The basic idea behind the concept of silicon-heterojunction is the formation of rectifying junctions by deposition of thin-film silicon onto crystalline (monocrystalline, multicrystalline) silicon wafers or ribbons (called substrates in the present paper). The substrate can be either p — or n-type, and the silicon deposition may be applied either to one or to both of its surfaces. An example of an SHJ structure based on an n-type substrate is shown in Fig. 1

In the case of a p-type c-Si substrate, the structure is similar except for the realisation of the back contact. Here, the formation of a p-p+ homojunction for higher-efficiency devices is always achieved by depositing aluminium and subsequently sintering in order to allow the partial diffusion of aluminium atoms into the lattice. In fact the back contact in solar cells based on p-type crystalline silicon, cannot be realised by means of a heterojunction p-type c-Si / p-type a-Si:H. because the high valence band offset between p-type crystalline silicon and the p-type amorphous silicon severely reduces hole collection at the rear metal contact. This is reflected in a lower quantum efficiency and, consequently a lower short circuit current3

Not depending on the selected n — or p-type substrate, the reasons to develop SHJ instead of diffused-emitter cells lie in a number of advantages4 derived from the combination of features of thin-film and wafer technology, i. e.:

v A simple structure. No particularly cumbersome step is involved in the fabrication process.

v A low-temperature process. The process is simple and cost-effective. Since sample temperature does not surpass 200-250°C, the degradation of minority-carrier lifetime in the substrate is negligible, even for low-quality substrates. This feature is essential for the use of new-generation silicon materials.

v Surface passivation is concomitant with junction formation. The insertion of a very thin intrinsic layer in the interface is a very powerful tool for passivation and, therefore, for reducing interface recombination. Experimental data have shown that intrinsic amorphous silicon is more effective than silicon dioxide or iodine methanol in achieving silicon surface passivation4. Surface recombination velocities below 100 cm/s have been reported.

v A BSF (back-surface field) structure is easily created in the fabrication process. This is an additional consequence of the excellent passivating properties of intrinsic amorphous silicon.

v Stable cells. The Staebler-Wronski effect is not seen in SHJ cells5. Since the cell absorber is crystalline or multicrystalline silicon, the mechanisms involved in the Staebler-Wronski effect are not applicable.

v Improved high-temperature performance. The temperature coefficients of SHJ cells are smaller than those of diffused-emitter devices. Since actual operating temperatures (between 35 and 40°C) lie well above the reference ones (25°C) this feature represents an enormous advantage in practical applications.

v Easiness to define emitter thickness very accurately. This allows to use thin new — generation substrates without the risk of unwanted diffusion of dopants into the absorber.

These advantages would be meaningless if the cells were limited in performance, but this is far from being the case. Sanyo has reported SHJ-cell efficiencies of 21% for devices being 100 cm2 in area6.