Concluding Remarks

Gavin Conibeer1 and Arthur Willoughby2

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Australia 2Faculty of Engineering and the Environment, University of Southampton, UK

This book aims to present the latest developments in high-efficiency photovoltaics, con­tributed by experts in the respective fields.

The physics of solar cells and of advanced concepts as presented by Jean-Francois Guille – moles, gives a very useful insight into the underlying mechanisms required for solar cells and their limiting efficiencies. The descriptions of multiple energy threshold devices and their potentials to increase efficiencies above the Schockley-Queisser limit are particularly useful. An understanding of the physical limits is essential to guiding progress in design and fabrication of devices. This is particularly true for the multiple energy level devices with their more complex physical principles.

The characterisation of solar cell materials presented by Daniel Bellet and Edith Bellet – Amalric, ideally describes the characterisation techniques necessary for development of high – efficiency devices. In particular, the techniques for determining important crystal properties with X-ray and Raman techniques are most useful. It is not common to have such a focus on characterisation in a volume such as this, but its importance in determining and allowing control of the physical parameters of materials and devices is essential to significant progress.

The present status of crystalline silicon cells presented by Martin Green outlines the dominant position of crystalline technologies in the market, and the fact that this will continue for at least another decade. The developments in mono-, multi – and pseudo-monocrystalline cells and modules are allowing an approach to the fundamental limits for silicon, whilst also driving down the cost of production significantly. In particular, developments in material quality, light trapping and carrier extraction are allowing continuing improvements in the efficiency of cast cell materials and this is set to continue as the competiveness in the market continues to increase.

Ruud Schropp continues the theme of silicon solar cells but with amorphous and micro­crystalline silicon materials. The inherent low material usage of thin-film deposition and the potential for cheap substrates make these technologies very competitive for cheap solar cell delivery. Efficiencies are boosted in double – and triple-junction micromorph and similar approaches. The differing morphology and defect density of amorphous materials and their characterisation presents challenges for strong light absorption and good carrier collection, but several p-i-n junction approaches with innovative characterisation techniques are proving successful in getting round these problems.

In the chapter on III-V solar cells, Nicholas Ekins-Daukes describes the large range of materials available in this group and their application to highly efficient crystalline cells. The roles of crystal quality, defects, hetero – and homojunctions are investigated. The very impor­tant area of multijunction III-V cells is explored with comparison of the several techniques used to balance the issues related to lattice matching and those of required bandgaps, with an excellent description of the route towards the highest efficiency solar cells in multijunction III-V materials.

In the next chapter on chalcogenide solar cells, Miriam Paire, Sebastian Delbois, Julien Vidal, Nagar Naghavi and Jean-Francois Guillemoles investigate the several different ana­logue materials of II-VI compounds. In a logical progression from CdTe to CuInGaSe2 (CIGS) to Cu2ZnSnS4 (kesterite) cells, the increasing number of elements allows greater flexibility in the control over bandgap, optical absorption, lattice spacing and material abun­dance. This group of materials also has produced the record thin-film cell at just over 20% for CIGS cells. Whilst the control of stoichiometry and morphology become more of a challenge with the increasing complexity, the abundance and toxicity issues of some of the elements are largely addressed by the recent move to the kesterite group of materials. Very promising absorption, carrier collection and efficiency properties are possible with the potential to move on to tandem and other multiple junctions to boost efficiencies further.

In the chapter on printed organic solar cells, Claudia Hoth, Andrea Seemann, Roland Steim, Tayebeh Amin, Hamed Azimi and Christoph Brabec change direction to look at organic materials and the many different aspects required for a full photovoltaic product. The potential for these materials to produce really cheap solar cells depends on implementation of good materials and devices in a full product, preferably as a printed product. The crucial importance of morphology and the ability to characterise this in the fabrication of bulk het­erojunctions that have all of good absorption, intimate mixing and good transport to contacts is investigated and put in the context of currently available and soon to be realised materials. The role of multiple-junction devices in further boosting efficiencies is also discussed and a case made for development of full OPV products.

In the final chapter on third-generation photovoltaics Gavin Conibeer addresses the poten­tial of advanced concepts to boost efficiencies through the use of multiple energy thresholds. Linking back to several of the physical concepts in Chapter 2, the concepts of using quantum confinement in nanostructures to engineer semiconductor bandgaps, up – or downconversion of incident photons, or the capture of excess carrier energy usually lost as heat are inves­tigated. Materials and practical approaches for each of these are explored and conclusions drawn on their near – or long-term viability.

This volume overall gives what we believe to be a good overview of the current status of photovoltaic development. It is not completely comprehensive with some technologies such as thin-film crystalline silicon and dye-sensitised cells not discussed in any detail, but all other technologies and to some extent their interaction with each other are covered. Comparisons can be drawn between the properties of bulk crystalline and thin-film materials particularly in terms of the light absorption and material usage of these approaches. The increasing move towards thinner and thinner layers of material and hence the importance of maximising absorption and hence light trapping makes such comparisons highly relevant. Similarly, the ‘engineering’ of solar cell materials’ properties either alloying in chalcopyrites to modify the bandgap or for lattice matching in III-Vs or through quantum confinement in nanostructures, are all examples of areas in which there is an overlap of expertise in the quite different materials groupings and in their preparation methods. These synergies and shared expertise can continue to give new insights and lead to greater progress on efficiency improvement and reduction of cost, both in financial and environmental terms, of new and improved photovoltaic technologies. We expect this volume to become dated as these technologies move forwards, but the underlying principles and the approaches towards exploiting these principles outlined in this volume will remain to be built on further.

[1] note that this can’t be compensated by the reduced radiative recombination that would be entailed, according to detailed balance, by a reduced absorptivity as shown by [Marti and Araujo, 1994].

[2] Early 2013, the record stabilised efficiency reported for a single junction |ac-Si:H p-i-n solar cell was 10.69%,

where the i-layer thickness was 1.8 цш (S. Hanni et al., Prog. Photovolt: Res. Appl21 (2013) 821).

[4] At the end of 2012, a stabilised efficiency of 13.44% was reported for an a-Si:H/|ac-Si:H/|ac-Si:H triple junction cell (S. W. Ahn et al., 20th European Photovoltaic Solar Energy Conference (2012) 3AO.5.1).

Solar Cell Materials: Developing Technologies, First Edition. Edited by Gavin Conibeer and Arthur Willoughby. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

[6] a-Si:H is an alloy of amorphous silicon with at least 10 at% hydrogen. The optical density and the bandgap of this material tend to be higher – refractive index 4 and bandgap 1.8 eV – than those of bulk Si – refractive index 3.6 and bandgap 1.1 eV. Giving a-Si:H a significantly shorter wavelength absorption edge and a pseudodirect bandgap, meaning that it is suitable as a thin-layer top cell material in a tandem cell.

[7] These selection rules arise from spin conservation on absorption of a photon; the excited singlet state containing two electrons of antiparallel spin and hence able to absorb a spin 1 photon, whereas in the excited triplet state the two electrons are of parallel spins and hence not strictly able to absorb or emit a photon.

[8] The partially delocalised nature of the organic species (e. g. benzene ring, porphyrins or fullerene molecules) used for this and most other organic electronic applications reduces this effect somewhat but does not give the same degree of delocalisation, and hence low exciton binding energy, as the completely dissociated electrons and holes in an inorganic semiconductor.

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