RESUMO
Recently, organic-inorganic halide perovskites have received attention for applications in solar cells. Measurements of high-quality single crystals reveal lower defect densities and longer carrier lifetimes than those of conventional thin films, which result in improved electrical and optical properties. However, single crystal surfaces are sensitive to exposure to ambient conditions, and degrade under long-term storage in air. The surface also shows differences from the bulk in terms of its optical and electronic characteristics. For a heterojunction device, the interface at the single crystal is important. Understanding the difference between the surface and bulk properties offers insights into device design. Here, we prepared non-sliced and sliced formamidinium lead iodide (FAPbI3; FA+ = HC(NH2)2+) single crystals with a bandgap of 1.4 eV, which matches well with the requirements for solar cell photoabsorption layers. We evaluate the energy level diagrams of the surface and bulk regions, respectively. Our data indicate that the valence band maximum of the surface region is at a higher energy level than that of the bulk region. We also discuss hypotheses for the well-known and unexplained phenomena (multiple bandgaps and bandgap narrowing) seen in the absorption and photoluminescence spectra of single crystals. We conclude that these effects are likely caused by a combination of the degraded surface, Rashba-splitting in bulk, and self-absorption by the single crystal itself.
RESUMO
High photovoltaic performance and light stability are required for the practical outdoor use of lead-halide perovskite solar cells. To improve the light stability of perovskite solar cells, it is effective to introduce a self-assembled monolayer (SAM) between the carrier transport layer and the perovskite layer. Several alternative approaches in their molecular design and combination with multiple SAMs support high photovoltaic conversion efficiency (PCE). Herein, we report a new structure for improving both PCE and light stability, in which the surface of an electron transport layer (ETL) was modified by combining a fullerene-functionalized self-assembled monolayer (C60SAM) and a suitable gap-filling self-assembled monolayer (GFSAM). Small-sized GFSAMs can enter the gap space of the C60SAM and terminate the unterminated sites on the ETL surface. The best GFSAM in this study was formed using an isonicotinic acid solution. After a light stability test for 68 h at 50 °C under 1 sun illumination, the best cell with C60SAM and GFSAM showed a PCE of 18.68% with a retention rate of over 99%. Moreover, following outdoor exposure for six months, the cells with C60SAM and GFSAM exhibited almost unchanged PCE. From the valence band spectra of the ETLs obtained using hard X-ray photoelectron spectroscopy, we confirmed a decrease in the offset at the ETL/perovskite interface owing to the additional GFSAM treatment on the C60SAM-modified ETL surface. Time-resolved microwave conductivity measurements demonstrated that the additional GFSAM improved electron extraction at the C60SAM-modified ETL/perovskite interface.