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

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.

Pulsed laser deposition growth of 3D ZnO nanowall network in nest-like structures by two-step approach

Mahmoud Saeed Qaid Mohammad Hezam, Gwénolé Jean Jacopin

The mechanism for the growth of 3D ZnO nanowall networks in nest-like structure by a pulsed laser deposition (PLD) technique have been investigated for dye sensitized solar cells (DSSCs) and biosensor applications. As complex oxides shift towards large volume productions and practical applications, PLD offers a good advantage over other techniques due to the scalability of the produced film/structures, good adhesion between nanostructures and substrates, and preservation of stoichiometric ratio of the target material. In this study, a two-step temperature-pressure approach has been used to grow ZnO nanostructures on Si(100) using PLD. Optimizing this technique, we have structured a 3D ZnO nanowall networks with nest-like feature without any catalyst. Investigating the effect of substrate temperature from 300 degrees C to 750 degrees C and O-2 background from 10 mTorr to 10 Torr, we have grown the nanowall structures by first depositing a thin ZnO seed layer on Si(100) at a substrate temperature of similar to 300 degrees C and a background oxygen pressure of 10 mTorr, subsequently growing the nanowalls at similar to 550 degrees C and 500 mTorr background. The relationship of the PLD parameters and the morphology of the grown nanowalls were examined by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-ray Diffraction (XRD), Atomic Force Microscope (AFM), Photoluminescence (PL) and X-ray Photoemission Spectroscopy (XPS). The grown ZnO nanowall networks are single-crystals and highly-oriented on the c-axis. The nest-like structures initially grow on the thermodynamically active sites along the grain boundaries of randomly oriented Zn-rich ZnO (01 (1) over bar0) plane, which undergoes self-nucleation on the amorphous layer of the Si (100) substrates. The growth mechanisms for the synthesis of the ZnO nanowalls network can be explained by the vapor-solid (VS) model. The PLD-grown 3D ZnO nanowall network has a very high surface-to-volume ratio and appears to be denser than the previously reported ZnO nanowall structures grown by other techniques. This makes the PLD-grown ZnO nanowall a very promising material as photoanodes in DSSCs and an excellent matrix for biomolecule binding for potential applications in biosensors. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier2015

Asymmetric Cathodoluminescence Emission in CH3NH3PbI3-xBrx Perovskite Single Crystals

Neha Arora, Mohammad Ibrahim Dar, Benoît Marie Joseph Deveaud, Mahmoud Saeed Qaid Mohammad Hezam, Gwénolé Jean Jacopin, Mohammad Khaja Nazeeruddin, Shaik Mohammed Zakeeruddin

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.

Amer Chemical Soc2016

Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a water-soluble lead compound. Replacing lead in the perovskite structure: with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PM, and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the Valence of Sr2+, results, in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite This is explained as a:consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.