High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

Abstract

In the present paper, the authors discuss the application of amorphous p-i-n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous/microcrystalline tandem ("micromorph") solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si:H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13mA/cm 2 for a total cell current density of 26mA/cm 2 .A detailed study is presented of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the µ 0 τ 0 parameter, which has been shown to be directly correlated to the cell performance.The paper discusses in detail the technological problems arising from the insertion of ilayers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p-i-n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures.Finally, the application of such optimized high-temperature amorphous cells as (a) bottom cells in a-Si:H/a-Si:H stacked cells and (b) as top cells in micromorph tandem cells is discussed. Current densities of 13mA/cm 2 in the top cell are available with a top cell i-layer thickness of only 250nm. The potential of such top cells for raising the stabilized efficiency of micromorph tandem cells is discussed.

Record amorphous silicon single-junction photovoltaic module with 9.1% stabilized conversion efficiency on 1.43 m2

“…It is well known that the implementation of a thin high band‐gap buffer layer at this interface improves the open‐circuit voltage. According to Platz , the mechanism leading to this improvement in open‐circuit voltage is that the band‐offset at the conduction band edge between the wide band‐gap buffer layer and the intrinsic absorber layer prevents electrons from diffusing back to the p‐doped layer and recombining instead of drifting to the n‐layer. Very often, a very highly diluted a‐Si:H layer alloyed with carbon is used as a buffer layer resulting in good initial performance.…”

Thin film silicon photovoltaics: Architectural perspectives and technological issues

“…The deposition temperature has been used elsewhere as a means to change the absorption of intrinsic amorphous silicon films [19]. In particular higher T S values are used to obtain a material with higher absorption coefficient.…”

“…However hydrogen dilution reduces the optical absorption, and this turns out in lower current densities [19]. Undiluted and H 2 -diluted a-Si have been considered as possible absorber layer in order to evaluate the effect of different bandgaps on the J-V characteristic.…”