Band gap optimization of the window layer in silicon heterojunction solar cells

• The analytical model of the open-circuit voltage is derived.
• The optimum value of Eg2 for silicon heterojunction solar cells is obtained at 1.8 eV.
• The mechanism that Eg2 operates on the properties of solar cells is illuminated.

The properties of silicon heterojunction solar cells differ with the band gap of the window layer. The analytical model of the open-circuit voltage for silicon heterojunction solar cells is first derived. Based on the analytical model and the carrier transport, the effect of the band gap of the window layer (Eg2) on the properties of silicon heterojunction solar cells and the mechanism are explored by a set of AMPS simulations. At non-negligible interface states, the increase of Eg2 leads to increase of the electric field in the c-Si depletion region, and then causes decrease of the effective interface recombination. The open-circuit voltage increases with Eg2 increasing, and the fill factor FF increases with Eg2 increasing at Eg2 ⩽ 1.8 eV. However, at Eg2 ⩾ 1.9 eV, the valence band offset barrier limits the carrier transport, and the S-shape in the J–V characteristics occurs. It results in the decrease of FF. The optimum band gap of the window layer is obtained ΔEV ≈ 0.5 eV for silicon heterojunction solar cells, especially at Eg2 = 1.8 eV for the offset ratio 3:1. And the effect of Eg2 on the properties of silicon heterojunction solar cells operates by: (1) decreasing photons absorbed in the window layer; (2) the recombination due to the interface traps is reduced due to enhanced electric field with increase in the band gap; and (3) the increase of the valence band offset influencing the electron transport.