Oleg Sergeev

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.

Cost-effective nanostructured thin-film solar cell with enhanced absorption

Nanostructured transparent conductive electrodes are highly interesting for efficient light management in thin-film solar cells, but they are often costly to manufacture and limited to small scales. This work reports on a low-cost and scalable bottom-up approach to fabricate nanostructured thin-film solar cells. A folded solar cell with increased optical absorber volume was deposited on honeycomb patterned zinc oxide nanostructures, fabricated in a combined process of nanosphere lithography and electrochemical deposition. The periodicity of the honeycomb pattern can be easily varied in the fabrication process, which allows structural optimization for different absorber materials. The implementation of this concept in amorphous silicon thin-film solar cells with only 100 nm absorber layer was demonstrated. The nanostructured solar cell showed approximately 10% increase in the short circuit current density compared to a cell on an optimized commercial textured reference electrode. The concept presented here...

Carrier collection losses in interface passivated amorphous silicon thin-film solar cells

In silicon thin-film solar cells the interface between the i- and p-layer is the most critical. In the case of back diffusion of photogenerated minority carriers to the i/p-interface, recombination occurs mainly on the defect states at the interface. To suppress this effect and to reduce recombination losses, hydrogen plasma treatment (HPT) is usually applied. As an alternative to using state of the art HPT we apply an argon plasma treatment (APT) before the p-layer deposition in n-i-p solar cells. To study the effect of APT, several investigations were applied to compare the results with HPT and no plasma treatment at the interface. Carrier collection losses in resulting solar cells were examined with spectral response measurements with and without bias voltage. To investigate single layers, surface photovoltage and X-ray photoelectron spectroscopy (XPS) measurements were conducted. The results with APT at the i/p-interface show a beneficial contribution to the carrier collection compared with HPT and no plasma treatment. Therefore, it can be concluded that APT reduces the recombination centers at the interface. Further, we demonstrate that carrier collection losses of thin-film solar cells are significantly lower with APT.

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.

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.

Effect of the Vertical Transportation Component of the TCO Layer on the Electrical Properties of Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells that consist of thin amorphous silicon layers and crystalline silicon substrate are known as the high-efficiency class of solar cells. To collect the charge carriers, transparent conductive oxide (TCO) layers are inserted in which the charge carriers are being either vertically or both laterally and vertically transported. In this study, we have investigated the effect of the vertical transportation component of aluminum-doped zinc oxide (AZO) layers on the electrical properties of the fabricated SHJ solar cells and its contribution to the total series resistance of the obtained devices. In order to separate the vertical from the lateral transportation, we have employed an AZO/Ag/AZO multilayer structure, which only allows the vertical transportation of the charge carriers within the AZO layers. Our results show that with increase in O2 flow, the reduction rate of the FF is about three times higher when both lateral and vertical conductions take place, compared with when only vertical conduction occurs. Moreover in the latter case, a reduction of ~ 6% in the FF value per unit increase of vertical resistivity is obtained. Finally, we validate our procedure by comparing the obtained experimental results with the theoretically modeled values. The validation delivered a good agreement.

Simulation of single-junction thin-film silicon solar cells with varying intrinsic layer thickness

Optical and electrical simulation is an economical method for optimizing the layer thicknesses of silicon based thin-film solar cells. However, the used electrical parameter set must be verified for different layer stack configurations to calibrate the modeling system. In this contribution we present an electrical parameter set as input data, which is able to model dark and illuminated IV-curves and EQE-spectra of a-Si:H single junction solar cells with different intrinsic layer thicknesses. Only few parameters are varied to align the experimental characteristics of the solar cells.