**Acknowledgements**

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

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

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

efficiency of SHJ solar cells in concentrated solar applications.

light is more challenging compared to SHJp solar cell structures.

**6. Conclusion**

86 Nanostructured Solar Cells

This work was supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and of the Slovak Academy of Sciences under project VEGA 1/0651/16 and Slovak Research and Development Agency under the contract APVV-15-0152.
