Optimized optical and morphological properties of thin MEH-PPV: PC71BM films deposited on glass substrates for photovoltaic applications

Organic solar cells (OSCs) made of at least two electronically dissimilar molecules have attracted a lot of attention due to their low-cost solution manufacturing and color tunability. Bulk-heterojunction active layer usually achieved through spin-coating provide an appealing technique. These cells generate Frenkel excitons through photo-absorption in one molecule combined with acceptor resulting in the formation of free charge carriers that emerge after exciton dissociation at the donor–acceptor interface. These processes highly depend on optimization of the blend composition and deposition parameters such that we form an interpenetrating bi-continuous network with the domain sizes roughly twice of the exciton diffusion length. The choice of materials plays a major role in ensuring that sufficient energy offset at the donor/acceptor interface leads to an efficient charge separation. The study focused on probing a blend of poly [2-methoxy-5-(2’-ethylhexyloxy)-1, 4-phenylenevinylene (MEH-PPV) and fullerene derivatives of phenyl-C71-butyric acid methyl ester (PC₇₁BM) forming a bulk heterojunction active layer. We investigate the optical and morphological properties of MEH-PPV: PC₇₁BM spin-coated films at varied spin-coating parameters such as spin-rates and spin-step as well as blend properties including solution concentration, deposition techniques and donor–acceptor blend ratios. We related how deposition parameters affect the exciton quenching capabilities at donor–acceptor interface, film-surface homogeneity and light absorption in deposited films. From the optimized results obtained, deep photoluminescence quenching was observed for MEH-PPV doped with 75% PC₇₁BM in addition to reduced optical band gap energy (1.88 eV), a signature of improved charge transfer rate and photon-absorption. The optimized MEH-PPV: PC₇₁BM systems are possible candidates for photovoltaic applications especially in the production of thin organic photovoltaic solar cells.