Controllable fixed charge densities of $TiO_2$-based passivation layer in c-Si solar cells

Abstract

Recently, the importance of renewable energy has been increasing due to the depletion of fossil fuels and environment pollution with green house effect. Among the many renewable energy, solar photovoltaic that provide harmless to environment and unlimited solar source has attracted considerable attention. Photovoltaic based on solar light is semiconductor device which convert incident light of sun to electricity with photoelectric effect. Although silicon solar cells with lower silicon wafer cost and stable performance occupy 90 % of photovoltaic industry, solar cells based on silicon wafer has high capital cost and low power conversion efficiency to commercialization of silicon solar cells. Therefore, it is important to high power conversion efficiency and reducing manufacturing cost for commercialization of photovoltaic. Surface passivation of crystalline silicon is a key enabler for achieving high efficiency and cost-effective c-Si solar cells. To achieve high efficiency of silicon solar cells, stack of $SiN_x$ anti-reflection coating (ARC) and $Al_2O_3$ passivation layer are required based on $p^+$-emitter/n-base silicon structure. However, stack films demands additional time and extra capital cost since complex process were used for deposition of films. Thus, the functional passivation layer that provide outstanding passivation property and suppress optical reflectance was required for cost-effective and high performance of silicon solar cell. $TiO_2$ film has attracted attention from 1970s as ARC of silicon solar cells, but unfortunately, poor passivation performance of $TiO_2$ passivation layer deposited by various method such as plasma-enhanced chemical vapor deposition (PECVD) and pulsed layer deposition were reported in the literature. Atomic layer deposition (ALD) was developed for $TiO_2$ passivation layer with negative fixed charge densities to provide field-effect passivation in recent years. Chapter 3 introduces the research on ALD $Al_2O_3$ films which has been widely used in PV industry and ALD $TiO_2$ films that are recently attracted attention for functional passivation layer. $Al_2O_3$ has excellent passivation characteristics by annealing process, but an additional anti-reflection film was needed due to the low refractive index. In contrast, $TiO_2$ has slightly lower passivation characteristics compared to $Al_2O_3$. However, high refractive index of film is suitable to minimize the loss due to reflection of silicon. Therefore, we studied functional Al-doped $TiO_2$ thin film through ALD super cycle process in Chapter 4. In chapter 4, we adjusted the amount of Al concentration in $TiO_2$ passivation layer from 0 to 15.5 % by controlling the cycle proportion of $Al_2O_3$ to $TiO_2$ in ALD process. Fixed charge densities of Al-doped $TiO_2$ film was increased from $–8 \cdot$ $10^{11}cm^{-2}$ to $–3 \cdot$ $10^{12} cm^{-2}$ by controlling the amount of Al concentration, leading to implied $V_{oc}$ up to 709 mV with 15nm thick Al-doped $TiO_2$ on n-Si by improving field-effect passivation. We then investigated the optical reflection losses of silicon by applying Al-doped $TiO_2$ passivation layer. The refractive index of Al-doped $TiO_2$ film was controlled from 1.9 to 2.3 and maintained high. As a result, the reflection of silicon was reduced from 35.78% to 10.24%. Therefore, we demonstrated that the ALD Al-doped $TiO_2$ functional passivation film is promising candidate for cost effective and high efficient Si solar cells based on the $p^+$-emitter/n-base structure with suppressed optical reflection losses and outstanding passivation characteristic. Also, the silicon photo-electrode with Al-doped $TiO_2$ passivation layer will provide stability with long time and high open circuit voltage.