**4. Front TCO/a-Si:H heterointerface**

in the amorphous emitter (formed by the intrinsic and doped parts) resulting in an increase of the diffusion voltage in a-Si:H part of the junction and in a decrease of carrier inversion at c-Si surface of the a-Si:H/c-Si interface with increased *d*a-Si:H(i). This conclusion is in accordance with experimental observation [38]. *V*OC decreases as a consequence of weaker carrier inversion. In accordance with this explanation, **Figure 5(b)** shows a more significant decrease of band banding in the c-Si and an increase of the band banding in the a-Si:H followed by a decrease of the carrier inversion at the interface for *d*a-Si:H(i) = 50 nm compared to the sample with *d*a-Si:H(i) = 10 nm. While the quality and thus passivation properties of the a-Si:H(i) layer increase with the thickness, careful tuning of the thickness and passivation ability is required to achieve high *V*OC and high output performance. The same principle can be applied to the

**Figure 5.** (a) *V*OC calculated as a function of a-Si:H(i) thickness, *d*a-Si:H(i) inserted at the front a-Si:H/c-Si of SHJp solar cell structure. *D*it is varied as a parameter in the simulations. (b) Band diagrams calculated for two values of *d*a-Si:H(i) and *D*it = 5 × 1011 cm–2 for SHJp solar cell structure. The inset shows the change in the carrier inversion (change in the distance

From the above discussion it is clear that high carrier inversion at the emitter/c-Si interface is crucial for high *V*OC and high output performance of the SHJ solar cell. The high carrier inversion in the SHJ solar cells can be attained through (i) modification of band alignment at the heterointerface or (ii) by a decrease of *D*it by optimizing the cleaning process or by insertion of a thin passivation a-Si:H(i) layer [6]. In following, we will discuss two alternative concepts of emitters which allow formation of high inversion at the emitter/c-Si interface and offer perspective to achieve high performance. The first one is the hetero-homojunction concept based on the field passivation effect [39, 40] and the second one is the use of alternative emitters

, which form the hole transport layers in

**3.4. Alternative concepts to obtain carrier inversion at emitter/c-Si interface**

of the conduction band level from the Fermi level at the heterointerface).

based on transition metal oxides TMO with high *W*<sup>f</sup>

SHJn structures [41].

SHJn structure.

80 Nanostructured Solar Cells
