**5. The role of interfaces in SHJ working under concentrated light**

Recently, possible utilization of silicon-based solar cells in light concentration applications became an attractive approach to increase the energy yield from such solar cell structures [51,

52]. Thus, it is of high interest to explore possible aspects connected with the SHJ solar cells for utilizations under concentrated light. Due to the formation of heterojunctions between a-Si:H layers and the c-Si absorption layer, the carrier transport has to overcome barriers at the front and back interfaces of the SHJ structure. Such barriers can significantly affect the collection of photo-generated carriers and thus the solar cell performance at high light intensity. Moreover, the increased light intensity absorbed by the solar cell represents a considerable amount of energy which is partially transformed to thermal energy and causes an increase of cell temperature. Because of this, the impact of the elevated temperature of such a solar cell is considered in the simulations as well. **Figure 7(a)** shows the efficiency as a function of concentrated light expressed in the suns (1 sun = 1000 W/m2 ) calculated at 300, 340 and 380 K for both SHJn and SHJp structures. As can be seen, the efficiency at 1 sun decreases with temperature for both SHJ structures. Such decreases are due to the increase of the saturation current caused by an increase of the intrinsic carrier concentration in the c-Si. Saturation current lowers the *V*OC (see Eq. 5), which consequently results in a decrease of efficiency. In general, the increase of light concentration causes an increase of the light generation *g* and excess concentration of carriers Δ*p =* Δ*n*, thus results in an increases of *V*OC according to Eqs. (1) and (2) for SHJp and SHJn, respectively (see Section 2). Simulated results revealed that the efficiency of SHJ structures reach the maximum value at particular light concentration and then starts to decrease. With increased temperature the maximum of the efficiency is shifted to higher values of light concentrations. The temperature dependence of efficiency suggests that the source of efficiency drop at higher light concentrations is the presence of barriers for carrier transport which are partially overcome at higher temperatures by thermionic emission. Such carrier transport limitations are reflected also in FF. **Figure 7(b)** shows FF calculated as a function of light concentration for both SHJn and SHJp structures. The FF exhibits a similar trend to *V*OC, and decreases significantly at high light concentrations. This drop is more relevant for SHJp structure. Considering the band diagrams of both SHJ structures (see **Figures 2a** and **b**), it can be suggested that different barriers are limiting carrier transport in SHJp and SHJn solar cells. In the case of SHJn, the photo-generated holes are collected through the front heterointerface and photo-generated electrons are collected through the back surface field (BSF) formed in our case by the c-Si/a-Si:H(n) contact. For SHJn structure, the valence band offset at the front a-Si:H/c-Si Δ*E*<sup>V</sup> attains considerable higher values of 0.55 eV compared to the conduction band offset Δ*E*<sup>C</sup> = 0.15 eV at the back BSF contact. It can be assumed that, due to the higher barrier, the front a-Si:H/c-Si heterointerface will be the main limitation factor for the transport of photo-generated carriers. In the case of SHJp structure, photo-generated electrons are collected through the front a-Si:H(n)/c-Si(p) contact, while photo-generated holes are collected through the back c-Si(p)/a-Si:H(p) BSF contact. Δ*E*<sup>C</sup> for minority electrons at the front heterointerface is around 0.15 eV, while the barrier for holes Δ*E*BSF can reach values around 0.7 eV. Because of this, it can be assumed that the back contact is the limiting factor for the carrier transport for SHJp structure.

ASA simulation was carried out to confirm the negative impact of the front Δ*E*<sup>V</sup> and back Δ*E*BSF barrier for carrier transport of SHJn and SHJp structures, respectively. **Figure 8(a)** shows the simulated efficiency as a function of light concentration for SHJn at 300 K with considered variation in Δ*E*V from 0.65 to 0.45 eV. Δ*E*V has a negligible impact on the efficiency at 1 sun light concentration. With the increase of the light concentration the efficiency exhibits a decrease, which is more relevant for higher Δ*E*V values. We can assume that such a decrease of efficiency is connected with limitation of carrier transport through Δ*E*V barrier. **Figure 8(b)** shows the efficiency of SHJp calculated as a function of light concentration with a varied barrier for holes Δ*E*BSF. The results show that the onset of the efficiency decrease is shifted to higher light concentrations with an increase of Δ*E*BSF. Further simulations revealed (not shown in this chapter) that varying of the front Δ*E*C has no impact on the efficiency behaviour with the change of light concentration. Such trends justify the back Δ*E*BSF barrier to be responsible for the limitation of carrier transport and efficiency losses at high light concentrations of SHJp solar cell structure.

52]. Thus, it is of high interest to explore possible aspects connected with the SHJ solar cells for utilizations under concentrated light. Due to the formation of heterojunctions between a-Si:H layers and the c-Si absorption layer, the carrier transport has to overcome barriers at the front and back interfaces of the SHJ structure. Such barriers can significantly affect the collection of photo-generated carriers and thus the solar cell performance at high light intensity. Moreover, the increased light intensity absorbed by the solar cell represents a considerable amount of energy which is partially transformed to thermal energy and causes an increase of cell temperature. Because of this, the impact of the elevated temperature of such a solar cell is considered in the simulations as well. **Figure 7(a)** shows the efficiency as a

and 380 K for both SHJn and SHJp structures. As can be seen, the efficiency at 1 sun decreases with temperature for both SHJ structures. Such decreases are due to the increase of the saturation current caused by an increase of the intrinsic carrier concentration in the c-Si. Saturation current lowers the *V*OC (see Eq. 5), which consequently results in a decrease of efficiency. In general, the increase of light concentration causes an increase of the light generation *g* and excess concentration of carriers Δ*p =* Δ*n*, thus results in an increases of *V*OC according to Eqs. (1) and (2) for SHJp and SHJn, respectively (see Section 2). Simulated results revealed that the efficiency of SHJ structures reach the maximum value at particular light concentration and then starts to decrease. With increased temperature the maximum of the efficiency is shifted to higher values of light concentrations. The temperature dependence of efficiency suggests that the source of efficiency drop at higher light concentrations is the presence of barriers for carrier transport which are partially overcome at higher temperatures by thermionic emission. Such carrier transport limitations are reflected also in FF. **Figure 7(b)** shows FF calculated as a function of light concentration for both SHJn and SHJp structures. The FF exhibits a similar trend to *V*OC, and decreases significantly at high light concentrations. This drop is more relevant for SHJp structure. Considering the band diagrams of both SHJ structures (see **Figures 2a** and **b**), it can be suggested that different barriers are limiting carrier transport in SHJp and SHJn solar cells. In the case of SHJn, the photo-generated holes are collected through the front heterointerface and photo-generated electrons are collected through the back surface field (BSF) formed in our case by the c-Si/a-Si:H(n) contact. For SHJn structure, the valence band offset at the front a-Si:H/c-Si Δ*E*<sup>V</sup> attains considerable higher values of 0.55 eV compared to the conduction band offset Δ*E*<sup>C</sup> = 0.15 eV at the back BSF contact. It can be assumed that, due to the higher barrier, the front a-Si:H/c-Si heterointerface will be the main limitation factor for the transport of photo-generated carriers. In the case of SHJp structure, photo-generated electrons are collected through the front a-Si:H(n)/c-Si(p) contact, while photo-generated holes are collected through the back c-Si(p)/a-Si:H(p) BSF contact. Δ*E*<sup>C</sup> for minority electrons at the front heterointerface is around 0.15 eV, while the barrier for holes Δ*E*BSF can reach values around 0.7 eV. Because of this, it can be assumed that the back contact

) calculated at 300, 340

function of concentrated light expressed in the suns (1 sun = 1000 W/m2

84 Nanostructured Solar Cells

is the limiting factor for the carrier transport for SHJp structure.

ASA simulation was carried out to confirm the negative impact of the front Δ*E*<sup>V</sup> and back Δ*E*BSF barrier for carrier transport of SHJn and SHJp structures, respectively. **Figure 8(a)** shows the simulated efficiency as a function of light concentration for SHJn at 300 K with considered variation in Δ*E*V from 0.65 to 0.45 eV. Δ*E*V has a negligible impact on the efficiency at 1 sun light

**Figure 7.** (a) Efficiency *η* and (b) FF calculated as a function of light concentration at temperatures 300, 340 and 380 K for SHJn (solid lines) and SHJp (dashed lines) structures.

**Figure 8.** (a) Efficiency *η* calculated as a function of light concentration for different values of Δ*E*<sup>V</sup> of SHJn solar cell structure. (b) Efficiency *η* calculated as a function of light concentration for different values of Δ*E*BSF of SHJp solar cell structure.

From the above discussion it is clear that the presence of barriers for carrier transports has to be taken into account when the SHJ is designed for light concentration applications. While amorphous silicon forms higher Δ*E*V than Δ*E*C with c-Si, the barriers for collections of holes are the main source of carrier transport limitations. In the case of SHJn structure such a barrier is formed at the front a-Si:H/c-Si interface while for SHJp structure this barrier is placed at the back c-Si/a-Si:H BSF contact. Due to the presence of thermionic emission causing a temperature-dependent carrier transport mechanism through such barriers, adjustment of the working temperature with light concentration has to be considered in order to attain the highest possible efficiency of SHJ solar cells in concentrated solar applications.

Our recent study shows that the higher operation temperature has a beneficial effect not only in enhancement of the carrier transport through barriers formed by the a-Si:H/c-Si interface but also decreases the negative impact of the parasitic Schottky barrier at the TCO/a-Si:H interface [53]. The negative influence of such barriers is more significant for SHJn structure, where the Schottky barrier depletes the emitter and increases the negative influence of Δ*E*V. Thus, the optimization of SHJn solar cell structures for solar applications under concentrated light is more challenging compared to SHJp solar cell structures.
