Metastability and reliability of CdTe solar cells

Abstract

Thin-film modules of all technologies often suffer from performance degradation over time. Some of the performance changes are reversible and some are not, which makes deployment, testing, and energy-yield prediction more challenging. Manufacturers devote significant empirical efforts to study these phenomena and to improve semiconductor device stability. Still, understanding the underlying reasons of these instabilities remains clouded due to the lack of ability to characterize materials at atomistic levels and the lack of interpretation from the most fundamental material science. The most commonly alleged causes of metastability in CdTe devices, such as ‘migration of Cu’, have been investigated rigorously over the past fifteen years. Still, the discussion often ended prematurely with stating observed correlations between stress conditions and changes in atomic profiles of impurities or CV doping concentration. Multiple hypotheses suggesting degradation of CdTe solar cell devices due to interaction and evolution of point defects and complexes were proposed, and none of them received strong theoretical or experimental confirmation. It should be noted that atomic impurity profiles in CdTe provide very little intelligence on active doping concentrations. The same elements could form different energy states, which could be either donors or acceptors, depending on their position in crystalline lattice. Defects interact with other extrinsic and intrinsic defects; for example, changing the state of an impurity from an interstitial donor to a substitutional acceptor often is accompanied by generation of a compensating intrinsic interstitial donor defect. Moreover, all defects, intrinsic and extrinsic, interact with the electrical potential and free carriers so that charged defects may drift in the electric field and the local electrical potential affects the formation energy of the point defects. Such complexity of interactions in CdTe makes understanding of temporal changes in device performance even more challenging and a closed solution that can treat the entire system and its interactions is required.

Wide Band Gap Chalcogenide Semiconductors

“…Successful demonstration of such contacts includes Cu/Au bi-layers, 446 Cu-containing graphite paste, 434 Cu 2 Te, 447 CuCl 2 and Cu-doped ZnTe 448 . However, Cu may also be responsible for degradation, 449 so prospective area of research is finding a p-type non-Cu buffer layer that makes an Ohmic contact with CdTe.…”

Beyond 30% Conversion Efficiency in Silicon Solar Cells: A Numerical Demonstration

“…Moreover, thick silicon solar cells suffer from unavoidable losses in power conversion efficiency due to non-radiative recombination of photo-generated charge carriers during their relatively long path to electrical contacts at the extremities of the cell. These deficiencies have sparked broad interest in a variety of thin-film solar materials including CdTe , GaAs , perovskites and various polymers 1–3 . Due to the indirect band gap nature of c – Si , thin-film silicon has not been considered a viable competitor to these alternative materials.…”

Complex Investigation of High Efficiency and Reliable Heterojunction Solar Cell Based on an Improved Cu2O Absorber Layer

“…The HASS test also establishes the probable rate of solar cell failure within the operation temperature range of 50-100 °C (Figure 17b). It is remarked that the The degradation degree of the analyzed solar cell can be characterized by the temperature influence on the device [34][35][36] and is obtained for two cases: (1) under stress conditions (black curve) and (2) under standard conditions (blue curve) (Figure 16b).…”