3.1 Optimization of thickness of a-Si:H(i) layer
Figure 4 shows the variation of a-Si:H(i) layer thickness with solar cell parameters. The thickness of a-Si:H(i) layer was varied from 3 to 20 nm. The thickness of p-layer was fixed at 5 nm for this i-layer optimization. The Voc of 763.3 mV, Jsc of 42.17, FF 85.77 and ɳ of 27.6 obtained for 3 nm of i-layer. It was found that the Voc values are not changed up to 10 nm and slightly increased at 15 and 20 nm of i-layer. The Voc of solar cells mainly depends on bulk and interface defects and also recombination centers. It is observed that the Jsc values are decreased as increase in thickness of i-layer. The Jsc values are decreased to 40.41 from 42.17 mA/cm2. This is mainly due to increased absorption of photons in the i-layer while entering into c-Si substrate. As a results in less number of free electron-hole generation. After generation, free carriers travel through i-layer to reach metal contacts, in this process, these free carriers recombine in the i-layer due to large minority carrier path length of i-layer [37]. The FF values are not changed up to 10 nm of i-layer and beyond that FF values are decreased due to increased resistance of thick i-layer. The maximum efficiency of 27.6% was obtained at 3 nm of i-layer. These values are decreased as increase in thickness of i-layer due to decrease of Jsc and FF of solar cells. The efficiency of 23.62% was obtained at 20 nm thick i-layer.
3.2 Optimization of thickness of a-Si:H(p) layer
Figure 5 shows the variation of a-Si:H(p) layer thickness with solar cell parameters. The thickness of a-Si:H(p) layer was varied from 3 to 20 nm and thickness of i-layer was fixed at 3nm, where obtained best efficiency was 27.6%. The Voc of 764.8 mV, Jsc of 43.15, FF of 85.71 and ɳ of 28.28% were obtained for 3 nm of p-layer. In order to match the more realistic experimental bandgap value of p-layer, the bandgap of p-layer was kept at 1.8 eV. The maximum value of Voc (764.8 mV) was obtained at 3 nm of p-layer and it was decreased as increase in the thickness of p-layer. The reason is that increased defect density in the p-layer with increase in the thickness of p-layer [27, 33]. The Voc decreased from 764.8 mV to 760.2 mV as increase of p-layer thickness of 3- 20 nm respectively. The high Jsc of 43.15 mA/cm2 obtained at 3 nm of p-layer thickness. Absorption losses were reduced with less thickness p-layer. The higher bandgap p-layer allows more number photons to reach c-Si substrate, which results in high Jsc values in the solar cells. The FF values are not changed much as increase in thickness. The increase in Voc, Jsc and FF values of solar cells resulting in an increased efficiency of solar cells with decreasing thickness of p-layer. The low value of efficiency 24.21% obtained at 20 nm thick p-layer.
3.3. Influence of thickness of interface layer at different defect density
The c-Si(n) interface was introduced between i-layer and c-Si substrate in the c-Si/a-Si:H heterojunction solar cells. The defect density on surface and bulk of the c-Si wafer is not equally distributed and this defect density may vary with fabrication, wafer processing and thickness of wafer [5, 6, 41]. Usually, thickness of the wafer is more at centre and less at edges of diameter. In order to study the effect of defect density and thickness on c-Si/a-Si:H heterojunction solar cells, the defect density and thickness of c-Si(n) interface layer was varied from 109 -1014 cm-3 and 5 nm-1 µm respectively. The 250 µm thick c-Si was used in this study. The 5-10 nm, 50 nm thin and 0.1 to 1 µm thick interface layer was introduced at different defect density (109 -1014 cm-3).
Figure 6 (a-f) show the performance of c-Si/a-Si:H heterojunction solar cell with c-Si(n) interface layer thickness at defect density (109-1014 cm-3) of interface. Figure 6a shows the solar cell parameters as a function of the thickness at the defect density 109 cm-3 of c-Si(n) interface. The Voc is not changed up to 150 nm of interface layer and then it was decreased at 0.25 µm, further, Voc was constant till 1 µm thick interface layer. The Jsc values are decreased from 38.83 to 35.14 mA/cm2 as thickness increased. Absorption losses in solar cells were increased by increasing thickness of interface layer. The changes in FF are very less with thickness at the defect density of 109 cm-3 of interface. The resulting efficiency has decreased. The achieved Voc, Jsc, FF, ɳ is 761.7 mV, 38.83 mA/cm2, 86.11%, 25.47% and 760.2, 35.14, 85.9%, 22.94% at 5 nm and 1 µm thick interface layer respectively. In previous section, it is observed that even 20 nm thick a-Si:H layer has huge impact on the performance of the solar cell. In Figure 6b, at the defect density of 1010 cm-3, the performance of solar cells has not changed and almost equally comparable with the performance at the defect of 109 cm-3. At the defect density of 1011 cm-3 in Figure 6c, the Voc (761.7 mV) has not changed till 150 nm thick layer and later decreased gradually. The Voc has decreased to 758.6 mV at 1 µm thick interface layer and Jsc and FF have followed similar trend of earlier solar cells. Slight decrease in efficiency has been observed at 1 µm of interface layer due to decrease in Voc. The achieved Voc, Jsc, FF, ɳ is 761.7 mV, 38.83 mA/cm2, 86.11%, 25.47% and 758.6, 35.14, 85.83%, 22.88% at 5 nm and 1 µm thick interface layer respectively at the defect of 1011 cm-3 of interface. In Figure 6d, the performance of the c-Si/a-Si:H solar cell have not changed till 150 nm thick at defect density of 1012 cm-3. Then slightly decreased at 250 and 450 nm thickness. The Voc and efficiency have decreased to 750 mV and 22.88% with 1012 cm-3 at 1 µm of interface layer respectively. The solar parameters were not changed at 5 and 10 nm even at high defect density of 1013 cm-3 (Figure 6e). The Voc and FF have not changed at thin layer of 5 nm and 10 nm and then Voc of 4 mV and 8 mV decreased at 50 nm and 100 nm respectively at the defect of 1013 cm-3 of interface (Figure 6e). The Voc has been decreased significantly as increase in thickness beyond 100 nm and severely affected with 1013 cm-3 at 1 µm of interface layer. Very low Voc of 713 mV observed at 1 µm and the Jsc has not changed at high defect density as variation of thickness of interface layer. The FF values are decreased up to 100 nm and then slightly increased further increase in the thickness of the interface layer.
The efficiency of cells decreased with thickness and low efficiency of 20.54% observed at 1 µm. The solar cell parameters were not significantly changed at 5 and 10 nm of c-Si interface layer even at high defect density of 1014 cm-3 (Figure 6f). The Jsc values are not changed with defect density. The FF values are decreased and then increased slightly. The Voc was significantly decreased from the thickness of 150 nm to 1 µm. The lowest Voc and ɳ of 652.3 mV and 18.47% were observed with 1 µm respectively. The achieved Voc, Jsc, FF, ɳ is 761.7 mV, 38.83 mA/cm2, 86.09%, 25.46% and 652.3, 35.05, 80.76%, 18.47% at 5 nm and 1 µm thick interface layer respectively at the defect of 1014 cm-3 of interface.
3.4. Influence of defect density of interface layer at different thickness
Figure 7 (a-i) show the solar cell parameters as a function of c-Si(n) interface layer defect density at different thickness of interface layer respectively. Solar cell parameters were estimated by varying defect density from 109 to 1014 cm-3 at different thicknesses such as 5, 10, 50, 150, 200, 450, 750 and 1000 nm. It was observed that at 5 and 10 nm thick, solar cell parameters Voc, Jsc, FF and efficiency were not changed for defect density (109 to 1014 cm-3) of c-Si(n) interface layer (Figure 7(a, b)). This means that defect density is not affecting at very thin interface layer. The obtained values Voc, Jsc, FF, ɳ at 1014 cm-3 are 761.7 mV, 38.83 mA/cm2 ,86.09%, 25.46% and 761.7 mV, 38.75 mA/cm2, 85.95%, 25.37% for 5, 10 nm respectively (Figure 7(a, b)). In Figure 7(c, d), the Voc, Jsc, FF and ɳ values are not changed up to 1012 cm-3 of defect density and the performance of the solar cell was detiorated as further increase in the defect density. The defect density of 1012 cm-3 is maximum limit for 50, 100 and 150 nm thick interface layer for less changes in solar cell parameters. The obtained values of Voc, Jsc, FF, ɳ at 1012 cm-3 are 761.7 mV, 38.26 mA/cm2, 85.81%, 25% and 760.2 mV, 37.77 mA/cm2, 85.76% FF, 24.62%, 760.2 mV, 37.38 mA/cm2, 85.6%, 24.32% for 50, 100 and 150 nm respectively (Figure 7(c-e)). The performance of solar cellls was not changed in the range of 109 to1011 cm-3 of defect density and beyond these density Voc, FF and ɳ has decreased at thickness of 250 nm (Figure 6f). In Figure 7(g, h), the efficiency has decreased about 2% at 109 and 1010 cm-3, this continued up to 1012 cm-3 of defect density. The obtained values of Voc, Jsc, FF, efficiency at 1010 cm-3 are 760.2 mV, 36.02 mA/cm2, 85.98%, 23.54% and 760.2 mV, 35.42 mA/cm2, 85.91%, 23.13% for 450 and 750 nm respectively. The obtained low values of Voc, Jsc, FF, ɳ at 1014 cm-3 are 671.1 mV, 36.01 mA/cm2 ,81.21%, 19.62% and 658.6.2 mV, 35.37 mA/cm2, 81.24%, 18.93% for 450 and 750 nm respectively. The current density has not changed by varying defect density from 109 to 1014 cm-3 at each thickness of interface. In Figure 7i, at 1000nm, the Voc, FF and efficiency values have been changed from 760.2 to 653.2 mV, 85.9 to 80.76% and 22.94 to 18.47% for 109 to 1014 cm-3 of defect density. It is found that the solar cell performance was not deterioted with very thin (5 and 10 nm) c-Si(n) interface layer for defect density in the range of 109 to 1014 cm-3. For the 50-150 nm thick interface layer, the solar cell parameters values decreased slightly due to thikness of interface layer and however peroformance was not effected by defect density in the range 109 to 1012 cm-3 and beyond this defect density, solar cell parameter values are decreased. The performance of solar cells was not affected till the defect density of 1012 cm-3 for the thickness of inetrafce layer of 250 and 450 nm . The solar cell performance was severly deterioted by very thick interface layer even at low defect density.
The current density of solar cells was decreased as thickness increased and it was not changed with defect density of interface layer. For very thin interface layer, the Voc and FF, efficiency values are not changed by varying defect density. However, the Voc values are severely affected by high defect density for very thin interface layer. In very thin layer, generated charge carriers crossed junction without recombining at interface. Whereas in the case of thick and very thick interface layer, recombination centres increased and generated and seperated charge carriers recombined at interface layer before crossing the junction and reaching metal contacts. The FF values are decreased as increase in thickness of interface layer due to increased resistance losses in the solar cells. Resulting efficiency of solar cells was decreased as increase in thickness and defect density of interface layer.