We report our effort to unravel the origin of efficient operation of nonfullerene organic solar cells (OSCs), based on a poly[4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)](PTB7-Th):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6-b′]dithiophene (ITIC) blend system. The effects of buildup of space charges, charge extraction, and bimolecular recombination processes on the performance and the stability of PTB7-Th:ITIC-based regular and reverse configuration OSCs are analyzed. It is found that the high-performing inverted PTB7-Th:ITIC OSCs benefit from the combined effects of (1) suppression of bimolecular recombination enabled by an augmented effective internal electric field and (2) improvement of charge extraction by avoiding the holes passing through ITIC-rich region, which would otherwise occur in a regular configuration cell. The inverted PTB7-Th:ITIC OSCs possess a significant improvement in the cell stability and a high power conversion efficiency of ∼8.0%, which is >29% higher than that of an optimized regular configuration control cell (6.1%).

•Two n-type polymers P(PDI-BDT-Ph) and P(PDI-BDT-Th) with PDI as acceptor unit and BDT as donor unit were synthesized.•The two polymer films possess the same LUMO level of −3.89 eV, and low bandgaps of 1.65 and 1.55 eV respectively.•The all-PSCs based on PTB7-Th:P(PDI-BDT-Ph) showed a PCE of 4.31% with a Jsc of 11.94 mA cm−2 and a Voc of 0.81 V.Two n-type conjugated D-A copolymers with perylene diimide (PDI) as acceptor unit and benzodithiophene (BDT) as donor unit, P(PDI-BDT-Ph) and P(PDI-BDT-Th), were synthesized and applied as electron acceptor in all-polymer solar cells (all-PSCs). P(PDI-BDT-Ph) and P(PDI-BDT-Th) films exhibit similar absorption spectra in the visible region with optical bandgap (Eg) of 1.65 eV and 1.55 eV respectively, and the identical LUMO level of −3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor demonstrated a power conversion efficiency (PCE) of 4.31% with a short-circuit current density (Jsc) of 11.94 mA cm−2, an open-circuit voltage (Voc) of 0.81 V, and a fill factor (FF) of 44.49%. By contrast, the corresponding all-PSCs with P(PDI-BDT-Th) as acceptor showed a relative lower PCE of 3.58% with a Jsc of 11.36 mA cm−2, Voc of 0.79 V, and FF of 40.00%.Two n-type conjugated polymers P(PDI-BDT-Ph) and P(PDI-BDT-Th) were synthesized and all-all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor showed a PCE of 4.31% with a Jsc of 11.94 mA cm−2 and a Voc of 0.81 V.

•Efficient nonfullerene PSC based on PBPD-Th: ITIC was developed.•The PSC showed a high PCE of 10.8% with a Voc of up to 1.01 V−2.•The PSC has a smaller Eloss of 0.54 eV than the empirically low threshold of 0.6 eV.In this work, we developed high efficiency nonfullerene PSCs based on a wide bandgap polymer donor PBPD-Th with a structure of meta-alkoxyphenyl benzodithiophene-alt-thienyl benzodithiophene-4,8-dione and a low bandgap small molecule acceptor ITIC. The PBPD-Th shows a strong absorption in the short wavelength of 300–650 nm with a bandgap of 1.91 eV, which is complementary with that of ITIC (1.55 eV). Moreover, PBPD-Th possesses a deep HOMO level of −5.42 eV, which is beneficial to obtain high open-circuit voltage (Voc) in PSCs. As a result, the optimal PSCs based on PBPD-Th: ITIC showed a power conversion efficiency (PCE) of 10.8% with a Voc of up to 1.01 V, a high short-circuit current density (Jsc) of 18.3 mA cm−2 and a fill factor (FF) of 59%, under the illumination of AM 1.5 G, 100 mW cm−2. Notably, the energy loss (Eloss) of the optimal PSCs is 0.54 eV, which is smaller than the empirically low threshold of 0.6 eV. The PCE of 10.8% is one of the highest values reported in the literatures for the PSCs with Voc over 1.0 V.Download high-res image (194KB)Download full-size image

A 1,1′-vinylene-fused indacenodithiophene (IDTV) donor unit with 22 π-conjugated electrons was synthesized. A ladder-type D–A copolymer PIDTV-ffBT using IDTV as the donor unit and 5,6-difluorobenzothiadiazole (ffBT) as the acceptor unit was developed for application as a donor material in polymer solar cells (PSCs). Compared to other analogue polymers, PIDTV-ffBT possesses a two-dimensional conjugated multi-electron fused ring, excellent planarity and close π–π stacking, leading to a higher light harvesting coefficient, an enhanced charge carrier mobility of 0.032 cm2 V−1 s−1 and improved photovoltaic performance. The PSCs based on PIDTV-ffBT:PC71BM achieved a promising power conversion efficiency (PCE) of 7.3% with a high short-circuit current density (Jsc) of 17.1 mA cm−2. These results indicate that the introduction of the 1,1′-vinylene-fused system into IDTV for ladder-type polymers is an effective strategy to enhance the light absorption coefficient and improve charge carrier mobility for high efficiency PSCs.

A new conjugated polymer (PBFZ-OP) based on meta-alkoxy-phenyl-substituted benzodithiophene (BDT-m-OP) and difluorobenzotriazole (FBTZ) was designed and synthesized for application as a donor material in non-fullerene polymer solar cells (PSCs). The polymer exhibits a strong absorption in the range of 300–620 nm with a wide bandgap of 1.99 eV, which is complementary to that of the small molecule acceptor ITIC. Meanwhile, it also possesses a deeper HOMO energy level of −5.33 eV and a higher hole mobility of 7.28 × 10−4 cm2 V−1 s−1. The PSCs based on PBFZ-OP:ITIC showed a higher power conversion efficiency (PCE) of 10.5% with a Voc of 0.91 V, Jsc of 18.7 mA cm−2, and FF of 61.8% in comparison with the PCE of 5.5% with a Voc of 0.73 V, Jsc of 13.1 mA cm−2, and FF of 57.8% for J52-based PSCs. The results indicate that PBFZ-OP is a promising wide bandgap polymer donor for the photovoltaic application in non-fullerene PSCs.

In this work, a new wide-bandgap polymer, PSBZ, based on thienyl substituted benzodithiophene (BDTT) as the donor unit and difluorobenzotriazole (BTz-2F) as the acceptor unit was synthesized for photovoltaic applications. Compared to the analogous polymer J61 with linear dodecylthio side chains in the BDTT unit and a long 2-hexyldecyl side chain in BTz-2F, PSBZ possesses branched 2-butyloctyl side chains to increase steric hindrance of the BDTT unit and a short 2-butyloctyl side chain to decrease steric hindrance of the BTz-2F unit for more efficient charge separation and transport in the devices. As a result, PSBZ exhibited stronger π–π interaction and smaller stacking spacing leading to a higher extinction coefficient of 1.48 × 105 cm−1 and a high hole mobility of 8.56 × 10−3 cm2 V−1 s−1. Compared to the analogous polymer J61 with a power conversion efficiency (PCE) of 9.53% and a short-circuit current density (Jsc) of 17.43 mA cm−2, the PSBZ:ITIC-based polymer solar cells yielded a higher PCE of 10.5% with a higher Jsc of 19.0 mA cm−2. The results show that our design strategy is successful for improving photovoltaic performance by side chain engineering.

In this work, efficient nonfullerene (NF) polymer solar cells (PSCs) based on a polymer donor PM6 containing a fluorinated-thienyl benzodithiophene unit and a small molecule acceptor ITIC were developed. PM6 possesses a strong absorption in the short wavelength region of 300–685 nm with a large bandgap of 1.80 eV, which is complementary to that of ITIC (1.55 eV) and facilitates achieving high short-circuit current (Jsc) in PSCs. Moreover, PM6 shows a deep HOMO level of −5.50 eV, a strong crystallinity and a dominant face on packing, which helps to achieve a high open-circuit voltage (Voc) and fill factor (FF) in PSCs. As a result, the PM6:ITIC-based PSCs obtained a power conversion efficiency (PCE) of 9.7% with a Voc of up to 1.04 V, a Jsc of 16.0 mA cm−2 and a FF of 58%, under the illumination of AM 1.5G, 100 mW cm−2. Notably, the energy loss (Eloss) of the PSCs is as low as 0.51 eV, which is smaller than the empirically low threshold of 0.6 eV. The PCE of 9.7% is one of the highest values reported in the literature for PSCs with a Voc over 1.0 V and an Eloss less than 0.55 eV.

•A novel wide bandgap conjugated polymer (PSTZ) was synthesized.•Two compatible non-fullerene acceptors (IDIC and ITIC) were combined as acceptor alloy.•The PSCs based on PSTZ: ITIC: IDIC (1:0.1:0.9) exhibited a higher PCE of 11.1%.In this work, we fabricated high efficient ternary PSCs based on two compatible non-fullerene acceptors (IDIC and ITIC) with similar chemical structures and one new D-A-type polymer (PSTZ) donor. By inserting ITIC into the binary PSTZ:IDIC system, the active layer show smooth and gradient energy levels, improved crystallinity and optimized morphologies, which results in efficient exciton separation, charge transport and collection. As a result, the optimal ternary PSCs based on PSTZ:ITIC:IDIC (1:0.1:0.9) exhibited a higher PCE of 11.1% with a Voc of 0.953 V, Jsc of 17.4 mA cm−2 and FF of 66.9% in comparison with those of binary PSC systems based on PSTZ:IDIC (PCE of 8.06%) or PSTZ:ITIC (PCE of 8.13%). These results indicate that combining two compatible nonfullerene acceptors with similar structures to fabricate ternary PSCs should be a promising strategy to enhance the photovoltaic performance of the PSCs.A new wide bandgap copolymer PSTZ was synthesized and applied for photovoltaic devices. The ternary non-fullerene PSCs based on PSTZ as donor and the alloy of two compatible nonfullerene acceptors ITIC and IDIC as acceptor exhibited superior photovoltaic performance with a PCE of 11.1%, under the illumination of AM 1.5 G, 100 mW cm−2.Download high-res image (202KB)Download full-size image

In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 16.4 mA cm−2, and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm2. These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.

A new conjugated copolymer, PBTF-OP, based on meta-alkoxy-phenyl-substituted benzodithiophene (BDT-m-OP) and 2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT) was designed and synthesized for application as the donor material in polymer solar cells (PSCs). PBTF-OP possesses a similar molecular structure to the well-known polymer PTB7-Th but different conjugated side chains on the BDT unit: meta-alkoxy-phenyl side chains for PBTF-OP and alkylthienyl side chains for PTB7-Th. Compared with PTB7-Th, PBTF-OP exhibits absorption with some blue shifts, while it possesses a deeper HOMO energy level of −5.45 eV and a slightly enhanced hole mobility of 1.25 × 10−3 cm2 (V−1 s−1) versus a HOMO energy level of −5.30 eV and a hole mobility of 1.11 × 10−3 cm2 (V−1 s−1) for PTB7-Th. The PSCs based on PBTF-OP:PC71BM showed a higher power conversion efficiency (PCE) of 9.0% with a higher Voc of 0.86 V in comparison with a PCE of 8.3% and a Voc of 0.78 V for PTB7-Th. The results indicate that side chain engineering of BDT-based copolymers is an effective way to improve photovoltaic performance of polymer donors.

We synthesized a wide-bandgap conjugated polymer (named PTZ1) based on thienyl-substituted benzodithiophene as the donor unit and thiazolothiazole as the acceptor unit for photovoltaic applications. The polymer exhibits a desirable broad bandgap of 1.97 eV with the maximum absorption edge of 630 nm, a deep highest occupied molecular orbital (HOMO) energy level of −5.31 eV and a relatively high hole mobility of 3.86 × 10−3 cm2 V−1 s−1. Consequently, single junction PSCs based on PTZ1 exhibit outstanding performance with a PCE of 7.7% and a high Voc of 0.94 V, which are among the highest values for PSCs based on conjugated polymers with a broad bandgap near to 2.0 eV. The excellent performance is attributed to the high polymer crystallinity, favorable backbone orientation and continuous interpenetrating network in blend films. The tandem PSCs based on PTZ1 as the donor material in the front cell exhibited a high PCE of 10.3% with a Voc of 1.65 V.

Two electron-donating sides chain (octylthio and methoxy) were strategically attached on BDTT unit to develop two new polymers (PBDTT-OS-TT-EF and PBDTT-OS-TT-CF) as donor materials for polymer solar cells. PBDTT-OS-TT-EF and PBDTT-OS-TT-CF show broad absorption spectra with narrow optical band-gap of 1.52 and 1.56 eV, and low-lying HOMO energy level of −5.34 and −5.46 eV, respectively. A high Voc of 0.92 V was obtained from the PSC devices based on PBDTT-OS-TT-EF. Particularly, the Voc of PSC device based on PBDTT-OS-TT-CF is as high as 1.00 V, which is the highest value for the devices based on TT-base polymers. The results indicate that the engineering two electron-donating side chains in BDTT unit is an effective strategy to narrow the band-gap and manipulate the energy levels of polymers, which can be potentially applied to design polymer donor materials with high Voc values.Two electron-donating sides chain (octylthio and methoxy) were strategically attached on BDTT unit to develop two new polymers (PBDTT-OS-TT-EF and PBDTT-OS-TT-CF) as donor materials for polymer solar cells. PBDTT-OS-TT-EF and PBDTT-OS-TT-CF possess narrow band-gap and low-lying HOMO energy levels. A high Voc of 0.92 V was obtained from the PSC devices based on PBDTT-OS-TT-EF. Particularly, the Voc of PSC device based on PBDTT-OS-TT-CF is as high as 1.00 V, which is the highest value for the devices based on TT-base polymers.

•Two 2D-conjugated D-A copolymers, PBDTT-S-DPP and PBDTSe-S-DPP with alkylthio substituent, were synthesized.•The side chain engineering plays important roles in affecting the absorption and HOMO energy levels of the polymers.•The PSCs based on PBDTT-S-DPP as donor and PC71BM as acceptor demonstrate PCE of 5.62% with relatively higher Voc of 0.79 V.Two new 2D-conjugated D-A copolymers, PBDTT-S-DPP and PBDTSe-S-DPP, based on benzodithiophene (BDT) donor unit with alkylthio-thiophene or alkylthio-selenophene conjugated side chains and 2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DPP) acceptor unit, were synthesized for the application as donor materials in polymer solar cells (PSCs). The two polymers were characterized by absorption spectroscopy, cyclic voltammetry, thermogravimetric analysis, theoretical calculation with density functional theory, X-ray diffraction and photovoltaic measurements. The results show that the alkylthio-thiophene/selenophene side groups on BDT unit and intramolecular hydrogen bonding interaction in DPP acceptor unit play important roles in affecting the absorption, HOMO energy levels, molecular planarity and the crystallinity of the polymers. The PSCs based on PBDTT-S-DPP or PBDTSe-S-DPP as donor and PC71BM as acceptor demonstrate power conversion efficiency (PCE) of 5.62% and 5.01%, with relatively higher Voc of 0.79 V and 0.76 V, respectively.Two new 2D-conjugated D-A copolymers, PBDTT-S-DPP and PBDTSe-S-DPP, based on 2D-BDT donor unit and DPP acceptor unit, were synthesized for the application as donor materials in polymer solar cells (PSCs). The results reveal that integrating the 2D-BDT with alkylthio side chain and DPP acceptor unit in one polymer can effectively lower the HOMO energy level, improve the coplanarity and increase the crystallinity of the polymers. The PSCs based on PBDTT-S-DPP or PBDTSe-S-DPP/PC71BM demonstrated PCE of 5.62% and 5.01%, with a higher Voc of 0.79 V and 0.76 V, respectively.

New D–A copolymers, PTPD-DT and PTPD-DFDT, based on a thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor unit and 2,2′-bithiophene (DT) or 3,3′-difluoro-2,2′-bithiophene (DFDT) donor units, were designed and synthesized for application as donor materials in polymer solar cells (PSCs). A control polymer PTPD-DT with a similar structure but without fluorine substitution on the 2,2′-bithiophene (DT) unit was also synthesized for comparison. Compared with PTPD-DT, the polymer PTPD-DFDT with fluorine substitution on the DT unit shows a lower HOMO energy level of −5.55 eV, more broad absorption in the wavelength range from 300 to 700 nm, greater coplanarity and crystalline structure. The PSCs based on PTPD-DFDT/PC71BM demonstrated a power conversion efficiency of 5.52%, with a higher open-circuit voltage of 0.96 V. Furthermore, PTPD-DFDT exhibits a simpler molecular structure and easier synthesis steps, which is beneficial for mass production in future.

A new 2D-conjugated copolymer (PBDTSe-S-TT) based on alkylthio-selenophene substituted benzodithiophene (BDTSe-S) and 2-ethylhexyl 4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT) was designed and synthesized for application as a donor material in polymer solar cells (PSCs). PBDTSe-S-TT shows a broad absorption in the wavelength range from 300 to 800 nm, and a lower highest occupied molecular orbital (HOMO) energy level of −5.33 eV. The PSCs based on PBDTSe-S-TT as donor and [6, 6]-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptor with 3% DIO as additive exhibited a power conversion efficiency (PCE) of 7.57%, under the illumination of AM 1.5 G, 100 mW cm−2. These results indicate that attaching an alkylthio-selenophene side chain in 2D-conjugated polymers could be an alternative method to enhance the Voc and PCE of the PSCs.

•Two novel copolymers, with priorities in light absorption and hole mobility, were applied as HTL materials in perovskite solar cells.•To balance the charge transportation, pristine C60 was employed as interlayer at the cathode side.•With the two above-mentioned interlayers, a PCE as high as 15.83% was achieved.The interlayers, including hole transporting layer (HTL) and electron transporting layer (ETL), segregating photoactive layer and the electrodes play an important role in charge extraction and transportation in perovskite solar cells (pero-SCs). Two novel copolymers, PDTSTTz and PDTSTTz-4, for the first time were applied as HTL in the n-i-p type pero-SCs, with the device structure of ITO/compact TiO2/CH3NH3PbI3-xClx/HTL/MoO3/Ag. The highest occupied molecular orbitals (HOMO) levels of PDTSTTz and PDTSTTz-4 exhibit a suitable band alignment with the valence band edge of the perovskite. Both of them lead to improved device performances compared with reference pero-SCs based on P3HT as HTL. To further balance the charge extraction and the diffusion length of charge carriers, pristine C60 was introduced at the cathode side of the pero-SCs, working together with TiO2 as ETL. With insertion of both the HTL and ETL, the performance of pero-SCs was greatly enhanced. The optimized devices exhibited impressive PCEs of 14.4% and 15.8% for devices based on PDTSTTz and PDTSTTz-4. The improved performance is attributed to better light harvest ability, decreased interface resistance and faster decay time due to the introduction of the interlayers.

We report the cooperative assembly of the fullerene-styrene cyano-(octyloxy)benzene triad (PCBB-CN-C8) and poly(3-hexylthiophene) (P3HT) to form an active layer of polymer solar cells (PSCs) with a well-defined microstructure and an enhanced stability of morphology. A favorable synergistic effect of the three functional moieties (C60, styrene cyano and tri(octyloxy) chains) in PCBB-CN-C8 can not only induce P3HT to assemble into long-range ordered periodic fibrils giving an interpenetrating network but also can form PCBB-CN-C8 crystallized domains without the need for external treatment. The characterization of the microstructure and morphology of P3HT:PCBB-CN-C8 blend films by two-dimensional grazing incidence X-ray diffraction, transmission electron microscopy and atomic force microscopy reveals that the P3HT fibrils possess a highly crystallized lamellar phase, and the spacing of the periodic P3HT fibrils is approximately 10 nm depending on the PCBB-CN-C8 crystallites, which fill in the P3HT interpenetrating network. Bulk heterojunction PSCs based on P3HT:PCBB-CN-C8 exhibit an improved open-circuit voltage and an excellent power conversion efficiency of 4.20%, which is greater than that of control PSCs based on P3HT:PCBB-C8 and the devices based on P3HT:PCBM with thermal annealing. We believe that the cooperative assembly of the active layer using the synergistic effect of the fullerene triad is a general approach that can be used to develop external treatment-free technology and improve the morphological stability of the active layer and photovoltaic performance.

A novel n-type two-dimensional (2D) conjugated polymer based on bithienyl-benzodithiophene (BDT) and perylene diimide (PDI), P(PDI-BDT-T), was synthesized by Stille coupling for application as an acceptor material in all-polymer solar cells (all-PSCs). P(PDI-BDT-T) exhibits broad absorption in the visible region with an optical bandgap (Eg) of 1.64 eV, and a LUMO level of −3.89 eV, which is similar to and slightly higher than that of PCBM, indicating that the polymer is a suitable acceptor to replace PCBM in PSCs. PSCs with P(PDI-BDT-T) as the acceptor and PTB7-Th as the donor demonstrated a power conversion efficiency (PCE) of 4.71% with a Jsc of 11.51 mA cm−2, Voc of 0.80 V, and FF of 51.1%. Meanwhile, the PCE of the PSCs based on the acceptor of a corresponding 1D-conjugated polymer P(PDI-BDT-O) with an alkoxy side chain on the BDT unit was only 2.75% with a Jsc of 10.14 mA cm−2, Voc of 0.72 V, and FF of 37.6%. These results indicate that the 2D-conjugated P(PDI-BDT-T) is a promising acceptor material for all-PSCs.

A new planar D2-A-D1-A-D2 structured organic molecule with bithienyl benzodithiophene (BDT) as central donor unit D1 and fluorine-substituted benzothiadiazole (BTF) as acceptor unit and alkyl-dithiophene as end group and donor unit D2, BDT-BTF, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDT-BTF shows a broad absorption in visible region, suitable highest occupied molecular orbital energy level of −5.20 eV, and high hole mobility of 1.07 × 10–2 cm2/(V s), benefitted from its high coplanarity and strong crystallinity. The OSCs based on BDT-BTF as donor (D) and PC71BM as acceptor (A) at a D/A weight ratio of 3:1 without any extra treatment exhibit high photovoltaic performance with Voc of 0.85 V, Jsc of 10.48 mA/cm2, FF of 0.66, and PCE of 5.88%. The morphological study by transmission electron microscopy reveals that the blend of BDT-BTF and PC71BM (3:1, w/w) possesses an appropriate interpenetrating D/A network for the exciton separation and charge carrier transport, which agrees well with the good device performance. The optimized D/A weight ratio of 3:1 is the lowest acceptor content in the active layer reported so far for the high-performance OSCs, and the organic molecules with the molecular structure like BDT-BTF could be promising high-performance donor materials in solution-processable OSCs.Keywords: active layer with low fullerene content; coplanarity; crystallization; D2-A-D1-A-D2 organic molecules; organic solar cells

•The non-fullerene PSCs based on PTZ6: ITIC exhibit a PCE of 10.3%.•The PSCs based on PTZ6: PC71BM show a PCE of 8.1%.•A novel wide bandgap conjugated polymer (PTZ6) with 2.0 eV was synthesized.An efficient wide bandgap conjugated polymer (PTZ6) based on alkoxylphenyl substituted benzodithiophene as donor unit and bithiazole as acceptor unit was developed for polymer solar cells (PSCs). The polymer exhibited a wide bandgap of 2.0 eV with strong absorption in the range of 300–620 nm, and a low-lying highest occupied molecular orbital (HOMO) energy level of −5.36 eV. The PSCs based on PTZ6: PC71BM show a PCE of 8.1% with a Voc of 0.96 V, a Jsc of 10.9 mA cm−2 and a high FF of 76.7%, which is among the highest values for the fullerene PSCs based on conjugated polymer donors with bandgap near to 2.0 eV. Moreover, for this blend system, the photovoltaic performance of the devices changes little when the active layer thickness increases from 90 nm to 220 nm. More importantly, the non-fullerene PSCs based on PTZ6: ITIC exhibit a PCE of 10.3% with a high Voc of 1.01 V, which should be the best value for the non-fullerene PSCs with the Eloss less than 0.6 eV to date. Our results indicate that PTZ6 is a promising wide bandgap polymer donor for the photovoltaic application in PSCs.A novel wide bandgap conjugated polymer PTZ6 with an Eg of 2.0 eV is designed and synthesized for photovoltaic application. PSCs based on PTZ6: PC71BM and PTZ6: ITIC blend films exhibit high PCEs of up to 8.1% and 10.3%, respectively, under the illumination of AM 1.5 G, 100 mW cm−2.

We report the cooperative assembly of the fullerene-styrene cyano-(octyloxy)benzene triad (PCBB-CN-C8) and poly(3-hexylthiophene) (P3HT) to form an active layer of polymer solar cells (PSCs) with a well-defined microstructure and an enhanced stability of morphology. A favorable synergistic effect of the three functional moieties (C60, styrene cyano and tri(octyloxy) chains) in PCBB-CN-C8 can not only induce P3HT to assemble into long-range ordered periodic fibrils giving an interpenetrating network but also can form PCBB-CN-C8 crystallized domains without the need for external treatment. The characterization of the microstructure and morphology of P3HT:PCBB-CN-C8 blend films by two-dimensional grazing incidence X-ray diffraction, transmission electron microscopy and atomic force microscopy reveals that the P3HT fibrils possess a highly crystallized lamellar phase, and the spacing of the periodic P3HT fibrils is approximately 10 nm depending on the PCBB-CN-C8 crystallites, which fill in the P3HT interpenetrating network. Bulk heterojunction PSCs based on P3HT:PCBB-CN-C8 exhibit an improved open-circuit voltage and an excellent power conversion efficiency of 4.20%, which is greater than that of control PSCs based on P3HT:PCBB-C8 and the devices based on P3HT:PCBM with thermal annealing. We believe that the cooperative assembly of the active layer using the synergistic effect of the fullerene triad is a general approach that can be used to develop external treatment-free technology and improve the morphological stability of the active layer and photovoltaic performance.

A new conjugated copolymer, PBTF-OP, based on meta-alkoxy-phenyl-substituted benzodithiophene (BDT-m-OP) and 2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT) was designed and synthesized for application as the donor material in polymer solar cells (PSCs). PBTF-OP possesses a similar molecular structure to the well-known polymer PTB7-Th but different conjugated side chains on the BDT unit: meta-alkoxy-phenyl side chains for PBTF-OP and alkylthienyl side chains for PTB7-Th. Compared with PTB7-Th, PBTF-OP exhibits absorption with some blue shifts, while it possesses a deeper HOMO energy level of −5.45 eV and a slightly enhanced hole mobility of 1.25 × 10−3 cm2 (V−1 s−1) versus a HOMO energy level of −5.30 eV and a hole mobility of 1.11 × 10−3 cm2 (V−1 s−1) for PTB7-Th. The PSCs based on PBTF-OP:PC71BM showed a higher power conversion efficiency (PCE) of 9.0% with a higher Voc of 0.86 V in comparison with a PCE of 8.3% and a Voc of 0.78 V for PTB7-Th. The results indicate that side chain engineering of BDT-based copolymers is an effective way to improve photovoltaic performance of polymer donors.

In this work, a new wide-bandgap polymer, PSBZ, based on thienyl substituted benzodithiophene (BDTT) as the donor unit and difluorobenzotriazole (BTz-2F) as the acceptor unit was synthesized for photovoltaic applications. Compared to the analogous polymer J61 with linear dodecylthio side chains in the BDTT unit and a long 2-hexyldecyl side chain in BTz-2F, PSBZ possesses branched 2-butyloctyl side chains to increase steric hindrance of the BDTT unit and a short 2-butyloctyl side chain to decrease steric hindrance of the BTz-2F unit for more efficient charge separation and transport in the devices. As a result, PSBZ exhibited stronger π–π interaction and smaller stacking spacing leading to a higher extinction coefficient of 1.48 × 105 cm−1 and a high hole mobility of 8.56 × 10−3 cm2 V−1 s−1. Compared to the analogous polymer J61 with a power conversion efficiency (PCE) of 9.53% and a short-circuit current density (Jsc) of 17.43 mA cm−2, the PSBZ:ITIC-based polymer solar cells yielded a higher PCE of 10.5% with a higher Jsc of 19.0 mA cm−2. The results show that our design strategy is successful for improving photovoltaic performance by side chain engineering.

A novel n-type two-dimensional (2D) conjugated polymer based on bithienyl-benzodithiophene (BDT) and perylene diimide (PDI), P(PDI-BDT-T), was synthesized by Stille coupling for application as an acceptor material in all-polymer solar cells (all-PSCs). P(PDI-BDT-T) exhibits broad absorption in the visible region with an optical bandgap (Eg) of 1.64 eV, and a LUMO level of −3.89 eV, which is similar to and slightly higher than that of PCBM, indicating that the polymer is a suitable acceptor to replace PCBM in PSCs. PSCs with P(PDI-BDT-T) as the acceptor and PTB7-Th as the donor demonstrated a power conversion efficiency (PCE) of 4.71% with a Jsc of 11.51 mA cm−2, Voc of 0.80 V, and FF of 51.1%. Meanwhile, the PCE of the PSCs based on the acceptor of a corresponding 1D-conjugated polymer P(PDI-BDT-O) with an alkoxy side chain on the BDT unit was only 2.75% with a Jsc of 10.14 mA cm−2, Voc of 0.72 V, and FF of 37.6%. These results indicate that the 2D-conjugated P(PDI-BDT-T) is a promising acceptor material for all-PSCs.

We synthesized a wide-bandgap conjugated polymer (named PTZ1) based on thienyl-substituted benzodithiophene as the donor unit and thiazolothiazole as the acceptor unit for photovoltaic applications. The polymer exhibits a desirable broad bandgap of 1.97 eV with the maximum absorption edge of 630 nm, a deep highest occupied molecular orbital (HOMO) energy level of −5.31 eV and a relatively high hole mobility of 3.86 × 10−3 cm2 V−1 s−1. Consequently, single junction PSCs based on PTZ1 exhibit outstanding performance with a PCE of 7.7% and a high Voc of 0.94 V, which are among the highest values for PSCs based on conjugated polymers with a broad bandgap near to 2.0 eV. The excellent performance is attributed to the high polymer crystallinity, favorable backbone orientation and continuous interpenetrating network in blend films. The tandem PSCs based on PTZ1 as the donor material in the front cell exhibited a high PCE of 10.3% with a Voc of 1.65 V.

A 1,1′-vinylene-fused indacenodithiophene (IDTV) donor unit with 22 π-conjugated electrons was synthesized. A ladder-type D–A copolymer PIDTV-ffBT using IDTV as the donor unit and 5,6-difluorobenzothiadiazole (ffBT) as the acceptor unit was developed for application as a donor material in polymer solar cells (PSCs). Compared to other analogue polymers, PIDTV-ffBT possesses a two-dimensional conjugated multi-electron fused ring, excellent planarity and close π–π stacking, leading to a higher light harvesting coefficient, an enhanced charge carrier mobility of 0.032 cm2 V−1 s−1 and improved photovoltaic performance. The PSCs based on PIDTV-ffBT:PC71BM achieved a promising power conversion efficiency (PCE) of 7.3% with a high short-circuit current density (Jsc) of 17.1 mA cm−2. These results indicate that the introduction of the 1,1′-vinylene-fused system into IDTV for ladder-type polymers is an effective strategy to enhance the light absorption coefficient and improve charge carrier mobility for high efficiency PSCs.