Advanced Concept Photovoltaics
The photovoltaic power conversion process unfortunately suffers from significant losses, due to fundamental issues like the transparency of material to particular wavelength ranges, and the use of the energy of light absorbed. The goal of a photovoltaic device is remarkably simple; absorb as much light as possible, generating excited charge carriers and then to sweep these excited carriers to an external circuit to generate power. As the performance of conventional solar cells reaches closer to theoretical limits the search is on for new paradigms, where energy conversion mechanisms can be harnessed to exceed existing limits. Some of the research being pursued includes:
Efficiency Limits for Novel Device Proposals
Figuring out the limits to power conversion efficiency is a key step in determining if an idea is worth pursuing, but it can be challenging sometimes to determine just what those limits are. In particular, for proposed devices like hot carrier solar cells, which are proposed to harness the excess energy of electrons when absorbed (this energy is normally lost to us), can be troublesome to assess. Some of the cases analyzed have included multi-junction solar cells, up to 8 junctions, intermediate band solar cells under AM1.5 spectrum (a standard terrestrial spectrum used to assess solar cell performance), as well recent work looking at multiple exciton generation (MEG) solar cells [1]. The model proposed includes an important factor called microscopic reversibility, a derived internal quantum efficiency plot is shown below left, which agrees in form with experimental results reported in the literature. Our paper has set new efficiency limits for MEG solar cells, check it out at Applied Physics Letters.
Additionally we have looked at the intriguing question of what a hot carrier solar cell does in the dark if we apply a voltage [3]. Our results show that a device that is a hot carrier solar cell will behave very differently when a voltage is applied in the dark. A plot taken from our paper in Progress in Photovoltaics is shown where these differences are shown is below on the right.
Nanowires as Hot Carrier Solar Cells
A key focus of my research is exploring the exploitation of thermally based energy transitions to enhance solar energy conversion. A key theme is the links between thermoelectrics and hot carrier solar cells, a proposed device that reduces thermalisation losses in solar cells. Recent experimental results from a collaboration with the Linke Group at Lund University using InAs nanowires containing InP energy barriers, was reported in ACS Nano Letters, and showed photocurrent reversal with excitation wavelength, and open circuit voltages exceeding the limits predicted by conventional solar cells. The main results form this paper are shown below, more details can be found in our paper in Nano Letters. We are currently setting up time resolved optical analysis capability to be able to probe the hot carrier dynamics in nanowires, not just in terms of time evolution, but also spatially i.e. we will watch how hot carriers diffuse in the nanowires. Recently, a review of hot carrier results in nanowires was published [2], which gives an excellent overview of key results and some of the insights into how nanowires might be more conducive to extracting hot carriers for power conversion applications.
Investigation of Sb containing barriers in quantum-dot based solar cells
The InAs/GaAsSb system is of significant interest for realising the advanced concept intermediate-band solar cell. This materials system has some advantages over other quantum dot and quantum well systems, namely the observation of a Zero Valence Band Offset between InAs and GaAsSb. This important to minimise energy losses for holes and to get closer to being a true three level system, my research confirmed the presence of a zero valence band offset in this system for Sb content of 14%, using time-resolved and temperature-dependent photoluminescence. In a follow up study power dependent data strengthened the conclusion that a type I to type II heterojunction transition was taking place as Sb content increased through the 12 to 16% region. The presence of Sb in the barriers of the InAs/GaAsSb quantum dot system has also proven to lead to very high quantum dot number density and reduced size inhomogeneity.
Designing Phonon Cavities for long lifetime phonon modes
Phonons are the quantised lattice vibration present in semiconductor lattices due thermal energy, but are also generated by optical absorption processes. The phonons present and their interactions with optical absorption as well as the energy relaxation mechanism in semiconductors are of real interest when looking at the fundamentals of power conversion devices like solar cells. Typically these interaction are viewed as something to be minimised or mitigated as typically they lead to energy loss. But understanding the processes in detail can allow us to formulate designs that can harness these processes, rather than try to minimise their negative impacts. Towards these ends we have designed, fabricated, and characterised phonon cavity structures realised in AlAs/GaAs. The cavity is a variant on a typical Bragg reflector where the reflectivity is designed to vary adiabatically (smoothly rather than abruptly like in conventional designs). This has allowed us to produce samples that show enhanced phonon modes at the frequencies we have designed for, as well as long coherence times (i.e. the phonons stay in the mode we want for a longer time). Below is shown a summary of the designs used, as well as the ultra-fast vibration spectroscopy used to observe the phonon modes (essentially we measure the vibration of the sample surface from the phonon modes by detecting the slight changes in reflectivity).
Relevant published works:
[1] Microscopic reversibility demands lower open circuit voltage in multiple exciton generation solar cells,
A. Pusch, S. P. Bremner, M. J. Y. Tayebjee, N. J. Ekins-Daukes
Applied Physics Letters, 118 (15), 151103, 2021.
[2] Hot-carrier optoelectronic devices based on semiconductor nanowires,
J. Fast, U. Aeberhard, S. P. Bremner, H. Linke
Applied Physics Reviews, 8 (2), 021309, 2021.
[3] Optoelectronic reciprocity in hot carrier solar cells with ideal energy selective contacts,
A. Pusch, M. Dubajic, M. P. Nielsen, G. J. Conibeer, S. P. Bremner, N. J. Ekins-Daukes
Progress in Photovoltaics, 29, 433-444, 2021.
[4] Long‐Lived Acoustic Phonon and Carrier Dynamics in III–V Adiabatic Cavities
M. Hanif, M. Dubajic, S. J. Sreerag, R. N. Kini, G. J. Conibeer, M. P. Nielsen, S. P. Bremner
Advanced Functional Materials, 34(39), 2404299, 2024.
If this type of work is of interest to you then you can also check out the QPV group, here at UNSW. I work with the people in this group regularly and we have many interesting projects going on.
