Abstract
Over the past decade, organic photovoltaics (OPVs) experienced significant development in performance reaching the threshold needed for commercialization [power conversion efficiencies (PCE) ˃10%]. Due to their light weight, flexibility and low cost, OPV technology is highly promising in terms of industrial aspects. Besides, OPVs have the advantage of being able to be fabricated on flexible substrates which facilitate the use of mass production techniques such ink jet printing or various roll-to-roll techniques. All these attractive advantages urged researchers worldwide to address challenges facing OPVs technology commercialization such as achieving high performances, replacing toxic processing solvents and upscaling with low costs. In this thesis, we introduced two main issues discussing processing techniques for high device performances and replacing toxic processing solvents with more eco-friendly counterparts. Solvent additive with "dual functionality" has been introduced in small-molecule solar cells with significant enhancement in performance. The first function of this additive was that it controlled the morphology: the addition of 0.1% CP3MS (the additive) was sufficient to improve the film’s crystallinity and morphology. The second function was the spontaneous migration of the CP3MS molecules from the bulk to the interface between the active layer and the Al cathode, forming an ultrathin interlayer that acted as a buffer layer. PCE enhanced from 2.75% for devices without additives to 4.55% for devices containing 0.1% CP3MS. As promising approach to widen the light absorption in organic solar cells, "ternary approach" has been introduced as technique to enhance device performance. We combined two small-molecule donors with acceptor to form all-small molecule ternary solar cells. Improvement in device performance mainly attributed to the complementary absorption of the two donors. After addition of the molecular donor as third component to form the ternary blend, the crystallinity and morphology of the active layer were enhanced. This means that the molecular donor in the ternary blend has effective role on both the absorption and the morphology. Devices produced PCE of 6.3% in ternary blend system relative to 4.55% in binary blend system. As most of highest OPVs performances achieved using halogenated toxic solvents which are big obstacle toward industrialization and mass production. Replacing these toxic solvents with more eco-friendly solvents is urgent need for the upscaling OPVs. Toluene (Tol), a halogen-free solvent, was employed in the fabrication of molecular solar cells, achieving power conversion efficiency PCE higher than that obtained when using chlorinated counterparts. Halogen-free solvents have been also used for solvent vapor annealing (SVA) to control the morphology. We succeeded to achieve one of the highest PCE in molecular solar cells processed form halogen-free solvents (˃ 7%). The enhancement arose mainly from the improvement in the fill factor, due to morphological tailoring and favorable phase separation. PCE higher than 7% is one of the highest achieved so far when using halogen-free solvents for molecular solar cells processing A new green solvent, cyclopentyl methyl ether (CPME), has been also introduced as successful alternative to replace toxic halogenated solvent for efficient molecular solar cells. Tol as co-solvent has been introduced with various amounts in CPME forming different green solvent mixtures. Tol incorporation as a co-solvent with CPME combined with thermal annealing effect led to PCE of greater than 8% which is the highest to date for molecular solar cells processed from green solvent mixtures. Morphology plays the key role in the performance improvement under the effects of co-solvent and thermal annealing.