Organic-inorganic lead halide perovskites have become the focus of intense research due to their outstanding performance in hybrid photovoltaic devices. One essential component for attaining stable and high efficient solar cell is the employed hole transporting material (HTM).
Although one significant HTM design parameter for maximizing the open circuit voltage (VOC) is the position of the Highest Occupied Molecular Orbital (HOMO), correlation is not always found among the published results. In this collaborative work between groups with expertise in organic synthesis (IMDEA-Nanociencia), devices physics (ICIQ) and interface properties (ICMAB-CSIC) we assess the possible reasons for the differences observed in open circuit voltage VOC in mixed cation CsFAMAPbIBr perovskite solar cells by comparing four different HTMs (namely TAE-1, TAE-3, TAE-4 and the prevalently used spiro-OMeTAD).
By means of photo-induced charge extraction, we have obtained an indication of the HOMO level position of the HTM when layered in a solar cell stack. Kelvin probe force microscopy, employed for measuring the work function in the functional devices with the different of HTM layers, confirms deviations in the relative alignment. We show that the shift of the energy levels of the TAE molecules upon contact with the perovskite layer, together with changes in recombination rate measured the photo-induced transient photovoltage, influences the measured VOC values. This work unveils the importance of the interface between the HTM and the perovskite. Therefore the design of a HTM that can reach the expected maximum theoretical efficiency in perovskite solar cells will require fine-tuning of the energetics at the interface, without increasing the interfacial carrier recombination processes between the HTM and the semiconductor perovskite.
This article was included in the 2019 Energy and Environmental Science HOT Articles
Energy Alignment and Recombination in Perovskite Solar Cells: Weighted Influence on the Open Circuit Voltage
Ilario Gelmettia, Núria F. Montcada, Ana Pérez-Rodríguez, Esther Barrena, Carmen Ocal, Inés García-Benito, Agustín Molina-Ontoria, Nazario Martín, Anton Vidal-Ferrana, Emilio Palomares
Energy and Environment Science 12, 1309-1316, 2019
Figure: (Top) Kelvin Probe Force Microscopy measurements for surfaces of pristine CsFAMAPbIBr and the different HTM devices (spiro-OMeTAD, TAE-1, TAE-3 and TAE-4). The vacuum level shifts are obtained by extracting the diverse work function from the parabolic data. (Bottom) Molecular representation of the HTM devices.