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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT(a) Annealing effects on theabsorbance of P3HT/PCBMblended layers on glass substrate(temperatures from 125°C to 145°Cfor 5 to 15 minutes duration). (b)Average absorbance compared fordifferent annealing temperaturesand duration. The optimumannealing conditions were foundas 125°C for 15 minutes for the bestquality layer.Figure 4.4.2.4AFM images of surface morphologyof P3HT layer on glass substrate (a)as spin coated; and (b) annealedat 140°C for 10 minutes. Afterannealing the layer structurebecome more clearly visible. Thesurface roughness is also reducedby annealing.Figure 4.4.2.3Voc and fill-factor (FF) of our OPVcells, fabricated with differentdonor type materials of P3HT.Table 4.4.2.1homogeneous surface,while annealing produces acoarser textured surface withobvious phase segregation.The majority of device studieson P3HT:PCBM blends utilizeannealing to increase deviceefficiency. This is believed tobe due to changes in boththe morphology of the activelayer whereby P3HT and PCBMself-organize into nanoscaledomains as well as the betterintermolecular alignment ofthe P3HT giving higher holemobilities.In our studies, we havefound that annealing thedevice after the applicationof the cathode improvesthe efficiency of the devicemuch more significantly thatannealing before the cathodedeposition. The reason forthis is believed to be reduced mobility of the PCBMmolecules under the increased confinement of thecathode. In our studies we have explored a rangeof anneal temperatures and find that the optimumannealing temperature depends upon the blendratio, and in general lies in the range of 120°C to150°C. The optimum anneal temperature for ourblend appears to be ~125°C (for 15 minutes) – avalue subsequently used in the remainderof the optimization experiments detailedbelow. An improved transport propertyof the active layer is also observed whichleads to a reduced series resistance and inturn a better device performance. Recently,microwave annealing has been proposedas an alternative thermal treatmentmethod [4.4.2.5].Figure 4.4.2.4 shows the absorbancespectra of the blended active layer of P3HT/PCBM, annealed at different temperature and fordifferent durations. The absorbance spectra almostremain the same under all annealing temperaturesand times. However, the intensity of the spectrachanges. As mentioned, the optimum annealingtemperature was found as 125°C for 15 minutes forthe best quality layer.We have started to fabricate our OPV cells withaverage Voc ≈ 0.6 V, FF ≈ 70 % and Jsc ≈ 10 mA/cm 2 .The average conversion efficiency is around 3 - 4%.The measured Voc of our OPV devices as a functionof light intensity is shown in Figure 4.4.2.5.We have used different types of P3HT to fabricateour OPV cells. Their corresponding Voc and FF arerecorded in Table 4.4.2.1.4.4.2.5 Light Trapping in OPVGiven the high absorption coefficient of organicmaterials, 100 nm thick organic layers are typicallysufficient to absorb most of sunlight. The diffusionlength of excitons in the organic materials istypically in the range of 5–10 nm. A partialsolution to the problem of low charge carrier andexciton mobility is the reduction of the activelayer thickness, that also implies the loss of a largefraction of the impinging light. The use of a lighttrapping systems has been actively investigated toincrease the total path length of light into the activematerial without needing to increase its physicalthickness. The concept is to induce sunlight to travela longer distance in the active layer by a multi-passpath in order to exploit most of the radiation.Many different light trapping schemes have beenproposed to enhance the quantity of light absorbedin OPV cells such as metal gratings [4.4.2.6], buriednanoelectrodes [4.4.2.7] and multireflectionstructures [4.4.2.8]. However, the requirement thatthe feature sizes of the scattering structures becomparable to the film thickness of the OPV cells,i.e., 50–300 nm, and at the same time larger than thewavelength of light to effectively alter the photonpropagation direction is intrinsically contradictory.The electrical properties can be negatively affectedby the introduction of light scattering elements indirect contact with such thin active layers: defectsand shorts may easily occur. Alternate approaches56
light intensity (Suns)100.0010.001.000.100.01DataVocMPPV_n1 fitV_n1VxFitV_0.1RshV_n2 fitV_n2ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT0.000.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80V oc (V)Open circuit voltage (Voc) of OPV devices as a functionof light intensity. The Voc increases logarithmicallywith light intensity as for conventional silicon solarcells.Figure 4.4.2.5in improving absorption efficiency have exploitedthe properties of surface plasmons (SPs). SPs areelectromagnetic surface waves confined to a metaldielectricinterface by coupling to the free electronplasma in metals [4.4.2.9]. The evanescent nature ofSPs permits the manipulation and enhancement ofoptical fields below the diffraction limit, allowingthe use of thin OPV layers without sacrificing theirabsorption potential. The integration of plasmonicstructures with OPVs has been previously examinedin the context of metallic nanoparticles, which canbe used to increase optical absorption. We have alsostarted working on the plasmonic light trappingsystem to increase the efficiency of our OPV cells.4.4.2.6 Simulation WorkThe Centre has purchased an upgrade of theGaussian Density Functional Theory (DFT) programpackage to Gaussian09. We can model largersystems in the so-called ONIOM model whichconsists of up to three model shells of differentaccuracy. The highest accuracy is limited to themolecule of interest, while the lower accuracyregions are bigger and cover the electronicenvironment of the species under investigation.This can include neighbouring molecules for chargetransfer as well as electrodes. An improved tool forcalculating infrared- and Raman-spectra is includedas well, providing an important link to experimentalobservations at the organic species. The presentstrengths include a dedicated ab-initio molecularsimulation capability (Fig. 4.4.2.7) presently rated at1080 gigaflops but with upgrading to 1.6 teraflopsplanned for the near-term. To date, this facilityhas been used for ab-initio modelling of siliconquantum dots in a dielectric environment. Theextended capability for excited state modellingshould allow the screening of new candidate OPVmaterials for both performance and durability.Other strengths include past experience with anumber of engineered vertical junction structuresand with light-trapping, not yet fully exploitedin OPV devices. The Centre has pioneered lighttrappingin both wafer-based and silicon thinfilmdevices, as well as the use of plasmonicsfor light-trapping in thin, plane-parallelphotovoltaic structures [4.4.2.10].The Centre is also particularly interested inhybrid organic/inorganic systems as a way ofimproving both performance and stability.Some work in this area has already commencedas reported below. Hybrid solar cells are a mixerof nanostructures of both organic and inorganicmaterials. They combine the unique propertiesof inorganic semiconductor nanoparticles withproperties of organic/polymeric materials.Inorganic semiconductor nanoparticles or quantumdots may have high absorption coefficients andparticle size induced tunability of the optical bandgap.Band-gap tuning in inorganic nanoparticleswith different nanoparticle sizes can be used forrealization of device architectures, such as tandemsolar cells in which the different bandgaps canbe obtained by modifying only one chemicalcompound. Thus, the organic/inorganic hybridconcept for photovoltaic solar cells is becomingattractively interesting in recent years. Thesolubility of the n-type and p-type componentsis an important parameter in the construction ofhybrid solar cells processed from solutions.4.4.2.7 Ordered Nanoparticle ArraysFor Hybrid Organic/InorganicSolar CellsExperimental work on fabrication of highlyordered arrays of nanoparticles as absorbermaterials originally for hot carrier solar cellshas been initiated as a potential means to realiseordered superlattice structures. In this work, the aimis to establish a fabrication system for depositingsequential monolayers of nanoparticles withuniform shells.We have installed a Langmuir-Blodgett (LB) systemfor fabrication of highly ordered nanoparticlemonolayers as shown in Fig. 4.4.2.8. The LBtechnique leads to the development of orderedmonolayers at an air-water interface whileexploiting the self-organization mechanism ofAbsorbance spectra of conventionalOPV cell (ITO/PEDOT:PSS/P3HT:PCBM/Al without TiOx spacerlayer and with TiOx layer.Figure 4.4.2.6Dedicated Linux 64 bit cluster forab-initio molecular simulationspresently rated at 1080 gigaflops.Figure 4.4.2.757
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