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Abstract:

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have demonstrated excellent optical properties and an encouraging potential for optoelectronic applications; however, mixed-halide perovskites, especially CsPb(Cl/Br)₃ NCs, still show lower photoluminescence quantum yields (PL QY) than the corresponding single-halide materials. Herein, anhydrous oxalic acid is used to post-treat CsPb(Cl/Br)₃ NCs in order to initially remove surface defects and halide vacancies, and thus, to improve their PL QY from 11% to 89% for the emission of 451 nm. Furthermore, due to the continuous chelating reaction with the oxalate ion, chloride anions from the mixed-halide CsPb(Cl/Br)₃ perovskite NCs could be extracted, and green emitting CsPbBr₃ NCs with PL QY of 85% at 511 nm emission are obtained. Besides being useful to improve the emission of CsPb(Cl/Br)₃ NCs, the oxalic acid treatment strategy introduced here provides a further tool to adjust the distribution of halide anions in mixed-halide perovskites without using any halide additives.

Abstract:

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible and efficient solar cells for a wide range of applications. Physical vapour deposition by co-evaporation of precursors is a method which results in very smooth and pin-hole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) has proved extremely challenging. We show that the established method of controlling the evaporation-rate of MAI with quartz micro balances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from MAI batch-to-batch and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapour deposition will allow high solar cell device yields even if the purity of precursors change from run to run.

Abstract:

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies1,2,3,4. Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites5,6,7,8,9, optimizing the interfaces within the device structures10,11,12,13, and using new encapsulation techniques14,15. However, further improvements are required in order to deliver a longer-lasting technology. Ion migration in the perovskite active layer—especially under illumination and heat—is arguably the most difficult aspect to mitigate16,17,18. Here we incorporate ionic liquids into the perovskite film and thence into positive–intrinsic–negative photovoltaic devices, increasing the device efficiency and markedly improving the long-term device stability. Specifically, we observe a degradation in performance of only around five per cent for the most stable encapsulated device under continuous simulated full-spectrum sunlight for more than 1,800 hours at 70 to 75 degrees Celsius, and estimate that the time required for the device to drop to eighty per cent of its peak performance is about 5,200 hours. Our demonstration of long-term operational, stable solar cells under intense conditions is a key step towards a reliable perovskite photovoltaic technology.