Study On Pefrormance Improvement Of Small Molecular Organic Photovoltaic

Energy plays a vital role on the world economy and the whole humankind, theuse of safe, clean, widely distributed and inexhaustible solar energy is an importantmethod for the human survival and sustainable development. In1986, C. W. Tangfrom Kodak firstly introduced the donor-acceptor heterojunction concept to getorganic photovoltaic cell（OPV）. OPV with the advantages of thin, light, flexible, largearea and low-cost preparation, has gained the extensive attention of researchers andcompanies. To early2012, the German the Heliatek company reported a stackedsmall-molecule organic photovoltaic cell with the conversion efficiency up to9.8%.To further improve the photoelectric conversion efficiency of OPV is extremelynecessary for which can be used on a large scale. The series resistance of OPV is animportant reason which reduces the conversion efficiency. In this article,4,4’,4"-tris{N-（3-methylphenyl）-N-phenylamin}triphenylamine（m-MTDATA） dopedMoO_x was used as the hole transport layer（HTL） of organic solar cells to reduce theseries resistance, then, to improve its photoelectric conversion efficiency. In theexperiment, we prepared the device with the structure ofITO/m-MTDATA:MoO_x（3:1）（30nm）/CuPc（20nm）/C60（40nm）/Bphen（8nm）/LiF（0.8nm）/Al（100nm）, which showed the parameters Voc=0.40V, J_sc=6.59mA/cm²andFF=0.55, the photoelectric conversion efficiency was1.46%, under AM1.5100mW/cm²solar illumination. The efficiency of the device was improved by38%compared with the device without hole transport layerITO/CuPc（20nm）/C₆0（40nm）/Bphen（8nm）/LiF（0.8nm）/Al（100nm）. The improvementof the device performance can be attributed to the addition of m-MTDATA:MoO_x（3:1）（30nm） hole transport layer reduced the contact resistancebetween the ITO electrode and the organic layer, thus reducing the overall deviceseries resistance and improved the efficiency of the device.During the experiment, we found that the device with the undoped m-MTDATAused as HTL device showed the S-shape J-V characteristics curve, which meansthere was hole accumulation at the m-MTDATA/CuPc interface. the charge transfercomplex generated in MoO_x doped m-MTDATA improved the hole mobility in them-MTDATA, the hole mobility of HTL and CuPc layer matched, eliminating the holeaccumulation at the CuPc and HTL interface. The doped HTL maked the holestransport through the hole transport layer better, and it is easier for the ITO anode tocollect the holes, and improve the efficiency of the device. We also studied theinfluence on device performance with different MoO_xdoping concentration, weprepared a series of solar cell devices with different MoO_xdoping concentration HTL,the ratio of m-MTDATA:MoO_xis1:1,2:1,3:1, and5:1, respectively. The experimentproved that the m-MTDATA:MoO_xwith the doping concentration ratio of3:1devicehas the best performance.After the work above, we used TPTPA/C70which got a better carrier transportproperties and optical absorption spectra to fabricate organic solar cell devices.Device with the structure of ITO/m-MTDATA: MoO_x（3:1）（30nm）/TPTPA （30nm）/C70（50nm）/Bphen （8nm）/LiF （0.8） nm/Al showed open-circuit voltageVoc=0.82V, short-circuit current Jsc=7.61mA/cm²device fill factor FF=0.49underAM1.5100mW/cm²solar illumination, the photoelectric conversion efficiency of thedevice reached3.06%, which is a high photoelectric conversion efficiency.In this paper, we also used the method of phosphorescence sensitization to solvethe problem that the active layer in organic solar cell devices has a short excitondiffusion length which is not enough for the light to be absorbed efficiently. Theauthor used NPB（20nm）, NPB:10wt%Ir（ppy）3（5nm）/NPB（15nm）,NPB:10wt%Ir（ppy）3（10nm）/NPB（10nm）, NPB:10wt%Ir（ppy）3（15nm）/NPB（5nm）,NPB:10wt%Ir（ppy）3（20nm） as the donor part of the OPV. The device with Ir（ppy）3doped NPB showed an increase on photoelectic current, with the thickness of Ir（ppy）3 doped NPB increasing. we think that Ir（ppy）3phosphorescence sensitized thephotoinduced singlet excitons, which were converted to triplet excitons, so that moreexciton can reach the interface of the heterojunction and be separated into the freecarriers, the experiment showed that phosphorescence sensitization is an effectivemethod to incease the efficiency of OPV.