Increasing the Reverse Bias Breakdown Potential of Perovskite Solar Cells with a Conformal SnOx Barrier Layer

Reverse bias can occur in a photovoltaic module when a subset of cells are shaded while other series connected cells are still under illumination. Conventionally, bypass diodes are placed in parallel within a module to avoid detrimental power dissipation across the shaded cells. While most commercial silicon solar cells exhibit a high breakdown voltage (V_BD) above &gt;|-10 V|, perovskite cells have been shown to exhibit low breakdown voltages of ~ -1V to -5V [1], [2]. This low breakdown voltage, while hypothetically beneficial as it reduces the overall power dissipated during a shading event, conversely, requires more bypass diodes per module, with modeling suggesting approximately two cells per bypass diode [1]. An additional and more pressing concern is that lead-halide perovskite devices are highly susceptible to reverse bias degradation pathways not just from the traditional hot spot formations, but also from ion migration and halide oxidation defect formation[3]. These defects are fundamentally interrelated to the nature of the reverse bias tunnel breakdown process. Due to the unequal distribution and mobility of positively and negatively charged ions, there can be a much larger field at the anodic interface (ETL) and, therefore, the probability of holes tunneling will be much larger than electrons [3]. This charge imbalance drives hole driven redox reactions that are linked to device degradation. Recent work utilizing in situ cross sectional KPFM under reverse bias potentials showed a change in band energetics after a reverse bias voltage of -3V. This change was partially reversable after several minutes of forward biasing, thus providing proof of quasi-reversable reactions during reverse bias [4]. We theorize that the change is only quasi reversable because of the loss of volatile species, such as I₂, and the migration of ions outside of the perovskite active area. Minimizing the loss of volatile species and migrations of ions in the device is critical to its stability.<br/>In this study we have taken a deep dive into the intricacies of reverse bias degradation for P-I-N lead halide perovskite solar cells. By developing a robust ALD diffusion barrier of SnOx we can significantly mitigate the damage of reverse bias degradation without compromising the electronic performance of the charge transport layer. The ALD barrier layer not only reduces metal ingress from the top electrode that can shunt the devices in the device but also serve to reduce the loss of volatile oxidized iodide/iodine species. Without diffusion barriers, devices rapidly shunt at voltages &lt; |-1 V| while cells with a barrier layer have been shown to be stable at -3 V for over one hour. By implementing the ALD SnOx barrier layer, halide ion migration is mitigated. These results show a promising pathway to extending the reverse bias stability which is critical to its deployment in modules [5].<br/>[1] E. J. Wolf <i>et al.</i> , “Designing Modules to Prevent Reverse Bias Degradation in Perovskite Solar Cells when Partial Shading Occurs,” <i>Solar RRL</i>. 2021, doi: 10.1002/solr.202100239.<br/>[2] A. R. Bowring <i>et al.</i>, “Reverse Bias Behavior of Halide Perovskite Solar Cells,” <i>Adv. Energy Mater.</i>, vol. 8, no. 8, p. 1702365, Mar. 2018, doi: 10.1002/aenm.201702365.<br/>[3] L. Bertoluzzi <i>et al.</i>, “Incorporating Electrochemical Halide Oxidation into Drift-Diffusion Models to Explain Performance Losses in Perovskite Solar Cells Under Prolonged Reverse Bias,” <i>Under Rev.</i>, vol. 2002614, pp. 1–9, 2020, doi: 10.1002/aenm.202002614.<br/>[4] I. E. Gould, C. <i>et al.</i>, “In-Operando Characterization of P-I-N Perovskite Solar Cells Under Reverse Bias,” pp. 3–5.<br/>[5] M. V. Khenkin <i>et al.</i>, “Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures,” <i>Nat. Energy</i>, vol. 5, no. 1, pp. 35–49, 2020, doi: 10.1038/s41560-019-0529-5.