Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.176
Publication date: 21st February 2018
Non-radiative losses limit attainable open-circuit voltage (Voc) in perovskite solar cells (PSCs) and hinder the breaking of current power conversion efficiency (PCE) record. A deeper understanding of the existing non-radiative pathways in PSCs requires an in-situ optoelectronic characterization of working devices. Based on differential charging and transient photovoltage decay, we investigated the evolution of Voc by measuring the density of charge carriers and their overall recombination lifetime in working devices under various illumination intensities. The mechanism of hole transport materials (HTMs) affecting Voc is studied in inverted CH3NH3PbI3 (MAPI) solar cells based on PEDOT:PSS, PTAA and PTPD. Voc shows little correlation with the HOMO energy of HTMs, as PEDOT:PSS demonstrates the deepest HOMO level but the lowest Voc and thus PCE (0.86 V, 12.9%) compared with PTAA (1.07V, 17.0%) and PTPD (1.09V, 18.5%) cell. Such a large loss in Voc is otherwise associated with extraction of photo-generated holes from perovskite, leading to electron-hole recombination primarily occurring at PEDOT:PSS/perovskite heterojunction. In contrast hole extraction by PTAA and PTPD is less efficient and electrons recombine with holes mainly in bulk perovskite. Recombination at HTL/perovskite interfaces reduces the effective electronic band gap of the cell compared with recombination occurring in bulk. It is observed that density of photo-generated charges is comparable in the two devices but Voc loss is mainly due to reduced potential energy for given charge density. The overall charge recombination lifetime in the complete cell is only slightly increased by the interfacial recombination, which is not the main cause of Voc loss. In addition, extraction of holes is due to more p-type nature (higher work function) and higher conductivity of PEDOT:PSS compared with PTAA/PTPD, which causes stronger band bending at heterojunction but is not regarded as beneficial in PSCs. The results we showed suggest that charge extraction by the transport layers is detrimental to device performance. Employing charge transport layer with relatively low intrinsic carrier density enhances bulk recombination of electrons and holes, which should be a viable strategy to achieve high Voc in PSCs.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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