The Study Of Organic Semiconductor Optoelectronic Devices-Bulk-heterojunction Solar Cell And Photo-detector

Polymer bulk-heterojunction photovoltaic devices, such as solar cells (PSCs) andpolymer photo detectors (PPDs), are a new gengeration of semiconductor devices, whichpossess multiple advantages such as flexibility, light weight, low cost, easiness of fabrication,so on and so forth. But the poor performance of the polymer photovoltaic devices preventstheir commercialization. In this work, we investigated and identified the influence of certainfactors on the performance of PSCs and PPDs. We show that the devices' performance can bepotentially improved by addressing these factors. The work was divided into five partsmainly.In the first part, we found that the performance of polymer solar cells based on newpolysilafluorene derivative PSiF-BBT blended with PC61BM is doubled when usingchloroform, instead of chlorobenzene, as the processing solvent. Through a seriescomparative studies on the external quantum efficiencies and carrier mobilities of the devices,as well as the optical absorption and nano-scale morphology of the active layers, we foundthat the absorption efficiency enhancement of the polymer phases in the chloroform-processedblended film is responsible for such improvement. The polymer phase is fiber-like with sizeof²0nm, and is homogeneously distributed in the chloroform-processed film in TEM data.This result showed the solvent effect has great influence on this polysilafluorene-derivativesystem, which dramatically affects the short circle current due to the changes of the opticalabsorption of the film. As polysilafluorene derivative is a potential electron donor material,this discovery can be of great important for the optimization of such devices.In second part,we doped an amino functionalized fullerence derivative (PC61PN) intoP3HT: PC61BM solar cells,in order to study the reason for its low performance. We obtainedand analyzed results of the photovoltaic performances, holes and electrons transportcharacters and light-intensity-dependent current-voltage characteristics. We attribute the lowperformance is due to the significant decrease in the hole mobility, and the existence ofhigh-order recombination. Moreover, we found that the current in hole-only devices showedcharacteristics of space charge limited current with exponential-distribution hole traps. Thiswork shows the influence of polar group—amino group to devices, and provide someimportant inspiration to the synthesis of water/achohol soluble materials and the optimizationof devices.In the third part, we realized for the first time an inverted visible-near infraredphoto-detector by using ITO/Interlayer modified layer/Active layer/MoO₃/Al, whose spectral response extends to1150nm. The active layer consists of a narrow bandgap polymer NIR-2Ablended with PC61BM, and new water-soluble conjugated polymers―PF6NSO andPF6NO25Py, were used for ITO modification layers. The resultant detectivities and lineardynamic range of these two kinds of inverted structure photo-detectors are not good, whichmay be due to the low external quantum efficiency (EQE) and high dark current. Also, wefabricated traditionally-structured PPD, with aluminum, calcium/aluminum, polyelectrolyte/aluminum and methol-treated aluminum as cathodes. In the working bias range, the traditionalaluminum devices have similar dark current to the inverted devices. But the inverted deviceshave higher EQE. Other traditional devices have high rectification ratio up to¹04.5, which isconsiderably better than inverted devices. This result show the by depressing the dark current,it is possible to obtain high-performance polymer photodetectors.In the fourth part, oleic acid,1,8-diiodooctane and1,8-octanedithiol were used asadditives in the processing solvent of material PSiF-DTA: PC71BM. The fill factor and powerconversion efficiency were improved by controlling the morphology of active layer. Throughcomparing the dark current, we found this owes to the decrease of the open circuit voltage.And small red-shift of absorption spectra from the controlled film to the film processed withadditives was observed. The results indicated the molecular distance is shortened, and as aresult, the interaction becomes stronger. Moreover, the light-dependent current-voltagecharacters showed first-order recombination at the short-circuit-current condition. On theother hand, we found the build-in potentials were proportional to the light intensity, which isdifferent from other linear type polymer donor material systems. We deduced that there wasanother Voc influence factor which was related to light intensity in2D polymer materialsystems.2D polymer donor is a potential photovoltaic material, but hindered by the low FF,performance of such devices is below expectation. Our experiments showed additives canobviously improve FF.In the fifth part, we applied a cationic polyelectrolyte PFPSO₃Na (poly (9,9-bis(4'-sulfonatobutyl) fluorene-alt-co-1,4-phenylene) as an interlayer between the active layerand cathode, with PFO-M3: PC61BM and PFO-DBT: PC61BM as controlled devices. Theresultant performances of these two devices are both improved. The former gained a higherfill factor, and the PCE increased to2.34%from2.08%; while the latter gained larger shortcircuit current and higher open circuit voltage, PCE increased to1.98%from1.53%. We alsofound that Li2CO₃doping has no effects on the devices. The results indicated that the role ofpolyelectrolyte in solar cells is different from that in the organic light emitting diodes.