Paul Pistor

Monitoring the Phase Formation of Coevaporated Lead Halide Perovskite Thin Films by in Situ X-ray Diffraction

Perovskite solar cells based on (CH3NH3)Pb(I,Cl)3 have recently demonstrated rapidly increasing cell efficiencies. Here, we show progress identifying phases present during the growth of (CH3NH3)Pb(I,Cl)3 perovskite thin films with the vacuum-based coevaporation approach using two sources under varying deposition conditions. With in situ X-ray diffraction, crystalline phases can be identified and monitored in real time. For different (CH3NH3)I-to-PbCl2 flux ratios, two distinct (CH3NH3)Pb(IxCl(1-x))3 phases with high (x > 0.95) and with lower (x < 0.5) iodine content as well as a broad miscibility gap in-between were found. During post deposition annealing we observe recrystallization and preferential orientation effects and finally the decomposition of the perovskite film to PbI2 at temperatures above 200 °C.

Structural investigation of co-evaporated methyl ammonium lead halide perovskite films during growth and thermal decomposition using different PbX2 (X = I, Cl) precursors

While the progress in device development of perovskite solar cells is rapidly evolving, details of the film formation and the interplay of processing parameters, structural and compositional properties of deposited phases and their stability are still under dispute. Here we present a detailed structural analysis of methylammonium lead halide (I, Cl) films by in situ X-ray diffraction during their growth and thermal recrystallization up to their decomposition. MAPbI3 films grown by co-evaporating MAI and PbI2 are compared to MAPbI3(Cl) films derived from an evaporation route using MAI and PbCl2 precursors. The main differences observed between the two routes are varying crystal structures at room temperature and differently limited process windows, but similar overall growth, recrystallization and decomposition features. The preferential orientation of the pure MAPbI3 is shown to depend on the applied molar precursor flux ratio and can additionally be modified by thermal annealing.

Advanced Raman Spectroscopy of Methylammonium Lead Iodide: Development of a Non-destructive Characterisation Methodology.

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. However, the stability of perovskite films and respective solar cells is still an open point of concern and calls for advanced characterization methods. In this work, we identify appropriate measurement conditions for a meaningful analysis of spin-coated absorber-grade perovskite thin films based on methylammonium (MA) lead iodide (MAPbI3) by Raman spectroscopy. The material under investigation and its derivates is the most commonly used for high efficiency devices in the literatures and has yielded working solar cell devices with efficiencies around 10% in our laboratory. We report highly detailed Raman spectra obtained with excitation at 532 nm and 633 nm and their deconvolution taking advantage of the simultaneous fitting of spectra obtained with varying excitation wavelengths. Finally, we propose a fast and contactless methodology based on Raman to probe composition variations and/or degradation of these perovskite thin films and discuss the potential of the presented technique as quality control and degradation monitoring tool in other organic-inorganic perovskite materials and complete solar cell devices.

Cu 2 ZnSnSe 4 -Based Solar Cells With Efficiency Exceeding 10% by Adding a Superficial Ge Nanolayer: The Interaction Between Ge and Na

Recently, beneficial effects of the incorporation of small amounts of Ge into Cu2ZnSnSe4 (CZTSe)-based solar cells have been reported, showing that the presence of Ge can enhance the crystalline properties of CZTSe, assisting the grain growth, leading to high-efficiency devices. In this study, we prepare CZTSe layers by a sequential process consisting of the sputtering of metallic stacks followed by a reactive annealing under Se atmosphere, previously adding different Ge nanolayers on top (from 0 to 50 nm). The present work is focused on the study of the interaction between germanium and sodium. As is widely known, Na is a very important dopant in kesterite, which plays an essential role in the doping level control. We demonstrate that during the annealing process, a Ge-Se liquid phase is formed which dissolves preferably Na-related phases modifying the content of this last element in the CZTSe absorber and impacting notably on the electrical properties of the layers and, concomitantly, on the performance of the devices. We support our Ge-Na interaction model with experiments using Na-free substrates, showing the importance of accurately controlling the Na content when Ge is used to increase the efficiency of CZTSe-based solar cells.

Raman scattering quantitative assessment of the anion composition ratio in Zn(O,S) layers for Cd-free chalcogenide-based solar cells

This work reports the use of Raman scattering for the chemical characterization of Zn(O,S) layers that are being developed as a Cd-free alternative for the buffer layer in advanced chalcogenide solar cells. The nanometric thickness of these layers requests the use of resonant excitation conditions, which are strongly sensitive to the alloy composition. In this study Raman spectra were measured with different excitation wavelengths (325 nm, 532 nm) on a set of reference samples with chemical compositions covering the whole range from stoichiometric ZnS to stoichiometric ZnO. The results show the existence of a strong linear dependence of the frequency of the ZnO-like peak on the alloy composition, which provides a simple methodology for the quantitative assessment of the chemical composition in almost all the composition region. In the case of samples with a S-rich composition (0.5 ≤ S/(S + O) ≤ 1), the analysis of the relative intensity of the ZnS like peak allows for a complementary assessment of the S/(S + O) content ratio. The characterization of layers grown under conditions similar to those used for the fabrication of chalcogenide solar cells has allowed the demonstration of the viability of the proposed methodology for the non-destructive chemical assessment of these advanced buffer layers.

Large performance improvement in Cu2ZnSnSe4 based solar cells by surface engineering with a nanometric Ge layer

This work presents a radically new approach based on the application of very small Ge quantities on the CZTSe surface (from 1 nm to 25 nm thick Ge layers), allowing for a liquid assisted improved crystallization due to the formation of a Ge-Se (Se-rich) liquid phase. This modification improves the charge transport properties at this interface and consequently the devices voltage and electro-optical properties in general. Using TEM and TOF-SIMS we demonstrate that Ge is barely incorporated into the absorber; nevertheless we observe a drastic increase of the VOC of the solar cells (from 405 mV for the reference to 470 mV for the best Ge modified one). This in turn has a large impact on the performance, increasing it from 7.0% (reference) to 10.1% (Ge modified), which sets a new record efficiency for a Ge containing kesterite and a VOC among the highest obtained for Se-based kesterite solar cells. First characterizations indicate that this is related to an improved grain growth assisted via Ge-Se liquid phases, the minimization of Sn-reduced species and the formation of Ge-O nano-clusters. Our approach not only allows to go towards high efficiency concepts and to contribute to solve the voltage deficit problems in kesterites, but also opens new perspectives for the possible band-gap engineering of kesterite devices with very low Ge concentrations.

Research Update: Stable single-phase Zn-rich Cu2ZnSnSe4 through In doping

Alloying in the system Cu2ZnSnSe4–CuInSe2–ZnSe (CZTISe) is investigated experimentally and theoretically. The goal is to distinguish single-phase and multi-phase regions within the Cu2ZnSnSe4-2CuInSe2-4ZnSe pseudo-ternary phase diagram. CZTISe thin films are prepared by co-evaporation of the chemical elements and are investigated in real-time during growth using in situ angle dispersive X-ray diffraction. The focus is mainly on thin films along the Cu2ZnSnSe4–2CuInSe2 isopleth with small ZnSe addition as well as on films along the Cu2ZnSnSe4-4ZnSe isopleth with small CuInSe2 addition. For both cases, ab initio calculations with density-functional theory are performed to estimate the stability of the alloy with respect to the formation of secondary phases. Both in experiment and calculation, we find a surprisingly large single-phase region in the Cu2ZnSnSe4 corner of the pseudo-ternary phase diagram slightly off the Cu2ZnSnSe4-4ZnSe isopleth. This may help avoiding secondary phase formation under Zn-rich c...

Resonant Raman scattering based approaches for the quantitative assessment of nanometric ZnMgO layers in high efficiency chalcogenide solar cells

This work reports a detailed resonant Raman scattering analysis of ZnMgO solid solution nanometric layers that are being developed for high efficiency chalcogenide solar cells. This includes layers with thicknesses below 100 nm and compositions corresponding to Zn/(Zn + Mg) content rations in the range between 0% and 30%. The vibrational characterization of the layers grown with different compositions and thicknesses has allowed deepening in the knowledge of the sensitivity of the different Raman spectral features on the characteristics of the layers, corroborating the viability of resonant Raman scattering based techniques for their non-destructive quantitative assessment. This has included a deeper analysis of different experimental approaches for the quantitative assessment of the layer thickness, based on (a) the analysis of the intensity of the ZnMgO main Raman peak; (b) the evaluation of the changes of the intensity of the main Raman peak from the subjacent layer located below the ZnMgO one; and (c) the study of the changes in the relative intensity of the first to second/third order ZnMgO peaks. In all these cases, the implications related to the presence of quantum confinement effects in the nanocrystalline layers grown with different thicknesses have been discussed and evaluated.

Thermal stability and miscibility of co-evaporated methyl ammonium lead halide (MAPbX3, X = I, Br, Cl) thin films analysed by in situ X-ray diffraction

We present the identification of crystalline phases by in situ X-ray diffraction during growth and monitor the phase evolution during subsequent thermal treatment of CH3NH3PbX3 (X = I, Br, Cl) perovskite thin films. The thin films are prepared by vacuum-based two-source co-evaporation using various methyl ammonium (MA) halide and lead halide (PbX2) precursors. The single halide perovskite materials MAPbI3, MAPbBr3 and MAPbCl3 are prepared without secondary phases and an upper thermal limit for decomposition into the corresponding lead halides is established. We show that at a substrate temperature of 120 °C, the halide in MAPbI3/MAPbBr3 thin films can be completely and reversibly exchanged upon exposure to the opposite MA halide. We monitor the temporal evolution of the conversion process in situ and discuss differences in the forward and backward conversion. For the deposition of mixed MAPb(I,Br)3 perovskite thin films, different growth routes are suggested and evaluated in terms of growth with single phases or phase segregations. Our results are discussed in a broader context considering I/Br miscibility. Finally we propose a new growth route for the synthesizes of single phase mixed MAPb(I1−xBrx)3 thin films in the range from x = 0.3 to 1 by two source co-evaporation and discuss the implication of our results.

Crystal Phases and Thermal Stability of Co-evaporated CsPbX3 (X = I, Br) Thin Films

We present the growth, phase transitions, and thermal decomposition of CsPbX3 (X = I, Br) thin films monitored by in situ X-ray diffraction (XRD). The perovskite films are prepared in vacuum via co-evaporation of PbX2 and CsX (X = I, Br) onto glass substrates. In situ X-ray diffraction allows the observation of phase transitions and decomposition while the samples are heated with a linear temperature ramp. Our experiments reveal the decomposition route for the CsPbX3 perovskites in high vacuum, with a much higher stability than their hybrid organic-inorganic MAPbX3 counterparts. We also observe the response of a black CsPbI3 thin film to exposure to ambient air at room temperature using the same XRD system. Exposing the black CsPbI3 to ambient air leads to the formation of yellow orthorhombic δ-CsPbI3, whose crystal structure could be identified by its X-ray diffraction pattern. Additionally, the linear coefficients of expansion are determined for δ-CsPbI3 and the (020)-orientation of CsPbBr3.