Characterization of meh-ppv: pc71bm blend thin films Deposited on glass substrates by spin-coating and drop-casting techniques for photovoltaic applications

Abstract

The surge in population growth and modernity since the pre-industrial era has seen an increase in anthropogenic greenhouse gas emissions resulting from increased use of fossil fuels. As a result, there is increase in demand for an alternative clean, sustainable, economical and renewable energy based on photovoltaic technology (PV). This is aimed at reducing greenhouse gas emissions and global average temperature. Kenya receives adequate sunshine, and therefore, photovoltaic technology is the way to go. First generation solar cells technology based on crystalline and polycrystalline silicon solar cells have been widely used to produce electricity for domestic and industrial uses due to their high efficiency of about 20 %. However, their high cost of production is still a challenge. The search for alternative absorber materials have led to intensive research in organic photovoltaic solar cells, which are composed of at least two electronically dissimilar molecules that form a donor-acceptor system. Donor-acceptor molecules form bulk heterojunction, which has been proven to be the best option for a low-cost production of thin film solar cells. Optimizing the active layer blend composition and deposition parameters to improve the layer's properties is an essential stage in producing thin film solar cells with increased efficiency. The choice of materials also plays a major role in ensuring that sufficient energy offset at the donor-acceptor interface take place. The use of fullerene derivatives as electron acceptors has gained popularity due to their favorable electron affinity. These ball-like fully conjugated structures have strong electron affinity and unipolar electron transport that promotes delocalization of electrons. This study focuses on characterization of a blend between poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and fullerene derivatives of phenyl-C71-butyric acid methyl ester (PC71BM) for PVs. The optical and morphological properties of MEH-PPV: PC71BM films were investigated at different spin-coating speed, solution concentration, annealing temperature, acceptor percentage composition in the blend and deposition methods. From the optimized results obtained, deep photoluminescence quenching was observed for MEH-PPV doped with 75 % PC71BM, a signature of improved charge transfer rate. In addition, the band gap energy was reported to reduce to 1.8775 eV, indicating better photon-absorption. It was also found that high spin-coating speeds enhanced film-surface homogeneity and improve light absorption in deposited films. An optimized MEH-PPV: PC71BM system was obtained at spin-coating speed of 2000 revolutions per minute, a thickness of 113 nm, a concentration of 16 mg/ml and an annealing temperature of 150 oC. This system is intended for photovoltaic applications especially in the production of improved thin organic photovoltaic solar cells.