to PCDTBT and then transported along the channels formed by PCDTBT to anode. Consequently, the champion PCE of ternary solar cells arrives to 7.62% with 9 wt% SQ doping ratio in donors, which is much higher than the PCE of 6.54% for PCDTBT:PC71BM-based or 1.95% for SQ:PC71BM-based solar cells.Efficient ternary solar cells achieve a champion PCE of 7.62% by doping 9 wt% SQ in donors, which exhibits a 16.5% PCE improvement compared with the corresponding PCDTBT:PC71BM-based binary solar cells.

A series of ternary polymer solar cells (PSCs) were fabricated based on two narrow bandgap polymers PTB7 and PBDT-TS1 as the donors and PC71BM as the acceptor. The performances of the ternary PSCs monotonically increased along with the PBDT-TS1 doping ratio, up to 80 wt% in the donors. The optimum power conversion efficiency (PCE) achieved for the ternary PSCs was 7.91% with an open circuit voltage (VOC) of 0.76 V, a short circuit current density (JSC) of 18.85 mA cm−2 and a fill factor (FF) of 55.2% for the active layers with an 80 wt% PBDT-TS1 doping ratio in the donors. The optimized ternary PSCs show a 12.8% improvement compared to the optimum PCE of 7.01% for the binary PSCs with PBDT-TS1 as the donor and a 28.2% improvement compared with the optimum PCE of 6.17% for the binary PSCs with PTB7 as the donor. The FFs of all the ternary PSCs are larger than 54%, indicating efficient charge carrier transport channels in the ternary active layers. The energy or charge transfer between PTB7 and PBDT-TS1 should be neglected according to the investigation of the photoluminescence spectra of the blend films and the current density–voltage (J–V) curves of devices without the acceptor in the active layers. The ternary PSCs should be parallel-linkage structures with the donors independently working with the acceptor, which may be the most promising strategy for obtaining highly efficient ternary PSCs.

Incorporation of a solution processed PFN interfacial layer has been demonstrated as an efficient method to improve the performance of polymer solar cells (PSCs). The champion power conversion efficiency (PCE) of PSCs with PTB7-Th:PC71BM as the active layer was increased from 6.13% to 7.72% by directly spin-coating PFN methanol solution on the surface of active layers or to 8.50% for the active layer with 4 min PFN methanol solution soaking. The champion PCE of PSCs was further increased to 8.69% for the active layers with a two-step treatment, 4 min methanol soaking and then directly spin-coating PFN methanol solution. A 12.6% PCE improvement was obtained by using the two-step strategy compared with directly spin-coating PFN methanol solution on the active layers. Methanol soaking of the active layers plays the key role in forming the more ideal vertical phase separation for efficient exciton dissociation, charge carrier transport and collection. An ultrathin PFN interfacial dipole layer can be obtained by directly spin-coating PFN methanol solution. The two-step strategy may provide a simple and effective method to finely optimize the phase separation and obtain an ultrathin PFN interfacial dipole layer for the performance improvement of PSCs.

Recently, power conversion efficiency (PCE) of organic solar cells has been increased up to about 10% by using solvent additives or mixed solutions to elaborately adjust phase separation which is a great challenge for large scale production. We report a champion PCE of 7.40% for solution-processed small molecule ternary SMPV1:DIB-SQ:PC71BM solar cells by only adjusting the DIB-SQ doping ratio in donors without any treatments on the blend solutions or active layers. The champion PCE of ternary solar cells is larger than the champion PCEs sum (6.98%) of binary solar cells with SMPV1:PC71BM or DIB-SQ:PC71BM as the active layers. The PCE improvement should be attributed to the synergistic enhancement of photon harvesting, exciton dissociation, charge carrier transport and collection by the appropriate DIB-SQ doping ratio in donors. The experimental results on morphology, crystallinity, phase separation and charge carrier mobility of active layers well support the PCE improvement in ternary solar cells with a 10 wt% DIB-SQ doping ratio in donors.

Highly sensitive polymer photodetectors (PPDs) are successfully achieved with a broad spectral response range from UV light to the near infrared region (NIR) based on P3HT:PTB7-Th:PC71BM as the active layer. The highest external quantum efficiency (EQE) values of the PPDs with P3HT:PC71BM (100:1) as the active layer are 90700% and 84100%, corresponding to 390 nm and 625 nm light illumination under a −25 V bias, respectively. The spectral response range of the PPDs can be extended to the NIR by doping narrow band gap polymer PTB7-Th into P3HT:PC71BM as the active layer. The highest EQE values of the PPDs with P3HT:PTB7-Th:PC71BM (50:50:1) as the active layer are around 38000% in the spectral range from 625 nm to 750 nm under a −25 V bias. The high EQE values of the PPDs should be attributed to three points: (i) the rather weak dark current due to the relatively large hole injection barrier; (ii) the enhanced hole tunneling injection due to the interfacial band bending, which is induced by trapped electrons in PC71BM near the Al cathode; and (iii) the efficient hole-only transport in the active layers with the rather low PC71BM content. The broad spectral response range is due to the contribution of PTB7-Th exciton dissociation on the number of trapped electrons in PC71BM near the Al cathode.

We present a smart strategy to simultaneously increase the short circuit current (Jsc), the open circuit voltage (Voc), and the fill factor (FF) of polymer solar cells (PSCs). A two-dimensional conjugated small molecule photovoltaic material (SMPV1), as the second electron donor, was doped into the blend system of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl (PC71BM) to form ternary PSCs. The ternary PSCs with 5 wt % SMPV1 doping ratio in donors achieve 4.06% champion power conversion efficiency (PCE), corresponding to about 21.2% enhancement compared with the 3.35% PCE of P3HT:PC71BM-based PSCs. The underlying mechanism on performance improvement of ternary PSCs can be summarized as (i) harvesting more photons in the longer wavelength region to increase Jsc; (ii) obtaining the lower mixed highest occupied molecular orbital (HOMO) energy level by incorporating SMPV1 to increase Voc; (iii) forming the better charge carrier transport channels through the cascade energy level structure and optimizing phase separation of donor/acceptor materials to increase Jsc and FF.Keywords: charge carrier transfer; energy level; energy transfer; polymer solar cells; ternary strategy

A series of photomultiplication (PM)-type polymer photodetectors (PPDs) were fabricated with polymer poly(3-hexylthiophene)–[6,6]-phenyl-C71-butyric acid methyl ester (P3HT–PC71BM) (100:1, w/w) as the active layers, the only difference being the self-assembly time of the active layers for adjusting the P3HT molecular arrangement. The grazing incidence X-ray diffraction (GIXRD) results exhibit that P3HT molecular arrangement can be adjusted between face-on and edge-on structures by controlling the self-assembly time. The champion EQE value of PPDs, based on the active layers without the self-assembly process, arrives at 6380% under 610 nm light illumination at −10 V bias, corresponding to the face-on molecular arrangement of P3HT in the active layers. The EQE values of PPDs were markedly decreased to 1600%, along with the self-assembly time up to 12 min, which should be attributed to the variation of absorption and hole transport ability of the active layers induced by the change of P3HT molecular arrangement. This finding provides an effective strategy for improving the performance of PM-type PPDs by adjusting the molecular arrangement, in addition to the enhanced trap-assisted charge-carrier tunneling injection.Keywords: mobility; photomultiplication; polymer photodetectors; self-assembly; tunneling injection

A smart strategy is reported to obtain photomultiplication (PM) type polymer photodetectors (PPDs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) which are commonly used in polymer solar cells. The PPDs with 1 wt % PC61BM exhibit a champion EQE of 37,500% under 625 nm illumination with an intensity of 8.87 μW cm–2 at −19 V bias. The PM phenomenon of PPDs with rather low PC61BM doping ratios should be attributed to the enhanced hole tunneling injection assisted by trapped electrons in PC61BM near the Al cathode, which can be completely demonstrated from (i) turning distribution of electron traps by changing P3HT:PC61BM doping weight ratios from 200:1 to 1:1; (ii) adjusting interfacial barrier width by inserting LiF layer between the active layer and the Al cathode.Keywords: band bending; organic photodetector; photomultiplication; trap; tunneling injection

•The champion PCE of ternary solar cells was increased to 6.49%.•Charge transfer between donors is beneficial to the photoelectric conversion.•Photon harvesting, charge transport and collection were synergistically improved.A series of polymer solar cells (PSCs) were fabricated with narrow band gap polymer poly{[4,9-dihydro-4,4,9,9-tetra(4-hexylbenzyl)-s-indaceno[1,2-b:5,6-b0]-dithiophene-2,7-diyl]-alt-[2,3-bis(3-(octyloxy)phenyl)-2,3-dihydro-quinoxaline-2,20-diyl] (PIDTDTQx), small molecule material 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIB-SQ), or their blend as electron donor, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor. The champion power conversion efficiency (PCE) of ternary PSCs arrives to 6.49% with a short circuit current density (JSC) of 11.56 mA/cm2 and a fill factor (FF) of 66% when the DIB-SQ doping ratio in donors is 9 wt%. The champion PCE values of binary PSCs with PIDTDTQx:PC71BM or DIB-SQ:PC71BM as the active layers are 5.47% or 1.78%, respectively. An apparent PCE improvement of 18.6% was obtained from the optimized ternary PSCs compared with binary PSCs with PIDTDTQx:PC71BM as the active layers. The underlying reason of PCE improvement was investigated from the absorption spectral complementary, photoluminescence emission quenching, intermolecular charge transfer, and the balance of charge carrier transport in ternary active layers.

●DIO solvent additive is not almighty for PCE improvement of PSCs.●Morphology of active layer can be effectively adjusted by DIO additive and ethanol treatment.●The PCE was decreased from 4.57% to 1.96% by adding DIO additive and recovered to 3.71% by ethanol treatment.Solvent additive 1,8-diiodooctane (DIO) is not almighty for improving the performance of polymer solar cells (PSCs). In this paper, the effect of solvent additive DIO and ethanol treatment on the performance of polymer solar cells (PSCs) with poly{[4,9-dihydro-4,4,9,9-tetra(4-hexylbenzyl)-s-indaceno[1,2-b:5,6-b′]-dithiophene-2,7-diyl]-alt-[2,3-bis(3-(octyloxy)phenyl)-2,3-dihydro-quinoxaline-2,2′-diyl] (PIDTDTQx) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layer was investigated. The power conversion efficiency (PCE) was decreased from 4.57% to 1.96% by adding 4 vol% DIO solvent additive for the active layer processed with 1,2-dichlorobenzene (DCB) as solvent. The negative effect of DIO on PCE from 1.94% to 1.18% of PSCs processed with chlorobenzene (CB) as solvent was further demonstrated. The PCE values of PSCs with DIO additive can be effectively increased from 1.96% to 3.71% for DCB as solvent and from 1.18% to 1.89% for CB as solvent by ethanol treatment on the active layer. The crystalline and morphology of active layers play the key role in determining the performance of PSCs.

A series of polymer solar cells (PSCs) were fabricated with indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor and with PBDT-TS1 as an electron donor. The donor/acceptor (D/A) phase separation was adjusted with different solution processing methods, consisting of cool (room temperature, 20 °C) solution, hot (70 °C) solution and the solutions with solvent additive 1,8-diiodideoctane (DIO). The champion PCE of PSCs with ICBA or PC61BM as an electron acceptor is 4.32% or 5.97% for the active layers prepared from hot solution with DIO additive or cool solution with DIO additive, respectively. The improved PCEs should be attributed to the optimized D/A phase separation in the active layers by adjusting the redistribution of PC61BM or the ICBA among the PBDT-TS1 networks. The degree of phase separation of the active layers with different acceptors was evaluated according to the current density–voltage (J–V) curves of hole-only and electron-only devices. The distribution of PC61BM or ICBA molecules in the normal direction can be simply judged from the symmetry degree of J–V curves of electron-only devices measured under the forward and reverse bias.

We report polymer photodetectors (PPDs) with an evident photomultiplication (PM) phenomenon, based on a sandwich structure ITO/PEDOT:PSS/P3HT:PC71BM(100:1)/Al. A similar device structure has been reported in our previous work, showing great potential as a new type of high performance PPD. However, we found more interesting new phenomena from these PPDs. Solid evidence is provided to prove the existence of photogenerated electron transport in the almost hole-only active layer under an applied bias. The transport of photogenerated electrons leads to electron accumulation near the Al electrode and the electron redistribution, which strongly affect the EQE spectral shape and the transient response of the PPDs. Our conclusion is further confirmed by confirmatory devices with a structure of Al(1)/P3HT:PC71BM(100:1)/Al(2). EQE spectra and transient Jph curves of the confirmatory devices accord well with our speculation. This discovery may provide a new insight to increase the response speed of PM type PPDs by adjusting the photogenerated electron distribution in the active layer. Considering that the PM type PPDs have much higher EQE than the traditional organic photodetectors, the improvement may further extend its potential applications with low cost.

Polymer solar cells (PSCs), with poly(diketopyrrolopyrrole-terthiophene) (PDPP3T):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layers, were fabricated using solutions of different temperatures. The best power conversion efficiency (PCE) of the PSCs prepared using a hot solution was about 6.22%, which is better than 5.54% for PSCs prepared using cool (room temperature) solutions and 5.85% for PSCs prepared using cool solutions with a 1,8-diiodooctane (DIO) solvent additive. The underlying reasons for the improved PCE of the PSCs prepared using a hot solution could be attributed to the more dispersive donor and acceptor distribution in the active layer, resulting in a better bi-continuous interpenetrating network for exciton dissociation and charge carrier transport. An enhanced and more balanced charge carrier transport in the active layer is obtained for the PSCs prepared using a hot solution, which can be determined from the J–V curves of the related hole-only and electron-only devices.

•D/A intermolecular arrangement strongly determines OSCs performance.•Large VOC of 1.15 V was obtained for the OSCs based on ICBA as acceptor.•The champion PCE of 7.05% was obtained for the cells with PC71BM as acceptor.A series of solution processed organic solar cells (OSCs) were fabricated with a two-dimensional conjugated small molecule SMPV1 as electron donor and fullerene derivatives PC71BM or ICBA as electron acceptor. The champion power conversion efficiency (PCE) of OSCs arrives to 7.05% for the cells with PC71BM as electron acceptor. A relatively large open circuit voltage (VOC) of 1.15 V is obtained from cells using ICBA as electron acceptor with an acceptable PCE of 2.54%. The fill factor (FF) of OSCs is 72% or 61% for the cells with PC71BM or ICBA as electron acceptor, which is relatively high value for small molecule OSCs. The relatively low performance of OSCs with ICBA as electron acceptor indicates that ICBA cannot play positive role in photoelectric conversion processes, which is very similar to the phenomenon observed from the OSCs with high efficient narrow band gap polymers other than P3HT as electron donor, the underlying reason is still in debate. The SMPV1 has strong self-assemble ability to form an ordered two dimensional lamellar structure, which provides an effective platform to investigate the effect of electron acceptor chemical structure on the performance of OSCs. Experimental results exhibit that ICBA molecules may prefer to vertical cross-intercalation among side chains of SMPV1, PC71BM molecules may have better miscibility with SMPV1 in the active layer. The different donor/acceptor (D/A) intermolecular arrangement strongly influences photon harvesting, exciton dissociation and charge carrier transport, which may provide a new sight on performance improvement of OSCs by adjusting D/A intermolecular arrangements.

•Synthesis of polymers of [1,2,5]thiadiazolo[3,4-g]quinoxaline and BDT and NDF.•Bandgaps of 1.03–1.10 eV for the polymers.•Improved PCE for P3HT devices by doping 9 wt% new polymers.A new strong electron-deficient acceptor unit, 4,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TQxT), is designed. Three TQxT based new conjugated polymers have been further developed by alternating with benzo[1,2-b:4,5-b′]dithiophene or naphtho[1,2-b:5,6-b′]difuran. Featuring ultra low bandgaps of 1.03–1.10 eV, these polymers exhibit absorption spectra beyond near-infrared (NIR) region. The polymer:PC71BM (1:2) solar cells deliver a best power conversion efficiency (PCE) of 0.43%. The doping of 9 wt% PBDTT-TQxP into P3HT:PC71BM (1:2) blend, however, leads to a highest PCE of 3.58% for the resultant ternary solar cells, corresponding to ∼22% improvement in comparison to 2.93% PCE for P3HT:PC71BM binary cells.

A novel donor-acceptor (D-A) structural polymer PBDT-BTzQx-C12 consisting of benzodithiophene and triazoloquinoxaline units as donor and acceptor building blocks was synthesized. The PBDT-BTzQx-C12 was evaluated as complementary electron donor in polymer solar cells (PSCs) with poly(3-hexylthiophene) (P3HT) as electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor. The power conversion efficiencies (PCEs) of PSCs were improved from 3.18% to 3.54% by doping 2 wt. % PBDT-BTzQx-C12, corresponding to an approximately 11% PCE improvement. The performance improvement of ternary PSCs should be attributed to the increase of photon harvesting and the optimized phase separation of active layers by doping D-A structural PBDT-BTzQx-C12.

We present performance improved ternary bulk heterojunction polymer solar cells by doping a small molecule, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIB-SQ), into the common binary blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The optimized power conversion efficiency (PCE) of P3HT:PC71BM-based cells was improved from 3.05% to 3.72% by doping 1.2 wt % DIB-SQ as the second electron donor, which corresponds to ∼22% PCE enhancement. The main contributions of doping DIB-SQ material on the improved performance of PSCs can be summarized as (i) harvesting more photons in the low-energy range, (ii) increased exciton dissociation, energy transfer, and charge carrier transport in the ternary blend films. The energy transfer process from P3HT to DIB-SQ is demonstrated by time-resolved transient photoluminescence spectra through monitoring the lifetime of 700 nm emission from neat P3HT, DIB-SQ and blended P3HT:DIB-SQ solutions. The lifetime of 700 nm emission is increased from 0.9 ns for neat P3HT solution, to 4.9 ns for neat DIB-SQ solution, to 6.2 ns for P3HT:DIB-SQ blend solution.Keywords: energy transfer; low bandgap materials; polymer solar cells; ternary bulk heterojunction;

We present a route to successfully tackle the two main limitations, low open circuit voltage (Voc) and limited short circuit-density (Jsc), of polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) as an electron-donor. The indene-C60 bisadduct (ICBA) was selected as an electron acceptor to improve the open circuit voltage (Voc). The narrow band gap polymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C), as a complementary electron-donor material, was doped into the host system of P3HT:ICBA to form ternary cascade energy structured PSCs with increased Jsc. The power conversion efficiency (PCE) of P3HT:ICBA-based cells was improved from 3.32% to 4.38% by doping with 3 wt% PBDTTT-C with 1 min 150 °C annealing treatment. The 4.38% PCE of ternary PSCs is still larger than the 3.79% PCE of PSCs based on P3HT:ICBA with 10 minutes 150 °C annealing treatment.

A series of polymer solar cells (PSCs) based on poly (diketopyrrolopyrrole-terthiophene) (PDPP3T) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as active layer were fabricated to investigate the effect of 1,8-diiodooctane (DIO) on the performance of PSCs. The power conversion efficiency (PCE) of PSCs was increased from 3.77 % to 4.37 % for the cells with DIO additive. The underlying reason may be attributed to that DIO additive could make PC71BM more dispersive in the active layer, forming a better bi-continuous interpenetrating network for excition dissociation and charge carrier transport. Therefore, the short circuit current density (JSC) and fill factor (FF) was increased from 8.25 to 9.18 mA/cm2 and from 67.2 % to 70.0 % for the PSCs with DIO additive compared with PSCs without DIO additive.

•The PCE of ternary blend BHJ PSCs was increased to 2.48%, with 27% improvement compared with the control PSCs.•The improvement of PCE is attributed to the balance between photon harvest and charge carrier transport.•The ternary blend BHJ PSCs provides a simple method to widen spectral response range.The performance of polymer solar cells (PSCs), based on poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C71-butyric acid methyl ester (PC71BM), were improved by adding poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′](dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C). The absorption region of the ternary blend films is extended into near infrared region (NIR). Power conversion efficiency (PCE) of the ternary blend PSCs is improved to 2.48% with 6% PBDTTT-C, which is 27% higher than the binary blend PSCs. The dominant mechanism for the PCE improvement could be attributed to the well balance between photon harvest and charge carrier transport by doping proper concentration PBDTTT-C. The energy transfer or charge carrier transfer directly between P3HT and PBDTTT-C was investigated, which are found to be positive for the performance improvement of ternary blend PSCs.

•The PCE of solution-processed small molecular solar cells (SMSCs) arrives to 2.68%.•The roughness of MoO3 increases the interfacial area between anode and active layer.•JscVoc, and FF of cells with MoO3 as anode interfacial layer are improved about 54.7%, 26.9%, and 23.5%.A series of solution-processed small molecular solar cells (SMSCs) were fabricated with 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] (DIB-SQ) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as the active layer. The cells with MoO3 as anode interfacial layer and LiF as cathode interfacial layer have the maximum power conversion efficiency (PCE) 2.68%, with short circuit density (Jsc) of 8.2 mA/cm2, open circuit voltage (Voc) of 0.9 V and fill factor (FF) of 36.3% under 100 mW/cm2 illumination intensity. The key parameters, including JscVoc, and FF of cells with MoO3 as anode interfacial layer and LiF as cathode interfacial layer, are improved about 54.7%, 26.9%, and 23.5% compared with these of cells using PEDOT:PSS as anode interfacial layer and LiF as cathode interfacial layer. The underlying reasons are mainly attributed to the effect of MoO3 layer on charge carrier collection and morphology of active layer. The effect of illumination intensity on the performance of cells was also discussed in detailed.

Two-dimensional poly[fluorene-alt-bithiophene] backboned copolymers with phenylvinyl bridged accepting side chain exhibit tuned bandgap and absorption spectrum with different accepting units, which may be used as promising electroluminescent materials.Two-dimensional poly[fluorene-alt-bithiophene] backboned copolymers were prepared by introducing phenylvinyl bridged accepting side chain containing malononitrile, 1,3-indanedione, or 4-nitrophenyl acetonitrile accepting moiety. The structural, optical, electroluminescent, electrochemical properties of these polymers were studied in details. These polymers possess good thermal stability and low highest occupied molecular orbital (HOMO) level (−5.59 to 5.67 eV). Results show that the introduction of π-conjugated accepting side chains can effectively adjust the optical and electrochemical properties of the resulting polymers. These new alternating copolymers may be promising electroluminescent materials.Graphical abstract

Two-dimensional polyfluorenes bearing thienylenevinylene-bridged malononitrile (PF-BTDCN) or diethylthiobarbituric acid accepting side chains (PF-BTDTA) have been successfully prepared. The polymers were fully characterized for their physicochemical, electrochemical, and photovoltaic properties. These polymers exhibited greatly changed properties with the introduction of π-conjugated accepting side chains. The enhancement of current density for the bulk-heterojunction solar cells was observed when replacing the PEDOT:PSS interfacial layer with molybdenum oxide (MoO3). Photovoltaic solar cells with the configuration ITO/MoO3/polymer:PCBM/Al exhibited an efficiency of 3.13% and 1.72% for PF-BTDCN and PF-BTDTA, respectively. A morphology study revealed the existence of nanoscale phase separation with interpenetration networks between polymer and PCBM domains.

•Summary innovated and efficient light out-coupling technology.•Novel transparent conducting electrodes for flexible OLEDs.•Ideas for improving white emission OLEDs performance.Organic light-emitting diodes (OLEDs) are considered as an ideal in next generation of flat panel displays and solid state lighting source. Still, the stability and efficiency of OLEDs remain great challenges for its commercialization application. This article provides an overview on working principle of different kinds of luminescence, effective methods to improve quantum efficiency, recent progress of white emission OLEDs, novel types of transparency electrode for flexible OLEDs and the stability of OLEDs. A series of interesting and promising ideas to improve the performance of OLEDs are summarized from physical engineering based on the recent achievement of high brightness, high efficient and good stability of OLEDs.

The critical review on the recent development of novel narrow bandgap polymers for high-efficiency polymer solar cells concentrates on (i) the structural design of narrow bandgap polymers, which occupy a central place in recent advances in high-efficiency polymer solar cells, (ii) the intrinsic physics and chemistry of special properties, such as absorption, bandgap and energy levels, and (iii) the correlation of polymer structure and device fabrication with their photovoltaic performances. The statistical summaries of their device parameters are also discussed. The description of these structure–property correlations may guide the rational design of polymer structures and the reasonable evaluation of their photovoltaic performance.

Inverted polymer solar cells (IPSCs) were fabricated with cesium carbonate (Cs2CO3) modified indium tin oxide (ITO) substrates as the electrode and molybdenum trioxide (MoO3) modified Al as the anode. The Cs2CO3 dissolved in 2-ethoxyethanol was spin-coated on ITO substrates, showing snowflake-like morphology characterized by the scanning electron microscope (SEM). The absorption, X-ray diffraction as well as the morphology of the active layer were measured before and after annealing treatment. The IPSCs with annealing treatments on the active layers and MoO3 layers exhibited the maximum power conversion efficiency (PCE) approaching to 2%, with open circuit voltage (Voc) of 0.57 V, short circuit current density (Jsc) of 8.8 mA/cm2 and fill factor (FF) of 38.7%. The performance of IPSCs was dramatically decreased by annealing treatment after the deposition of Al cathode, which may be due to the diffusion of Al atom crossing the MoO3 layer forming new channels for charge carrier collection. However, the new channels are not beneficial to the charge carrier collection, which is demonstrated from that the Jsc of IPSCs was evidently decreased from 8.8 to 4.6 mA/cm2 by annealing treatment after deposition Al layer. The annealing treatment after deposition of MoO3 could improve the interfacial contact to aid in electron extraction.Highlights► PCE of inverted polymer solar cells (IPSc) with Cs2Co3 modified ITO as cathode and MoO3/Al as anode is approaching to 2%. ► The snowflake-like morphology of Cs2CO3 was investigated by SEM. ► The annealing treatment before deposition Al layer has positive effect on improvement of IPSCs performance. ► EDX results directly demonstrate the vertical phase separation of P3HT:PCBM induced by annealing treatment.

We studied the luminescent and photovoltaic properties of poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) based on ITO/PEDOT:PSS/F8T2/Bphen/LiF(0 or 1 nm)/Al and ITO/PEDOT:PSS/F8T2:PCBM/Bphen/Al. A stable and bright yellow emission was obtained from polymer F8T2, and the electroluminescence power reached 45 μW at a 15 V driving voltage. Polymer F8T2 shows a broad absorption band from 400 to 500 nm, and has a shorter absorption edge at about 560 nm compared to that of the typical electron donor P3HT (650 nm). The photoluminescence quenching of F8T2 occurs with only a small fraction of blended PCBM due to the effective exciton dissociation at the interface between F8T2 and PCBM. Polymer solar cells (PSCs) using F8T2:PCBM as the active layer show a low power conversion efficiency (PCE) of 0.10% with an open circuit voltage (Voc) of 0.91 V and short circuit current density (Jsc) of 0.23 mA/cm2. The PSCs using F8T2:P3HT:PCBM as the active layer have a Voc of 0.85 V and Jsc of 3.02 mA/cm2, improving the PCE by about 0.90%. We attribute the improved cell performance to the higher number of photons harvested by P3HT molecules.

Organic photovoltaic cells (OPVs) have attracted more and more attention due to its highly potential application to solve the energy crisis considering its advantages, such as low cost and ease of large area production. The power conversion efficiency (PCE) of OPVs has undergone a more than nine-fold increase from ∼1.0% by Tang in 1986 to 9.2% in 2010 announced by Mitsubishi Chemical. The major challenges of obtaining high efficiency OPVs are the synthesis of new narrow band gap materials, controlling molecular arrangement, designing novel configuration cells for better photon harvesting in the active layer. In the article, we summarized the recent progress of novel narrow band gap photovoltaic materials and the effective methods to control the morphology of donor and acceptor in the blend films for high performance of OPVs.

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C60 butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I2) doping concentrations. The short circuit current density (Jsc) was increased to 8.7 mA/cm2 from 4 mA/cm2, meanwhile the open circuit voltage (Voc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.Highlights► Power conversion efficiency of P3HT:PCBM organic solar cells was increased through optimal iodine doping concentration. ► Short circuit current density was increased to 8.7 mA/cm2 from 4 mA/cm2 when iodine doping concentration is 5%. ► These improvements can be attributed to the increase of absorption intensity and range, and better charge carrier transport and collection induced by iodine doping. ► A series of characterizations were carried out to clarify the function of iodine doping in the polymer solar cells.

The luminescence processes of metal complexes are complicated by intramolecular charge (energy) transfer from the metal to the ligand or from the ligand to the metal. The charge transfer strongly influences the excited state of the ligand and its luminescence characteristics. The luminescence characteristics of tris(8-hydroxyquinoline) aluminum (Alq3) and tris(8-hydroxyquinoline) gallium (Gaq3) are investigated to reveal the effect of the metal ion on the ligand. Emission from the complexes shows a significant red shift as the size of the metal ion increases from Al to Ga because of more efficient charge transfer from the metal to the ligand. Theoretical calculations on the structure and transition characteristics of the excited states of Alq3 and Gaq3 were performed. The calculated emission wavelength agrees with the experimental value and the effect of the metal electron cloud on the emission wavelength is clarified.

In poly(3-hexylthiophene) mixed with phenyl C61-butyric acid methyl ester heterojunction polymer solar cells, organic small molecular pentacene was introduced as the interfacial layer between PEDOT:PSS coated ITO substrates and polymer layer. It is found that the short circuit current density and power conversion efficiency were distinctly improved due to the introduction of the nanostructural pentacene interlayer. The nearly 100% power conversion efficiency improvement was obtained on the cells with a 4 nm pentacene interlayer, which benefits from the increased short circuit current from 2.34 mA/cm2 to 5.76 mA/cm2. The morphology of different thicknesses of pentacene thin films was observed by atomic force microscopy. The effect of pentacene interlayer's thickness on the distribution of light in the active layer was simulated by using a transfer matrix mode.

Wide band gap semi-conductor zinc sulfide (ZnS) nanocolumn arrays were prepared on the glass side of indium tin oxide (ITO) substrates by glancing angle deposition (GLAD) technology. The scanning electron microscopy (SEM) images show the formation of ZnS nanocolumn arrays when the glancing angle was set to 85°; however, continuous ZnS films without any evident nanostructures were fabricated under normal deposition. The transmitting ability of ITO substrates coated with ZnS nanocolumn arrays is improved in comparison to bare ITO substrates and continuous ZnS films coated ITO substrates in the visible range. Organic light-emitting diodes (OLEDs) were simultaneously fabricated on these three kinds of substrates. The electroluminescence (EL) intensity of OLEDs based on the ITO substrate coated with ZnS nanocolumn arrays was about 1.2 times bigger than the devices based on the other substrates (bare ITO substrates and continuous ZnS films coated ITO substrates) under the same driving voltage. The improvement of EL intensity should be ascribed to the enhancement of the light extraction by the nanocolumn arrays effect.

The effect of an ultra-thin molybdenum trioxide (MoO3) layer thickness inserted between the indium tin oxide (ITO) substrate and copper phthalocyanine (CuPc) layer on the performance of organic photovoltaic devices (OPVs) was studied. Experimental results demonstrate that the short-circuit current density (Jsc) was decreased slightly with the increase of MoO3 thickness; meanwhile, the fill factor (FF) was increased from 53.5 to 57.7%, respectively, leading to the improved power conversion efficiency with the optimal thickness of MoO3 (1 nm). The experimental results also reveal that the Ohmic contact is formed with the deposition of MoO3. Further, the effect of the MoO3 layer was checked from the variation of the performance of OPVs under different illumination intensities. It was found that the MoO3 layer could effectively prevent exciton quenching at the ITO anode side, resulting in the small variation of the FF for the devices with the MoO3 layer compared to the devices without the MoO3 layer under high illumination intensity.

A series of Co-doped ZnO thin films have been prepared by direct current reactive magnetron sputtering on glass substrates. The structural characterization by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) gave no evidence of second phase formation. The qualitative composition and chemical state were characterized by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectronic spectrometry (XPS), respectively. The results confirmed that Co was incorporated as Co3+, occupying the Zn2+ sites in ZnO’s wurtzite structure. The luminescence properties of the films were investigated by room temperature photoluminescence, and the optical properties were studied by optical transmittance. The magnetic analysis was carried out at room temperature and at 50 K by Quantum Design MPMS (SQUID) XL. The results showed that all the Co-doped ZnO thin films prepared by direct current magnetron sputtering were not ferromagnetic above 50 K.

In this paper, the influence of the ultraviolet (UV)–ozone treatment of indium tin oxide (ITO) surface and the active layer post-annealing treatment on the performance of organic solar cells were investigated. Bulk heterojunction organic solar cells based on the blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were fabricated. It is found that the devices with the UV–ozone treatment for 5 min on ITO substrates show the better performance, compared with the devices without this treatment. The results demonstrate that the short-circuit current density (Jsc) and fill factor (FF) could be improved by the post-annealing treatment. The devices with both treatments together show the best performance, with the increase of Jsc from 2.68 mA/cm2 to 4.13 mA/cm2 and the enhancement of FF from 32.2% to 38.8%. Therefore, the power conversion efficiency is improved from 0.62% to 1.08%. The morphology of the active layers with and without the post-annealing treatment was characterized by atomic force microscopy.

There is an emission peak at 494 nm in the electroluminescence (EL) of PVK [poly(n-vinylcarbazole)]: Eu(o-BBA)3(phen) besides PVK exciton emission and Eu3+ characteristic emissions. Both the peaking at 494 nm emission and PVK emission influenced the color purity of red emission from Eu(o-BBA)3(phen). In order to restrain these emissions and obtain high intensity red emission, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyljulolidy-9-enyl)-4Hpyran (DCJTB) and Eu(o-BBA)3(phen) were co-doped in PVK solution and used as the active emission layer. The EL intensity of co-doped devices reached to 420 cd/m2 at 20 V driving voltage. The chromaticity coordinates of EL was invariable (x = 0.55, y = 0.36) with the increase of driving voltage. For further improvement of EL intensity, organic–inorganic hybrid devices (ITO/active emission layer/ZnS/Al) were fabricated. The EL intensity was increased by a factor of 2.5 [(420 cd/m2)/(168 cd/m2)] when the Eu complex was doped with an efficient dye DCJTB, and by a factor of ≈4 [(650 cd/m2)/(168 cd/m2)] when in addition ZnS layer was deposited on such an emitting layer prior to evaporation of the Al cathode.

The critical review on the recent development of novel narrow bandgap polymers for high-efficiency polymer solar cells concentrates on (i) the structural design of narrow bandgap polymers, which occupy a central place in recent advances in high-efficiency polymer solar cells, (ii) the intrinsic physics and chemistry of special properties, such as absorption, bandgap and energy levels, and (iii) the correlation of polymer structure and device fabrication with their photovoltaic performances. The statistical summaries of their device parameters are also discussed. The description of these structure–property correlations may guide the rational design of polymer structures and the reasonable evaluation of their photovoltaic performance.

•The PCE of solution-processed small molecular solar cells (SMSCs) arrives to 2.68%.•The roughness of MoO3 increases the interfacial area between anode and active layer.•Jsc Voc, and FF of cells with MoO3 as anode interfacial layer are improved about 54.7%, 26.9%, and 23.5%.A series of solution-processed small molecular solar cells (SMSCs) were fabricated with 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] (DIB-SQ) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as the active layer. The cells with MoO3 as anode interfacial layer and LiF as cathode interfacial layer have the maximum power conversion efficiency (PCE) 2.68%, with short circuit density (Jsc) of 8.2 mA/cm2, open circuit voltage (Voc) of 0.9 V and fill factor (FF) of 36.3% under 100 mW/cm2 illumination intensity. The key parameters, including Jsc Voc, and FF of cells with MoO3 as anode interfacial layer and LiF as cathode interfacial layer, are improved about 54.7%, 26.9%, and 23.5% compared with these of cells using PEDOT:PSS as anode interfacial layer and LiF as cathode interfacial layer. The underlying reasons are mainly attributed to the effect of MoO3 layer on charge carrier collection and morphology of active layer. The effect of illumination intensity on the performance of cells was also discussed in detailed.

•The PCE of ternary blend BHJ PSCs was increased to 2.48%, with 27% improvement compared with the control PSCs.•The improvement of PCE is attributed to the balance between photon harvest and charge carrier transport.•The ternary blend BHJ PSCs provides a simple method to widen spectral response range.The performance of polymer solar cells (PSCs), based on poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C71-butyric acid methyl ester (PC71BM), were improved by adding poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′](dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C). The absorption region of the ternary blend films is extended into near infrared region (NIR). Power conversion efficiency (PCE) of the ternary blend PSCs is improved to 2.48% with 6% PBDTTT-C, which is 27% higher than the binary blend PSCs. The dominant mechanism for the PCE improvement could be attributed to the well balance between photon harvest and charge carrier transport by doping proper concentration PBDTTT-C. The energy transfer or charge carrier transfer directly between P3HT and PBDTTT-C was investigated, which are found to be positive for the performance improvement of ternary blend PSCs.Download full-size image

We present a route to successfully tackle the two main limitations, low open circuit voltage (Voc) and limited short circuit-density (Jsc), of polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) as an electron-donor. The indene-C60 bisadduct (ICBA) was selected as an electron acceptor to improve the open circuit voltage (Voc). The narrow band gap polymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C), as a complementary electron-donor material, was doped into the host system of P3HT:ICBA to form ternary cascade energy structured PSCs with increased Jsc. The power conversion efficiency (PCE) of P3HT:ICBA-based cells was improved from 3.32% to 4.38% by doping with 3 wt% PBDTTT-C with 1 min 150 °C annealing treatment. The 4.38% PCE of ternary PSCs is still larger than the 3.79% PCE of PSCs based on P3HT:ICBA with 10 minutes 150 °C annealing treatment.

The dynamic drying process of the active layer should play a vitally important role in determining the performance of polymer solar cells (PSCs). Donor molecular packing and acceptor redistribution can be optimized by two successive post-treatments on the active layer. The blend films were freshly prepared by spin-coating method and then immediately transferred to a covered glass Petri dish to allow self-assembly of the donor molecules. The films were then treated with methanol or PFN–methanol solution to adjust the acceptor redistribution. In this study, power conversion efficiencies (PCEs) of PSCs with PffBT4T-2OD:PC71BM as the active layer were improved from 6.74% to 8.75% by employing 80 min for self-assembly and 20 s of methanol soaking. The PCE was improved even further to 9.72% by inserting a PFN interfacial layer. The performance improvement was mainly attributed to the optimized PffBT4T-2OD molecular packing during the self-assembly process, ideal vertical phase separation driven by methanol soaking and efficient charge collection by insertion of a PFN interfacial layer. The molecular packing and vertical phase separation were characterized by grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The experimental results solidly supported the effectiveness of the step-by-step optimization strategy.

Herein, a liquid crystal material, BTR, was elaborately selected as the third component to complement PTB7-Th for the fabrication of highly efficient ternary polymer solar cells (PSCs). Via incorporating 10 wt% BTR in their donors, the champion power conversion efficiency (PCE) of the PSCs increased from 10.08% to 10.83%, resulting from the enhanced short circuit current (JSC) of 19.23 mA cm−2 and fill factor (FF) of 72.21%. The small amount of incorporated BTR may prefer to distribute in the PTB7-Th networks and has good miscibility with PC71BM. The excitons on BTR may dissociate into free charge carriers at the BTR/PC71BM interfaces and also transfer their energy to PTB7-Th through Förster resonance energy transfer, resulting in improved exciton utilization. Moreover, the molecular arrangement and morphology of the active layers could be optimized by incorporating appropriate amount of BTR as a nucleating agent, also leading to enhanced stability of the ternary PSCs. The positive effects of BTR on the performance improvement of PSCs were confirmed from the inverted and conventional structures of the cells.

It is a great challenge to obtain narrowband and photomultiplication (PM) type organic photodetectors (OPDs) without optical filters due to the broad absorption range and large exciton binding energy of organic materials. Narrowband OPDs with the special structure of ITO/PFN-OX/P3HT:PC61BM (100:1,wt/wt)/Al were successfully fabricated with different active layer thicknesses, exhibiting a tunable response window and PM phenomenon under bi-directional bias. The OPDs exhibit U-shaped EQE spectra with two narrowband response windows under forward bias and a single narrowband response window under reverse bias. The best EQE of the optimized OPDs with a 4.0 μm thick active layer approaches 7160% or 8180% for 340 nm or 650 nm light illumination at 60 V and 1640% for 665 nm light illumination at −60 V, respectively. The most important features of the optimized OPDs is that the full width at half-maximum (FWHM) of their response windows is less than 30 nm under bi-directional biases, which can be well maintained at large bias. The PM type narrowband OPDs working at bi-directional bias are the first to be reported with a tunable response range, further indicating that the density of the electrons trapped in PC61BM near the hole injection electrode plays the key role in determining the interfacial band bending for hole tunneling injection from the external circuit.

Solution-processed small molecule solar cells (SMSCs) are fabricated based on DRCN5T:PC71BM as active layers, the power conversion efficiency (PCE) is markedly increased from 3.63% to 9.11% for the active layers undergoing up-side-down thermal annealing and solvent vapor annealing post-treatments. The PCE improvement should be attributed to the appropriate phase separation consisting of enhanced crystallinity of donor and purified acceptor domain at nanoscale. The nematic liquid crystal small molecule BTR is selected as the second donor and morphology regulator to prepare ternary SMSCs. The champion PCE of ternary SMSCs was improved to 10.05% by mixing 1.5 wt% BTR, which corresponds to a 10.3% PCE improvement compared with the optimized binary SMSCs. The performance improvement is mainly attributed to the further optimized phase separation and complementary photon harvesting between DRCN5T and BTR, which could be well demonstrated from absorption spectra, two dimensional grazing incidence X-ray diffraction (2D-GIXD) and transmission electron microscopy (TEM).

Photon harvesting and phase separation can be synergistically optimized by using ternary strategy with appropriate DIB-SQ-doped Si-PCPDTBT:PC71BM as active layer. The power conversion efficiency (PCE) of ternary organic photovoltaic cells (OPVs) exhibits an increasing and then decreasing trend dependent on the increase in DIB-SQ doping ratio. The PCE of OPVs was increased from 5.52% to 6.18% by doping 3 wt% DIB-SQ while keeping the donor/acceptor doping weight ratio at 1:1.5. The PCE improvement can be attributed to the better trade-off between photon harvesting and phase separation in the ternary active layers. The degree of phase separation strongly influences energy or charge transfer between donors and charge transport in the ternary active layers. The dynamic process between donors was investigated according to J–V curves of cells without acceptor under light illumination. Experimental results fully confirm that this ternary strategy should have a brilliant future due to about 12% PCE improvement compared with Si-PCPDTBT:PC71BM-based binary OPVs.

A series of polymer solar cells (PSCs) were fabricated with indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor and with PBDT-TS1 as an electron donor. The donor/acceptor (D/A) phase separation was adjusted with different solution processing methods, consisting of cool (room temperature, 20 °C) solution, hot (70 °C) solution and the solutions with solvent additive 1,8-diiodideoctane (DIO). The champion PCE of PSCs with ICBA or PC61BM as an electron acceptor is 4.32% or 5.97% for the active layers prepared from hot solution with DIO additive or cool solution with DIO additive, respectively. The improved PCEs should be attributed to the optimized D/A phase separation in the active layers by adjusting the redistribution of PC61BM or the ICBA among the PBDT-TS1 networks. The degree of phase separation of the active layers with different acceptors was evaluated according to the current density–voltage (J–V) curves of hole-only and electron-only devices. The distribution of PC61BM or ICBA molecules in the normal direction can be simply judged from the symmetry degree of J–V curves of electron-only devices measured under the forward and reverse bias.

We report polymer photodetectors (PPDs) with an evident photomultiplication (PM) phenomenon, based on a sandwich structure ITO/PEDOT:PSS/P3HT:PC71BM(100:1)/Al. A similar device structure has been reported in our previous work, showing great potential as a new type of high performance PPD. However, we found more interesting new phenomena from these PPDs. Solid evidence is provided to prove the existence of photogenerated electron transport in the almost hole-only active layer under an applied bias. The transport of photogenerated electrons leads to electron accumulation near the Al electrode and the electron redistribution, which strongly affect the EQE spectral shape and the transient response of the PPDs. Our conclusion is further confirmed by confirmatory devices with a structure of Al(1)/P3HT:PC71BM(100:1)/Al(2). EQE spectra and transient Jph curves of the confirmatory devices accord well with our speculation. This discovery may provide a new insight to increase the response speed of PM type PPDs by adjusting the photogenerated electron distribution in the active layer. Considering that the PM type PPDs have much higher EQE than the traditional organic photodetectors, the improvement may further extend its potential applications with low cost.

Polymer solar cells (PSCs), with poly(diketopyrrolopyrrole-terthiophene) (PDPP3T):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layers, were fabricated using solutions of different temperatures. The best power conversion efficiency (PCE) of the PSCs prepared using a hot solution was about 6.22%, which is better than 5.54% for PSCs prepared using cool (room temperature) solutions and 5.85% for PSCs prepared using cool solutions with a 1,8-diiodooctane (DIO) solvent additive. The underlying reasons for the improved PCE of the PSCs prepared using a hot solution could be attributed to the more dispersive donor and acceptor distribution in the active layer, resulting in a better bi-continuous interpenetrating network for exciton dissociation and charge carrier transport. An enhanced and more balanced charge carrier transport in the active layer is obtained for the PSCs prepared using a hot solution, which can be determined from the J–V curves of the related hole-only and electron-only devices.

Four isostructural donor–acceptor alternating polymers of benzodithiophene (BDT)/naphthodifuran (NDF) and benzoselenadiazole (BSe)/benzothiadiazole (BT) have been developed and evaluated for organic photovoltaics. The substitution of one-atom (Se for S) in the accepting units exerts remarkable impact on the optoelectronic properties of polymers. Extended absorption, narrowed bandgap and higher HOMO energy levels were observed for Se-containing polymers in comparison to their S-containing counterparts. Theoretical calculations confirmed the measurable effect on energy levels as found in experimental studies. Bulk-heterojuction solar cells based on the BDT–BSe copolymer and [6,6]-phenyl-C71-butyric acid methyl ester (1:2, w/w) blend films deliver the best PCE of 5.40%. BSe-based polymers showed enhanced photovoltaic performances than BT-based polymers. The device performance is found to be strongly dependent on the processing conditions and morphology of the active layers.

A series of ternary polymer solar cells (PSCs) were fabricated based on two narrow bandgap polymers PTB7 and PBDT-TS1 as the donors and PC71BM as the acceptor. The performances of the ternary PSCs monotonically increased along with the PBDT-TS1 doping ratio, up to 80 wt% in the donors. The optimum power conversion efficiency (PCE) achieved for the ternary PSCs was 7.91% with an open circuit voltage (VOC) of 0.76 V, a short circuit current density (JSC) of 18.85 mA cm−2 and a fill factor (FF) of 55.2% for the active layers with an 80 wt% PBDT-TS1 doping ratio in the donors. The optimized ternary PSCs show a 12.8% improvement compared to the optimum PCE of 7.01% for the binary PSCs with PBDT-TS1 as the donor and a 28.2% improvement compared with the optimum PCE of 6.17% for the binary PSCs with PTB7 as the donor. The FFs of all the ternary PSCs are larger than 54%, indicating efficient charge carrier transport channels in the ternary active layers. The energy or charge transfer between PTB7 and PBDT-TS1 should be neglected according to the investigation of the photoluminescence spectra of the blend films and the current density–voltage (J–V) curves of devices without the acceptor in the active layers. The ternary PSCs should be parallel-linkage structures with the donors independently working with the acceptor, which may be the most promising strategy for obtaining highly efficient ternary PSCs.

Highly sensitive polymer photodetectors (PPDs) are successfully achieved with a broad spectral response range from UV light to the near infrared region (NIR) based on P3HT:PTB7-Th:PC71BM as the active layer. The highest external quantum efficiency (EQE) values of the PPDs with P3HT:PC71BM (100:1) as the active layer are 90700% and 84100%, corresponding to 390 nm and 625 nm light illumination under a −25 V bias, respectively. The spectral response range of the PPDs can be extended to the NIR by doping narrow band gap polymer PTB7-Th into P3HT:PC71BM as the active layer. The highest EQE values of the PPDs with P3HT:PTB7-Th:PC71BM (50:50:1) as the active layer are around 38000% in the spectral range from 625 nm to 750 nm under a −25 V bias. The high EQE values of the PPDs should be attributed to three points: (i) the rather weak dark current due to the relatively large hole injection barrier; (ii) the enhanced hole tunneling injection due to the interfacial band bending, which is induced by trapped electrons in PC71BM near the Al cathode; and (iii) the efficient hole-only transport in the active layers with the rather low PC71BM content. The broad spectral response range is due to the contribution of PTB7-Th exciton dissociation on the number of trapped electrons in PC71BM near the Al cathode.

A series of solvent additive-free polymer solar cells (PSCs) were fabricated with PBDT-TS1:PC71BM (1:2, wt/wt) as the active layers; the performance of the PSCs can be enhanced by using MeOH:CH2Cl2 or MeOH:CB mixed solvent spin-coating treatment on the active layers. The maximum power conversion efficiency (PCE) of PSCs was increased from 6.69% to 7.21% or 7.76% by spin-coating the mixed solvents onto the active layers, respectively. The PCE improvement may be attributed to the increase in PC71BM elevated toward the top surface of the active layers during the mixed solvent spin-coating treatment. The redistribution of PC71BM in the active layers could also be indirectly confirmed from the variation of photoluminescence spectra and water contact angles of the corresponding films, as well as the J–V curves of electron-only devices.

Incorporation of a solution processed PFN interfacial layer has been demonstrated as an efficient method to improve the performance of polymer solar cells (PSCs). The champion power conversion efficiency (PCE) of PSCs with PTB7-Th:PC71BM as the active layer was increased from 6.13% to 7.72% by directly spin-coating PFN methanol solution on the surface of active layers or to 8.50% for the active layer with 4 min PFN methanol solution soaking. The champion PCE of PSCs was further increased to 8.69% for the active layers with a two-step treatment, 4 min methanol soaking and then directly spin-coating PFN methanol solution. A 12.6% PCE improvement was obtained by using the two-step strategy compared with directly spin-coating PFN methanol solution on the active layers. Methanol soaking of the active layers plays the key role in forming the more ideal vertical phase separation for efficient exciton dissociation, charge carrier transport and collection. An ultrathin PFN interfacial dipole layer can be obtained by directly spin-coating PFN methanol solution. The two-step strategy may provide a simple and effective method to finely optimize the phase separation and obtain an ultrathin PFN interfacial dipole layer for the performance improvement of PSCs.

Recently, power conversion efficiency (PCE) of organic solar cells has been increased up to about 10% by using solvent additives or mixed solutions to elaborately adjust phase separation which is a great challenge for large scale production. We report a champion PCE of 7.40% for solution-processed small molecule ternary SMPV1:DIB-SQ:PC71BM solar cells by only adjusting the DIB-SQ doping ratio in donors without any treatments on the blend solutions or active layers. The champion PCE of ternary solar cells is larger than the champion PCEs sum (6.98%) of binary solar cells with SMPV1:PC71BM or DIB-SQ:PC71BM as the active layers. The PCE improvement should be attributed to the synergistic enhancement of photon harvesting, exciton dissociation, charge carrier transport and collection by the appropriate DIB-SQ doping ratio in donors. The experimental results on morphology, crystallinity, phase separation and charge carrier mobility of active layers well support the PCE improvement in ternary solar cells with a 10 wt% DIB-SQ doping ratio in donors.