Room Temperature as a Goldilocks Environment for CH3NH3PbI3 Perovskite Solar Cells: The Importance of Temperature on Device Performance
Terrestrial applications of solar cells during day-night cycling as well as operation in winter and summer involve substantial temperature variations, which influence the photophysics as well as the charge separation and transport properties in the various materials employed in a device. In this study, the optical absorption of methylammonium lead iodide (MAPbI(3)) and the device performance of MAPbI(3) solar cells have been investigated in an extended temperature range between -190 and 80 degrees C. The optical properties were found to change by only a small amount in that temperature range. The device performance did, however, show more dramatic changes and decreased in a reversible manner for temperatures both higher and lower than room temperature. For temperatures up to 80 degrees C and down to -80 degrees C, the drop in performance was up to 25% compared to the room temperature value. Given thermal stability and reversible device performance, this is probably not a showstopper for terrestrial applications of perovskite solar cells but should be considered when evaluating the total energy yield under outdoor operations. At temperatures of 100 degrees C and below, which are relevant for outer atmosphere and space applications, the performance decreases rather dramatically and approaches zero at even lower temperature. Irreversible changes set in for temperatures above 50 degrees C. In addition, the hysteresis decreases at reduced temperatures. As the effects for the absorption properties are minor, the decrease in performance can be attributed to a temperature induced limitation in the transport and extraction of the photogenerated charge carriers which is seen as a strong increase of the series resistance at reduced temperature. The drop of the photovoltage for temperatures below -100 degrees C might be related to reduced charge carrier separation in the perovskite due to excitonic effects and a lower dielectric constant.