NOVAGAINS

While silicon-based solar cell technologies dominate the photovoltaic (PV) market today, their performance is limited. Indeed, the world record efficiency for Si-based PVs has been static at 25% for several years now. III-V multijunction PVs, on the other hand, have recently attained efficiencies > 40% and new record performances emerge regularly. Although tandem PV geometries have been developed combining crystalline and amorphous silicon, it has not been possible so far to form devices with efficiencies to rival III-V multijunctions. NOVAGAINS aims to benefit from combining the maturity of the Si technology with the potential efficiency gains associated with IIIV PV through the development of a novel tandem PV involving the integration of an InGaN based junction on a monocrystalline Si junction by means of a compliant ZnO interfacial template layer which doubles as a tunnel junction. Although the (In)GaN alloy has been used extensively in LEDs, its’ use in solar cell technology has drawn relatively little attention. Nevertheless, the InGaN materials system offers a huge potential to develop superior efficiency PV devices. The primary advantage of InGaN is the direct-band gap, which can be tuned to cover a range from 0.7 eV to 3.4 eV. As such, this is the only system which encompasses as much of the solar spectrum. Indeed, the fact that InGaN can provide such tunability of the bandgap means that PV conversion efficiencies greater than 50% can be anticipated. Unfortunately, it is very difficult to grow GaN based films of high materials quality directly on Si because they do not have a good crystallographic match. ZnO can be grown more readily on such substrates, however, because of its’ more compliant nature. Indeed, well-crystallized and highly-oriented ZnO can even be grown directly on the native amorphous SiO2 layer. Since ZnO shares the same wurtzite structure as GaN and there is less than 2% lattice mismatch it has been demonstrated that it is then possible to grow InGaN/GaN epitaxially on ZnO/Si using the specialized know-how offered by the consortium. Modeling indicates that when optimized, stacked InGaN and Si cells coupled by tunneling through a ZnO interlayer offer the perspective of tandem cells with overall solar conversion efficiencies in excess of 30%.