Nonhalogen Solvent‐Processed Asymmetric Wide‐Bandgap Polymers for Nonfullerene Organic Solar Cells with Over 10% Efficiency

Abstract

Two new wide‐bandgap D–A–π copolymer donor materials, PBDT‐2TC and PBDT‐S‐2TC, based on benzodithiophene and asymmetric bithiophene with one carboxylate (2TC) substituent are synthesized by a facile approach for fullerene‐free organic solar cells (OSCs). The combination of one carboxylate‐substituted thiophene with one thiophene bridge in the backbone substantially reduces the steric hindrance, thereby favoring a planar geometry for efficient charge transport and molecular packing. A reasonable highest‐occupied‐molecular‐orbital energy level in relation to that of the acceptor and balanced hole and electron transport are observed for both polymers. This asymmetric structure unit is flexible and versatile, allowing the absorption, energy levels, and morphology of the blend films to be tailored. Fullerene‐free OSCs based on PBDT‐S‐2TC:ITIC achieve a high power conversion efficiency of 10.12%. More impressively, a successful nonhalogen solvent‐processed solar cell with 9.55% efficiency is also achieved, which is one of the highest values for a fullerene‐free OSC processed using an ecofriendly solvent.

Asymmetric Wide‐Bandgap Polymers Simultaneously Improve the Open‐Circuit Voltage and Short‐Circuit Current for Organic Photovoltaics

“…Synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms is found to significantly reduce the HOMO level to −5.55 eV for PBDT‐F‐2TC and −5.57 eV for PBDT‐SF‐2TC, which can well‐match with the IT‐4F acceptor. Furthermore, the maximum extinction coefficients are also improved compared to our previously reported PBDT‐2TC and PBDT‐S‐2TC counterpart . The PCEs (10.75%) with both high V OC (0.91–0.92 V) and high J SC (17.45–18.90 mA cm −2 ) for PBDT‐F‐2TC, and the PCEs (11.07%) with both high V OC (0.93–0.94 V) and high J SC (19.17–20.73 mA cm −2 ) for PBDT‐SF‐2TC are achieved.…”

“…The HOMO level of PBDT‐F‐2TC is higher than that of PBDT‐SF‐2TC, which proved that the introduction of a fluorine atom in the side chain of the BDT unit down‐shifts the HOMO level. The simulated HOMOs of PBDT‐2TC and PBDT‐S‐2TC counterpart without fluorine atom are −4.74 and −4.80 eV, respectively . Therefore, the newly designed donors have deeper HOMOs than our previously reported polymer donors PBDT‐2TC and PBDT‐S‐2TC.…”

“…Moreover, the extinction coefficients of PBDT‐F‐2TC and PBDT‐SF‐2TC films at 540 and 542 nm are calculated to be 9.31 × 10 4 cm − 1 and 1.34 × 10 5 cm − 1 , respectively, as shown in Figure S10, Supporting Information. Thereby, the two copolymers have stronger light capture ability than their counterparts PBDT‐2TC (8.6 × 10 4 cm −1 ) and PBDT‐S‐2TC (1 × 10 5 cm −1 ) without fluorine atom . Figure S11, Supporting Information, shows a typical temperature‐dependent aggregation phenomenon of both copolymers .…”

“…Thereby, the two copolymers have stronger light capture ability than their counterparts PBDT‐2TC (8.6 × 10 4 cm −1 ) and PBDT‐S‐2TC (1 × 10 5 cm −1 ) without fluorine atom . Figure S11, Supporting Information, shows a typical temperature‐dependent aggregation phenomenon of both copolymers . During the cooling process from 95 to 25 °C, a main peak together with a shoulder peak emerges, corresponding to the aggregates in the two polymer solution.…”

“…Recently, we introduced asymmetric bithiophene with one carboxylate (2TC) substituent to synthesis two copolymers PBDT‐2TC and PBDT‐S‐2TC . The combination of 2TC into the copolymer donors not only can achieve a high V OC , but also can facilitate charge transport due to its highly planar structure.…”