Bonded Multijunction Photovoltaics to Improve Efficiency and Performance

Abstract

Sun emits approximately 3.8x10 26 Joules per second and it produces more energy than global population demand in an hour. However, converting solar energy to electric energy is not trivial due to two major limitations. First, solar energy is not power dense, only ~1000 Watt/m 2 . Second, solar energy is spread in very wide spectrum from 250nm to 2500 nm. Both problems can be solved with the combination of multijunction photovoltaic device and concentrator photovoltaic system (CPV). CPV systems use low cost light-gathering material to increase effective energy density where multijunction devices can efficiently convert solar energy by significantly reduce thermalization loss and non-absorption loss using multiple materials with different bandgaps in the device. Multijunction devices improved significantly with the advancement in epitaxial growth where devices with low defect density can be achieve. However, optimal bandgap selection cannot be achieved by lattice matched material or conventional material alone, therefore, growth techniques such as metamorphic growth and novel materials such as dilute nitride material were used in various current state-of-art triple junction deices. As the number of junctions increases, low defect density and lattice matching can still be challenging even with metamorphic growth and novel materials. Wafer bonding is an approach to combine dissimilar materials together without lattice matching constrain. Direct bonded photovoltaic devices were explored by other researchers, however process compatibility and electrical performance were generally limited by the bonding process and direct bonded devices were not intended for high illumination concentration. Indirect bonding with conductive assisting layer was also attempted in which optical loss at the bonding layer and electrical performances were the limiting factors. A novel bonding approach will be presented in this talk. The interface consists of metal and dielectric where the metal will provide the necessary electrical coupling and mechanical coupling and the dielectric provide the optical, thermal and some mechanical coupling. Metal topology is designed to be a “pillar-array” that can achieve minimal optical and electrical loss at the interface. This bonding method could enables a fully lattice-matched two-terminal four-junction device which consist invert grown 1.85 eV GaInP/ 1.42 eV GaAs cell and an upright lower Eg3 eV GaInAsP/ 0.74 eV GaInAs aimed for concentrator application. It also have the possibility of integrating an InGaN top cell forming a five junction device with expected efficiency >50%. Prove of concept devices such has GaAs cell bonded on GaAs wafer, GaAs/ GaInAs bonded two junction devices will be presented along with detailed fabrication process, related damages and losses. A bonded GaInP/ GaAs on GaInAs three junction device with antireflection coating demonstrated Voc = 2.70 V, Jsc = 12.66 mA/cm 2 , FF = 83.0% and efficiency = 28.39 % under 1 sun condition which confirms the potential for high performance integration of photovoltaic devices. This material is based upon work supported as part of the Center for Energy Efficient Materials (CEEM), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001009. E.E. Perl is supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144085