InP/GaInP Nanowires for Tandem Junction Solar Cells : Growth, Processing, and Characterization

Abstract: Semiconductor nanowire solar cells have achieved comparable efficiency to their planar counterparts with substantial reduction of material consumption. Tandem geometry is a path towards even higher efficiency. However, extensive studies are needed to reach this goal. My study started from the building blocks of the materials used in the upper cell, namely GaInP nanowires. The growth and doping control was the focus. We achieved well-controlled GaInP nanowire arrays and effective doping control of both n-type and p-type which is needed for use in the tandem junction solar cells.In a tandem junction solar cell, a tunnel junction which is virtually a degenerately doped pn-junction is a critical component. It provides the connection between adjacent subcells with low-resistivity and optical-transparency. With the knowledge obtained from our doping evaluation studies, we established the growth of InP/GaInP heterojunction nanowires with degenerate doping achieved at the material heterointerface which is the prerequisite for the tunneling effect. The tunnel diodes were realized in both forward (InP/GaInP) and reverse (GaInP/InP) directions. This enabled the tandem structures in the forward (InP subcell (bottom)/GaInP subcell (top)) and reverse (GaInP subcell (bottom)/InP subcell (top)) direction. (To enable the use of the reverse tandem structure, peel-off of the nanowires is needed since the higher-bandgap GaInP subcell should be on the top of the tandem structure in applications.)When the tunnel junction is incorporated into multiple pn-junction structures, the growth of the complex structure can bring extra influence to the entire device performance due to the VLS growth mechanism of nanowires. To simplify the diagnostics of tandem junction structures, we designed a test structure which connects a tunnel junction back to back with a p-i-n junction (namely a ‘n-pin’ structure). The n-pin structure has both ends of n-type, facilitating Ohmic contact formation. We found that a strong memory effect from the firstly-grown segment can non-intentionally dope the following segments. To overcome this challenge, compensation doping will probably be needed. The transfer of large area nanowire arrays to cheap substrates can significantly reduce the material cost. We developed an effective technology to embed the nanowire arrays into a polymer film and peel the film off from the native substrate. These studies pave the way to the realization of the first InP/GaInP nanowire tandem junction solar cell. The peel-off technique will also be of broad interests for reducing the fabrication cost of other nanowire devices, such as light emitting diodes and photodetectors.