Fermi Level Engineering of the Perovskite and Electron Transport Layer Interface through Charge Transfer Doping
Perovskite solar cells (PSCs) have captured the focus of the photovoltaic field due to their ease of fabrication and high power conversion efficiencies of ~22%. In PSCs, the perovskite active material is sandwiched between electron and hole transport layers that serve to extract photogenerated charge carriers and to avoid direct contact between the perovskite layer and metal electrodes in order to prevent degradation. Organic semiconductors are attractive candidates for transport layers because of their low temperature processing and scalability. The major obstacle to use of organic semiconductors in PSCs is their low carrier mobility and carrier concentration, which cause them to act as a parasitic series resistance in devices. In order to overcome transport issues, doping of organic transport layers is performed to passivate trap sites and increase the concentration of free carriers.
To date, there have been few reports on understanding how the presence of small molecule dopants can influence the perovskite active layer. Small molecule dopants that have been developed to dope organic semiconductors are also candidates for charge transfer to MAPbI3. The impact of direct charge transfer from small molecules to MAPbI3 was investigated as a model system to provide a means to understand the more complex heterointerface of doped organic blends at perovskite surfaces. We studied the impact of surface doping on passivation of electron traps in MAPbI3. Using spectroscopy and photoconductivity measurements, we determined that at moderate degrees of doping the Fermi level shift will passivate this recombination center. At high degrees of doping, there is an overabundance of charge beyond what is necessary to passivate surface traps, creating an additional pathway for bimolecular recombination. In addition, excessive charge leads to chemical damage of the surface of MAPbI3, resulting in the release of mobile I- ions. Given these design principles, the impacts of doping the organic electron transport layer on solar cell performance was investigated. The power conversion efficiency of PSCs showed a rise and fall in performance with increasing doping concentration of the electron transport layer, consistent with observations from surface doping. Optimal solar cell performance was observed at moderate doping concentrations of 1 wt%. At this doping concentration, evidence of trap passivation was observed by higher open circuit voltages (VOC) and longer carrier lifetimes. Numerous studies in literature on the impact of doping the electron transport layer have identified a range of factors influencing performance, including changes to carrier mobility, resistance mismatch, energy level alignment, and morphology. This work indicates that the impacts of interfacial charge transfer on surface recombination should be added to this list of considerations.