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Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se2 (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.

In this paper, we studied the effect of rubidium fluoride (RbF) post-deposition treatment (PDT) on the properties of Cu(In,Ga)Se2 (CIGS) solar cells. Specifically, the recombination mechanisms were analyzed by a series of characterizations including thermal and optical defect spectroscopies, temperature dependent current density-voltage measurements, and time resolved photoluminescence. It was found that the main effect of RbF PDT on the solar cell was an increase of the open circuit-voltage, Voc, by 30 mV due to a decrease of the values of the diode quality factor and reverse saturation current. Recombination mechanisms were identified as being in the CIGS space charge region, likely at the grain boundaries and near the CIGS surface. Breakdown of contributions to the Voc increase showed that part of it is due to an increase of the majority carrier concentration (16 mV) and another to the increase in the minority carrier lifetime (1 mV). The latest is mostly due to a reduction in the EV +0.99 eV deep-level trap density. An additional CIGS surface modification (contributing 13 mV), observed by the secondary ion mass spectrometry, is essential to explain the full change in Voc.

Alkali ions are key to improving the performance of polycrystalline Cu(In,Ga)Se2 (CIGS) solar cells. Typically, a soda–lime glass substrate acts as an intrinsic source for many alkali ions. For these ions to reach the CIGS layer, diffusion through a metallic back contact is necessary. Typically deposited via sputtering, the morphology of this metallic back contact is dependent on multiple deposition parameters, including substrate temperature. By preparing films with varying deposition parameters, and utilizing both, multiple material characterizations (X-ray diffraction, scanning electron microscopy, and secondary ion mass spectrometry) and numerical modeling, we demonstrate here that effective paths of diffusion are just as important as diffusion rate for the alkali ions. As the substrate temperature increases, the mechanism that hinders the ability for alkali ions to diffuse switches effectively from diffusion rate to effective path of diffusion. It is shown that the lack of viable diffusion paths at substrate temperatures above 100 °C becomes the dominant factor for the transport of alkali ions.

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities (NT) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration (NA) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage (VOC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in VOC. A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

In this paper, the concept of using solid oxide photoelectrochemical cells (SOPCs) to split water at high temperature using visible sunlight was experimentally investigated, in order to search for the appropriate semiconductor materials. High temperature photoelectrochemical water splitting physically broadens the selection of potential applicable semiconductor materials and enables more visible sunlight absorption. This newly conceived concept provides a unique pathway for solar hydrogen production, compared to conventional photoelectrochemical cells (PECs) that require aqueous environments, though experimental validation is needed. The main focus of this study was on the fabrication and evaluation of high temperature diodes suitable for use as the photocatalytic/oxygen electrodes of SOPCs. The rectifying characteristics of TiO2 based diodes, including Ag/TiO2, LSM/TiO2, LSC/TiO2, and LSCF/TiO2, were investigated at elevated temperatures. Among those diodes, only LSM/TiO2 demonstrated acceptable rectifying properties up to 450degC, indicating that such configuration may be applicable to the proposed SOPC. The results also excluded Schottky diodes from the candidates due to their rapidly degrading rectifying behaviors at elevated temperatures. Candidate semiconductors for the photovoltaic (PV) part of the cell and electrolytes for the electrolysis part were also identified and summarized in this paper for future investigations.

In situ and ex situ characterization were performed to assess the effects of potassium fluoride postdeposition treatment (KF PDT) on Cu(In,Ga)Se2 (CIGS) films and solar cells. Real-time spectroscopic ellipsometry enabled the in situ observation of the modification of the CIGS dielectric function and surface roughness thickness during KF PDT. In addition, deep-level transient spectroscopies (DLTS) and deep-level optical spectroscopies and scanning-DLTS enabled the identification of the nature and location of trap levels. Two main traps were present in both films near Ev + 0.56/0.61 eV and Ev + 0.99 eV, but with clearly lower concentrations for the latter in the KF PDT film. Finally, devices were fabricated, and higher efficiencies were found for the KF PDT devices. SCAPS modeling indicates that the traps identified by deep-level spectroscopies are in part responsible for these changes.

Alternative deposition methods and materials are of interest for the fabrication of thin film solar cells since they offer potential enhancements for either low cost, high speed or high efficiency but also because they can help in better understanding the underlying physical and chemical processes that could lead to the next generation of solar cells. In this study, we will present new results on the deposition of ZnS by atomic layer deposition (ALD) as an alternate to CdS deposited by chemical bath deposition. More specifically, in-situ real time and ex-situ measurements by spectroscopic ellipsometry will be performed, allowing for the analysis of the growth processes as a function of deposition parameters. These measurements also allow for a parameterization of the dielectric functions of ZnS and the evolution of its grain size and band gap as a function of thickness. These measurements will then be correlated with ex-situ measurements, such as XRD, AFM and T&R, which is expected to validate the model used for spectroscopic ellipsometry data analysis.

In view of the large-scale utilization of Cu(In,Ga)Se2 (CIGS) solar cells for photovoltaic application, it is of interest not only to enhance the conversion efficiency but also to reduce the thickness of the CIGS absorber layer in order to reduce the cost and improve the solar cell manufacturing throughput. In situ and real-time spectroscopic ellipsometry (RTSE) has been used conjointly with ex situ characterizations to understand the properties of ultrathin CIGS films. This enables monitoring the growth process, analyzing the optical properties of the CIGS films during deposition, and extracting composition, film thickness, grain size, and surface roughness which can be corroborated with ex situ measurements. The fabricated devices were characterized using current voltage and quantum efficiency measurements and modeled using a 1-dimensional solar cell device simulator. An analysis of the diode parameters indicates that the efficiency of the thinnest cells was restricted not only by limited light absorption, as expected, but also by a low fill factor and open-circuit voltage, explained by an increased series resistance, reverse saturation current, and diode quality factor, associated with an increased trap density.

In this paper, the concept of using solid oxide photoelectrochemical cells (SOPCs) to split water at high temperature using visible sunlight was experimentally investigated, in order to search for the appropriate semiconductor materials. High temperature photoelectrochemical water splitting physically broadens the selection of potential applicable semiconductor materials and enables more visible sunlight absorption. This newly conceived concept provides a unique pathway for solar hydrogen production, compared to conventional photoelectrochemical cells (PECs) that require aqueous environments, though experimental validation is needed. The main focus of this study was on the fabrication and evaluation of high temperature diodes suitable for use as the photocatalytic/oxygen electrodes of SOPCs. The rectifying characteristics of TiO2based diodes, including Ag/TiO2, LSM/TiO2, LSC/TiO2, and LSCF/TiO2, were investigated at elevated temperatures. Among those diodes, only LSM/TiO2demonstrated acceptable rectifying properties up to 450°C, indicating that such configuration may be applicable to the proposed SOPC. The results also excluded Schottky diodes from the candidates due to their rapidly degrading rectifying behaviors at elevated temperatures. Candidate semiconductors for the photovoltaic (PV) part of the cell and electrolytes for the electrolysis part were also identified and summarized in this paper for future investigations.

Alternative deposition methods and materials are of interest for the fabrication of thin film solar cells since they offer potential enhancements for either low cost, high speed or high efficiency but also because they can help in better understanding the underlying physical and chemical processes that could lead to the next generation of solar cells. In this study, we will present new results on the deposition of ZnS by atomic layer deposition (ALD) as an alternate to CdS deposited by chemical bath deposition. More specifically,in-situreal time and ex-situmeasurements by spectroscopic ellipsometry will be performed, allowing for the analysis of the growth processes as a function of deposition parameters. These measurements also allow for a parameterization of the dielectric functions of ZnS and the evolution of its grain size and band gap as a function of thickness. These measurements will then be correlated with ex-situmeasurements, such as XRD, AFM and T&R, which is expected to validate the model used for spectroscopic ellipsometry data analysis.