Category Third Generation Photovoltaics
9.4.1 Case with Filters
The simplest case to analyse is that with filters and independent voltage sources to bias the LED and solar cell as shown in Fig. 9.6.
Fig. 9.6: Thermophotonic conversion, with narrow pass filter and independent power supplies.
The power supply to the LED boosts the light emission compared to thermal emission. Since VH will, for an ideal device, be less than EG /q, the photons emitted by this device will have more energy than that drawn from this supply, with the difference drawn from the heat source. An energy balance gives:
Qh + vh1h/a = Eh – ec
where Elh and Ec are the photon energy fluxes emitted and A is the device area. In the radiative limit, a particle balances gives:
lH/(qA) = INh – Nc
where Nh and Nc are the photon fluxes emitted by the two devic...Read More
Since the cell/receiver combination can be designed to convert at close to the Carnot efficiency, it might be thought that illuminating the receiver with sunlight would lead immediately to efficiencies equal to the photothermal conversion limits of Sect. 9.2. However, an additional requirement is to match the optimal rate of supply of energy from the sun to the receiver with the optimal rate from the receiver to the cell. The cell operates at close to the Carnot limit only when the energy reaching it is suppressed, using either large band-gaps or selective sources or filters.
This mismatch can be accommodated by using different areas for the absorber, emitter and cell. Some simple extensions of the flat absorber treated so far are shown in Fig. 9.5...Read More
8.3.1 Black-Body Source
If the receiver is assumed to have ideal black-body properties on both sides, heat will be extracted from it by net radiative transmission to the cell. The analysis of this case obviously is closely related to that for solar energy conversion where the sun is the black-body radiator, except photons can be recycled. Assuming the ideal case where the cell reflects all photons of energy below its bandgap but absorbs all photons of energy above it, a particle balance leads to the following expression for power output per unit area:
P = qV[N(EG,<*,0,TR )- N(EG,~,qV, Tc )]
The net rate of heat supply from the receiver becomes:
Q = [E (Eg,",0,Tr) – E (EG,~,qV, Tc)]
Hence the conversion efficiency of heat from the absorber becomes:
n = N(Eg,~,0,Tr )[ 1 -N(EG,~,qV, Tc )/N(Eg...Read More
To get a feel for desirable absorber properties, the problem treated in Exercise 2.2 will be examined in more detail. This problem involved the system of Fig. 9.3, where the maximum solar conversion efficiency was calculated for a system based on a receiver with good absorption properties on one side, and low emission properties on the other.
Fig. 9.3 : Photothermal system based on the conversion of heat collected by an absorber at the Carnot efficiency.
For direct sunlight conversion (f = fc), the maximum possible conversion efficiency for black-body absorption properties is given by the familiar result:
П = [1 -(Tr/Ts)4 ](1 – TA/TR) (9.1)
The first term represents the net radiative heat input to the receiver (that from the sun minus that re-emitted), while the secon...Read More
One way of reducing the energy loss on light absorption that occurs in a conventional cell is to reduce the average energy of the photon absorbed by it. This is the basic idea behind thermophotovoltaic conversion, schematically shown in Fig. 9.1. Sunlight (or heat from another source) is absorbed in a receiver that is thereby heated to a reasonably high temperature, generally much lower than that of the sun. The heated receiver then radiates energy to a photovoltaic cell. Although many of the photons emitted by the receiver may be too low in energy to be used by the cell, these can be reflected by the cell back to the receiver, helping to maintain its temperature. As a result, these photons are not necessarily wasted. Light emitted by the cell is also not wasted...Read More
A disadvantage of introducing impurities in the bulk of the cell is that they are likely, in practice, to introduce additional non-radiative recombination paths (Guttler and Queisser 1970). However, if the associated optical excitations are decoupled from the cell, this objection can be overcome (Trupke et al. 2002a; 2002b). Surprisingly, a down-converter can be placed on the front surface of a cell, creating two photons from high energy photons without significantly interfering with the transmission of lower energy photons into the cell (Trupke et al. 2002a). An up-converter placed on the rear of the cell can create one high energy photon by a multi-step excitation involving two lower energy photons...Read More
The previous theory can be extended to the impurity photovoltaic effect of Fig. 8.1 by noting a theorem proposed elsewhere (Green 2001). Entropy generation is minimised if the level of excitation within the absorber (as measured by the chemical potential associated with the inverse spontaneous emission process) is constant over the absorption volume.
The general proof of this proposition is quite unwieldy (Green 2002) although it is simpler to prove in specific examples (e. g., Exercise 8.2).
For the case of a single impurity level, the consequence of this theorem is that the occupancy of each separate defect site must be equal for limiting performance under the general set of assumptions used throughout this book...Read More
Several ideas for implementing multigap cells have already been discussed (Fig. 8.7). Are there other ideas that might allow extension of the approach to devices with more than three bands?
One possibility is to chose existing semiconductor material where the band structure consists of several narrow bands. For example, the elemental semiconductors Se and Te are thought to have such a band structure, due to the splitting of 3 p-levels that are degenerate (i. e., have the same energy) in isolated atoms of the material. Various binary semiconductors including I-VII and I3-VI compounds are also thought to have similar properties (Berger 1997).
One difficulty with these being semiconductors is that excitations between fully empty or between two full bands are likely to be weak...Read More