Analytical model for the optical functions of amorphous semiconductors and its applications for thin film solar cells (original) (raw)
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We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (ϵ1,ϵ2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (ϵ1,ϵ2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (ϵ1,ϵ2) can be simulated assuming a single Lorentz oscillator. The expression developed here provides an improved description of ϵ2 (or α) in the below-band gap and near-band gap regions compared with previous approaches. Although the expression is more complicated analytically, it has numerous applications in the analysis and simulation of thin film a-Si:H based p-i-n and n-i-p multilayer photovoltaic devices. First, we describe an approach whereby, from a single accessible measure of the optical band gap, the optical functions can be generated over the full solar spectrum for a sample set consisting of the highest quality intrinsic a-Si:H based alloys prepared by plasma-enhanced chemical vapor deposition using the principle of maximal H2 dilution. Second, we describe quantitatively how such an approach can be modified for sample sets consisting of lower quality alloy materials. Finally, we demonstrate how the generated optical functions can be used in simulations of the absorption, reflection, and quantum efficiency spectra of a-Si:H based single-junction and multijunction solar cells. © 2002 American Institute of Physics.
The Effect of Hydrogenated Amorphous Silicon on the Optical Properties of Solar Cells
2017
Hydrogenated amorphous silicon (a-Si:H) produced by plasma enhanced chemical vapor deposition (PECVD) is a very interesting material due to the possibility of controlling the energy band gap and the electrical properties by means of the alloy composition. In this work we focused on the intrinsic layer because it is responsible for light absorption and the subsequent charge carrier generation/separation and for photovoltaic stability under continuous illumination (Staebler–Wronski) effect. We have prepared six sample of a-Si:H (i-layer) A, B, C, D, E, F with varied H2 diluted (40, 50, 60, 70 sccm) and the deposition time is varied (30, 40 minutes) and the flow of SiH4 was fixed at 20 sccm. The pressure of chamber MPZ (modular process zones) before deposition process about 5×10 Torr. While during deposition process is 530 mTorr, the deposition temperature is 2700C and RF power is 1.8 watt. To evaluation the optical properties of the thin film we used multiple reflection method (NanoCa...
Two-dimensional computer modeling of single junction a-Si:H solar cells
2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009
A two dimensional physically-based computer simulation of single junction pin amorphous silicon solar cells is presented using Sentaurus, TCAD by Synopsys Inc. The simulation program solves the Poisson, the continuity, and the current density equations by using a standard procedure for amorphous materials, including the continuous density of state model, Shockley-Read-Hall and Auger recombination mechanisms, and computes the generation function of electron-hole pairs from the optical parameters of each layer. The dependence of these optical parameters with the photon energy has been included, taking into account the doping level, thickness of each layer and their effect on cell efficiency. The simulator is applied to the analysis of a pin single junction a-SiC:H/a-Si:H/a-Si:H solar cell, obtaining results comparable to one dimensional simulation results using AMPS-1D. More advanced simulation models for novel solar cell devices such as tandem cell are in progress, with the aim of achieving an optimal design of solar cells based on amorphous materials or micro-/nanocrystalline layer.
Key issues for accurate simulation of a-Si:H / c-Si heterojunction solar cells
Energy Procedia, 2011
Accurate simulation of a-Si:H / c-Si heterojunction (HET) solar cells is mandatory for acquiring a deeper understanding of device physics, better knowledge of material properties, and thus improving solar cells efficiency towards the 26% theoretical limit. The purpose of this paper is to provide relevant guidelines and to highlight key issues for accurate and physicallybased HET solar cells simulation. The need for a 2D simulation approach is demonstrated, together with an accurate description of the device optical performance. For the first time, a unified set of models and material parameters is proposed for reproducing experimental IV characteristics under illumination and obscurity conditions, considering state-of-the-art material parameters and localized defects. Finally, the key role of solar cell simulation is demonstrated for further device optimization.
Thin Solid Films
The relation between the open-circuit voltage (V oc) of hydrogenated amorphous silicon (a-Si:H) solar cells and the band gap of the absorber layer has been investigated by changing the substrate temperature of the absorber layer. By decreasing the substrate temperature from 200°C to 150°C the V oc has increased by 0.05V. However, the temperature dependence of the V oc is larger than the change of the optical band gap of individual intrinsic layers, which indicates that also other absorber layer properties play an important role. Using simulations the effect of changing the mobility gap of as well the absorber layer as the buffer layer at the p-i interface on the performance of a-Si:H solar cells has been investigated. The recombination rate profile in the solar cell at the V oc is a useful tool to analyze and optimize different parts of the solar cell to obtain a high V oc. The simulations confirm the trend of increasing the V oc when implementing absorber layers with a higher mobility gap. The simulations demonstrate that increasing the mobility gap of the buffer layer at the p-i interface results in a further improvement of the V oc , however an optimal value of the mobility gap is found with respect to the conversion efficiency.
Optical Modeling of a-Si Solar Cells
MRS Proceedings, 1999
We describe applications of PV Optics to analyze the behavior of a metallic back-reflector on an a-Si solar cell. The calculated results from PV Optics agree well with the measured data on solar cells. Several unexpected results obtained from these calculations are qualitatively explained.
Proceedings of the 8th Spanish Conference on Electron Devices, CDE'2011, 2011
Simulation data of the performance of amorphous silicon (a-Si:H) thin film solar cells using the software package Sentaurus TCAD (Synopsis Inc.) are presented. The Sentaurus software is configured with standard theoretical models describing e.g. the density of states in the mobility gap of a-Si:H, generation/recombination statistics, optical data of a-Si:H thin films etc. to calculate illuminated current voltage curves and the respective spectral response for the initial and degraded state of the solar cell. For the selected physical properties of the solar cell the simulation data predicts a maximum of the efficiency for an intrinsic a-Si:H layer thickness between 200-250 nm. Furthermore, a guideline for the optimization of the p-doped layer thickness and the doping concentration is given.