Simultaneous Performance and Stability Improvement of Ternary Polymer Solar Cells Enabled by Modulating the Molecular Packing of Acceptors

Abstract

Nanoscale morphology of the active layer plays a crucial role in the power conversion efficiency (PCE) and stability of polymer solar cells (PSCs). Blending the photoactive layer with a third component to produce a ternary system is considered a reliable approach to tune the nanomorphology, thereby improving the device performance. Herein, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene)‐co‐(1,3‐di(5‐thiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PBDB‐T): 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) solar cells doped with a third small molecule are systematically investigated, namely, (5Z,5′Z)‐5,5′‐((7,7′‐(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(benzo[c]1,2,5]thiadiazole‐7,4‐diyl))‐bis(methanylyl‐idene))bis(3‐ethyl‐2‐thioxothiazolidin‐4‐one) (FBR). Owing to the wide optical bandgap of FBR, blending PBDB‐T:ITIC with FBR increases the device's light‐harvesting capability in the short wavelength range (400–550 nm), which improves the short circuit current. Differential scanning calorimetry and grazing incidence wide angle X‐ray scattering analyses reveal that the FBR exhibits impressive miscibility with ITIC, leading to the formation of ITIC:FBR alloys. Optimum performance is achieved with a PBDB‐T:ITIC:FBR (1:0.8:0.2) cell, which yields a PCE of 11.17%, demonstrating a 10% improvement relative to the PBDB‐T:ITIC binary cell. Crucially, the ternary solar cells also show improved device stability, which is attributed to the formation of ITIC:FBR alloys suppressing the crystallization of ITIC. This study provides deep insights into the performance‐ and stability‐related improvements available to PSCs devices that incorporate a third conjugated small molecule.