Rational Design Of Highly Efficient Sulfur Containing Small Molecular Donor Materials For Organic Photovoltaic Cells

Harvesting sunlight energy is regarded as being one of the most important ways to dealgrowing global energy demands using photovoltaic technology. Organic photovoltaics (OPV)is a field of applied research which has been growing speedily in the last decade leading to acurrent record value of power-conversion efciency (PCE) of10%. One major motive forthis boom is a potentially low-cost production of solar modules on flexible polymer substrate.However, the PCE of OPVS based on the small molecular donors is comparably smaller thanthose based on polymer. The lower PCE is confined because of the poor mismatch betweenthe absorption spectra of the small organic molecules and that of sun which in turn causesenergy lose. To this end, the research activities include the designing and investigation ofsmall branched or star shape donor molecules based on the different molecular strategies,configurations, topologies, donor-acceptor ratios and compositions by using theoreticaldensity functional theory (DFT) approaches. The aim of the research was to design optimalorganic donor small molecules with improved parameters like appropriate energy levels withrespect to the acceptor molecules, strong visible light absorption and sufficient high chargecarrier mobility, which can be used as potential candidates for small molecular OPV withhigher PCE.The first part of thesis focuses to screen out proper building blocks for the design oforganic solar cell (OSC) donor materials. Hence, Varying ratio of D/A fragments, topologiesand their effects on the optical and electronic properties of a series of conjugated donormolecules (DmAnwhere m=1-4and n=1-7while D=benzodithiophene and A=benzooxadiazole) are explored for OPV applications. An increase in the ratio of acceptorfragments lowers the LUMO energy level and narrows the Eg for the designed molecules. Inthe case of molecules containing more vertically bonded acceptor fragments with donorfragment, a significant red shift in the absorption wavelength of the final donor molecules isobserved as compared to linearly bonded ones. While, the linear binding side of donorfragments assisted considerably intensify the absorption bands. D-A-D topology moleculesexhibit more significant optical and electronic characteristics than those of D-D topology. Inaddition, D-A-D topology shows prominent affect over the factor of having more verticalbonded fragments in terms of opto-electro properties. All donor molecules (m=2-4) of D-A-Dtype exhibit lower ?hthan those of1donor containing (DAn) molecules. Nevertheless, D-Dtype molecules show only lower ?ethan DAnmolecules due to the presence of second donorfragment. Furthermore, charge transfer phenomenon is also shape dependent, branched or anisotropic X, H, ?, n, and square shapes display higher charge transfer rate thancorresponding linear isomers because of having better dimensionality. Finally, the designeddonor molecules and matched acceptor molecules hold immense potential to construct solarcells devices.Secondly, employing a double overlapping wave band strategy based on DFT, five X-shape anisotropic low energy gap donor compounds (D1-D5) have been designed for solarcells applications possessing good solubility. A series of new PDIs acceptor molecules arebuilt to match each designed donor in terms of frontier molecular orbital energy levels bytuning substituents at P1and P2positions of PDI1. The designed donors consist of centralelectron donor fragment benzodithiophene (DF), electron accepting fragments (A1to A5) andterfuran and ethynyl-terfuran bridges (B1and B2respectively). The absorption bands of thedesigned donors based on TD-DFT not only cover the visible region but also extend toinfrared region of spectrum. The multibranched ?-conjugated B12-DF-B22donor fragmentprovides the strong and broad short wavelength ?�?*absorption while the anisotropicmultibranched intramolecular charge transfer (ICT) between the B12-DF-B22donor and Afavors fragment the strong and broad middle and long wavelength absorption. In addition,PDIs also exhibit complementary absorptions in the visible range of solar spectrum. Amongdesigned donors, D1exhibits ideal lowest band gap, FMO energy levels and exclusivebroadest absorption because of the strongest electron withdrawing fragment. The lower ?evalues as compared to ?hillustrate that these five donors would be favorable for electrontransfer. The carrier mobility of D1in the crystalline state has been predicted using P21/cspace group. D1displays higher carrier mobilities for ?e=2.00cm2/V.s and ?h=1.7�10-2cm2/V.s. The calculated Voc of D1is1.02V. The designed donors and PDIs acceptors aresuitable and recommended for high performance solar cell devices.Thirdly, with the aim to investigate further the effect of molecular anisotropy on theabsorption properties of3D multi-branched compounds, we have designed and investigatedconjugated three and four arms star shape molecules with bithiophene as donor (DF)fragment connected to pyridine-thiadiazole as acceptor fragment (AF) via ethyne as ?-spacer(Ps) in the arms linking with different central core atoms. The central cores in three and fourarms molecules are N, B and C, Si respectively. A combination of density functional theory(DFT) and Time-dependent-DFT (TD-DFT) approaches is applied to understand the effect ofdifferent central cores on the optical, electronic and charge transport properties in twodifferent topologies i.e.�core-D-?-A� and �core-A-?-D�. HOMO energy levels of the �core- D-?-A� type molecules (N3-Mol, B3-Mol, Si4-Mol and C4-Mol) are more ideal related tothose of �core-A-?-D� type molecules (N3-RMol, B3-RMol, Si4-RMol and C4-RMol). The?maxvalues of �core-A-?-D� type molecules are significantly red shifted than those of �core-D-?-A� type molecules. Three arms N3-Mol and N3-RMol display the largest ?maxamong therespective designed molecules. Interestingly, B3-RMol and C4-RMol show more a less same?maxwavelength. However, in case of three and four arms molecules absorption bands of Band Si cores containing molecules are of strong intensity as compared to those containing Nand C cores respectively. The molecules C4-RMol, Si4-RMol, B3-RMol and N3-RMol showa red shift of59,14,28and39nm than ?maxof C4-Mol, Si4-Mol, B3-Mol and N3-Mol. Bothreorganization energy and mobility results reveal that four arms molecules are better chargetransport materials than three arms molecules because of better dimensionality. Thus, opticalelectronic and charge transport properties analysis confirms that these designed three and fourarms molecules can act as promising donor materials for organic solar cells. We believe thatthe present results can also provide a potential information to develop small molecular donorsfor highly efficient organic photovoltaics.