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

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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.