Bolko von Roedern

Temperature‐induced changes in the performance of amorphous silicon multi‐junction modules in controlled light‐soaking

We extended our light-soaking tests of amorphous silicon (a-Si) modules, which were previously stabilized in light-soaking experiments conducted at 50°C and 35°C module temperatures for a combined total of 1840 h, to further light exposure at a lower temperature (25°C). We definitively establish that changes in the exposure temperature significantly modify module performance. Furthermore, we have determined that subsequent degradation observed upon light exposure at a lower temperature is completely recovered to start-of-test values within the accuracy of our measurements, as the operating temperature is increased again to 50°C. The behavior is qualitatively similar in all modules tested, suggesting that the phenomena observed are governed by fundamental a-Si properties. This conduct is consistent with the annual variations observed for a-Si modules and systems deployed in the field, at climates that undergo significant temperature variations between winter and summer seasons. Because we have emulated the annual winter–summer oscillations twice, in a controlled-climate, light-soaking chamber and because module performance was always determined from simulator measurements performed at standard conditions, it is hard to defend the notion that the changes observed are either spectral or spectrally induced in nature. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.

Shortfall of defect models for amorphous silicon solar cell performance

It is suggested that the prevailing models for the Staebler–Wronski effect are incorrect because they ignore the effects of charged dangling bonds. The degradation behavior of material parameters such as photoconductivity or midgap defect densities does not allow us to predict either the magnitude or the kinetic behavior of solar cell degradation.

“The role of polycrystalline thin-film PV technologies in competitive PV module markets”

This paper discusses the developments in thin-film PV technologies. It provides an outlook on future commercial module efficiencies achievable based on todays knowledge about champion cell performance. It also provides a relative cost comparison of thin-film and wafer/ribbon based Si PV modules. In 2007, about 65% of the modules produced in the US were thin-film modules when amorphous silicon modules are also considered.

Determination of drift mobility and lifetime for dominant charge carriers in polycrystalline CuInSe2 by photomixing

Drift mobility and lifetime for the dominant charge carriers in polycrystalline CuInSe2 were determined for the first time by a photomixing technique. Evidence for a continuous distribution of localized states in the band gap near the extended states was provided. The temperature dependence of the photoconductive charge transport was found to be determined by multiple trapping processes.

Can the Staebler-Wronski effect account for the long-term performance of a-Si PV arrays?

We suggest a model for the Staebler-Wronski degradation of a-Si based solar cells that can account for long-term performance observations of deployed a-Si photovoltaic modules. The model suggests that the stabilization of the Staebler-Wronski degradation does not occur because an equilibrium between light-induced degradation and thermal or light-induced recovery is reached. Rather, stabilization occurs because the degradation phenomenon itself is self-limiting, i.e., once enough degradation has been introduced, the degradation process diminishes. This model shows that the module performance and the long-term power output of an a-Si PV array depend not only on the operating conditions, but also, on the temperature history of light exposure. This makes it difficult to accurately predict the exact amount of degradation in the field.

How do Buffer Layers Affect Solar Cell Performance and Solar Cell Stability

Buffer layers are commonly used in the optimization of thin-film solar cells. For CuInSe2and CdTe-based solar cells, multilayer transparent conductors (TCOs, e.g., ZnO or SnO2) are generally used in conjunction with a CdS heterojunction layer. Optimum cell performance is usually found when the TCO layer in contact with the CdS is very resistive or almost insulating. In addition to affecting the open-circuit voltage of a cell, it is commonly reported that buffer layers affect stress-induced degradation and transient phenomena in CdTe- and CuInSe2-based solar cells. In amorphous silicon solar cells, light-induced degradation has a recoverable and a nonrecoverable component too, and the details of the mechanism may depend on the p-type contact layer. Because of the similarity of transients and degradation in dissimilar material systems, it is suggested that degradation and recovery are driven by carriers rather than by diffusing atomic species. The question that must be addressed is why, not how, species diffuse and atomic configurations relax differently in the presence of excess carriers. In this paper, I suggest that the operating conditions of a cell can change the carrier transport properties. Often, excess carriers may enhance the conductance in localized regions (“filaments”) and buffer layers; limiting current flow into such filaments may therefore control the rate and amount of degradation (or recovery).

Fast Changes in a-Si:H Solar Cells After Severe Light-Soaking

The fast changes observed in the stabilized state of a-Si:H solar cells and modules at temperatures below 70°C are inconsistent with the commonly accepted picture of “defect annealing.” The fast changes observed in the stabilized state, for example when the temperature is altered, are explained in terms of converting the charge state of the dangling bond defects that are already present in the material. It is suggested that the slow degradation of solar cells arises from the creation of new defects and can be described by fitting stretched exponential curves to the solar cell performance data.

The photovoltaic technology incubator project

The Photovoltaic (PV) Technology Incubator project facilitates a companys transition from developing a PV module or cell prototype to pilot and full-scale manufacturing. The PV Technology Incubator project receives funds from the U.S. Department of Energy (DOE), through the National Renewable Energy Laboratory (NREL). The PV Incubator project targets small businesses that have demonstrated a proof-of-concept/process in a lab, but still need to overcome significant barriers before they can achieve commercialization. After 18 months of funding, a successful project is expected to have a commercially viable product and be capable of approximately 2–3 MW/ year pilot production. It is anticipated that these companies will begin entry into the market by 2011.

Advances in photovoltaics at NREL

This paper discusses the critical strategic research and development issues in the development of next-generation photovoltaic technologies, emphasizing thin-film technologies that are believed to ultimately lead to lower production costs. The critical research and development issues for each technology are identified. An attempt is made to identify the strengths and weaknesses of the different technologies, and to identify opportunities for fundamental research activities suited to advance the introduction of improved photovoltaic modules.This paper discusses the critical strategic research and development issues in the development of next-generation photovoltaic technologies, emphasizing thin-film technologies that are believed to ultimately lead to lower production costs. The critical research and development issues for each technology are identified. An attempt is made to identify the strengths and weaknesses of the different technologies, and to identify opportunities for fundamental research activities suited to advance the introduction of improved photovoltaic modules.

Progress of the PV Technology Incubator project towards an enhanced U.S. manufacturing base

In this paper, we report on the major accomplishments of the U.S. Department of Energys (DOE) Solar Energy Technologies Program (SETP) Photovoltaic (PV) Technology Incubator project. The Incubator project facilitates a companys transition from developing a solar cell or PV module prototype to pilot- and large-scale U.S. manufacturing. The project targets small businesses that have demonstrated proof-of-concept devices or processes in the laboratory. Their success supports U.S. Secretary of Energy Steven Chus SunShot Initiative, which seeks to achieve PV technologies that are cost-competitive without subsidies at large scale with fossil-based energy sources by the end of this decade. The Incubator Project has enhanced U.S. PV manufacturing capacity and created more than 1200 clean energy jobs, resulting in an increase in American economic competitiveness. The investment raised to date by these PV Incubator companies as a result of DOEs