Design And Synthesis Of Novel Organic Photovoltaic Donor Materials And Investigation Of Device Optimization

We designed and synthesized series of novel organic donor materials towards the application in organic solar cells (OSCs), their thermal stability, optical and electrochemical and solid packing properties were investigated. Then the power conversion efficiencies (PCEs) of these donors were dissucssed based on OSC device studies. In order to further improving the PCE, we tend our attentions to device optimization and mechanism studies. Firstly, we attempted several electron-transporting layers (ETLs), and the PCE was improved to8.32%. Then we fabricated and optimized the inverted device, and the PCE of the small molecule based inverted device was improved to8.84%. Primary studies on the working mechanism and stabilities in OSCs were also performed, and then we have dissussed the design of the high performance donors based on the four years’theoretical calculations. Abstracts of each part are presented as following:Firstly, two novel copolymers PBDT-DEAITN and PBDT-DOAITN containing the same backbone of isothianaphthene (ITN) quinoidal unit and benzodithiophene (BDT) donor units with different side chains are synthesized and their organic photovoltaic (OPV) devices are fabricated. Though these two polymers have rather low optical band gaps,1.52and1.58eV for PBDT-DEAITN and PBDT-DOAITN, respectively, their OPV cells based on PC61BM as the acceptor exhibit rather limited power conversion efficiencies of1.25and1.20%under an AM1.5G simulated solar light. The reasons for the low OPV performance are investigated by structure modeling calculation, X-ray diffraction, atomic force microscopy and space charge limited current measurements et al. Relatively large dihedral angles between BDT and ITN units are obtained as33.66°and34.35°for PBDT-DEAITN and PBDT-DOAITN, respectively. This causes the polymer packing less ordered in solid state and lower hole mobility, thus result in poor OPV performance. The differences among the representative quinoidal polymers in literatures are discussed based on theoretical dihedral angles and dipole calculation results, and planarity became the focus of attention in our future donor molecule design. Secondly, three quinquethiophene derivatives with different end groups of octyl2-cyanoacetate (DCA05T),3-ethylrhodanine (DERHD5T) and2H-indene-1,3-dione (DIN5T) are synthesized in order to obtain the higher open circuit voltage (Voc) than their septithiophene analogs. The photovoltaic performance of these three molecules as donors and fullerene derivatives as the acceptors in bulk heterojunction solar cells are studied by using the simple solution spin-coating fabrication process. Among them, DERHD5T shows Voc as high as1.08volt and power conversion efficiency of4.63%under AM1.5G irradiation (100mW cm-2). To our knowledge, only very few compounds including polymers have been reported with Voc over1volt for solution processed BHJ solar cells. Meanwhile, DCAO5T and DIN5T exhibit PCE of3.27%and4.00%, with Voc of0.78V and0.88V. The reasons for the high Voc were investigated by the theoretical simulations and DERHD5T showed the weakest interaction with PC61BM, therefore could reduce the reverse dark saturation current density and improve the Voc. The consistent results have been obtained in comparison with experimental measurements. Future work would be focus not only to decrease the HOMO level of donor material and increase the LUMO level of acceptor materials to increase the Voc, but also to weaken the interactions between donor and acceptor materials.Thirdly, three different solution-processed ETLs-PFN, ZnO NPs and LiF on the performance of small molecule-based devices with DR3TBDT:PC71BM blend as the active layer have been investigated and compared, and power conversion efficiencies (PCEs) of8.32%,7.30%and7.38%are achieved under AM1.5G irradiation (100mW cm-2), respectively. To the best of our knowledge, this PCE of8.32%is among the highest efficiencies reported for small molecule and polymer-based solar cells. The mechanism for the ETL-induced enhancement has been studied and it is found that different ETLs have significantly different impact on the device performance and the clearly improved performance from PFN is attributed to the combination of reduced bimolecular recombination and increased effective photon absorption in the active layer.Fourthly, we have fabricated the inverted small molecule solar cell based on DRCN7T:PC71BM blend as the active layer. The power conversion efficiency has reached8.84%, with Voc of0.91V, Jsc of14.28mA cm’2and FF of0.68under AM1.5G irradiation (100mW cm-2), this is the new world record for the solution-processed inverted small molecule solar cells. Meanwhile, we have prepared the conventional device based on the same conditions, achieved PCE of8.06%and Jsc of13.07mA cm-2, with no obvious change on Voc and FF compared with the inverted device. In order to investigate the reasons for the enhancement of Jsc, therefore the PCE, we have performed the optical simulations and light-intensity experiment on the conventional and inverted device. All these indicate that the inverted device structure could increase the real absorption in the active layer. The inverted solar cells showed enhanced life time as we have expected, which exhibited PCE over8%after stored in air for103days with encapsulation. This indicates that small molecule solar cells could achieve the comparable stability with polymer solar cells.Fifthly, we have compared various functionals and basis sets on the prediction of structures and properties for organic photoelectric materials, and found that B3LYP/6-31G*and PBE1PBE/6-31G*could meet the basic demands for experimental chemists. Especially that PBE1PBE/6-31G*could provide the comparable results with experimental results, like the optimized geometry of the organic semiconductors, HOMO level, absorption spectrum and the reorganization energy during charge transfer et al. According to the Marcus electron transfer theory, we have correlated the power conversion efficiencies of the oligothiophene derivatives developed by our group and the respective theoretical hole reorganization energies. We found that smaller hole reorganization energy would facilitate both charge transport in donor domains and charge transfer between the donor/acceptor interfaces, therefore increasing the charge transfer (ηCD) and charge collection at the electrodes (ηCC), and also the exciton dissociation (ηCD) during the photoinduced electron-transfer process, thus contributed to the improved power conversion efficiency. This has profound guiding significance in future organic donor material design.