Novel Active-layer Materials For Organic Photovoltaics And Thin-film Morphology Study

Organic semiconductors show tremendous interests from academic and industrial communities due to their potentials to be integrated into electronic devices with mechanical properties through low-cost solution processing.One of such devices is organic photovoltaic(OPV),which recently has achieved great progress with power conversion efficiency exceeding 17%.Despite these assets,the developments of device efficiencies are still highly dependent on the material evolutions for photoactive layers.Moreover,the relationship between film morphology and device performance based on the new materials is also urgent to be understood in order to pave the way for the application of OPVs in the near future.This dissertation focuses on two sections: i)synthesis of novel fluorinated benzothiadiazole based small molecules and the study of their film morphology and photovoltaic performance;ii)thin-film morphology manipulation and mechanism study based on novel polymer blending for all-polymer solar cells.Specifically,the dissertation contains four chapters as follows.In chapter 2,we synthesized a series of fluorinated benzothiadiazole based acceptor-donor-acceptor type small molecules.The incorporation of fluorines shows great impacts on the torsion of ending groups,electron transitions under excited state,absorption and energy levels of the resulting molecules.Using the fluorinated molecule as the third component,a ternary blend film of is fabricated,showing complementary absorption and cascade energy level alignment.Tuning the fraction of the molecule in ternary blend can manipulate the film phase separation and the solar cell efficiency.In comparison with the isomeric molecules,the regiochemistry of fluorine introduction can alter the molecular geometries and electrostatic potential.Processed with 1,8-diiodooctane(DIO),the molecules show substantially distinct in absorption changes.Photoactive layers are fabricated using poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-4,8-di(thiophe n-2-yl)-2,6-dioctyl-5H-pyrrolo[3,4-f]benzotriazole-5,7(6H)-dione(PTz BI)as the donor and the molecules as the acceptors for OPV devices.The strong molecular crystallization tendency induced by regiochemistry leads to the large-scale phase separation in blending,which is deleterious to the photovoltaic properties.Such results indicate that control over the component compatibility inside blending is basically dependent on the crystallization and aggregation of the materials.Thus,the correlation between the crystallization and the compatibility of the materials is established.In chapter 3,we constructed a ternary all-polymer blend based on poly[4,8-bis(5-((4-ethyloctyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-4,7-di(thi ophen-2-yl)-5,6-difluoro-2-(6-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)hexyl)-2H-benzo[d][1,2,3]triazole]:poly[2,6’-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene-alt-4,6’-3-fl uoro-2-((2-ethylhexyl)carboxylate)thieno[3,4-b]thiophene]: poly[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl-alt-5,5’-(2,2’-bithiophene)](SC3:PCE10:N2200)for all-polymer solar cells.The addition of PCE10 can effectively adjust the film absorption and the photovoltaic properties.With multiple morphology characterizations,PCE10 is found to curtail the polymer aggregations and the nuclei formations,which ultimately determine the structural ordering and phase separation in the blending.In ternary blend,N2200 crystallite fibers inter-dispersing across the polymer mixing matrix with characteristic d-spacing assist in improving the solar cell efficiency.These results highlight how a polymer compatibilizer impacts the chain aggregations and crystallite growth in polymer mixing,and correlate the crystallization dynamics of polymer mixtures during film drying with the morphology in solid state.In chapter 4,PTz BI:N2200 blends are fabricated by spin-coating and slot-die printing for all-polymer solar cells,respectively.The printed device shows similar efficiency to the spin-coated device.With optimized by adding solvent additive DIO during printing,efficiency of printed active layer is further improved.Real-time grazing-incidence wide-angle X-ray scattering(GIWAXS)demonstrates the addition of DIO would not only postpone the polymer nuclei formation and growth,but also swell the polymer aggregations,which ultimately promote the polymer crystallization.GIWAXS,transmission electron microscopy(TEM),and resonant soft X-ray scattering(RSo XS)indicate printing with DIO would show multi-length phase separation with N2200 crystalline fibers in-between the polymer mixing domains.Such morphology suppresses the trap-assisted charge recombination and renders the transport balance.Thus,the correlation of the solvent additive with the polymer crystallization dynamic from solution to solid state as well as the final film morphology is determined.In chapter 5,three solvents,chlorobenzene,chloroform,and carbondisulfide,are utilized for solvent vapor annealing(SVA)applied to PTz BI:N2200 blending.The SVA can trigger the N2200 crystallite fiber growth and the transformation of local molecular orientation is revealed.With multi-dimensional correlation analysis for the morphological and device parameters,the local orientation of the naphthalenediimide(NDI)sub-fragments in N2200 conjugated backbone shows highly correlated with device short-circuit current density.This might be attributed to the weakened Coulomb bonding force of excitons at the donor/acceptor(D/A)interface when the NDIs are edge-on to the PTz BI,and consequently,the free-carrier charge generation and device current are fully facilitated.These results exemplify how the in-chain subunit orientation in conjugated polymers influence the resulting photovoltaic parameters,and finally establish the relationship between the molecular orientations at the polymer donor/acceptor interface and the solar cell performances.