Performance and stability of PTB7:PC71BM based polymer solar cells, with ECZ and/or PVK dopants, under the application of an external electric field

Abstract

The effect on the J–V behavior of polymer solar cells (PSCs) based on the active layer PTB7:PC71BM under a previous application of an external electric field (Eext) is presented. The active layer is doped with 0.01–0.04 wt. ratio (with respect to PTB7) of poly(9-vinylcarbazole) (PVK) as a photoconductor, and/or 0.25–0.5 wt. ratio of 9-ethylcarbazole (ECZ) as plasticizer. The general PSC structure was Glass-ITO/PEDOT:PSS/PTB7:PC71BM:dopant/PFN/FM where dopant means PVK, ECZ or a mixture of them. Field’s metal (FM) is an eutectic alloy with a melting point above 62 °C, which is deposited in a vacuum free atmosphere. Electric fields of 5 or −70 V/μm (forward and reversed polarity, respectively) are applied. For PSCs doped with PVK, under forward polarity, it is observed a remarkable variation (36 %) on Jsc: from 7.7 to 4.9 mA/cm2, while Voc remains almost constant: ~0.76 V; whereas with the reversed polarity, a drastic variation of 95 % is observed on Voc: from 0.76 to 0.04 V; while Jsc value changes from 7.4 to 5.6 mA/cm2 (24 %). On the other hand, PSCs doped with the combination of ECZ and PVK, an increase of 10 % in Jsc is reached for reversed bias (Eext = −70 V); and without applying any field, and keep them under N2 atmosphere, a slower photovoltaic degradation, with respect to that one from all the other studied samples, is shown over 57 days of monitoring them: FF variation is of just 6 % and PCE decays only 21 %.