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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTRecent improvements in independently confirmedefficiency for small area (≤ 1cm 2 ) organic solar cells(extracted from Green et al. [4.4.2.2]).Figure 4.4.2.1University Staff• Dr Ashraf Uddin• Prof. Martin Green• A/Prof. Gavin ConibeerSenior FellowDr Dirk KönigResearch FellowDr Shujuan HuangResearch AssistantXiaohan YangPostgraduate Research Students• Lara Treiber• Xiaohan Yang• Miga Jung• Xiaolei Liu• Kim Taekyun• Fred Oi• Matthew WrightUndergraduate Thesis Students• Thoo Chee Hong• Matthew Wright• Xinda Yang• Jizhong Yao• Adam Sen Ann Ong• Lihui Song• Buyi Yan• Viren NathooTaste of Research StudentSolomon Freer4.4.2.1 IntroductionA new organic photovoltaic (OPV) researchlab has been established at Level-10 inthe UNSW Chemical Science Buildingto accommodate work on organicsemiconductor materials and OPV devices.Today this lab is equipped with basicfacilities organised with support from anARC Discovery Grant, UNSW equipmentgrant, a family grant from the Green family,a Faculty grant and with the support ofthe ARC Photovoltaic Centre of Excellence.Currently, the research team consists ofthree academics, two research fellows, oneresearch assistant, 6 PhD, one M.Eng. and 8honours students.We have already produced some meaningfuldata from our research work. Two journalpapers, three conference proceedingpapers and one patent are already inpreparation. The nanoscale texture or thinfilm morphology of the donor/acceptorblends used in most OPV devices is acritical variable that can dominate boththe performance of new materials beingoptimized in the lab and efforts to movefrom laboratory-scale to factory-scaleproduction. Although OPV conversionefficiencies (up to 8.3%) have improvedsignificantly in recent years, progress inmorphology optimization still occurs largelyby trial and error, in part because much ofour basic understanding of how nanoscalemorphology affects the optoelectronicproperties of these heterogeneous organicsemiconductor films has to be inferredindirectly from macroscopic measurements.The morphology of the active film is avery important factor in producing highefficiency devices.Films spin-coated from blend solutionsundergo separation of the donor/acceptorphases. The scale of the phase separationdepends on the solvent, solubility of thematerials and parameters of the spincoatingprocess such as the spin speedand temperature. If the morphologycould be controlled on a molecularscale the efficiency of charge separationand transport could be expected to besubstantially higher. Now we are workingfor the morphology optimization oforganic films for OPV devices in respect tofundamental issues such as light trapping,control of electronic structure at filmsinterfaces, exciton dissociation and carriertransport for photovoltaic operation toachieve our goals.The detailed physical understanding of OPVdevices still lags behind their application,on account of fundamental differences inthe optoelectronic properties of organicmaterials compared to conventionalsemiconductors. This lack of understandinglimits the scope of material and devicedesign. Among those features thatdistinguish OPV devices from inorganicsemiconductor based devices are: (i) thecharges and excited states are localizedon individual molecules or molecularsegments, with the result that charge andenergy transport processes are relativelyslow; (ii) the dielectric permittivity is low,leading to stronger space charge effects;(iii) the organic semiconductor materialsare electronically disordered, dispersingthe rates of charge transfer and transportprocesses; (iv) the active layers are oftenheterogeneous, either as multi-componentfilms or because of nonuniform molecularordering; and (v) the organic semiconductoris usually not doped, thus precluding theconditions that allow charge dynamics tobe linearized in the description of devicephysics. The need for appropriate device54
(a) The schematic band energystructure of bulk-heterojunctionorganic cell used to produce theresults of Fig. 4.4.2.1; (b) (left)a schematic showing randomdistribution of donor (P3HT) andacceptor (PCBM) regions in theblended bulk-heterojunctionorganic solar cell, (right) aschematic diagram of systematicalignment of donor andacceptor layers.ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTFigure 4.4.2.2physics for organic semiconductor materialsis of particular relevance to the organic bulkheterojunction solar cell.4.4.2.2 OPV Conversion EfficiencyInterest in OPV with increasing conversion efficiencyhas grown exponentially over recent years. Thekey development in the OPV device is the bulkheterojunction cell [4.4.2.1] obtained by blendingdonor and acceptor layers, a device structure wellsuited to the associated short excitonic diffusionlengths. The workhorse for OPV research hasbecome polymer-fullerene bulk heterojunctiondevices fabricated by coating a thin layer ofPEDOT:PSS on the top of cleaned ITO substratesfollowed by a blend of PCBM/P3HT (or other donorpolymer), then by vacuum evaporation of Al andannealing. Both packaging and cell efficiency hadimproved by 2006 to the stage where independentmeasurement of cell efficiency was both feasibleand warranted, with subsequent efficiencyimprovements well documented, after reviewby a team that includes one of the present teammembers [4.4.2.2]. Using the above structure,Konarka, USA established 4.8% efficiency for a tiny0.14 cm 2 cell in July 2005. Konarka increased thisto 5.15% in December 2006 with 5.24% postedfor a 0.7 cm 2 cell in July 2007. Companies such asPlextronics have reported roughly comparableresults using similar materials with 5.4% efficiencyconfirmed in July 2007 for a 0.1 cm 2 device, 6.0% inAugust 2008 for an even smaller 0.04 cm 2 cell and2.0% as recently as 28 January 2009 for a 224 cm 2module. OPV device developer Solarmer Energyachieved 7.9% efficiency in December 2009 [4.4.2.3].In November 2010, Konarka achieved their highestOPV efficiency 8.3%. The history of OPV conversionefficiency is shown for small area (≤ 1cm 2 ) cells inFigure 4.4.2.1.4.4.2.3 Standard OPV StructuresThe fullerene based acceptor material (PCBM) isinterspersed with a donor polymer, commonlyP3HT, with line-bond diagrams for these materialsand the energy band diagram of the standard“bulk heterojunction” cell structure shown in Fig.4.4.2.2(a). From this baseline device structure, theCentre believes it has several strengths that willallow it to contribute to the ongoing cell efficiencyevolution documented in Fig. 4.4.2.1 and to thedevelopment of more stable and durable devices.The challenge in the OPV is that absorbing light inan organic donor material produces coulombicallybound excitons that require dissociation at thedonor/acceptor interface. The energy-level offsetof the heterojunction (donor/acceptor interface)is also believed to play an important role for thedissociation of bound excitons in OPV cells. Anefficient exciton separation into free charge carriersat the interface of an acceptor and donor materialswill increase the photocurrent of the solar cell[4.4.2.4]. The transport of excitons to the interfacealso limits the conversion efficiency of OPV. Thedetails of the exciton transport mechanism in OPVare still not well understood. There is a need for moresystematic study to gain in-depth understandingof the mechanisms of light absorption, excitondissociation and charge transport, particularly theirrelationships with molecular and morphologicalstructures of the materials as well as the nano-scalearchitectural design of the devices. Figure 4.4.2.2(b)shows the two examples of donor/acceptor blendingdesign (left) and systematic alignment of donorand acceptor layers for bulk-heterjunction organicsolar cells.We believe that the conversion efficiency of OPV canbe increased to over 10% for possible commercialapplications through in-depth understanding ofthe light harvesting behaviour of organic materialsand by selecting suitable materials and new devicearchitectures.4.4.2.4 Active Layer AnnealingThermal annealing is an effective method thatincreases the polymer crystallinity and improvesthe phase segregation of donor and acceptordomains. The surface morphology is also affectedby annealing as shown in Fig. 4.4.2.3. Usually,as-cast P3HT:PCBM films have a featureless and55
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