Päivikki Repo

Wave optical simulation of the light trapping properties of black silicon surface textures.

Due to their low reflectivity and effective light trapping properties black silicon nanostructured surfaces are promising front side structures for thin crystalline silicon solar cells. For further optimization of the light trapping effect, particularly in combination with rear side structures, it is necessary to simulate the optical properties of black silicon. Especially, the angular distribution of light in the silicon bulk after passage through the front side structure is relevant. In this paper, a rigorous coupled wave analysis of black silicon is presented, where the black silicon needle shaped structure is approximated by a randomized cone structure. The simulated absorptance agrees well with measurement data. Furthermore, the simulated angular light distribution within the silicon bulk shows that about 70% of the light can be subjected to internal reflection, highlighting the good light trapping properties.

Surface Passivation Properties of

Atomic layer deposited hafnium oxide is shown to provide good surface passivation of low resistivity, n-type crystalline Si wafers after a low temperature anneal. The surface passivation is related to a fixed negative charge, as well as an excellent interface with the crystalline Si wafer. In this paper, the influence of four deposition parameters on the HfO2 passivation properties, namely precleaning, precursors, deposition temperature, and postannealing temperature, is discussed. Minority carrier lifetimes of 1.9 ms (surface recombination velocity (SRV) 7.7 cm/s) on float zone n-type wafers and 1.7 ms (SRV 11 cm/s) on Czochralski n-type wafers, under optimized deposition conditions and a postannealing process, have been measured. A significant improvement of the surface passivation is observed after 100 h light soaking, resulting in a carrier lifetime of 2.5 ms. Fitting of the results by a two-defect charge trapping/detrapping model indicates that additional light-induced negative charges enhance the field effect passivation, which is also consistent with the experimental results. Due to its high refractive index and the obtained good surface passivation of Si wafers, HfO2 has a great potential as a surface passivation material, e.g., in the fabrication of high-efficiency Si solar cells.

{\textrm{HfO}}_{2}

The authors show here that the passivation quality of Al2O3 is highly sensitive to the surface condition prior to the atomic layer deposition, affecting especially the thermal stability of the film. Pretreatments like diluted HCl bath or preheating at 200 °C both improved significantly the passivation quality and thermal stability of the films. In addition, the authors observed that a thin chemical SiO2 layer resulting from diluted HCl solves the blistering problem often encountered in H2O based atomic layer deposited process. Finally, the authors show that the chemical oxide protects the surface from contaminants, enabling long storage times in a dirty ambient between the cleaning and the film deposition.

Thin Film on n-Type Crystalline Si

For thin silicon solar cells, standard pyramidal textures cannot be used as antireflection structures due to their thicknesses of several micrometres. In this paper, we investigate two alternative front side antireflection surfaces, which in combination with diffractive rear side structures could lead to low surface reflectance and advanced light trapping: firstly, planar multi-layer antireflection coatings, optimized using experimentally determined refractive indices, have been produced. The resulting hemispheric reflectance weighted with the AM1.5g spectrum (280 nm to 1000 nm) for two and three optimized planar layers were 3.67 % and 3.52 %, respectively, slightly higher than for a sample with inverted pyramids with 3.09 %. Secondly, black silicon was investigated. This acicular nanostructured silicon surface has a very low reflectance over a wide range of the spectrum (weighted reflectance 1.21 %). Reflection and transmission measurements show that a black silicon surface leads to scattering and light trapping. As we see significant absorption for energies below the silicon bandgap for all samples with structured surfaces, possible mechanisms leading to these measurement results were analysed.

Effect of substrate pretreatments on the atomic layer deposited Al2O3 passivation quality

Atomic layer deposited aluminum oxide (Al2O3) has in recent years proven to be a promising surface passivation material for crystalline silicon solar cells. However, blistering in Al2O3 films is a common problem deteriorating the surface passivation quality. Here, blistering is studied from material aspects including film thickness, film composition and postdeposition heat treatment. We show how thicker films, higher annealing temperatures and longer annealing times lead to more severe blistering and demonstrate how blistering can be avoided by using either O3 as the oxidant or depositing a thin TiO2 layer at the silicon interface.

Front side antireflection concepts for silicon solar cells with diffractive rear side structures

Commercial photodiodes suffer from reflection losses and different recombination losses that reduce the collection efficiency of photogenerated charge carriers. Recently, we realized a near-ideal silicon photodiode, which steps closer to the physical performance limits of silicon photodiodes than any other silicon photodiode realized before. Our device exhibits an external quantum efficiency above 95% over the wavelength range of 235 – 980 nm, and provides a very high response at incident angles of up to 70 degrees. The high quantum efficiency is reached by 1) virtually eliminating front surface reflectance by forming a “black silicon” nanostructured surface having dimensions in the range of wavelength of optical light and 2) using an induced junction for signal collection, formed by negatively charged alumina, instead of a conventional doped p-n junction. Here, we describe the latest efforts in further development of the photodiode technology. In particular, we report improvements both in the short wavelength response via better control of the surface quality, and superior response to photons with energies close to the silicon bandgap.

Effect of ALD reactants on blistering of aluminum oxide films on crystalline silicon

Commercial photodiodes suffer from reflection losses and different recombination losses that reduce the collection efficiency. Recently, we realized a near-ideal silicon photodiode that exhibits an external quantum efficiency above 95% over the wavelength range of 235 – 980 nm, exceeds 100% below 300nm, and provides a very high response at incident angles of up to 70 degrees. The high quantum efficiency is reached by 1) virtually eliminating front surface reflectance by forming a “black silicon” nanostructured surface having dimensions proportional to the wavelength of light to be detected and 2) using an induced junction for signal collection instead of a conventional doped p-n junction, virtually eliminating Auger recombination at the light entry surface. This recombination prevention is especially important in ultraviolet detection since ultraviolet photons are absorbed very close to device surface, where conventional photodiodes have high doping concentration causing loss of signal, but induced junction diode is able to collect virtually all charge carriers generated. In this paper, we analyse the performance of our photodiodes under ultraviolet radiation.