Electron-beam induced degradation in CdTe photovoltaics

Abstract

We used electron beam induced current (EBIC) to measure degradation of CdTe photovoltaic cells. We have observed that: (i) the EBIC signal shows a considerable, continuous degradation depending on the electron-beam current, scan area, energy, and sample treatment; (ii) the characteristic degradation time fluctuates between different spots on the same sample; and (iii) grain boundary regions are the most effective collectors of the electron-beam generated charge carriers. Our phenomenological model relates the observed degradation to defects caused by the electron-beam generated electrons and holes.

Numerical analysis of degradation kinetics in CdTe thin films

“…By attributing the device degradation to the defect increment, one can consider the effect of any particular stress condition on the device parameters since, practically, it has been shown that the defect creation/annihilation is unequal under a specific situation. Electron beam induced current (EBIC) measurements and ALT experiments revealed that the devices significantly degrade under annealing at high temperatures whereas they recover when they are short-circuited under dark rest conditions (Harju et al, 2000;Rimmaudo et al, 2014;Fisher et al, 2013). Thus we consider the device parameter variation under specific stress conditions and track their effect on the carrier/defect density across the cell.…”

“…m = 1 and m = 2 for a linear and quadratic kinetic profile of n, respectively. We consider three similar excess charge carrier profiles based on a quantitative model that was evaluated in connection with EBIC measurements in CdTe PV devices (Harju et al, 2000). From (1) we conclude that the excess charge carriers, generated by any stress condition, cause formation of new defects.…”

“…Then, the new defect density will change the device behavior by changing the device parameters and band diagrams (Makhamn et al, 2010;Myong, 2007). The kinetics of the excess carrier creation is coupled to the excess defect creation via linear or quadratic order profiles (Harju et al, 2000;Shvydka et al, 2003;Stutzmann et al, 1985) …”

“…However, Cu diffusion into a CdTe layer and its accumulation at the junction (CdS/CdTe) impose variations in the carrier density, band diagrams, and doping/defect concentrations (Paudel et al, 2010;Plotnikov et al, 2011). Hence, one can use the latter parameters to numerically analyze and model the degradation rate of the device parameters under a specific stress condition (Harju et al, 2000;Shvydka et al, 2003). In a few available modeling approaches, it was assumed that any stress condition changes the carrier density, which, in turn, changes the defect density and interacts with the atomic structure.…”

Photoassisted physical vapor epitaxial growth of semiconductors: a review of light-induced modifications to growth processes

“…There exists some evidence of such carrier-induced defect formation extending into the slightly-more-covalent II-VI compounds such as CdTe. 52 , 53 , 54 Also, AX defect formation (AX being the acceptor analog of DX centers) in CdTe has been predicted computationally and recently observed. 555657 This process of Schottky or Frenkel pair formation upon carrier capture can be considered as a more pronounced configuration change into adjacent lattice positions compared to the relaxations accompanying transitions to the deeper state in DX centers for III-V and II-VI materials.…”