Department of Energy Engineering (Energy Engineering)Now organic solar cells exhibited excellent device performance with power conversion efficiency over 16%. Therefore, it has a great potential for various practical application such as large-area flexible devices, indoor photovoltaic applications, colorful devices, and so on. Fundamental study is important for further development of organic solar cells. Charge carrier recombination and transport properties should be investigated which is related to the photocurrent and/or energy losses leading to poor device performance. This thesis deals with a relationship between solvent additives, morphology and bimolecular recombination by using diphenyl ether and diphenyl sulfide as universal and non-halogenated solvent processing additives.(Chapter 2 and 3) Diphenyl ether acts like theta solvent to photovoltaic polymers, helps to form ideal bulk-heterojunction film morphologies and suppress bimolecular charge recombination regardless photovoltaic polymers. Diphenyl sulfide exhibited fast and field-independent photocurrent saturation with negligible bimolecular recombination led to efficient charge separation and collection, which resulted in the highest power conversion efficiency up to 9.08% in PTB7-Th:PC71BM devices. Charge recombination and transport characteristics are different with incident light intensities. In chapter 4, device properties are investigated under various light intensities with three semi-crystalline polymers by modulating the intra- and intermolecular noncovalent coulombic interactions. With the polymers which exhibited compact molecular packing structures, high power conversion efficiency was achieved even in low light intensity. P2FDTBTBO devices exhibited a low efficiency of 3.69% under standard light. However the efficiency was dramatically enhanced by 2.3 times (8.33% PCE) under dim light, showing negligible decrease in open-circuit voltage and remarkable increase in fill factor, which is due to the exceptionally high Rsh of over 1000 k?? cm2. This work provides an important tips to further optimize organic solar cells for indoor applications with low-power electronic devices such as Internet of Things (IoT) sensors, etc. Conventional bulk-heterojunction (BHJ) organic solar cells (OSCs) generally guarantee high power conversion efficiency, but poor reproducibility of active layer???s morphologies and complicated device optimization processes limit further commercialization. In chapter 5, bilayer organic solar cells (OSCs) are characterized with 5 different non-fullerene acceptors which are distinguished by Stokes shift. Bilayer-heterojunction was formed using orthogonal solvent system for donor and acceptor, and it was confirmed by various experimental techniques. ITIC-Th1, IDIC and its derivatives (Stokes shift 38~74 nm) exhibits 9-11% PCEs in bilayer-heterojunction OSCs with high fill factor ~0.70, whereas NIDCS-HO (Stokes shift 133 nm) exhibits poor PCE around 2%. The high efficiencies were enabled by their efficient self F??ster energy transfer (FRET) with long exciton diffusion lengths (20-30 nm), and long-range FRET from donor to acceptor (with significant spectral overlap between emission of donor and absorption of acceptor). The bilayer devices have negligible bimolecular and monomolecular recombination. This work suggest that small Stokes shift materials have a great potential for high-performance bilayer OSCs with long exciton diffusion lengths.clos
