Organic Solar Cells
Organic photovoltaic devices have attracted scientists in recent years who want to harness solar energy in an eco-friendly and cost-effective way. Organic solar cell (OSC) technology has many advantages when compared with older silicon-based solar cell technology. While current silicon-based solar cells boast conversion efficiencies that exceed 10 percent, reaching even 12 percent, some researchers predict OSCs will reach 15-20 percent efficiency. Also, fabrication costs of OSCs are lower because processing is much simpler.
OSC structure and materials allow the formation of flexible, semi-transparent devices. The typical photoactive blend layer in OSC cells consists of conjugated polymer donor (D) and molecular acceptor (A). The polymer donor serves as the main solar light absorber and as the hole transporting phase, whereas the small molecule transports electrons. Fullerenes are the most investigated acceptors for OSCs due to their LUMO energy level of ~3.8–4.2 eV, reversible electrochemical reduction, excellent electron transport characteristics and anisotropic charge transport. However, it is difficult to tune fullerenes’ electronic structure and they have low absorption in the visible region. Fullerene acceptors have poor photochemical stability in air, and their spherical geometry and limited molecular weight increase the morphological instability of blends with linear polymers. Lately, several research groups have begun investigating non-fullerene molecular acceptors due to progress in D-A polymer architecture. The figure below shows examples of acceptor polymers for non-fullerene bulk heterojunction (BHJ) OSCs. Recent results indicate breakthroughs for non-fullerene OSCs. Zhao-Hui Wang et al. present non-fullerene BHJ OSCs base at diimide derivatives as acceptors with power conversion efficiency (PCE) up to 6%. Also, other groups have obtained high-performance OPV based at diimides. Janssen et al. successfully applied as acceptor conjugated polymers based on the diketopyrrolopyrrole with PCE up to 2.9%. Conjugated polymers based on benzothiadiazole units are another example of non-fullerene BHJ.
The efficient charge-carrier generation and collection, comparable to those of polymer/fullerene solar cells were found to be the main reasons for the high device performance. The ideal polymers for OSCs should exhibit narrow band gap, broad absorption in the ultraviolet-visible range, high absorption coefficients, high carrier mobility, and appropriate HOMO and LUMO energy levels.
Moreover, several processes such as: light absorption to create hole-electron pairs (excitons); exciton diffusion to the donor-acceptor interface and splitting to free carriers; migration of holes (in the donor) and electrons (in the acceptor) toward the contacts for collection due to the built-in electric potential/field must be optimised. Therefore, an investigation of non-fullerene acceptors will enrich the diversity of acceptors to match the present high-performance donors, which may eventually give rise to higher PCEs via proper selection and combination of donors and acceptors.
Via the ExCEED project, it is anticipated the ERA Chair Team will investigate functional pi-conjugated molecules and macromolecules as non-fullerene materials that could be used in photovoltaic cells. An important issue for any HBJ cell to be considered is the actual nanoscale morphology of the blend that can be assessed with state-of-the-art TEM, AFM, and grazing incidence X-ray techniques. Furthermore, it is expected the ERA Chair Team will train postgraduates in electrochemical methods, which could be used to determine oxidation and reduction potentials and stability of organic compounds. They will show several spectro-electrochemical measurements in order to determine basic parameters such as: energy gap or changes in spectra during electrochemical doping. They will conduct complex in situ studies of the doping process of investigated systems using EPR coupled with electroanalytical and UV-Vis-NIR techniques or RAMAN and IR spectroscopies.
Additionally, the ERA Chair Team will also examine the redox mechanism, as well as intermediate and possible by-products of the doping process. This will be conducted in static and dynamic analyses of investigated monomers, oligomers and chemically formed coatings by Electrochemical Impedance Spectroscopy involving best-fitting of equivalent circuit models in an attempt to identify the key electrical features of investigated organic films.