By Surbhi JainSep 21 2022Reviewed by Susha Cheriyedath, M.Sc.
In an article recently published in the journal ACS Energy Letters, researchers discussed the utility of a universal grain cage for mixed-halide inorganic perovskite solar cells to prevent halide segregation.
Study: A Universal Grain “Cage” to Suppress Halide Segregation of Mixed-Halide Inorganic Perovskite Solar Cells. Image Credit: Perutskyi Petro/Shutterstock.com
Power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PSCs) has been documented to be close to the best monocrystalline silicon solar cell, but there is less room for PCE increase due to the Shockley–Queisser (SQ) limit. Making multi-junction tandem cells to increase solar spectrum overlap and decrease thermalization loss is one of the possible solutions to this technological constraint.
The mixed-halide perovskites stand out in achieving this goal when taking into account their bandgap tunability, which, regrettably, causes substantial halide segregation under continuous illumination. Therefore, to apply wide band gap and mixed-halide perovskite in tandem models, an efficient technique to reduce this photo-demixing tendency must be developed.
A few mechanisms that explain the driving thermodynamic force for halide segregation have so far been identified. These mechanisms offer suggestions for logically developing materials to harden the soft perovskite lattice. All of these scenarios have the same premise of considering the vacancy-assisted migration process while shutting down the ion-to-ion or ion-to-vacancy migration channels. The iodide ions suffer from the grain-to-boundary diffusion and nucleate at grain boundaries to generate an iodide-rich phase. To prevent iodide ion accumulation and thereby inhibit halide segregation, it is crucial to precisely manage the grain boundary within the perovskite layer.
About the Study
In this study, the authors discussed a general caging technique to prevent halide segregation during the synthesis of mixed-halide perovskite. This technique involved the in situ production of conjugated covalent organic frameworks (COFs), which are catalyzed by PbX2 (X = Br and I). The strong electron-donating property of COFs was demonstrated through theoretical analysis and thorough investigation to efficiently harden the soft lattice and obstruct the passage of iodide ions from bulk to the grain boundary, slowing the light-induced halide-demixing process.
The team demonstrated that the non-radiative recombination was much decreased, which increased efficiency up to 11.50% for an inorganic CsPbIBr2 perovskite solar cell and 14.35% for a CsPbI2Br cell with a longer shelf life and higher photostability. A grain cage was proposed to in situ form 2D conjugated COFs at perovskite grain surface and boundaries using two organic small molecules, 4,4′-biphenyldicarboxaldehyde (BPDA) and 4,4′,4′′-(1,3,5-Triazine-2,4,6-triyl)trianiline (TAPT), into precursor solution. Because of the increased ion migration activation energy and vacancy formation energy brought on by the strong interaction between perovskite and COFs, the PbX2-catalyzed COFs not only contributed to the lateral diffusion defensive matrix between the interconnecting grains but also successfully passivated the harmful flaws to lessen non-radiative recombination.
The researchers created strong mixed-halide PSCs with increased PCEs of 11.50% for the CsPbIBr2 PSC, which was entirely inorganic, and 14.35% for the CsPbI2Br cell using the proposed grain caging technique. The finest PSC free of encapsulation exhibited a 28-fold increase in photostability under standard sun illumination under ambient circumstances, in addition to exceptional storage stability over 60 days. Inorganic CsPbIBr2 was chosen as a prototype to investigate the impact of COFs on phase deterioration.
The optimized device's defect density was 1.98 x 1016 cm3, which was significantly less than the control device's density of 2.11 x 1016 cm3. This was made possible by lowering the trap filling limit voltage (VTFL) from 1.23 to 1.15 V. A significantly higher contact potential difference (CPD) of 50.9 mV for COF-perovskite film than for control film implied that there were more holes at the surface after illumination.
The champion PSC with 0.10 mg mL-1 concentration achieved a PCE of 11.50%, a Voc of 1.327 V, a short-circuit current density (Jsc) of 11.95 mA cm2 and a fill factor (FF) of 72.55% after the optimization of the COF dosage in the perovskite film by regulating the monomer amount in the precursor solution. For I-rich CsPbI2Br PSC, an improved PCE of up to 14.35% was attained, which proved the superiority of the grain cage to support the practical implementation of the wide band gap PSC in tandem models.
After 250 hours of maximum power point tracking under steady AM1.5G solar spectrum illumination, the optimized device still retained 75.3% of its initial efficiency whereas the control device degraded to almost zero after 100 hours, which extended the T80 lifetime by approximately 28 times.
In conclusion, this study discussed a universal grain cage approach of in situ production of organic COFs at the grain boundaries and perovskite surface through the insertion of monomer molecules into the perovskite precursor solution for operationally stable and highly effective wide-bandgap PSCs.
Due to the higher ion migration activation energy and vacancy formation energy caused by PbX2's catalysis, COFs were chemically bound to the perovskite lattice to concurrently suppress halide segregation and passivate the defects. As a result, the non-radiative recombination loss was reduced, resulting in the champion PCEs of 11.50% and 14.35% for the inorganic CsPbIBr2 and CsPbI2Br PSCs based on carbon electrodes, respectively. In addition to having exceptional long-term resistance to heat and humidity, the resulting PSC had a 28-fold increase in photostability.
The authors mentioned that the proposed approach offers the chance to address the drawbacks of wide-bandgap perovskites and improve their ultimate implementation in tandem and semi-transparent photovoltaics, taking into account the planned application in other mixed-halide systems.
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Zhang, J., Duan, J., Guo, Q., et al. A Universal Grain “Cage” to Suppress Halide Segregation of Mixed-Halide Inorganic Perovskite Solar Cells. ACS Energy Letters, 7, 3467-3475 (2022). https://pubs.acs.org/doi/10.1021/acsenergylett.2c01771
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