Synthesis And Photovoltaic Properties Of D-A Type Small Molecules For Solution-processed Solar Cells

With the increasing consumption of traditional energy, such as petroleum and coal,Search for new and renewable energy has become a worldwide issue. Solar energyhas attracted much attention due to its non-polluting and inexhaustible. In the past fewdecades, organic solar cell (OSCs) has attracted more and more attention because ofits simple preparation process, inexpensive, light weight and flexible etc. Based on theactive layer, bulk heterojunction (BHJ) organic solar cells can be divided into thepolymer BHJ solar cells and small molecule BHJ solar cells. Solution-processed solarcells based on π-conjugated polymer and fullerene derivatives have reached the powerconversion efficiency (PCE) of over9%, and the PCE of solution-processed OSCsbased on D-A low band-gap small molecules have already exceeded8%. Compared topolymer solar cells, small molecule organic solar cells have some advantages:convenient synthesis, good reproducibility, defined structure, facile characterizationand purification.Based on the donor-acceptor (D-A) designing concept, a series of small organicmolecules are synthesized and as donor materials for solution-processed BHJ organicsolar cells in this thesis.In the second chapter, thienoisoindigo is introduced into the small moleculesstructure. The thienoisoindigo was thienopyrrolone units by replacing the outerphenyl rings of isoindigo with thiophenes. The thiophene ring shows stronger electrondonating properties than benzene ring, the absorption of TIDO2T in film was broadenup to871nm. The test results indicate that the absorption spectrum ofthienoisoindigo-based small molecule can be extended to the near infrared （NIR） incomparison with isoindigo. The optical band-gap of TIDO2T is1.42eV. Thecorresponding HOMO energy level is-4.87eV, the LUMO level of TIDO2T is-3.45eV. The TIDO2T based organic solar cell was fabricated by spin-coating method. Thedevices show a power conversion efficiency of1.14%with an open-circuit voltage(Voc) of0.44V, short circuit current density (Jsc) of7.76mA cm-2and fill factor (FF)of33%. To the best of our knowledge, this is the first report about thienoisoindigobased small molecules for solution-processed OSCs.In the third chapter, a solution-processed small molecule DTTz-DTBTT, using a thiazolothiazole (Tz) unit as the core, has been synthesized though the Stille reactionand employed as electron donors for small molecule OSCs. The small moleculeexhibits good thermal stability, excellent solubility and film forming properties. Theoptical band-gap of DTTz-DTBTT is1.65eV. The corresponding HOMO energylevel is-5.05eV, the LUMO level is-3.07eV. A PCE of1.64%is obtained with a Vocof0.66V, Jscof5.11mA/cm2and FF of48%. The results indicate that the moleculebased on two different acceptors is an effective strategy for the design of smallmolecules donor materials.In the fourth chapter, the bandgap of the small molecule is reduced through theintroduction of two-dimensional benzodithiophene (BDT) units, and this is beneficialfor a high short-circuit current and open circuit voltage. Introduction of a strongelectron-withdrawing octyl cyanoacetate and BTA unit, the HOMO and LUMO levelsof the molecules are effectively regulated, and the energy levels of the small moleculeand PC61BM match better. The corresponding HOMO and LUMO energy level are-5.13eV and-3.31eV. Consequently, using the simple solution spin-coating fabricationprocess, OSCs based on D(CATBTzT)BDT:PC61BM exhibit PCE of3.61%with a Vocof0.93V which is consistent with the literature. The solar cells also can show a highFF up to72%.In the fifth chapter, two small molecules DRTBTTBDT （1D） andDRTBTTBDTT （2D） based on BDT and benzothiadiazole (BT) have beensynthesized through palladium-catalyzed Stille coupling reactions. The optical bandgaps of DRTBTTBDT and DRTBTTBDTT are estimated to be1.80eV and1.81eV,respectively. The corresponding HOMO energy level of DRTBTTBDT andDRTBTTBDTT are-5.09eV and-5.13eV. The LUMO level of DRTBTTBDT andDRTBTTBDTT are found to be-3.28eV and-3.33eV. The HOMO and LUMOenergy levels of small molecules are effectively tuned by introducing of2D BDTunits and a relatively high Vocis also obtained. By fine-tuning of the material structure,a current or voltage can be improved. The photovoltaic properties of DRTBTTBDTTare investigated in typical BHJ devices using PC61BM as acceptor. A PCE of1.01%isobtained with a Vocof0.70V, Jscof4.15mA/cm2and FF of35%. A largest mobilityvalue of0.004and0.016cm2/Vs have been obtained with100°C annealing.2Dsmall molecules exhibit a relatively high mobility.