**4.1. Front TCO/a-Si:H: impact of parasitic Schottky barrier**

The above simulation study revealed that *V*OC is strongly determined by the properties at the front a-Si:H/c-Si heterointerface. Defect states and band alignment affect the distribution of the charge in the SCR and thus directly influence the electric field and carrier inversion at the heterointerface. The photo-generated carriers are collected by the front TCO and metal contacts. While TCO can be considered as a degenerated semiconductor, the properties at TCO/ a-Si:H have to be also considered for carrier transport [44–46]. The most critical aspect for carrier transport is the possible presence of a parasitic Schottky barrier at the TCO/a-Si:H interface which can arise due to an inappropriate work function of TCO, *W*TCO [24]. *W*TCO depends on the material used as a TCO as well as on the deposition conditions used for preparation. For example, in the case of ITO work functions of 4.2–5.3 eV were reported [45, 47]. The most common way to modify *W*TCO is by controlling the oxygen pressure or pre- and post-deposition annealing [46]. It was shown that the parasitic Schottky barrier at TCO/a-Si:H interface of value *Φ*TCO = 0.35 eV can reduce the conversion efficiency by more than 40% due to the deteriorated light current-voltage characteristics, which follows the so-called S-shape [48]. The *Φ*TCO is partially affected by the carrier doping in the TCO layer, which shifts the Fermi level and thus affects the band alignment at the TCO/a-Si:H. Depending on the magnitude of *Φ*TCO, different carrier transport mechanisms should provide a good contact TCO/a-Si:H. In the case of low *Φ*TCO, thermionic emission should take place as a dominant transport mechanism of carriers. In the case of high *Φ*TCO, tunnelling should take place to assist in the carrier transport. For tunnelling to be active it is necessary to have a high doping at both adjacent parts of the junction [46]. Recently, it was shown that high doping of TCO can result in lowering of the passivation and thus decrease the carrier inversion at the a-Si:H/c-Si, resulting in a decrease of *V*OC and output performance [46]. Because of this, it is necessary to carefully consider not only the *W*TCO but also the appropriate carrier doping to achieve a loss-free TCO/ a-Si:H interface.

In the following simulation, the TCO is considered as a metal contact and the impact is simulated of low parasitic *Φ*TCO at the TCO/a-Si:H emitter on the performance of SHJp solar cell. The aim of this simulation is to describe the impact of the *Φ*TCO on the carrier inversion at the a-Si:H/c-Si of SHJp solar cells with conclusions which can be extended to the SHJn solar cell. **Figure 6(a)** shows *V*OC simulated as a function of emitter layer thickness *d*emitt and as a function of *Φ*TCO. To model the high and low quality of a-Si:H/c-Si interface, two values of negligible low *D*it = 109 and high 5 × 1011 cm–2 were adopted in the simulations. For the low value of *D*it a negligible change of *V*OC with *Φ*TCO is observed. On the other hand, the change of *V*OC with *Φ*TCO is more relevant for high values of *D*it. With the increase of *d*emitt the influence of *Φ*TCO on *V*OC becomes negligible. *Φ*TCO has an impact only on SHJp with a high value of *D*it and low *d*emitt. To explain such a behaviour, **Figure 6(b)** shows the band diagrams of SHJp structures with *Φ*TCO = 0.2 eV, *D*it = 5 × 1011 cm–2 simulated for *d*emitt = 1 and 8 nm. For comparison reasons, the band diagram of SHJp with *d*emitt = 8 nm and without parasitic Schottky barrier is shown as well. The band lines for both *d*emitt were aligned to have the heterointerface at the same distance. As can be seen, the structures with *Φ*TCO = 0 eV and *Φ*TCO = 0.2 eV simulated with *d*emitt *=* 8 nm exhibit the same carrier inversion at the silicon surface of the a-Si:H/c-Si interface. The carrier inversion is, however, significantly lowered when *d*emitt decreases to 1 nm. The parasitic *Φ*TCO forms SCR at the TCO/a-Si:H contact and thus is the source of an electric field with opposite direction to the electric field at the a-Si:H/c-Si junction. In the case of low *D*it the strong carrier inversion at the c-Si surface of a-Si:H/c-Si interface screens the charge and electric field in the SCR of *Φ*TCO. As a result, *Φ*TCO has only a negligible impact on the band bending as well as on the carrier inversion and *V*OC (**Figure 6b**). For *D*it = 5 × 1011 cm–2 the carrier inversion at the a-Si:H/c-Si is significantly lowered due to the presence of *Q*<sup>i</sup> . For such conditions, the distribution of the electric field in the a-Si:H emitter is more sensitive to *Φ*TCO. In case of high *d*emitt, the free carriers in the emitter can screen the impact of *Φ*TCO and the electric field formed in the SCR of *Φ*TCO barrier, thus no relevant decrease of carrier inversion at a-Si:H/c-Si is observed. With a decrease of *d*emitt the SCR of *Φ*TCO can reach the SCR of SHJp. For such a case, the electric field of *Φ*TCO lowers the diffusion potential of a-Si:H/c-Si and the parasitic Schottky barrier attracts the holes from the c-Si. As a result, carrier inversion at the interface decreases, leading into a decrease of *V*OC and thus the overall performance decreases. Simulation results revealed that the negative influence of the parasitic*Φ*TCO is due to the change of the carrier inversion at the a-Si:H/c-Si interface caused by the electric field of SCR at TCO/a-Si:H contact. Such a change is, however, possible only for low emitter thicknesses which have not sufficient charge for screening of *Φ*TCO. Obviously, the doping of the emitter layer, in other words, the concentration of free carriers will also affect the screening ability of the emitter. With decrease of the doping, *Φ*TCO will have more significant impact on the carrier inversion at the a-Si:H/c-Si interface and thus will more rapidly deteriorate the output performance.

preparation. For example, in the case of ITO work functions of 4.2–5.3 eV were reported [45, 47]. The most common way to modify *W*TCO is by controlling the oxygen pressure or pre- and post-deposition annealing [46]. It was shown that the parasitic Schottky barrier at TCO/a-Si:H interface of value *Φ*TCO = 0.35 eV can reduce the conversion efficiency by more than 40% due to the deteriorated light current-voltage characteristics, which follows the so-called S-shape [48]. The *Φ*TCO is partially affected by the carrier doping in the TCO layer, which shifts the Fermi level and thus affects the band alignment at the TCO/a-Si:H. Depending on the magnitude of *Φ*TCO, different carrier transport mechanisms should provide a good contact TCO/a-Si:H. In the case of low *Φ*TCO, thermionic emission should take place as a dominant transport mechanism of carriers. In the case of high *Φ*TCO, tunnelling should take place to assist in the carrier transport. For tunnelling to be active it is necessary to have a high doping at both adjacent parts of the junction [46]. Recently, it was shown that high doping of TCO can result in lowering of the passivation and thus decrease the carrier inversion at the a-Si:H/c-Si, resulting in a decrease of *V*OC and output performance [46]. Because of this, it is necessary to carefully consider not only the *W*TCO but also the appropriate carrier doping to achieve a loss-free TCO/

In the following simulation, the TCO is considered as a metal contact and the impact is simulated of low parasitic *Φ*TCO at the TCO/a-Si:H emitter on the performance of SHJp solar cell. The aim of this simulation is to describe the impact of the *Φ*TCO on the carrier inversion at the a-Si:H/c-Si of SHJp solar cells with conclusions which can be extended to the SHJn solar cell. **Figure 6(a)** shows *V*OC simulated as a function of emitter layer thickness *d*emitt and as a function of *Φ*TCO. To model the high and low quality of a-Si:H/c-Si interface, two values of

value of *D*it a negligible change of *V*OC with *Φ*TCO is observed. On the other hand, the change of *V*OC with *Φ*TCO is more relevant for high values of *D*it. With the increase of *d*emitt the influence of *Φ*TCO on *V*OC becomes negligible. *Φ*TCO has an impact only on SHJp with a high value of *D*it and low *d*emitt. To explain such a behaviour, **Figure 6(b)** shows the band diagrams of SHJp structures with *Φ*TCO = 0.2 eV, *D*it = 5 × 1011 cm–2 simulated for *d*emitt = 1 and 8 nm. For comparison reasons, the band diagram of SHJp with *d*emitt = 8 nm and without parasitic Schottky barrier is shown as well. The band lines for both *d*emitt were aligned to have the heterointerface at the same distance. As can be seen, the structures with *Φ*TCO = 0 eV and *Φ*TCO = 0.2 eV simulated with *d*emitt *=* 8 nm exhibit the same carrier inversion at the silicon surface of the a-Si:H/c-Si interface. The carrier inversion is, however, significantly lowered when *d*emitt decreases to 1 nm. The parasitic *Φ*TCO forms SCR at the TCO/a-Si:H contact and thus is the source of an electric field with opposite direction to the electric field at the a-Si:H/c-Si junction. In the case of low *D*it the strong carrier inversion at the c-Si surface of a-Si:H/c-Si interface screens the charge and electric field in the SCR of *Φ*TCO. As a result, *Φ*TCO has only a negligible impact on the band bending as well as on the carrier inversion and *V*OC (**Figure 6b**). For *D*it = 5 × 1011 cm–2 the carrier inversion at the a-Si:H/c-Si is significantly lowered due to the

and high 5 × 1011 cm–2 were adopted in the simulations. For the low

. For such conditions, the distribution of the electric field in the a-Si:H emitter

is more sensitive to *Φ*TCO. In case of high *d*emitt, the free carriers in the emitter can screen the impact of *Φ*TCO and the electric field formed in the SCR of *Φ*TCO barrier, thus no relevant decrease of carrier inversion at a-Si:H/c-Si is observed. With a decrease of *d*emitt the SCR of

a-Si:H interface.

82 Nanostructured Solar Cells

negligible low *D*it = 109

presence of *Q*<sup>i</sup>

**Figure 6.** (a) *V*OC calculated as a function of *d*emitt of SHJp solar cell structure. *Φ*TCO is varied as a parameter and two values of *D*it are used in the simulations. (b) Band diagrams calculated for two values of *d*emitt and *Φ*TCO = 0.2 eV for SHJp solar cell structure. The band diagrams are aligned to place the heterointerface at the same distance. The inset shows the change in the carrier inversion.

Similar effect of *Φ*TCO is presented in SHJn structure. Comparing SHJn and SHJp structures, the main difference is in the dopation type of amorphous emitter and thus required *W*<sup>f</sup> of TCO to obtain good TCO/a-Si:H contact. Due to the presence of n-type a-Si:H emitter in SHJp solar cell, the TCO lower than at least 4.5 eV is required [24]. Typically, TCO materials have *W*<sup>f</sup> higher than 4.5 eV [49, 50], which make the design of SHJp more challenging and require higher thicknesses or higher doping of a-Si:H emitter layer. In case of SHJn solar cells, the minimal *W*<sup>f</sup> = 5.1 eV is required to obtain good TCO/a-Si:H contact [24], resulting in the lower technological obstacles for preparation of good TCO/a-Si:H contact.
