Sonya Calnan

PECVD Intermediate and Absorber Layers Applied in Liquid-Phase Crystallized Silicon Solar Cells on Glass Substrates

Liquid-phase crystallized silicon absorber layers have been applied in heterojunction solar cells on glass substrates with 10.8% conversion efficiency and an open-circuit voltage of 600 mV. Intermediate layers of SiOx, SiNx, and SiOxNy, as well as the a-Si:H precursor layer, were deposited on 30 cm × 30 cm glass substrates using industrial-type plasma-enhanced chemical vapor deposition equipment. After crystallization on 3cm × 5cm area using a continuous-wave infrared laser line, the resulting polysilicon material showed high material quality with large grain sizes.

Implications of TCO Topography on Intermediate Reflector Design for a-Si/μc-Si Tandem Solar Cells—Experiments and Rigorous Optical Simulations

The influence of the transparent conducting oxide (TCO) topography was studied on the performance of a silicon oxide intermediate reflector layer (IRL) in a-Si/μc-Si tandem cells, both experimentally and by 3-D optical simulations. Therefore, cells with varying IRL thickness were deposited on three different types of TCOs. Clear differences were observed regarding the performance of the IRL as well as its ideal thickness, both experimentally and in the simulations. Optical modeling suggests that a small autocorrelation length is essential for a good performance. Design rules for both the TCO topography and the IRL thickness can be derived from this interplay.

FEM-based optical modeling of silicon thin-film tandem solar cells with randomly textured interfaces in 3D

Light trapping techniques are one of the key research areas in thin film silicon photovoltaics. Since the 1980s randomly rough textured front transparent oxides (TCOs) have been the methods of choice as light trapping strategies for thin-film devices. Light-trapping efficiency can be optimized by means of optical simulations of nano-structured solar cells. We present a FEM based simulator for 3D rigorous optical modeling of amorphous silicon / microcrystalline silicon tandem thin-film solar cells with randomly textured layer interfaces. We focus strongly on an error analysis study for the presented simulator to demonstrate the numerical convergence of the method and investigate grid and finite element degree refinement strategies in order to obtain reliable simulation results.

Thin-film a-Si:H solar cells processed on aluminum-induced texture (AIT) glass superstrates: prediction of light absorption enhancement

Light scattering superstrates are important for thin-film a-Si:H solar cells. In this work, aluminum-induced texture (AIT) glass, covered with nonetched Al-doped ZnO (AZO), is investigated as an alternative to the commonly used planar glass with texture-etched AZO superstrate. Four different AIT glasses with different surface roughnesses and different lateral feature sizes are investigated for their effects on light trapping in a-Si:H solar cells. For comparison, two reference superstrates are investigated as well: planar glass covered with nonetched AZO and planar glass covered with texture-etched AZO. Single-junction a-Si:H solar cells are deposited onto each superstrate, and the scattering properties (haze and angular resolved scattering) as well as the solar cell characteristics (current-voltage and external quantum efficiency) are measured and compared. The results indicate that AIT glass superstrates with nonetched AZO provide similar, or even superior, light trapping than the standard reference superstrate, which is demonstrated by a higher short-circuit current Jsc and a higher external quantum efficiency. Using the trapped light fraction δ, a quantity based on the integrated light scattering at the AZO/a-Si:H interface, we show that Jsc linearly increases with δ in the scattering regime of the samples, regardless of the type of superstrate used.

Modification of light scattering properties of boron doped zinc oxide grown by Low Pressure Chemical Vapour Deposition using wet chemical etching

The best light scattering properties for ZnO:B films grown by Low Pressure Chemical Vapor Deposition (LPCVD) require low doping in the precursors and films. Such films must be thicker than 2 µm to achieve sheet resistances ≤ 20 Ω/sq, suitable for thin film silicon solar cells. In this study, ZnO:B films, grown by LPCVD with different doping levels, were etched using several solutions so as to modify their surface morphology in a quick and simple way. The haze ratio, at 800 nm, of the films increased from values below 10 % to about 20 % after etching. Observation of the SEM micrographs confirmed that the film surface had been modified by the etching process. Highly doped films could be etched to achieve a high haze while keeping the sheet resistance below 15 Ω/sq. Tandem junctions consisting of amorphous silicon p-i-n and micro-crystalline silicon p-i-n solar cells grown on etched highly doped ZnO:B films exhibited higher open circuit voltage than those on a natively textured lower doped films. Further optimization of both the etching process and the starting ZnO:B material is expected to lead to higher solar cell efficiency. These results show that the optimum thickness of ZnO:B front contacts in thin film silicon can be reduced, potentially cutting production time and material costs.

Optimization of PECVD process for ultra-thin tunnel SiO x film as passivation layer for silicon heterojunction solar cells

Ultra-thin silicon oxide (a-SiOx:H) films have been grown by means of plasma enhanced chemical vapor deposition (PECVD) to replace the standard hydrogenated amorphous silicon (a-Si:H) passivation layer for silicon heterojunction solar cells to reduce parasitic absorption. Additionally, silicon oxide surfaces are well known as superior substrates for the nucleation enhancement for nanocrystalline silicon doped films. Symmetrical passivation samples were fabricated with variable a-SiOx:H layers with a thickness of 10-1.5 nm and characterized after several annealing steps (25-650 °C). The best value reached so far on <;100> oriented Si wafers is: implied open circuit voltage of 686 mV and minority carrier lifetime of 1.6 ms after annealing at 300 °C. Such values were found to be reproducible even for ultra-thin a-SiOx:H layers (1.5 nm).

Detailed comparison of transparent front contacts for thin film silicon solar cells

In this contribution we compare the optical, electronic and surface morphological properties of various textured transparent conducting oxides (TCO) based on zinc oxide and fluorine doped tin oxide. Since the TCO material properties tend to be interrelated, comparison of such films is a rather complex exercise. The TCO films were characterised by atomic force microscopy, scanning electron microscopy, spectrophotometry, angle resolved scattering, four point probe and Hall Effect measurements. Thereafter, an attempt was made to correlate the TCO material properties with the corresponding current-voltage characteristics and quantum efficiency in actual solar cells. It was found that the solar cells were more sensitive to changes in the optical properties of the TCO substrates than to those in the sheet resistance. Based on these results, we recommend that when optimising TCO films for thin film silicon production, the first priority should be to obtain the highest transmittance possible and then to tune the electrical properties accordingly.