Thin-film solar cells based on polycrystalline absorbers have reached very high conversion efficiencies of up to 23-25%. In order to elucidate the limiting factors that need to be overcome for even higher efficiency levels, it is essential to investigate microscopic origins of loss mechanisms in these devices. In the present work, a high efficiency (21% without anti-reflection coating) copper indium gallium diselenide (CIGSe) solar cell is characterized by means of a correlative microscopy approach and corroborated by means of photoluminescence spectroscopy. The values obtained by the experimental characterization are used as input parameters for two-dimensional device simulations, for which a real microstructure was used. It can be shown that electrostatic potential and lifetime fluctuations exhibit no substantial impact on the device performance. In contrast, nonradiative recombination at random grain boundaries can be identified as a significant loss mechanism for CIGSe solar cells, even for devices at a very high performance level.

A new method for enhancing the charge separation and photo-electrochemical stability of CuBi 2 O 4 photoelectrodes by sequentially depositing Bi 2 O 3 and CuO layers on fluorine-doped tin oxide substrates with pulsed laser deposition (PLD), followed by rapid thermal processing (RTP), resulting in phasepure, highly crystalline films after 10 min at 650 °C, is reported. Conventional furnace annealing of similar films for 72 h at 500 °C do not result in phasepure CuBi 2 O 4 . The combined PLD and RTP approach allow excellent control of the Bi:Cu stoichiometry and results in photoelectrodes with superior electronic properties compared to photoelectrodes fabricated through spray pyrolysis. The low photocurrents of the CuBi 2 O 4 photocathodes fabricated through PLD/RTP in this study are primarily attributed to their low specific surface area, lack of CuO impurities, and limited, slow charge transport in the undoped films. Bare (without protection layers) CuBi 2 O 4 photoelectrodes made with PLD/RTP shows a photocurrent decrease of only 26% after 5 h, which represents the highest stability reported to date for this material. The PLD/RTP fabrication approach offers new possibilities of fabricating complex metal oxides photoelectrodes with a high degree of crystallinity and good electronic properties at higher temperatures than the thermal stability of glass-based transparent conductive substrates would allow.

Achieving high power conversion efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells grown at low temperature is challenging because of insufficient thermal energy for grain growth and defect annihilation, resulting in poor crystallinity, higher defect concentration, and degraded device performance. Herein, the possibilities for high‐performing devices produced at very low temperatures (≤450 °C) are explored. By alloying CIGS with Ag by the precursor layer method, (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells grown at about 450 °C reach an efficiency of 20.1%. Only a small efficiency degradation (0.5% and 1.6% absolute) is observed for ACIGS absorbers deposited at 60 and 110 °C lower substrate temperature. CIGS devices exhibit a stronger efficiency degradation, driven by a decrease in the open‐circuit voltage (VOC). The root cause of the VOC difference between ACIGS and CIGS devices is investigated by advanced characterization techniques, which show improved morphology, reduced tail states, and higher doping density in ACIGS absorbers. The proposed approach offers several benefits in view of depositions on temperature‐sensitive substrates. Increased Cu diffusion promoted by Ag allows end‐point detection in the three‐stage process at the substrate temperatures below 300 °C. The modified process requires minimal modification of existing processes and equipment and shows the potential for the use of different flexible substrates and device architectures.

Thin‐film solar cells based on Cu(In,Ga)Se2 (CIGSe) absorber layers reach conversion efficiencies of above 20%. One key to this success is the incorporation of alkali metals, such as Na and K, into the surface and the volume of the CIGSe thin film. The present work discusses the impact of Na and K on the grain‐boundary (GB) properties in CIGSe thin films, i.e., on the barriers for charge carriers, Φb, and on the recombination velocities at the GBs, sGB. First, the physics connected with these two quantities as well as their impact on the device performance are revised, and then the values for the barrier heights and recombination velocities are provided from the literature. The sGB values are measured by means of a cathodoluminescence analysis of Na‐/K‐free CIGSe layers as well as on CIGSe layers on Mo/sapphire substrates, which are submitted to only NaF or only KF postdeposition treatments. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 103–104 cm s−1, irrespective of whether CIGSe thin films are Na‐/K‐free or Na‐/K‐containing.

In Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open‐circuit voltage (Voc). In the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and Voc losses in solar cells with various [Ga] in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron‐beam‐induced current measurements were employed to study the local difference in the width of the space‐charge region (wSCR). It is shown that the amplitude of fluctuations in the wSCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy‐dispersive X‐ray spectroscopy and cathodoluminescence measurements reveal that band‐gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.