Planar versus mesoscopic perovskite microstructures: The influence of CH3NH3PbI3 morphology on charge transport and recombination dynamics

Abstract

Perovskite solar cells (PSCs) employing planar and mesoscopic architectures have both resulted in high efficiency devices. However, there is presently a limited understanding of the inherent advantages of both systems, particularly in terms of the charge transport and recombination dynamics. In the present study we characterize the relative benefits of the two most prominent CH 3 NH 3 PbI 3 morphologies, primarily through time-resolved microwave conductivity (TRMC) and time-resolved photoluminescence (TRPL) measurements. The comparatively large perovskite grains, typical of planar assemblies, exhibited higher charge mobilities and slower trap-mediated recombination compared to the mesoscopic architectures. These findings reveal the injurious influence of grain boundaries on both charge transport and recombination kinetics, and suggest an innate advantage of planar morphologies. However, through impedance spectroscopy (IS) measurements, mesoscopic architectures were found to limit the interfacial recombination at the transparent conductive oxide (TCO) substrate. The lessons learnt through the characterization measurements were subsequently utilized to produce an optimized cell morphology, resulting in a maximum conversion efficiency of 16%.

Directing nucleation and growth kinetics in solution-processed hybrid perovskite thin-films

“…4a-d, respectively. Regions of contrasting intensity between neighboring grains and grain-clusters can be clearly seen in the PL maps, as identified previously [25,42]. Firstly, it is important to note the differing intensity scales for each image in Fig.…”

“…Yet, one of the most important connections between the perovskite microstructure and the optoelectronic performance is arguably related to the perovskite grain size and charge transport/recombination kinetics. Several studies have revealed clear correlations between the perovskite microstructure, charge-carrier mobilities and charge-carrier lifetimes, implying that large perovskite crystals with a low density of grain boundaries are advantageous [22][23][24][25]. These findings are in keeping with the relatively high mobilities and low trap-state densities observed in single-crystal organic-inorganic perovskites [26].…”

“…Previous reports have also noted the low PL intensities characteristic of grain boundaries [42] and have highlighted the deleterious effects of grain boundaries on the charge-carrier lifetimes [25]. PL decays for each of these morphology types are presented in Fig.…”

“…However, classical atom-mediated growth models are unable to satisfactorily explain the polycrystalline nature of the resulting perovskite microstructure observed in the present work. Polycrystalline microstructures have similarly been observed in hybrid organic-inorganic perovskite dendrites [16,25], despite the comparatively low heterogeneous nucleation rates. Other recent studies have also revealed highly-oriented growth of perovskite domains within dendritic morphologies [38], involving the growth mechanisms associated with perovskite dendrites that appear to be very similar to those outlined above.…”

Impact of sol aging on TiO2 compact layer and photovoltaic performance of perovskite solar cell

“…Comparison of the PL in Fig. 7b reveals that the samples with compact films made by sol aged for 4 h show the most obvious quenching effect, indicating that the exciton separation and transfer is most effective [37]. The biggest roughness of the TiO2 film from 4 h aged sol may provide the best contact between the TiO2 and perovskite film and offer the most effective electron transfer, and consequently reduce charge recombination at the interface between the perovskite and FTO or HTM.…”