No abstract

Accounting for the perovskite ionic transport and reactions reveals the importance of the electron blocking (hole transporting) layer in determining device stability.

The mixed ionic–electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts—such as hysteresis or device degradation—leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼10 21 cm –3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

Ion migration into blocking layers toward the metallic electrodes is studied within a semiconductor device model framework. We find that ion leakage into the blocking layers and their accumulation at the electrode interface are significantly affected by the electronic injection barrier at the contact. Specifically, we find that if the device structure promotes, under light, hole (electron) accumulation within the perovskite layer, these excess holes (electrons) would release an almost equivalent number of cations (anions) into the transport layers toward the contacts. Our analysis suggests that it would be beneficial to include intentional doping of the blocking layers and that it should follow the “just enough” strategy.

Ion diffusion into the BLs changes the device from symmetric-intrinsic to asymmetric-doped with all internal parameters changed.

The polycrystalline nature of perovskite thin films suggests that the nonradiative recombination losses may be grain size dependent. We use measured grain size distributions of methylammonium lead triiodide layers to describe the macroscopic solar cell as composed of multiple single grain size cells operating in parallel. Using a model, we show that the grain size distribution results in spatial dispersion of the local open-circuit voltage (0.9–1.15 V), fill factor (50–82%), and power conversion efficiency (7.5–19%). When the device is held at open-circuit voltage, there is significant current exchange between large and small grains. Smaller grains are a “parasitic shunt” for the larger grains.

No abstract

Defect chemistry plays a major role in the operation of perovskites cells. On one hand they are behind the remarkable self-healing,[1] while on the other they dictate the level of traps, non-radiative recombination,[2] as well as promote ion migration.[3] Moreover, once ions reach the electrodes the device degradation is accelerated.[4, 5] Combining the various reports it seems that if the perovskite would not lose its constituents it would self-heal and if it do lose them it is game-over. We claim that ion migration into the blocking layer is the most important electrochemical process standing in the way of long-term stability. As we will show at the presentation, a significant part of the electrochemistry behind ion leakage and consequently electrode reactions, is captured by a semiconductor device model that accounts for mixed electronic-ionic conduction.[6] While the mere transport of ions introduces no device degradation, introducing oxidation/reduction reactions at the electrodes reproduces the effect of light & bias on device stability. Lastly, we’ll discuss if it is possible to design the device to mitigate this ion leakage out of the perovskite layer. References [1] D. R. Ceratti et al., "Self-Healing Inside APbBr3 Halide Perovskite Crystals," Advanced Materials, vol. 30, no. 10, p. 1706273, 2018, doi: 10.1002/adma.201706273. [2] J. M. Azpiroz, E. Mosconi, J. Bisquert, and F. De Angelis, "Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation," Energy Environ. Sci., 10.1039/C5EE01265A vol. 8, no. 7, pp. 2118-2127, 2015, doi: 10.1039/C5EE01265A. [3] D. Meggiolaro, E. Mosconi, and F. De Angelis, "Formation of Surface Defects Dominates Ion Migration in Lead-Halide Perovskites," ACS Energy Letters, vol. 4, no. 3, pp. 779-785, 2019/03/08 2019, doi: 10.1021/acsenergylett.9b00247. [4] F. Galatopoulos, I. T. Papadas, G. S. Armatas, and S. A. Choulis, "Long Thermal Stability of Inverted Perovskite Photovoltaics Incorporating Fullerene-Based Diffusion Blocking Layer," Advanced Materials Interfaces, vol. 5, no. 20, p. 1800280, 2018/10/01 2018, doi: 10.1002/admi.201800280. [5] B. Rivkin, P. Fassl, Q. Sun, A. D. Taylor, Z. Chen, and Y. Vaynzof, "Effect of Ion Migration-Induced Electrode Degradation on the Operational Stability of Perovskite Solar Cells," ACS Omega, vol. 3, no. 8, pp. 10042-10047, 2018/08/31 2018, doi: 10.1021/acsomega.8b01626. [6] S. Bitton and N. Tessler, "Electronic-ionic coupling in perovskite based solar cells: Implications for device stability," Applied Physics Letters, vol. 117, no. 13, p. 133904, 2020, doi: 10.1063/5.0023902.