Novel Approaches in the Design of Donor-Acceptor Oligomeric and Polymeric Materials for Photovoltaic Applications: D/A Blend versus Self-assembly of D/A by Covalent or Non-Covalent Interaction

Abstract

Organic semiconducting materials involving small -conjugated molecules as well as polymers are fast growing members in the family of potential materials for harvesting non-conventional source of energy like the solar radiation.1-8 Organic photovoltaic cells (OPVs) are a revolutionary technology which converts the solar radiations into electrical energy using organic/polymeric semiconducting material (active layer). The technology has become very promising due to its low fabrication cost, lightweight, large area and mechanical flexibility. The major breakthrough in OPV was created by Tang et al. who reported the first hetero junction organic solar cell with power conversion efficiency (PCE) ~ 1 %.1,9Although intensely researched for more than two decades, the efficiency of organic solar cell remains lower compared to efficiency based on inorganic crystalline silicon which has reached up to ~ 25 %. However, promising breakthroughs in terms of better device performance efficiencies, device designs as well as longer outdoor performance of test modules have kept the field ongoing.10-13 The basic OPV device architecture consists of a photo-active layer of organic /polymeric semiconductor sandwiched between indium tin oxide (ITO) electrode (anode) and metal electrode (cathode).3 Tang implemented modifications involving a bilayer heterojunction OPV device comprising of p-type donor and n-type acceptor as active layer, which improved the device performance.1,9 The working principle of such device involves the light absorption by active layer which lead to creation of electron-hole pair i.e. exciton. The next step is the dissociation of excitons; which recombined by emitting photon or non-radioactive recombination if eventual separation to its component electron and hole did not occur within the short exciton diffusion lenght.1,3, 6-8 Once the charges are separated into electrons and hole, they move towards the respective electrode and finally charge collection leads to electric current. The charge extraction is driven by internal electric field in cell caused by different work function of electrodes. It is widely accepted that the exciton diffusion at D/A interface is the most important step that depends upon how the donor and acceptor are arranged in the nanoscopic length scale of about 10-20 nm which is critical to the exciton life time. Generally, the life time of exciton is very short with diffusion length limited to 5-20 nm after which they will recombine and fail to generate free charges.3, 6, 8 The exciton diffusion is considered to be the most important aspect during design of active layer of solar cell. Therefore, morphology of the active layer component (donor-acceptor) plays a vital role in the device efficiency of solar cells.14