Increasing the stability of perovskites is essential for their integration in commercial photovoltaic devices. Halide mixing is suggested as a powerful strategy toward stable perovskite materials. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate. Here we successfully determine the exact location of the I and Cl anions in the  CH3NH3PbBr3–y I y and CH3NH3PbBr3–z Cl z mixed halide perovskite lattices and correlate it with the enhanced stability we find for the latter. By combining scanning tunneling microscopy and density functional theory, we predict that, for low ratios, iodine and chlorine incorporation have different effects on the electronic properties and stability of the CH3NH3PbBr3 perovskite material. In addition, we determine the optimal Cl incorporation ratio for stability increase without detrimental band gap modification, providing an important direction for the fabrication of stable perovskite devices. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a half-cell device architecture. Our findings provide an answer to the current debate on halide incorporation and demonstrate their direct influence on device stability.

Organic−inorganic hybrid perovskites (OHPs) have garnered much attention among the photovoltaic and light-emitting diode research community due to their excellent optoelectronic properties and low-cost fabrication. Defects in perovskites have been proposed to affect device efficiency and stability and to have a potential role in enabling ion migration. In this study, the dynamic behavior and electronic properties of intrinsic defects in CH 3 NH 3 PbBr 3 (MAPbBr 3 ) were explored at the atomic scale. We use scanning tunneling microscopy to show unambiguously the occurrence of vacancy-assisted transport of individual ions as well as the existence of vacancy defect clusters at the OHP surface. We combine these observations with density functional theory (DFT) calculations to identify the mechanisms for this ion motion and show that ion transport energy barriers, as well as transport mechanisms, at the surface depend on crystal direction. DFT calculations also reveal that vacancy defect clusters can significantly modify the local work function of the perovskite surface, which is then expected to alter interfacial charge transport in a device. Our work provides a microscopic insight into the mechanism of ion migration in OHPs and also delivers the useful information for device improvement from the perspective of interface engineering.

Carbon electrode-based perovskite solar cells (PSCs) with low-cost and long-term stability have been recognized as a competitive candidate toward future practical applications. However, energy level mismatch and ineffective hole extraction at the carbon electrode/perovskite interface limit device performance. Herein, we develop a low-cost carbon-based electrode that utilizes a cheap small-molecule semiconductor copper phthalocyanine (CuPc) as both the interface modifier and dopant. The resultant planar PSC yields a power conversion efficiency of 14.8%, ∼30% higher than that based on the bare carbon electrode. This is due to higher work function and better hole extraction properties of the CuPc-modified carbon electrodes. The simple modification process of the carbon electrode has potential applications for large-scale fabrication. We further applied such electrodes in large-area solar modules and flexible solar cells, demonstrating their capability of upscaling and flexibility.

preparation can lead to a large variety of film morphologies.OHPs have impressive optoelectronic properties, ranging from high absorption coefficient and suitable bandgap for visible light, [5] low exciton binding energy, [6,7] high carrier mobility, and long carrier diffusion lengths, [8] all of which make this class of materials especially suitable for PV applications. It is of the utmost importance to understand the effect that film morphology has on these critical properties, and subsequently on standard solar cell device parameters such as photocurrent, photovoltage, and PCE. OHP devices are also known to degrade under prolonged exposure to environmental factors including moisture, iodine vapor, high temperature, and sunlight. [9][10][11][12] Understanding how and why the film morphology and performance changes over time is also of key interest.Perovskite films are often characterized using conventional techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), quantum efficiency measurements, and I-V curve measurements, which lack high spatial resolution. However, scanning probe microscopy (SPM) techniques offer a unique advantage to correlate changes in optoelectronic properties and PV performance parameters with morphological features on the nanoscale. SPM techniques have been employed to characterize other PV technologies such as silicon, copper indium gallium selenide (CIGS), CdTe, GaAs, and organic solar cells. [13][14][15][16][17][18][19][20][21][22] Open-circuit voltage variation due to grain orientation and grain boundaries was revealed. [13] Moreover, SPM has been used to investigate the interfacial properties in polycrystalline Si [14] as well as in blended [15] and phase-segregated [16] organic solar cells. Furthermore, scanning-probe techniques demonstrated their ability to study molecules used for dye sensitized solar cells down to the atomic level. [20,21] In addition, SPM techniques can provide insight into phenomenon specific to OHPs. This includes film growth variation depending on fabrication technique, film degradation due to environmental factors, and undesirable current-voltage hysteresis possibly induced by ion migration within the OHP film. A wide variety of SPM techniques have been used to explore heterogeneities in OHP thin film properties (Figure 1a-c), including topographic atomic force micro scopy (AFM), Kelvin probe force microscopy (KPFM), scanning tunneling microscopy (STM), conductive-AFM (c-AFM), and scanning near-field optical microscopy (SNOM).To meet the increasing energy demands of the growing society, environmentally friendly and renewable energy sources are needed. Organicinorganic halide perovskites are a promising class of materials for building solar cells due to their easy fabrication, flexibility, and bandgap tunability. The highest efficiency achieved with these materials in the lab is comparable to conventional silicon solar cells currently on the market. However, their commercialization is hampered by certain challenges, such as stabil...

Incorporation of a certain amount of Cl ions into methylammonium lead iodide (MAPbI3) perovskite films and how these incorporated Cl ions affect the structural and electronic properties of these films...