The Impact of Device Polarity on the Performance of Polymer–Fullerene Solar Cells

• Published in: Advanced energy materials Vol. 8; no. 22
• Main Authors: Li, Mengmeng, Li, Junyu, Di Carlo Rasi, Dario, Colberts, Fallon J. M., Wang, Junke, Heintges, Gaël H. L., Lin, Baojun, Li, Weiwei, Ma, Wei, Wienk, Martijn M., Janssen, René A. J.
• Format: Journal Article
• Language: English
• Published: 06.08.2018
• Subjects: device polarity; light absorption; morphology control; polymer solar cells
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201800550

Diketopyrrolopyrrole (DPP)‐conjugated polymers are a versatile class of semiconductors for application in organic solar cells because of their tunable optoelectronic properties. A record power conversion efficiency (PCE) of 9.4% was recently achieved for DPP polymers, but further improvements are required to reach true efficiency limits. Using five DPP polymers with different chemical structures and molecular weights, the device performance of polymer:fullerene solar cells is systematically optimized by considering device polarity, morphology, and light absorption. The polymer solubility is found to have a significant effect on the optimal device polarity. Soluble polymers show a 10–25% increase in PCE in inverted device configurations, while the device performance is independent of device polarity for less soluble DPP derivatives. The difference seems related to the polymer to fullerene weight ratio at the ZnO interface in inverted devices, which is higher for more soluble DPP polymers. Optimization of the nature of the cosolvent to narrow the fibril width of polymers in the blends toward the exciton diffusion length enhances charge generation. Additionally, the use of a retroreflective foil increases absorption of light. Combined, the effects afford a PCE of 9.6%, among the highest for DPP‐based polymer solar cells. Polymer solubility critically affects the optimal device polarity for diketopyrrolopyrrole based polymer solar cells. Inverted devices show superior performance for polymers with good solubility, while the performance is independent of device polarity for less soluble polymers. Optimization of the morphology using a cosolvent and applying a retroreflective foil to enhance light absorption further improve the power conversion efficiency to 9.6%