Passivation at the interface between liquid‐phase crystallized silicon and silicon oxynitride in thin film solar cells

Abstract

The passivation quality at the interface between liquid-phase crystallized silicon (LPC-Si) and a dielectric interlayer (IL) was investigated in terms of the defect state density at the IL/LPC-Si interface (D it ) as well as the effective fixed charge density in the IL (Q IL,eff ). Both parameters were obtained via high-frequency capacitance-voltage measurements on developed metal-insulator-semiconductor structures based on a molybdenum layer sandwiched between the IL and the glass substrate. D it and Q IL,eff were correlated to the open circuit voltage (V oc ) and the integrated external quantum efficiency (J sc,EQE ) obtained on corresponding solar cell structures as well as to V oc and J sc,EQE results based on two-dimensional simulations. We found that D it was reduced by one order of magnitude using a hydrogen plasma treatment (HPT) at 400°C. Irrespectively of the HPT, Q IL,eff was > 10 12 cm À2 . We suggest that field-effect passivation dominates chemical passivation at the IL/n-type LPC-Si interface. We attribute the significant enhancement of V oc and J sc,EQE observed after HPT on n-type LPC-Si solar cells mainly to improvements of the passivation quality in the n-type LPC-Si bulk rather than at the IL/n-type LPC-Si interface. For p-type absorbers, the HPT did not improve V oc and J sc,EQE significantly. We propose that this is because of an insufficient passivation of bulk defects by positively charged hydrogen, which dominates in p-type silicon, in combination with an insufficient interface passivation.

Liquid phase crystallized silicon – A holistic absorber quality assessment

“…For the doping series, 15 µm and for the thickness variation 14 and 33 µm of nominally intrinsic Si were deposited on 1.1 mm Corning Eagle XG glass using high rate electron beam evaporation. Prior to Si deposition a multifunctional intermediate layer (IL) comprised of SiOx/SiNx/SiOx was deposited as described elsewhere [12,13,16]. Amongst others, this IL is crucial for final device performance, since it is the only option to control the passivation and stop impurity diffusion at the glass/LPC-Si interface because later this interface is buried and inaccessible.…”

“…Different final doping concentrations were achieved by varying the phosphine gas flow during deposition. The actual liquid phase crystallization procedure was performed using a line-shaped laser at a wavelength of 808 nm in vacuum [12]. The absorber is molten entirely, allowing the dopants in the doping layer to disperse homogeneously throughout the absorber.…”

“…Present efficiencies on LPC-Si are 13.2 % on below 15 µm thick absorbers using an interdigitated back contact (IBC) system and up to 15.9 % on full-emitter test structures [11]. LPC-Si has shown a steep learning curve and progress in recent years, amongst other, thanks to a continuous improvement of the quality of the interface between Si and the supporting glass by an intermediate layer (IL) stack [12,13] and hydrogen plasma passivation techniques [14]. To push performance even further, a thorough material assessment is necessary to find possible levers for improvement.…”

Passivation of Liquid‐Phase Crystallized Silicon With PECVD‐SiNx and PECVD‐SiNx/SiOx

“…The capacitance across the structure varies with applied voltage which is dedicated to the behavior of the semiconductor space charge layer at the IL/LPC‐Si interface. Previously, we applied this method successfully to demonstrate that defect states at the interface between N/O/ON IL stacks and LPC‐Si are reduced strongly when a postdeposition hydrogen plasma treatment is performed …”

Applicability of an economic diode laser emitting at 980 nm for preparation of polycrystalline silicon thin film solar cells on glass

“…Recently, the material quality of such polycrystalline silicon (pc‐Si) became comparable with conventional multicrystalline wafers, e.g., indicated by high open circuit voltages of the prepared solar cells . Solar conversion efficiencies up to 13.2% were demonstrated . Moreover, the high temperature load due to the laser treatment is very close to the sample surface and rests only for some milliseconds which facilitate the use of low cost glasses as substrates.…”