Enhancing Thermal Stability of Perovskite Solar Cells with a Polymer Through Grain Boundary Passivation

Organic-inorganic halide perovskite solar cells have emerged as a promising photovoltaic technology due to their superb power conversion efficiency (PCE) and very low material costs. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45 °C higher than ambient temperatures when operating under direct sunlight. In this thesis, the thermal stability of perovskite solar cells was primarily investigated. Initially, we systematically studied the effects of heating and cooling processes on the principal photovoltaic performance of perovskite solar cells by combining temperature-dependent J-V, steady-state PL, UV-VIS and time-resolved lifetime decay measurements. In particular, we have observed the dynamic evolution of degraded crystallinity, increased charge trapping, deep trap depth and PbI2 phase. During the heating process, the thermal degradation of the perovskite film was observed at 70 ° C or higher. An increase in the disordered phase of the perovskite film involved a drastic increase in charge trapping and the development of a deeper trap depth. Interestingly, we observed that the degradation of the perovskite film persisted even after the temperature was dropped, which led to irreversible J-V characteristics of the perovskite solar cell. Later, we introduced a polymer layer of PMMA which improved thermal stability for more than 1000hrs at 85°C. Without PMMA, host-casted MAPbI3 films suffered rapid thermal degradation, forming a number of pin-holes at GBs and then extending into GIs. Rapid thermal degradation of perovskite GBs without PMMA may be due to the rich moisture chemical structure of hydrated (CH3NH3)4PbI6•H2O. At the elevated temperature, hydrated (CH3NH3)4PbI6•H2O grain boundaries might suffer from moisture-assisted decomposition, forming a number of pin-holes at GBs. Conversely, we observed high thermal stability of perovskite films by introducing PMMA to induce marked thermal stability at GBs. It is believed that the excellent hygroscopicity of PMMA played an active role in absorbing moisture from hydrated (CH3NH3)4PbI6•H2O GBs and driving them out through the GB channel. We believe that continuous functionalization of perovskite GBs or crosslinking perovskite GBs with PMMA molecules might drastically render perovskite GBs chemically robust, resilient, and heat-resistant. Moreover, we mixed inorganic cesium (Cs) cation into the perovskite, which improved thermal stability at a higher temperature of 120°C. Finally, we have fabricated perovskite solar cells in an antisolvent method in which the perovskite film does not contain deeper grain boundary like hot-casted perovskite thin film. Also, we introduced a polymer (polyimide) on the top of the perovskite solar cell which has a large contact angle and glass transition temperature. Consequently, perovskite solar cells with polyimide showed thermal stability without any efficiency decrement more than 30 days

A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells

Perovskite solar cells have achieved photo-conversion efficiencies greater than 20%, making them a promising candidate as an emerging solar cell technology. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45°C higher than ambient temperatures when operating under direct sunlight. Herein, recent advances in resolving thermal stability problems are highlighted through literature review. Moreover, the most recent and promising strategies for overcoming thermal degradation are also summarized

A deconvoluted PL approach to probe the charge carrier dynamics of the grain interior and grain boundary of a perovskite film for perovskite solar cell applications

We explore a new characterization approach capable of probing the grain interior (GI) and grain boundary (GB) of a CH3NH3PbI3-xClx perovskite thin film. In particular, we have found that the photoluminescence (PL) spectrum observed for a CH3NH3PbI3-xClx perovskite thin film is asymmetric, and can be deconvoluted using a bi-Gaussian function, representing the ordered and disordered phases of the perovskite film. In order to understand the origin of the ordered and disordered phases of the perovskite film, two-dimensional (2D) PL mapping was performed to resolve the PL spectra at the nanoscale level. Quantitative analysis of the local PL spectra revealed that the ordered phase originated from the GIs while the disordered phase mainly came from the GBs. In particular, power-dependent PL measurements of the deconvoluted PL spectra revealed that smaller grained perovskites showed defect-mediated recombination at GBs but exciton-like transitions at GIs. In contrast, perovskite films with large grains followed an excellent power law, showing exciton-like recombination at both GIs and GBs. As expected, perovskite solar cells fabricated with large grains showed an increased efficiency with higher light absorption and higher charge extraction efficiency. This journal is © the Owner Societies 201718101sciescopu

Unveiling the irreversible performance degradation of organo-inorganic halide perovskite films and solar cells during heating and cooling processes

While organo-inorganic halide perovskite solar cells show great potential to meet future energy needs, their thermal instability raises serious questions about their commercialization viability. At present, the stability of perovskite solar cells has been studied under various environmental conditions including humidity and temperature. Nonetheless, understanding of the performance of CH3NH3PbI3-xClx perovskite solar cells is limited. This study reports the irreversible performance degradation of CH3NH3PbI3-xClx perovskite solar cells during the heating and cooling processes under AM 1.5 and unveils what triggers the irreversible performance degradation of solar cells. Particularly, the primary cause of the irreversible performance degradation of CH3NH3PbI3-xClx is quantitatively analyzed by monitoring in real time the development of deteriorated crystallinity, charge trapping/detrapping, trap depth, and the PbI2 phase, namely a critical signal of perovskite degradation while varying the temperature of the perovskite films and solar cells. Most surprisingly, it is revealed that the degradation of both perovskite films and solar cells was triggered at similar to 70(circle)C. Remarkably, even after the device temperature cooled down to room temperature, the degraded performance of the solar cells persisted with increasing charge trapping and further development of the PbI2 phase. Identification of the irreversible performance degradation of perovskite solar cells provides guidance for future development of more stable perovskite solar cells. This journal is © the Owner Societies 20173