**6.3 Relative influence of different parameters on the performance of HIT cells**

In this section we make a comparative study of the influence on HIT cell performance, of the Nss on the surface of the c-Si wafer, the lifetime () of the minority carriers in c-Si, and the surface recombination speeds (SRS) of free carriers at the contacts. The sensitivity to the first two is shown in Table 11. For all the cases studied here, the P layer has an activation energy of 0.3 eV and a surface band bending 0.21 eV.

We note that when the defect density on the surfaces of the c-Si wafer is low, there is some sensitivity of the solar cell output to . In fact the conversion efficiency increases by ~3.22% and ~2.47% in double P-c-Si and N-c-Si HIT cells respectively as varies from 0.1 ms to 2.5 ms. By contrast there is a huge sensitivity to Nss, as already noted in sections 4.2, 4.3 and 5.3; the performance of the HIT cell depending entirely on this quantity when it is high, with no sensitivity to (Table 11). The lone exception is the Nss on the rear face of N-c-Si, to which solar cell output is relatively insensitive as already noted

Finally, the minority carrier SRS at the contacts, that regulates the back diffusion of carriers, has only a small influence in these double HIT cells. The majority carrier SRS does not affect cell performance up to a value of 103 cm/s, except the SRS of holes at the contact that is the

Computer Modeling of Heterojunction

carriers in the c-Si wafer (Table 6).

ms and its doping 1016 cm-3.

**8. Acknowledgements** 

A. for many helpful discussions.

**7. Conclusions** 

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 297

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

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.

(mA cm-2)

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

Double HIT on P-c-Si 37.76 694 0.828 21.72 Double HIT on N-c-Si 38.89 701 0.814 22.21

The authors wish to express their gratitude to Prof. Pere Roca i Cabarrocas of LPICM, Ecole Polytechnique, Palaiseau, France for providing all the experimental results on "HIT" cells on P-types wafers, that have been simulated in this article. We are also grateful to him for many in-depth discussions and constant encouragement during the course of this work. The authors also wish to thank Prof. C. Baliff, of IMT, University of Neuchâtel, Switzerland, M. Nath of the Energy Research Unit, IACS, Kolkata, India and J. Damon-Lacoste of TOTAL, S.

Voc

(mV) FF

(%)

Type Jsc


Table 11. Sensitivity of double HIT solar cell output parameters to Nss on the front and rear surfaces of the c-Si wafer and minority carrier life-time ().

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 conditions can also be critical for good solar cell performance.

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.
