Current–Voltage Analysis of a-Si and a-SiGe Solar Cells Including Voltage-dependent Photocurrent Collection

Abstract

The current–voltage (J(V)) data measured in light and dark from a‐Si‐based solar cells have been analyzed to yield six parameters that completely specify the illuminated J(V) curve from reverse bias to beyond open‐circuit voltage (Voc). A simple photocurrent collection model is used, which assumes drift collection in a uniform field. The method has been applied to J(V) data from over 20 single‐junction a‐Si or a‐SiGe devices from five laboratories measured under standard simulated sunlight. Very good agreement is obtained between measured and calculated J(V) performance with only one adjustable parameter, the ratio of collection length to thickness Lc/D. Some of these devices have also been analyzed after extended light soaking or under filtered illumination. The effect of the voltage‐dependent photocurrent collection on the fill factor and Voc is considered in detail. Results under 1 sun illumination for both a‐Si and a‐SiGe devices are consistent with hole‐limited collection. Photocurrent collection in very thin devices (D ∼ 0.1 μm), or thicker devices under blue light, may be strongly influenced by interface recombination or back diffusion. The flatband voltage (Vfb) is dependent on the intensity and spectrum of illumination, and hence is not a fundamental device property and is not equivalent to the built‐in potential. The Voc is limited by Vfb, not junction recombination current Jo, in typical devices. The illuminated solar cell performance is nearly independent of the forward diode current for low values of Lc/D, as occurs after light soaking or with a‐SiGe. The model is also useful to investigate the intensity dependence of the fill factor and to predict the influence of Lc/D and Vfb on solar cell performance. © 1997 John Wiley & Sons, Ltd.

Low-mobility solar cells: a device physics primer with application to amorphous silicon

“…The improvement may be satisfactorily explained using Eq. (17). With the 50 meV bandtail, the output power saturates for thicknesses greater than about 200 nm, as would be anticipated given the collection width (about 250 nm) illustrated in Fig.…”

“…This approach is incorrect for very lowmobilities; it assumes that the electric-field is unmodified by light, which is certainly not true for the present models. In addition, two earlier analyses have assumed a uniform electric field [16,17]. These analyses apply to cells in which the carrier mobility is large enough that it does not limit the power output; in this paper we have chosen to examine the low-mobility region.…”

Amorphous Silicon–Based Solar Cells

“…The field becomes stronger near the p-layer. At the highest intensity, the fully collected photocurrent density is 11.5 mA/cm 2 , which is about the same as that for solar illumination neglect other aspects of power loss in the cell, which apply when field collapse may be neglected [127,128], in particular for lower intensities.…”

“…For weakly absorbed illumination (5000/cm -corresponding to a photon energy of 1.8 eV in Figure 12.2), power saturation occurs when the intrinsic layer is about 300-nm thick. This collection length [128] originates in the region where field collapse had occurred in Figure 12.16. The collapsed electric field is strongest near the p-layer and weaker near the n-layer.…”

Influence of electric field distortion and i-layer quality on the collection function of drift-driven a-Si:H solar cells

“…In the case of a-Si:H solar cells, however, it is more di cult to make reasonable approximations by which the non-linearly coupled transport equations can be solved. As a ®rst-order approximation the uniform ®eld approach [2] has been used to describe many of the basic features of photocurrent collection in a-Si:H solar cells [3,4]. Here, we present an analytical approach which takes into account distortions of the electric ®eld.…”