Are the Current Models Helpful to Understanding Staebler-Wronski Degradation?
- PDF / 167,402 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 102 Downloads / 159 Views
A4.41.1
Are the Current Models Helpful to Understanding Staebler-Wronski Degradation? Bolko von Roedern National Renewable Energy Laboratory (NREL) 1617 Cole Blvd., Golden, CO 80401-3393, U.S.A. ABSTRACT This contribution reviews the compatibility of Staebler-Wronski models with experimental data and observations. The review will show that neither the “bond-breaking models” (originally proposed by Dersch and Stutzmann) nor the “defect conversion” models (originally proposed by Adler) can explain all observations on films and/or solar cells. It has been well accepted for some time that experimental stress and recovery phenomena, both on films and devices, always identify both “slow” and “fast” degradation and recovery mechanisms. It is argued that the quintessential understanding of the Staebler-Wronski mechanisms will come from identifying a fundamental physical process that provides a quantitative understanding of the “coupling” between the slow and fast mechanisms.
INTRODUCTION Most work on attempting to model and explain the Staebler-Wronski degradation of amorphous silicon (a-Si:H) materials and devices has sought to explain the observed degradation either in terms of a Si-Si bond-breaking process creating metastable dangling bonds, first proposed by Dersch et al. [1], or as a defect-conversion model, first proposed by Adler [2]. For years it was believed that the neutral dangling bond, whether light-induced or “native,” was the most important factor for controlling material phenomena such as photoconductivity (pc) or device behavior such as solar cell fill factors. Although I pointed out some time ago that such an interpretation was invalid [3], it took another decade to generally accept the lack of such correlation between (neutral) dangling bond densities, photoconductivity, and solar cell fill factors [4]. In 1992 and 1993, several papers studying solar cells reported interesting intensity, bias, and temperature behavior of the light induced degradation phenomena from studying solar cell behavior. First, the work by Yang et al. studying solar cell degradation as a function of applied electrical bias revealed that the degradation is not driven by electron-hole recombination events [5]. This assumption is usually taken for granted in published bond-breaking theories. Second, studies carried out at different light intensities showed that solar cell stabilization was not an equilibrium between degradation and thermal annealing. Rather, an “overshoot” was observed when a cell, stabilized at a lower value after high-intensity light-soaking, was subsequently operated at lower intensity [6, 7]. These and earlier studies [8] clearly suggested that lightsoaking creates a fast, easy to recover degradation and a slow, below 70 oC irrecoverable degradation. To save the assumption that stabilization arose from and was a balance between degradation and annealing, a light-induced annealing mechanism was postulated at this time [9]. Ever since fast and slow effects were reported on both solar cell and material properties,
Data Loading...
