Effects of nonuniformity in thin-film photovoltaics

Abstract

We discuss the physical origin and effects of micrononuniformities on thin-film photovoltaics. The key factors are the large device area and the presence of potential barriers in the grain boundaries (for polycrystalline films) and in device junctions. We model the nonuniformity effects in the terms of random microdiodes connected in parallel through a resistive electrode. The microdiodes of low open circuit voltages are shown to affect macroscopically large regions. They strongly reduce the device performance and induce its nonuniform degradation in several different modes. We support our predictions by experiments, which show that the device degradation is driven by the light-induced forward bias and is spatially nonuniform.

Universality of non-Ohmic shunt leakage in thin-film solar cells

“…8,9,13,18,19 Some equivalent circuits incorporating a parasitic weak diode have also been proposed to account for these nonlinear shunts. [19][20][21] However, these macroscopic, circuit level models cannot account for the microscopic nature of shunt paths. The physical origin of shunt conduction paths have also been explored in the literature.…”

“…The analysis of shunt leakage phenomena is hampered, in part, because most models of the dark current typically focus on the forward bias current at one temperature only. 12,14,20,25 A detailed characterization of the shunt current, considering the entire voltage range at different temperatures, is usually not performed. Moreover, the picture of shunt leakage has been fragmented because the nature of shunt current has been assumed to be unique for solar cell technologies.…”

Ordered CdTe/CdS Arrays for High-Performance Solar Cells

“…Many studies have identified various efficiency loss mechanisms, 7,8 one of which is the nonuniformity of the crystals in polycrystalline films. 2,[9][10][11][12] In this paper, the phrase ''random polycrystalline'' refers to the situation in which the morphology and crystal quality of the grains are uncorrelated or only slightly correlated. The term ''nonuniformity'' describes a wide variation in the morphology and crystal quality of the grains.…”

The use of 2^nd and 3^rd level correlation analysis for studying degradation in polycrystalline thin-film solar cells

“…The effects of back contact and processes on cell durability have been discussed for example in [3][4][5]. More recently, the detrimental effects of localized shunts [6], the general effects of film micrononuniformities [7], and the effect of cell-fabrication processing through formal design-of-experiments methodologies [8] have been reported.…”