Search

The optical performance of micromorph solar cells employing mixed-phase n-SiO x as advanced n-layer in the top junction has been systematically investigated with efficient state of the art ZnO front electrodes characterized by different scattering properties. The layer is applied with the dual function of n-layer and intermediate reflector. Enhanced top cell current complemented by significant dependence on the front texture has been observed. With smoother (locally flat-like) front electrode, interference effects are present with adverse impact on the spectral response of the bottom cell. Moving to rougher substrates these effects are strongly attenuated thus fully disclosing the strength of the present scheme, which is the simultaneous enhancement of both top cell current and total current. Quantitatively the gain is impacted by the front electrode morphology. Guidelines for a cross-optimization of texture and dual-function n-SiO x properties/thickness are supplied in order to take full advantage from this design. [ABSTRACT FROM AUTHOR]

We have investigated the application of ultrathin lithium fluoride (LiF) interlayers for effective light harvesting in hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) tandem solar cells. It is proved that the LiF interlayers are not suitable for intermediate reflectors of the tandem solar cells despite their low refractive index and low lateral conductivity. A poor vertical conductivity leads to the formation of a highly resistive tunnel junction. On the contrary, novel hydrogenated n-type silicon-oxide (n-SiO x :H)/LiF back reflectors are successfully employed in the tandem solar cells, reducing plasmonic absorption losses in nanotextured Al back contacts and providing effective refractive index grading. It is found that the ultrathin LiF interlayer mitigates nanotextures of the Al back contact. The spectral response of μc-Si:H bottom cells is markedly elevated in a near-infrared wavelength region. As a result, a conversion efficiency is improved by 8.1% compared to the reference cell with a conventional zinc oxide (ZnO) back reflector thanks to an increase in a short-circuit current by 5.5%. Consequently, the initial efficiency of 10.4% is attained. [ABSTRACT FROM AUTHOR]

ABSTRACT Optical and electrical simulations were carried out for thin film silicon solar tandem cells with intermediate reflector layer (IRL) between top and bottom cell and compared with experimental external quantum efficiency and current voltage characteristics results. Reference data were collected from a series of tandem cells with different thicknesses of the top cell absorber layer (160-240 nm), the bottom cell absorber layer (1750-2100 nm), and the transparent conductive oxides based IRL (10-80 nm). It turned out that for capturing correctly the influence of the IRL on the light management as a function of the IRL thickness, the conventional semicoherent approach is not sufficient. Whereas the optical properties of a very thin IRL are governed by interference effects that are best calculated using a fully coherent model, increasingly thicker IRL show a more and more incoherent behavior. By taking into account, the interface morphology and angular light distribution within the cell stack an algorithm for the effective IRL reflectivity was proposed that explains the experimental findings very well. The consecutive electrical simulations were carried out with the device simulator ASA. The dependence of short circuit current density jsc and fill factor FF on the thickness dIRL of the IRL is in qualitative agreement between simulation and experiment showing coincident extrema in jsc( dIRL) and FF( dIRL) at the current matching point. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: A simulated and experimental investigation of the trade-off between light trapping and current matching in p-i-n structured a-Si:H/μc-Si:H tandem solar cells is presented, which aims to address the limited short circuit current density (J sc) that results from the low long-wavelength light scattering of the fluorine-doped tin oxide (SnO2:F) substrates typically used. To this end, the mismatch of the J sc between the top and bottom cells is reduced by utilizing a ZnO:Al substrate with optimized long-wavelength light scattering properties as the front contact, thereby improving the response of the bottom cell at the expense of the lower top cell's J sc yet. A trade-off between the top and bottom cell's light response is subsequently found with SnO2 or ZnO:Al as a substrate, by introducing an n-type μc-SiOx intermediate reflector (IR) between the two component cells. An initial efficiency based on an approximate current matching of 11.90% is achieved for a-Si:H/μc-Si:H tandem solar cell by adopting a magnetron-sputtered and texture-etched ZnO:Al substrate and an optimized n-type μc-SiOx IR. [Copyright &y& Elsevier]

The recent surge of unveiled US patents on the intermediate reflectors for thin-film silicon (Si) photovoltaic (PV) devices reflects the paramount importance of light trapping to improve the conversion efficiency. Here, the recent patent issues on the intermediate reflectors of thin-film Si PV devices are reviewed. Highly transparent and conductive metal oxide intermediate reflectors have the advantage of the higher efficiency for the fabricated multi-junction solar cells compared to the Si alloy intermediate reflectors. However, their high lateral electrical conductivity leads to the lateral shunting during the monolithic series integration of segments. To avoid the lateral shunt creations, an additional laser scribe or a coating process that induces a high production cost is necessary. In addition, a low conversion efficiency for hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline Si (μc-Si:H) double-junction PV modules employing a metal oxide intermediate reflector stems from the decrease in the active area as a result of the additional process. Meanwhile, double-junction PV modules employing an n-type hydrogenated microcrystalline Si oxide (n-μc-SiOx:H) intermediate reflector provide a higher conversion efficiency. Since the Si alloy intermediate reflector can avoid the lateral shunting, it may be a promising option for cost-effective mass production of large-area thin-film Si multi-junction PV modules. Although the developed intermediate reflectors have the high potential, the current status is limited at the research and development (R&D) level. Therefore, the up-scaling with the low cost, high throughput, and high yield is a key technological mission for mass production. [ABSTRACT FROM AUTHOR]

Abstract: State-of-the-art optical trapping designs for tandem thin film silicon solar cells in the superstrate configuration commonly feature two elements: a textured transparent conductive oxide front contact and a low refractive index interlayer between top and bottom cell. We investigate more advanced super light trapping schemes that have been designed and implemented in thin film tandem junctions at different levels in the solar cells to further enhance light trapping capabilities. Regularly patterned nano-imprinted substrates offer an additional capability to tune the substrate morphology and optimize it for enhanced solar cell performance. We show that the onset of defect formation in thin film layers can be controlled and that the spectral response in the infra-red part of the spectrum is increased. Intermediate reflectors with ultra-low refractive indeces and plasmonic properties lead to increased light confinement in top cells. Results show that reducing the refractive index below 1.5 still leads to a substantial current increase in the top cell. These innovative designs improve the output current in amorphous/microcrystalline tandem devices with thin photo-active layers. In addition, they surprisingly enhance the electrical parameters of the tandem cells with a record open circuit voltage of 1.47V. Plasmonic effects of metal nano-particles embedded in interlayers do not compromise the electrical solar cell parameters and similarly strengthen light confinement in the top cell, however detrimental effects are observed in the bottom cell current output. [Copyright &y& Elsevier]

Microcrystalline silicon oxide (µc-SiO x :H) alloys prepared by plasma enhanced chemical vapor deposition (PECVD) represent a versatile material class for opto-electronic applications especially for thin-film and wafer based silicon solar cells. The material is a phase mixture of microcrystalline silicon (µc-Si:H) and amorphous silicon oxide (a-SiO x :H). The possibility to enhance the optical band gap energy and to adjust the refractive index over a considerable range, together with the possibility to dope the material p-type as well as n-type, makes μc-SiO x :H an ideal material for the application as window layer, as intermediate reflector (IR), and as back reflector in thin-film silicon solar cells. Analogously, μc-SiO x :H is a suitable material for p- and n-type contact layers in silicon hetero junction (SHJ) solar cells. The present paper gives an overview on the range of physical parameters (refractive index, optical band gap, conductivity) which can be covered by this material by variation of the deposition conditions. The paper focuses on the interdependence between these material properties and optical improvements for amorphous silicon/microcrystalline silicon (a-Si:H/µc-Si:H) tandem solar cells prepared on different substrates, such as Asahi (VU) and sputtered ZnO:Al. It gives a guideline on possible optical gains when using doped µc-SiO x :H in silicon based solar cells. As intermediate reflector in a-Si:H/µc-Si:H tandem cells µc-SiO x :H leads to an effective transfer of short circuit current generation from the bottom cell to the top cell resulting in a possible thickness reduction of the top cell by 40%. Within another series of solar cells shown in this paper a short circuit current density of 14.1mA/cm2 for an a-Si:H/µc-Si:H tandem solar cell with a µc-SiO x :H intermediate reflector is demonstrated. A SHJ solar cell on a flat (non-textured) wafer using p- and n-type doped µc-SiO x :H contact layers with an effective area efficiency of 19.0% is also presented. [ABSTRACT FROM AUTHOR]

Abstract: We focus on the use of mixed-phase phosphorus-doped SiO x (n-SiO x ) as advanced n-layer in both the junctions of micromorph tandem solar cells. The material, deposited by plasma enhanced chemical vapor deposition at two pressure values, has been fully characterized. Direct evidence of phase separation into nanocrystalline Si and amorphous oxygen-rich phases is reported, accompanied by information on the optical properties of the material as a whole and the two phases, as gathered by spectroscopic ellispometry. The multifunctional role of the n-SiO x layer, providing for beneficial optical effects at both the intermediate and back reflecting stage in micromorph solar cells, is established. In particular superior performance (improved fill factor and enhanced bottom solar cell spectral response) is demonstrated with use at the rear side of the device in combination with a simple Ag back reflector, with respect to the standard device employing the conventional ZnO/Ag reflector. [Copyright &y& Elsevier]

Abstract: In this paper, we investigate tandem amorphous/microcrystalline silicon solar cells with asymmetric intermediate reflectors grown in the n–i–p substrate configuration. We compare different types of substrates with respect to their light-trapping properties as well as their influence on the growth of single-junction microcrystalline cells. Our most promising back reflector combines a textured zinc oxide film grown by low-pressure chemical vapor deposition, a silver film for reflection, and a zinc oxide buffer layer. Grown on this substrate, microcrystalline cells exhibit excellent response in the infrared while keeping high open-circuit voltage and fill factor, leading to efficiencies of up to 10.0%. After optimizing the morphology of the asymmetric intermediate reflector, we achieve an n–i–p micromorph solar cell stabilized efficiency of 11.6%, using 270nm and 1.7μm of silicon for the absorber layer of the amorphous top cell and the microcrystalline bottom cell, respectively. Using this original device architecture, we reach efficiencies close to those of state-of-the-art n–i–p and p–i–n micromorph devices, demonstrating a promising route to deposit high-efficiency thin-film silicon solar cells on opaque substrates. [Copyright &y& Elsevier]

In this paper, we report on the development of an n-type silicon oxide intermediate reflector (SOIR) for a-Si:H/µc-Si:H tandem solar cells produced in an industrial-type AKT1600 PECVD reactor. A comparison to a tunnel recombination junction with µc-SiOx in the p-layer is made. Lower fill factors, resulting from the implementation of the SOIR, could be avoided by the deposition of a thin n-doped microcrystalline silicon (µc-Si) recombination layer after the SOIR. A cell efficiency of 9.5% after 168 h of light soaking at 50 °C and 1 sun was reached on commercially available SnO2:F front TCO, which is a 2% relative increase over a similar cell without the SOIR. Possible explanations for the role of this recombination layer are discussed (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: The use of zinc oxide (ZnO) based intermediate reflector (ZIR) in micromorph solar cells using low pressure chemical vapor deposition (LPCVD) was investigated. The influences of deposition temperature and dopant gas concentration on grain size and lateral electrical conductivity measurements are presented. Further ZIR deposition conditions were then directly evaluated in micromorph solar cell devices. Their electrical performances were compared to reference cells and cells incorporating silicon oxide based intermediate reflector. It is shown that both reduced ZIR deposition temperature and increased total flow rate allow for better performing devices with increased shunt resistance, as further supported by lock-in thermography shunt imaging. Relative micromorph efficiency increase of above 7% is shown with thin ZnO layers, along with absence of loss or even small increase of total current in the whole structure compared to cells without intermediate reflector. [Copyright &y& Elsevier]

Abstract: We have developed zinc oxide (ZnO) film and CO2 plasma treatment for the use as an intermediate layer between top and bottom cell in order to improve performance of micromorph silicon solar cells. The CO2 plasma treatment was performed by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique, and the ZnO interface layer was deposited by DC-magnetron sputtering method. Effects of both techniques on the cell performance were comparatively investigated. We found that the ZnO interface layer and CO2 plasma treatment were effective in enhancing V oc, J sc as well as FF of the cells as the same. The micromorph solar cells using an optimized ZnO interface layer and the CO2 plasma treatment indicated initial conversion efficiency of 11.4% and 11.2%, respectively. Experimental results indicated that the CO2 plasma treatment technique is more suitable for using in cell fabrication process than the ZnO interface layer since it is simpler and has no negative impact of possible shunts. [ABSTRACT FROM AUTHOR]