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

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.