Influence of Chemical Composition and Structure in Silicon Dielectric Materials on Passivation of Thin Crystalline Silicon on Glass

Abstract

In this study, various silicon dielectric films, namely, a-SiOx:H, a-SiNx:H, and a-SiOxNy:H, grown by plasma enhanced chemical vapor deposition (PECVD) were evaluated for use as interlayers (ILs) between crystalline silicon and glass. Chemical bonding analysis using Fourier transform infrared spectroscopy showed that high values of oxidant gases (CO2 and/or N2), added to SiH4 during PECVD, reduced the Si-H and N-H bond density in the silicon dielectrics. Various three layer stacks combining the silicon dielectric materials were designed to minimize optical losses between silicon and glass in rear side contacted heterojunction pn test cells. The PECVD grown silicon dielectrics retained their functionality despite being subjected to harsh subsequent processing such as crystallization of the silicon at 1414 °C or above. High values of short circuit current density (Jsc; without additional hydrogen passivation) required a high density of Si-H bonds and for the nitrogen containing films, additionally, a high N-H bond density. Concurrently high values of both Jsc and open circuit voltage Voc were only observed when [Si-H] was equal to or exceeded [N-H]. Generally, Voc correlated with a high density of [Si-H] bonds in the silicon dielectric; otherwise, additional hydrogen passivation using an active plasma process was required. The highest Voc ∼ 560 mV, for a silicon acceptor concentration of about 10(16) cm(-3), was observed for stacks where an a-SiOxNy:H film was adjacent to the silicon. Regardless of the cell absorber thickness, field effect passivation of the buried silicon surface by the silicon dielectric was mandatory for efficient collection of carriers generated from short wavelength light (in the vicinity of the glass-Si interface). However, additional hydrogen passivation was obligatory for an increased diffusion length of the photogenerated carriers and thus Jsc in solar cells with thicker absorbers.

Interface passivation of liquid‐phase crystallized silicon on glass studied with high‐frequency capacitance–voltage measurements

“…Further details about the interlayer and absorber preparation for LPC‐Si solar cells on glass are described in Refs. .…”

“…A more detailed description is supplied in Ref. [7]. The absorption coefficient spectra for the SiON films listed in Table 1 are shown in Fig.…”

“…So far, the investigations of the IL/LPC‐Si interface have been restricted to indirect measurements of the solar cell parameters open circuit voltage and internal quantum efficiency and it was found that the IL/LPC‐Si interface has a major impact on the solar cell performance . For the direct analysis of the passivation quality at the IL/LPC‐Si interface, the defect state density at the interface ( D it in eV −1 cm −2 ) as well as the effective charge density in the IL ( Q IL,eff in cm −2 ) are crucial parameters .…”

“…Furthermore, we investigated the effect of an additional direct hydrogen plasma treatment (HPT) for 30 min at 400 8C on D it and Q IL,eff . The positive effect of the HPT on the LPC-Si solar cell performance was shown before [5,7], but up to now the effect of this treatment on the IL/LPC-Si interface is not fully understood.…”

“…Further details about the interlayer and absorber preparation for LPC-Si solar cells on glass are described in Refs. [5,7]. Figure 1 displays the MIS structure which was fabricated to realize C-V measurements on IL/LPC-Si stacks.…”