The application of molybdenum oxide (MoOx) as a hole-selective contact for silicon-based solar cells has been explored due to superior optical transmittance and potentially leaner manufacturing compared to fully amorphous silicon-based heterojunction (SHJ) devices. However, the development of MoOx contacts has been hampered by their poor thermal stability, resulting in a carrier selectivity loss and an S-shaped IV curve. The aim of this study is to understand the influence of different passivating interlayers on the carrier selectivity of hole-selective MoOx contacts for crystalline silicon (c-Si) solar cells. We highlight the effect of different interlayers on the surface passivation quality, contact selectivity, and the thermal stability of our MoOx-contacted devices. The interlayers studied are intrinsic hydrogenated amorphous silicon (a-Si:H(i)), thermally grown ultrathin SiO2, and a stack consisting of an ultrathin SiOy and Al2O3 layer. Additionally, we simulate the interacting interlayer properties on the carrier selectivity of our MoOx contacts using a simplified model. Among these interlayers, the Al2O3/SiOy stack shows to be a promising alternative to SiO2 by enabling efficient transport of holes while being able to sustain an annealing temperature of at least 250 °C underlining its potential in module manufacturing and outdoor operation.@en

In this work, the applicability of pulsed laser deposition (PLD) of transparent conductive oxides (TCOs) on high-quality ultra-thin poly-Si based passivating contacts is explored. Parasitic absorption caused by poly-Si layers can be minimized by reducing the poly-Si layer thickness. However, TCO deposition on poly-Si contacts, commonly by sputtering, results in severe deposition-induced damage and further aggravates the surface passivation for thinner poly-Si layers (<20 nm). Although a thermal treatment at elevated temperature (∼350 °C) can be used to partially repair the surface passivation quality, the contact resistivity severely increases due to the formation of a parasitic oxide layer at the poly-Si/ITO interface. Alternatively, we show that PLD TCOs can be used to mitigate the damage on ultra-thin (∼10 nm) poly-Si layers. Further improvement in poly-Si contact passivation can be achieved by increasing the deposition pressure while low contact resistivities (∼45 mΩ cm²) and good thermal stability (up to 350 °C) are achieved with a PLD indium-doped tin oxide (ITO) layer on high-quality ultra-thin poly-Si(n⁺) contacts. This allows for the application of a highly transparent front side contact by combining the excellent opto-electrical properties of a PLD ITO film with a 10 nm thin poly-Si contact.

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In this study, undoped hydrogenated amorphous silicon (a-Si:H) thin films deposited under moderate dilution ratios of silane by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) have been investigated using steady-state photoconductivity and improved dual beam photoconductivity (DBP) methods to identify changes in multiple gap states in annealed and light-soaked states. Four different gap states were identified in annealed state named as A, B, C, and X states. The peak energy positions of these Gaussian distributions are consistent with those recently identified by Fourier transform photocurrent spectroscopy (FTPS). After in situ light soaking, their density increases with different rates as peak energy positions and half-widths remain unaffected. The electron-occupied A and B states located below the dark Fermi level and their density and ratios in the annealed and light-soaked states correlate well with those defects detected by time-domain pulsed electron paramagnetic resonance (EPR) experiments. The A, B, and X states located closer to the middle of the bandgap anneal out at room temperature in dark and define the "fast"states. However, the C states show no sign of room temperature annealing such that they must define the "slow"states in undoped a-Si:H. The results found in this study indicate that the anisotropic disordered network is a more appropriate model than previously proposed defect models based on the continuous random network to define the nanostructure of undoped a-Si:H, where multiple defects, D0 and non-D0 defects, can be identified by using the improved DBP method.

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Hydrogenation of polycrystalline silicon (poly-Si) passivating contacts is crucial for maximizing their passivation performance. This work presents the application of Al2O3 prepared by atomic layer deposition as a hydrogenating capping layer. Several important questions related to this application of Al2O3 are addressed by comparing results from Al2O3 single layers, SiNx single layers, and Al2O3/SiNx double layers to different poly-Si types. We investigate the effect of the Al2O3 thickness, the poly-Si thickness, the poly-Si doping type, and the postdeposition annealing treatment on the passivation quality of poly-Si passivating contacts. Especially, the Al2O3/SiNx stack greatly enhances the passivation quality of both n+ and p+ doped as well as intrinsic poly-Si layers. The Al2O3 layer thickness is crucial for the single-layer approach, whereas the Al2O3/SiNx stack is less sensitive to the thickness of the Al2O3 layer. A thicker Al2O3 layer is needed for effectively hydrogenating p+ compared to n+ poly-Si passivating contact. The capping layers can hydrogenate poly-Si layers with thicknesses up to at least 600 nm. The hydrogenation-enhanced passivation for n+ poly-Si is found to be more thermally stable in comparison to p+ poly-Si. These results provide guidelines on the use of Al2O3 capping layers for poly-Si contacts to significantly improve their passivation performance.

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