• 
  Lang, Felix Christian: Stability and properties of methylammonium lead iodide solar cells. , Berlin, Technische Universität, Diss., 2017

Open Access Version

Abstract:
Within this thesis, methylammonium lead iodide (CH3NH3PbI3), a hybrid perovskite was studied. Polycrystalline thin-films, processed from solution, reached low defect concentrations of 6.0∙1015 cm-3 and diffusion lengths of 200 nm. Investigating various device structures, a perovskite solar cell with a stabilized efficiency of 12 % was demonstrated. Combining a band gap of 1.6 eV and a low sub-band gap absorption, CH3NH3PbI3 is a promising top-cell candidate for the integration with bottom-cells from crystalline silicon (c Si) or copper-indium gallium di-selenides (CIGS). The combination forms a tandem solar cell, which has the potential to outperform the individual single junction solar cells. The integration requires the deposition of a contact, with high electric conductivity and high transparency onto the perovskite top cell. Conventional sputter deposition of transparent conductive oxides was found to deteriorate the topmost perovskite and organic layers. To tackle this challenge, large area graphene grown by chemical vapor deposition was implemented onto the perovskite solar cell. The developed water-free transfer process paved the way for a defect free implementation. The measured charge collection efficiencies and open-circuit voltages were identical to those of reference devices using opaque Au electrodes. Absorbing less than 2.7 % of the incident light, the graphene electrode enabled a perovskite top cell with an optical transmission of 64.3 % below the perovskite band gap. These graphene-contacted perovskite top-cells were the premise for perovskite/silicon tandem solar cells with 13.2 % power conversion efficiency. Thin-film tandem solar cells, comprising a CH3NH3PbI3 top-cell and a radiation hard CIGS bottom-cell are attractive for space applications since they can be thin, lightweight, flexible, and efficient. The ability of a perovskite solar cell to withstand the harsh radiation environment in space, consisting mainly of high-energy protons was demonstrated. The J–V characteristics were recorded in situ during irradiation with protons at 68 MeV of energy. The perovskite layers were found to withstand proton doses of 1012 p/cm2, which, by far, exceeds the threshold at which c-Si begins to degrade. For higher doses, localized defects were generated throughout the CH3NH3PbI3 absorber, eventually reducing the short-circuit current by 20 % at a proton dose of 1013 p/cm2. A self-healing mechanism commenced after terminating the proton irradiation. To explain the degradation and the observed recovery a microscopic mechanism was introduced. Despite the high radiation hardness, perovskite solar cells tend to decompose under the influence of moisture, heat, oxygen, and illumination. Although there has been considerable progress in the past years, reported perovskite solar cells barely reach lifetimes of 1,000 h. The industry standard, set by commercial silicon modules, in contrast, exceeds 50,000 h. The various mechanisms that are responsible for the degradation of perovskite solar cells were reviewed and their impact examined. Most importantly, hybrid perovskites exhibit several photo-induced degradation mechanisms. Illumination, for example, induces a phase separation of blended perovskites. Furthermore, photo-generated charge carriers can be trapped in the antibonding N H orbitals, which dissociates the organic cations. Microscopic mechanisms, their implications, and strategies to overcome these instabilities are discussed.