Book Description

Thin film solar cells based on hybrid organic-inorganic perovskites (HOIPs) have become highly attractive over the past several years due to a high solar to electric power conversion efficiencies (PCEs). Perovskite materials based on methylammonium lead iodide (CH3NH3PbI3, MAPbI3) possess high optical absorption coefficients, long minority carrier lifetimes and diffusion lengths, and desirable optical band gaps, and carrier collection in these materials can be highly efficient when they are paired with appropriate electron and hole transport materials (ETMs and HTMs), respectively. Additionally, perovskite solar cells (PSCs) can be fabricated via a variety of solution-based routes, which are suitable for low-cost, large area manufacturing. The combination of these attributes gives PSCs an advantage over currently available commercial photovoltaic (PV) technologies. Understanding the nucleation and growth mechanisms, and controlling the grain size and crystallinity in the solution-processed fabrication of perovskite thin films are important to prepare electronic-quality materials for PV applications. We investigated the nucleation and growth mechanisms of MAPbI3 formed in a two-step solution process. To prepare the MAPbI3 films, PbI2 films were spin-coated and then were reacted with methylammonium iodide (MAI) in the isopropanol (IPA) solution at various concentrations. We showed that the conversion rate, grain size, and morphology of MAPbI3 perovskite films depend on the concentration of the MAI solution. Three distinct perovskite formation behaviors were observed at various MAI concentrations, and a tentative model was proposed to explain the reaction mechanisms. The nucleation and growth process of MAPbI3 can be significantly changed by adding divalent metal salts into the MAI solution. We showed that the incorporation of Cd2+ ions significantly improved the grain size, crystallinity, and photoexcited carrier lifetime of MAPbI3. Formation of (CH3NH3)2CdI4 (MA2CdI4) perovskite in the solution by reacting the MAI and Cd2+ is the key for this nucleation and growth change. Devices prepared using this approach showed a significant improvement in the PCE relative to control devices prepared without Cd2+ addition. The improved optoelectronic properties are attributed to a Cd-modified film growth mechanism that invokes low dimensional Cd-based perovskites. In addition to the Cd2+, Zn2+ and Fe2+ also have the potential to change the nucleation and growth process of MAPbI3 formation, to improve the material quality. Formation of Cd-based perovskites, once the Cd2+ ions contacted with MAI, successfully applied in the cadmium telluride (CdTe) solar cell technology to form a Te layer on the CdTe surface, that would reduce the Schottky barrier height and band bending at the back contact, reducing the recombination at the back junction, and thus improve the device efficiency. We found that Cd can be selectively extracted from the CdTe surface by reacting MAI thin films with the CdTe surface, forming MA2CdI4 perovskite. MA2CdI4 is soluble in IPA, therefore can be rinsed out, leaving a Te layer behind on the CdTe surface. MAI treated CdTe devices showed a reduction in the barrier height at the back contact for both Au and transparent indium tin oxide (ITO) electrodes as calculated from the temperature dependent J-V measurements, resulting higher photovoltaic parameters of open circuit voltage (VOC), fill factor (FF), and PCE relative to the control devices. In addition, only a ~6% reduction in transmittance in the near infrared (NIR) region occurred in the devices with an ITO back electrode due to the MAI treatment, indicating this can be potentially used for the fabrication of high performance transparent CdTe solar cells that use in tandem solar cell or window applications.