Tunable Conductivity of MAPbI₃ via Surface Doping from Small Molecules

Lead halide perovskites have gained enormous attention as a low-cost and solution-processable active material for solar cells. The performance of hybrid-halide solar cells is dependent on the nature of interfaces. In order to improve device performance, the physical and chemical properties of these interfaces continue to be explored. Organic hole and electron transport materials are often employed as electron- and hole-blocking layers. In order to optimize charge-extraction in the device, these organic layers can be doped using organic small molecules. However, to date there has been little work carried out on the details of doping at these interfaces. Due to a lack of surface states and undercoordinated Pb²⁺ ions on the perovskite surface, it is possible for charge transfer to take place from small molecule n-type dopants to MAPbI₃. Here we study the change in electrical properties of MAPbI₃ by modulation-doping the film with an organic dopant molecule: cobaltocene. By varying the amount of cobaltocene deposited, the conductivity of thin films are observed to be tunable over several orders of magnitude. We observe a tunable shift in the valence band edge of up to 0.7 eV upon doping, illustrating that charge transfer doping allows for control over the interfacial energy levels.