The present review rationalizes the information spread in the literature concerning the use and role of buffer layers in polymer solar cells. Usual device structures include buffer layers, both at the anode and at the cathode interface, mainly to favour charge collection and extraction, but also to improve the device's overall performance. Buffer layers are actually essential for achieving highly efficient polymer solar cells and can no more be considered as ''optional'', thus the need and importance of understanding their properties and role. The aim of this review is to give the reader an overview of this topic and to provide a practical and useful tool for the daily activities of researchers in the field of polymer photovoltaics.

Pag. 5General criteria used in the preparation of Tables, Figures and Schemes.

Organic photovoltaic (OPV) devices, in particular polymer solar cells, made by solution processed organic materials have shown great promise as a disruptive technology for affordable electricity. Even though recent advances look impressive on paper, until now the commercialization of OPV has been hampered by the difficulty of converting lab produced "champion" cell figures into reliable industrial-scale product performances. A key factor to achieve this condition is to develop OPV materials (polymer donors, acceptors, buffer materials, electrodes materials and encapsulants) exhibiting the required technical and economic characteristics to be conveniently used in an industrial environment. The well established strategies for the design of materials for efficient lab-scale OPV devices are not sufficient when largearea printed panels are concerned. A number of additional requirements, normally not addressed in the laboratory context, must be met: the materials must be easily accessible as pure compounds in few synthetic steps from cheap starting compounds, need to be stable and soluble enough to afford ink formulations processable with roll-to-roll compatible equipment; solvent and solvent additives should be easily removable after printing, and possibly should be environmentally friendly compounds; the layers should achieve a stable morphology under mild conditions (low temperatures and short times); the above mentioned materials can be screened on glass substrates, but should be finally tested on plastic films, protected through a scalable encapsulation technique. The more researchers adhere to these guidelines, the greater the possibility for OPV to demonstrate at last its enormous potential on the industrial scale. Broader contextThe global electric energy demand in 2010 was about 21 400 TW h (source: IEA). Renewable energies accounted for almost one-h of the total energy production. While solar energy was only around 1% of this portion, over the past ve years it has averaged an annual growth rate of over 50%. Growth has been mostly concentrated in a few countries, where PV generates today a few percent of total yearly electricity production. High cost, along with the intermittency of the solar radiation, is one of the main limiting factors, therefore much effort must be spent to overcome these problems. Polymer-based organic solar cells have the potential to be cost-effective and lightweight solar energy converters, with a promising energy balance. Their function is based on the photoinduced electron transfer from a polymeric donor to an acceptor, generally a fullerene derivative. In principle, they can be fabricated by low temperature printing or coating processes from solvent-based inks, which are compatible with exible plastic substrates. Fair power conversion efficiencies ($10%), comparable with amorphous silicon, have been recently achieved in the laboratory, by the combination of improved materials engineering, ne control of the photoactive layer morphology and use of more sophisticated device archit...

The interlayer materials used in non-fullerene based solar cells are reviewed and the peculiar mechanisms operating in these systems are discussed.

Polymersolar cells have gained wide interest in the past few years for their potential in the field of large-area and low-cost photovoltaic devices. Thanks to rather simple treatments developed in the new millennium, the morphology of polymer solar cells has been optimized at the nanoscale level, leading to high efficient charge-carrier photogeneration and collection. Power conversion efficiency up to 6% and 6.5% have been reported in the literature for solution-processed polymer solar cells in single-junction and tandem configuration, respectively, and a record efficiency of 6.77% has been recently announced. After an introduction into the operational principles and device structure of polymer solar cells, this paper provides an overview of the last-years research activity. In particular, the different treatments successfully performed on polymer active layers, and their beneficial effects on the overall device efficiency are discussed. Subsequently, some significant examples of photoactive materials will be examined, outlining the foremost structure−properties relationships. Some directions for further enhancement of the performance of polymer solar cells will be also introduced, mainly through the fine-tuning of the electronic properties of the active materials

We present the results of a thorough molecular modeling study of several alkylthiophene-based oligomers and polymers. In particular, we consider two polymers whose limit-ordered crystal structures have been recently reported by our group, on the basis of powder X-ray data analysis: poly(3-(S)-2-methylbutylthiophene) (P3MBT) and form I' of poly(3-butylthiophene) (P3BT). We first describe the development of a series general purpose force fields for the simulation of these and related systems. The force fields incorporate the results of ab initio calculations of the bond torsion energies of selected oligomers and differ in the set of atomic charges used to represent the electrostatic interactions. We then present the results of an extensive validation of these force fields, by means of molecular mechanics (MM) energy minimizations and molecular dynamics (MD) simulations of the crystal structures of these oligomers and polymers. While our "best" force field does not outperform the others on each of the investigated systems, it provides a balanced description of their overall structure and energetics. Finally, our MM minimizations and MD simulations confirm that the reported crystal structures of P3MBT and P3BT are stable and correspond to well-defined energetic minima. The room-temperature MD simulations reveal a certain degree of side-chain disorder, even in our virtually defect-free polymer crystal models.

The preparation of various molecules taken as representative examples of some of the main classes of molecular donors for organic solar cells is discussed in order to assess the complexity and possibilities of scaling-up their synthesis.

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