ABSTRACT
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 â¼70 °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.
ABSTRACT
Carbon is inherently abundant in nature and relatively inexpensive, which can potentially reduce the manufacturing cost of solar cells. In recent years, carbon has been used as a hole transport layer or counter electrode in perovskite solar cells. Herein, we demonstrate that carbon can also be used as a charge transport layer capable of enhancing the energy conversion efficiency of a CH3NH3PbI3-xClx solar cell when carbon is combined with PCBM. Particularly, we have been able to deposit an ultra-flat carbon layer using an e-beam irradiation method, which exhibited much better conductivity than the competitive PCBM/C60 layer. In addition, quantitative analysis of interfacial charge dynamics shows that the quenching efficiency of PCBM/carbon is comparable to that of PCBM/C60 but better interface defect passivation and improved series and shunt resistances were observed when PCBM/carbon was employed. For the photovoltaic performance, the reference perovskite solar cell fabricated from the widely used PCBM/C60 has a power conversion efficiency (PCE) of 14% while the perovskite solar cell with PCBM/carbon has an increased PCE of 16%. Our results demonstrate the potential of the use of cost-effective carbon for perovskite solar cells, which could reduce production costs.
ABSTRACT
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.
