Design, Synthesis And Performance Of Organic And Polymeric Photovoltaic Materials

Four series of new organic and polymeric photovoltaic materials as photoactive layer, which are named as perylenetetracarboxylates, poly (phenylene ethynylene) alternating copolymer, poly (phenylene vinylene) alternating copolymer and low bandgap poly (thiophene methines), were designed and synthesized, and characterized by 1H NMR, IR spectroscopy and elemental analysis. Their thermal, UV-Vis absorption, photoluminescence (PL) and electrochemical properties were discussed. Photovoltaic devices based on the configuration of ITO/PEDOT-PSS/active layer/Ba/Al were fabricated. The photophysics of active layer, photovoltaic performance of these devices and the effect of device annealing on morphology of active layer were also discussed.Perylenetetracarboxylates (TMEP, TPrEP, TPeEP, THEP, TOEP, TDdEP, TCHEP, TBEP) have four electron-withdrawing ester groups suited to electron transport, approximately planar structure and are easily soluble in conventional organic solvents. Photovoltaic devices based on perylenetetracarboxylates acted as electron acceptors blended with poly (3-hexylthiophene)(P3HT) and MEH-PPV acted as electron donors, respectively, were fabricated. The energy conversion efficiency ( n e) of the device based on perylenetetracarboxylates and P3HT is 9.5 X 10~3% under AM 1.5 (78.2mW/cm2), which is above one order of magnitude higher compared to that of the pure P3HT device, and also enhanced by a factor of 6 compared with that of device based on perylene diimide blended with P3HT. After annealing at 180(under N2 atmosphere), devices based on perylenetetracarboxylates blended with MEH-PPV showed higher ne of 5.6X 10'3% and external quantum efficiency (EQE) of 0.5%, which is enhanced by a factor of 5 and 29 with regard to that of non-annealing devices, which due to forming an electron conducting perylenetertracarboxylates crystal network after annealing. This crystal network of perylenetetracarboxylates can make the charge to easily find a percolation path to the electrode and be collected. However, the PL intensity of perylenetetracarboxylates and MEH-PPV composite film was notAbstractcompletely quenched due to that perylenetetracarboxylates are high fluorescent dyes. In this case, the photoinduced charge transfer and separation are not effective and subsequently the lower n e for the devices.Poly (phenylene ethynylene) alternating copolymer containing electron donor and electron acceptor units were synthesized by cross-coupling polycondensation with Pd(PPh3)2Cl2 and phase transfer catalysts. The copolymer containing benzothiadiazole electron acceptor units is named as PPE-BT and containing 1,4-bis ( 3 -cyano-p-styryl)benzene as PPE-DCNTB. Due to effective charge transfer between the electron donor and electron acceptor segments within the copolymer main chain, a significant red-shifted absorption compared with poly (phenylene ethynylene) homopolymer was observed, which resulted in enhancement of optical harvesting efficiency in photovoltaic devices. PPE-BT and PPE-DCNTB exhibited higher oxidation potential (Eonox > 1.0V vs. Ag/AgCl) and reduction potential (Eonred > -0.8V vs. Ag/AgCl). The copolymers could be acted as electron donors when blended with PCBM (fullerene derivative), and also as electron acceptors when blended with MEH-PPV. Three photovoltaic devices were fabricated. Their configurations are ITO/ PEDOT-PSS/PPE-BT+PCBM(l/4,w/w)/Ba/Al,ITO/PEDOT-PSS/MEH-PPV+PPE-BT(l/l,w/w)/Ba/Al and ITO/PEDOT-PSS/MEH-PPV+PPE-DCNTB(l/l,w/w)/Ba/Al, respectively. All of devices show photovoltaic effect. Among them, device based on PPE-BT and PCBM shows the highest ne of 0.084% and EQE of 2.7% which is enhanced by a factor of 23 compared with that of the pure PPE-BT device. When PPE-BT and PPE-DCNTB blended with MEH-PPV donor, the ne is 3.67X 10'3% and 2.54X10"3%, respectively, which are lower due to that electrons mobility are inferior than holes within these copolymers and electrons would be trapped before reaching the electrode.In order to step further enhance the conjugating properties of the polymer, pol...