Wanbin Li

High Efficiency Nonfullerene Polymer Solar Cells with Thick Active Layer and Large Area

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.

Efficient ternary blend all-polymer solar cells with a polythiophene derivative as a hole-cascade material

Ternary blending is one of the effective strategies to broaden the complementary absorption range and smooth the energy level at the donor/acceptor interface for achieving high efficiency bulk heterojunction (BHJ) polymer solar cells (PSCs). In this study, we report efficient ternary blend all-polymer solar cells (all-PSCs) with complementary absorption bands based on two polymer donors PTB7-Th and PBDD-ff4T and one polymer acceptor N2200. The polythiophene derivative PBDD-ff4T as a hole-cascade material plays a bridging role in energy levels between PTB7-Th and N2200, and thus provides more efficient channels for charge transfer. The ternary all-PSCs with 10 wt% PBDD-ff4T content show efficient photon harvesting, enhanced charge mobility and better active layer morphology due to the induced crystallization of PTB7-Th by the inserted PBDD-ff4T in the donor domains. As a result, the device without any extra treatments exhibits an optimized power conversion efficiency (PCE) of 7.2% with an open circuit voltage (Voc) of 0.82 V, a short circuit current density (Jsc) of 15.7 mA cm−2, and a fill factor (FF) of 56%. While the PCEs are 5.9% and 4.2% for the all-PSCs based on the binary blends PTB7-Th:N2200 and PBDD-ff4T:N2200, respectively. This PCE of 7.2% is one of the highest values reported in the literature so far for ternary all-PSCs and polythiophene derivative-based all-PSCs.

Synthesis and photovoltaic properties of an n-type two-dimension-conjugated polymer based on perylene diimide and benzodithiophene with thiophene conjugated side chains

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 wide-bandgap conjugated polymer for highly efficient inverted single and tandem polymer solar cells

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.

High-performance nonfullerene polymer solar cells based on a fluorinated wide bandgap copolymer with a high open-circuit voltage of 1.04 V

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.

Efficient polymer solar cells based on a copolymer of meta-alkoxy-phenyl-substituted benzodithiophene and thieno[3,4-b]thiophene

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.

Efficient non-fullerene polymer solar cells based on a wide bandgap polymer of meta-alkoxy-phenyl-substituted benzodithiophene and difluorobenzotriazole

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.

A trifluoromethyl substituted wide bandgap conjugated polymer for non-fullerene polymer solar cells with 10.4% efficiency

A new wide bandgap copolymer (PBZ-m-CF3) based on meta-trifluoromethyl-p-alkoxyphenyl substituted benzodithiophene (BDTP-m-CF3) and difluorobenzotriazole (FBTZ) was successfully designed and synthesized for efficient non-fullerene polymer solar cells (PSCs). As a comparative material, a wide bandgap copolymer (PBZ1) based on p-alkoxyphenyl substituted benzodithiophene (BDTP) and FBTZ was synthesized by the same method. It was found that introducing trifluoromethyl into the meta-position of the phenyl group led to PBZ-m-CF3 possessing a deeper HOMO energy level of −5.49 eV, a wider optical bandgap (Eoptg) of 1.99 eV with a higher extinction coefficient of 6.51 × 104 cm−1 at 533 nm and a suitable hole mobility of 7.86 × 10−4 cm2 V−1 s−1 in comparison with PBZ1 (a HOMO level of −5.27 eV, an Eoptg of 1.96 eV with an extinction coefficient of 5.23 × 104 cm−1 at 539 nm and a hole mobility of 7.23 × 10−4 cm2 V−1 s−1). Non-fullerene bulk heterojunction PSCs based on PBZ-m-CF3 as the donor and ITIC as the acceptor were fabricated by using toluene as the solvent and showed a higher PCE of 10.4% with a high Voc of 0.94 V, a Jsc of 18.4 mA cm−2, and a FF of 60.2% compared with PSCs based on PBZ1: ITIC (a PCE of 5.8% with a Voc of 0.74 V, a Jsc of 15.7 mA cm−2, and a FF of 49.8%). These results demonstrate that a benzodithiophene unit with meta-trifluoromethyl-p-alkoxyphenyl side chains is a promising candidate as an electron-rich building block for high performance PSCs.

Improved photocurrent and efficiency of non-fullerene organic solar cells despite higher charge recombination

Organic solar cells (OSCs) with a fused-ring dye, ITIC, and fullerene derivative PC71BM as the acceptor materials were fabricated. Compared to PC71BM-based cells, which reach a power conversion efficiency of 6.91%, the ITIC device shows a significantly higher power conversion efficiency (9.20%). The broader absorption range of ITIC helps to improve the short-circuit current density, which leads to better device efficiency. Electrical characterization, including electrical simulations and impedance analysis, was performed to investigate the physical processes in these OSCs. The results suggest that the charge-recombination loss in the non-fullerene device is even higher than that in PC71BM cells. Morphological analysis reveals that a poor phase-separation in the photoactive layer is responsible for the large recombination loss. Our results indicate that fused-ring acceptor materials are promising candidates for high-efficiency OSCs. Furthermore, if recombination loss could be suppressed effectively, the energy conversion efficiency of non-fullerene organic solar cells can be even higher.

A wide-bandgap polymer based on the alkylphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene unit with high power conversion efficiency of over 11%

A novel wide-bandgap conjugated polymer (PTZP) based on alkylphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-P) as the electron-rich unit and thiazolo[5,4-d]thiazole (TTz) as the electron-deficient unit was designed and synthesized for the non-fullerene polymer solar cell (PSCs) application. The polymer exhibited a wide bandgap of 2.01 eV with a strong absorption in the range of 300–620 nm, which was complementary with that of the fused-ring small molecule acceptor (SMA; 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile, IDIC). Also, the polymer exhibits a deep highest occupied molecular orbital (HOMO) energy level of −5.41 eV. Furthermore, the polymer film possesses strong crystallinity and dominated face-on stacking with a small d-spacing of 3.65 A, resulting in a high hole mobility of 4.01 × 10−3 cm2 V−1 s−1. The optimal PSCs based on the PTZP:IDIC blend showed a high PCE of 11.8% with an open-circuit voltage (Voc) of 0.90 V, a short-circuit current density (Jsc) of 17.9 mA cm−2 and a fill factor (FF) of 73.3%. Moreover, the device with an active layer thickness of up to 200 nm or area of up to 0.81 cm2 exhibited outstanding performance, with PCE of over 10%, resulting from the excellent molecular stacking. These results revealed that PTZP will be a promising conjugated polymer for the fabrication of efficient large-area PSCs.