Molecular Design Of Novel Low Band Gap Polymers And Singlet-fission Molecules For Organic Photovoltaics

Organic photovoltaics are a type of solar cells, which use organic electronics toproduce electricity from sunlight by the photovoltaic effect. Compared to inorganicphotovoltaic cells, organic solar cells are particularly attractive because of their ease ofprocessing, mechanical flexibility, and potential for low cost fabrication of large areas.Although significant progress has been made of the power conversion efficiency, thereare many obstacles and issues in the application and commercialization of organicphotovoltaics. The most important challenge is still the efficiency of converting solarenergy into electrical power. To improve the efficiency of organic photovoltaics, it iscrucial to design and synthetize the excellent materials. The low band gap conjugatedpolymers, particularly donor-acceptor polymers, usually have good performances onthe charge transport characteristics of carries and absorption spectrum. The singletfission can allow a singlet excited molecule share its energy with a neighbor ground-state molecule to produce two triplet excited molecules, producing triplet yields as highas200%. Thus, low band gap conjugated polymers and small molecules with singletexciton fission are considered as admirable candidates to improve the power conversionefficiency. Now, theoretical calculation is playing an important role in researching therelationship between the structures and properties of materials and predicting novelphotovoltaic materials. In this thesis, we adopt Density Functional Theory and Time-dependent Density Functional Theory to design and investigate a series of novel lowband gap polymers and small molecules with efficient singlet fission. Firstly, we studythe relationship between structure and property of D-A conjugated polymers and aim toinvestigate the essential function of donor-acceptor structure. Secondly, we design andstudy some molecules, which have potential to possess singlet fission. We hope to understand the singlet fission deeply, and provide theoretical guidance for synthesisworkers. This research is summarized as follows:1. In chapter3, we systematically investigate a series of donor-acceptorcopolymers by using density functional theory. We consider thiophene rings as donor,with9different acceptors linked to the donor, aiming to provide compelling insightsinto the strategy for fine-tuning of the electronic properties of donor-acceptorconjugated copolymers. In order to gain a fundamental understanding, the HOMO,LUMO, band gap, UV-vis spectrum, reorganization energy and excited states arecalculated. The results indicate that the acceptor units introduced present a large tuningcapacity in frontier orbital energies, which focus on effecting the lowest unoccupiedmolecular orbit. It also turns out that the Se atom is good for low band gap copolymers.2. In chapter4, we employ the spin-projected unrestricted Hartree-Fock method(PUHF) to evaluate the multiple diradical characters yi. In this section, a series ofheteroatom-doping π-conjugated acenes which contains N, O, S and Se atoms arestudied. The results turn out that such heteroacenes have good diradical characterswithout obvious tetraradical character (y1/y0>0.2and0.2<y0<0.6). The excitationenergies of S1, T1and T2state are calculated by means of the time-dependent densityfunctional theory method with the Tamm Dancoff approximation. On the basis of thediradical character properties and energy level matching conditions, E(S1)≥2E(T1) andE(T2)≥2E(T1), our calculated results demonstrate that heteroacenes are possiblecandidates for energetically efficient singlet fission. The chalcogen-containingheteroacenes are better than N-doped acenes as singlet-fission materials.3. In chapter5, a family of polycyclic aromatic hydrocarbons (PAH) areinvestigated due to their promising applications in materials science and molecularelectronics. It refers to the zethrene molecules, including zethrene (Ze), heptazethrene(HZe), octazethrene (OZe). The family of Z-shaped polycyclic hydrocarbons can beenseemed as two phenalenes are head-to-head fused together with or without a benzenoidspacer. These molecular subunits are key syntons to develop Kekulébiradicals. Theopen-shell character has been proved to helpful to two-photon absorption and tends tobe short-lived and instability. In this section, we systematically investigate the biradical character and physical properties of zethrene derivatives bearing donor/acceptorsubstituents at7,14-positions. The results reveal the7,14-substituted zethrenederivatives are more stable and promising to apply in singlet fission. Our research alsoindicates the heptazethrene with electron-withdrawing substituents are excellentmaterials with singlet exciton fission.