Evaluation of light trapping structures for liquid-phase crystallized silicon on glass (LPCSG)

Abstract

Liquid‐phase crystallized silicon on glass (LPCSG) presents a promising material to fabricate high quality silicon thin films, e.g., for solar cells and modules. Using continuous wave line focus laser irradiation at 808 nm, about 10 μm thick microcrystalline silicon layers are fabricated by liquid‐phase crystallization of amorphous or nanocrystalline silicon layers deposited by electron beam evaporation on Borofloat 33 glass. To achieve high solar cell efficiencies with such thin silicon layers, effective light trapping structures at the silicon surface are needed to enhance the light absorption and thereby the current in the solar cell. At the same time, these surface structures must provide low surface recombination velocity to maintain high open circuit voltage (Voc). Light trapping structures in LPCSG absorber prepared by conventional KOH texturing and by nanowire structuring of the solar cell backside are investigated. The impact of structures on short circuit current density (Isc) is determined from optical measurements. As a new approach, effective carrier lifetime is measured in LPCSG absorbers using the quasi steady‐state photoconductance method to determine the impact of structuring on surface recombination and implied Voc of solar cell precursors. Carrier lifetimes in the range of 300–400 ns are measured indicating a carrier diffusion length of more than 20 μm, which is 2–3 times larger than the layer thickness. It is found that a slight pyramidal surface texture by KOH solution provides a high level of light trapping increasing Isc by 17–18% and maintaining high Voc (>600 mV). The potential for current enhancement of nanowire structuring is higher (≈20%), but further optimization of nanowire dimensions and of surface cleaning of nanowire structures is needed to overcome higher surface recombination and the resulting Voc losses.

Multicrystalline Silicon Thin‐Film Solar Cells Based on Vanadium Oxide Heterojunction and Laser‐Doped Contacts

“…This parameter determines the mean path that minority carriers (holes) can travel before recombining. The expected carrier diffusion length in the silicon thin‐film absorbers is in the range of several 10 μm, which is long enough to collect minority carriers in the thin (below 10 μm) absorbers, when they are photogenerated over the emitter regions . In contrast, the photogenerated carriers out of these regions are further away from the collector contact, recombining in the material bulk and, thus, base contact regions are not active for carrier collection.…”

“…The expected carrier diffusion length in the silicon thin-film absorbers is in the range of several 10 μm, which is long enough to collect minority carriers in the thin (below 10 μm) absorbers, when they are photogenerated over the emitter regions. [24,25] In contrast, the photogenerated carriers out of these regions are further away from the collector contact, recombining in the material bulk and, thus, base contact regions are not active for carrier collection. Moreover, the lateral transport of minority carriers is jeopardized by the high density of grain and twin boundaries observed.…”

“…This bulk lifetime value agrees well with the previous works on similar substrates. [24,25] Moreover, using this range for τ B and a hole diffusion constant of 7.34 cm 2 s À1 , we calculate a diffusion length range of 7.66-8.56 μm. This range agrees well with the aforementioned discussion, being longer than the substrate thickness and too short to collect carriers out of the emitter-contact regions.…”

“…Consequently, a combination of both strategies could be a feasible solution to fully exploit the potential of laser‐doped contacts on LPC thin films. Additionally, the insertion of a suitable light trapping for the reported concept would also increase the efficiency …”

“…Additionally, the insertion of a suitable light trapping for the reported concept would also increase the efficiency. [24]…”