Effect of Active Layer Thickness on the Performance of Polymer Solar Cells Based on a Highly Efficient Donor Material of PTB7-Th

One of the challenges for the commercialization of polymer solar cells (PSCs) is the difficulty in fabricating a homogeneous and pinhole-free, thin active layer through a large-scale, high-throughput, roll-to-toll manufacturing process. On the other hand, thick active layers in current PSCs generally result in low power conversion efficiency (PCE). Here, we reported the effect of active layer thickness on the performance of the PSC device based on the state-of-the-art blends of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-uorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The results showed that although the short-circuit current density (J(SC)) was increased in the device with thicker active layer the PCE was decreased due to the drastically declined fill factor (FF), which offset the improved light absorption from using thicker films. Optical modeling using the transfer matrix method (TMM) and analysis of the fitted Shockley diode equation of illuminated current density-voltage (J-V) characteristics indicated that the decrease of FF for thicker PTB7-Th:PC71BM solar cells was ascribed to the inefficient charge carrier transport and collection, which resulted from relatively low electron mobility and the increased interface defect states. Based on these results, the devices with higher PC71BM content were fabricated to facilitate the electron transport, which allowed an overall increase of the efficiency to 8.15% for the device with a 270 nm thick active layer due to the significantly improved FF.