Third Generation Photovoltaics
- Buried Contact Group
- High Energy Efficiency Group
- Thin Film Group
- Silicon Photonics
- Collaborative Research
- Facilities & Infrastructure
University Staff:
Dr. Gavin Conibeer (Group Leader)
Dr. Richard Corkish
Prof. Martin Green
Research and Postdoctoral Fellows:
Dr. Robert Bardos
Dr. Patrick Campbell (Part Time)
Dr. Kylie R. Catchpole
Dr. Ximing Dai
Dr. Tammy Humphrey
Dr. Tom Puzzer (Part Time)
Dr. Bryce Richards, BSc
Dr. Thorsten Trupke
Research Associates:
Yidan Huang
The work of the Third Generation Strand has continued to build on the work of the Third Generation Special Research Centre. The principal objective is to significantly, rather than merely incrementally, improve photovoltaic cell performance beyond that of present devices.
The term Third Generation derives from the progression of photovoltaic technologies. First generation refers to high quality and hence low defect single crystal photovoltaic devices these have high efficiencies and are approaching the limiting efficiencies for single band gap devices. However, such devices are labour and energy intensive and are not likely to get lower than US$1/W (see below). Second generation technology involves low cost and low energy intensity growth techniques such as vapour deposition and electroplating. Such processes can bring costs down to a little under US$0.50 but because of the defects inherent in the lower quality processing methods, have much reduced efficiencies compared to First Generation.
Efficiency and cost projections for first-, second-
and third-generation photovoltaic technology
(wafers, thin-films, and advanced thin-films, respectively).
Third Generation Approaches to Tackling the Major Losses in PV Cells
Third Generation concepts are based on devices that can exceed the theoretical solar conversion efficiency limit for a single energy threshold material. This was calculated in 1961 by Shockley and Queisser as 31% under 1 sun illumination and 40.8% under maximal concentration of sunlight (46,200 suns, which makes the latter limit more difficult to approach than the former).
Loss processes in a standard solar cell: (1) non-absorption
of below band gap photons; (2) lattice thermalisation loss;
(3) and (4) junction and contact voltage losses; (5) recombination
loss.
The routes to exceeding the Shockley-Queisser limit address the below band gap and the thermalisation loss mechanisms in the above figure. Means to tackling these have been variously quoted as falling into three generic categories, namely: multiple energy threshold devices; modification of the incident spectrum; and use of excess thermal generation to enhance voltages or carrier collection. Specific directions for experimental research on each of these were set for the Third Generation SRC in 2002. These continue to be the most promising routes to achieving the objectives of the Third Generation Strand. The specific technologies chosen address these generic approaches and respectively are:
Bandgap engineering of silicon based material is a very promising route towards third generation Si photovoltaic devices such as silicon based tandem solar cells and energy selective contacts for hot carrier solar cells. This project focuses on the fabrication and characterisation of silicon nanostructures consisting of quantum well (QW) or quantum dot (QD) superlattices to achieve such band gap control.
Theoretical work in the Centre has shown that modification of the solar spectrum can boost the efficiency of solar cells if the bandwidth of the light incident on the cell can be made narrower than the solar spectrum. Luminescent materials are being investigated that either absorb one high energy photon and emit more than one low energy photon just above the band gap of the solar cell (down-converters) OR that absorb more than one low energy photon below the band gap of the cell and emit one photon just above the band gap (up-converters). The important property for these devices is high quantum efficiency meaning that they must be very radiatively efficient. Experimental work is concentrating on Up converters as any conversion of low energy light that is normally wasted can in principle increase efficiency. External quantum efficiencies of 1% have been measured in our work for such materials. This 1% extra is small as yet but nonetheless represents extra photons above the band gap and hence is promising.
The Hot Carrier Cell tackles the major PV loss mechanism of thermalisation of carriers. The underlying concept is to slow the rate of photoexcited carrier cooling, caused by phonon interaction in the lattice, to allow time for the carriers to be collected whilst they are still “hot” thus enhancing the voltage of a cell. Work in this area includes investigation of Selective Energy Contacts and Hot Carrier Absorbers.
A spin-off of previous theoretical work has included the application of the concept of energy-selective electron transport, used in hot carrier solar cells, to thermoelectrics and thermionics. This has led to the discovery of two types of equivalence that simplify the description of this class of devices.
Additionally there are two other project areas being pursued within the Strand:
Thermophotonics is an extension of the concept of Thermophotovoltaics where photovoltaic conversion by a receiver cell of radiation from an emitter, which could be heated by various sources. In a thermophotonic system, a heated light-emitting diode radiates towards a PV cell connected to a load.
Surface plasmon resonances on metal nanoparticles can scatter incident light into guided modes of a thin semiconductor layer. Applications include novel light-trapping for very thin silicon films that are too thin for conventional light-trapping structures.