Buried wurtzite structures composed by stacking faults of the {111} planes in zinc-blende and {112} planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of stacking fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co-evaporation of Cu(In,Ga)Se-2-a photovoltaic absorber material showing record efficiencies of up to 22.9% for thin film solar cells a reduction of stacking faults occurs at the transition from a Cu-poor to a Cu-rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X-ray diffraction during annealing of Cu-poor CuInSe2 thin films that stacking faults can be strongly reduced through annealing, without passing through a Cu-rich film composition. We simulate the evolution of the X-ray diffraction stacking fault signal with a simple numerical model of grain growth driven by stacking fault energy and grain boundary curvature. The results support the hypothesis that the stacking fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for stacking fault reduction and could be adapted for polycrystalline thin films with similar morphology.

Because of its high mobility, hydrogen-doped indium oxide (In2O3H) has a high potential as front contact in thin film or Si heterojunction solar cells. Certain growth conditions are required to process high-mobility In2O3:H. In Cu(In,Ga)Se-2 (CIGS) devices, the growth of In2O3:H is influenced by the existing sublayers. Therefore, the intention of the present study is to investigate these influences on In2O3:H, deposited by pulsed direct current magnetron sputtering onto bare glass substrates or Zn(O,S) and ZnO layers, as they are commonly used in GIGS solar cells. The amorphous Zn(O,S) and the crystalline ZnO films were deposited by radio frequency sputtering onto planar glass as well as rough GIGS samples. On the basis of X-ray diffraction and transmission electron microscopy measurements, the structure of the as-grown In2O3:H films was evaluated. X-ray diffraction patterns show amorphous growth on glass and Zn(O,S) and a higher crystallinity of In2O3:H films grown on ZnO layers. The preferred orientation of In2O3:H films changes from (222) to (400) when grown on ZnO layers with small grains. A pronounced crystalline growth leads to a reduction in charge carrier density and electron mobility. This was found for crystalline grown In2O3:H on planar glass as well as on rough GIGS samples in combination with a ZnO layer. A post deposition thermal treatment leads to the crystallization of amorphous phases and reduces strain in crystalline grown films, increasing the electron mobility for all films. However, the electrical properties of the crystalline-grown In2O3:H films did not improve sufficiently.

We study the sequential fabrication of Cu(In,Ga)Se-2 (CIGSe) absorber layers by using an atmospheric pressure selenization with a process duration of only a few minutes and the utilization of elemental selenium vapor from independent Se sources. This technology could proof to be an industrially relevant technology for the fabrication of thin-film solar cells. Controlling the amount of Se provided during the selenization of metal precursors is shown to be an effective measure to adjust the Ga in-depth distribution. A reduced Se supply for CIGSe formation leads to a more homogeneous Ga distribution within the absorber. The underlying growth dynamics is investigated by interrupting the selenization at different times. At first, CIGSe formation occurs in accordance with previously suggested growth paths and Ga segregates at the Mo back contact. Between 520 and 580 degrees C, the growth dynamics differs distinctly, and In and Ga distribute far more uniformly within the absorber depth. We also studied the impact of the precursor architecture. The best performing precursor in terms of efficiency of the respective solar cells was a multilayer with 22 In/CuGa/In triple layers. Simple bilayers stacks lead to films of higher roughness and correlated shunting. By optimizing the precursor architecture and the Ga in-depth distribution in the CIGSe layer, a conversion efficiency of up to 15.5% (active area) could be achieved. To our knowledge, this is the highest reported efficiency for sulfur free CIGSe-based solar cells utilizing fast (few minutes) atmospheric processes and elemental Se vapor. Copyright (C) 2017 John Wiley & Sons, Ltd.

A remarkable discrepancy between the optically active bandgap of Cu(In,Ga)(S,Se)(2) absorber layers for thin film solar cells and the minimum bandgap as determined via elemental depth profiling has been observed in this study. This behavior occurs in absorbers sequentially grown by sulfurization after selenization of metal precursors and is demonstrated and explored in the following, using glow discharge optical emission spectroscopy, external quantum efficiency and Raman scattering. Furthermore this mismatch is explained by investigating the microscopic elemental distributions using transmission electron microscopy. It turns out, that sulfur is-on a microscopic scale-inhomogeneously distributed in the bulk of the absorber and solely present in areas near the absorber surface itself and at inner surfaces, e.g. in voids in the bulk. (C) 2017 Elsevier B.V. All rights reserved.

Sequential Cu(In,Ga)Se2 fabrication with a thermally activated reaction of Cu-In-Ga metal precursor layers in chalcogen atmosphere is an industrially attractive route for preparation of Cu(In,Ga)Se2 absorber based thin film solar cells. Recent results show that controlling the selenium supply during rapid thermal processing has a huge impact on absorber growth. Especially a two stage process applying a first annealing step with or without Se at temperatures up to 400°C was shown to have a positive effect on the elemental in-depth distribution. However, during this annealing, lateral phase separation, dewetting and coarsening may occur in the metal phase, leading to lateral non-uniformity of the absorber. In this study we show how the dewetting can be strongly decreased by adjusting the precursor architecture, applying faster heating rates and NaF addition on top of a precursor. In contrast, NaF deposited underneath the precursor increases the dewetting effect. Further we show that lateral phase separation during annealing increases with temperature and leads to phase domain sizes of several micrometers at 580°C.

Atomic layer deposition of Zn(O,S) is an attractive dry and Cd-free process for the preparation of buffer layers for chalcopyrite solar modules. As we previously reported, excellent cell and module efficiencies were achieved using absorbers from industrial pilot production. These absorbers were grown using a selenization/sulfurization process. In this contribution we report on the interface engineering required to adapt the process to sulfur-free multi source evaporated absorbers. Different approaches to a local sulfur enrichment at the heterojunction have been studied by using surface analysis (XPS) and scanning transmission electron microscopy. We correlate the microstructure and element distribution at the interface with device properties obtained by electronic characterization. The optimized completely dry process yields cell efficiencies >16% and 30 × 30 cm2 minimodule efficiencies of up to 13.9% on industrial substrates. Any degradation observed in the dry heat stress test is fully reversible after light soaking.

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se) 2 thin films for high‐efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep‐defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect‐free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se) 2 thin films with two‐dimensional device simulations suggest that these defects are one origin of the reduced open‐circuit voltage of the photovoltaic devices. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

We investigated hydrogenated nanocrystalline silicon (nc-Si:H) films as doped emitter layers for silicon heterojunction solar cells. Firstly, we focused on the effect of the nc-Si:H deposition conditions and film growth on the intrinsic hydrogenated amorphous silicon passivation layer ((i)a-Si:H) underneath. Three different p-doped emitters were compared: nc-Si:H, nc-SiOx:H, and a-Si:H. We found that the nc-Si:H and nc-SiOx:H growth enhances the passivation of the epitaxy-free (i)a-Si:H layer, yielding implied open circuit voltages above 730mV. Secondly, for (p)nc-Si:H emitters, we observed a trade-off between fill factor (FF) and open circuit voltage (V-oc) by using two types of (i)a-Si:H films. A slight epitaxy of the (i)layer seems to promote the rapid nucleation of nc-Si:H, thereby positively affecting the FF (79.5%) and series resistance but reducing V-oc (670mV). Contrarily, on well-passivating (i)a-Si:H the nc-Si:H nucleation is more difficult resulting in S-shaped I-V curves, presumably due to low built-in voltage and a poor emitter/TCO contact. To circumvent this dilemma, a CO2 plasma treatment is used to oxidize the a-Si:H surface before the nc-Si:H emitter deposition thereby enhancing nucleation. Accordingly, a FF of 74.5% with V-oc of 727mV is reached in the best device, yielding a conversion efficiency of 21%. HR-TEM micrograph of the front layer stack of the solar cell. The image shows a region close to the valley between two pyramids. From bottom to top: c-Si substrate, (i)a-Si:H passivation layer showing epitaxially grown regions, (p)nc-Si:H emitter layer, and In2O3:Sn (ITO). Yellow lines highlight layers and individual crystals. Silicon zone axis orientation is < 101 >.

Polycrystalline thin-film solar cell absorbers exhibit complex structure-property relationships. Their microstructure is affected by sophisticated synthesis processes and influences the photovoltaic performance. In Cu(In,Ga)Se-2 - currently presenting the highest efficiencies of all thin film absorber materials - the evolution of the [Cu]/([In] + [Ga]) ratio prior to the final Cu-poor composition is crucial for the efficiency of layers deposited by co-evaporation. A precise understanding of the effect of Cu on the microstructure is necessary to simplify the deposition process and bridge the gap between champion cell and module performance. In the present investigation, domain size growth, stacking fault annihilation and strain relaxation during low-temperature CuInSe2 co-evaporation are shown to correlate with Cu deposition, implying a driving force for grain growth induced by diffusion.Synchrotron based x-ray diffraction and fluorescence permit to study the microstructural evolution of the thin film in situ during Cu-Se deposition in a specially adapted process chamber. Repeated interruptions of the Cu evaporation reveal the dependency of the microstructure evolution on Cu deposition. Diffusion-induced grain boundary migration (DIGM) - hitherto not considered in chalcopyrite thin film deposition - is proposed as a mechanism by which Cu deposition drives grain growth at temperatures considerably below the threshold for thermal activation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Small (∼5 nm), Cu- and Sn-rich nanoparticles play a key role in initiating the growth of micrometer-sized Cu2ZnSn(S,Se)4 grains.

We report the development of Cd-free buffers by atomic layer deposition for chalcopyrite-based solar cells. Zn(O,S) buffer layers were prepared by atomic layer deposition on sequentially grown Cu(In,Ga)(Se,S)(2) absorbers from Bosch Solar CISTech GmbH. An externally certified efficiency of 16.1% together with an open circuit voltage of 612mV were achieved on laboratory scale devices. Stability tests show that the behavior of the ALD-Zn(O,S)-buffered devices can be characterized as stable only showing a minor drift of the open circuit voltage and the fill factor. Copyright (c) 2015 John Wiley & Sons, Ltd.

Thin-film solar cells based on Cu(In,Ga)Se2 (CIGSe) reach high power-conversion efficiencies in spite of large dislocation densities of up to 1010–1011 cm−2. The present work gives insight into the structural and compositional properties of dislocations in CIGSe thin films, which are embedded in a complete solar cell stack. These properties are related to the average electrical potential distributions obtained by means of inline electron holography. At a part of the dislocations studied, the average electrostatic potential shows local minima, all with depths of about −1.4 V. The measured average electrostatic potential distributions were modeled in order to reveal possible influences from strain fields, excess charge, and also compositional changes at the dislocation core. Cu depletion around the dislocation core, as evidenced by atom-probe tomography, explains best the measured potential wells. Their influences of the strain field around the dislocation core and of excess charge at the dislocation core are small. A structural model of dislocations in CIGSe thin films is provided which includes a Cu-depleted region around the dislocation core and gives a possible explanation for why decent photovoltaic performances are possible in the presence of rather large dislocation densities.

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We present a-Si:H/mu c-Si:H tandem solar cells on laser textured ZnO:Al front contact layers. Direct pulsed laser interference patterning (DLIP) was used for writing arrays of one-dimensional micro gratings of submicron period into ZnO:Al films. The laser texture provides good light trapping which is indicated by an increase in short-circuit current density of 20% of the bottom cell limited device compared to cells on planar ZnO:Al. The open-circuit voltage of the cells on laser textured ZnO:Al is almost the same as for cells on planar substrates, indicating excellent growth conditions for amorphous and microcrystalline silicon on the U-shaped grating grooves. DLIP is a simple, single step and industrially applicable method for large area periodic texturing of ZnO:Al thin films. ((c) 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)