Silicon heterojunction solar cells (SHJ) are currently one of the most promising solar cell technologies in the world. The SHJ solar cell is based on a crystalline silicon (c-Si) wafer, passivated on both sides with a thin intrinsic hydrogenated amorphous silicon (a-Si:H) layer. Subsequently, p-type and n-type doped layers are deposited on the passivated wafer as the emitter and the back surface field of the solar cell. The first study of the a-Si:H/c-Si heterojunction for photovoltaic application was more than 40 years ago. During these years the SHJ solar cell technology has been investigated extensively, approaching the theoretical efficiency limit of single junction silicon solar cells. Although SHJ solar cell technology has been extensively studied and has demonstrated such a good performance, there are still aspects of the device operation not well understood. This thesis discusses certain aspects of SHJ solar cells and has a main focus on the surface passivation of the c-Si wafer. In this way it aims to contribute to the understanding of SHJ solar cell fabrication and operation, helping to improve the SHJ solar cell performance. ... Read more

In this work, we use intrinsic hydrogenated amorphous silicon oxide layers (a-SiOx:H) with varying oxygen content (cO) but similar hydrogen content to passivate the crystalline silicon wafers. Using our deposition conditions, we obtain an effective lifetime (τeff) above 5 ms for cO ≤ 6 at. % for passivation layers with a thickness of 36 ± 2 nm. We subsequently reduce the thickness of the layers using an accurate wet etching method to ∼7 nm and deposit p- and n-type doped layers fabricating a device structure. After the deposition of the doped layers, τeff appears to be predominantly determined by the doped layers themselves and is less dependent on the cO of the a-SiOx:H layers. The results suggest that τeff is determined by the field-effect rather than by chemical passivation. ... Read more

This work studies the dependency of the effective lifetime on the a-Si:H layer thickness of c-Si substrates passivated with intrinsic a-Si:H. This is experimentally investigated by using a soft wet-etching method that enables accurate control of the a-Si:H layer thickness. In this way, variations in the effective lifetime down to thicknesses of a few nanometers are studied, while excluding effects originating from the deposition conditions of a-Si:H when samples of different thicknesses are fabricated. For thin passivation layers, results show a strong thickness dependency of the effective lifetime, which is mainly influenced by the recombination at the external a-Si:H surfaces. For thicker passivation layers, the effective lifetime is predominantly determined by the bulk a-Si:H and/or c-Si defect density. During the etching of the a-Si:H passivation layers, a gradient in the Cody gap for our samples is observed. This gradient is accompanied by a stronger decrease in the effective lifetime and is attributed to a decrease in the a-Si:H band gap and valence band offset. The observed changes in lifetime with a-Si:H layer thickness are supported with AFORS-HET simulations. When a gradient in the a-Si:H passivation layer band gap is used, simulations can reproduce the experimental results. ... Read more

Light-induced effects on the minority carrier lifetime of silicon heterojunction structures are studied through multiple-exposure photoconductance decay (MEPCD). MEPCD monitors the effect of the measurement flash from a photoconductance decay setup on a sample over thousands of measurements. Varying the microstructure of the intrinsic hydrogenated amorphous silicon (a-Si:H) used for passivation of n-Type crystalline silicon (c-Si) showed that passivating films rich in voids produce light-induced improvement, while denser films result in samples that are susceptible to light-induced degradation. Light-induced degradation is linked to an increase in dangling bond density at the a-Si:H/c-Si interface, while light-induced improvements are linked to charging at the a-Si:H/c-Si interface. Furthermore, doped a-Si:H is added to make samples with an emitter and back surface field (BSF). These doped layers have a significant effect on the light-induced kinetics on minority carrier lifetime. Emitter samples exhibit consistent light-induced improvement, while BSF samples exhibit light-induced degradation. This is explained through negative charging at the BSF and positive charging at the emitter. Full precursors with a BSF and emitter exhibit different kinetics based on which side is being illuminated. This suggests that the light-induced charging at the a-Si:H/c-Si interface can only occur when a-Si:H has sufficient generation. ... Read more

This work adapts a model to simulate the carrier injection dependent minority carrier lifetime of crystalline silicon passivated with hydrogenated amorphous silicon at elevated temperatures. Two existing models that respectively calculate the bulk lifetime and surface recombination velocity are used and the full temperature dependency of these models are explored. After a thorough description of these temperature dependencies, experimental results using this model show that the minority carrier lifetime changes upon annealing of silicon heterojunction structures are not universal. Furthermore, comparisons of the temperature dependent model to using the room temperature model at elevated temperatures is given and significant differences are observed when using temperatures above 100 °C. This shows the necessity of taking temperature effects into account during in-situ annealing experiments. ... Read more

A hybrid tandem solar cell consisting of a thin-film, nanocrystalline silicon top junction and a siliconheterojunction bottom junction is proposed as a supporting solar cell for photoelectrochemical applications.Tunneling recombination junction engineering is shown to be an important consideration indesigning this type of solar cell. The best hybrid cell produced has a spectral utilization of 30.6 mA cm2a JSC of 14 mA cm2, a VOC of 1.1 V, a fill factor of 0.67 and thus an efficiency of 10.3%. A high solar-tohydrogenefficiency of 7.9% can be predicted when using the hybrid cell in conjunction with current a-SiCphotocathode technology. ... Read more