Asymmetric Cathodoluminescence Emission in CH3NH3PbI3-xBrx Perovskite Single Crystals

Probing local emission properties of organic-inorganic lead halide perovskite material can provide evidence regarding the photovoltaic performance of perovskite solar cells. Herein, cathodoluminescence, which has the potential to resolve emission characteristics in the nanoregime, has been exploited to carry out temperature-dependent studies on individual well-faceted CH3NH3PbI3-xBrx perovskite single crystals. The spatial distribution of emission recorded at 4 and 300 K reveals that the periphery of the perovskite crystals radiates predominantly, which establishes that such an unusual emission characteristic is independent of the crystallographic phase of CH3NH3PbI3-xBrx crystals. Investigation based on scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy deduces that the asymmetric cathodoluminescence is associated with the nonhomogeneous distribution of methylammonium cations in CH3NH3PbI3-xBrx single crystals. These results emphasize the unraveling of a correlation between the composition and spectroscopic properties of perovskite crystals in the nanoregime, which eventually can influence the overall photovoltaic performance of the devices based on them.

Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells

Marine Eva Fedora Bouduban, Juan-Pablo Correa-Baena, Ulf Anders Hagfeldt, Elham Halvani Anaraki, Tor Jesper Jacobsson, Jacques-Edouard Moser, Kurt Schenk, Joël Teuscher, Wolfgang Richard Tress

Lead halide perovskites have over the past few years attracted considerable interest as photo absorbers in PV applications with record efficiencies now reaching 22%. It has recently been found that not only the composition but also the precise stoichiometry is important for the device performance. Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3)x(CH(NH2)2)(1–x)PbBryI(3–y). In this work a broad range of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photo electron spectroscopy (PES), transient absorption spectroscopy (TAS), UV–vis, electroluminescence (EL), photoluminescence (PL), and confocal PL mapping have been used to further understand the importance of remnant PbI2 in perovskite solar cells. Our best devices were over 18% efficient, and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent dropped, which could be attributed to accumulation of organic species at the grain boundaries, low charge carrier mobility, and decreased electron injection into the TiO2. The PbI2-deficient compositions did, however, also have advantages. The record Voc was as high as 1.20 V and was found in PbI2-deficient samples. This was correlated with high crystal quality, longer charge carrier lifetimes, and high PL yields and was rationalized as a consequence of the dynamics of the perovskite formation. We further found the ion migration to be obstructed in the PbI2-deficient samples, which decreased the JV hysteresis and increased the photostability. PbI2-deficient synthesis conditions can thus be used to deposit perovskites with excellent crystal quality but with the downside of grain boundaries enriched in organic species, which act as a barrier toward current transport. Exploring ways to tune the synthesis conditions to give the high crystal quality obtained under PbI2-poor condition while maintaining the favorable grain boundary characteristics obtained under PbI2-rich conditions would thus be a strategy toward more efficiency devices.

American Chemical Society2016

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Neha Arora, Mohammad Ibrahim Dar, Gwénolé Jean Jacopin, Shaik Mohammed Zakeeruddin

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2) 2PbBr(3) perovskite films deposited under different yet optimized conditions using X-ray scattering and cathodoluminescence spectroscopy, respectively. X-ray scattering shows that a CH3NH3PbI3 film involving spin coating of CH3NH3I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH3NH3PbI3 film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH3NH3PbBr3, while the crystallinity decreases when the HBr solution is used in a single-step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH2)(2)PbBr3 devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.

Wiley-Blackwell2017

Insights about the Absence of Rb Cation from the 3D Perovskite Lattice: Effect on the Structural, Morphological, and Photophysical Properties and Photovoltaic Performance

Neha Arora, Silvia Binet, Mohammad Ibrahim Dar, Michael Graetzel, Gwénolé Jean Jacopin, Ryusuke Uchida, Shaik Mohammed Zakeeruddin

Efficiencies >20% are obtained from the perovskite solar cells (PSCs) employing Cs+ and Rb+ based perovskite compositions; therefore, it is important to understand the effect of these inorganic cations specifically Rb+ on the properties of perovskite structures. Here the influence of Cs+ and Rb+ is elucidated on the structural, morphological, and photophysical properties of perovskite structures and the photovoltaic performances of resulting PSCs. Structural, photoluminescence (PL), and external quantum efficiency studies establish the incorporation of Cs+ (x < 10%) but amply rule out the possibility of Rb-incorporation into the MAPbI(3) (MA = CH3NH3+) lattice. Moreover, morphological studies and time-resolved PL show that both Cs+ and Rb+ detrimentally affect the surface coverage of MAPbI(3) layers and charge-carrier dynamics, respectively, by influencing nucleation density and by inducing nonradiative recombination. In addition, differential scanning calorimetry shows that the transition from orthorhombic to tetragonal phase occurring around 160 K requires more thermal energy for the Cs-containing MAPbI(3) systems compared to the pristine MAPbI(3). Investigation including mixed halide (I/Br) and mixed cation A-cation based compositions further confirms the absence of Rb+ from the 3D-perovskite lattice. The fundamental insights gained through this work will be of great significance to further understand highly promising perovskite compositions.

WILEY-V C H VERLAG GMBH2018

Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells

Marine Eva Fedora Bouduban, Juan-Pablo Correa-Baena, Ulf Anders Hagfeldt, Elham Halvani Anaraki, Tor Jesper Jacobsson, Jacques-Edouard Moser, Kurt Schenk, Joël Teuscher, Wolfgang Richard Tress

American Chemical Society2016