Narayan Ramesh

Predicting the Reliability of Polyisobutylene Seal for Photovoltaic Application

Polyisobutylene (PIB) or butyl rubber has been used widely in applications such as construction materials, adhesives and sealants, agricultural chemicals, medical devices, personal care products, and fuel additives. Due to the unique low gas permeability, flexibility, and excellent weathering resistance, PIB or PIB based materials are frequently employed in photovoltaic (PV) industry as sealant to protect the electrical assembly in the package as well as moisture sensitive PV cells from aggressive environments. Long term behavior of the PIB sealant within the operating temperature range of the PV devices thus becomes a critical factor to the reliability of the device. In this paper, an experimental study of the temperature dependent fatigue behavior of a PIB based joint is presented. A finite element model capturing the joint region geometry is developed and an approach to estimate lifetime is proposed.

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.

A study of thermal, voltage, and photoinduced effects on the external quantum efficiency of CuInGaSe 2 (CIGS) photovoltaic devices

The purpose of this study was to assess the performance of CuInGaSe2 (CIGS) solar cells ranging from 6.9-11.2% efficiency to identify means of improving device performance. One of the main drivers for this work was to take an independent look at elevated operating temperatures associated with BIPV products like Dows POWERHOUSE™ shingle system for performance differences as a function of system operating temperature independent of increasing efficiency to help future product definition. For three of the four samples we observed a clear lack of dark-light current-voltage (JV) superposition and correspondingly via external quantum efficiency (EQE) measurements an electrical field dependent carrier collection response in the blue regime. By inference this performance indicates the presence of impurities in the device, likely in the CdS film that could present an opportunity to further improve the CIGS device performance.

Methodology for delivering reliable CIGS based building integrated photovoltaic (BIPV) products

A key challenge currently limiting the wide spread acceptance of Cu(In,Ga)Se2 (CIGS) thin-film photovoltaic technologies in building integrated photovoltaic (BIPV) systems is the demonstration of product reliability in accelerated testing to support rapid product improvement cycles and new product introduction. To augment multi-year & geographically diverse real world performance a priori, one must adopt a creative approach to ensure rapid product introduction of new, highly reliable solar PV systems. Here, we present a synopsis of Dow Solars reliability philosophy that utilizes multi-stress testing, a combination of accelerated and real world conditions, to provide predictive life stress relationships for CIGS based BIPV system level product performance. In addition, the methodology we present here includes a proactive philosophy of identifying and isolating individual reliability failure mechanisms in PV technologies. This philosophy enables significantly shorter development cycles and the obtainment of meaningful product performance feedback. The approach, which is balanced between accelerated testing and field testing data, may be utilized to establish lifetime performance of any PV technology.

Assembly processes for thin film CIGS solar cells: Approaches for improving interconnect repeatability and costs

We have developed new assembly equipment for manufacturing interconnected CIGS solar cell strings that delivers improved control of mechanical tolerances and improved form factor flexibility. The assembly approach is based upon the use of pallets that ensure alignment of solar cells and ribbons during the assembly and curing process. An overview of the assembly equipment is provided. Also discussed are the results of an improved multi-wire interconnect technology that provides a 10% reduction in series resistance relative to standard strings with interconnects provided via electrically conductive adhesive.