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.

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

A three‐ to four‐fold increase in power conversion efficiency is reported for poly(3‐alkylthiophene)/fullerene solar cells by simply heating the device to 55 °C for 30 min. This behavior may be due to enhanced diode properties, plus a higher degree of order within the material. The Figure shows the soluble fulleropyrrolidine used as electron acceptor.

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