For example, grain size and/or grain boundary can affect the types and amounts of defects 16, 18, 19, 20. Some of these material issues are closely correlated to one another. Additionally, inorganic materials that are used in conjunction with halide perovskite layers for charge transport and extraction can result in extra interface defects 5, 22, 23. From a materials point of view, these issues arise from diverse morphological and compositional variations of halide perovskite materials 6, 16, 17, 18, 19, crystallization kinetics of halide perovskite grains 5, 16, 18, 19, 20, 21, various defects and/or remnant of precursors 16, 17, 18, 19, 20, and internal redistribution of ionic species within halide perovskite layers 6, 16, 17, 18, 19. Previous studies report various degrees of success in improving hysteresis, reproducibility, and stability 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, however, these issues remain to be fully resolved. Although PSCs can have power conversion efficiency (PCE) as high as 25.2% 4, they have inherent issues of large hysteresis, poor reproducibility, and limited stability. In recent years, a perovskite solar cells (PSCs) emerged as an intriguing new addition to the list of novel solar cells (SCs) that either challenge or complement already mature silicon SCs 1, 2, 3. The presence of multiple types of defects was corroborated by findings from equivalent-circuit analysis of impedance spectra. These ideality-factor values were consistent with those representing the intensity dependence of loss-current ratio estimated by using a constant internal-quantum-efficiency approximation. However, at high intensities, another type of defect not only took over monomolecular recombination, but also dominated bimolecular recombination to result in the ideality factor of ~2.0. At low intensities, monomolecular recombination occurred due to one of these defects in addition to bimolecular recombination to result in the ideality factor of ~1.7. Intensity dependence of ideality factor led us to the conclusion that there were two other types of defects that contributed mostly as recombination centers. Diode-like currents were analysed using a modified Shockley-equation model, the validity of which was confirmed by comparing measured and estimated open-circuit voltages. The variation of power-law exponent of SCLC showed that charge trapping by defects diminished as intensity increased, and that drift currents became eventually almost ohmic. Measured J- V curves consisted of space-charge-limited currents (SCLC) in a drift-dominant range and diode-like currents in a diffusion-dominant range. If you vary the distance from the light source to the photoresistor the intensity should fall off as the square of the distance.We investigated operation of a planar MAPbI 3 solar cell with respect to intensity variation ranging from 0.01 to 1 sun. Where I is the light intensity, R is the resistance, and R D is the resistance in the dark. The reciprocal of the resistance (after you subtract the reciprocal of the dark resistance) is proportional to the light intensity. You should get about a factor of 10 or so difference. Take the largest one of the lot and use the ohm-meter to measure the resistance both pointed toward the light and then covered up. Radio Shack has several models in the $20 region. It's a must for every young experimenter. If you don't have one you should buy one. You'll also need a multimeter to measure the resistance of the device before and after the light is shone on it. A 5-pack of them ,stock number 276-1657, costs about 3 bucks. A cheap and straightforward way is to go down to Radio Shack and buy a CdS (Cadmium Sulfide) photoresistor pack. A photographic light meter will work but may cost you $100 or so. You'll need some sort of light measuring device.
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