Abstract
Nonfullerene acceptors (NFAs)‐based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors‐based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 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) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady‐state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC‐based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC71BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC71BM yield a better PCE of 11.4% without any additive or annealing treatment.
Highlights
Introduction components inNFA-based systems, there exist two possible charge transfer channels— electron transfer from DIn recent several years, nonfullerene acceptors (NFAs)-based to A and hole transfer from A to D via highest occupied solution-processed bulk heterojunction (BHJ) organic solar molecular orbital (HOMO) energy levels
PBBF11 was synthesized via one-pot Stille coupling reaction, and its synthetic route is shown in Figure S1 in the Supporting Information
It is noticed that the HOMO offset (ΔEHOMO) between PBBF11 and ITIC is negligible as only 0.03 eV, which is enough to achieve efficient hole transfer from ITIC to PBBF11
Summary
Nonfullerene acceptors (NFAs)-based organic solar cells (OSCs) have recently drawn considerable research interests; their excitonic molecular structures, allowing for a fine control of optical bandgap (Eg), energy levels, light absorption, and crystallinity dynamics seems quite different than that of fullerene acceptors-based devices and remains to be largely explored. Wang and coworkers found that the charge recombination loss in the ITIC-based OSCs was considerably worse than that in FA-based counterparts due to the poor phase-separation morphology induced by well miscibility of polymer donor and ITIC.[18] One feasible approach to solve the problem is the construction of ternary blend OSCs by judiciously selecting three components and tuning their ratio that can synergistically optimize film morphology, enhance optical absorption, and promote charge transport, which has attracted increasing research interest.[19,20,21,22,23,24,25,26,27,28] This contribution aims to gain a better understanding of efficient charge transfer and address the issue of morphologyinduced charge recombination in NFA-based OSCs. We first designed and synthesized the random terpolymer donor based on one benzodithiophene (BDT) donor moiety and two wellchosen acceptor units of benzodithiophene-4,8-dione (BDD)[29] and fluorine-substituted benzotriazole (FTAZ).[30] At a molar ratio of BDT:BDD:FTAZ = 2:1:1, the resulting terpolymer donor named as PBBF11 exhibits complementary light absorption and very small HOMO/LUMO energy offsets with ITIC, the latter of which can still ensure efficient hole and electron transfer between them. The optimal ternary OSC based on the PBBF11:ITIC:PC71BM (1:1:0.3, wt%) blend delivers an outstanding PCE of 11.4%
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