Semi-transparent perovskite solar cells are highly attractive for a wide range of applications, such as bifacial and tandem solar cells; however, the power conversion efficiency of semi-transparent devices still lags behind due to missing suitable transparent rear electrode or deposition process. Here we report a low-temperature process for efficient semi-transparent planar perovskite solar cells. A hybrid thermal evaporation–spin coating technique is developed to allow the introduction of PCBM in regular device configuration, which facilitates the growth of high-quality absorber, resulting in hysteresis-free devices. We employ high-mobility hydrogenated indium oxide as transparent rear electrode by room-temperature radio-frequency magnetron sputtering, yielding a semi-transparent solar cell with steady-state efficiency of 14.2% along with 72% average transmittance in the near-infrared region. With such semi-transparent devices, we show a substantial power enhancement when operating as bifacial solar cell, and in combination with low-bandgap copper indium gallium diselenide we further demonstrate 20.5% efficiency in four-terminal tandem configuration.

Thin film solar cells with a Cu(In,Ga)Se2 (CIGS) absorber layer achieved efficiencies above 20%. In order to achieve such high performance the absorber layer of the device has to be doped with alkaline material. One possibility to incorporate alkaline material is a post deposition treatment (PDT), where a thin layer of NaF and/or KF is deposited onto the completely grown CIGS layer. In this paper we discuss the effects of PDT with different alkaline elements (Na and K) on the electronic properties of CIGS solar cells. We demonstrate that whereas Na is more effective in increasing the hole concentration in CIGS, K significantly improves the pn-junction quality. The beneficial role of K in improving the PV performance is attributed to reduced recombination at the CdS/CIGS interface, as revealed by temperature dependent J-V measurements, due to a stronger electronically inverted CIGS surface region. Computer simulations with the software SCAPS are used to verify this model. Furthermore, we show that PDT with either KF or NaF has also a distinct influence on other electronic properties of the device such as the position of the N1 signal in admittance spectroscopy and the roll-over of the J-V curve at low temperature. In view of the presented results we conclude that a model based on a secondary diode at the CIGS/Mo interface can best explain these features.

The introduction of a KF post-deposition treatment (KF PDT) of Cu(In,Ga)Se 2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7% for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali-free, flexible plastic substrates, a postdeposition treatment to add Na was already successfully applied. However with the introduction of additional KF under comparable conditions, distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more details the intrinsically different role of both alkali-treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed, and further evidences for the formation of a K-containing layer on the CIGS surface with increased surface reactivity are given. These results shall serve as a basis for the further understanding of the effects of alkali PDT on CIGS and help identifying research needs to achieve even higher efficiencies.

This review summarizes the current status of Cu(In,Ga)(S,Se) 2 (CIGS) thin film solar cell technology with a focus on recent advancements and emerging concepts intended for higher efficiency and novel applications. The recent developments and trends of research in laboratories and industrial achievements communicated within the last years are reviewed, and the major developments linked to alkali post deposition treatment and composition grading in CIGS, surface passivation, buffer, and transparent contact layers are emphasized. Encouraging results have been achieved for CIGS-based tandem solar cells and for improvement in low light device performance. Challenges of technology transfer of lab's record high efficiency cells to average industrial production are obvious from the reported efficiency values. One section is dedicated to development and opportunities offered by flexible and lightweight CIGS modules.

and has yielded 10.3% effi cient solar cells with a V oc defi cit of 0.60 V [ 10 ] or recently, even up to 11.8% measured on active area. [ 11 ] Some commonly reported problems of the DMSO-processed kesterite layers are their high porosity, nonuniformity, and numerous grain boundaries that can lead to undesirable recombination. [ 12 ] Here, we employ a three-stage annealing process under controlled selenium atmosphere in an SiO x coated graphite box to drastically improve the grain size and morphology of the absorber layer. Importantly, the V oc defi cit can be reduced to 0.57 V, which appears to be one of the lowest values reported for kesterite devices. Systematic electrical characterization of absorbers and fi nished solar cells with time-resolved photoluminescence (TRPL), temperature-dependent currentvoltage measurements ( JV-T ), and admittance spectroscopy (AS) are used to identify the reasons of the improved voltage. Figure 1 shows the scanning electron microscopy (SEM) cross sections of four different Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorbers A-D yielding effi ciencies from 6.6% to 10.1% (total cell area of 0.3 cm 2 including metal grid lines). The annealing conditions are varied from uncoated graphite box (sample A,B) to SiO x -coated graphite box (sample C,D) and two-stage temperature gradient (sample A,C) to three-stage temperature gradient (sample B,D); temperature gradients are presented in Figure S1 (Supporting Information). The selenization of sample A was conducted in an uncoated graphite box employing a two-stage temperature gradient, and the absorber layer exhibits a distinct bilayer structure with a thick small-grain bottom layer. [ 13 ] Sample B was selenized in an uncoated graphite box similar to A but employing a three-stage temperature gradient. The SEM cross section shows an improved crystallization and grain size in both upper crust and bottom layer. However, the distinct bilayer structure of the absorber layer remains. The selenization of sample C was conducted in an SiO x -coated graphite box using the two-stage process, and the morphology of the fi lm exhibits a comparably thin upper layer with small grains, but an improved crystallization in the bottom layer in contrast to sample A. Finally, sample D was selenized in the SiO x -coated graphite box with the three-stage temperature gradient and shows an overall improved crystallization with large grains and a signifi cant reduction of the small-grain bottom layer. X-ray diffraction (XRD) pattern in Figure 2 b shows a double kesterite refl ex at 53.4° for all samples, indicating two regions with different S/(S + Se) ratio in the absorber layer. Grazing incidence XRD with varying incident angles confi rms that the region with lower S/(S + Se) ratio coincides with the upper crust and the region with higher S/(S + Se) ratio belongs to the small-grain bottom layer. The refl exes corresponding to the higher S/(S + Se) ratio extenuate with the shift from uncoated On the way towards a marketable and industrially-relevant photo voltaic technology, ke...

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se2 thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (<30 nm) on the faceted, rough surface of polycrystalline chalcogenide thin films. The method, based on selective dissolution of self-assembled and well-defined alkali condensates in water, opens up new research opportunities toward development of thin film solar cells with enhanced efficiency.

Doping the Cu(In,Ga)Se 2 (CIGS) absorber layer with alkaline metals is necessary to process high efficiency solar cells. When growth of CIGS solar cells is performed on soda-lime glass (SLG), the alkaline elements naturally diffuse from the substrate into the absorber layer. On the other hand, when CIGS is grown on alkaline free substrates, the alkaline metals have to be added from another source. In the past, Na was believed to be the most important dopant of the alkaline elements, even though K was also observed to diffuse into CIGS from the SLG. Recently, the beneficial effect of a post deposition treatment with KF was pointed out and enabled the production of a 20.4% CIGS solar cell grown at low substrate temperature (<500 C). However, possible negative effects of the presence or addition of the alkaline impurities during the low temperature growth process were observed for Na, but were not investigated for K so far. In this study, we investigate in detail the role of K on the defect formation in CIGS layers deposited at low temperature on alkaline free polyimide with intentional addition of K during selected time intervals of the CIGS layer growth. By means of admittance spectroscopy and deep level transient spectroscopy, we identify a deep minority carrier trap at around 280 meV below the conduction band E C in CIGS layers grown with K. Its influence on recombination and minority carrier lifetime in the absorber layer is investigated with external quantum efficiency measurements and time-resolved photoluminescence. Furthermore, to support the experimental findings device simulations were performed using the software SCAPS. V

Among the thin‐film solar cell technologies, Cu(In,Ga)Se2‐based solar cells demonstrate the highest efficiencies, where the recent boost in efficiency is triggered by a KF postdeposition treatment (PDT). In this contribution, Cu(In,Ga)Se2‐based solar cells are fabricated using RbF PDTs after absorber layer growth with varying substrate and RbF source temperature. The electronic charge transport properties of the solar cell devices are investigated using temperature‐dependent current–voltage analysis and admittance spectroscopy. To investigate the observed transport barriers, a novel concept based on the differential series resistance is proposed. This approach is supported by simulations of current–voltage curves, which reproduce qualitatively experimental data. Experimentally, two parallel conduction paths are found, which act as barriers with different activation energies and impede the charge carrier transport. Both the thickness and height of these barriers increase with an increasing amount of incorporated Rb and can lead to losses in the fill factor and power conversion efficiency at room temperature. Etching in HCl prior to CdS buffer layer deposition reduces the barrier width and can recover these losses.