**5.3 Effect of the defect density on the front and rear faces of the N-type c-Si wafer**

The sensitivity of the solar cell output of HIT cells on N-type wafers to the surface defect density (Nss) at the amorphous/crystalline interface is given in Table 9. All aspects of the solar cell output appear to be highly sensitive to the Nss on the front surface (on the side of the emitter layer) of the N-type c-Si wafer; however the sensitivity to Nss on the rear face is weak and is limited to the condition when these defects are very high. We have also given in Table 8, the values of the corresponding recombination speeds at the a-Si:H /c-Si front and the c-Si/a-Si:H rear heterojunctions, as calculated by ASDMP, under AM1.5 illumination and short circuit condition. We find that for a well-passivated front interface (Nss ≤ ~3x1011 cm-2) the recombination speed at this heterojunction is less than 10 cm/sec (Table 8), in good agreement with measured interface recombination speeds (Dauwe et al, 2002).


Table 8. Sensitivity of the solar cell output to the defect density (Nss) in thin surface layers (DL) on the front and rear faces of the c-Si wafer in N type double HIT solar cells. The Player thickness is 6.5 nm. The recombination speeds of holes (Sp - at the front DL) and electrons (Sn – at the rear DL), calculated under AM 1.5 light and 0 volts, are also shown.

In Fig.9 (a) we plot the light J-V characteristics and in Fig. 9 (b) the band diagram for various values of Nss on the front face of the c-Si wafer. We find that for a very high defect density on the surface of the c-Si wafer, the depletion region in the N-c-Si wafer completely vanishes, while the emitter P-layer is depleted (Fig. 9b). With a high Nss on the c-Si wafer, the holes left behind by the electrons flowing into the P-layer during junction formation, are localized on its surface, leading to a high negative field on the wafer surface and little field penetration into its bulk (Fig. 10a). Hence the near absence of the depletion zone in N-c-Si and a strong fall in Voc for the highest Nss (1013 cm-2).

solar cell performance of varying the defect density in this layer itself. For this purpose, we have assumed its thickness to be 6 nm (as in case "Double") where the best passivation of Nss has been attained (Table 7). An increase in the defect density in the I-a-Si:H layer may affect the defect density (Nss) on c-Si, but in this study we assume Nss to be constant. We have found (Rahmouni et al, 2010) that unless the defect density of this intrinsic layer is greater than 3x1017 cm-3, no significant loss of cell performance occurs. Similar conclusions

**5.3 Effect of the defect density on the front and rear faces of the N-type c-Si wafer**  The sensitivity of the solar cell output of HIT cells on N-type wafers to the surface defect density (Nss) at the amorphous/crystalline interface is given in Table 9. All aspects of the solar cell output appear to be highly sensitive to the Nss on the front surface (on the side of the emitter layer) of the N-type c-Si wafer; however the sensitivity to Nss on the rear face is weak and is limited to the condition when these defects are very high. We have also given in Table 8, the values of the corresponding recombination speeds at the a-Si:H /c-Si front and the c-Si/a-Si:H rear heterojunctions, as calculated by ASDMP, under AM1.5 illumination and short circuit condition. We find that for a well-passivated front interface (Nss ≤ ~3x1011 cm-2) the recombination speed at this heterojunction is less than 10 cm/sec (Table 8), in good

agreement with measured interface recombination speeds (Dauwe et al, 2002).

1010 2.89x104

Sn at back (DL) (cm/s)

1.5x1011 4.20 37.00 0.712 0.799 21.03 1012 24.73 37.24 0.636 0.695 16.46 2x1012 202.62 37.37 0.596 0.470 10.47 1013 1.16x103 18.83 0.544 0.160 1.64

Table 8. Sensitivity of the solar cell output to the defect density (Nss) in thin surface layers (DL) on the front and rear faces of the c-Si wafer in N type double HIT solar cells. The Player thickness is 6.5 nm. The recombination speeds of holes (Sp - at the front DL) and electrons (Sn – at the rear DL), calculated under AM 1.5 light and 0 volts, are also shown.

In Fig.9 (a) we plot the light J-V characteristics and in Fig. 9 (b) the band diagram for various values of Nss on the front face of the c-Si wafer. We find that for a very high defect density on the surface of the c-Si wafer, the depletion region in the N-c-Si wafer completely vanishes, while the emitter P-layer is depleted (Fig. 9b). With a high Nss on the c-Si wafer, the holes left behind by the electrons flowing into the P-layer during junction formation, are localized on its surface, leading to a high negative field on the wafer surface and little field penetration into its bulk (Fig. 10a). Hence the near absence of the depletion zone in N-c-Si

Jsc (mA cm-2)

1010 2.89x104 37.00 0.712 0.799 21.03 1011 2.37x104 36.99 0.711 0.799 21.01 1012 1.95x104 36.98 0.696 0.797 20.51 1013 1.00x104 35.45 0.609 0.779 16.82

Voc

(volts) FF (%)

36.96 0.720 0.801 21.32

Nss at back (DL) (cm-2)

and a strong fall in Voc for the highest Nss (1013 cm-2).

Nss at front (DL) (cm-2)

Sp at front (DL) (cm/s)

1010 3.62

1.5x1011 4.20

have been reached in the case of HIT cells on P-type c-Si wafers.

Fig. 9. (a) The light J-V characteristics and (b) the band diagram under AM1.5 light bias and 0 volts for different values of Nss on the front face of the N type c-Si wafer..

In Fig. 10 (b) we plot the trapped hole population over the front part in N-c-Si double HIT cells under AM1.5 bias light at 0 volts. We note that when Nss on the front c-Si wafer surface is the highest (1013 cm-2), there is a huge concentration of holes at the amorphous / crystalline (a-c) interface on the c-Si wafer side, where the high surface defect density exists (dashed line, Fig. 10b).

Fig. 10. Plots of (a) the electric field on the holes and (b) the trapped hole density over the front part of the device as a function of position in the entire device under illumination and short-circuit conditions, in N-c-Si HIT cells for different densities of defects on the front face of the c-Si wafer. The amorphous/crystalline (a-c) interface is indicated on (a) and (b).

The hole pile-up at the amorphous / crystalline interface slows down the arrival of holes to the front contact (the collector of holes), and attracts photo-generated electrons, i.e., encourages their back diffusion towards the front contact. The result is that the electrns back-diffuse towards the front contact and recombine with the photo-generated holes resulting in poor carrier collection (Rahmouni et al, 2010). Thus Jsc and FF fall sharply for high values of Nss on the front surface of c-Si (Table 8). In fact we may arrive at the same conclusion also from Fig. 9 (b), which shows that for Nss = 1013 cm-2, there is almost no band bending or electric field in the c-Si wafer (the main absorber layer) so that carriers cannot be collected, resulting in the general degradation of all aspects of solar cell performance.

Computer Modeling of Heterojunction

**Ev**

**Ev**

**Ev**

**Ev**

**(a)**

**Free hole density (cm-3)**

performance.

**10<sup>17</sup>**

**10<sup>18</sup>**

**10<sup>19</sup>**

**10<sup>20</sup>**

Eμ (P) (eV)

held constant at 0.22eV.

Eac

**300 300.01 300.02**

**Position (microns)**

 **= 0.41 eV, Eac = 0.3 eV** 

 **= 0.56eV, Eac = 0.3 eV** 

 **= 0.56 eV, Eac = 0.4 eV** 

 **= 0.64 eV, Eac = 0.4 eV** 

(eV) ΔEv (eV) Jsc

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 293

It is for this reason that a transition from a front to a double HIT structure does not appreciably improve cell performance for P-c-Si HIT cells. The accumulated holes at the c-a interface, furthermore, repel the approaching holes and encourage photo-generated electron back diffusion, resulting in increased recombination, that reduces even Jsc for the highest ΔEv (Table 9, Fig. 11b). Finally, for high hole pile-up, the amorphous BSF is screened from the rest of the device, so that the large variation of its band gap and activation energy (Table 9) fails to alter the Voc of the device. The best double HIT performance is attained when the mobility gap (ΔE) of the amorphous BSF P-layer is ≤ 1.80 eV and Eac = 0.3 eV (Table 9).

Fig. 11. Variation of (a) the free hole population near the c-Si/ amorphous BSF interface and (b) the light J-V characteristics for different valence band discontinuities (Ev) and activation energies (Eac) of the P-BSF layer in double P-c-Si HIT solar cells. ΔEc = 0.22eV in all cases.

**-40**

**-20**

**(b)**

**0**

**J (mA cm-2)**

**20**

**40**

**c-a interface**

Table 10 shows the effect of the variation of the emitter P-layer mobility gap, activation energy and the valence band discontinuity at the a-c interface on N-c-Si double HIT cell

(mA cm-2)

1.75 0.3 0.41 38.06 670 0.818 20.86 1.75 0.4 0.41 38.14 652 0.681 16.93 1.80 0.3 0.46 38.10 671 0.811 20.75 1.90 0.3 0.56 38.22 677 0.705 18.25 1.90 0.4 0.56 38.38 674 0.463 11.98 1.98 0.4 0.64 28.18 732 0.184 3.79 Table 10. Variation of solar cell output parameters with mobility gap (E), activation energy (Eac), and ΔEv at the emitter P-a-Si:H/c-Si interface in double N-c-Si HIT solar cells. ΔEc is

Table 10 indicates that for valence band offsets up to 0.51 eV, and Eac (P) ≤ 0.3 eV, the FF is high, indicating that the majority of the holes photo-generated inside the c-Si wafer, can surmount the positive field barrier due to the a-Si/ c-Si valence band discontinuity by

Voc

(mV) FF

(%)

**-0.2 0 0.2 0.4 0.6 0.8**

**V (volts)**

On the other hand Table 8 indicates that there is little sensitivity of the solar cell output to the defect states on the rear face of the wafer, except at the highest value of Nss. To explain this fact, we note that the recombination over the rear region is determined by the number of holes (minority carriers) that can back diffuse to reach the defective layer. Not many succeed in doing so, since the high negative field due to the large valence band discontinuity at the c-Si/ a-Si rear interface pushes the holes in the right direction, in other words, towards the front contact. Therefore the defects over this region cannot serve as efficient channels for recombination, and there is no large difference between the recombination through these states for different values of Nss (Table 8). Moreover the conduction band discontinuity at the c-Si/ a-Si interface is about half that of the valence band discontinuity. Since the mobility of electrons, relative to that of holes, is also much higher, clearly this reverse field due to the conduction band discontinuity poses little difficulty for electron collection even when the defect density at this point is high, except when Nss≥ 1013 cm-2, from which point the solar cell performance deteriorates.

### **6. Comparative study of the performances of HIT solar cells on P- and N-type c-Si wafers**

Using parameters extracted by our modeling (given in Tables 3), we have made a comparative study between the performances of HIT solar cells on 300 m thick textured Pand N-type c-Si wafers (for more details refer to Datta et al, 2010).

#### **6.1 Sensitivity of amorphous/crystalline band discontinuity in the performances of HIT solar cells**

Since the band gap, activation energy of the amorphous layers and the band discontinuities at the amorphous/crystalline interface are interlinked, we treat these sensitivity calculations together. For HIT cells on P-c-Si, the large valence band discontinuity (ΔEv) on the emitter side prevents the back-diffusion of holes and has a beneficial effect. Keeping this constant, we varied the mobility gap and therefore the conduction band discontinuity (ΔEc) on the emitter side. We find that a ΔEc upto 0.3 eV, does not impede electron collection, but instead brings up both Jsc and Voc, due to an improved built in ptential (Vbi).

However high ΔEv at the crystalline/amorphous (c-a) interface on the BSF side of P-c-Si double HIT cells (Table 9), impedes hole collection, resulting in a pile up of holes on the c-Si side of this band discontinuity (Fig. 11a) and a consequent sharp fall in the FF and S-shaped J-V characteristics for high ΔEv, especially when the activation energy of the P-a-Si:H layer is also high (Fig. 11b).


Table 9. Variation of solar cell output with mobility gap (E), activation energy (Eac) and ΔEv (P-c-Si/P-a-Si:H BSF interface) in double P-c-Si HIT solar cells. ΔEc is held constant at 0.22eV.

292 Solar Cells – Thin-Film Technologies

On the other hand Table 8 indicates that there is little sensitivity of the solar cell output to the defect states on the rear face of the wafer, except at the highest value of Nss. To explain this fact, we note that the recombination over the rear region is determined by the number of holes (minority carriers) that can back diffuse to reach the defective layer. Not many succeed in doing so, since the high negative field due to the large valence band discontinuity at the c-Si/ a-Si rear interface pushes the holes in the right direction, in other words, towards the front contact. Therefore the defects over this region cannot serve as efficient channels for recombination, and there is no large difference between the recombination through these states for different values of Nss (Table 8). Moreover the conduction band discontinuity at the c-Si/ a-Si interface is about half that of the valence band discontinuity. Since the mobility of electrons, relative to that of holes, is also much higher, clearly this reverse field due to the conduction band discontinuity poses little difficulty for electron collection even when the defect density at this point is high, except when Nss≥ 1013 cm-2,

**6. Comparative study of the performances of HIT solar cells on P- and N-type** 

Using parameters extracted by our modeling (given in Tables 3), we have made a comparative study between the performances of HIT solar cells on 300 m thick textured P-

**6.1 Sensitivity of amorphous/crystalline band discontinuity in the performances of** 

Since the band gap, activation energy of the amorphous layers and the band discontinuities at the amorphous/crystalline interface are interlinked, we treat these sensitivity calculations together. For HIT cells on P-c-Si, the large valence band discontinuity (ΔEv) on the emitter side prevents the back-diffusion of holes and has a beneficial effect. Keeping this constant, we varied the mobility gap and therefore the conduction band discontinuity (ΔEc) on the emitter side. We find that a ΔEc upto 0.3 eV, does not impede electron collection, but instead

However high ΔEv at the crystalline/amorphous (c-a) interface on the BSF side of P-c-Si double HIT cells (Table 9), impedes hole collection, resulting in a pile up of holes on the c-Si side of this band discontinuity (Fig. 11a) and a consequent sharp fall in the FF and S-shaped J-V characteristics for high ΔEv, especially when the activation energy of the P-a-Si:H layer is

> Jsc (mA cm-2)

1.75 0.3 0.41 36.70 649 0.810 19.28 1.75 0.4 0.41 36.69 647 0.688 16.34 1.80 0.3 0.46 36.70 649 0.807 19.21 1.90 0.3 0.56 36.70 649 0.762 18.14 1.90 0.4 0.56 36.68 649 0.484 11.51 1.98 0.4 0.64 27.45 649 0.171 3.04 Table 9. Variation of solar cell output with mobility gap (E), activation energy (Eac) and ΔEv (P-c-Si/P-a-Si:H BSF interface) in double P-c-Si HIT solar cells. ΔEc is held constant at 0.22eV.

Voc

(mV) FF

%

from which point the solar cell performance deteriorates.

and N-type c-Si wafers (for more details refer to Datta et al, 2010).

brings up both Jsc and Voc, due to an improved built in ptential (Vbi).

ΔEv (eV)

**c-Si wafers** 

**HIT solar cells** 

also high (Fig. 11b).

Eμ (P) (eV)

Eac (eV) It is for this reason that a transition from a front to a double HIT structure does not appreciably improve cell performance for P-c-Si HIT cells. The accumulated holes at the c-a interface, furthermore, repel the approaching holes and encourage photo-generated electron back diffusion, resulting in increased recombination, that reduces even Jsc for the highest ΔEv (Table 9, Fig. 11b). Finally, for high hole pile-up, the amorphous BSF is screened from the rest of the device, so that the large variation of its band gap and activation energy (Table 9) fails to alter the Voc of the device. The best double HIT performance is attained when the mobility gap (ΔE) of the amorphous BSF P-layer is ≤ 1.80 eV and Eac = 0.3 eV (Table 9).

Fig. 11. Variation of (a) the free hole population near the c-Si/ amorphous BSF interface and (b) the light J-V characteristics for different valence band discontinuities (Ev) and activation energies (Eac) of the P-BSF layer in double P-c-Si HIT solar cells. ΔEc = 0.22eV in all cases.

Table 10 shows the effect of the variation of the emitter P-layer mobility gap, activation energy and the valence band discontinuity at the a-c interface on N-c-Si double HIT cell performance.


Table 10. Variation of solar cell output parameters with mobility gap (E), activation energy (Eac), and ΔEv at the emitter P-a-Si:H/c-Si interface in double N-c-Si HIT solar cells. ΔEc is held constant at 0.22eV.

Table 10 indicates that for valence band offsets up to 0.51 eV, and Eac (P) ≤ 0.3 eV, the FF is high, indicating that the majority of the holes photo-generated inside the c-Si wafer, can surmount the positive field barrier due to the a-Si/ c-Si valence band discontinuity by

Computer Modeling of Heterojunction

similar pattern as Fig. 15.

surface band bending changes.

of 0.3 eV and a surface band bending 0.21 eV.

solar cell output is relatively insensitive as already noted

**-40**

**-20**

**0**

**J (mA cm-2)**

**20**

**40**

Fig. 13 indicates that both Voc and FF fall off for

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 295

where E(P) and Eac(P) represent respectively the mobility band gap and the activation energy of the P-layer, and 'sbb' is the surface band bending due to a Schottky barrier at the TCO/P interface. With a change of the work function of the TCO, it is this 'sbb' that varies. In this section we study the dependence of the solar cell output to changes in this surface band bending. We hold the band gap and the activation energies of the P-layer constant at 1.75 eV and 0.3 eV respectively, so that the TCO work function has a direct effect on the front contact barrier height. The results are summarized in Fig. 15. For these sensitivity calculations we have chosen the thickness of the P-layer to be 15 nm (Rahmouni et al, 2010).

We have also studied the effect of changing the rear P-a-Si:H BSF/TCO barrier height ,

**1.35 eV 1.24 eV 1.05 eV 0.85 eV**

Fig. 13. The current density - voltage characteristics under AM1.5 light and 0 volts for different front contact barrier heights. The band gap, the activation energy and the thickness of the P-layer are held constant at 1.75 eV, 15 nm and 0.3 eV respectively, so that only

**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

**0 0.2 0.4 0.6 0.8 1**

**V (volts)**

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

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

in P-c-Si HIT cells. The variation in the current-density – voltage characteristics follow a

*b0* ≤ 1.05 eV.

*bL* ,

thermionic emission and get collected at the front ITO/ P-a-Si:H contact. However solar cell performance deteriorates both with increasing band gap and increasing Eac of the P-layer. The latter is only to be expected as it reduces the built-in potential.

Fig. 12 (a) shows the effect on the energy band diagram of increasing the P-layer band gap (therefore of increasing ΔEv, since ΔEc is held constant) and the activation energy. Increasing ΔEv at the P-a-Si:H/N-c-Si interface results in hole accumulation and therefore a fall in FF for ΔEv 0.56 eV, for a P-layer activation energy of ~0.3 eV, due to the reverse field it generates; that is further accentuated when Eac is high (Table 10). van Cleef et al (1998 a,b) have also shown that for a P-layer doping density of 9x1018 cm-3 (same as ours – Table 3, giving Eac = 0.3 eV) and for ΔEv = 0.43 eV, normal J-V characteristics are achieved at room temperature and AM1.5 illumination, and that "S-shaped" characteristics begin to develop at higher ΔEv and Eac. In our case, for ΔEv 0.60 eV, Fig. 12(c) indicates that free holes accumulate over the entire c-Si wafer, resulting in a sharp reduction of the electric field and flat bands over the depletion region, on the side of the N-type c-Si wafer (Fig. 12b). This fact results in a sharp fall in the FF and conversion efficiency (Table 10). In fact under this condition, the strong accumulation of holes on c-Si, can partially deplete even the highly defective P-layer, resulting in a shift of the depletion region from c-Si to the amorphous emitter layer (Fig. 12a). This also means that the carriers can no longer be fully extracted at 0 volts, resulting in a fall in Jsc (Table 10). We have found that the current recovers to the normal value of ~36 mA cm-2 only at a reverse bias of 0.3volts (Datta et al, 2010). Modeling indicates that for improved performance of N-c-Si HIT cells, the valence band offset has to be reduced by a lower emitter band gap, unless the tunneling of holes exists.

Fig. 12. Variation of (a) the band diagram under AM1.5 light and 0 volts and (b) the free hole population under the same conditions, as a function of position in the N-c-Si HIT device for different valence band discontinuities (Ev) and activation energies (Eac) of the emitter layer.

#### **6.2 Sensitivity of the solar cell output to the front contact barrier height.**

The front TCO/P-a-Si:H contact barrier height, *<sup>b</sup>*0 in N-type HIT cells is determined by the following expression:-

$$
\phi\_{b\,0} = E\_{\mu}(P) - E\_{ac}(P) - sbb\,\prime\,\tag{3}
$$

thermionic emission and get collected at the front ITO/ P-a-Si:H contact. However solar cell performance deteriorates both with increasing band gap and increasing Eac of the P-layer.

Fig. 12 (a) shows the effect on the energy band diagram of increasing the P-layer band gap (therefore of increasing ΔEv, since ΔEc is held constant) and the activation energy. Increasing ΔEv at the P-a-Si:H/N-c-Si interface results in hole accumulation and therefore a fall in FF for ΔEv 0.56 eV, for a P-layer activation energy of ~0.3 eV, due to the reverse field it generates; that is further accentuated when Eac is high (Table 10). van Cleef et al (1998 a,b) have also shown that for a P-layer doping density of 9x1018 cm-3 (same as ours – Table 3, giving Eac = 0.3 eV) and for ΔEv = 0.43 eV, normal J-V characteristics are achieved at room temperature and AM1.5 illumination, and that "S-shaped" characteristics begin to develop at higher ΔEv and Eac. In our case, for ΔEv 0.60 eV, Fig. 12(c) indicates that free holes accumulate over the entire c-Si wafer, resulting in a sharp reduction of the electric field and flat bands over the depletion region, on the side of the N-type c-Si wafer (Fig. 12b). This fact results in a sharp fall in the FF and conversion efficiency (Table 10). In fact under this condition, the strong accumulation of holes on c-Si, can partially deplete even the highly defective P-layer, resulting in a shift of the depletion region from c-Si to the amorphous emitter layer (Fig. 12a). This also means that the carriers can no longer be fully extracted at 0 volts, resulting in a fall in Jsc (Table 10). We have found that the current recovers to the normal value of ~36 mA cm-2 only at a reverse bias of 0.3volts (Datta et al, 2010). Modeling indicates that for improved performance of N-c-Si HIT cells, the valence band offset has to

The latter is only to be expected as it reduces the built-in potential.

**-1.5 -1 -0.5 0 0.5 1 1.5 2**

following expression:-

**Energy (eV)**

**0.001 0.01 0.1 1 10 100 1000**

**Position (microns)**

The front TCO/P-a-Si:H contact barrier height,

**EV**

**EV**

**EV**

**EV**

**=0.41 eV, Eac=0.3 eV**

be reduced by a lower emitter band gap, unless the tunneling of holes exists.

**=0.56 eV, Eac=0.3 eV**

**=0.56eV, Eac=0.4eV**

**=0.64 eV, Eac=0.4 eV**

**6.2 Sensitivity of the solar cell output to the front contact barrier height.** 

Fig. 12. Variation of (a) the band diagram under AM1.5 light and 0 volts and (b) the free hole population under the same conditions, as a function of position in the N-c-Si HIT device for different valence band discontinuities (Ev) and activation energies (Eac) of the emitter layer.

**107**

**1010**

**1013**

**Free hole density (cm-3)**

**1016**

**1019**

<sup>0</sup> () ()

*<sup>b</sup> E P E P sbb*

*<sup>b</sup>*0 in N-type HIT cells is determined by the

**0.001 0.01 0.1 1 10 100 1000**

**(b) a-c interface**

**Position (microns)**

*ac* , (3)

**(a)**

where E(P) and Eac(P) represent respectively the mobility band gap and the activation energy of the P-layer, and 'sbb' is the surface band bending due to a Schottky barrier at the TCO/P interface. With a change of the work function of the TCO, it is this 'sbb' that varies. In this section we study the dependence of the solar cell output to changes in this surface band bending. We hold the band gap and the activation energies of the P-layer constant at 1.75 eV and 0.3 eV respectively, so that the TCO work function has a direct effect on the front contact barrier height. The results are summarized in Fig. 15. For these sensitivity calculations we have chosen the thickness of the P-layer to be 15 nm (Rahmouni et al, 2010). Fig. 13 indicates that both Voc and FF fall off for *b0* ≤ 1.05 eV.

We have also studied the effect of changing the rear P-a-Si:H BSF/TCO barrier height , *bL* , in P-c-Si HIT cells. The variation in the current-density – voltage characteristics follow a similar pattern as Fig. 15.

Fig. 13. The current density - voltage characteristics under AM1.5 light and 0 volts for different front contact barrier heights. The band gap, the activation energy and the thickness of the P-layer are held constant at 1.75 eV, 15 nm and 0.3 eV respectively, so that only surface band bending changes.
