Silicon heterojunction solar cells: Excellent candidate for low light illuminations

Abstract

The current solar cells and modules are marketed according to the behaviour at standard test conditions (STC), however, these devices are more often operated at lower irradiance levels. The dependence on illumination stems mainly from the voltage at the open circuit and at the maximum power point. The latter might also be strongly influenced by serial resistance, but that is more an engineering problem not addressed here. More fundamentally, the voltage is determined by quasi-Fermi levels at the contacts. In the case of unconstrained conductivity between the absorber and electrode, the quasi-Fermi levels are flat, and determined by their splitting in the absorber. Our analysis shows that the modulation doping mechanism working in Si heterojunction solar cell between the doped amorphous Si layer with a higher bandgap and crystalline absorber with a lower bandgap can, for certain parameter settings, lead to strongly depleted contact layer that limits conduction. This is a prerequisite for decoupling the quasi-Fermi level between contact and absorber and offers the possibility for additional voltage increase. Based on this understanding, we simulate a heterojunction solar cell with a varying thickness and doping of amorphous silicon p-type contact. We demonstrate that for a certain combination of thinner or lower-doped contact, higher efficiency at low illumination can be achieved compared to the technological baseline. This is fully in line with the experimental findings. This analysis is crucial not only for using solar cells for indoor applications but also for designing photovoltaic modules optimized for low irradiance, potentially increasing the level of self-sufficiency of buildings.