1 Thickness optimization of p-cSi silicon wafer:
In this section, thickness of p-type crystalline silicon wafer is optimized. Manufacturing of solar cells using thin p-cSi wafer can be cost effective up to certain extent at industrial scale. Though nowadays n-cSi ingots are also reasonable in cost compared to p-cSi and wafer of around 200 µm thickness are in use at industrial scale. However, through our simulation we have obtained that its thickness can further be reduced to upto 70 µm without affecting its performance. All other parameters were in accordance with the values reported in Table-1. Efficiency rapidly increases with thickness initially, begins to saturate after 70 µm and remains constant upto 500 µm. This can be well explained as initially as thickness increases, more number of charge carriers are generated and contribute to flow of charge across p-n junction. However, after certain thickness, charge carriers generated does not travel across junction due to thickening of wafer and less diffusion length of charge carriers in silicon wafer which is around 100 µm. When thickness of silicon wafer gets thicker than its diffusion length, charger carriers generated does not travel upto p-n junction and gets recombined and hence charge carriers generated having diffusion length more than 100 µm does not participate in charge collection. Thus, efficiency begins to saturate after 70 µm. Also, film gets flexible when its thickness reduces beyond 50 µm. Experimentally, an ultrathin flexible film of thickness 45 µm has been fabricated by  using Cu-assisted chemical etching of bulk c-Si producing conversion efficiency of over 17%. Also, mechanical stress, multi flexure, fracture and static test has been conducted by  for flexible thin films based crystalline silicon solar cells where thickness of flexible thin film varying from 20 µm to 50 µm. Thus, an ultrathin stable film can be utilized for silicon based HJ solar cells. Hence, the thickness of p-type silicon wafer at 70 µm is optimized in this work. Similar pattern is also observed for short circuit current (Jsc) curve. This behaviour is well illustrated in Voc vs thickness curve figure: 2(a), Jsc vs thickness curve figure: 2(b) and fill factor (FF) vs thickness curve figure: 2(c). Voc, Jsc and FF values remains nearly constant from 70 µm to 500 µm; Voc and Jsc rapidly decreases on further reducing the thickness of p-cSi layer which accounts for the decrease in efficiency. Fill factor firstly increases slightly and then decreases but this increase is less significant than rapid decrease in Voc and Jsc which results in decrease in efficiency. Hence, the thickness of p-cSi layer is optimized at 70 µm to obtain an efficiency of 12.84% for AZO layer and 12.16% for TiO2 layer based p-cSi HJ solar cells.
2 Thickness optimization of ultra-thin n-type AZO and TiO2 layer:
Thickness of AZO and TiO2 layer is varied from 0.5 nm to 10 nm and open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and efficiency is recorded with respect to thickness of AZO & TiO2 layer, Figure 3 (a) to (d) represents curve for each respectively. All other parameters were in accordance with the values reported in Table-1. There is no effect of thickness variation of TiO2 layer on Voc and it remains constant at 546.1 mV. Same pattern is observed for AZO layer as well, However, there is slight enhancement in Voc at 3 - 4 nm, giving maximum Voc of 549.2 mV below 3 nm. Short circuit current linearly decreases with increase in its thickness giving maximum Jsc at 0.5 nm. However, due to practical limitations, deposition of layers using modern techniques is limited upto 3 nm for stable film and hence Jsc at 3 nm giving short circuit current of 29.12 mA for ultra-thin AZO layer and 29.02 mA for ultra-thin TiO2 layer based solar cell is considered as optimized short circuit current . This behaviour is expected, since thinning of emitter layer will contribute to flow more number of charge carriers generated in absorber layer and easy transport across layer due to decrease in resistance, thus recombination rate also decreases; hence resulting in enhancement of short circuit current with decrease in thickness of emitter layer. Fill factor also decreases linearly with increasing thickness. This can be attributed due to decrease of series resistance as we decrease its thickness. Hence, overall efficiency increases linearly with decreasing thickness and gives maximum efficiency of 12.84% for ultra-thin n-type AZO layer and 12.59% for ultra-thin n-type TiO2 layer based HJ solar cell.
3 Doping concentration of donors (Nd)
Doping of donor concentration plays an important role in enhancing the charge carriers which resulted in improved efficiency. Figure 4 (a) to (d) represents the variation of Voc, Jsc, FF and efficiency curve with respect to doping concentration of donors respectively, where Nd is varied from 1014 cm-3 to 1018 cm-3. Doping can be achieved in order of 1020 cm-3 using various techniques like ion implantation, mixed molecular monolayer doping technique etc. . Introduction of doping concentration of donors doesn’t affect open circuit voltage (Voc) much and it nearly remains constant for both, AZO and TiO2 layer excluding for TiO2 at lower concentration. Short circuit current (Jsc) effectively decreases with increase of doping concentration of donors, giving maximum short circuit current of 30.1 mA and 30.3 mA for AZO and TiO2 layer respectively. Maximum fill factor at 1015 cm-3 for TiO2 layer and at 1016 cm-3 for AZO layer is observed and it nearly remains unaffected on further increasing doping of donors. Similar pattern is followed in efficiency curve as well, where maximum efficiency of 13.13% for AZO layer and 13.21% for TiO2 is reported at doping concentration of 1×1015 cm-3 and 1×1014 cm-3 respectively. Performance of device degrades as doping concentration increases beyond order of 1015 which can be due to introduction of defect states in AZO and TiO2 layer at higher doping concentration limiting its performance. With increase in defect states in space charge region, it can create conduction channel across it and leakage current starts flowing which degrades the performance of device. Hence, efficiency decreases with further increase of doping concentration for AZO layer beyond 1015 cm-3. Similar pattern is followed for TiO2 layer as well, where efficiency remains nearly constant upto 1015 cm-3 and decreases thereafter due to introduction of defect states at higher concentration.
4 Effective conduction band density
During carrier charge transport, effective conduction band density and effective valence band density are the major density of states that plays important role during charge transport. Figure 5.1 to 5.4 represents the variation of Voc, Jsc, FF and efficiency curve with respect to effective conduction band density respectively. All other parameters were in accordance with the values reported in Table-1. Effective conduction band density is varied from 1015 cm-3 to 1019 cm-3 as shown in figure 5(a) to 5(d). Similar variation in tail density of states from 1013 cm-3 to 1021 cm-3 has been reported in . Open circuit voltage remains constant on varying conduction band density for both, AZO & TiO2 layer. Short circuit current linearly decreases slightly on increasing Nc giving maximum Jsc of 30.4 mA at 1015 cm-3 for TiO2 layer and 30.08 mA at 1019 cm-3 for AZO layer based solar cell. Fill factor is significantly influenced with Nc and thus contributes more to efficiency. Hence, efficiency follows the pattern like fill factor. Efficiency increases initially exhibiting best efficiency at around 1016 cm-3 and remains nearly constant thereafter which can be attributed to increase in shunt resistance and decrease in series resistance initially thus affecting fill factor of device accordingly. After certain level, there is no significant change in shunt resistance and series resistance with change in effective conduction band density resulting in nearly constant fill factor, thus efficiency also gets saturated. However, initial increase in efficiency is more significant in case of TiO2 based solar cell than compare to AZO based silicon solar cell and further study needs to be carried out for more fundamental reason behind this. Maximum efficiency of 13.24% for AZO layer and 13.21% for TiO2 layer at 5 × 1015 cm-3 and 1 × 1017 cm-3 respectively is obtained for AZO/Si and TiO2/Si based HJ solar cells respectively.
5 Texturing angle
Efficiency of solar cell can be significantly enhanced through texturing . Many reports are there on pyramidal and inverted pyramid textured silicon wafer solar cell. Texturing angle is varied from 0° to 89° as shown in figure 6. Voc increases linearly with texturing angle giving maximum open circuit voltage 555.5 mV at 89° for both, AZO layer based and TiO2 layer based silicon HJ solar cells. Similar trend is followed in Jsc curve giving maximum short circuit current of 37.81 mA and 38.92 mA at 89° for AZO layer based and TiO2 layer based silicon HJ solar cells. Fill factor nearly remains constant and varies from 80.25% to 80.96% for entire range of texturing angle. Efficiency follows the same curve, giving maximum efficiency of 17% for AZO layer and 17.5% for TiO2 layer at texturing angle of 89°. Efficiency increases linearly with texturing angle due to increment in close packing density of textured surface which increases the internal multiple reflection of illuminated light and helps to enhance absorption loss, thus contributes in collecting more charge carriers . Thus efficiency of modelled AZO/Si HJ solar cell is significantly increased. Hence all the parameters of TiO2, AZO layer and p-cSi wafer layer is optimized to attain maximum efficiency of 17% for AZO/Si and 17.5% for TiO2/p-cSi heterojunction solar cells.