Performance of p- and n-side illuminated microcrystalline silicon solar cells following 2 MeV electron bombardment

Abstract

The impact of defects on the performance of p- and n-side illuminated microcrystalline silicon solar cells is investigated. The absorber layer spin density NS is controlled over some two orders of magnitude by electron bombardment and subsequent annealing steps. At increased NS (between 3 × 1016 and 1018 cm−3), performance of n-side illuminated cells is much more strongly reduced relative to p-side illuminated cells, particularly with regard to short circuit current density. Quantum efficiency measurements indicate a corresponding strong asymmetry in wavelength-dependence, which has been successfully reproduced by numerical device simulations.

Dependence of open circuit voltage in a-Si:H and μc-Si:H solar cells on defect density in absorber layer varied by 2 MeV electron bombardment

“…The electron bombardment creates defects in every layer of the solar cell stack including glass substrate. Nevertheless, we expect that the photovoltaic performance is predominantly determined by the increase in the defect density of absorber layer rather than other functional layers of a cell [9][10][11].…”

“…Both the solar cells and ESR samples underwent identical 2 MeV electron bombardment with fluence of 9.4 × 10 16 e/cm 2 at approximately 100 K, and subsequent stepwise annealing as described in our previous publications [8][9][10][11]16]. In the following discussion the spin density, N S , measured in the reference samples, is referred to as the defect density in the absorber layer.…”

“…In the experiment, the spin density, N S , measured with electron spin resonance (ESR), was taken as a measure of the defect density. Both solar cells and ESR samples are exposed to the same electron bombardmentannealing procedure, which enables us to investigate the recombination-related variations of cell performance [9][10][11][12]. In the present study we use data on both defect density [12] and illumination intensity dependencies obtained on the same set of a-Si:H and c-Si:H solar cells to investigate the relation between the absorber layer defect density and V OC using the simplest theoretical formulation given earlier for analysis.…”

“…Defect density increase in a-Si:H may be achieved with prolonged illumination [7], whereas c-Si:H is stable to the light soaking, and thus requires other methods to be modified. In our previous works [8][9][10][11][12] we presented experiments, utilizing 2 MeV electron irradiation and successive annealing, to achieve a defect density in a-Si:H and c-Si:H material and solar cells, which can be varied over several orders of magnitude. In the experiment, the spin density, N S , measured with electron spin resonance (ESR), was taken as a measure of the defect density.…”

Bifacial microcrystalline silicon solar cells with improved performance due to μc-SiOx:H doped layers

“…This is because the hole drift mobility and mobilitylifetime product in c-Si:H films are much higher than those in a-Si:H films [3]. It was recently shown that the performance of p-side and n-side illuminated solar cells with microcrystalline silicon absorber layers is quite similar provided that the defect density of the absorber layer is sufficiently low [4]. High efficiency c-Si:H solar cells with illumination from the n-side have been demonstrated with highly transparent n-type alloys as window layers, for example, silicon carbide [5] or microcrystalline silicon oxide (c-SiO x :H) [6].…”