S Roberts

ACE Designs: the beauty of rear contact solar cells

Presents an outline of the work done in the EC co-funded project ACE Designs. The objective of this project was to develop rear contact solar cell designs and to demonstrate their applicability as an alternative crystalline silicon technology for industrial module production. An overview of the results is given with links to the most relevant, publications for further details. The most important result of this project was that rear contact solar cells are a feasible, attractive and cost effective alternative to the well-known front contacted solar cell.

Large area multicrystalline silicon buried contact solar cells with bulk passivation and an efficiency of 17.5

The purpose of this study was the development of a processing sequence for buried contact solar cells on multicrystalline silicon (mc-Si). The applied process includes mechanical V-texturing for the reduction of reflection losses as well as bulk passivation by a remote hydrogen plasma source. Record high efficiencies of 17.5% (V/sub /spl prop//=628 mV, J/sub sc/=36.3 mA/cm/sup 2/, FF=76.8%, independently confirmed at FhG-ISE) have been obtained on Polix mc-Si on a solar cell area of 144 cm/sup 2/. The high J/sub sc/ results from low shadowing and reflection losses, high bulk diffusion lengths and from a selective emitter structure. Hydrogenation was investigated for Baysix mc-Si and led to an increase in V/sub /spl prop// of 5-11 mV and in J/sub sc/ of 0.3-0.6 mA/cm/sup 2/, which were caused by an increase in the effective diffusion length of 40-50 /spl mu/m. It has been demonstrated, that hydrogenation from a PECVD SiN/sub x/ layer applied to screen printed solar cells could be more effective than remote plasma hydrogenation in BCSCs.

Prospects for high efficiency silicon solar cells in thin Czochralski wafers using industrial processes

Lower PV systems cost can be achieved if less silicon material is used in modules and if higher solar cell efficiencies can be achieved cost effectively. In this study the efficiency limits of mass production high efficiency laser grooved buried grid solar cells have been modelled for thinner and thinner wafers. PC1D modelling has been coupled with a 3D ray tracing simulation RAYN to predict cells performance. Given suitable surface passivation, light trapping and minority carrier lifetime, solar cell efficiency can actually increase with decreasing wafer thickness. Cells were made by the RP-PERC process, using thinned industrial grade Czochralski silicon wafers. Cells (4cm/sup 2/) of over 20% efficiency were fabricated in wafers with a final thickness of 115 /spl mu/m. Standard production LGBG cells had poor BSFs but on process optimisation nearly 17% efficiency were made in 140 /spl mu/m micron wafers on an industrial production line.

Laser Grooved Buried Contact Solar Cells for Concentration Factors up to 100x

The laser grooved buried contact (LGBG) crystalline silicon solar cell is an attractive technology for the production of low-cost concentrator cells. Due to the high-conductivity buried front contact, the metallization pattern may be readily adapted to handle the larger current densities produced at higher concentrations whilst minimizing shading. In the 1990s an efficiency of 18% at 30X concentration without prismatic covers was demonstrated in the EUCLIDES concentrator system. A matrix of cell process conditions has been investigated in order to optimize the emitter and front contact design of the LGBC cell for concentration factors of 50-100X. Efficiencies over 18% at 50X concentration have been measured on 2.56 cm2 cells. Factors limiting the efficiency are discussed and processing improvements are suggested

Low cost, 100/spl times/ point focus silicon concentrator cells made by the LGBC process

LGBC silicon solar cells have demonstrated efficiencies up to 20% when used in linear focus concentrating systems up to 20/spl times/ concentration. Small area cells of 1.2 cm/sup 2/ have been cut from these cells and tested up to 100/spl times/ concentration for point focus applications. A potential to achieve 20% efficiency at 100/spl times/ has been recognised and independent results to date have shown an efficiency approaching 18% at 100/spl times/. Based on simple variations of a mass production process, analysis indicates that the manufacturing cost for such cells can be below

Process optimisation for coloured laser grooved buried contact solar cells

0.15/Wp.

Towards 20% efficient silicon solar cells manufactured at 60 MWp per annum

The use of photovoltaic modules in architectural applications is now firmly established and large modules of glass-glass construction produced specifically for the BIPV market are available. However, the range of solar cell colours and shapes currently offered by suppliers is still very limited and this is a barrier to the widespread use of PV modules as constructional components. Initial investigations of the colour and efficiency of Laser Grooved Buried Contact (LGBC) solar cells as a function of the thickness of the LPCVD silicon nitride antireflection coating were reported in the late 1990s, but the subsequent commercialisation of coloured cell products has been limited in part by the difficulty in controlling the uniformity and reproducibility of colour in large scale cell production. The aim of the present work is to understand and control the processes that affect the thickness and hence colour of the silicon nitride ARC. Process conditions were optimised to enable the formation of antireflection coatings with thicknesses in the range 90 nm to 400 nm. LGBC solar cells were fabricated in 5 colours on both non-textured Cz and partially textured multicrystalline wafers. Good uniformity of colour was achieved both across individual cells and throughout whole process runs. Laser scribing was used to produce cells in a range of shapes which, in conjunction with the choice of colours, demonstrates the potential for novel BIPV applications.