Study of structure-property-performance relationships for organic thin-film transistors and polymeric solar cells
Organic electronics has great potential for fabricating low cost, flexible and large-area devices. Despite the rapid development, several main challenges of the field need to be addressed in both organic conjugated polymer and small molecules based devices, including organic thin-film transistors (OTFTs) and polymer solar cells (PSCs). This dissertation first explores two approaches to align small molecule crystals and improve surface coverage. The controlled evaporative self-assembly (CESA) method is combined with binary solvent system using small molecule SMDPPEH to control the crystal growth. By optimizing the two solvent ratios, well-aligned SMDPPEH crystals with significantly improved areal coverage were achieved. Also, polymer additives can be added into small molecule to control crystal alignment. As a result, mobilities are at least 10 times higher than that from spin-coated film. The SMDPPEH based OTFTs exhibit a mobility of 1.6×10-2 cm2/Vs, which is the highest mobility from SMDPPEH ever reported. The donor-acceptor vertical composition profile on the performance of the P3HT/PCBM based organic bulk heterojunction solar cells was studied. In this simulation study, variety of donor-acceptor vertical configurations was investigated for both regular and inverted PSC structures. The physical mechanisms behind the diversification of open circuit voltage, short circuit current, and fill factor, and thus power conversion efficiency from various vertical configurations are explained. The effect of vertical composition profile from the study could serve as guidance for experimental optimization of organic bulk heterojunction solar cells. Also, morphology variation of ZnO electron transport layer from atomic layer deposition and sol-gel methods on the performance of organic inverted solar cells were investigated. AFM and SEM were utilized to characterize the morphology of ZnO thin films and nanorods so as to explain the efficiency difference. The final part of the work demonstrates one-step multi-layer pattern transfer to make organic solar cells on rigid and flexible substrates. A multi-layer inking and stamping, a cost-efficient, purely additive pattern transfer technique, was developed to fabricate PSCs. GLYMO is added into PEDOT:PSS hole transport layer and its effect on PSC performance and pattern transfer yield was investigated to reach overall PSC efficiency and high yield pattern transfer.