**6. Conclusion**

This chapter was devoted to a-Si:H/c-Si and TCO/a-Si:H heterointerfaces forming the front emitter stack with the aim to explain the influence of such heterointerfaces on *V*OC and output performance of SHJp and SHJn solar cells. It was shown that the carrier inversion at the c-Si surface of a-Si:H/c-Si plays a key role for *V*OC and the output performance. Various properties affecting the carrier inversion in the SHJ solar cells were analysed by means of numerical simulation leading to several conclusions. Low defect states at the interface as well as large band offset for minority carriers at a-Si:H/c-Si heterojunction are crucial to achieve strong carrier inversion and high *V*OC. The insertion of an a-Si:H(i) passivation layer provides a decrease of the defect states at the interface; however, careful tuning of the passivation layer thickness is required to achieve a strong passivation effect with a negligible negative effect of the potential drop over this passivation layer. The Schottky barrier at the TCO/a-Si:H interface acts as a parasitic junction with opposite direction of the electric field to the electric of a-Si:H/ c-Si junction. In the case of weak carrier inversion and small emitter thickness, the effect of the parasitic Schottky barrier is not screened by the charge in the emitter or minority carriers in the inversion layer and the Schottky barrier deteriorates the performance of SHJ solar cell. The simulation of SHJ structures at concentrated light conditions revealed a crucial effect of the barriers for hole collection on the efficiency. Tuning of such barriers together with tuning of the operation temperature is required to achieve a high performance of SHJ solar cells under concentrated light conditions. Due to the higher valence band offset compared to the conduction band offset at the a-Si:H/c-Si interface, higher carrier inversion is observed at the front heterointerface of SHJn solar cells leading to higher *V*OC and lower sensitivity to defect states at the heterointerface for SHJn solar cells compared to SHJp solar cell. Two alternative concepts with the ability to provide high carrier inversion at the heterointerfaces were presented. The first one is based on the field effect passivation provided by insertion of a highly doped c-Si layer at the interface and the second one is based on the replacement of a-Si:H emitter by metal

oxide with high *W*<sup>f</sup> , which provides favourable band alignment for formation of strong carrier inversion at the heterointerface.
