Kambulakwao Chakanga

Three dimensional optical modeling of amorphous silicon thin film solar cells using the finite-difference time-domain method including real randomly surface topographies

In this paper, modeling of light propagation in silicon thin film solar cells without using any fitting parameter is presented. The aim is to create a realistic view of the light trapping effects and of the resulting optical generation rate in the absorbing semiconductor layers. The focus is on real three dimensional systems. Our software Sentaurus tcad, developed by Synopsys, has the ability to import real topography measurements and to model the light propagation using the finite-difference time-domain method. To verify the simulation, we compared the measured and simulated angular distribution functions of a glass/SnO2:F transparent conducting oxide system for different wavelengths. The optical generation rate of charge carriers in amorphous silicon thin film solar cells including rough interfaces is calculated. The distribution of the optical generation rate is correlated with the shape of the interface, and the external quantum efficiencies are calculated and compared to experimental data.

Optimizing Folded Silicon Thin-Film Solar Cells on ZnO Honeycomb Electrodes

A promising approach for low-cost nanostructured thin-film solar cells with enhanced absorption is the fabrication of zinc oxide (ZnO) honeycomb electrodes in a combined bottom-up process of nanosphere lithography and electrochemical deposition. To optimize the honeycomb structures, we investigate thin hydrogenated amorphous silicon (a-Si:H) solar cells (with 100 nm absorber thickness) on honeycomb electrodes with different periodicities in optical and electrical simulations; whereas the electrical performance is not significantly affected with changing periodicity, the short-circuit current density is reduced for increasing honeycomb diameter due to increased parasitic absorption of the electrochemically deposited ZnO. Furthermore, we demonstrate that for micromorph tandem solar cells with an intrinsic layer thickness of hydrogenated microcrystalline silicon (μc-Si:H) of >500 nm, a focusing effect occurs, which leads to a strong enhancement in the quantum efficiency in the microcrystalline bottom solar cell.

Laser textured substrates for light in-coupling in thin-film solar cells

Abstract. In this work, we investigate the use of a picosecond (ps) laser used for monolithic connection to texture three commercially available and frequently used multicomponent glasses, Corning EAGLE XG®, Schott BOROFLOAT® 33 and Saint-Gobain SGG DIAMANT®. The results show that the ablated crater profile and degree of texturing are glass composition dependent. This might be attributed to the different laser-induced electron collision times and recombination rates, and thus the critical electron density evolution leading to ablation. The surface texture is altered from periodic to random with decreasing scribing speed. The transmission of the textured substrates gradually decreases, whereas the multireflection on the surface increases as a consequence of the topological and morphological changes. The angular resolved measurements illustrate that the textured glass substrates scatter the light toward greater angles, which is necessary to increase the effective optical path in the absorber layer. Simulation results show that textured glass increases the absorption in the absorber material and the slightly modified refractive index region around the crater does not counteract the light in-coupling effect. The results suggest that these substrates can be used in various photovoltaic technologies and show potential for the application of alternative front contacts, such as carbon nanotubes.

Optical modeling of light trapping in thin film silicon solar cells using the FDTD method

In this paper we present our recent work on modeling the light propagation in silicon thin film solar cells. The aim is to create a realistic view of the light trapping effects and of the resulting optical generation rate. The focus is on real three dimensional systems. Our software Sentaurus TCAD, developed by Synopsys, has the ability to import real topography measurements and to model the light propagation with the Finite-Difference Time-Domain method (FDTD).

Textured substrates for light in-coupling in thin-film solar cells

The current world population of 7.2 billion is predicted to increase to 9.6 billion in 2050,1 and world energy consumption is projected to grow by 56% between 2010 and 2040 to 820 quadrillion British thermal units (Btu).2 Although fossil fuels constitute almost 80% of energy use,2 they are depleting, and it is becoming more urgent to ensure that energy supply is sustainable. Renewable energies already belong to the world’s fastest-growing energy sources and for electricity generation are increasing by 2.8% per year.2 The most important renewable energy sources are hydro, wind, and photovoltaic (PV).3 Silicon wafer-based modules dominate the PV market with an 86% market share. The other 14% belongs to thin-film PV,4 some of which is based on hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon ( c-Si:H), which share the advantage of silicon’s abundance as a raw material. An additional advantage is that less silicon is required than for conventional wafer silicon solar cells. Thus, silicon-based thin-film technology has the potential for low-cost, rapid, largearea industrial production and low energy-payback time.5 Current solar cell efficiencies are 10.1% for single-junction a-Si:H in the p-i-n configuration (meaning a combination of positively doped, intrinsic, and negatively doped silicon),6 10.8% for single-junction c-Si:H p-i-n,7 and 12.3% for tandem a-Si:H/ c-Si:H (micromorph).8 Light trapping has proven a useful tool for increasing efficiency. The concept of light trapping, also termed light in-coupling, is illustrated in Figure 1(a). Incorporating a textured surface increases the effective light path and hence absorption in the silicon absorber material.9 Consequently, even thinner absorber layers can be used, an advantage for amorphous silicon, which suffers from a light-induced Figure 1. Schematic of a silicon thin-film p-i-n (a combination of positively doped, intrinsic, and negatively doped silicon) solar cell with (a) a textured transparent conductive oxide (TCO) front contact9 and (b) a textured glass substrate. c-Si: Microcrystalline silicon. a-Si: Amorphous silicon.

Computational investigation of silicon thin-film solar cells withgrating structures fabricated by holographic lithography

Light trapping due to rough interfaces is a common and industrially applied technique to enhance cell performance in silicon thin-film solar cells. The induced scattering enhances the absorption and consequently the conversion efficiency of the device. Periodic structures promise to further enhance the light trapping, allowing a beneficial reduction of the absorber layer thickness. In this work, solar cells with transparent front contacts with a two-dimensional (2D) grating structure produced by holographic lithography are investigated. The grating structures are characterized by various means and the results are used to calibrate finite-difference time-domain (FDTD) simulations. With the computational method, the influence of the grating height on the solar cell performance is investigated.