Narrow-Bandgap InGaAsP Solar Cell with TiO₂ Carrier-Selective Contact

Carrier-selective contacts offer promising opportunities for solar cells. By alleviating the need for p–n junctions and acting as passivation layers, they significantly simplify the device design and fabrication. Herein, this strategy is applied to a narrow-bandgap (≈0.91 eV) InGaAsP solar cell. Such a solar cell, lattice-matched to InP, possesses a bandgap ideal for the bottom subcell of a tandem cell. It is shown that TiO₂ forms an electron-selective contact to InGaAsP. The TiO₂/InGaAsP solar cell exhibits a short-circuit current density of 35.2 mA cm⁻², an open-circuit voltage of 0.49 V, and an efficiency of 8.9%. The cell J–V characteristics and quantum efficiency highlight the beneficial aspect of TiO₂ as a passivating layer for InGaAsP. The reduced open-circuit voltage and lower response at longer wavelengths, on the other hand, indicate that the quaternary alloy material quality could be further improved to increase the carrier diffusion length. Nevertheless, the performance of this simplified electron-selective contact solar cell structure is comparable to conventional p–n junction 1 eV InGaAsP solar cells reported in the literature, highlighting the promise toward lower-cost photovoltaic tandem cells.