S.H. Demtsu

Quantification of losses in thin-film CdS/CdTe solar cells

Quantification of solar cell losses can identify promising pathways for further cell improvements. This paper expands earlier work and applies it specifically to CdS/CdTe cells. For the analysis we have defined four cells: The Target cell is one that should be possible with current industrial processes. The Production cell is typical of todays production. The Record cell has the highest efficiency (16.5%) reported to date. The Ideal cell has the highest theoretical performance for CdTe. The systematic technique of separating losses, referred to as third level metrics, breaks current, voltage, and fill-factor losses down into their individual loss mechanisms. The losses are expressed both as the deficiency in the specific parameter and as the impact on cell efficiency. The latter allows clear identification of the most significant losses.

Effect of back-contact copper concentration on CdTe cell operation

CdTe solar cells were fabricated with five different concentrations of copper, including zero, used in back-contact formation. Room-temperature J-V curves showed progressive deterioration in fill factor with reduced copper. J/sub SC/ and QE were similar for all Cu-levels. Capacitance measurement suggested enhanced intermixing at the back contact with copper present. Photocurrent mapping was much less uniform for reduced-Cu cells. Elevated-temperature stress induced very little change in J-V when sufficient Cu was used in the contact.

Role of Copper in the Performance of CdS/CdTe Solar Cells

The performance of CdS/CdTe solar cells made with evaporated Cu as a primary back contact was studied through current-voltage (JV) at different intensities, quantum efficiency (QE) under light and voltage bias, capacitance-voltage (CV), and drive-level capacitance profiling (DLCP) measurements. The results show that while modest amounts of Cu enhance cell performance, excessive amounts degrade device quality and reduce performance. The analysis is supported with numerical simulations to reproduce and explain some of the experimental results

Intrinsic stability of thin-film CdS/CdTe modules

Stability of thin-film CdTe module is crucial to the advancement of the technology. Long-term stability studies are carried out under a variety of accelerated test conditions. Accelerated-life-tests (ALT) are performed in the laboratory to accelerate degradation and hence quantify performance change in reasonably short periods of time. ALT studies attempt to simulate the conditions a solar module experiences outdoors during its lifetime, and ALT results are correlated to outdoor test results to estimate module service lifetimes, nameplate parameters, and product warranty. To accelerate degradation, laboratory experiments are often performed at elevated temperature, voltage bias and under continuous illumination. In this study laboratory size cells (0.5 cm2), mini-modules (15 cm × 15 cm) and full-size encapsulated modules (60 cm × 120 cm) that consist of monolithically interconnected cells were subjected to different levels of illumination, elevated temperatures and electrical biases for extended periods of time. Changes in efficiency, fill-factor (FF), open-circuit voltage (Voc) and short-circuit current (Isc) as a function stress time and stress conditions are discussed.