Trap States, Electric Fields, and Phase Segregation in Mixed‐Halide Perovskite Photovoltaic Devices

• Published in: Advanced energy materials 2020-03, Vol.10 (9), p.n/a
• Main Authors: Knight, Alexander J., Patel, Jay B., Snaith, Henry J., Johnston, Michael B., Herz, Laura M.
• Format: Article
• Language: English
• Subjects: Charging; Crystal defects; Current carriers; Defects; Electric fields; Energy gap; Grain boundaries; halide segregation; Interstitials; Lattice vacancies; Percolation; Perovskites; Photoluminescence; Photovoltaic cells; photovoltaic devices; Quantum efficiency; Solar cells; trap states
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201903488

Mixed‐halide perovskites are essential for use in all‐perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed‐halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed‐halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge‐carriers in the perovskite, a charge‐neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA+ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge‐carriers can be extracted from the iodide‐rich low‐bandgap regions of the phase‐segregated perovskite formed under illumination, suggesting the existence of charge‐carrier percolation pathways through grain boundaries where phase‐segregation may occur. Mixed‐halide perovskites are essential for use in all‐perovskite or perovskite–silicon tandem solar cells. Through photoluminescence measurements and electric field application, three distinct defect species are found responsible for charge‐carrier trapping, halide segregation, and electric field screening, respectively, within MAPb(Br0.5I0.5)3 materials. External quantum efficiency measurements highlight that charge‐carriers can be extracted from the low‐bandgap regions of the phase‐segregated perovskite formed under illumination.