R.A.C.M.M. van Swaaij

Optical modeling of a-Si:H solar cells with rough interfaces: Effect of back contact and interface roughness

An approach to study the optical behavior of hydrogenated amorphous silicon solar cells with rough interfaces using computer modeling is presented. In this approach the descriptive haze parameters of a light scattering interface are related to the root mean square roughness of the interface. Using this approach we investigated the effect of front window contact roughness and back contact material on the optical properties of a single junction a-Si:H superstrate solar cell. The simulation results for a-Si:H solar cells with SnO2:F as a front contact and ideal Ag, ZnO/Ag, and Al/Ag as a back contact are shown. For cells with an absorber layer thickness of 150–600 nm the simulations demonstrate that the gain in photogenerated current density due to the use of a textured superstrate is around 2.3 mA cm−2 in comparison to solar cells with flat interfaces. The effect of the front and back contact roughness on the external quantum efficiency (QE) of the solar cell for different parts of the light spectrum was de...

A scattering model for nano-textured interfaces and its application in opto-electrical simulations of thin-film silicon solar cells

We present a scattering model based on the scalar scattering theory that allows estimating far field scattering properties in both transmission and reflection for nano-textured interfaces. We first discuss the theoretical formulation of the scattering model and validate it for nano-textures with different morphologies. Second, we combine the scattering model with the opto-electric asa simulation software and evaluate this combination by simulating and measuring the external parameters and the external quantum efficiency of solar cells with different interface morphologies. This validation shows that the scattering model is able to predict the influence of nano-textured interfaces on the solar cell performance. The scattering model presented in this manuscript can support designing nano-textured interfaces with optimized morphologies.

Application of plasmonic silver island films in thin-film silicon solar cells

Silver nanoparticles can be used as light scattering elements for enhancing solar cell energy conversion efficiencies. The objective of our work is to gain more insight into the optical properties of silver nanoparticle films and their effect on the performance of solar cells. We use a common self-assembly technique to fabricate a range of silver island films on transparent substrates and measure their reflectance and transmittance for visible and near infrared light. We demonstrate that it is possible to represent silver island films by an effective medium with the same optical properties. The observed strong dependence on illumination side of the reflectance and absorptance, attributed to driving field effects, is reproduced very well. Thin-film silicon solar cells with embedded silver island films were fabricated and it was found that their performance is reduced due to parasitic absorption of light in the silver island film. Simulations of these solar cells, where the silver island film is represented as an effective medium layer, show a similar trend. This highlights the importance of minimizing parasitic absorption.

Determination of the mobility gap of intrinsic μc-Si:H in p-i-n solar cells

Microcrystalline silicon (?c-Si:H) is a promising material for application in multijunction thin-film solar cells. A detailed analysis of the optoelectronic properties is impeded by its complex microstructural properties. In this work we will focus on determining the mobility gap of ?c-Si:H material. Commonly a value of 1.1?eV is found, similar to the bandgap of crystalline silicon. However, in other studies mobility gap values have been reported to be in the range of 1.48–1.59?eV, depending on crystalline volume fraction. Indeed, for the accurate modeling of ?c-Si:H solar cells, it is paramount that key parameters such as the mobility gap are accurately determined. A method is presented to determine the mobility gap of the intrinsic layer in a p-i-n device from the voltage-dependent dark current activation energy. We thus determined a value of 1.19?eV for the mobility gap of the intrinsic layer of an ?c-Si:H p-i-n device. We analyze the obtained results in detail through numerical simulations of the ?c-Si:H p-i-n device. The applicability of the method for other than the investigated devices is discussed with the aid of numerical simulations.

Local structure and bonding states in a‐Si1−xCx:H

Infrared spectra of a‐Si1−xCx:H deposited in a glow discharge of a silane/methane mixture have been measured. Comparison with elastic recoil detection and Rutherford backscattering spectrometry shows that the mean number of hydrogen atoms attached to silicon per silicon atom ([HSi]/[Si]) increases with higher carbon content and that more Si—H2 bonding configurations are formed. Hydrogen is preferentially bonded in (Si—H2)n clusters, which partly explains the observed apparent shift of the Si—H stretching mode to higher energy. The remaining contribution to this shift is believed to result from Si—H on surfaces of voids instead of an inductive effect. From composition measurements we observe that for each carbon atom, three hydrogen atoms are incorporated in the material, suggesting that during deposition carbon is initially incorporated in CH3 groups. However, the mean number of C—H bonds per carbon atom decreases from about 2.2±0.4 to 1.4±0.3 with increasing carbon content, indicating that the majority o...

Quadruple-junction thin-film silicon-based solar cells with high open-circuit voltage

We have fabricated a-SiOx:H/a-Si:H/nc-Si:H/nc-Si:H quadruple-junction thin-film silicon-based solar cells (4J TFSSCs) to obtain high spectral utilization and high voltages. By processing the solar cells on micro-textured superstrates, extremely high open-circuit voltages for photovoltaic technology based on thin-film silicon alloys up to 2.91 V have been achieved. Optical simulations of quadruple-junction solar cells using an advanced in-house model are a crucial tool to effectively tackle the challenging task of current matching among the individual sub-cells in such devices. After optimizing the optical design of the device and the absorber thicknesses, an energy conversion efficiency of 11.4% has been achieved. The open-circuit voltage, short-circuit current density, and fill factor were 2.82 V, 5.49 mA/cm2, and 73.9%, respectively. Based on this demonstration, strategies for further development of highly efficient 4J TFSSCs are proposed.

Spatial effects on ideality factor of amorphous silicon pin diodes

We analyze dark current-voltage and short-circuit current versus open-circuit voltage measurements of hydrogenated amorphous silicon pin and nip diodes. The ideality factor of ∼1.4 is independent of the thickness of the intrinsic layer, indicating that recombination occurs in the p/i interface region rather than in the bulk. These results can be simulated accurately when the defect-pool model is used. Lateral amorphous silicon diodes with a uniform defect density throughout the intrinsic region have ideality factors, which do depend on the size of the intrinsic region and exceed the theoretical value of 2. We present a model to explain the ideality factor and to verify the results with numerical simulations. Values smaller than 2 are caused by unequal shifts of the quasi-Fermi levels for electrons and holes; values larger than 2 arise when the recombination is spread out over a wide region. In that case the distance between the quasi-Fermi levels is smaller than the applied voltage.

Angular resolved scattering measurements of nano-textured substrates in a broad wavelength range

The angular intensity distribution (AID) is a major parameter for evaluating scattering of light by surface-textured thin films. We discuss how the AID can be determined in the near ultraviolet, the visible and the near infrared and evaluate the used method by comparing the obtained measurement results to the results obtained with other methods. Measuring the AID in a broad wavelength range is of great use for the solar cell community, because textured thin films are widely used to enhance the photocurrent in thin-film solar cells.

Structural, compositional and optical properties of hydrogenated amorphous silicon-carbon alloys

Abstract Steady-state optical modulation spectroscopy (OMS) measurements have been carried out on well characterized hydrogenated amorphous silicon-carbon alloy layers. A series of samples with methane-to-silane gas flow ratios varying from 0.1 to 0.7 was deposited at 250°C using the conventional r.f. glow-discharge deposition method. In order to characterize the material, we investigated the structural, compositional and optical properties of these alloys by means of Fourier-transform infrared spectroscopy, Raman spectroscopy, elastic recoil detection, Rutherford back-scattering spectrometry and optical transmission and reflection spectroscopy. OMS measurements were performed at room temperature and at T 50 K. From room-temperature data the energy positions of the dangling-bond states were deduced. Transition energies involving D° states are found to shift to higher values with increasing carbon content, while transition energies involving D− states remain almost constant. At lower temperatures, band-ta...

Designing optimized nano textures for thin-film silicon solar cells.

Thin-film silicon solar cells (TFSSC), which can be manufactured from abundant materials solely, contain nano-textured interfaces that scatter the incident light. We present an approximate very fast algorithm that allows optimizing the surface morphology of two-dimensional nano-textured interfaces. Optimized nano-textures scatter the light incident on the solar cell stronger leading to a higher short-circuit current density and thus efficiency. Our algorithm combines a recently developed scattering model based on the scalar scattering theory, the Perlin-noise algorithm to generate the nano textures and the simulated annealing algorithm as optimization tool. The results presented in this letter allow to push the efficiency of TFSSC towards their theoretical limit.