Metastability of SiNx/a-Si:H crystalline silicon surface passivation for PV application

In this paper we investigate one of the most promising solution for crystalline silicon (c-Si) surface passivation in solar cell fabrication consisting in a double layer of intrinsic amorphous silicon (a-Si:H) and amorphous silicon nitride (SiNx) obtained in PECVD system. Indeed this has led to a minority carrier lifetime up to 0.5 ms, on both p- and n-type doped c-Si substrates. We have found out that the passivation mechanism is mainly provided by the superficial field effect induced by the charge of hydrogen ions within both materials incorporated during the double layer deposition. But this high amount of hydrogen induces metastability due to ions mobility and Si–H bond breaking by high energy photons exposure. This strongly affects the passivation quality of the double layer as evident from Capacitance–Voltage measurements performed on Al/SiNx/a-Si/c-Si Metal Insulator Semiconductor and photo-conductance decay evaluated on both side passivated c-Si wafer. Particular thermal annealing procedure can be adopted at the end of the layers growth to reduce the metastability and increase the passivation quality of c-Si surfaces. This double passivation layer has been demonstrated effective to increase the photovoltaic performances of silicon based solar cell as surface passivation either on front emitter either between local back contacts.