In situ observation of heat-induced degradation of perovskite solar cells

The lack of thermal stability of perovskite solar cells is hindering the progress of this technology towards adoption in the consumer market. Different pathways of thermal degradation are activated at different temperatures in these complex nanostructured hybrid composites. Thus, it is essential to explore the thermal response of the mesosuperstructured composite device to engineer materials and operating protocols. Here we produce devices according to four well-established recipes, and characterize their photovoltaic performance as they are heated within the operational range. The devices are analysed using transmission electron microscopy as they are further heated in situ, to monitor changes in morphology and chemical composition. We identify mechanisms for structural and chemical changes, such as iodine and lead migration, which appear to be correlated to the synthesis conditions. In particular, we identify a correlation between exposure of the perovskite layer to air during processing and elemental diffusion during thermal treatment. Solar cells based on lead halide perovskite composites have become increasingly popular in the past few years owing to a combination of low synthesis cost and high power conversion efficiency, with certified values in excess of 20% (refs 1,2,3,4,5). However, the stability of such devices is a concern—it is well known that heating at or above around 85 ∘C, a temperature close to those reached during normal operation in full sunlight, performance degrades rapidly, and such instability is exacerbated by exposure to moisture; systematic thermal and ageing studies are required to understand such degradation processes. Changes happen in both the organic and inorganic components of the cells; the resilience of the perovskite layer, in particular, is expected to become a limiting factor once different hole-conducting materials (or hole-conductor-free cells) are developed. To overcome this limitation, it is vital to understand the degradation pathways of the structures involved, which here are observed at nanometre-scale spatial resolution in situ, inside an analytical scanning transmission electron microscope (STEM), while the composition is monitored with elemental mapping through energy-dispersive X-ray analysis (EDX). The analysis of such devices is challenging owing to several factors. The spatial dimensions relevant to the fabrication and the operation of the cells are in the 1–100 nm range, and the materials are easily damaged by exposure to an electron beam in a TEM, requiring careful tuning of the electron dose. The system also includes organic and inorganic components in an assembly with complex chemistry and morphology. Finally, the rapid changes to the devices in air and the low degradation temperatures pose an additional challenge to the experiment, which needs to be timed appropriately and carefully executed. The monitoring of this process is made possible by combining several recent advances in TEM technology. The use of high-brightness electron guns and detectors with large collection areas allows the fast acquisition of high-quality EDX maps with limited electron dose on the sample; the signal-to-noise ratio of the maps can be further increased by applying denoising algorithms (PCA, principal components analysis) within an open-source software suite. The development of novel in situ heating holders for TEM, based on micro-heaters and featuring high stability and fast response, was also crucial—in particular, the holder used here allows very precise control (sub-degree) at values just above room temperature, as well as providing fast heating and cooling (a few seconds for the temperatures in use in this paper). The good spatial stability of the holder is crucial in acquiring EDX maps.G.D., S.C., and C.D. acknowledge funding from ERC under grant number 259619 PHOTO EM. C.D. acknowledges financial support from the EU under grant number 312483 ESTEEM2. F.M., L.C. and A.D.C. acknowledge funding from “Polo Solare Organico” Regione Lazio, the “DSSCX” MIURPRIN2010 and FP7 ITN “Destiny”. G.D and S.C. thank Dr. Francisco de la Peña and Dr. Pierre Burdet for assistance with PCA analysis.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nenergy.2015.1

Interface and Composition Analysis on Perovskite Solar Cells.

Organometal halide (hybrid) perovskite solar cells have been fabricated following four different deposition procedures and investigated in order to find correlations between the solar cell characteristics/performance and their structure and composition as determined by combining depth-resolved imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and analytical scanning transmission electron microscopy (STEM). The interface quality is found to be strongly affected by the perovskite deposition procedure, and in particular from the environment where the conversion of the starting precursors into the final perovskite is performed (air, nitrogen, or vacuum). The conversion efficiency of the precursors into the hybrid perovskite layer is compared between the different solar cells by looking at the ToF-SIMS intensities of the characteristic molecular fragments from the perovskite and the precursor materials. Energy dispersive X-ray spectroscopy in the STEM confirms the macroscopic ToF-SIMS findings and allows elemental mapping with nanometer resolution. Clear evidence for iodine diffusion has been observed and related to the fabrication procedure.We acknowledge Lucio Cinà, Simone Casaluci, Stefano Razza and Narges Yaghoobi Nia for the technical support, “Polo Solare Organico” Regione Lazio, the “DSSCX” MIUR-PRIN2010 and FP7 ITN “Destiny” for funds. G.D., S.C. and C.D. acknowledge funding from ERC under grant number 259619 PHOTO EM. C.D. acknowledges financial support from the EU under grant number 312483 ESTEEM2.This is the final version of the article. It was first available from ACS via http://dx.doi.org/10.1021/acsami.5b0803

Pinhole-free perovskite films for efficient solar modules

We report on a perovskite solar module with an aperture area of 4 cm2 and geometrical fill factor of 91%. The module exhibits an aperture area power conversion efficiency (PCE) of 13.6% from a current–voltage scan and 12.6% after 5 min of maximum power point tracking. High PCE originates in pinhole-free perovskite films made with a precursor combination of Pb(CH3CO2)2·3H2O, PbCl2, and CH3NH3I

High-Efficiency Perovskite Solar Cell Based on Poly(3-Hexylthiophene): Influence of Molecular Weight and Mesoscopic Scaffold Layer

Here, we investigated the effect of the molecular weight (MW) of poly 3-hexylthiophene (P3HT) hole-transport material on the performance of perovskite solar cells (PSCs). We found that by increasing the MW the photovoltaic performances of the cells are enhanced leading to an improvement of the overall efficiency. P3HT-based PSCs with a MW of 124 kDa can achieve an overall average efficiency of 16.2 %, double with respect to the ones with a MW of 44 kDa. Opposite to spiro-OMeTAD-based PSCs, the photovoltaic parameters of the P3HT-based devices are enhanced by increasing the mesoporous TiO2 layer thickness from 250 to 500 nm. Moreover, for a titania scaffold layer thickness of 500 nm, the efficiency of P3HT-based PSCs with high MW is larger than the spiro-OMeTAD based PSCs with the same scaffold layer thickness. Recombination reactions of the devices were also investigated by voltage decay and electrochemical impedance spectroscopy. We found that the relationship between P3HT MW and cell performance is related to the reduction of charge recombination and to the increase of the P3HT light absorption by increasing the MW

Progress, highlights and perspectives on NiO in perovskite photovoltaics

The power conversion efficiency of NiO based perovskite solar cells has recently hit a record 22.1%. Here, the main advances are reviewed and the role of NiO in the next breakthroughs is discussed

Gold and iodine diffusion in large area perovskite solar cells under illumination.

Operational stability is the main issue hindering the commercialisation of perovskite solar cells. Here, a long term light soaking test was performed on large area hybrid halide perovskite solar cells to investigate the morphological and chemical changes associated with the degradation of photovoltaic performance occurring within the devices. Using Scanning Transmission Electron Microscopy (STEM) in conjunction with EDX analysis on device cross sections, we observe the formation of gold clusters in the perovskite active layer as well as in the TiO2 mesoporous layer, and a severe degradation of the perovskite due to iodine migration into the hole transporter. All these phenomena are associated with a drastic drop of all the photovoltaic parameters. The use of advanced electron microscopy techniques and data processing provides new insights on the degradation pathways, directly correlating the nanoscale structure and chemistry to the macroscopic properties of hybrid perovskite devices.European Research Council (291522), European Research Council (259619

Physical and chemical properties and degradation of MAPbBr3 films on transparent substrates

To date, the potential exploitation of hybrid organic-inorganic perovskites (HOIPs) in photovoltaic technologies has been significantly hampered by their poor environmental stability. HOIP degradation can be triggered by conventional operational environments, with excessive heating and exposure to oxygen and moisture significantly reducing the performances of HOIP-based solar cells. An imperative need emerges for a thorough investigation on the impact of these factors on the HOIP stability. In this work, the degradation of methylammonium lead bromide (CH3NH3PbBr3) thin films, deposited via spin-coating on indium tin oxide (ITO) and strontium titanate (STO) substrates, was investigated by combining Raman and ultraviolet-visible (UV-Vis) absorption spectroscopy, as well as optical and fluorescence microscopy. We assessed the physical and chemical degradation of the films occurring under diverse preservation conditions, shedding light on the byproducts emerging from different degradation pathways and on the optimal HOIP preservation conditions