Conjugated polymers with three components, P1-1 and P1-2, were prepared by one-pot Stille polymerization. The two-component polymer P1-0 is only composed of a 5-fluoro-6-alkyloxybenzothiadiazole (AFBT) acceptor unit and a thiophene donor unit, while the three-component polymers P1-1 and P1-2 contain 10% and 20% 5,6-difluorobenzothiadiazole (DFBT), respectively, as the third component. The incorporation of the third component, 5,6-difluorobenzothiadiazole, makes the side chains discretely distributed in the polymer backbones, which can enhance the π–π stacking of polymers in film, markedly increase the hole mobility of active layers, and improve the power-conversion efficiency (PCE) of devices. Influence of the third component on the morphology of active layer was also studied by X-ray diffraction (XRD), resonant soft X-ray scattering (R-SoXS), and transmission electron microscopy (TEM) experiments. P1-1/PC71BM-based PSCs gave a high PCE up to 7.25%, whereas similarly fabricated devices for P1-0/PC71BM only showed a PCE of 3.46%. The PCE of P1-1/PC71BM-based device was further enhanced to 8.79% after the use of 1,8-diiodooctane (DIO) as the solvent additive. Most importantly, after the incorporation of 10% 5,6-difluorobenzothiadiazole unit, P1-1 exhibited a marked tolerance to the blend film thickness. Devices with a thickness of 265 nm still showed a PCE above 8%, indicating that P1-1 is promising for future applications.Keywords: film thickness; morphology; power-conversion efficiency; side chains; solar cells; three-component conjugated polymers;

Novel polymers comprising a 3-fluoro-5-alkylthiophenyl benzodithiophene donor unit and a 5-fluoro-6-alkoxy (or alkylthio)-2,1,3-benzothiadiazole (BT) acceptor unit were synthesized. Both POF and PSF possess low HOMO and LUMO energy levels due to the incorporation of fluorine atoms. Additionally, alkoxy and alkylthio substitution on the BT unit also had a great influence on the molecular packing and the energy level of the resulting polymers. The introduction of the alkylthio side chains on the BT unit of PSF led to a significant downshift of the HOMO energy level in comparison to that of POF with an alkoxy substituent due to the weaker electron-donating properties of the sulfur atom than that of oxygen. However, the steric hindrance caused by the large sulfur atoms resulted in reduced planarity of the backbone of PSF, which might influence the charge transport and the morphology of the blend film. As a result, POF based NF-PSCs exhibited a PCE of 7.28%, with a Voc of 0.86 V, a Jsc of 14.9 mA cm−2, and an FF of 0.47, while a low PCE of 1.55% with a Voc of 0.95 V, a Jsc of 5.6 mA cm−2, and an FF of 0.29 was obtained for PSF based non-fullerene polymer solar cells (NF-PSCs). Therefore, the side chain engineering of the donor polymer is crucial for maximizing both Jsc and Voc values to achieve high performance polymer solar cells.

•Three novel copolymers based on DTQx and IDT were synthesized.•PHF based PSCs gave a PCE of 7.2% with fluorination on side chain of DTQx.•Fluorination position and number result in varied morphology of active layers.To explore the influence of fluoro substitution position and number on optical, electrochemical and photovoltaic properties, three novel donor-acceptor (D-A) alternative copolymers (PHF, PFH and PFF) were synthesized by Stille polycondensation of 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx) acceptor unit and indacenodithiophene (IDT) donor unit. As films, PHF and PFF comprising two fluoro substituents on the lateral phenyl groups displayed a broad absorption ranging from 350 to 700 nm; whereas PFH containing two fluorine atoms on the polymer main chain exhibited a slightly narrower absorption ranging from 350 to 650 nm. In addition, fluoro substitution on the polymer main chain can lower the HOMO level of the resulted polymers. As expected, PFH and PFF possess deeper HOMO energy level than PHF. Polymer solar cells (PSCs) were fabricated with these three polymers as donor materials and PC71BM as acceptor material. PHF based PSCs gave a power conversion efficiency (PCE) of 7.2% with a Voc of 0.84 V, a Jsc of 12.46 mA/cm2 and an FF of 0.69. And PFH based PSCs showed a PCE of 6.19% with a Voc of 0.93 V, a Jsc of 9.57 mA/cm2 and an FF 0.70. However, a PCE of only 2.9% with a Voc of 0.92 V, a Jsc of 4.61 mA/cm2 and an FF of 0.68 was obtained for PFF based PSCs. Transmission electron microscopy (TEM) and resonant soft X-ray scattering (R-SoXS) studies indicated that the introduction of four fluorine atoms at each repeating unit can spoil the morphology of active layer. These results highlight the importance of fluorination position and number to the performance of PSCs.Three novel donor-acceptor (D-A) alternative copolymers PHF, PFH and PFF were synthesized and applied for polymer solar cells (PSCs) to explore the influence of fluoro substitution position and number on optical, electrochemical and photovoltaic properties. PHF, PFH and PFF based PSCs gave power conversion efficiencies of 7.2%, 6.19% and 2.9%, respectively. TEM and R-SoXS studies indicated that the introduction of four fluorine atoms at each repeating unit can spoil the morphology of active layer.Download high-res image (249KB)Download full-size image

•Two nonfullerene acceptors with different linkage, PDI-V and PDI-E were synthesized.•The impact of the linkage between two PDI units on the photovoltaic performance were investigated.•The PCE for PSCs based on PTB7-Th:PDI-V is almost two times higher than that of PTB7-Th:PDI-E based devices.•It gains deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs.Vinylene (V)- and ethynylene (E)-bridged perylene diimide dimers (PDI-V and PDI-E) were designed, synthesized and used as nonfullerene acceptors for polymer solar cells. Our researches revealed that the linkage between two PDI units has a great impact on the molecular geometry, the optical properties, the blend film morphology, the molecular packing orientation, and the photovoltaic performance. Computational calculations via density functional theory (DFT) showed that PDI-E and PDI-V possessed planar and twisted geometric structures, respectively. TEM investigations showed that PTB7-Th:PDI-V based blend film exhibited a uniform morphology with small domain size and PTB7-Th:PDI-E based one showed apparent phase separation with large domain size. GIWAXS results revealed that the PDI-V can influence PTB7-Th to take on a face-on orientation, which is beneficial for vertical charge transport to increase Jsc. A PCE of 4.51% with a Voc of 0.76 V, a Jsc of 10.03 mA cm−2, and an FF of 0.59 was obtained for PSCs based on PTB7-Th:PDI-V, which is almost two times higher than that of PTB7-Th:PDI-E based devices, which showed a PCE of 2.66%, a Voc of 0.66 V, a Jsc of 7.33 mA cm−2, and an FF of 0.55. These results help to gain deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs.Download high-res image (167KB)Download full-size image

This data contains additional data related to the article “Influence of Substrate Temperature on the Film Morphology and Photovoltaic Performance of Non-fullerene Organic Solar Cells” (Jicheng Zhang et al., In press) [1]. Data include measurement and characterization instruments and condition, detail condition to fabricate norfullerene solar cell devices, hole-only and electron-only devices. Detail condition about how to control the film morphology of devices via tuning the temperature of substrates was also displayed. More information and more convincing data about the change of film morphology for active layers fabricated from different temperature, which is attached to the research article of “Influence of Substrate Temperature on the Film Morphology and Photovoltaic Performance of Non-fullerene Organic Solar Cells” was given.

Four small molecular acceptors (SM1–4) comprising a central benzene core, two thiophene bridges and two 1,8-naphthalimide (NI) terminal groups were designed and synthesized by direct C–H activation. SM1 has a planar chemical structure and forms H-aggregation as films. By attachment of different substituents on the central benzene ring, the dihedral angles between the two NI end groups of SM1–4 gradually increased, leading to a gradual decrease of planarity. SM1–4 all possess a high-lying LUMO level, matching with wide band gap (WBG) polymer donors which usually have a high-lying LUMO level. When used in OSCs, devices based on SM1 and WBG donor PCDTBT-C12 gave higher electron mobility, superior film morphology and better photovoltaic performance. After optimization, a PCE of 2.78% with a Voc of 1.04 V was achieved for SM1 based devices, which is among the highest PCEs with a Voc higher than 1 V. Our results have demonstrated that NI based planar small molecules are potential acceptors for WBG polymer based OSCs.Keywords: C−H activation; H-aggregation; high open circuit voltage; nonfullerene acceptor; organic solar cells; planar small molecules; wide band gap polymer

Two novel polymers PTFBDT-BZS and PTFBDT-BZO with 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as the donor unit, benzothiadiazole or benzooxadiazole as the acceptor unit, and thiophene as the spacer have been synthesized and used as donor materials for polymer solar cells (PSCs). These two polymers exhibited wide optical band gaps of about 1.8 eV. PSCs with the blend of PTFBDT-BZS:PC71BM (1:2, by weight) as the active layer fabricated without using any processing additive and any postannealing treatment showed power conversion efficiency (PCE) of 8.24% with an open circuit voltage (Voc) of 0.89 V, a short circuit current (Jsc) of 12.67 mA/cm2, and a fill factor (FF) of 0.73 under AM 1.5G illumination, indicating that PTFBDT-BZS is a very promising donor polymer for PSCs. The blend of PTFBDT-BZO:PC71BM showed a lower PCE of 5.67% with a Voc of 0.96 V, a Jsc of 9.24 mA/cm2, and an FF of 0.64. One reason for the lower PCE is probably due to that PTFBDT-BZO has a smaller LUMO offset with PC71BM, which cannot provide enough driving force for charge separation. And another reason is probably due to that PTFBDT-BZO has a lower hole mobility in comparison with PTFBDT-BZS.Keywords: bulk heterojunction; donor materials; fluorinated polymers; polymer solar cells; wide band gap polymers

•Three novel polymers have branched side chains with varied branching positions.•The bifurcation positions influnce intermolecular stacking and film morphology.•The bifurcation positions influnce charge mobility and photovoltaic performance.•The best PCE of 5.67% is achieved by adjusting the bifurcation points position.Three novel copolymers (PTTAFBT-C0, PTTAFBT-C1 and PTTAFBT-C2) based on thieno[3,2-b]thiophene and 5-alkoxy-6-fluorobenzo-[c][1,2,5]thiadiazole (AFBT) bearing branched alkoxy chains with varied branching positions are synthesized. The influences of the bifurcation positions on intermolecular stacking, charge mobility, film morphology and photovoltaic performance are systematically investigated. 2D-GIWAXS analyses of the optimized polymer:PC71BM blend films exhibit the crystallinity increases from PTTAFBT-C0 to PTTAFBT-C1 and PTTAFBT-C2 is more prone to form the edge-on orientation than PTTAFBT-C1, which result in a dramatic difference in film morphologies. TEM image of PTTAFBT-C1:PC71BM blend films exhibits a suitable morphology with favorable interpenetrating networks, which is in favor of high performance. The best PCE of 5.67% with a device configuration of ITO/PEDOT: PSS/PTTAFBT-C1:PC71BM/LiF/Al under AM 1.5G solar radiation (100 mW cm−2) is achieved. These results indicate that higher PCE can be obtained by adjusting the bifurcation points of the branched side chains away from the polymer backbone.

The use of environmentally friendly halogen-free organic solvents for the fabrication of polymer solar cells will be of great importance for future practical applications. In this work, a new alternative conjugated polymer with 3,4-bis(octyloxy)-phenyl substituted benzo[1,2-b:4,5-b]dithiophene as the donor unit and benzo[c][1,2,5]thiadiazole as the acceptor unit was synthesized and used as the donor material for polymer solar cells. This polymer showed good solubility in halogen-free solvents such as toluene, o-xylene and so on. The blend film morphology, charge mobility and photovoltaic performance were investigated in halogen-free solvents. The photovoltaic devices fabricated from o-xylene with N-methyl-2-pyrrolidone as additive provided the best power conversion efficiency of 4.57%, comparable to that fabricated from halogenated solvents such as 1,2-dichlorobenzene/1,8-diiodooctane with a power conversion efficiency of 4.33%. Our results demonstrate that halogen-free solvents are promising for the fabrication of high efficiency polymer solar cells.

A novel small molecule NI-T-NI with a thiophene core and two 1,8-naphthalimide terminal groups was synthesized via direct C–H activation and used as the acceptor for polymer solar cells. NI-T-NI exhibits a good crystallinity and can form H-aggregates in the solid state. NI-T-NI has a rather high-lying LUMO level, which is beneficial for achieving a high Voc. In cooperation with a high-lying LUMO level polymer PCDTBT-C12, a PCE of 2.01% with a high Voc of 1.30 V has been achieved. As far as we know, a Voc of 1.30 V is the highest value reported for single junction organic solar cells. Our results have demonstrated that 1,8-naphthalimide could be a useful building block for the synthesis of promising acceptor materials for polymer solar cells.

Three novel copolymers P1–3 with alkylthiophenyl substituted benzodithiophene as the donor unit, thiophene as the spacer, and benzothiadiazole as the acceptor unit have been designed, synthesized, and used as donor materials for polymer solar cells. Polymer solar cells with P3:PC71BM blends as the active layer exhibited a high power conversion efficiency (PCE) of 7.7% and a good tolerance to the change of film thickness. PCE higher than 7.3% can be obtained with the thickness of the active layer ranging from 90 to 380 nm, indicating that P3 is a very promising donor material for practical application.

Two conjugated polymers with benzodithiophene derivatives as the donor unit and planar difluoro-substituted dibenzo[a,c]phenazine as the acceptor unit P1, P2-L and P2-H were designed, synthesized, and used as the donor material in polymer solar cells. P2-H exhibits the highest hole mobility of 1.54 × 10−2 cm2 V−1 s−1. Polymer solar cells (PSCs) with a blend of P1:PC71BM (1:1.5, by weight) as the active layer show the highest power conversion efficiency (PCE) of 6.0% with an open circuit voltage (Voc) of 0.74 V, a short circuit current (Jsc) of 12.50 mA cm−2, and a fill factor (FF) of 0.65.

•Poly(fluorinated acenaphthoquinoxaline-co-benzodithiophene) was synthesized and PCE of 4.72% was obtained for solar cells.•Polymer solar cells with the blend of P1 and PC71BM as the active layer showed a PCE of 4.72%.•P1 based devices have the highest PCE report for acenaphtho[1,2-b]quinoxaline based polymer solar cells.A new low band gap polymer P1 with fluorinated acenaphtho[1,2-b]quinoxaline (AQ) as the acceptor unit and benzo[1,2-b:4,5-b′]dithiophene as the donor unit has been designed and synthesized. Comparing with its non-fluorinated analog polymer P2, P1 shows a lower band gap of 1.76 eV with a deeper HOMO energy level of −5.54 eV. Polymer solar cells with the blend of P1 and PC71BM as the active layer without the additive or annealing showed a PCE of 4.72%, which was higher than P2 based devices with a PCE of 1.65%. Obviously, P1 can endow devices with a Voc of 0.77 V and a Jsc of 10.75 mA cm−2, which are higher than that of P2 based devices with a Voc of 0.75 V and aJsc of 4.41 mA cm−2, such an enhanced photovoltaic performance can be attributed to the broader absorption, the improvement of film morphology, and the higher charge mobility. Thus, fluorinated AQ can be a useful acceptor unit to construct narrow band gap conjugated polymers. To the best of our knowledge, this is the highest PCE report for AQ based polymer solar cells.

9-Arylidene-9H-fluorene containing donor–acceptor (D–A) alternating polymers P1 and P2 were synthsized and used for the fabrication of polymer solar cells (PSCs). High and low molecular weight P1 (HMW-P1 and LMW-P1) and high molecular weight P2 were prepared to study the influence of molecular weight and the position of alkoxy chains on the photovoltaic performance of PSCs. HMW-P1:PC71BM-based PSCs fabricated from 1,2-dichlorobenzene (DCB) solutions showed a power conversion efficiency (PCE) of 6.26%, while LMW-P1:PC71BM-based PSCs showed poor photovoltaic performance with a PCE of only 2.75%. PCE of HMW-P1:PC71BM-based PSCs was further increased to 6.52% with the addition of 1,8-diiodooctane (DIO) as the additive. Meanwhile, PCE of only 2.51% was obtained for P2:PC71BM-based PSCs. The results indicated that the position of alkoxy substituents on the 9-arylidene-9H-fluorene unit and the molecular weight of polymers are very crucial to the photovoltaic performance of PSCs.Keywords: benzothiadiazole; bulk heterojunction; conjugated polymers; donor−acceptor alternating polymers; polymer solar cells; Suzuki polycondensation;

Three donor–acceptor (D–A) alternating conjugated polymers with silafluorene as the donor unit, 5-alkyloxy-6-fluorobenzothiadiazole as the acceptor unit, and thiophene as the spacer has been synthesized and used as donor materials for polymer solar cells (PSCs). The introduction of a fluorine atom on the benzothiadiazole unit can lower the HOMO and LUMO energy level of the resulted polymers to afford higher open circuit voltage (Voc); whereas the introduction of a flexible alkyloxy chain on benzothiadiazole unit can increase the solubility of the resulted polymers without interfering the close packing of polymer chains in the solid state. High molecular weight polymers P-1a, P-1b, and P-1c, which are fully soluble in 1,2-dichorobenzene (DCB) at elevated temperature, have been prepared by Suzuki polycondensation. Among these polymers, P-1c exhibited the highest hole mobility up to 1.36 × 10–2 cm2 V–1 s–1. PSCs based P-1b:PC71BM demonstrated the highest Voc up to 0.98 V. P-1a:PC71BM based PSCs gave the highest power conversion efficiency (PCE) of 6.41%, which is the highest value among solar cells with benzothiadiazole- and silafluorene-containing polymers as the donor material.

Two new triindole-cored star-shaped molecules SM-1 and SM-2 have been designed and synthesized, and their optical, electrochemical, thermal, transport and photovoltaic properties have been investigated in detail. SM-1 and SM-2 exhibited good thermal stability, intensive absorption in a broad region, and relatively high hole mobility. Photovoltaic performances of these two molecules were investigated by fabricating bulk heterojunction solar cell devices with a blend film of SM-1:PC71BM or SM-2:PC71BM as the active layer. Organic solar cells (OSCs) based on SM-1:PC71BM and SM-2:PC71BM gave power conversion efficiencies (PCEs) of 2.05% and 2.29%, respectively. A PCE of 2.29% is the best result for all the reported triindole-based photovoltaic materials, indicating that triindole-based small molecules could become promising donor materials for solution-processed OSCs.

Alcohol soluble fullerene derivative (FN-C60) has been synthesized and used as a cathode interfacial layer for high-efficiency polymer solar cells (PSCs). To examine the function of the FN-C60 interfacial layer, polymer solar cells were fabricated with blends of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM as the active layer. In comparison to the bare Al electrode, power conversion efficiencies (PCEs) of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM based PSCs were increased from 3.50 to 4.64%, 4.69 to 5.25%, 2.70 to 4.60%, and 1.52 to 2.29%, respectively, when FN-C60/Al was used as the electrode. Moreover, the overall photovoltaic performances of PSCs with the FN-C60/Al electrode were better than those of cells with LiF/Al electrode, indicating that FN-C60 is a potential interfacial layer material to replace LiF.Keywords: alcohol soluble; fullerene derivative; interfacial layer; photovoltaic performance; polymer solar cells; power conversion efficiency;

A novel donor–acceptor (D–A) copolymer PDFCDTBT with 3,6-difluoro substituted carbazole as the donor unit and dialkoxy substituted benzothiadiazole as the acceptor unit has been synthesized and used as a donor material for bulk heterojunction polymer solar cells (BHJ PSCs). PDFCDTBT possesses a band gap of 1.75 eV, a low-lying HOMO energy level of −5.23 eV, and a good thermal and electrochemical stability. In comparison with the corresponding non-fluoro substituted HXS-1, which has a HOMO energy level of 5.21 eV, a LUMO energy level of 3.35 eV, and an optical band gap of 1.86 eV, the incorporation of two fluoro atoms in the carbazole donor unit lowers the HOMO and the LUMO energy levels of the polymer, which results in simultaneously decreasing the band gap of the polymer and increasing the Voc of polymer solar cells. The fluoro-containing polymer PDFCDTBT also shows strong intramolecular interactions and forms close packing in the solid state. Polymer solar cells based on PDFCDTBT and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) demonstrate a power conversion efficiency (PCE) of 4.8% with a Voc of 0.91 V, a Jsc of 9.5 mA cm−2, and an FF of 0.55. In comparison with HXS-1, the better stability, higher Voc, and narrower band gap indicate that PDFCDTBT is a very promising donor material for high efficiency polymer solar cells.

High molecular weight thiophene-containing conjugated polymers were successfully synthesized by Suzuki polycondensation of aryl dibromides and 2,5-thiophenebis(boronic acid) derivatives by using a new thiophene-containing bulky phosphorous compound as the ligand for a zerovalent palladium catalyst.

6,7-Dialkoxy-2,3-diphenylquinoxaline based narrow band gap conjugated polymers, poly[2,7-(9-octyl-9H-carbazole)-alt-5,5-(5,8-di-2-thinenyl-(6,7-dialkoxy-2,3-diphenylquinoxaline))] (PCDTQ) and poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(5,8-di-2-thinenyl-(6,7-dialkoxy-2,3-diphenylquinoxaline))] (PFDTQ), have been synthesized by Suzuki polycondensation. Their optical, electrochemical, transport and photovoltaic properties have been investigated in detail. Hole mobilities of PCDTQ and PFDTQ films spin coated from 1,2-dichlorobenzene (DCB) solutions are 1.0 × 10−4 and 4.1 × 10−4 cm2V−1 s−1, respectively. Polymer solar cells were fabricated with the as-synthesized polymers as the donor and PC61BM and PC71BM as the acceptor. Devices based on PCDTQ:PC71BM (1:3) and PFDTQ:PC71BM (1:3) fabricated from DCB solutions demonstrated a power conversion efficiency (PCE) of 2.5% with a Voc of 0.95 V and a PCE of 2.5% with a Voc of 0.98 V, respectively, indicating they are promising donor materials.

A new alternating copolymer (PSFDTBT) based on spirobifluorene, thiophene, and benzothiadiazole units has been synthesized. PSFDTBT has an optical band gap of 1.97 eV with the low-lying HOMO energy level at −5.4 eV. The hole mobility of the pristine PSFDTBT film spin-cast from o-dichlorobenzene (DCB) solution is 7.26 × 10–3 cm2 V–1 s–1 with on/off ratios in the order of 105. Polymer solar cell devices based on the blend films of PSFDTBT and PC71BM show a high open-circuit voltage of 0.94 V and a power conversion efficiency of 4.6% without any post-treatment. All the device measurements were performed in air without encapsulation. This is the first report on spirobifluorene-based conjugated polymers used for polymer solar cells, demonstrating the great potential of spirobifluorene moiety as an electron-donating unit for the construction of main chain donor–acceptor alternating conjugated polymers for high performance polymer solar cells.

Three D–A alternating copolymers P1–3 with 3,7-linked 2,8-bis(alkoxy)dibenzothiophene as the donor unit and benzothiadiazole (P1 and P2) or 3,4-bis(octyloxy)benzothiadiazole (P3) as the acceptor unit have been designed and synthesized. P1–3 show two broad absorption peaks in the visible region, and the internal charge transfer (ICT) absorptions at about 530 nm in solutions and 560 nm in films of P3 are much stronger than that of P1 and P2. All the polymers show narrow band gaps below 2.0 eV and the low-lying HOMO energy levels of approximately −5.30 eV. The hole mobilities of polymer films spin-cast from 1,2-dichlorobenzene (DCB) solutions are 3.0 × 10–4, 2.7 × 10–4, and 2.3 × 10–3 cm2 V–1 s–1 for P1, P2, and P3, respectively. Under simulated solar illumination of AM 1.5G (100 mW/cm2), a PCE of 4.48% with a Voc of 0.83 V, a Jsc of 9.30 mA/cm2, and an FF of 0.58 has been achieved for PSCs with the P3:PC71BM blend (1:3, by weight) as the active layer in simply processed devices, whereas after the optimization, PCEs of only 1.02% and 1.71% have been obtained for P1- and P2-based devices, respectively. This is the first report on dibenzothiophene-based conjugated polymers used for high efficiency polymer solar cells, demonstrating that photovoltaic performance can be improved by fine-tuning the conjugated polymer structure.

A novel donor–acceptor copolymer containing 9-alkylidene-9H-fluorene unit in the main chain, poly[9-(1′-hexylheptylidene)-2,7-fluorene-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dialkoxy-2′,1′,3′-benzothiadiazole)] (PAFDTBT), has been synthesized and evaluated in bulk heterojunction polymer solar cells (BHJ PSCs). The polymer possesses a low band gap of 1.84 eV, a low-lying HOMO energy level (5.32 eV), and excellent solubility in common organic solvents. PSCs based on PAFDTBT and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) demonstrate a power conversion efficiency (PCE) of 6.2% with a high fill factor (FF) of 0.70, which indicates that 9-alkylidene-9H-fluorene can be a very useful building block for constructing narrow band gap conjugated polymers for high-efficiency BHJ PSCs.

•A novel polymer (PBO) comprising alkylthiophenyl substituted BDT and BOz was synthesized.•PBO possesses deep HOMO energy level and medium band gap.•PCE higher than 7% was achieved for PBO:PC71BM or PBO:ITIC based devices.A novel medium band gap copolymer (PBO) based on benzodithiophene (BDT) and benzoxadiazole (BOz) was synthesized and applied in polymer solar cells (PSCs). The introduction of two alkylthio side chains onto the BDT unit can endow the resulting polymer (PBO) with good solubility in solutions and good crystallinity in the solid state. The low HOMO level of −5.52 eV and the broad absorption ranging from 350 to 700 nm make PBO a promising donor material for PSCs. Power conversion efficiency higher than 7% has been achieved for devices using PBO as the donor material and PC71BM or ITIC as the acceptor material.A novel medium band gap copolymer (PBO) based on benzodithiophene (BDT) and benzoxadiazole (BOz) was synthesized by judiciously selecting chalcogen atom and applied in polymer solar cells (PSCs). Power conversion efficiency higher than 7% has been achieved by using PBO as the donor and PC71BM or ITIC as the acceptor.

A novel small molecule NI-T-NI with a thiophene core and two 1,8-naphthalimide terminal groups was synthesized via direct C–H activation and used as the acceptor for polymer solar cells. NI-T-NI exhibits a good crystallinity and can form H-aggregates in the solid state. NI-T-NI has a rather high-lying LUMO level, which is beneficial for achieving a high Voc. In cooperation with a high-lying LUMO level polymer PCDTBT-C12, a PCE of 2.01% with a high Voc of 1.30 V has been achieved. As far as we know, a Voc of 1.30 V is the highest value reported for single junction organic solar cells. Our results have demonstrated that 1,8-naphthalimide could be a useful building block for the synthesis of promising acceptor materials for polymer solar cells.

Novel polymers comprising a 3-fluoro-5-alkylthiophenyl benzodithiophene donor unit and a 5-fluoro-6-alkoxy (or alkylthio)-2,1,3-benzothiadiazole (BT) acceptor unit were synthesized. Both POF and PSF possess low HOMO and LUMO energy levels due to the incorporation of fluorine atoms. Additionally, alkoxy and alkylthio substitution on the BT unit also had a great influence on the molecular packing and the energy level of the resulting polymers. The introduction of the alkylthio side chains on the BT unit of PSF led to a significant downshift of the HOMO energy level in comparison to that of POF with an alkoxy substituent due to the weaker electron-donating properties of the sulfur atom than that of oxygen. However, the steric hindrance caused by the large sulfur atoms resulted in reduced planarity of the backbone of PSF, which might influence the charge transport and the morphology of the blend film. As a result, POF based NF-PSCs exhibited a PCE of 7.28%, with a Voc of 0.86 V, a Jsc of 14.9 mA cm−2, and an FF of 0.47, while a low PCE of 1.55% with a Voc of 0.95 V, a Jsc of 5.6 mA cm−2, and an FF of 0.29 was obtained for PSF based non-fullerene polymer solar cells (NF-PSCs). Therefore, the side chain engineering of the donor polymer is crucial for maximizing both Jsc and Voc values to achieve high performance polymer solar cells.

Three novel copolymers P1–3 with alkylthiophenyl substituted benzodithiophene as the donor unit, thiophene as the spacer, and benzothiadiazole as the acceptor unit have been designed, synthesized, and used as donor materials for polymer solar cells. Polymer solar cells with P3:PC71BM blends as the active layer exhibited a high power conversion efficiency (PCE) of 7.7% and a good tolerance to the change of film thickness. PCE higher than 7.3% can be obtained with the thickness of the active layer ranging from 90 to 380 nm, indicating that P3 is a very promising donor material for practical application.

Two new triindole-cored star-shaped molecules SM-1 and SM-2 have been designed and synthesized, and their optical, electrochemical, thermal, transport and photovoltaic properties have been investigated in detail. SM-1 and SM-2 exhibited good thermal stability, intensive absorption in a broad region, and relatively high hole mobility. Photovoltaic performances of these two molecules were investigated by fabricating bulk heterojunction solar cell devices with a blend film of SM-1:PC71BM or SM-2:PC71BM as the active layer. Organic solar cells (OSCs) based on SM-1:PC71BM and SM-2:PC71BM gave power conversion efficiencies (PCEs) of 2.05% and 2.29%, respectively. A PCE of 2.29% is the best result for all the reported triindole-based photovoltaic materials, indicating that triindole-based small molecules could become promising donor materials for solution-processed OSCs.