Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

Abstract

Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation of such films as window layers—without degraded carrier collection—demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

“…The effect is more pronounced for the n-SHJ cell compared with the n-hybrid cell. The nonlinearity of FF and η versus temperature was previously observed in such contacts 25,28,53 and is commonly observed in solar cells incorporating thermionic barriers. Generally, increasingly linear behaviour can be achieved by increasing the conductivity of the contact layer 28 and the use of thinner intrinsic buffer layers 25 , for which the challenge is to maintain sufficient surface passivation.…”

Temperature‐dependent performance of silicon heterojunction solar cells with transition‐metal‐oxide‐based selective contacts

“…This trend is often attributed to thermionic barriers at the heterojunctions of these cells. 2,59 The decrease of FF of the MoOx-and TiOx-based solar cells in the temperature range from 40 to 70 °C is less pronounced compared to that of the SHJ solar cell. Note that for all the cells, the decreasing trend of the FF is different from that of the pFF as a function of temperature.…”

Sonochemical Modification of SiGe Layers for Photovoltaic Applications

“…[5] Heterostructures with a-Si can offer considerably higher separation lengths for photogenerated charge carriers. [6] Moreover, a-Si layers have been widely used as a top surface passivation coating. [7][8][9] To reduce the surface and interface trap density and improve the quality of the device, plasma hydrogenation [10][11][12][13] or H 2 O vapor treatment [14] can be used to passivate the front and rear surfaces of various solar cell structures.…”