Impact of Monovalent Cation Halide Additives on the Structural and Optoelectronic Properties of CH$_{3}$NH$_{3}$PbI$_{3}$ Perovskite

Abstract

The influence of monovalent cation halide additives on the optical, excitonic, and electrical properties of CH$_{3}$NH$_{3}$PbI$_{3}$ perovskite is reported. Monovalent cation halide with similar ionic radii to Pb$^{2+}$, including Cu$^{+}$, Na$^{+}$, and Ag$^{+}$, have been added to explore the possibility of doping. Significant reduction of sub-bandgap optical absorption and lower energetic disorder along with a shift in the Fermi level of the perovskite in the presence of these cations has been observed. The bulk hole mobility of the additive-based perovskites as estimated using the space charge limited current method exhibits an increase of up to an order of magnitude compared to the pristine perovskites with a significant decrease in the activation energy. Consequentially, enhancement in the photovoltaic parameters of additive-based solar cells is achieved. An increase in open circuit voltage for AgI (≈1.02 vs 0.95 V for the pristine) and photocurrent density for NaI- and CuBr-based solar cells (≈23 vs 21 mA cm$^{−2}$ for the pristine) has been observed. This enhanced photovoltaic performance can be attributed to the formation of uniform and continuous perovskite film, better conversion, and loading of perovskite, as well as the enhancement in the bulk charge transport along with a minimization of disorder, pointing towards possible surface passivation.