Abstract
In this thesis, I focus on the material characterization and the device optimization of small molecule organic solar cells (SMOSCs). In the first part, I briefly review the history and development of organic solar cells (OSCs), followed by working mechanisms, material preparation, device structures and measurement of OSCs. In the second part of thesis, before evaluting new donor compounds for SMOSCs, I review some previous reports of SMOSCs employing symmetrical or unsymmertrical small molecular donors. Among our symmetrical donor compounds, PyCN, a donor with the acceptor-acceptor-donor-acceptor-acceptor (A-A-D-A-A) molecular structure, shows the best performance in SMOSCs by utilizing the planar mixed heterojunction (PMHJ) strcuture. The optimized blend ratio is PyCN:C70 = 1.5:1 (by volume), giving a power conversion efficiency (PCE) of up to 4.3% with an open circuit voltage (Voc) of 0.95 V, short circuit current density (Jsc) of 12.50 mA/cm2, fill factor (F.F.) of 0.37. On the other hand, TDP with donor-acceptor-acceptor (D-A-A) molecular structures shows the most promising characteristics among our unsymmetrical donor systems. The TDP:C70 (1:2) PMHJ device exhibits the best performance of a PCE up to 5.6% with a Voc of 0.94 V, Jsc of 11.34 mA/cm2, F.F. of 0.52. In the third part, the electron transporting materials, TmPyPB, B3PyPB, BCP, BP4mPy, HATCN, DPPS, ET-7 and TC-1108 had been examined for the role of electron transporting layer (ETL) in OSCs. Among them, TmPyPB possesses the advantages of good thermal stability and high electron mobility, which make it a good ETL candidate for OSCs. In the long-term light soaking test, the TmPyPB-based cells also showed longer lifetime and less deterioration than the tranditional BCP-based cells. At the last part of this section, by using AC impedance analysis, I show that the TmPyPB-based cells exhibit the lowest AC resistance among all devices. In the fourth part, silver nanowire (AgNW) was used to replace indium tin oxide (ITO) as a transparent electrode for the OSCs. The P3HT:PCBM OSCs employing AgNW show a PCE up to 4%, which is highly comparable to the ITO-based reference cells. In the last part, I developed a physical vapor deposition technique for solar active layer deposition. In the priliminary test, DBP thin films and the bilayer heterojunction solar cells were fabricated using this deposition method.