Preface: phys. stat. sol. (a) 205/12
Authors: Wehrspohn Ralf B. , Gombert Andreas , Carius Reinhard , Rau Uwe 
Published: December 2008

Photon Management in Solar Cells

By definition, photovoltaics is the transformation of the electro‐magnetic energy of solar light into electricity. Thus, photons stand at the beginning and electrons stand at the end of this value creation chain, respectively. Thus, photovoltaics investigates and optimizes all processes taking place in‐between the advent of a photon on the surface and the output of electrical energy at the terminals of the solar cell or module. It is instructive to note that the maximum efficiency of photovoltaic energy conversion as described by the Shockley–Queisser limit makes no specific assumption on those processes in‐between, aside from perfection: Any arriving photon generates an elementary electrical charge that is readily available at the electrical terminals of the device. The only physical quantity entering in the Shockley–Queisser theory and thereby connecting the photonic input with the electronic output is a threshold energy, often the band gap energy of the semiconductor used for the solar cell. This energy defines the energetic range of usable photons, simultaneously the available voltage at the terminals, and finally the maximum power conversion efficiency.

If perfect photovoltaic materials were abundantly available as well as straight‐forward methods to combine them into perfect photovoltaic devices with marginal cost, any further photovoltaic research would be pointless. As a matter of fact, this situation is still somewhat ahead because almost perfect materials and devices, though yet feasible to a certain extent, are still far too expensive for large scale energy production. Meanwhile photovoltaics must cope with imperfections and this immediately implies a clever management of both particles involved in the process, electrons as well as photons.

This special issue of physica status solidi (a) contains eleven invited papers from the 398th Wilhelm and Else Heraeus Seminar ‘Photon Management in Solar Cells’ which took place from 29 October to 1 November 2007 in the Physikzentrum Bad Honnef, Germany. This seminar was jointly organized by two German national projects ‘Nanosun’ funded by the German Science Foundation (DFG, project number pak88) and ‘Nanovolt’ funded by the German Federal Ministry of Education and Research (BMBF, project number 03SF0322H). The term ‘photon management’ was coined in an early stage of these projects and has become nowadays almost commonplace for recent efforts to improve solar cells by means of modern optics including diffractive optics, photonic crystals and plasmonics.

The fundamental interdependence of optical and electronic properties in solar cells, the reciprocity of photovoltaic and electroluminescent action, is subject of the paper by Thomas Kirchartz and Uwe Rau. Tom Markvart investigates the entropy generation by non‐radiative and by radiative recombination in solar cells. Andreas Gombert and Antonio Luque review the current state‐of‐the‐art of photonics in photovoltaics and describe future requirements. The following contributions present theoretically and experimentally novel concepts of photon management in solar cells. Karsten Bittkau and coworkers revisit randomly textured substrates, Matthias Kroll, Stephan Fahr et al. 1D and 2D diffractive structures in solar cells, and Andreas Bielawny et al. intermediate reflectors based on 3D photonic crystals for thin film solar cells. External light management by optically functionalized fluorescence collectors is subject of the contribution by Jan Christoph Goldschmidt and coworkers. Upconversion systems based on rare‐earth‐ion doped phosphors and glass ceramics for solar cells are discussed by Stefan Schweizer et al. Ultra‐light trapping by direction selective filters for solar cells is presented by Carolin Ulbrich et al. and the application of plasmonics to photovoltaics by Florian Hallermann, Carsten Rockstuhl et al. Finally, a review on light trapping in organic solar cells and future requirements is presented by Michael Niggemann and coworkers.

We would like to thank the German Science Foundation (DFG), the German Federal Ministry of Education and Research (BMBF) and the Wilhelm and Else Heraeus Foundation for financial support. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)