A Study of Solution-processed Small Molecules and Fullerene Derivatives for Bulk Heterojunction Solar Cells

The dissertation is mainly focused on designing and developing the novel solution-processed small molecule donors and different fullerene derivative acceptors for organic solar cell (OSC) application. Besides, the interaction between the donor and acceptor and the morphological difference after the film formation of active layer were studied. This study further combines other techniques, including the thermal treatment and ternary blend, to further improve the cell performance of OSCs. In the first part (Chapter 3), two novel, symmetrical, and linear A–D–A-type π-conjugated donor molecules (BDT6T, BDTCN) were strategically designed and convergently synthesized, each containing a planar electron-rich 2-octylthiene-2-yl–substituted benzodithiophene (BDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. Both of these materials were thoroughly characterized, and the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties were investigated. Solution-processed bulk heterojunction organic solar cells were fabricated by incorporating BDT6T and BDTCN. Among these tested devices, the one containing BDT6T and PCBM in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (JSC) of 9.08 mA/cm2, an open circuit voltage (VOC) of 0.90 V, and an impressive fill factor (FF) of 0.66 under AM 1.5G irradiation (100 mW/cm2). The FFs of these solution-processed small molecule organic solar cells (SMOSCs) are outstanding when compared with those recently reported for benzodithiophene (BDT)-based SMOSCs, due to the high crystallinity and excellent stacking properties of the BDT-based compounds. In the second part (Chapter 4), the conventional bulk heterojunction (BHJ) solar cells incorporating the small molecule donor (2Z,2´E)-dioctyl 3,3´-(5´´,5´´´´´-(4,8-bis(5-octylthiophen-2-yl)benzo[1,2-b:5,4-b´]dithiophene-2,6-diyl)bis(3,4´,4´´-trioctyl-[2,2´:5´,2´´-terthiophene]-5´´,5-diyl))bis(2-cyanoacrylate) (BDT6T) and [6, 6]-Phenyl-C61-butyric acid methyl ester (PCBM) acceptor archive the power conversion efficiency (PCE) of 5.26%. Replacing PCBM with indene-C60 bisadduct (ICBA) which possesses a higher lowest unoccupied molecular orbital (LUMO) level enhanced an open-circuit voltage (VOC) from 0.90 V to 0.97 V, but decreased the short-circuit current density (JSC) and fill factor (FF), leading to a low cell efficiency of 2.68%. It was found that BDT6T:ICBA film reveals a well-mixed morphology due to lower surface energy difference (∆γ) than BDT6T:PCBM, which is attributed to less thermodynamic driving force for phase separation. After the thermal annealing, the morphology of BDT6T:ICBA displayed the suitable phase separation for effective charge transport rather than the serve aggregation from BDT6T:PCBM and showed the enhanced PCE of 4.03% with VOC of 1.04 V, JSC of 8.07 mA cm-2, and FF of 0.48. It is therefore concluded that the value of ∆γ not only provides the useful information for synthetic strategies, but also manifests a promising strategy to improve the ICBA-based small molecules solar cells. Ternary bulk heterojunctions (BHJs) are platforms that can improve the power conversion efficiencies of organic solar cells. In Chapter 5, we report an all-small-molecule ternary BHJ solar cell incorporating [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and ICBA as mixed acceptors and the conjugated small molecule BDT6T as a donor. When incorporating a 15% content of ICBA relative to PC71BM, the ternary BHJ solar cell reached a power conversion efficiency of 6.36% with a short-circuit current density (JSC) of 12.00 mA/cm2, an open-circuit voltage (VOC) of 0.93 V, and a fill factor of 0.57. The enhancement in efficiency, relative to that of the binary system, resulted mainly from the increased value of JSC, attributable to not only the better intermixing for the donor and acceptor that improved charge transfer but also the more suitable morphology for efficient dissociation of excitons and more effective charge extraction. Our results suggest that there is great potential for exceeding the efficiencies of binary solar cells through adding a third component, without sacrificing the simplicity of the fabrication process.