Charge Transfer States in D-A Excitonic Solar Cells: Photophysical Characterization and Loss Mechanisms for Charge Generation

Alternative energy sources are in great demand to support the economic growth of our society. During the past decades the world has been assaulted by the idea of a possible exhaustion of our fossil fuel supply in a relative short period of time, dramatically increasing energy costs as it becomes more and more scarce. The way to face this problem is obviously to look for environment friendly energy sources that satisfy the criteria of being i) cheap, ii) environmentally benign and iii) inexhaustible. Among the ones that satisfy the criteria is the use of sunlight to produce energy by the photovoltaic effect.Recently much attention has been focused on organic, polymeric or hybrid systems for photovoltaic operation. Such solar cells need to be cheap and easy to produce for practical applications. In addition, questions of energy conversion efficiency and long-term stability need to be addressed. Two distinct, but complementary, methodologies have emerged; mesoscopic dye sensitized (DSC) and bulk organic (or polymeric) heterojunction (BHJ) solar cells.In the BHJ configuration, a mixture of a conjugated polymer (donor) and charge acceptor, usually a fullerene derivative, are sandwiched between two metallic electrodes, one of which is transparent. This architecture has the advantage of giving the possibility of process and assemble the active layer using a single step from solution, and make use of classical printing techniques on different types of substrates, avoiding high temperatures and more expensive deposition methods.Organic solar cells differ from their inorganic counterparts by producing bound electron-hole pairs (excitons) upon light absorption; these excitons, as a result of the low dielectric constants of the active medium, show a considerable electron-hole binding energy, around 0.4 eV, and as a consequence, exciton dissociation occurs only at the interface between two materials of different electron affinities, working as electron donor (D) and the electron acceptor (A), which yield a driving force for charge separation.Understanding the fundamental electronic interactions between the D and A materials as well as the role of the composite film morphology, device architecture and processing conditions, is crucial to achieve high efficiencies. Over the last few years progress has been mostly achieved through the understanding of the importance of the active layer morphology, especially the type of solvent used, polymer regioregularity and film annealing conditions to the device final performance.In the proposed research, several electron donor and acceptor materials will be investigated in order to unravel the processes that control the formation and recombination of free charge carriers in organic photovoltaic devices. A particular focus of the research will be the investigation of conjugated materials containing heavy atom complexes, which give rise to an intrinsic mechanism that promotes the formation of triplet excitons. This very rapidly and efficiently converts all polymer singlets into polymer triplets that can be used as electron donors in photovoltaic operation. A detailed investigation of these materials as donor materials in BHJ solar cells is still lacking, and the concept of using long lived triplet excitons in charge generation deserves further attention, particularly in order to clarify the effect of the increasing mixing of singlet and triplet states on the energy of the CT state and geminate back electron transfer.

The aim of the research project is to perform a comprehensive photophysical investigation of processes that govern the efficiency of photoinduced charge generation in molecular materials. This is aimed at the emergent photovoltaic market based on organic materials. Together with academic Institutions and Laboratories, companies working in this sector will be the main beneficiaries. However, the general public and everyone concerned with environment and energy efficiency policies will also benefit from the research outputs. Both polymers and small molecules have been used in organic photovoltaics (OPV). While the efficiencies of organic based devices is still low when compared with the more commonly used, but more costly, a-Si or CIGS thin film approaches, the possibility of using flexible, cheap, lightweight nanostructured organic semiconductor materials opens a completely new route to produce low-cost photovoltaic devices for use in portable applications, as for example, smart lamps, photoactive textiles, laptops or mobile phones. More and more companies are being attracted to the OPVs market. Plextronics a materials production company is developing its OPV niche. Heliatek has investments from Bosh and BASF to develop OPV solar panels, in Wales, G24i has built an OPV facility using technology licensed from Gretzel, and Konarka has recently announced a line of solar panels for use in portable applications. A complete understanding of the mechanism that controls the charge generation in molecular materials is still lacking, and the factors that ultimately govern the generation of free charges upon light absorption in molecular materials are still not completely understood. This is crucial to highlight targets for new material structures, and to improve device efficiencies from 5% to 10%, the aimed target that will allow establishing OPV as a valuable alternative in the photovoltaic market and challenge silicon based technology. This research aims to help make this step forward. Academics and companies concerned with the photovoltaic technology will be able to immediately incorporate the research outputs and establish new synthetic targets, alternative device fabrication steps and device structures in order to create devices of social, economic and commercial interest. The Organic Electroactive Materials Group in the Durham University, where the Dr Dias is currently developing his scientific activity, has a long track record of collaboration with industry and other academic institutions and laboratories, both in the UK and abroad, Dr Dias has worked with CDT and Thorn for 5 years and currently holds a position partially funded by CDT. The existing links will be exploited to maximize the impact of the present proposal. If new triplet route works, this could rapidly be taken into PETEC or Konarka for device development, giving UK a strong position in this future market. This will be a good return to the EPSRC investment. Finally, physics students of our department will also be direct beneficiaries of the present research project through the realization of workshops, postgraduate lectures and final year research projects into the field of organic photovoltaics.