Polymer Solar Cells—Interfacial Processes Related to Performance Issues

Harnessing solar energy with solar cells based on organic materials (in particular polymeric solar cells) is an attractive alternative to silicon-based solar cells due to the advantages of lower weight, flexibility, lower manufacturing costs, easier integration with other products, low environmental impact during manufacturing and operations and short energy payback times. However, even with the latest efficiencies reported up to 17%, the reproducibility of these efficiencies is not up to par, with a significant variation in the efficiencies reported across the literature. Since these devices are based on ultrathin multilayer organic films, interfaces play a major role in their operation and performance. This review gives a concise account of the major interfacial issues that are responsible for influencing the device performance, with emphasis on their physical mechanisms. After an introduction to the basic principles of polymeric solar cells, it briefly discusses charge generation and recombination occurring at the donor-acceptor bulk heterojunction interface. It then discusses interfacial morphology for the active layer and how it affects the performance and stability of these devices. Next, the formation of injection and extraction barriers and their role in the device performance is discussed. Finally, it addresses the most common approaches to change these barriers for improving the solar cell efficiency, including the use of interface dipoles. These issues are interrelated to each other and give a clear and concise understanding of the problem of the underperformance due to interfacial phenomena occurring within the device. This review not only discusses some of the implemented approaches that have been adopted in order to address these problems, but also highlights interfacial issues that are yet to be fully understood in organic solar cells

XPS, UV–Vis, FTIR, and EXAFS Studies to Investigate the Binding Mechanism of N719 Dye onto Oxalic Acid Treated TiO2TiO_{2} and Its Implication on Photovoltaic Properties

The anchoring mechanism of N719 dye molecules on oxalic acid treated TiO2 (OA-TiO2) electrodes has been investigated using extended X-ray absorption fine structure (EXAFS) measurements, Fourier transform infrared spectroscopy (FTIR), UV−vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). The FTIR spectroscopy of OA-TiO2 electrodes revealed that the oxalic acid dissociates at the TiO2 surface and binds through bidentate chelating and/ or bidentate bridging. Analyses of EXAFS, FTIR, UV−vis, and XPS measurements of N719 dye loaded onto OA-TiO2 revealed that the binding of N719 molecules takes place via interaction between the Ru atom of the dye and O− of bidentate bridged oxalate ions at the TiO2 surface. This mechanism is quite different from the binding of N719 onto untreated TiO2 (WO-TiO2) surface, where −COOH and SCN groups of the dye directly bind to the TiO2 surface. The analyses of UV−vis data show that the amount of N719 dye loading onto OA-TiO2 surface is much higher than that onto the native TiO2 surface. In addition, the incident photon-to-current conversion efficiency (IPCE) measurements show that the presence of oxalate ions between the dye and TiO2 surface favors efficient electron transfer and therefore improvement in device efficiency. The dye-sensitized solar cells fabricated using N719 dye sensitized onto OA-TiO2 showed an efficiency of ∼4.6%, which is significantly higher than that based on a WO-TiO2 electrode (∼3.2%)

Interfacial charge trapping in the polymer solar cells and its elimination by solvent annealing

The PCDTBT:PCBM solar cells were fabricated adopting a tandem layer approach to investigate the critical issues of charge trapping, radiation absorption, and efficiency in polymer solar cells. This layered structure was found to be a source of charge trapping which was identified and confirmed by impedance spectroscopy. The low efficiency in multilayered structures was related to trapping of photo-generated carriers and low carrier mobility, and thus an increased recombination. Solvent annealing of the structures in tetrahydrofuran vapors was found beneficial in homogenizing the active layer, dissolving additional interfaces, and elimination of charge traps which improved the carrier mobilities and eventually the device efficiencies