**7. Conclusions**

296 Solar Cells – Thin-Film Technologies

P-c-Si 4x1011 1011 0.1 36.22 604 0.794 17.37

N-c-Si 4x1011 1011 0.1 38.39 631 0.767 18.58

Table 11. Sensitivity of double HIT solar cell output parameters to Nss on the front and rear

hole-collector. Hole collection (at the rear contact in P-c-Si HIT and at the front in N-c-Si HIT) is already somewhat impeded by the large valence band discontinuity at the amorphous/ crystalline interface and the lower mobility of holes relative to electrons; hence a low value of SRS of holes at the contacts is expected to have a disastrous influence on hole collection. The effect of lowering Sp0 for N-c-Si HIT cells is shown in Fig. 14, and is seen to lead to S-shaped J-V characteristics with a sharp fall in the FF when reduced to ≤ 104 cm/sec. In fact when sputtering ITO onto c-Si substrates coated with a-Si:H (intrinsic and doped) films, we sometimes obtain a rather degraded P/ITO interface, where the surface recombination speed is probably reduced. Therefore, Fig. 14 indicates that ITO deposition

Fig. 14. The sensitivity of the illuminated J-V characteristic under AM1.5 light and shortcircuit condition, to the surface recombination speed of the holes at the ITO/P front contact.

**10<sup>7</sup> cm/sec 10<sup>6</sup> cm/sec 10<sup>5</sup> cm/sec 10<sup>4</sup> cm/sec**

**0 0.4 0.8**

**V (volts)**

Jsc (mA cm-2)

Front Rear (%)

3x1013 1011 0.5 37.24 472 0.626 11.00

4x1011 3x1013 0.5 5.68 572 0.154 0.50

3x1013 1011 0.5 11.54 537 0.208 1.29

4x1011 3x1013 0.5 37.04 615 0.763 17.39

Voc (mV)

0.5 36.61 649 0.808 19.19 2.5 36.68 687 0.817 20.59

2.5 37.17 471 0.626 10.96

2.5 5.59 572 0.153 0.49

0.5 39.03 658 0.783 20.13 2.5 39.20 678 0.792 21.05

2.5 11.58 537 0.207 1.29

2.5 37.08 616 0.763 17.44

FF

(ms)

Type Nss (cm-2)

surfaces of the c-Si wafer and minority carrier life-time ().

conditions can also be critical for good solar cell performance.

**J (mA cm-2)**

We have studied the performance of HIT cells on P-and N-type c-Si wafers, using detailed computer modeling. In order to arrive at a realistic set of parameters that characterize these cells, we have modeled several experimental results. We find that the major breakthroughs in improving the performance of these cells having textured N-type c-Si as the absorber layer, come from the introduction of an amorphous BSF layer, by passivating the defects on the c-Si wafer surface and, to a lesser extent, by improving the lifetime of the minority carriers in the c-Si wafer (Table 6).

Modeling indicates that both types of HIT cell output is very sensitive to the defects on the surface of the c-Si wafer, and good passivation of these defects is the key to attaining high efficiency in these structures. An exception to this rule is the defects on the rear face of c-Si in N-type HIT cells, to which there is not much sensitivity. The amorphous/crystalline valence band discontinuity also has a strong impact. In particular, large ΔEv at the emitter Pa-Si:H/N-c-Si contact leads to S-shaped J-V characteristics, unless tunneling of holes takes place; while that at the P-c-Si/P-BSF contact reduces the FF in double P-c-Si HIT cells. It is for this reason that a transition from a front to double HIT structure on P-c-Si does not produce the spectacular improvement observed for N-type HIT cells (Table 6). Solar cell output is also influenced to some extent by the minority carrier lifetime in c-Si. In Table 12 we compare the performance of a P-type and an N-type HIT cell, with low Nss on the wafer surface, and realistic input parameters. We find that the N-type HIT cell shows better performance than a P-c-Si HIT cell with a higher Voc and conversion efficiency, because of a higher built-in potential in the former. However, the fill factor of N-c-Si HIT cells is lower than in P-type HIT cells due to the assumption of ΔEv > ΔEc, resulting in the holes facing more difficulty in getting collected at the front contact in the former case. This fact has also been pointed out by other workers (Stangl et al, 2001, Froitzheim et al, 2002). In P-type HIT cells, the electrons are collected at the front contact and have to overcome the relatively low ΔEc at the crystalline/amorphous interface so that its FF is higher than in N-c-Si HIT.


Table 12. Comparison of the performance of P-type and N-type double HIT cells, with optimized parameters. The life time of minority carriers in the c-Si wafer in both cases is 2.5 ms and its doping 1016 cm-3.
