Buried Contact Solar Cells (BCSC)

The buried contact solar cell (BCSC) process has been developed at the University of New South Wales [41, 105]. Many aspects of the BCSC structure and its processing have been extensively described in the technical literature [105, 106, 107, 108, 109, 110, 111, 112]. A laboratory efficiency as high as

21.3 % on small area FZ material has been reported [113]. The buried contact solar cell is discussed in more detail in Chapter II-b5. A conventional commercial BCSC processing sequence licensed to many solar cell manufactures is presented in Table 3.

Table 2 Processing sequence for a thin screen-printed multicrystalline silicon solar cell based on a shallow homogeneous emitter, SiNx-passivation and Al-BSF without wafer warping [103]

Step no: Process description [3] [4]

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Figure 6 Cross section of an advanced thin screen printed solar cell with shallow homogeneous emitter and Al-BSF.

The buried contact solar cell structure embodies almost all characteristic features of high efficiency laboratory cells described earlier: shallow emitter diffusion with a very good surface passivation by a thick thermal oxide, a very fine metallization line width, front-contact passivation by heavy diffusion in the contact area, and Back Surface Field. One of the important processing steps is the growth of a very thick thermal oxide on the top surface which simultaneously acts as a diffusion mask, a plating mask and a surface passivation layer.

This process, however, has its disadvantages when commercial applications are considered: a large number of lengthy processing steps at high temperaturefabove 950°C for a longer total time up to 16 hours), expensive equipment, many careful pre-cleaning steps making the process complex and labour intensive [114]. Although the buried contact solar cell process has been licensed to many leading solar cell manufacturers, only one has succeeded

Table 3 Conventional commercial process sequence of buried contact solar cell [111]

Step no: Process description

1. Saw damage etching and random texturing

2. Light n+ diffusion over the whole surface

3. Growing of thick thermal oxide

4. Mechanical or laser groove formation

5. Groove damage etching and cleaning

6. Second heavy diffusion in grooved areas only

7. Aluminium evaporation on a back side

8. High temperature A1 alloying

9. Electroless plating of nickel, sintering and etching

10. Electroless plating of copper and silver

11. Edge junction isolation by laser scribing introducing it into a large volume production by simplification of many processing steps [115]. Production efficiencies close to 17% are obtained on CZ-Si. Application of a lab scale buried contact process on lower quality multicrystalline wafers resulted in top efficiencies of 17.6% (144 cm2 cell) and 17.9% (25 cm2 cell) [116].

A simplified buried contact solar cell process has been proposed [112]. The aim of this process simplification is to suit infrastructure and equipment existing already in many solar cell production plants based on screen printed contacts. The simplified process relies on a homogeneous emitter diffusion instead of the selective emitter scheme [117]. For Czochralski and multicrystalline materials, the advantage of the selective emitter structure is rather limited due to the dependence on the base quality. The thermal oxidation, which is a long process with a high thermal budget, can be replaced by TiOx or SiNx-coatings. Novel uses of Ti02 are under study [118]. Cell results are scarce for these new simplified process steps. Other efforts in the field of buried-contact cells are focussed on double-sided structures, thinner wafers, new rear surface passivation (e. g. B-diffusion) [119] and environmentally friendly processing. More data on the buried contact approach can be found in [120] and Chapter II-b5.