Study On Fabrication Of The Novel Organic Polymer Solar Cells

Bulk heterojunction polymer solar cells （BHJ-PSCs）, which are promising in terms oftheir potential advantages of low cost, light weight, good mechanical flexibility, andsimplicity of manufacture, have attracted considerable research interest in recent years. BHJsolar cells are prepared from a p （or n）-type of organic semiconductor and an electronacceptor （or donor）. Among a variety of photoactive donor–acceptor composites, the blend ofpoly（3-hexylthiophene）（P3HT） and [6,6]-phenyl-C61-butyric acid methyl ester （PCBM） hasbeen extensively investigated since cells based on such a composite have a reproduciblepower conversion efficiency （PCE） of3–5%. Although the maximum PCE of PSCs increasedsignificantly during the past years, which have the potential to compete effectively withalternative solar cell technologies, it is still an urgent issue to explore new types ofphotovoltaic materials.Here, we designed and synthesized two novel conjugated polymers based on EDTT andEDOT with BDT or isoindigo, respectively, and explored their thermal, optical,electrochemical, and photovoltaic properties. The main results are as follows:To begin with, PBDTEDOT and PBDTEDTT were synthesized by Stille cross-couplingreactions in presence of Pd2（dba）3as catalyst and P（o-tol）3as ligand. TGA indicates that the5%weight loss of the PBDTEDOT and PBDTEDTT took place at321°C and331°C,respectively. In dilute chloroform solutions, the absorption peaks of PBDTEDOT andPBDTEDTT locate at502nm,494nm, respectively. In films, the peak of absorption ofPBDTEDOT and is at about524nm, followed by PBDTEDTT at537nm. The UV-visabsorption spectra of PBDTEDTT in films are broader and red-shifted relative to thosemeasured in solution. This behavior is due to the enhanced intermolecular interactionsbetween the polymer chains and the planarization effect of the π-conjugated polymerbackbone, which enable the polymer chains to self-assemble into a well-ordered nanostructurein the solid state. The HOMO energy levels of PBDTEDOT and PBDTEDTT are-4.98eV,-5.18eV, respectively, whereas the LUMO energy levels are-2.86eV,-3.23eV, respectively.The traditional solar cell devices of the PBDTEDOT were fabricated and the maximum PCEof0.19%was obtained. The PCE of2.4%for PBDTEDTT polymer was obtained with aopen-circuit voltage of0.69V, short circuit current density of6.62mA cm-2, and fill factor of 53%, when the device was made at a donor/acceptor weight ratio of2:1in CB at aconcentration of10mg mL-1spun cast at1000rpm and annealed at120oC before LiF/Aldeposition. The PCE of inverted cells were lower compared to those conventional ones. Thesurfaces of PBDTEDOT:PC60BM （1:1） and PBDTEDTT:PC60BM （2:1） composite thin filmsare very smooth and their root-mean-square （rms） of roughness is about0.6nm. The numberaverage molecular weights （Mn） of PBDTEDTT was22kDa （PDI2.1）. The typical FT-IRspectra of PBDTEDTT showed characteristic bands associated with C-O at around1047cm-1.The conventional cells were fabricated, well encapsulated and put in the air to estimate thestability of the device. J_sc and PCE show around17%decrease after480h of ageing in the air.However, Voc and FF undergo negligible changes （less than4%decrease）.In addition, P（iI-EDOT） and P（iI-EDTT） were obtained through the palladium-catalyzedSuzuki coupling method in presence of Pd2（dba）3as catalyst, P（o-tol）3as ligandand Aliquat336as phase transfer catalyst. TGA indicates that the5%weight loss of the P（iI-EDOT） tookplace at385°C whereas the polymer of P（iI-EDTT） started degradation at temperatures above351°C. The absorption peaks of P（iI-EDOT） locate at696nm,697nm, respectively in dilutechloroform solutions and films whereas the peaks of absorption of P（iI-EDTT） are at about612nm in dilute chloroform solutions, followed by626nm in films. The HOMO energylevels and LUMO energy levels of P（iI-EDOT） are-4.97eV,-3.48eV, respectively and thecorresponding energy levels to P（iI-EDTT） are-5.28eV,-3.90eV, respectively. Thephotovoltaic performance test showed that the maximum PCE of P（iI-EDOT） and P（iI-EDTT）were0.35%and0.30%. The atomic force microscopy （AFM） images of P（iI-EDOT）:PC60BMand P（iI-EDTT）:PC60BM blending films are tested and their root-mean-square （rms） ofroughness is less than0.6nm. The number average molecular weights （Mn） of P（iI-EDOT）and P（iI-EDTT） were20kDa （PDI2.2） and19kDa （PDI2.3）, respectively. The characteristicabsorption peak of the C=O stretching in the P（iI-EDOT） and P（iI-EDTT） were exhibited ataround1693cm-1. The stability of the devices was also evaluated using conventional cellarchitecture. When these devices were well encapsulated, they showed a good stability.