Towards Hot-Carrier Photovoltaics in Nanowires with Epitaxially Defined Potential Barriers

Abstract

In this work, we present the design, fabrication, and characterization of InAs-based nanowire heterostructures with epitaxially defined InP and GaAs potential barrier segments, aimed at understanding and demonstrating the functionality of a double-barrier nanowire heterostructure as a hot-carrier photovoltaic, in which InP and GaAs barriers encapsulate an InAs absorber region, where hot carriers are locally excited using plasmonic nanoantennas. Such a device is expected to enhance the quantum efficiency of nanowire-based hot-carrier photovoltaics by enabling energy- and carrier-selective charge extraction.

First, the conduction band barrier heights of the InP and GaAs segments were experimentally determined using temperature-dependent current–voltage measurements and Arrhenius analysis based on thermionic emission. Barrier heights of ΦInP = 0.431 eV and ΦGaAs = 0.398 eV were extracted for the InP and GaAs barriers, respectively.

For the optoelectronic characterization, a custom setup was built to perform IV measurements under light of controlled wavelength and polarization. First, single-barrier devices were characterized under global illumination, confirming that both InP and GaAs barriers allow for optically induced charge-carrier separation. Further, devices with plasmonic nanoantennas were investigated, which were shown to locally enhance absorption in the nanowire segment between the antennas under polarized light. In this context, InP single-barrier devices with nanoantennas exhibited a behavior consistent with internal photoemission across the full spectral range, suggesting a barrier height below 0.45 eV, in agreement with the aforementioned barrier height
measurements. However, the GaAs single-barrier and the double-barrier device showed strong hysteresis and offset voltage fluctuations, attributed to parasitic capacitance in the measurement setup. Accordingly, no conclusive results regarding the characteristics and performance of these devices could be obtained.

Although the full functionality of the double-barrier device could not yet be demonstrated, this study provides important insights into the potential barrier structure and confirms the ability of the individual barriers to separate charge carriers. It also supports the feasibility of employing plasmonic nanoantennas for absorption localization in the intended double-barrier device, advancing the path toward higher-efficiency hot-carrier photovoltaics beyond the Shockley-Queisser limit.

Popular Abstract

As the global demand for sustainable energy alternatives continues to grow, researchers are striving to push the boundaries of photovoltaic (PV) technology. This technology converts sunlight into electricity using devices known as solar cells. Traditional solarcells, so-called single pn-junction solar cells, lose a significant portion of the incoming solar energy. This loss is described by the Shockley–Queisser limit, which places an upper bound on the efficiency at approximately 30%. Hence, novel PV technologies are required to overcome this limit.

In a semiconductor, the material that solar cells are made of, pairs of charge-carrying particles are generated when light of sufficient energy is shined on the material. These so-called charge carriers, namely electrons and holes, have opposite charges and are free to move within the material. Their motion or flow leads to the generation of an electric current, which can be harvested for electricity. To generate electron-hole pairs in a semiconductor, the incoming light must have a certain minimum energy, the so-called bandgap energy. When light with energy exceeding the bandgap energy is absorbed, the charge carriers possess excess energy. These high-energy carriers are referred to as hot carriers. In traditional solar cells, only the bandgap energy is harvested, while the excess energy of hot carriers is lost, which limits the efficiency of such devices. Hot-carrier PVs aim to overcome this limitation by also harvesting the excess energy of hot carriers.

The operation of hot-carrier PVs relies on energy-selective filters, which allow only charge carriers with a certain (excess) energy to pass. One example of such a filter is a potential barrier, which can only be crossed by carriers with energy higher than the barrier. In this study, we implement such barriers by embedding a material segment with a larger bandgap energy between regions of material with a smaller one. This structure is realized within nanowires, nanoscale semiconductor rods, typically a few hundred nanometres wide, which are epitaxially grown. During this process, the material can be varied, enabling the fabrication of the desired barriers.

The functionality of such nanowire barrier structures as hot-carrier PVs has already been shown in the past. However, their efficiency was significantly below that of conventional PV devices. Therefore, in this study, we aim to fabricate and pave the way for characterization of a double-barrier nanowire structure that includes one barrier for the extraction of hot-holes and one barrier for the extraction of hot-electrons. This structure is expected to significantly advance the efficiency of such nanowire-based hot-carrier PVs on the path towards high-efficiency hot-carrier solar cells beyond the Shockley-Queisser limit.

• author: Peterkes, Noah
• supervisor: Heiner Linke
• organization: Department of Physics; Solid State Physics
• course: FYSM64 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: solid state physics, solar energy conversion, hot carriers, III−V nanowire heterostructures, plasmonics, optoelectronics, master thesis
• language: English
• id: 9202544
• date added to LUP: 2025-06-23 08:55:23
• date last changed: 2025-06-23 08:55:23