Solar cells incorporating metal‐halide perovskite (MHP) semiconductors are continuing to break efficiency records for solution‐processed solar cell devices. Scaling MHP‐based devices to larger area ...prototypes requires the development and optimization of scalable process technology and ink formulations that enable reproducible coating results. It is demonstrated that the power conversion efficiency (PCE) of small‐area methylammonium lead iodide (MAPbI3) devices, slot‐die coated from a 2‐methoxy‐ethanol (2‐ME) based ink with dimethyl‐sulfoxide (DMSO) used as an additive depends on the amount of DMSO and age of the ink formulation. When adding 12 mol% of DMSO, small‐area devices of high performance (20.8%) are achieved. The effect of DMSO content and age on the thin film morphology and device performance through in situ X‐ray diffraction and small‐angle X‐ray scattering experiments is rationalized. Adding a limited amount of DMSO prevents the formation of a crystalline intermediate phase related to MAPbI3 and 2‐ME (MAPbI3‐2‐ME) and induces the formation of the MAPbI3 perovskite phase. Higher DMSO content leads to the precipitation of the (DMSO)2MA2Pb3I8 intermediate phase that negatively affects the thin‐film morphology. These results demonstrate that rational insights into the ink composition and process control are critical to enable reproducible large‐scale manufacturing of MHP‐based devices for commercial applications.
The addition of the correct amounts of dimethyl sulfoxide (DMSO) with 2‐methoxyethanol (2‐ME) perovskite precursor ink is a crucial step toward reproducible slot‐die coatings and highly efficient perovskite solar cells. Through observing the drying process of 2ME‐DMSO inks from in situ X‐ray diffraction experiments, it is demonstrated that 11.77 mol% DMSO favorably affects thin film growth.
The formation mechanism and the evolution of optoelectronic properties during annealing of chlorine-derived methylammonium lead iodide (MAPbI 3−x Cl x ) are investigated in detail combining in situ ...and ex situ optical and structural characterization. Using in situ optical reflectometry we are able to monitor the evolution of the MAPbI 3−x Cl x phase as a function of time and processing temperature. The formation kinetics is fitted using an improved Johnson–Mehl–Avrami–Kolmogorov model and a delayed formation of MAPbI 3−x Cl x is found when chlorine is present in the precursor. This is verified by X-ray diffraction and X-ray fluorescence measurements. From absolute photoluminescence measurements we determine the implied V oc during film formation, which exhibits a maximum at a specific time during the annealing process. In conjunction with ex situ time-resolved photoluminescence we deduce a decrease in the net doping density for increased annealing times, while the minority carrier lifetime stays constant. We thus demonstrate the potential of in situ optical spectroscopy to monitor and tailor the electronic properties of hybrid perovskites directly during film growth, which can be easily applied to different growth recipes and synthesis environments.
Understanding the nanoscale structure and dynamics of supramolecular hydrogels is essential for exploiting their self-healing mechanisms. We describe here nanostructural evolution and self-healing ...mechanism of hydrogels formed from in situ generated hydrophobically modified hydrophilic polymers and wormlike sodium dodecyl sulfate (SDS) micelles. We observe a conformational transition in wormlike SDS micelles upon addition of hydrophobic as well as hydrophilic monomers. Several hundred nanometer long SDS micelles completely disappear after the monomer addition, in favor of spherical micelles with a radius of 2.4 nm. After conversion of the monomers to hydrophobically modified polymer chains via micellar copolymerization, the spherical shape of the micelles remains intact but the radius increases to 2.8 nm. The interconnected spherical mixed micelles consisting of SDS and hydrophobic blocks of the polymer self-assemble to form a layered hydrogel structure. Self-healing response of the damaged hydrogel samples begins by reshaping the injured area into circular holes and ends by complete healing due to the intra- and interlayer mobility of the mixed micelles, respectively.
The selenization of Cu-Zn-Sn-S nanocrystals is a promising route for the fabrication of low-cost thin film solar cells. However, the reaction pathway of this process is not completely understood. ...Here, the evolution of phase formation, grain size, and elemental distributions is investigated during the selenization of Cu-Zn-Sn-S nanoparticle precursor thin films by synchrotron-based in situ energy-dispersive X-ray diffraction and fluorescence analysis as well as by ex situ electron microscopy. The precursor films are heated in a closed volume inside a vacuum chamber in the presence of selenium vapor while diffraction and fluorescence signals are recorded. The presented results reveal that during the selenization the cations diffuse to the surface to form large grains on top of the nanoparticle layer and the selenization of the film takes place through two simultaneous reactions: (1) a direct and fast formation of large grained selenides, starting with copper selenide which is subsequently transformed into Cu2ZnSnSe4; and (2) a slower selenization of the remaining nanoparticles. As a consequence of the initial formation of copper selenides at the surface, the subsequent formation of CZTSe starts under Cu-rich conditions despite an overall Cu-poor composition of the film. The implications of this process path for the film quality are discussed. Additionally, the proposed growth model provides an explanation for the previously observed accumulation of carbon from the nanoparticle precursor beneath the large grained layer.
Cu2SnS3 (CTS) is starting to gain interest in the PV research community as an alternative earth abundant absorber for thin film photovoltaics. In this work, the structure, morphology and the ...composition of the CTS absorbers as well as their influence on the optoelectronic properties of the solar cells are analysed. The synthesis of Cu-Sn-S thin films by co-evaporation at a nominal temperature of 400 °C is presented. A combination of X-ray diffraction, Raman and UV-Vis spectroscopy suggests that the Cu2SnS3 is crystallising in a cubic structure with disorder in the Cu and Sn sites, leading to substantial band tailing.
The best device was fabricated from absorbers exhibiting a Cu/Sn ratio of approximately 1.7 and had an efficiency of 1.8%, a short circuit current of 28 mA cm−2, and an open circuit voltage of 147 mV with a fill factor of 42.9%. From the quantum efficiency measurement, we estimate a band gap of 1.06 eV for the CTS absorber material. Capacitance-voltage measurements show charge carrier concentrations between 4 and 6 × 1016 cm−3.
•Cu2SnS3 thin films deposited by co-evaporation.•We fabricated a Cu2SnS3 thin film solar cell.•The Cu2SnS3 absorber layer crystallises in a disorder cubic structure.
Photo‐ and charge‐carrier‐induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors for optoelectronic applications. For mixed iodide/bromide perovskites, the ...compositional instability due to light‐ or electrical bias induced phase‐segregation restricts the exploitation of the entire bandgap range. Previous experimental and theoretical work suggests that excited states or charge carriers trigger the process, but the exact mechanism is still under debate. To identify the mechanism and cause of light‐induced phase‐segregation phenomena, the full compositional range of methylammonium lead bromide/iodide samples are investigated, MAPb(BrxI1‐x)3 with x = 0…1, by simultaneous in situ X‐ray diffraction (XRD) and photoluminescence (PL) spectroscopy during illumination. The quantitative comparison of composition‐dependent in situ XRD and PL shows that at excitation densities of 1 sun, only the initial stage of photo‐segregation is rationalized with the previously established thermodynamic models. However, a progression of the phase segregation is observed that is rationalized by considering long‐lived accumulative photo‐induced material alterations. It is suggested that (additional) photo‐induced defects, possibly halide vacancies and interstitials, need to be considered to fully rationalize light‐induced phase segregation and anticipate the findings to provide crucial insight for the development of more sophisticated models.
Photo‐ and charge‐carrier induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors. It is uniquely apparent during the light‐induced phase‐segregation of mixed iodide/bromide perovskites. Here, its mechanism and cause is investigated by simultaneous in situ X‐ray diffraction and photoluminescence spectroscopy during illumination for the full compositional range of MAPb(BrxI1‐x)3 with x = 0…1.
The quick‐EXAFS (QEXAFS) method adds time resolution to X‐ray absorption spectroscopy (XAS) and allows dynamic structural changes to be followed. A completely new QEXAFS setup consisting of ...monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending‐magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland). The monochromator uses Si(111) and Si(311) channel‐cut crystals mounted on one crystal stage, and remote exchange allows an energy range from 4.0 keV to 32 keV to be covered. The spectral scan range can be electronically adjusted up to several keV to cover multiple absorption edges in one scan. The determination of the Bragg angle close to the position of the crystals allows high‐accuracy measurements. Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50 Hz resulting in a time resolution of 10 ms, using both scan directions of each oscillation period. The carefully developed low‐noise detector system yields high‐quality absorption data. The unique setup allows both state‐of‐the‐art QEXAFS and stable step‐scan operation without the need to exchange whole monochromators. The long‐term stability of the Bragg angle was investigated and absorption spectra of reference materials as well as of a fast chemical reaction demonstrate the overall capabilities of the new setup.
The quick-EXAFS (QEXAFS) method adds time resolution to X-ray absorption spectroscopy (XAS) and allows dynamic structural changes to be followed. A completely new QEXAFS setup consisting of ...monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending-magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland). The monochromator uses Si(111) and Si(311) channel-cut crystals mounted on one crystal stage, and remote exchange allows an energy range from 4.0keV to 32keV to be covered. The spectral scan range can be electronically adjusted up to several keV to cover multiple absorption edges in one scan. The determination of the Bragg angle close to the position of the crystals allows high-accuracy measurements. Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50Hz resulting in a time resolution of 10ms, using both scan directions of each oscillation period. The carefully developed low-noise detector system yields high-quality absorption data. The unique setup allows both state-of-the-art QEXAFS and stable step-scan operation without the need to exchange whole monochromators. The long-term stability of the Bragg angle was investigated and absorption spectra of reference materials as well as of a fast chemical reaction demonstrate the overall capabilities of the new setup.
Secondary phases zinc sulfide/selenide and copper sulfide in Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) thin film samples are investigated by X-ray absorption near edge structure (XANES) analysis at the ...chalcogen K-edges. Because of the formation of secondary phases the composition of the kesterite phase can deviate significantly from the total sample composition. For a large set of non-stoichiometric samples we find that the cation ratios of the kesterite phase never exceed Zn/Sn = 1 even for Zn-rich CZTS and CZTSe, with all excess Zn being contained in secondary phases. For CZTS the cation ratios are found to be additionally constrained by Cu/Sn ≤ 2, which means that Cu-excess always leads to the formation of CuxS secondary phases. These results give clear bounds on the Cu-rich and Zn-rich sides of the single phase region in polycrystalline CZTS/Se thin films.
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