Theoretical Design And Investigation Of Anisotrpic&Isotropic Compounds For Photovoltaic Application

Organic solar cells (OSCs) of tremendous potential in the world’s energy strategy havebecome the research interest of scientific community. However, the power conversionefficiencies of small molecules based OSCs (SM-OSCs) have been largely limited due to themismatch between the absorption spectra of most organic small molecules and solar emissionspectrum. To solve this problem, on the basis of high mobility building blocks, we havesystematically investigated the effect of different molecular topologies, compositions, andconfigurations on the photo-physical properties planar/three-dimensional (3D) multi-branchedmolecules by using computational approaches. Molecular anisotropy/isotropy was found toimpart great impact on the molecular photo-physical properties. Besides, promisingcandidates were screened out, which were further modified under the concept of wavelengthtailoring for the design of donor materials for SM–OSCs with high performance. Thereforeour work provides a clue to the design of full-color absorption and high performanceSM-OSC donor materials allowing experimentalists to choose materials based on thepredicated properties. The main results are summarized as follows:A set of benzo[1,2,-b:4,5-b′]dithiophene (BDT) based anisotropic/isotropic branchedconjugated molecules with various topologies (Linear-, V-, T-, and X-shape) have been firstlydesigned and investigated to screen out proper building blocks for the design of organic solarcell (OSC) donor materials. With the revelation of how the interplay of various factorsincluding topology, spacer, and molecular anisotropy/isotropy results in the broad and strongfull-colour absorptions in multi-branched compounds for the first time, a clue for designinghigh performance small molecular donor materials based on BDT as the branching centre,versus specific acceptor materials, such as fullerene derivatives, was proposed as thecombination of anisotropic multi-branched branches and triple-bond spacers on benzene ringof BDT. The anisotropic molecular skeleton favours the broad absorptions. The symmetricbranching positions (the two substitution positions on benzene or thiophene rings) of BDTshould be simultaneously substituted with near-degenerated Bs, where two types of chemicalpotential distributions of BDT and Bs are effective in achieving the broad and strongabsorptions: Bs on benzene> BDT> Bs on thiophene; Bs on thiophene> BDT> Bs onbenzene. This clue is further confirmed by the design of e3-X (on the basis of modifications inthe molecular anisotropy of selected X shape building blocks), which exhibits comparablelow-lying HOMO, narrow energy gap, broad and intense absorptions, and anisotropic highcharge carrier mobilities in its crystalline phase associated with low reorganization energies.Furthermore, good performance might be achieved for a solar cell from e3-X: PCBM withthe maximum open circuit predicted at1.64V theoretically.A strategy used to design novel high performance donor molecules for organic solar cellshas been proposed as using double overlapping wave bands for broad and intensiveabsorption based on three types of high mobility building blocks, i.e., the multibranchingelectron-rich fragment (DF), the different electron-excessive π-bridges (BF), and the proper electron-deficient fragment (AF). The multibranched π-conjugated DF-(BFi)ndonor fragmentprovides the strong and broad short and middle wavelength π-π*absorption, while theanisotropic multibranched intramolecular charge transfer between the DF-(BFi)ndonorfragment and AFs favours for the strong and broad middle and long wavelength absorption.This concept is confirmed by the theoretical design of the planar X-shaped(AF-BF1)2-DF-(BF2-AF)2conjugated donor molecules constructed bybenzo[1,2,-b:4,5-b′]dithiophene as the DF, bithiophene (BF1) and ethynyl-bithiophene (BF2)as anisotropic multiple BFs,, and cyano substituted thiadiazolo[3,4-c]pyridine as AFs versusproper perylene diimide derivatives, as the acceptor material. Our theoretical results obtainedwith the DFT and TD-DFT approaches for the electronic and spectroscopic properties as wellas the reorganization energies reveal that the designed molecules are highly promisingcandidates toward the high performance solar cell materials (i.e., exhibiting a strong andbroad spectroscopic absorption, high charge carrier mobility, and possessing a narrow energygap as well as appropriate FMO energy levels versus specific acceptors)With the aim to investigate the effect of molecular anisotropy on the absorption propertiesof3D multi-branched compounds, we have designed and investigated series of star-shapedcompounds comprising N, Si as core, oligo-thiophene as branches. Using PBE0/6-31G(d) andTD-PBE0/6-31G(d) methods，we found that the electronic transitions from branches to Si areresponsible for the strong absorptions in the long wavelength, while the electron transitionswithin the Bs are for the absorptions in short wavelength. Besides, compounds with branchesof different length tend to have comparably broad and strong absorptions. We thus proposedthat target3D multi-branched compounds based SM-OSC donor materials should possessanisotropic branches and electron-donating atom as core.To obtain3D multi-branched compounds with suitable electronic energy levels and strongand broad absorption, we proposed to construct (AF-BF1)1-X-(BF2-AF)2type multi-branchedcompounds, where X is N/C/Si, BFis the conjugated bridges, and AFis the electron-acceptingfragment. This concept is to utilize the anisotropic molecular platform accounting for thebroad absorption spectrum shapes while further red-shift the absorption by the introduction ofintra-molecular charge transfer between the central (BF1)1-X-(BF2)2and AF. We take the N,oligo-thiophene, and N cyano substituted benzo[c][1,2,5]thiadiazole as primary buildingblocks, to validate the reliability of our design concept. Our results confirm that theas-designed multi-branched compounds possess broad absorptions, high-charge carriermoblities, and suitable electronic energy levels verse typical acceptor, PBCM, which promisesit as an potential organic solar cell donor materials.