Synthesis and Characterization of Low Cost CH3NH3PbI3 Solar Cells using Metal Oxide:2D Composite Electron Selective Contacts

Abstract

Objective of this work was to develop a processing protocol to develop devices with Hole Transport Layer (HTL) free architectures in ambient air conditions of high humidity. This would result in considerable cost savings and rapid commercialization of this technology. We developed a synthesis technique allowing us to achieve reliable and reproducible films of CH3NH3PbI3 while processing under ambient air conditions. Effect of ambient air synthesis on morphology and photoconductivity was investigated to reach to an optimum CH₃NH₃I concentration for our devices. Morphology and photoconductivity measurements determined that a concentration of 0.050 M of CH₃NH₃I in iso-propanol offered the best compromise between grain size and photoconductivity. Photoconductivity measurements were recorded for both room and elevated temperatures. This ensured that the impedance measurements for completed devices at elevated temperatures truly reflected device artifacts. The efficiencies achieved for devices with HTL free architecture were low. We experimented by varying the compositions of electron selective contacts to achieve reasonable working efficiencies for our devices. Investigations with sol gel TiO2 compact film achieved an efficiency of 3.74 %. We incorporated ZnO compact film through sol gel chemistry and achieved an efficiency of 3.03 %. In order to increase the efficiency of devices we investigated the completed devices through impedance spectroscopy and identified that if the series resistance component could be lowered and recombination resistance increased we could increase the efficiencies working in the regime we identified at the outset. Material of choice to achieve these properties have been GO with its remarkable electronic and optical properties. We dispersed GO in ZnO and achieve an efficiency of 4.74 %. Investigation of TiO2 - GO composite was more challenging owing to the stringent requirement of GO dispersions in TiO2 sol. This process was not reproducible and the results achieved were less than perfect. We devised a novel synthesis route to achieve reliable and vii reproducible TiO2 - GO composite by in situ incorporation of GO during the sol gel reaction. Devices using this composite as electron selective contacts achieved an efficiency of 5.9 %. Another approach to enhance the efficiency of these devices in this synthesis regime was to increase the absorption of these devices using 2D material with superior absorption properties. CH3NH3PbI3 was identified as having good absorption in the middle of the solar spectrum while the absorption falls off at the edges of the spectrum. We investigated MoS2 composite with TiO2 as electron selective contact allowing greater photon harvesting toward the lower wavelength region of the solar spectrum. We again had to develop a novel synthesis technique to reliably imbed few to monolayer sheets of MoS2 in TiO2 matrix, a requirement critical to the working of this concept as the band gap of MoS2 shifts from 1.85 eV of monolayer to 1.2 eV for bulk. We achieved an efficiency of 4.3 % for devices employing this composite as electron selective contact.