The impact of the Al and Ge ratio in the Al-Ge pastes are investigated for fabricating the single crystalline Si1-xGex thick layers on large area Si substrates by screen-printing metallization process. From X-ray reciprocal space maps, Ge fraction in the fabricated Si1-xGex thick layers are found to increase up to 40% with increasing the Ge ratio in the Al-Ge pastes. On the other hand, the interface of the Si and Si1-xGex layers are getting winding with increasing the Ge ratio in the Al-Ge pastes. The Al-Si-Ge phase diagram indicated that uniform SiGe layer can be fabricated by adjusting the Al-Ge ratio in the pastes within the liquid phase region.
We report on a simple liquid-phase epitaxy (LPE) of SiGe on Si (100) substrate based on printing and firing. LPE was performed using an Al-Ge mixed paste screen-printed on a Si (100) substrate followed by annealing above the Al-Ge eutectic temperature in air or in an Ar atmosphere. In the case of annealing in air at 800 °C, SiGe was formed between the mixed paste and the surface of the Si substrate. However, a wave-like oxide film was confirmed at the SiGe/Si interface, which hindered the growth of SiGe. On the other hand, annealing in the Ar atmosphere at 800 °C led to the successful formation of a more continuous, thicker SiGe film by suppressing the oxidation of Al. It can be concluded that LPE growth using printing and firing of Al-Ge mixed paste is a suitable method to grow SiGe with a low-cost and simple high-speed process.
The authors discuss industry related approaches at Fraunhofer ISE for bifacial p-type silicon solar cells, taking into account the well-known “passivated emitter and rear cell” (PERC), “passivated emitter and rear totally diffused” (PERT) and “passivated emitter and locally diffused” (PERL) architectures. In the case of PERC, challenges in terms of alignment, printability and the importance of bifaciality are addressed. In the case of PERT, a co-diffusion process is utilized to form the emitter and the back surface field simultaneously avoiding also critical shunts that can arise at the edges of such devices. For the PERL technology, the industrial feasible pPassDop approach is discussed. We report on front side energy conversion efficiencies for PERC of 21.4%, PERT of 20.5%, and PERL of 19.8%. Furthermore, bifaciality factors for PERC of 0.7, for PERT of 0.86, and for PERL of 0.89 are presented.
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