Rebekah K. Feist

Examination of lifetime-limiting failure mechanisms in CIGSS-based PV minimodules under environmental stress

In our study, Shell Solar Industries (SSI) minimodules were subjected to dry heat (85°C), damp heat (85°C/100% RH), and anaerobic/aerobic 85°C water baths. After 168 hrs exposure to moisture-containing environments, the SSI power generation decreased by over 50% of that of the original state. Analytical characterization performed before and after the exposure identified degradation of the Al:ZnO and Mo layers as likely device failure routes. To elucidate the observed degradation mechanism, individual Al:ZnO and Mo films were sputtered onto borosilicate glass and exposed to both 85°C/100% RH and a room temperature water bath. After 24 hrs the resistivity and optical transmission of the Al:ZnO films increased significantly following both exposure methods. XPS surface analysis of the films revealed changes in the O to Zn bonding ratio suggesting film hydration may have occurred. In addition, after 48 hours by both exposure methods the Mo films corroded, and the film resistivities increased. Our results show Al:ZnO layer degradation limits the lifetime of CIGSS based PV devices, whereas Mo degradation is considered a non-lifetime-limiting failure.

Structure optimization for a high efficiency CIGS solar cell

This paper uses numerical simulation to study the effects of Ga concentration profile on the performance of CuIn1−xGaxSe2 (CIGS) solar cell, including the effects of acceptor type Cu antisite defects whose concentration depends on Ga composition. These defects are the dominant deep traps in the CIGS material system. The concentration and spatial distribution of these traps affect the solar cell performance. The trap density model used in this work follows experimental reports in the literature. The trap concentration is 4.3×1015 cm−3 for CIS (x=0) and decreases to 1.2×1014 cm−3 when the Ga mole fraction, x, reaches 0.24. The trap concentration increases exponentially above x=0.30. Applying this model to solar cells with uniform composition absorber layer predicts that the power conversion efficiency reaches a maximum value of 14.6%, at x=0.24 and decreases with increasing Ga content above x=0.30, in good agreement with experimental results. When this model is used to simulate a solar cell where the Ga composition in the absorber layer is graded, the electric field produced by compositional grading improves the efficiency because of the reduced recombination rate. However compositions where x is higher than 0.45 lead to a drop in performance due to the high trap density and shorter lifetime. Both grading from the CdS/CIGS interface (forward grading), and back grading where the Ga concentration increases from the junction into the CIGS film were studied. In forward grading, the maximum efficiency is achieved when the Ga concentration is graded such that x decreases from 0.35 at the surface to 0.24 at 0.4 µm into the CIGS film. In back grading, the maximum efficiency is achieved when x increases from 0.45 at the surface to 0.5 at 0.4 µm into the CIGS film.

Further investigation of the lifetime-limiting failure mechanisms of CIGSS-based minimodules under environmental stress

The lifetime-limiting failure mechanisms of CuInGaSSe (CIGSS) solar devices made by Shell Solar Industries (SSI) were investigated. In our study, SSI minimodules were exposed to dry-heat 85°C, damp-heat 85°C/85% RH and aerobic and anaerobic room temperature and 85°C water baths. After 200 hours exposure to moisture-containing environments, the average device performance decreased by more than 30% that of the initial state. The observed degradation was primarily due to losses in short circuit current density (Jsc) and fill factor (FF). The as-received device layers were relatively dense and free of voids. Interestingly, unreacted Cu-Ga particulates were found at the Mo-CIGSS interface. Corresponding with these particulates, CIS-rich defects were found within the bulk CIGSS material. Post-environmental weathering, Kirkendall-like voids were found in the Al:ZnO and CdS layers of these devices. Additionally, ToF-SIMS analysis revealed the Cu-Ga/CIS defects were enriched in Na and O. Our results indicate that in addition to moisture-induced failure of the window layers, the unreacted Cu-Ga particulates and the corresponding CIS-defects may facilitate moisture and/or oxygen-induced failure of these devices.

Product Reliability and Thin-Film Photovoltaics

Despite significant growth in photovoltaics (PV) over the last few years, only approximately 1.07 billion kWhr of electricity is estimated to have been generated from PV in the US during 2008, or 0.27% of total electrical generation. PV market penetration is set for a paradigm shift, as fluctuating hydrocarbon prices and an acknowledgement of the environmental impacts associated with their use, combined with breakthrough new PV technologies, such as thin-film and BIPV, are driving the cost of energy generated with PV to parity or cost advantage versus more traditional forms of energy generation. In addition to reaching cost parity with grid supplied power, a key to the long-term success of PV as a viable energy alternative is the reliability of systems in the field. New technologies may or may not have the same failure modes as previous technologies. Reliability testing and product lifetime issues continue to be one of the key bottlenecks in the rapid commercialization of PV technologies today. In this paper, we highlight the critical need for moving away from relying on traditional qualification and safety tests as a measure of reliability and focus instead on designing for reliability and its integration into the product development process. A drive towards quantitative predictive accelerated testing is emphasized and an industrial collaboration model addressing reliability challenges is proposed.

Tin dioxide as an alternative window layer for improving the damp-heat stability of copper indium gallium diselenide solar cells

The authors demonstrate a new copper indium gallium diselenide solar cell architecture by replacing the ZnO in the traditional design with SnO2. The open circuit voltages and efficiencies of the solar cells made with ZnO and SnO2 were the same indicating favorable band alignment. The solar cells made with SnO2 showed significantly better damp-heat stability than those made with ZnO. The efficiency of solar cells made with SnO2 decreased less than 5% after 120 h at 85 °C and 85% relative humidity while the efficiency of solar cells made with ZnO declined by more than 70%.

High throughput methodology for evaluation of the moisture barrier performance of thin films for PV applications

We describe a high throughput methodology for evaluation of moisture barrier performance of thin films based on the change in optical density of Al/glass substrates during exposure to high temperature/humidity conditions. This approach has enabled a comparative analysis of hundreds of single and multilayer barrier films and has provided predictive models to identify key input variables that affect moisture barrier performance as well as candidates for protection of thin film solar cells. The data is also utilized to identify different degradation modes that can be correlated with film attributes. The methodology has proved to be valuable in other aspects of thin film lifetime studies, such as the evaluation of transparent conductive oxide material properties as window layers in solar cell applications.

Exploration of binary & ternary photosensitive thin film silver selenides: Prediction, preparation, and characterization

The solar power industry is enjoying rapid growth, but challenges remain to produce new photoactive materials with the proper balance of cost and efficiency. Much of the fundamental science concerning alternative materials has not yet been elucidated. Using ab initio density functional theory to predict band structure and densities of state for 3-D unit cells, four binary and ternary silver selenides have been identified as prime candidates for use as an absorber layer in photovoltaic cells. Thin films of these silver selenides were grown, and morphology, stoichiometry, and structure were analyzed using SEM, EDS, and XRD. Optoelectronic characterization of the films was performed using UV-Vis, Hall Effect, and photoluminescence measurements. These data were used to judge the quality of the materials and to confirm the accuracy of the predictive capability of calculations.

Development of a high-pressure CdS sputtering process for improved efficiency in CIGS-based photovoltaic devices

A new sputtered process for cadmium sulfide (CdS) has been developed that boosted efficiency of a PV device baseline (with single-stage, coevaporated CIGS) by ~ 2.7% (absolute) to an average of ~10%. The process is highly scalable to a roll-to-roll environment, with the maximum efficiency effect occurring at 0.1 mbar (~75 mTorr) sputtering pressure. The efficiency improvement is thought to be due to two main factors: composition and defects. The CdS film deposited at high pressure (HP) contains more oxygen (primarily as CdO) than one deposited at typical pressures, with oxygen content higher towards the CdS-CIGS interface. The HP process produces an interface with less CdS-CIGS intermixing, which results in a junction with ~ 5x fewer defects as measured by admittance spectroscopy. The performance improvement due to HP CdS occurs even with a very thin CdS layer (<;15 nm), thus greatly reducing the total amount of cadmium contained in the cells.

Comparison of solar cell device thermal degradation and low-irradiance performance

The thermal degradation, low-irradiance performance, IV and EQE characteristics of CuInGaSe2 (CIGS), c-Si, and GaAs photovoltaic devices are presented. Typically thin-film polycrystalline materials are hypothesized to be advantaged over monocrystalline materials due to their lower thermal degradation coefficient and improved low-irradiance performance. In this work the performance of these different solar cell materials was evaluated with the intent being to determine if these hypothesized performance distinctions exist. Such differences could indicate that solar cells exhibit optimum performance in specific climates.

Control of CIGS roughness by initial selenization temperature

When formed by coevaporation, copper indium gallium diselenide (CIGS) films are typically more smooth than those formed by a two-step, precursor-selenization process. While some amount of roughness is desirable for minimizing reflection, extreme roughness or very sharp features can create a challenge in sputtering uniform, thin, transparent conductive oxide layers on top of the cell. NuvoSun, Inc. has determined that the surface roughness of the CIGS layer can be controlled by the initial temperature at which the CIGS precursor (PC) film is first exposed to a selenium flux during selenization. The resulting CIGS films are similar in roughness to coevaporated CIGS films, and the TCO layers on these smoother devices have fewer cracks and defects.