Stable perovskite solar cells developed through structural simplification

Perovskite solar cells are promising low-cost and highly-efficient next-generation solar cells. The ad hoc Team on Perovskite PV Cells (Kenjiro Miyano, Team Leader) at the Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN) (Kohei Uosaki, Director-General), NIMS (Sukekatsu Ushioda, President), successfully developed perovskite solar cells with good reproducibility and stability as well as exhibiting ideal semiconducting properties (, "Simple characterization of electronic processes in perovskite photovoltaic cells"). Schematic diagram of a perovskite solar cell and its cross section viewed with a scanning electron microscope. Light passes through a transparent electrode and is absorbed by the perovskite layer, generating positive (holes) and negative (electrons) charges by means of photoexcitation. Electrons in the perovskite layer move to the PCBM electron transport layer. Holes travel through the hole transport layer and are extracted from the transparent electrode, generating electric power. Lead-halide-based perovskite (hereinafter simply referred to as perovskite) has been used as a solar cell material since six years ago. Perovskite solar cells are promising low-cost and highly-efficient next-generation solar cells because they can be produced through low-temperature processes such as spin coating, and generate a large amount of electricity due to their high optical absorption together with the high open-circuit voltage. As such, the research on perovskite solar cells is making rapid progress. In order to identify the semiconducting properties of perovskites and formulate guidelines for the development of highly efficient solar cell materials, NIMS launched an ad hoc Team on Perovskite PV Cells last October led by the deputy director-general of GREEN. While the conventional perovskite solar cells have demonstrated high conversion efficiency, they were not sufficiently stable plagued by their low reproducibility and the hysteresis in the current-voltage curves depending on the direction of the voltage sweeps. For this reason, the semiconducting properties of perovskites had not been identified. We successfully created reproducible and stable perovskite solar cells as follows;

• We created perovskite solar cells with a simplified structure while strictly eliminating moisture and oxygen by employing the fabrication technique we have developed for the organic solar cells in the past.
• We found that our perovskite solar cells are stable and we observed no hysteresis in the current-voltage curve. Furthermore, we found that the perovskite solar cell material serves as an excellent semiconductor with ideal diode properties.

We proposed an equivalent circuit model that explains the semiconducting properties of perovskites based on analysis of the internal resistance of perovskite solar cells. This model indicated the existence of a charge transport process derived from an impurity level between the conduction and valence bands in the perovskite layer. Due to this transport process, the efficiency of perovskite solar cells may be suppressed to some extent. In future studies, we will investigate into the cause of the impurity level and its influence on solar cells. In addition, we intend to remove the impurity level and improve the efficiency of the solar cells, thereby contributing to energy and environmental conservation. This study was conducted at GREEN as a part of the MEXT-commissioned project titled “Development of environmental technology using nanotechnology.”