The electroluminescence emission of crystalline silicon solar cells at near-bandgap wavelengths is investigated. We show that the intensity of the emitted luminescence at near-bandgap wavelengths is directly proportional to the collection diffusion length Lc which is a measure of bulk and rear surface recombination properties and determines the short circuit current of a solar cell illuminated with light of near-bandgap wavelengths. We provide experimental evidence for the determination of Lc by carrying out electroluminescence measurements on a set of 15 specially prepared monocrystalline silicon solar cells with different thicknesses. Moreover, we demonstrate and discuss the applicability of the proposed method to obtain images of the collection diffusion length Lc of multicrystalline silicon solar cells. The values determined by electroluminescence imaging coincide with values obtained from spectrally resolved quantum efficiency measurements with a relative accuracy of 13%.

We report on RISE-EWT (Rear Interdigitated Single Evaporation-Emitter Wrap-Through) solar cells on full area (12.5×12.5cm2) pseudo square boron doped Czochralski-grown silicon wafers. We investigate the main efficiency optimisation factors of these cells by investigating the dependence of RISE-EWT cell parameters on the base dopant concentration NA. We furthermore detail the effects of large feature sizes in base and emitter regions at the rear of the solar cell and investigate these effects with particular attention to the edge regions. EWT solar cells typically exhibit rather low fill factors. However, our results show that the improved fill factors can be achieved by increasing NA, which in return leads to optimised efficiency values. For our RISE-EWT solar cells made from boron doped Cz-Si wafers, this benefit is maintained even after light-induced degradation. Our investigation of edge area related effects shows the importance of proper cell design in these areas, leading to a further 2.8% absolute improvement in the fill factor. Combining increased base dopant concentration with optimised edge design, we achieve 19.0% efficiency on (12.5×12.5cm2) boron doped Cz silicon wafers before light-induced degradation, resulting in 18.1% efficiency in the light-degraded state.

In the ALBA-II project, Q-Cells SE, Bitterfeld-Wolfen, Germany, and the Institute for Solar Energy Research Hamelin, Emmerthal, Germany, are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralski (Cz) method forms the basis of this high-efficiency solar cell development as it offers high bulk carrier lifetimes. The EWT device structure allows us to employ a simplified process sequence compared with interdigitated back-contact back-junction solar cells. High open-circuit voltages of our solar cells are achieved by different passivation layers for base and emitter surfaces and picosecond laser ablated contact openings. An optimization of the resistances along the current paths in base and emitter leads to an improvement in fill factor (FF) over former EWT solar cells. Together with the inherently high current densities of EWT solar cells, we achieve on our small-area (4-cm2, designated area without busbars) cells a short-circuit current density JSC of 40.4 mA/cm2, an open-circuit voltage VOC of 661 mV, FFs well above 80%, and, thus, cell efficiencies of up to 21.6%.

Currently, the emitter-wrap-through (EWT) design of Si solar cells is being intensively investigated as a potential candidate for cheap, low-quality Si materials. So far, experimentally achieved energy conversion efficiencies have stayed unexpectedly far below the expectations of common device theory. Therefore, we analyze fabricated EWT cells in detail and refine device theory to account for the limiting loss mechanisms present only in EWT cells. By means of rigorous three-dimensional numerical device modeling, we show that the fill factor (FF) is significantly reduced, primarily due to a effect we call the via-resistance induced recombination enhancement effect. The FF is only secondarily reduced by the resistive losses in the vias where the emitter is wrapped through the cell. This implies that lowering the base resistivity will improve cell efficiency more effectively than lowering the resistance in the vias. Our simulations predict that the EWT design with a nonpassivated rear emitter and a homogeneous emitter diffusion leads to an efficiency improvement of about 1% absolute, as compared to the common front-junction design. This is so for excess carrier lifetimes in the bulk between 1 and 100 μs, which means also for multicrystalline cells.

Emitter wrap through solar cells (EWT) are known for exhibiting low fill factors (FF). In this contribution we explain the FF losses by a three-dimensional EWT simulation model. The origin of reduced FF of EWT solar cells can be explained by Via-resistance Induced Recombination Enhancement (VIRE-effect). A voltage drop along the EWT-via emitter enhances the injection of minority carriers at the front pn-junction which causes higher recombination currents compared to those at the rear pn-junction. We explain the principle how the VIRE- effect leads to a deformation of the I-V characteristics and discuss its influence by showing 3d-simulation regimes.

Phosphorus-doped n-type multicrystalline silicon (mc-Si) is frequently considered to be an alternative material compared to boron-doped p-type mc-Si which holds the biggest fraction of base materials in today's world- wide photovoltaic market. In this paper, we describe the lifetime evolution of n-type mc-Si during different processes, such as phosphorus diffusion and hydrogenation, by means of spatially resolved microwave-detected photoconductance decay lifetime measurements. In addition, the total metal concentrations in particular regions of the wafers are examined by an advanced vapour phase decomposition total reflection x-ray fluorescence (VPD- TXRF) method. The measured lifetimes correlate well with the total metal concentrations. After an optimized gettering and hydrogenation step, area-averaged lifetimes up to 900 µs are measured on the n-type mc-Si wafers with a resistivity of 4 Ωcm.

The goal of the project Alba is the development and fabrication of EWT solar cells on multi-crystalline silicon with a clearly increased cell efficiency compared to standard industrial Si cells. The project lays special focus on the development of industrially applicable processes. Exemplary realisation of advanced process steps for the fabrication of EWT cells which are suitable for mass production of high efficiency solar cells is shown in this paper. First results of the successful demonstration of the cell design on a laboratory scale are presented while the introduction of the cell concept into pilot production at Q-Cells's new Reiner Lemoine Research Centre is announced.

Low fill factors generally limit the efficiency of emitter-wrap-through (EWT) solar cells. Until now, a conventional series resistance limitation along the laser-drilled EWT vias has usually been assumed to be responsible for this effect. We demonstrate that the characteristic fill factor loss is caused by a crucial change in the diffusion currents inside the base, which are influenced by the conductivity along the laser-drilled EWT vias. In addition, we show that the EWT via conductivity influences the fill factor loss caused by an iron contaminated base. This result affects the proposition that the EWT design is suitable for multicrystalline silicon in which interstitial iron is known to be the main contaminant. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim