Abstract
In this thesis, we focus on material characteristics, device optimization and photonic designs of small molecule organic solar cells (SMOSCs). First, in the introduction, we briefly review some vacuum deposition SMOSCs and novel electron donors with push-pull chromophores. The developments of indium tin oxide (ITO) free organic solar cells are also introduced. In the second part of thesis, we study a tailor-made compound DTDCTP with donor-acceptor-acceptor (D-A-A) molecular structures as donor material for SMOSCs. By pairing it with C60 or C70, intriging morphology and charateristics are shown. The appearance of phase-separated domains in atomic force microscope images indicate that a preferred morphology was formed in the mixed layer and thus provided sufficient D/A interfaces for exciton dissociation and effective pathways for charge carrier transport. These characteristics together with balanced electron and hole mobilities in DTDCTP:C70 mixed layer (~ 3x10-5 cm2 V-1 s-1 at 700 (V/cm)1/2) contribute to a high short circuit current density (Jsc) of 12.1 mA/cm2 and a fill factor (F.F.) as high as 0.56. A very high open circuit voltage (Voc) is also obtained due to low–laying solid state HOMO level of DTDCTP (-5.46 eV). As a result, power conversion efficiency (PCE) up to 6.4% under AM 1.5G, 1 sun (100 mW/cm2) illumination are achieved in optimized cells. In the third part, DTDCTP base small-molecule organic solar cells with microcavity structures utilizing very thin solar-absorbing active layers are simulated and fabricated. By carefully fine-tuning the in-cell spacer layer and out-of-cell capping layer, highly efficient top-illuminated indium tin oxide-free solar cells are obtained with efficiencies up to 5.5% and 5% on glass and flexible polyethylene terephthalate substrates, respectively. In the last part of this thesis, a series of low donor concentration organic solar cells organic solar cells is investigated and the best device shows PCE of 2.8%