The Fabrication And Performance Of Organic Solar Cells Based On Series Solution-processed Small Molecular Squaraine Donors

There are three main ways to improve the power conversion efficiency(PCE) of organic photovoltaics (OPV):the development of new organic materials, improved process control and design novel device architecture. In this thesis, we first investigated the squaraine material SQ, then a family of new highly absorptive solution processable squaraine materials have been systematically synthesized and made to devices. Then we enhanced performance of the OPV via improving process control.First, we investigated the effects of the active layer thickness on the performance of the SQ-based OPV. The optimized thickness of the active layer is78nm, at which the device shows the best PCE2.84%among all devices. The highest short-circuit current density (Jsc)8.76mA/cm2is attributed to the balancing between the light absorption and the carrier transport. Then we enhanced performance of the OPV via the anode modification. The results show that poly (3,4-ethylenedioxythiophene):poly (styrenesulfonicacid)(PEDOT:PSS) deteriorates the performance of SQ:PC71BM based OPV, and MoO3is used to substitute PEDOT.PSS as an effective exciton-blocking layer. A higher open-circuit voltage (Voc) of0.95V is reached. The barrier matching between the energy-level of MoO3and HOMO of SQ enhances fill factor (FF) and Jsc· After being treated by UV-ozone for10min, a high PCE of3.99%is obtained. The operation mechanism of the thermal annealing is also straightened out. Jsc and FF reach the maximum values after annealing at70℃for10min. The results show that the hole mobility increases from2.26×10-6cm2/V s to3.34×10-6cm2/V s by using spacecharge-limited current (SCLC) model. It suggests that the charge transport properties are substantially improved. On the other hand, the enhanced roughness obtained from the AFM image results in optimized and stable phase separation for efficient charge separation and transport.Compared with symmetrical molecular structure squaraine (SQ, D-A-D), asymmetrical squaraines (ASQs, D-A-D’) could afford more advantages with respect to the fine-tuning of optoelectronic properties, and in addition, ASQs often exhibit good solubility in common organic solvents. So we designed and synthesized a variety of new solution-processable asymmetrical squaraines, and mainly studied on the ASQB and ASQC. The optimized weight ratio of ASQB:PC71BM is1:8, ASQB-based OPV shows a PCE of2.24%after annealing at70℃for10min. The optimized weight ratio of ASQC:PC71BM is1:5, ASQC-based OPV shows a Voc of1.12V and a PCE of2.82%after annealing at110℃for10min. This is, to our knowledge, the highest Voc (1.12V) of single OPV based on small molecule donors. After that, we investigate the effects of adding ASQB into a conventional P3HT:PC6,BM-based OPV. The X-ray diffraction pattern of P3HT:PCBM:ASQB films shows that ASQB does not hinder the crystallization of P3HT. Forster resonance energy transfer based systems enable the effective use of multiple donors, thereby bringing significant improvements in light absorption and conversion.The near infrared absorption of the blend was enhanced by the ASQB additive, resulting in an increased PCE from2.58%to3.22%.Two solution-processable asymmetrical squaraines, with cyclopenta[b]indolinyl (la) and cyclopenta[b]indolyl (1b) as end capper have been designed and synthesized. The PCE values of the OPV based on the squaraine dyes/PC71BM are4.01%for1a with a FF of0.50and2.22%for lb with a FF of0.33. The correlation between the FF of solar cell and material density, higher density and hence higher FF, may provide an easier molecular design concept to measure and manipulate for material chemist to improve charge transport. Then we provide a simple way to modify the ITO anode, which is compatible with the aqueous fabrication process of the PEDOT:PSS layer. ITO anode used in P3HT:PC61BM-based OPV is treated by KMnO4and hydrogen peroxide solution, respectively. The modification of ITO anode results in an enhancement of the PCE of the device, which is responsible for the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO4and hydrogen peroxide solution, and then the charge collection efficiency is improved. On the other hand, the ITO surface is smoothed by the treatment, which indicates that adhesion between ITO and PEDOT:PSS is improved by the treatment. After that, ITO anode used in ASQ-la-based OPV is treated by hydrogen peroxide solution too. The device with ITO anode treated by hydrogen peroxide show a better property than the traditional UV-ozone treatment device. To show that the solution modification of ITO treatment has a universal applicability in the ASQs system.