Organic Solar Cells Based on PTB7:PC₇₁BM with Cs₂CO₃ as a Cathode Buffer Layer

Abstract

Bulk heterojunction solar cells were fabricated using poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2- [(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) after a layer of poly(3,4- ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was deposited on an indium tin oxide (ITO)-coated glass substrate. The fabricated structures were glass/ITO/PEDOT:PSS/PTB7:PC₇₁BM/Cs₂CO₃/Al with or without the cesium carbonate (Cs₂CO₃) buffer layer, and the effect of the buffer layer on the performance of the solar cells was investigated. The Cs₂CO₃ layer with a thickness of 0.8 nm significantly increased the short circuit current density, open-circuit voltages and fill factors of the solar cells, presumably because of the better contact between the active layer and the cathode and the work function shift of the aluminum cathode. As a result, the power conversion efficiency increased from 1.09 to 2.12%. However, when thickness of the Cs₂CO₃ exceeded 2 nm, significant degradation was observed. From the results of a stability test, it was suggested that the inserted Cs₂CO₃ layer had a minimal influence on the lifetime of the unpakcaged devices.