Organic photovoltaic cell analysis through quantum efficiency and scanning tunneling microscopy of the donor/blend as an active film

Abstract

In this work is reported a comparison of the film morphology, film molecular ordering and X-ray diffraction pattern between two of the most common and efficient donor polymers used in organic photovoltaic (OPV) cells: PTB7 and PTB7-Th. These comparisons indicate that PTB7-Th film chains are somewhat thicker and less spaced than those for PTB7; also, PTB7-Th films have a slightly better organized structure and higher co-planarity, which could provide a possible better electrical charge transport. On the order hand, an analysis of the external/internal quantum efficiency (EQE/IQE) of OPVs, based on PTB7-Th, as a function of the active layer thickness ranging from 40 to 165 nm was carried out. It was used the bulk heterojunction architecture to fabricate OPVs cells under the configuration glass/ITO/PEDOT:PSS/PTB7-Th:PC71BM/PFN/FM (Field’s Metal: eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn by weight that melts at 62 °C). IQE spectra were determined by using the active layer absorption calculated through the transfer matrix method (TMM). Our results show a significant reduction of IQE when increasing the active layer thickness above 120 nm. IQE decreases, and consequently EQE and PCE, mainly due to the reduction in charge carriers collection probability. On the reversed side, when the active layer is very thin (< 70 nm), there exists also a decrease in the IQE values. A comparison between the experimental measurements and theoretical simulations (by TMM) is discussed in order to have better understanding of the OPVs performance.