Improved performance by morphology control via fullerenes in PBDT-TBT-alkoBT based organic solar cells

Abstract

In this work, we report improved performance by controlling morphology using different fullerene derivatives in poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thieno[3,2-b]thiophen-2-yl)-benzo[c][1,2,5]thiadiazole} (PBDT-TBT-alkoBT) based organic solar cells. PC₆₀BM and PC₇₀BM fullerenes were used to investigate the characteristic changes in morphology and device performance. Fullerenes affect device efficiency by changing the active layer morphology. PC₇₀BM with broader absorption than PC₆₀BM resulted in reduced device performance which was elucidated by the intermixed granular morphology separating each larger grain in the PC₇₀BM/polymer composite layer which created a higher density of traps. However after adding additive 1,8-diiodooctane (DIO), a fibrous morphology was observed due to the reduced solubility of the polymer and increased solubility of PC₇₀BM in chloroform. The fibrous morphology improved charge transport leading to an increase in overall device performance. Atomic force microscopy (AFM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV), and Kelvin probe force microscopy (KPFM) were used to investigate the nanoscale morphology of the active layer with different fullerene derivatives. For the PC₆₀BM based active layer, AFM images revealed a dense fibrous morphology and more distinct fibrous morphology was observed by adding DIO. The PC₇₀BM based active layer only exhibited an intermixed granular morphology instead of a fibrous morphology observed in the PC₆₀BM based active layer. However, addition of DIO into the PC₇₀BM based active layer led to fibrous morphology. When additive DIO was not used, a wider distribution of surface potential was observed for PC₇₀BM than the PC₆₀BM based active layer by KPFM measurements, indicating that polymer and fullerene domains are separated. When DIO was used, a narrower distribution of surface potential for both PC₇₀BM and PC₆₀BM based active layers was observed. Photo-CELIV experiments showed larger extracted charge carrier density and mobility in the PC₇₀BM/DIO film.