Staebler Wronski effect

Amorphous Silicon. Amorphous alloys made of thin films of hydrogenated siUcon (a-Si H) are an alternative to crystalline siUcon devices. Amorphous siUcon ahoy devices have demonstrated smah-area laboratory device efficiencies above 13%, but a-Si H materials exhibit an inherent dynamic effect cahed the Staebler-Wronski effect in which electron—hole recombination, via photogeneration or junction currents, creates electricahy active defects that reduce the light-to-electricity efficiency of a-Si H devices. Quasi-steady-state efficiencies are typicahy reached outdoors after a few weeks of exposure as photoinduced defect generation is balanced by thermally activated defect annihilation. Commercial single-junction devices have initial efficiencies of ca 7.5%, photoinduced losses of ca 20 rel %, and stabilized efficiencies of ca 6%. These stabilized efficiencies are approximately half those of commercial crystalline shicon PV modules. In the future, initial module efficiencies up to 12.5% and photoinduced losses of ca 10 rel % are projected, suggesting stabilized module aperture-area efficiencies above 11%. 

Efficiency is still low and a-Si lacks stability, as it degrades and loses efficiency when first exposed to sunlight (Staebler-Wronski effect). However, the efficiency stabilizes in time. 

An important drawback of a-Si H is its intrinsic metastability the electronic properties degrade upon light exposure. This was discovered by Staebler and Wron-ski [87, 88], and is therefore known as the Staebler-Wronski effect (SWE). This effect manifests itself by an increase in the density of neutral dangling bonds, A db, upon illumination, according to N, (t) oc where G is the generation rate... 

Illumination of solar cells causes a reduction of efficiency and fill factor, as a result of light-induced creation of defects (Staebler-Wronski effect. Section 1.1.2.5). This reduction is halted after several hundred hours of illumination. The reduction is correlated with solar cell thickness. A large intrinsic layer thickness leads to a large reduction of efficiency and fill factor compared to a small intrinsic layer thickness. The solar cell properties can be completely recovered by annealing at about 150°C. The open circuit voltage and short circuit current decrease only slightly. 

In 1977 it was observed that extended illumination with visible light of a-Si H produced a decrease in photoconductivity and dark conductivity (the Staebler-Wronski effect), which is reversible upon annealing, as shown in Fig. 7 (Staebler and Wronski, 1977,1980). The effect can be quite dramatic, producing a decrease in dark conductivity of over four orders of magnitude, though the extent of the decrease depends on the initial defect density and doping level of the sample. The degraded conductivity state is... 

Fig. 7. Decrease of room temperature dark conductivity (solid circles) and photoconductivity (solid line) during illumination (the Staebler-Wronski effect) with 200mW/cm2 heat-filtered white light (Steabler and Wronski, 1977).

T. Tiedje, Information about Band-Tail States from Time-of-Flight Experiments Arnold R. Moore, Diffusion Length in Undoped a-Si H W. Beyer and J. Overhof, Doping Effects in a-Si H H. Fritzche, Electronic Properties of Surfaces in a-Si H CR. Wronski, The Staebler-Wronski Effect... 

Staebler and Wronski (1977) were the first to observe light-induced changes in the properties of a-Si H. The Staebler-Wronski effect is now known to affect the performance of a-Si H solar cells through the creation of recombination centers and charged traps (Carlson et al., 1983b). These light-induced centers are metastable and can be annealed out at temperatures of 150 - 200,C. 

Staebler- Wronski Effect. Light (Staebler and Wronski, 1977) and electric fields (Ast and Brodsky, 1978) can induce changes in the electronic properties of a-Si H. Staebler and Wronski (1977) observed that the photoconductivity and the dark conductivity of undoped a-Si H decreased slowly during illumination. The fall in the photoconductivity is relatively small... 

Volume 21, Part C, is concerned with electronic and transport properties, including investigative techniques employing field effect, capacitance and deep level transient spectroscopy, nuclear and optically detected magnetic resonance, and electron spin resonance. Parameters and phenomena considered include electron densities, carrier mobilities and diffusion lengths, densities of states, surface effects, and the Staebler-Wronski effect. 

The following discussion concentrates on the creation of bulk dangling bond defects, which may not be the only process, but is almost certainly the dominant one. Dersch, Stuke and Beichler (1980) were the first to show that illumination causes an increase in the g = 2.0055 paramagnetic defect and concluded that the Staebler-Wronski effect was the creation of dangling bonds. The metastable defect creation and annealing is described by the potential well model shown in Fig. 6.1, except that the barrier is overcome by the recombination energy from... 

A point to note is that the defect creation at the interface is quite strongly thermally activated, unlike the Staebler-Wronski effect which is almost athermal. The likely explanation is that the light excitation provides all the energy needed to overcome the barrier, whereas the interface bias reduces the barrier by the shift of E., but a thermal activation energy is still needed. These different situations are illustrated in Fig. 6.25. 

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