This paper presents an overview of the research carried out by a European consortium with the aim to develop and test new and improved ways to realise dye-sensitized solar cells (DSC) with enhanced efficiencies and stabilities. Several new areas have been explored in the field of new concepts and materials, fabrication protocols for TiO2 and scatterlayers, metal oxide blocking layers, strategies for co-sensitization and low temperature processes of platinum deposition. Fundamental understanding of the working principles has been gained by means of electrical and optical modelling and advanced characterization techniques. Cost analyses have been made to demonstrate the potential of DSC as a low cost thin film PV technology. The combined efforts have led to maximum non-certified power conversion efficiencies under full sunlight of 11% for areas < 0 center dot 2 cm(2) and 10 center dot 1% for a cell with an active area of 1 center dot 3 cm(2). Lifetime studies revealed negligible device degradation after 1000hrs of accelerated tests under thermal stress at 80 degrees C in the dark and visible light soaking at 60 degrees C. An outlook summarizing future directions in the research and large-scale production of DSC is presented

The selective transport of electrons and holes to the two terminals of a solar cell is often attributed to an electric field, although well-known physics states that they are driven by gradients of quasi-Fermi energies. However, in an illuminated semiconductor, these forces are not selective, and they drive both charge carriers toward both contacts. This paper shows that the necessary selectivity is achieved by differences in the conductivities of electrons and holes in two distinct regions of the device, which, for one charge carrier, allows transport to one contact and block transport to the other contact.To clarify the physics, we perform numerical simulations of three different solar cell structures with asymmetric carrier conductivities. Two of them achieve the ideal conversion efficiency limit, despite the fact that the charge carriers flow against an internal electric field, proving that the latter cannot explain carrier separation. A third, i.e., conceptual structure, has no electric field at all but still works ideally as a solar cell. In conclusion, the different conductivities of electrons and holes in two regions of the device can be identified as the essential ingredient for charge carrier separation in solar cells, regardless of the existence of an electric field.

This work elucidates the impact of charge transport on the photovoltaic properties of organic solar cells. Here we show that the analysis of current–voltage curves of organic solar cells under illumination with the Shockley equation results in values for ideality factor, photocurrent and parallel resistance, which lack physical meaning. Drift-diffusion simulations for a wide range of charge-carrier mobilities and illumination intensities reveal significant carrier accumulation caused by poor transport properties, which is not included in the Shockley equation. As a consequence, the separation of the quasi Fermi levels in the organic photoactive layer (internal voltage) differs substantially from the external voltage for almost all conditions. We present a new analytical model, which considers carrier transport explicitly. The model shows excellent agreement with full drift-diffusion simulations over a wide range of mobilities and illumination intensities, making it suitable for realistic efficiency predictions for organic solar cells.

The surface defects of solution-processed ZnO films lead to various intragap states. When the solution-processed ZnO films are used as electron transport interlayers (ETLs) in inverted organic solar cells, the intragap states act as interfacial recombination centers for photogenerated charges and thereby degrade the device performance. Here we demonstrate a simple surface-passivation method based on ethanedithiol (EDT) treatment, which effectively removes the surface defects of the ZnO nanocrystal films by forming zinc ethanedithiolates.