Optimized utilization of NIR spectrum with interfacial TiO2/SiO2/TiO2 trilayer in hybrid-integrated multijunction architecture

To make full use of the NIR spectrum, the three-junction (3J) solar cells with bandgaps of 1.9/1.4/1.0eV have been transfer-printed on top of microscale Ge solar cells with two different interfacial materials. The interfacial material consisting of TiO2/SiO2/TiO2 trilayer has been used for the first time and hold about 15% averaged reflection comparable to As2Se3 in the NIR range of 1200–1800nm, which means that the trilayer is of high crystalline quality with gradually varied refractive index and acting as the role of light-trapping to reduce interfacial reflection. Measured EQE curves over a standard AM1.5D spectrum yield a short-circuit current density of 7mA/cm2 for Ge cells with both As2Se3 and trilayer, which means that the trilayer is qualified for replacing As2Se3 with outstanding antireflective properties. NIR optical images have confirmed the existence of air gaps between interfaces of some cells, which exhibit similar response to those without air gaps in the measurements of reflectance and EQE curves, showing great accommodation in mass production of this multijunction architecture.