Active thin film variation in OPV cells and analysis through external and internal quantum efficiency

In this work is reported an analysis of the external and internal quantum efficiency (EQE and IQE) and, its correlation with the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells, as a function of the active layer thickness. It was used the bulk heterojunction architecture under the configuration ITO/PEDOT:PSS/PTB7- Th:PC71BM/PFN/Field's Metal (FM) and the active film thickness range was 40-165 nm. FM is a eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn, that melt at 65°C and is easily deposited on top of the electron transport layer (ETL) at low temperature (~ 90 °C). EQE set up was a home-made and the IQE spectra for the active film thickness range were determined from EQE and net internal absorption spectra; net absorption was estimated through the transfer matrix method (TMM). It was observed a significant reduction of IQE with the increasing of the active layer thickness up to 120 nm. IQE decreases and consequently EQE and PCE too because of the reduction in charge carriers collection. It was calculated the short-circuit current density (Jsc) from these measurements and compared these values with those achieved by the J-V plots to verify EQE results. It is noteworthy that EQE takes into account the effect of optical losses by reflection and transmission, while IQE is related with the efficiency of the photons that are not reflected or transmitted out of the cell.