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We studied the impact of oxidizing pre-treatments (OPT) and post deposition annealing (PDA) on the passivation performance and the hydrogen distribution near the interface between crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H), the critical functional region in Si heterojunction solar cells. The OPT prior to deposition of the a-Si:H layer consists of immersing the c-Si substrates into hydrogen peroxide solutions, which forms a silicon oxide interlayer. Spectroscopic ellipsometry (SE) indicates that slightly thicker a-Si:H layers result from OPT. The refractive index and the extinction coefficient are increased by inserting the oxide interlayers, suggesting that less deficient and denser a-Si:H layers can be formed. Under optimum conditions, OPT leads to at least 2-fold improvement of the effective photo-generated carrier lifetime. PDA at 200 °C further improves the passivation performance of samples with an interlayer. Hydrogen profiling with nuclear reaction analysis clarifies that higher hydrogen concentrations are present around the heterointerfaces of samples with an interlayer and that these hydrogen concentrations are maintained after PDA. Our results suggest that the oxide interlayer can suppress hydrogen desorption in the initial growth stage of high-quality a-Si:H layers and during subsequent PDA, resulting in excellent passivation performance.

We report on the application of Bayesian optimization (BO), which could accelerate the time-intensive process optimization of many parameters, to fabrication of the high-performance titanium oxide/silicon oxide/crystalline silicon passivating contact. The process contains pre-deposition treatment to form SiOy interlayer, atomic layer deposition (ALD) of TiOx, and hydrogen plasma treatment (HPT) as post-process. We attempted to optimize seven parameters for ALD and HPT by dealing with samples treated by three kinds of chemical solutions in the same batch. This permits to perform BO for each structure at the same time and determine the superior pre-deposition treatment. Consequently, carrier selectivity S10 estimated by independent measurements of the saturation current density and contact resistance was significantly improved by BO of only 12 cycles and 10 initial random experiments. These results certify that BO could efficiently provide experimental conditions in multidimensional parameter space although we need to consider the impact of the metallization process on the passivation performance.

Variable-angle spectroscopic ellipsometry analysis is performed to study the impact of post-deposition annealing on the passivation performance of the heterocontacts consisting of titanium oxide and silicon oxide on crystalline silicon (c-Si) prepared by atomic layer deposition (ALD) for the development of high-performance ALD-TiOx/SiOy/c-Si heterocontacts. The highest lifetime of 1.8 ms is obtained for the TiOx/SiOy/c-Si heterocontacts grown at 175 °C after annealing at 275 °C for 3 min. With increasing annealing temperature, the TiOx layers of the TiOx/SiOy/c-Si heterocontacts become dominant. Furthermore, the amplitude of dielectric functions of the ALD-TiOx layer decreases as annealing temperature increases, which suggests that enhanced diffusion of Ti into SiOy interlayers at higher annealing temperature. The sufficient diffusion of Ti atoms into SiOy interlayers is caused by annealing at 275 °C for 3 min, yielding high-quality interface passivation.

We have fabricated p-type BaSi2 (p-BaSi2)/n-type crystalline Si (n-c-Si) heterojunction solar cells using thermal evaporation. To control the defect around the heterointerface, samples were fabricated by two methods using different current profiles during evaporation. The one-step method was designed to avoid supplying Ba-rich vapor, and the two-step method was designed to intentionally introduce Ba-rich vapor at the first step. Deep-level transient spectroscopy measurements revealed that defect densities in the n-c-Si side were almost the same for both samples. Open-circuit voltage (Voc) was successfully improved from 319 to 463 mV by the two-step method. This may be due to the increase of carrier density in p-BaSi2 prepared using a two-step method. As a result of optimization, the conversion efficiency of 6.23% was achieved by the heterojunction solar cells.

The impact of the implementation of magnesium interlayer and the layer thickness (tTiOx) of titanium oxide on the electrical properties of TiOx/SiOy/Si heterojunctions is investigated to improve electron transport for use in silicon heterojunction solar cells. The passivation performance is improved with increasing tTiOx. For the samples with Mg interlayer, ohmic contact can be attained for the thicker TiOx layer compared with the sample without Mg interlayer. Schottky contact is mitigated by the TiOx/SiOy stacking layers and Mg interlayer, attributed to the reduction of interfacial energy level by TiOx/SiOy stacking layers and enhanced downward band bending by Mg interlayer. The open-circuit voltage and fill factor of the solar cells are improved by inserting the TiOx/SiOy stack and the Mg interlayer, indicating that the electron selectivity is enhanced. Excellent surface passivation and transport properties can be achieved by controlling TiOx layer thickness and using the Mg interlayer.