Multijunction Thin-film Silicon Solar Cells Research Articles

Design Considerations of Efficient Photo-Electrosynthetic Cells and its Realization Using Buried Junction Si Thin Film Multi Absorber Cells

AbstractAs is obvious from previous work on semiconductor photoelectrochemistry, single junction semiconductors do not provide either the required maximum photovoltage or a high photocurrent for solar water splitting, which is required for efficient stand-alone devices. From these experiences we conclude, that multi-junction devices must be developed for bias-free water splitting. In this article we present our design considerations needed for the development of efficient photo-electro-synthetic cells, which have guided us during the DFG priority program 1613. At first, we discuss the fundamental requirements, which must be fulfilled to lead to effective solar water splitting devices. Buried junction and photoelectrochemical arrangements are compared. It will become clear, that the photovoltaic (PV) and electrochemical (EC) components can be optimized separately, but that maximized conversion efficiencies need photovoltages produced in the photovoltaic part of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. Therefore, in part 2 we will present the needs to develop appropriate interface engineering layers for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction (HER, OER), must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentials. The third part of our paper describes at first the demands and achievements on developing multijunction thin-film silicon solar cells. With different arrangements of silicon stacks a wide range of photovoltages and photocurrents can be provided. These solar cells are applied as photocathodes in integrated directly coupled PV-EC devices. For this purpose thin Pt and Ni catalyst layers are used on top of the solar cells for the HER and a wire connected RuO2counter electrode is used for the OER. Electrochemical stability has been successfully tested for up to 10,000 s in 0.1 M KOH. Furthermore, we will illustrate our experimental results on interface engineering strategies using TiO2as buffer layer and Pt nanostructures as HER catalyst. Based on the obtained results the observed improvements, but also the still given limitations, can be related to clearly identified non-idealities in surface engineering either related to recombination losses at the semiconductor surface reducing photocurrents or due to not properly-aligned energy states leading to potential losses across the interfaces.

Porous multi-junction thin-film silicon solar cells for scalable solar water splitting

Monolithic solar water splitting devices implemented in an integrated design approach, i.e. submerged in the electrolyte, pose a significant limitation when it comes to up-scaling. The ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This fact turns out to be a bottleneck for reaching high device efficiency and maintaining optimum performance upon up-scaling. In this paper, we propose a new device design for integrated monolithic solar water splitting based on porous multi-junction silicon solar cells. Simulation results highlight that porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. In particular, we show how micrometer scale pore dimensions could greatly reduce Ohmic losses, thereby minimizing overpotentials. A square array of holes with a diameter of 20 µm and a period of 100 µm was fabricated on single-junction and multi-junction amorphous and microcrystalline silicon solar cells. A small impact on the open circuit voltage (Voc) and short circuit current density (Jsc) was obtained, with porous triple junction solar cells reaching Voc values up to 1.98 V. A novel device design is proposed based on porous triple-junction silicon-based solar cells.

Illumination intensity and spectrum-dependent performance of thin-film silicon single and multijunction solar cells

The performance of a solar cell is inherently dependent on the illumination spectrum and intensity. Therefore standard characterization under AM1.5 illumination represents only one point in a large parameter space. In view of potential applications in portable electronics, obtaining reference data on the performance under varying light sources and illumination intensity for a comprehensive set of thin-film silicon solar cells (TFSC) is a primary motivation of our study. In addition, illumination dependencies of photovoltaic parameters provide deeper understanding of the operation and limitations of thin-film silicon solar cell for both indoor and outdoor applications. In this paper we assess the performance of single and multijunction TFSC with amorphous (a-Si:H) and microcrystalline (µc-Si:H) absorber layers under common light sources like light emitting diode (LED), halogen, fluorescent and solar simulator with AM1.5 spectrum. The illumination intensity of these light sources has been varied over several orders of magnitude (0.01–230mW/cm2) to explore a wide range of operating conditions for the solar cells. We observed expected increase in efficiency with increase of the illumination intensity (up to approximately 1 sun) of all cells and naturally strong dependence on the illumination spectrum. For example, the efficiency reached impressive 20% for a-Si:H cells under LED illumination while it is below 4% under halogen bulb. More detailed analysis performed in this work shows that the cell parameters are not always following diode model predictions mostly due to the bias-dependent collection in drift driven thin-film silicon solar cells. The results first of all provide reference for the assessment of energy gain from TFSC in various illumination conditions. It also provides better view into the limitations of TFSC and potentials for improvement especially for the non-standard illumination spectra and intensities.

Light Scattering Enhancement by Double Scattering Technique for Multijunction Thin-Film Silicon Solar Cells

Light trapping is an important technique to increase the efficiency of thin-film silicon solar cells. Textured surfaces are known to scatter sunlight while it passes through thin-film solar cells, thereby increasing the optical pathlength and, thus, the photon absorption in the devices. In this paper, microtextured glass superstrates were prepared by the aluminum-induced texturization (AIT) method. These superstrates achieve high transmission haze values of up to 60% while maintaining a high total optical transmission. We demonstrate that both the surface structure and the roughness of the textured glass surface can be controllably adjusted by changing the AIT process parameters. Approximately 900-nm-thick aluminum-doped zinc oxide (AZO) films are deposited onto the microtextured glass surfaces by magnetron sputtering and then further textured using wet-chemical etching in diluted HCl, creating an AZO surface that features both micrometer-scale and submicron-scale structures. Optical spectroscopy and goniophotometer measurements reveal that the light scattering capability of the substrates increases significantly due to the wet-chemical AZO texturization. The combination of microtextured AIT glass, together with the submicron-textured AZO, could be very attractive for high-efficiency double-junction micromorph thin-film Si solar cells, whereby the amorphous Si top cell benefits significantly from the AZO's submicron texture and the microcrystalline Si bottom cell benefits primarily from the microtextured glass surface.

High-Efficiency, Multijunction nc-Si:H-Based Solar Cells at High Deposition Rate

Hydrogenated nanocrystalline silicon (nc-Si:H) is a promising candidate to replace the hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin-film silicon solar cells due to its superior long-wavelength response and stability against light-induced degradation. Due to its indirect bandgap, the absorbing nc-Si:H layer needs to be much thicker than its amorphous counterpart. For nc-Si:H-based solar cells to be commercially viable, the challenge is to deposit the nc-Si:H layer at a high rate with good quality. In this paper, we report on the development of our proprietary high-frequency glow discharge deposition technology to fabricate high-efficiency, large-area, a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells at a high deposition rate >;1 nm/s. The National Renewable Energy Laboratory (NREL) has confirmed stable efficiency of 12.41% on a 1.05-cm2 solar cell. We have attained initial efficiency of 12.33% on an encapsulated cell of aperture area ~400 cm2 ; the corresponding stable efficiency is projected to be 11.7-11.9%.

12.0% Efficiency on Large-Area, Encapsulated, Multijunction nc-Si:H-Based Solar Cells

Because of its superior long-wavelength response and stability against light-induced degradation, hydrogenated nanocrystalline silicon (nc-Si:H) has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin-film silicon solar cells. In this paper, we report on the development of our proprietary high-frequency (HF) glow discharge deposition technology for nc-Si:H solar cells that has resulted in high-quality nc-Si:H materials with good spatial uniformity. We studied the HF-deposited nc-Si:H material using various analytical techniques. We fabricated a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells that are deposited on textured Ag/ZnO back reflectors. Large-area cells were fabricated and encapsulated using our proprietary lightweight flexible encapsulants. National Renewable Energy Laboratory (NREL) has confirmed (1) initial aperture-area efficiency of 11.8% on an 807.8-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> encapsulated cell and (2) stable aperture-area efficiency of 11.2% on a 400-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> encapsulated cell.

3-D optical modeling of single and multi-junction thin-film silicon solar cells on gratings

ABSTRACTThree-dimensional (3-D) optical modeling based on Finite Element Method of single, double, and triple junction thin-film silicon solar cells is presented. The combination of front periodic gratings with optimal geometrical parameters and rear ZnO/Ag reflector constitutes an efficient light trapping scheme for solar cells in superstrate (pin) configuration. The application of optimized trapezoidal 1-D and 2-D gratings resulted in 25.5% (1-D case) and 32.5% (2-D case) increase in photo-current density with respect to the flat solar cell. The application of inverted pyramidal 2-D gratings in double and triple junction silicon solar cells with very thin absorber layers resulted in a photo-current density &gt; 11 mA/cm2 and &gt; 9 mA/cm2, respectively.

Doped Microcrystalline Silicon Layers for Solar Cells by 13.56MHz Plasma-enhanced Chemical Vapour Deposition

Doped hydrogenated microcrystalline silicon thin films play a critical role in multi-junction thin-film silicon solar cells, because their crystallinity has a large influence on the properties of intrinsic microcrystalline silicon absorber layers grown on them. The doping efficiency of the doped layers depends strongly on their crystallinity and hence a high-crystallinity doped layer is desired. In this study, highly crystalline doped microcrystalline silicon films are formed on 300mm × 400mm glass substrates using a conventional parallel-plate PECVD reactor operated at 13.56MHz. Raman spectroscopy is used to analyse the crystallinity of the films. The conductivity of the films is measured using the co-planar electrode method. The effects of the deposition parameters on the Raman crystallinity and conductivity of the doped films are investigated. The RF power is found to play a key role for achieving a high crystallinity in the doped layers, whereby a high crystallinity can only be obtained within a narrow RF power range. The influence of the RF power on the lateral thickness uniformity of the deposited films is also examined. It is found that the RF power has a strong influence on the lateral uniformity of the deposited films, with intermediate power giving the best thickness uniformity.

Fabrication and characterisation of parallel multijunction thin film silicon solar cells

The parallel multijunction solar cell design offers the exciting possibility of high efficiency at low cost. To date, there has been no detailed report on the experimental characteristics of these devices. This paper reports on the beginnings of a detailed experimental investigation of the parallel multijunction solar cell. Progress is reported on the fabrication of parallel multijunction thin-film silicon solar cells (on inert single-crystal silicon substrates), specifically designed and fabricated to serve as experimental test-beds for the detailed study of cell performance limiting mechanisms. Of particular interest is the importance of junction space-charge-region recombination in heavily defected parallel multijunction cells.