wileyonlinelibrary.com Macromolecular Rapid Communications Review 1 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.201500398 1. Introduction The certified power conversion efficiencies (PCE) of polymer-fullerene bulk heterojunction (BHJ) solar cells [1] are now above 10 and 11% in single-junction [2] and tandem [3] architectures, respectively. This recent improve- ment in device performance is due to the design and syn- thesis of new donor–acceptor (D–A) polymers exhibiting energy levels and band gaps optimized for high PCE. [4] Some of these high-efficiency polymers are now available commercially in small quantities for laboratory-scale research and development. Quite often, these polymers are The molecular weight of an electron donor-conjugated polymer is as essential as other well- known parameters in the chemical structure of the polymer, such as length and the nature of any side groups (alkyl chains) positioned on the polymeric backbone, as well as their placement, relative strength, the ratio of the donor and acceptor moieties in the backbone of donor–acceptor (D–A)-conjugated polymers, and the arrangement of their energy levels for organic photovoltaic performance. Finding the “optimal” molecular weight for a specific con- jugated polymer is an important aspect for the development of novel photovoltaic polymers. Therefore, it is evident that the chemistry of functional conjugated polymers faces major chal- lenges and materials have to adopt a broad range of specifications in order to be established for high photovoltaic performance. In this review, the approaches followed for enhancing the molecular weight of electron- donor polymers are presented in detail, as well as how this influences the optoelectronic properties, charge transport properties, structural confor- mation, morphology, and the photovoltaic performance of the active layer. Systematic Analysis of Polymer Molecular Weight Influence on the Organic Photovoltaic Performance Athanasios Katsouras, Nicola Gasparini, Chrysanthos Koulogiannis, Michael Spanos, Tayebeh Ameri, Christoph J. Brabec, Christos L. Chochos,* Apostolos Avgeropoulos A. Katsouras, C. Koulogiannis, M. Spanos, Dr. C. L. Chochos, Prof. A. Avgeropoulos Department of Materials Science Engineering University of Ioannina Ioannina 45110, Greece E-mail: cchochos@cc.uoi.gr N. Gasparini, Dr. T. Ameri, Prof. C. J. Brabec Institute of Materials for Electronics and Energy Technology (I-MEET) Friedrich-Alexander-University Erlangen-Nuremberg Martensstraße 7, 91058 Erlangen, Germany Prof. C. J. Brabec Bavarian Center for Applied Energy Research (ZAE Bayern) Haberstrasse 2a, 91058 Erlangen, Germany Early View Publication; these are NOT the final page numbers, use DOI for citation !! Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201500398