Abstract
The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells.
Highlights
The device physics of pin solar cells involves – at a minimum – three different materials as well as the interfaces between them
Since the invention of hydrogenated amorphous silicon (a-Si:H) pin solar cells about thirty years ago, an enormous effort by scientists around the world has generated a correspondingly large number of papers detailing the optoelectronic properties of the materials
There have been a number of pioneering efforts to integrate this information into models for the device physics of the cells [1,2,3,4,5,6]
Summary
The device physics of pin solar cells involves – at a minimum – three different materials as well as the interfaces between them. For intrinsic a-Si:H, we summarize the many measurements of hole drift-mobilities; these mobilities are very low (typically less than 10À2 cm2/Vs), there have been promising reports of better mobilities These mobility measurements are sufficient to establish a ‘hole mobility limit’ for single-junction cells with an a-Si:H intrinsic layer, and we show that a-Si:H nip solar cells prepared at United Solar Ovonic Corp. It implies that dangling bond densities have been kept low enough that they do not dominate the drift of holes, and that the n and p layers are good enough to act as nearly ideal electrodes to the cell. We speculate that the lightsoaking process is ‘self-limiting’ in the cells we have studied, and we discuss an obvious – but insufficient – mechanism for self-limitation in the concluding section
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