The surface plasmon resonance (SPR) effect of metal nanoparticles (MNPs) is effectively applied on polymer solar cells (PSCs) to improve power conversion efficiency (PCE). However, universality of the reported results mainly focused on utilizing single type of MNPs to enhance light absorption only in specific narrow wavelength range. Herein, a surface-energy-induced dual MNP plasmon resonance by thermally evaporating method was presented to achieve the absorption enhancement in wider range. The differences of surface energy between silver (Ag), gold (Au), and tungsten trioxide (WO3) compared by contact angle images enable Ag and Au prefer to respectively aggregate into isolated islands rather than films at the initial stage of the evaporation process, which was clearly demonstrated in the atomic force microscopy (AFM) measurement. The sum of plasmon-enhanced wavelength range induced by both Ag NPs (350–450 nm) and Au NPs (450–600 nm) almost cover the whole absorption spectra of active layers, which compatibly contribute a significant efficiency improvement from 4.57 ± 0.16 to 6.55 ± 0.12% compared to the one without MNPs. Besides, steady state photoluminescence (PL) measurements provide strong evidence that the SPR induced by the Ag–Au NPs increase the intensity of light absorption. Finally, ultraviolet photoelectron spectroscopy (UPS) reveals that doping Au and Ag causes upper shift of both the work function and valence band of WO3, which is directly related to hole collection ability. We believe the surface-energy-induced dual plasmon resonance enhancement by simple thermally evaporating technique might pave the way toward higher-efficiency PSCs.Keywords: buffer layer; metal nanoparticles; photoluminescence; plasmon resonance; polymer solar cells; surface energy

Enhanced performance of polymer solar cells (PSCs) based on the blend of poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) is demonstrated by titanium dioxide (TiO2) interface modification via CuInS2/ZnS quantum dots (CZdots). Devices with a TiO2/CZdots composite buffer layer exhibit both a high short-circuit current density (Jsc) and fill factor (FF), leading to a power conversion efficiency (PCE) up to 7.01%. The charge transport recombination mechanisms are investigated by an impedance behavior model, which indicates that TiO2 interfacial modification results in not only increasing the electron extraction but also reducing impedance. This study provides an important and beneficial approach to develop high efficiency PSCs.

The surface plasmon resonance (SPR) effect based on noble metal nanoparticles (NPs) such as gold(Au) and silver(Ag) has been widely investigated and demonstrated to be a breakthrough technology to further improve the power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, diameter-controlled copper (Cu) NPs were intentionally introduced into an anode buffer layer of tungsten trioxide (WO3) by the thermally evaporating method, structuring a light trapping center to enhance light absorption of PSCs. The big difference of surface energy between WO3 and Cu can induce the growth process of Cu NPs from nucleation to isolated island by controlling evaporating thickness, yielding multiple SPR centers to radiate the electromagnetic wave toward the active layer. The steady-state photoluminescence (PL) results provide direct evidence that Cu NPs-induced SPR effect can effectively enhance light absorption of the active layer at visible wavelength, leading to a significant improvement of the short current density (Jsc) by 35.3% and PCE by 37.2% for devices with 3 nm Cu. Encouragingly, the inclusion of Cu NPs simultaneously decreases device resistance determined by complex impedance spectra. We believe the low-cost, simple-fabricated evaporating method combined with SPR enhancement can pave the way to high-performance PSCs.

Window application is the important aim for semitransparent solar cells (STPSC) investigation. Here, we demonstrate a method to achieve significantly improved color rendering index (CRI), depressed chromaticity difference (DC), and enhanced power conversion efficiency (PCE) simultaneously by introducing the one-dimensional photonic crystals (1DPCs) Bragg reflector structure onto the STPSC. The device performance is studied from aspects of color perception, electrical properties, and theoretical optical simulations. The STPSCs exhibit achromatic transparency nature color perceptions, especially for the STPSCs with 1DPCs (pairs ≥ 3) under AM 1.5G illumination light source, standard illuminant D65, and standard illuminant A. The excellent CRI is approaching 97 with lower DC about 0.0013 for the device with 5 pairs of 1DPC illumined by AM 1.5G illumination light source. At the same time, the PCE of STPSC devices with 5 pairs of 1DPC was improved from 4.87 ± 0.14% to 5.31 ± 0.13% compared to without. This method provides a facilitative approach to realizing excellent SPTSC window application.Keywords: CIE; CRI; DC; IDPCs; STPSC;

Generally, the surface plasmon resonance (SPR) effect of metal nanoparticles is widely applied on polymer solar cells (PSCs) to improve device performance by doping method into solution. Herein, a diameter-controlled thermally evaporation method was used to realize Au nanoparticles (Au NPs) doping into WO3 anode buffer layer in inverted PSCs. The surface energy differences between Au and WO3 inevitably lead to Au growing up through the process from nucleation, isolated island, aggregation of metal islands to continuous films along with the process of evaporation. The atom force microscopy (AFM) images indicate that critical thickness of Au film formation is 8 nm, which is in accordance with current density–voltage (J–V) and incident photon-to-electron conversion efficiency (IPCE) measurement results of optimal device performance. The power conversion efficiency (PCE) with 8 nm Au is dramatically improved from 4.67 ± 0.13% to 6.63 ± 0.17% compared to the one without Au. Moreover, the optical absorption enhancement is demonstrated by steady state photoluminescence (PL), which agrees well with transmission spectrum. The optical and electrical improvement all suggest that thermal evaporation is the appropriate method to further enhance device performance.Keywords: buffer layer; photoluminescence; polymer solar cells; surface plasmon resonance; thermally evaporation; transmittance

We demonstrate a novel solution-processed method to fabricate a stable anode buffer layer without any annealing process. As we know, buffer layers in polymer solar cells (PSCs) are always prepared by the traditional high-vacuum thermal evaporation or annealing-treated spin-coating methods, but the fabricating processes are complicated and time-consuming. Here, a solution method without any annealing to fabricate phosphomolybdic acid (PMA) as anode buffers is presented, which brings an obvious improvement of power conversion efficiency (PCE) from 1.75% to 6.57% by optimizing the PMA concentrations and interface pretreatment with device structure shown as ITO/TiO2/PCDTBT:PC70BM/PMA/Ag. The improvement is ascribed to the fine energy-level matching and perfect surface modification. This annealing-free method greatly simplifies the device fabrication process and supplies a wide way to achieve a large area fabrication for PSCs.Keywords: isopropyl alcohol; PCDTBT; phosphomolybdic acid; polymer solar cell

Herein, we demonstrate a kind of high performance semi-transparent polymer solar cell (STPSC) with a significantly improved color rendering index (CRI) and power conversion efficiency (PCE) by introducing one-dimensional photonic crystals (1DPCs), which are intentionally designed to strongly reflect the pristine weak absorbed light to flatten the concavo-convex transmittance spectrum of STPSCs. The transmitted light from the STPSC device with 4 pairs of 1DPCs under AM 1.5G illumination shows extraordinary color rendering capacities, which contribute an increased CRI from 79 to 91, combined with an enhanced PCE from 4.14% to 5.01% compared to devices without 1DPCs. The simultaneously improved optical and electrical performance suggests that STPSCs can provide a unique feature, which is suitable for building integrated photovoltaic applications.

Efficient semitransparent polymer solar cells (ST-PSCs) have been fabricated with one-dimensional photonic crystals (1DPCs) as a high reflector. The 1DPCs are composed of several pairs of WO3 (65.8 nm)/LiF (95.5 nm). By optimizing the pairs of WO3/LiF, 1DPCs can reflect the light back into the ST-PSCs due to the photonic band gap, when the high reflectance range of 1DPCs is matched with absorption spectrum of the active layer. ST-PSCs with 8 pairs of 1DPC exhibit an attractive performance. The short-circuit current density (Jsc) and power conversion efficiency (PCE), respectively, reach to 9.76 mA/cm2 and 5.16% compared to 8.12 mA/cm2 and 4.24% of the reference ST-PSCs without 1DPCs. A maximum enhancement of 20.2% in Jsc is obtained and the PCE increases by ∼21.7%. This approach provides a simple, fascinating and promising method to realize the highly efficient ST-PSCs toward applications.Keywords: distributed Bragg reflector; low-bandgap polymer solar cells; one-dimensional photonic crystals; reflectance; semitransparent; transmittance;

•Light harvesting is enhanced toward low IPCE region(400–500 nm).•A high efficiency of 4.84% with an enhancement of 14.2% is obtained.•The average transmittance is 25.4% in visible range(400–700 nm).In this study, we demonstrate a light harvesting method to achieve efficient semitransparent polymer solar cells (STPSCs) based on PCDTBT: PC70BM by employing one-dimensional photonic crystals (1DPCs) as reflector. The 1DPCs were constructed by N pairs of WO3/LiF, a pair means a combination of WO3 and LiF, including a layer of WO3 film and a layer of LiF film. By optimizing the number of pairs, a 1DPC with high reflectance region (400–500 nm), which is exactly matched with the weak region of the (incident photon-to-electron conversion) IPCE spectrum of the reference STPSC is obtained. In contrast, the excellent STPSC with 8 pairs of 1DPC exhibited a high efficiency about 4.84% and hold an average transmittance of 25.4% in the visible range (400–700 nm).

•1D DBR overcomes the contradiction between efficiency and transparency.•A maximum efficiency of 4.12% with an enhancement of 24.1% is obtained.•The average transmittance is 55.6% in 600–800 nm based on 1D DBR.We demonstrate that one-dimensional photonic crystals as distributed Bragg reflectors can effectively improve the performance of semitransparent polymer solar cells (PSCs) based on the blend of P3HT:ICBA. The one dimensional distributed Bragg reflectors (1D DBRs) are composed of N pairs of WO3/LiF which are thermally evaporated on Ag anode. Due to its photonic bandgap, 1D DBRs can reflect the light totally back into the PSCs when the high reflectance range of 1D DBRs is well matched with absorption spectrum of the active layer. A maximum power conversion efficiency (PCE) of 4.12%, a highest transmittance of 80.4% at 660 nm and an average transmittance of 55.6% in the wavelength range of 600–800 nm are obtained in the case of N = 8, corresponding enhancement of 24.1% in PCE when compared with the device without the 1D DBRs.Graphical abstract

•WO3 (40 nm)/Au (10 nm)/WO3 (20 nm) have fine optical and electric performance.•A high efficiency of 4.55% is obtained for ITO-free device.•Microcavity effect is investigated with 70 nm and 130 nm active layers.Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO3)/Au/WO3/PCDTBT: PC70BM/LiF/Al was fabricated and studied. The multilayer structure of WO3/Au/WO3 is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70 nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130 nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (Jsc max) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.Graphical abstract

•CuI was thermally evaporated as an anode buffer layers in PSCs.•The PCE with 2 nm CuI is 3.84%, which is 1.6 times higher than without.•The complex impedance spectra indicate that CuI can decrease the series resistance.The p-type semiconductor copper iodide (CuI) was incorporated into P3HT:ICBA bulk heterojunction polymer solar cells (PSCs) as a novel anode buffer layer. The power conversion efficiency of PSCs with 2 nm CuI is 3.84%, about 1.6 times higher than that of the reference device without CuI, which is due to the simultaneous enhanced photocurrent (Jsc), open current voltage (Voc) and fill factor (FF). The CuI buffer layer can module the Schottky barrier and form an ohmic contact at the P3HT:ICBA/Au interface, which act as both holes transport layer and electron block layer. The complex impedance spectra indicate that CuI buffer layer can also effectively decrease the series resistance of whole device.

•The one-dimensional photonic crystals with 8 pairs of WO3/LiF act as distributed reflectors.•The device with 8 pairs of WO3/LiF has an improvement of 28.1% in efficiency than without.•The average transmittance of the device with 8 pairs of WO3/LiF is almost 40% in 380–580 nm.One-dimensional photonic crystals (1DPCs) with the structure of (WO3/LiF)N are employed to simultaneously improve the efficiency and transmittance of low bandgap semitransparent polymer solar cells(PSCs). Within the photonic bandgap(PBG) of 580–780 nm, 1DPCs with 8 pairs of WO3/LiF act as distributed reflectors (DBR), which reflects light wavelength of 580–780 nm back into the PSCs for reabsorption by active layers. Power conversion efficiency (PCE) of 2.46% is obtained for the semitransparent PSCs and there is an improvement of 28.1% in the PCE when compared with that of the device without 1DPCs. Within the photonic passband of 380–580 nm, the 1DPCs act as antireflection coatings. An average transmittance of 40% is remained within the wavelength range and the value is improved by 33% when compared with that of the device without the 1DPCs. Finally, it is demonstrated that the efficiency and transmittance of the device are dependent on the number of the repeated period (N).

We demonstrate semi-transparent inverted polymer solar cells with transparent anodes, made of Vanadium pentoxide (V2O5)/silver (Ag)/V2O5. The inner V2O5 layer was introduced as a buffer layer to improve holes collection, while the outer V2O5 layer served as a light coupling layer to enhance optical transmittance of the device. The transmittance and reflectance of V2O5 (10 nm)/Ag (13 nm)/V2O5 (x = 20, 40, 60, 80 nm) electrode are measured and compared, and the dependence of the device performances on the thickness of the outer V2O5 layer was investigated. The results show that the maximum transmittance of 90%, which appears from 400 to 700 nm, is obtained when the thickness of outer V2O5 layer is 40 nm.Graphical abstractThe photovoltaic device has a structure of ITO/nc-TiO2/RR-P3HT: PCBM/V2O5/Ag/V2O5, and the influence of outer V2O5 on device performance are investigated in detail.Highlights► The inner V2O5 was introduced as a buffer layer to improve holes collection. ► The outer V2O5 layer served as a light coupling layer to enhance optical transmittance. ► A high transmission of 90% was achieved with a 40 nm outer V2O5 layer. ► The reflectance peaks are red shifted gradually when the thickness of the outer V2O5 increases.

•The one-dimensional photonic crystals with 8 pairs of WO3/LiF act as distributed reflectors.•The device with 8 pairs of WO3/LiF has an improvement of 28.1% in efficiency than without.•The average transmittance of the device with 8 pairs of WO3/LiF is almost 40% in 380–580 nm.One-dimensional photonic crystals (1DPCs) with the structure of (WO3/LiF)N are employed to simultaneously improve the efficiency and transmittance of low bandgap semitransparent polymer solar cells(PSCs). Within the photonic bandgap(PBG) of 580–780 nm, 1DPCs with 8 pairs of WO3/LiF act as distributed reflectors (DBR), which reflects light wavelength of 580–780 nm back into the PSCs for reabsorption by active layers. Power conversion efficiency (PCE) of 2.46% is obtained for the semitransparent PSCs and there is an improvement of 28.1% in the PCE when compared with that of the device without 1DPCs. Within the photonic passband of 380–580 nm, the 1DPCs act as antireflection coatings. An average transmittance of 40% is remained within the wavelength range and the value is improved by 33% when compared with that of the device without the 1DPCs. Finally, it is demonstrated that the efficiency and transmittance of the device are dependent on the number of the repeated period (N).Download full-size image

Enhanced performance of polymer solar cells (PSCs) based on the blend of poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) is demonstrated by titanium dioxide (TiO2) interface modification via CuInS2/ZnS quantum dots (CZdots). Devices with a TiO2/CZdots composite buffer layer exhibit both a high short-circuit current density (Jsc) and fill factor (FF), leading to a power conversion efficiency (PCE) up to 7.01%. The charge transport recombination mechanisms are investigated by an impedance behavior model, which indicates that TiO2 interfacial modification results in not only increasing the electron extraction but also reducing impedance. This study provides an important and beneficial approach to develop high efficiency PSCs.

Herein, we demonstrate a kind of high performance semi-transparent polymer solar cell (STPSC) with a significantly improved color rendering index (CRI) and power conversion efficiency (PCE) by introducing one-dimensional photonic crystals (1DPCs), which are intentionally designed to strongly reflect the pristine weak absorbed light to flatten the concavo-convex transmittance spectrum of STPSCs. The transmitted light from the STPSC device with 4 pairs of 1DPCs under AM 1.5G illumination shows extraordinary color rendering capacities, which contribute an increased CRI from 79 to 91, combined with an enhanced PCE from 4.14% to 5.01% compared to devices without 1DPCs. The simultaneously improved optical and electrical performance suggests that STPSCs can provide a unique feature, which is suitable for building integrated photovoltaic applications.