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27th European Photovoltaic Solar Energy Conference and Exhibition; 512-515, The use of quantum wells in photovoltaic devices are often discussed in terms of providing additional current to improve the efficiency of multi-junction photovoltaic devices. However, quantum wells also provide some freedom to manipulate the radiative recombination in the quantum well solar cell. Under concentrated sunlight, the quantum well solar cell becomes radiatively limited, so the voltage of the quantum well solar cell becomes dependent upon the radiative loss from the device, which in turn can be controlled by careful design of the solar cell. Anisotropic radiative recombination from a quantum well solar cell leads to a small voltage improvement in the solar cell since the recombination current is suppressed in directions where sunlight is not absorbed. Further, we show that strong radiative coupling between sub-cells takes place, resulting in a multi-junction solar cell that is tolerant to daily and seasonal changes to the solar spectrum.

Quantum well solar cells are the result of the application of nanotechnology to enhance of the efficiency of concentrator solar cells. The band gap of a quantum well (QW) solar cell can be adapted to the incident spectral conditions by tailoring the QW depth. The single-junction strain-balanced quantum well solar cell (SB-QWSC) has achieved an efficiency of 28.3%. The dominant loss mechanism at the high concentrator cell operating bias is due to radiative recombination, so a major route to further efficiency improvement requires a restriction of the optical losses. It has been found that (100) biaxial compressive strain suppresses a mode of radiative recombination in the plane of the QWs. As biaxial strain can only be engineered into a solar cell on the nanoscale, SB-QWSCs are seen to have a fundamental efficiency advantage over equivalent bulk cells. Strain-balanced quantum wells in multi-junction solar cells can current match the sub-cells without the introduction of dislocations. Quantum-well (QW) technology was proved to boos the efficiency of triple-junction cells by around 3 percent in absolute terms. In fact 40 percent median efficiency has been achieved across production wafers.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 52-56, The band gap of the quantum well solar cell can be tailored for the incident spectral conditions. We report on recent developments for single-junction and tandem strain-balanced quantum well solar cells at Imperial College London. The single-junction quantum well solar cell has attained an efficiency of 28.3 %. It has also been shown that the strain in the quantum wells leads to the suppression of a mode of radiative recombination, which is the dominant loss mechanism at the operating bias. As the requisite compressive biaxial strain can only be engineered into a solar cell on the nano-scale, strain-balanced quantum well solar cells are seen to have a fundamental efficiency advantage over equivalent bulk solar cells. Quantum wells in multi-junction solar cells can enable favourable band gap combinations to be reached and aid with current matching of the subcells without the introduction of dislocations. A dual-junction InGaP/GaAs solar cell with quantum wells in the bottom subcell has been grown and characterised, and efficiencies for the quantum well cell relative to a bulk control cell are predicted at two spectrally distinct locations.

The quantum well solar cell (QWSC) has been proposed as a route to higher efficiency than that attainable by homojunction devices. Previous studies have established that carriers escape the quantum wells with high efficiency in forward bias and contribute to the photocurrent. Progress in resolving the efficiency limits of these cells has been dogged by the lack of a theoretical model reproducing both the enhanced carrier gen- eration and enhanced recombination due to the quantum wells. Here we present a model which calculates the incremental generation and recombination due to the QWs and is verified by modelling the experimental light and dark current-voltage characteristics of a range of III-V quantum well structures. We find that predicted dark currents are significantly greater than experiment if we use lifetimes derived from homostructure devices. Successful simulation of light and dark currents can be obtained only by introducing a parameter which represents a reduction in the quasi-Fermi level separation., Comment: Preprint submitted to the 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, USA, Sept. 2000, pp. 1304-1307

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 3338-3342, Multijunction thin film modules are of ever greater interest, due to their high efficiency values recorded in the literature. Spectral response measurements on such devices require a careful usage of coloured bias light and also present additional difficulties when applied to large area modules: the method should avoid current limitation by the junction producing less current at a given wavelength, due to series connection of junctions in the device. In this work we present details of the method recently developed at ESTI for spectral response measurements of large area thin film multijunction modules. The mismatch factor calculation for the same devices is also shown: this is done by the convolution of the measured spectral response of each junction and of the reference cell with the spectrum of our Large Area Pulsed Solar Simulator and with the standard AM1.5 spectrum.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 332-335, Several works have appeared in the literature as a result of the joint activity of Imperial College London with the European Solar Test Installation and other European research centres on luminescent solar concentrators: some have been specifically related to indoor and outdoor characterisation. Within the characterisation processes, the external quantum efficiency is particularly important: by means of external quantum efficiency measurements, spectral mismatch correction can be performed, enabling performance predictions for any given irradiance conditions. In recent works an increase in module efficiency due to a diffusive reflector at the backside of the concentrator has been shown. In this work we present recent results on the impact of a non-index-matched backside diffusive reflector on the external quantum efficiency of a luminescent solar concentrator and therefore on the short-circuit current of the device: the result is particularly relevant in view of using luminescent solar concentrators in building integration of photovoltaics.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 19-21, The strain-balanced quantum well solar cell benefits from a fundamental efficiency enhancement due to anisotropic emission from the quantum wells. This anisotropy arises from a splitting of the valence band due to compressive strain in the quantum wells, suppressing a transition which contributes to emission from the edge of the quantum wells. The extent of the suppression has been calculated using k.p theory. Detailed balance modelling was used to determine the limiting efficiencies for cells containing quantum wells under varying degrees of strain, and cells with highly strained quantum wells were found to benefit from a greater efficiency enhancement relative to a cell that emits isotropically. Since biaxial strain in a solar cell can only be achieved on the nano-scale, nanostructured quantum well cells are seen to have a fundamental efficiency advantage over bulk semi-conductor solar cells.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 359-362, Luminescent solar concentrators (LSCs) are typically planar low-concentration systems that absorb sunlight over a large area and emit a red-shifted spectrum out of smaller surfaces, where solar cells can be attached. We present the study of a composite system, in which a linear geometrical concentrator is used as primary device to focus sunlight onto a cylindrical LSC encased in a transparent matrix. The idea behind this design is to reduce re-absorption losses, which generally limit the performance of LSCs. Experimental measurements on a cylindrical LSC were compared with a raytrace model, showing a good agreement. Further predictions were made based on the model. It was shown how the reduction of re-absorption losses is achieved by allowing the luminescence from the cylindrical core to travel in the transparent matrix. The proposed design can achieve high optical concentrations with the need for only one-dimensional tracking.

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, such that emitted light is trapped in the sheet and travels to the edges where it can be collected by solar cells. LSCs offer potentially lower cost per Wp. This paper reviews results mainly obtained within the framework of the Fullspectrum project. Two modeling approaches are presented, i.e., a thermodynamic and a ray-trace one, as well as experimental results, with a focus on LSC stability.

We present an efficiency analysis of ideal photovoltaic solar cells based on multi-intermediate band structures. It is shown that the difference between the thermodynamic limit of photovoltaic conversion and the limit of efficiency of traditional bulk semiconductor solar cells can be gradually bridged if an optimum energy band structure is achieved. Efficiency enhancement originates from photonic excitations among multiple energy bands. Several possible ways to design the optimum energy band structures are proposed.