COMMUNICATION (1 of 7)and CsSnI 3 has, to date, been less encouraging, with solar cell efficiencies of <5% for solution-processed thin film devices [10] that are likely limited by the low degree of crystalline ordering. [11] Indeed, structural ordering has been linked in traditional semiconductors to (a) carrier transport, where mobilities increase from amorphous-Si (1 cm 2 V −1 s −1 ) [12] to single crystalline Si (1400 cm 2 V −1 s −1 ), [13] (b) recombination rates, where unpassivated grain boundaries act as quenching sites for charge carriers and excited states, and (c) quantum confinement, which can make even Si an excellent NIR emitter with luminescent efficiency >60%. [14] These factors, among others, have motivated the recent interest in halide perovskite single crystal growth. [15] Thus, one of the main challenges for enhancing the properties of halide perovskites for high end optoelectronic applications is to obtain epitaxial crystalline films that can also be integrated into heteroepitaxial and quantum well structures. The epitaxial growth is a key step

To analyze the dominant recombination, researchers often consider the diode ideality factor (nid), determined from the fitting of a semi‐log plot of light intensity–dependent open‐circuit voltage (Voc(lnI/I0)) to a linear dependence. This value is called “nid,Voc”. Theoretically, nid is the exponential dependence factor in the recombination rate function of the split of quasi‐Fermi levels. This nid is called “nid,C”. Herein, correlations between nid,Voc, nid,C, and the dominant recombination are reconsidered using a validated numerical drift–diffusion model and a diode current analysis in perovskite solar cell devices having accumulations of charged defects near the carrier transporting interfaces. It is found that the interplay between the recombination processes affects the linearity of the Voc(lnI/I0) plots. Devices having a single dominant recombination process exhibit Voc(lnI/I0) plots that appear to be linear, resulting in nid,Voc ≈ nid,C of the dominant recombination. Conversely, bends in the Voc(lnI/I0) curves indicate that different (multiple) recombination mechanisms dominate at different light intensities, so nid,Voc is an effective nid of the total diode current whose value is not consistent with any nid,C values. This work provides more understanding of nid and how to interpret a Voc(lnI/I0) curve more correctly for the insights into recombination mechanisms.

High-performance lab-scale perovskite solar cells often have a precious metal as the top electrode. However, there are drawbacks to using metal top electrodes on a large scale, such as inducing degradation processes, requiring a high-temperature deposition process under vacuum, and having low scalability. Recently many studies have shown the potentials of using a carbon electrode because of its conductivity, flexibility, low cost, and ease of fabrication. This review article presents an overview of using carbon materials to replace the top electrode in perovskite photovoltaics. We discuss various fabrication techniques, various carbon-based device structures, and the advantages of using carbon materials. A collection of research works on device performance, large-scale fabrication, and device stability is presented. As a result, this review offers insight into the future of large-scale flexible solar cells.

Passivating electron‐transporting layers (ETLs) with alkali salts have demonstrated a facial approach that is essential in healing defective surfaces, consequently improving the functionality and stability of perovskite‐based solar cells (PSCs). Herein, the pseudohalide salt of sodium tetrafluoroborate, whose anions have a higher electronegativity than other halide salts (i.e., iodide and chloride), with the potential to passivate the surface of tin oxide while enhancing the optoelectronic properties of a perovskite film, is presented. Meanwhile, the density functional theory calculations show that BF4−/F− ions exhibit a robust ionic interaction with an uncoordinated Sn4+ site. In contrast, the Na ion is bound to an oxygen atom of the OH− group, which helps reduce surface defect states and improves charge transfer properties. Thus, the best PSC exhibits a current density of 23.51 mA cm−2, an open‐circuit voltage of 1.10 V, and an excellent fill factor of 80.48, providing an efficiency of 20.82%, which exceeds that of a control device (18.38%). Importantly, the retention of the power conversion efficiency on NaBF4‐based PSCs without encapsulation is 18.44% after 1000 h of aging under ambient conditions, whereas the retention of a control device is only 16.08%.

Organic–inorganic perovskite solar cells (PSCs), which have good environmental durability, are of great interest for practical applications. In this work, we show that a solution-processed MoO x layer acts as a buffer layer against high moisture stress to suppress defects in the perovskite and as a hole transport layer. The inversion of the photoinduced charge migration behaviors, that is, the electron preferentially moving toward the surface when MoO x is directly deposited onto the perovskite, is found to cause a significant loss in device functionality. The deposition of MoO x onto spiro-OMeTAD results in a lower photocurrent density–voltage (J–V) hysteresis behavior, a greatly enhanced electrical conductivity, and a significantly stabilized power conversion efficiency (PCE) when compared with those of devices without the MoO x layer. More importantly, the PCEs of the MoO x -based devices are retained at over 85% of their initial value, while only 75% is retained for a reference cell. This work highlights the facial fabrication approach of the solution-based MoO x layer and provides experimental evidence of the photogenerated charge migration behaviors on the perovskite/MoO x interface. This information would be beneficial for the further design and development of PSC technology.