Recovery of Indium-tin-oxide/silicon Heterojunction Solar Cells by Thermal Annealing

The emitter of silicon heterojunction solar cells consists of very thin hydrogenated amorphous silicon layers deposited at low temperature. The high sheet resistance of this type of emitter requires a transparent conductive oxide layer, which also acts as an effective antireflection coating. The deposition of this front electrode, typically by Sputtering, involves a relatively high energy ion bombardment at the surface that could degrade the emitter quality. The work function of the transparent conductive oxide layer could also significantly modify the band structure at the emitter. In this work, we study the particular case of p-type crystalline silicon substrates with a stack of n-doped and intrinsic amorphous silicon layers deposited by Plasma-Enhanced Chemical Vapor Deposition. The front electrode was an indium-tin-oxide layer deposited by Sputtering. The Quasi-Steady- State Photoconductance technique has been used to characterize the emitter quality by measuring the effective lifetime and the implicit open-circuit voltage. These measurements confirmed a strong degradation of the heterojunction after depositing the indium-tin-oxide layer. However, it is also shown that the initial degradation could be completely recovered by an adequate thermal treatment. In this sense, annealing times from 10 to 90 minutes at temperatures ranging from 100 to 160 °C have been studied, both in vacuum and inside an oven.