Complete Report - University of New South Wales

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