have to diffuse to dissociation sites where their charges ... substrates using simple, cheap and low-energy ... this domain, but it will certainly provide the reader.
PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2007; 15:657–658 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pip.807
Special Issue
EDITORIAL
Organic Solar Cells: Linking Nanoscale to Gigawatts? In these times of booming interest in photovoltaic technologies, organic solar cells take a special position. In itself ‘organic solar cells’ is already an ill-defined term, which deserves further clarification. Secondly, the physical phenomena which are at the basis of its photovoltaic operation and the tight link of its photovoltaic performance with nanomorphological features make it particularly attractive for the scientific community. Last but not least, from an economic perspective, the extremely high manufacturing throughput potential of these technologies bears the promise of a disruptive low-cost technology which attracts many investors. Based on all these perspectives, organic solar cells recently made their appearance in photovoltaic technology roadmaps. For all of these reasons, it was decided to publish a special issue of Progress in Photovoltaics on organic solar cells with the aim to shed more light on a number of scientific challenges associated with this emerging PV-technology like stability, link between nanomorphology and performance and to provide an outlook on potential efficiencies achievable with this technology. As mentioned above, the term ‘organic solar cells’ lacks a clear definition. The term often covers those photovoltaic technologies, which contain an organic material within the active layer of the photovoltaic device. The basic steps in photovoltaic conversion are light absorbance, charge carrier generation, charge carrier transport and extraction/injection of charge carriers through externally accessible contacts. More specifically, the term ‘organic solar cell’ is applicable whenever at least the two first steps are being realized by means of an organic layer. By this definition, full-organic devices as well as hybrid devices are being covered. The purpose of the organic material might be limited to increasing light absorbance as is the case in photo-electrochemical solar cells (Graetzel cells) or the organic layers also contribute to the transport of the photogenerated carriers. The latter is the case in e.g.
Copyright # 2007 John Wiley & Sons, Ltd.
bulk donor–acceptor heterojunction solar cells. In this issue we decided to limit ourselves to the approaches where the active layer only consists of organic layers. In organic semiconductors, absorption of photons leads to the creation of bound electron-hole pairs (excitons) with a binding energy far above the average thermal energy rather than free charges, as one is used to in the case of most inorganic solar cell structures. These excitons carry energy, but no net charge, and have to diffuse to dissociation sites where their charges can be separated and transported to the contacts. The breakthrough for efficient solar cells incorporating organic semiconducting materials came with the advent of concepts which were radically deviating from the planar hetero or homojunction solar cells. The generic idea behind these concepts is the existence of a bulk-distributed interface to capture the excited carrier and to increase the exciton dissociation rate. Organic semiconductors can be processed from solutions at or near room temperature on flexible substrates using simple, cheap and low-energy deposition methods such as spinning or printing thereby yielding cheaper devices. It will be of no surprise that the bulk donor–acceptor heterojunction approach based on polymeric donors and acceptors will be prominently present in this issue. The interest in this approach is fuelled by the recently reported efficiency results on polymeric multijunction solar cells and the appearance of first commercial products using the polymer bulk donor–acceptor heterojunction approach. It would however be unwise to limit ourselves to the polymeric approach. Also devices based on organic small molecules will be treated in this issue. It does honour to the fact that the first organic solar cells exceeding 1% efficiency were based on small molecules and the first organic multijunction solar cells were using small molecules. Moreover, one should remain aware that industrial manufacturing of organic solar cells based on vacuum evaporation or
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organic vapour phase deposition at atmospheric pressure of small molecules is not to be excluded. Finally, we also strived at including some of the approaches which could potentially boost the efficiency further like the use of plasmonic effects, although the benefits might not be limited to organic solar cells. The selected articles cover the frequently used polymer:PCBM system, polymer:polymer solar cells and small-molecule devices, transport and recombination in these systems including their optoelectronic
Copyright # 2007 John Wiley & Sons, Ltd.
EDITORIAL
behaviour on nanoscale, stability and efficiency potential calculations. We are aware that this only covers part of the issues and groundbreaking approaches in this domain, but it will certainly provide the reader with a sound base to get in touch with this rapidly expanding and exciting domain where nanoscale and the eventual perspective of gigawatt scale meet. J. Poortmans IMEC/PT/SOLO, Kapeldreef, 75, B-3001 Leuven, Belgium
Prog. Photovolt: Res. Appl. 2007; 15:657–658 DOI: 10.1002/pip
