Correlating high power conversion efficiency of PTB7:PC71BM inverted organic solar cells with nanoscale structures

• Published in: Nanoscale 2015-10, Vol.7 (38), p.15576-15583
• Main Authors: Das, Sanjib, Keum, Jong K, Browning, James F, Gu, Gong, Yang, Bin, Dyck, Ondrej, Do, Changwoo, Chen, Wei, Chen, Jihua, Ivanov, Ilia N, Hong, Kunlun, Rondinone, Adam J, Joshi, Pooran C, Geohegan, David B, Duscher, Gerd, Xiao, Kai
• Format: Article
• Language: English
• ISSN: 2040-3372
• DOI: 10.1039/c5nr03332b

Advances in material design and device engineering led to inverted organic solar cells (i-OSCs) with superior power conversion efficiencies (PCEs) compared to their "conventional" counterparts, in addition to the well-known better ambient stability. Here, we report an in-depth morphology study of the i-OSC active and cathode modifying layers, employing a model system with a well-established bulk-heterojunction, PTB7:PC71BM as the active layer and poly-[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as the cathode surface modifying layer. We have also identified the role of a processing additive, 1,8-diiodooctane (DIO), used in the spin-casting of the active layer to increase PCE. Using various characterization techniques, we demonstrate that the high PCEs of i-OSCs are due to the diffusion of electron-accepting PC71BM into the PFN layer, resulting in improved electron transport. The diffusion occurs when residual solvent molecules in the spun-cast film act as a plasticizer. Addition of DIO to the casting solution results in more PC71BM diffusion and therefore more efficient electron transport. This work provides important insight and guidance to further enhancement of i-OSC performance by materials and interface engineering.