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

Abstract

A trade‐off between open‐circuit voltage (V OC) and high short‐circuit (J SC) becomes one of the most vital problems limiting further improvement in polymer solar cells' (PSCs) efficiency. In this work, two asymmetric polymer donors PBDT‐F‐2TC and PBDT‐SF‐2TC are designed and synthesized. When blended with a state‐of‐the‐art acceptor IT‐4F with low lowest‐unoccupied molecular orbital level, simultaneously high V OC (up to 0.94 V) and J SC (up to 20.73 mA cm−2) are obtained for both copolymers. Note that the V OC value of 0.94 V is the highest value of PSCs based on IT‐4F reported so far. The simultaneously improved V OC and J SC in resulting devices are discovered from the deep highest‐occupied molecular orbital levels (−5.5 to −5.7 eV) and the hyperchromic effect of the polymers, the small driving force, and the small energy loss during the charge transfer, due to the synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms. More importantly, thanks to the asymmetric 2TC, both PBDT‐F‐2TC‐ and PBDT‐SF‐2TC‐based PSCs can be successfully processed by non‐halogenated solvent 1,2,4‐trimethylbenzene (TMB) to yield device efficiencies of 10.29% and 10.39%, respectively, which are the maximum values for non‐fullerene PSCs fabricated using the eco‐friendly solvent TMB.

Molecular Design of Efficient Chlorine‐ and Carboxylate‐Functionalized Donor Polymers for Nonfullerene Organic Solar Cells Enabling Processing with Eco‐Friendly Solvent in Air

“…As shown in Scheme S1 and S2, Supporting Information, the newly designed routes for each target compound have relatively low synthetic complexity compared with other possible synthetic routes. [ 27,39,40 ] As a result, the two brominated monomers BiT‐COOBOCl(in) and BiT‐COOBOCl(out) were successfully obtained with high overall yields (60.4% and 65.4%, respectively) via straightforward four‐step processes. The polymers were prepared via a typical Stille coupling reaction between the brominated monomers and F‐2DBDT, and then purified by successive Soxhlet extraction.…”

“…As a result, we can expect the chlorine‐ and carboxylate‐substituted moieties to have a stronger charge separation and lower HOMO level relative to that of reported polymers. [ 27 ]…”

“…Both polymers exhibited absorption bands at around 350 nm and 400–600 nm, corresponding to the π–π* transition and intramolecular charge transfer (ICT), respectively; these are complementary with IT‐4F, which has a strong absorption band in range of 600–800 nm. [ 18,19,27,29-31,33,34 ] As shown in Figure a, the molar absorption coefficients ( ε ) were calculated using the Beer–Lambert equation [ 23,32-34 ] from the UV–vis data recorded in chloroform solutions with different concentrations on the order of 10 −5 m . The maximum ε values were 1.82 × 10 4 and 8.25 × 10 4 m −1 cm −1 for P(F‐BiT)‐COOBOCl(in) and P(F‐BiT)‐COOBOCl(out), respectively.…”

“…[ 8,13,16–19,22–24 ] To address these issues, many researchers have proposed a simple design strategy with carboxylate‐functionalized heterocyclic rings as the acceptor unit. [ 25–31 ]…”

“…In 2019, Chen and co‐workers reported enhanced asymmetric polymers (PBDT‐F‐2TC and PBDT‐SF‐2TC; Figure 1c) by introducing fluorine and sulfur atoms on the 2DBDT unit, which efficiently lowered the HOMO level and increased the molar coefficient. [ 27 ] Optimized devices based on PBDT‐SF‐2TC blended with 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐ d :2′,3′‐ d ′]‐ s ‐indaceno[1,2‐ b :5,6‐ b ′]dithiophene (IT‐4F) showed PCEs of 11.07% and 10.29% when processed in CB and TMB, respectively. In the same year, our group reported a low‐cost wide‐bandgap donor polymer (P(Cl); Figure 1d) based on chlorine‐substituted thiophene, which was synthesized in only one step.…”