Gas chromatography–mass spectrometry analyses of encapsulated stable perovskite solar cells

Perovskite decomposition in detail Solar cells are subject to heating when operating in sunlight, and the organic components of hybrid perovskite solar cells, especially the commonly used methylammonium cation, can undergo thermal decomposition. Encapsulation can limit decomposition by bringing such reactions to equilibrium and can prevent exposure to damaging ambient moisture. Shi et al. examined several encapsulation schemes for perovskite films and devices by probing volatile products with gas chromatography–mass spectrometry (see the Perspective by Juarez-Perez and Haro). Pressure-tight polymer/glass stack encapsulation was effective in suppressing gas transfer and allowed solar cells containing methylammonium to pass harsh moisture and thermal cycling tests. Science, this issue p. eaba2412; see also p. 1309 Volatile emissions of encapsulated stable perovskite solar cells that passed harsh humidity and heating tests were analyzed. INTRODUCTION Although advances in materials and processing have led to remarkable advancements in the energy conversion efficiency of perovskite solar cells (PSCs), increasing from 3.8% to 25.2% in only 10 years, these solar cells cannot become commercially viable unless their underperforming durability is improved. The instability of perovskites must be addressed if PSCs are to compete with silicon technology, which currently offers a 25-year performance warranty. Previous approaches to this problem include the use of metal oxide barrier layers and butyl rubber sealants. Here, we report a low-cost polymer/glass stack encapsulation scheme that enables PSCs to pass the demanding International Electrotechnical Commission (IEC) 61215:2016 Damp Heat and Humidity Freeze tests. These tests help to determine whether solar cell modules can withstand the effects of outdoor operating conditions by exposing them to repeated temperature cycling (–40° to 85°C) as well as 85% relative humidity. Our airtight encapsulation scheme prevented moisture ingress. It was also effective in suppressing outgassing of decomposition products, which limits decomposition reactions of organic hybrid PSCs by allowing these reactions to come to equilibrium. The gas compositions were verified by gas chromatography–mass spectrometry (GC-MS). RATIONALE In the GC-MS technique, gas chromatography separates the components in a mixture, and the chemical identity of each component is determined with mass spectrometry. We could directly identify with high specificity the decomposition products of multi-cation perovskite precursors, of unencapsulated perovskite test structures, and of encapsulated full cells at elevated temperatures. The results allowed us to identify thermal degradation pathways by determining the outgassing products of mixed-cation perovskites during heating. We then used GC-MS to evaluate the effectiveness of different packaging techniques developed for PSCs. The packaging schemes were a polyisobutylene (PIB)–based polymer blanket encapsulation, a polyolefin-based blanket encapsulation, and a PIB edge seal. These packaging layers were then capped by a glass cover. For the edge seal, the decomposition gases inside the cell were sampled with a syringe. The feasibilities of these packaging techniques were also demonstrated by IEC photovoltaic module standard Damp Heat and Humidity Freeze testing. RESULTS Signature decomposition products such as CH3I, CH3Br, and NH3 were identified and decomposition pathways were proposed for CH3NH3I (MAI), HC(NH2)2I (FAI), CH3NH3Br (MABr), and mixed-cation and mixed-halide (FAI)0.85 + (MABr)0.15 perovskite precursors, including their secondary decomposition reactions at 350°, 140°, and 85°C. The GC-MS results confirmed that the Br-containing precursor was less prone to thermal decomposition than an I-containing precursor. Also, CsFAMA cells were found to outgas one-fifth as much decomposition product as their FAMA counterparts, which indicated that the Cs-containing cells had better thermal stability. Although the decomposition of FAI is reversible, the mixing of MA with FA precursors caused decomposition products to participate in the secondary reaction that was irreversible. This finding confirmed the disadvantage of mixing of MA with FA perovskite through the reduction in chemical stability. The blanket-encapsulated PSCs sustained no efficiency degradation after 1800 hours of Damp Heat testing or 75 cycles of Humidity Freeze testing. CONCLUSION GC-MS identified signature volatile products of the decomposition of organic hybrid perovskites under thermal stress, thereby informing decomposition pathways. The findings are important for developing potential cell-stabilizing strategies, given that cells in the field typically experience high operating temperatures. In addition, results of GC-MS confirm that the low-cost pressure-tight encapsulation we developed is effective in suppressing such outgassing and therefore decomposition reactions of PSCs. This encapsulation scheme is the simplest of all for perovskite cells to pass IEC photovoltaic module standard tests. Our approach can be applied to evaluating the effectiveness of other packaging approaches, as well as testing the effectiveness of coatings and material compositions aimed at limiting light and thermal degradation. Stable perovskite solar cells exceeding the requirements of the IEC 61215 Damp Heat and Humidity Freeze tests. Unencapsulated and encapsulated perovskite cells were analyzed by gas chromatography–mass spectrometry, detecting signature volatile products of the organic hybrid perovskite decomposition under thermal stress and confirming the effectiveness of the low-cost pressure-tight polymer/glass stack encapsulation schemes developed. Although perovskite solar cells have produced remarkable energy conversion efficiencies, they cannot become commercially viable without improvements in durability. We used gas chromatography–mass spectrometry (GC-MS) to reveal signature volatile products of the decomposition of organic hybrid perovskites under thermal stress. In addition, we were able to use GC-MS to confirm that a low-cost polymer/glass stack encapsulation is effective in suppressing such outgassing. Using such an encapsulation scheme, we produced multi-cation, multi-halide perovskite solar cells containing methylammonium that exceed the requirements of the International Electrotechnical Commission 61215:2016 standard by surviving more than 1800 hours of the Damp Heat test and 75 cycles of the Humidity Freeze test.