The goal of the GRANASOL project was to advance the research into low cost, high-efficiency III-V semiconductor NW solar cells. In the project we focused the work on fundamental studies on GaAs(Sb) NW/graphene hybrid devices and solar cells. This included synthesis of graphene and its transfer and use as a template for the growth of GaAs(Sb) NWs by molecular beam epitaxy (MBE). Different prototype GaAs(Sb) NW/graphene devices were fabricated including planar GaAs NW/graphene junctions and various GaAs NW/graphene solar cells.
We first developed a process of single GaAs NW/graphene hybrid devices with a planar junction configuration. A position-controlled micro transfer and imprinting technique that enabled us to choose and transfer a single NW (or graphene) selectively on a target graphene (or a single NW) for high quality junctions was developed. We carried out a thorough study on electrical properties of such devices on two different device configurations: a) a graphene bottom-contact device where the NW-graphene contact junction is formed by transferring a nanowire on top of graphene and b) a graphene top-contact device where the NW-graphene contact junction is formed by transferring graphene on top of an embedded NW. For the graphene top-contact devices, where graphene is one electrode and metal are the other electrode to a NW, and where both electrodes to a NW are graphene, were investigated and compared with conventional metal/p-GaAs NW devices. Conventional metal/p-GaAs NW contact devices were further enhanced in embedded and non-embedded NW device configurations. A significantly improved current in the embedded device configuration was explained with a "parallel resistors model" where the high resistance parts from the depletion layer formed by the metal-semiconductor Schottky contact and the low resistance parts with non-contacted facets of the hexagonal NWs were taken into consideration. Consistently, the non-embedded NW structure was found to be depleted much easier than the embedded NWs.
At the final stage of the GRANASOL project we focused the work on fabricating GaAsSb NW solar cell devices using both axial and radial p-i-n junctions, where devices were made with both metal and graphene electrodes optimized as described above.
In the case of radial junction GaAs NWs, we developed an approach to study radial junction GaAs NW solar cell devices from the analysis of its dark current-voltage characteristics through three different probing schemes: a) single NW device, b) NW array device and c) direct probing of individual as-grown NWs by using tungsten nano-probes in a focused ion beam (FIB) system. Each of these schemes helped us to eliminate one or more possible major issues in the device individually, and together they allowed us to understand the NW solar cell in order to maximize the solar cell efficiency. From the NW array solar cell device, a rectifying I-V was observed but a large leakage current at reverse bias was found. To investigate the origin of the leakage current, I-V characteristics of vertically standing NWs were measured individually by using a nano-probe technique. About 75% of the measured NWs showed very good rectifying I-V however, the other 25% of the measured NWs showed high leakage current or even non-rectifying type behavior due to some amount of shorting of the outer n-shell to the bottom substrate. This shorting of n-shell at the bottom of the NWs was found to be the reason for the high leakage current when a complete NW array solar cell device was measured.
In the case of axial junction GaAs NWs, an AlGaAs shell was grown to provide better passivation to the NW surface. After processing a full NW array device structure, current-voltage (I-V) characteristics was measured in the dark mode. However very weak diode rectification was observed in I-V and the leakage current was found to be high. It was found that the n-doped part of the NW is conducting significantly less current than the p-doped part. The results also suggested that an n-doped shell was grown radially during the axial growth of the n-doped part creating a shorting path. By finally employing an ex-situ shell etching process, clear rectification from an axial GaAs NW solar cells was achieved. In addition, we grew axial junction GaAs NW solar cell array on Si, made by an extra etching process of the n-GaAs shell at the bottom of the NWs out of the i- and p-GaAs segments of the as-grown NWs. At the end of the project we could obtain a GaAs NW solar cell with a photoconversion efficiency of 7 % under 1 sun condition.
Results from the GRANASOL project will be beneficial for future research studies in academia as well as for development work in industry to develop low cost high-efficiency nanowire/graphene hybrid solar cells. With further studies and development these solar cells may eventually outperform Si-solar cell technology and thereby has the potential to strongly contribute to the solution of the global energy and climate change problems in the future. By using III-V nanowire/graphene solar cells one can also expect a substantial positive environmental impact. This is especially true for nanowire solar cells grown on graphene as they are essentially "substrate-free" (i.e. reduced use of expensive and resource-limited compound substrates) and use a minimum amount of material in the active nanowires.
For future optoelectronic applications, III-V nanowires (NWs) are one of the most promising nanostructures among conventional inorganic semiconductor materials. Graphene is currently receiving enormous attention world-wide for several application areas as it has many unique properties that cannot be achieved in conventional inorganic semiconductors. Especially, due to high optical transparency, electric and thermal conduction, flexibility and demonstration of large scale production of graphene, it is now poised as a future low cost flexible electrode material for solar and display applications.
Our IP-protected discovery in 2010 of epitaxial growth of III-V semiconductor NWs on graphene imply that graphene can be used as an electrode and "substrate" sim ultaneously making the realization of novel semiconductor/grahene hybrid systems possible (III-V semiconductor NW/graphene hybrid). Most of all, by combining these two materials with each of their own supreme properties, a large impact for especially nano -optoelectronic device applications is expected. One of the most promising applications of III-V NW/graphene hybrid systems are for solar cells. In this project structured studies on such NW/graphene hybrid solar cell will be carried out. This includes th e synthesis of graphene to its transfer and use as a template for the growth of positioned epitaxial III-V NWs by molecular beam epitaxy. Different prototype NW/graphene hybrid solar cells will be fabricated and the efficiency will be measured in a solar simulator. The ultimate goal is to realize low cost, ultra-high efficiency III-V semiconductor NW solar cells. The low cost will come from minimal material usage and by using commercial roll-to-roll graphene as a substrate. Ultra-high efficiencies is base d on our recent simulations predicting efficiencies for radial p-n junction NW solar cells more than twice (~70%) the limit of planar junctions.