Study On Conducting Electrospun Fibers And Solution Processing Of Polymer Solar Cells

To realize the potential of polymer solar cells (PSCs), tremendous work has beendone in last two decades. Significant progress has been made in areas of molecularengineering of conjugated polymers, morphology control of active layers, interfacemodifications and transparent electrodes. State-of-the-art devices show powerconversion efficiencies (PCE) of over10%, which is considered the threshold tobreak through into commercial applications. Stability of PSCs becomes critical whenPCEs of devices and the physics mechanism are still important. Moreover, PSCsprocessed from solution is of great interest since they use techniques compatible withroll-to-roll large-area mass production. Materials for buffer layers and electrodesshould have chemical stability and solution processability. To this end, this paperfocuses on solution process of components of PSCs, including active layers, bufferlayers and electrodes. The study of cathode buffer layers and device physics are alsoconsidered. The paper content is as follows:1. Pristine electrospun fibers of poly [2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV) have been fabricated by electrospinning. Theelectrospinning of MEH-PPV in chloroform/isopropanol has superiority in devicefabrication because nonvolatile impurities are avoided. The key to improveelectrospinability is using a binary solvent system, in which a poor solvent wasintroduced to increase interchain interactions of MEH-PPV. At the same time, thepoor solvent also decreases surface tension and improves conductivity. Theultraviolet–visible (UV-Vis) spectra of pristine MEH-PPV show that increasedinterchain interactions are established when poor solvent is added. Polarizationanisotropy spectra of aligned pristine MEH-PPV were carried out. At emission peak,the emission anisotropy and polarization ratio show values of0.47and6.9,respectively, which are significantly higher than those of isotropic sample. Theanisotropy of aligned MEH-PPV electrospun fibers indicates that MEH-PPV polymerchain tends to align along the electrospun fiber axis. After incorporation of poly (ε-caprolactone)(PCL), MEH-PPV/PCL electrospun fibers show increased emissionanisotropy and polarization ratio of0.66and11. Since depolarization was depressedwith interchain energy transfer retarded. Diameters of pristine MEH-PPV electrospunfibers range from300nm to1100nm. And the device using MEH-PPV electrospinnigfiber as active layers has a low PCE. The low efficiency is from large thickness andthe non-continuity of electrospun films and uncontrolled phase separation. Theone-dimensional structure of conjugated polymers prepared by electrospinning ismore suitable for single fiber devices.2. Conducting networks for transparent electrodes are prepared by templates ofelectrospun polymer film. Electrospinning of PCL, Poly(vinylpolypyrrolidone)(PVP)and polyacryic acid(PAA) were carried out. Metals of Ag is evaporated onelectrospun films of Poly(vinylpolypyrrolidone)(PVP). Thereafter, polymer isremoved by solvent. The best Ag network has comparable conductance to ITO. Thetransmittance of such network is70%in the range of300nm~1100nm. The Agnetwork with optimized property meets the demand to be transparent electrodes. ThePCE is2.0%in P3HT:PC61BM devices using such Ag network, which is higher/lowerthan PCE of device using PH1000/ITO electrode. The large surface roughnessaccounts for the relative low PCE of device with Ag network. The thickness ofPH1000should be optimized to compensate the surface roughness of Ag network toreduce current leakage and to improve fill factors.3. A ferroelectric polymer of poly (vinylidenefluoride-hexafluoropropylene)P(VDF-HFP) with solution-processibility is used as electrode buffer layers forpolymer solar cells. P(VDF-HFP) can offer tunable dipole. It also has good filmformation, chemical stability and wide commercial availability. Measurements ofultraviolet photoelectron spectroscopy and work functions show that solventtreatment and P(VDF-HFP) introduce additional dipole on surface of thermalannealing P3HT:PC61BM films. As a result, improved cathode contacts are obtained.The device using P(VDF-HFP) has a higher PCE than device using LiF. P(VDF-HFP)was used as anode buffer layers of inverted structures, considering dipole tuningrealized by the response of P(VDF-HFP) to applied electric field. Such inverted devices had a low PCE in the beginning. The PCE increases gradually and thenbecomes comparable to PCE of MoO3evaporated device, with oxidation of Agelectrode and positive poling of P(VDF-HFP). The device using P(VDF-HFP) showsa superior stability to other devices. After120days, the device still has a PCE of2.4%, which is higher than MoO3evaporated device. To study the device physics,Integer Charge Transfer (ICT) model is used to understand improving electrodecontact with vacuum level shift realized by dipole tuning. Understanding the real roleof the P(VDF-HFP) in PSCs make it a promising and universal buffer layer viasimple regulation of dipole direction.4. We did some investigations on modification of cathode interface. For PSCswith inverted structures, plasma treatment of ZnO simultaneously improveopen-circuit voltage(Voc) and fill factor(FF), resulting a higher PCE. Plasmatreatment has the function of cleaning or creating oxygen-containing functional group.ZnO spin-coated with Poly(vinylidene fluoride)(PVDF) or P(VDF-HFP) make devicehave higher Voc and improved stability. Positive poling makes Voc increase further.Several materials were chosen as cathode buffer layers, including D-mannitol withhydroxyl, melamine with amino group and D-Serine with hydroxyl and carboxyl.NaCl, KBr, PVDF and P(VDF-HFP) were also used. D-mannitol shows an effect ofcathode buffer layers, and the device PCE is slightly lower than that of LiFevaporated device. NaCl, KBr, PVDF and P(VDF-HFP) make device showcomparable PCE to device using LiF. The incorporation of Ag plasmonic structuresnear cathode improve PCE of device by10%...