Precisely Controlling the Grain Sizes with an Ammonium Hypophosphite Additive for High‐Performance Perovskite Solar Cells

Abstract

A facile approach to precisely control the perovskite grain sizes is proposed and demonstrated for high‐performance photovoltaic (PV) solar cells. With the introduction of various amounts of NH4H2PO2 (AHP) additives into the PbI2/CH3NH3I precursors, the grain scale of CH3NH3PbI3 films can be finely turned from hundreds of nanometer to micrometer scale, allowing evaluating the effects of crystalline grain boundary on trap densities, charge recombination, and PV device performance. The X‐ray diffraction and X‐ray photoelectron spectroscopy measurements indicate that the formation of intermediates plays a key role in assisting the perovskite crystal growth. The optimized devices show much larger open‐circuit voltages (VOC) up to 1.10 ± 0.02 V and significantly enhance power conversion efficiencies (PCEs) of 16.5 ± 0.7%, as compared to the control devices with PCE of 9.4 ± 1.0% and VOC of 1.00 ± 0.03 V. Further investigations confirm that the boosted PV performance origins from the decreased defect densities due to enlarged grain sizes. It is also demonstrated that the approach is general and applicable to other perovskite systems, e.g., HC(NH2)2PbI3. The results suggest the promising application of AHP in achieving high‐performance perovskite PV devices, and shed light on understanding the grain boundary effects on perovskite optoelectronics.

Amphiphilic Carbon Dots Suppress Iodide Ion Migration in Perovskite Solar Cells

“…It is worth noting that the crystallite size does not correspond to the size of the perovskite grains and lies in the range of tens of nm, as previously reported. 60,61 Thus, the increase of the crystallite size for CD@FACsPbI 3 is well in line with the morphology study by AFM and SEM, as discussed above. Hydroxyl functional groups can form hydrogen bonds with defect sites of perovskite, and in particular with halides, through the interaction between hydrogen from the −OH group and electronegative I − ions from the perovskite layer.…”

Inverted Perovskite Solar Cells Based on Small Molecular Hole Transport Material C8‐Dioctylbenzothienobenzothiophene

“…Figure a and Table S2 displayed the X‐ray diffraction (XRD) patterns of the MAPbI 3 films on different substrates. The diffraction peaks at around 14.1 o can be ascribed to the (110) plane of MAPbI 3 crystal, and around 12.6 o is the diffraction peak of PbI 2 . The MAPbI 3 film on the 150 °C annealed C 8 ‐BTBT layer presents the highest intensity of the (110) diffraction peak and the weakest diffraction peak of PbI 2 , indicating the highest film quality, consistent with the SEM images shown in Figures and .…”

Small grains as recombination hot spots in perovskite solar cells

“…For example, Xu et al reported that increasing the grain size leads to an increase in V OC up to a point, followed by a decrease for the samples with the largest grains. 45 Similar results were also reported by Im et al, whose samples with medium-size grains resulted in the largest V OC while small and large grain sizes led to lower voltages. 49 In another work, the same group reported that devices with the smallest grain size led to the highest V OC .…”

“…Our results can explain the seemingly conflicting literature reports regarding the importance of grain boundaries: considering that only the average grain size was characterized and not the entire distribution, it is possible for samples with on average larger grains to exhibit a higher recombination, as long as the number of small grains is sufficiently high to mask the effect of larger grains. 45 Examination of the entire distribution of grain sizes (Figure S1) shows that an increase in average grain size is always accompanied by an increase in the width of the distribution. This highlights that experimental efforts should be devoted not only to increasing the average grain size but particularly to eliminating small grains from the film's microstructure.…”