Bulk heterojunction (BHJ) polymer solar cells (PSCs) that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based solar cells. A combination of novel polymer development, nanoscale morphology control and processing optimization has led to over 8% power conversion efficiencies (PCEs) for BHJ PSCs with a conventional device structure. Attempts to develop PSCs with an inverted device structure as required for achieving high PECs and good stability have, however, met with limited success. Here, we report that a high PCE of 8.4% under AM 1.5G irradiation was achieved for BHJ PSCs with an inverted device structure. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, leading to facilitate electron transport and suppress bimolecular recombination. These results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

Bulk-heterojunction polymer solar cells have emerged as an attractive type of cost-effective solar energy–electrical power transforming device. Recently, great progress in the development of new photo-harvesting materials and device optimizations have been achieved in this field, resulting in the significant improvement of the power conversion efficiencies of polymer solar cells from around 1% to higher than 8.0%. The rational design and fine tailoring of the molecular structures of donor polymers significantly contributed to these prominent advances. Among all kinds of donor polymers, push–pull conjugated polymers, which consist of alternating electron-rich and electron-deficient units have been most extensively developed and have dominated the library of donor materials for polymer solar cells, because their intrinsic optical and electronic properties can be readily tuned to the desired situation by controlling the intramolecular charge transfer from donor unit to acceptor unit. This review provides a brief overview of the recent development of push–pull conjugated polymers and their application in solar cells. The relationships between the materials' chemical structures and properties, such as absorption spectra, energy levels, mobilities and photovoltaic behaviors, were also discussed.

Water/alcohol-soluble conjugated polymers (WSCPs) and small molecules (WSCSs) are materials that can be processed from water or other polar solvents. They provide good opportunities to fabricate multilayer organic optoelectronic devices without interface mixing by solution processing, and exhibit a promising interface modification ability for metal or metal oxide electrodes to greatly enhance the device performance of solar cells. Moreover, owing to their intriguing processability, WSCPs and WSCSs have great potential for applying environmentally friendly processing technologies to fabricate solar cells. In this review, the authors give an overview of recent developments in WSCPs and WSCSs, including their molecular design, material synthesis, functional principles and application as interface modification layers and photoactive components in emerging photovoltaic technologies such as organic/polymer solar cells, organic–inorganic hybrid solar cells and dye-sensitised solar cells.

A non-fullerene acceptor with a high relative dielectric constant ( ε _r ) over 9 is developed. It offers an efficiency of 8.5%, which is the best result for organic solar cells employing high ε _r materials. Further research should focus on morphology optimization to make high ε _r practically useful in devices.