Enhanced Grain Size, Photoluminescence, and Photoconversion Efficiency with Cadmium Addition during the Two-Step Growth of CH3NH3PbI3

Control over grain size and crystallinity is important for preparation of methylammonium lead iodide (MAPbI(3)) solar cells. We explore the effects of using small concentrations of Cd2+ and unusually high concentrations of methylammonium iodide during the growth of MAPbI(3) in the two-step solution process. In addition to improved crystallinity and an enhancement in the size of the grains, time-resolved photoluminescence measurements indicated a dramatic increase in the carrier lifetime. As a result, devices constructed with the Cd-modified perovskites showed nearly a factor of 2 improvement in the power conversion efficiency (PCE) relative to similar devices prepared without Cd addition. The grains also showed a higher degree of orientation in the < 110 > direction, indicating a change in the growth mechanism, and the films were compact and smooth. We propose a Cd-modified film growth mechanism that invokes a critical role for low-dimensional Cd perovskites to explain the experimental observations.