OrganicDevicesLecture-7KelvinProbeStudyofBandBendingatOrganicSemiconductor/Metalinterface
Energylevelalignmentatorganic/metalinterfacesMott–Schottky(MS)Whenthetwosolidsmakecontact,theMSmodelassumesavacuumlevelalignmentrightattheinterfaceregionand(ii)bandbendinginthespacechargelayer(SCL)toachievethealignmentofthebulkFermilevelsbetweenthem.Thus,thebarrierheightforholeinjection,Inthermalequilibriumstate,Fermilevelisconstanteverywhereinthesystem.Thebuilt-inpotential(Vbi,thevoltagechangeacrossthespacechargelayer)isHowever,vacuumlevelalignmentdoesnotoccurinmostorganic/metalinterfacesduetotheformationofaninterfacedipoleinducingvacuumlevelshift(?).Hence,Vbihastobemodified
Doesbandbendingoccurinorganicsemiconductors?andIsFermilevelalignmentachievedforanorganicsystem?Organicsemiconductorsbandgapenergy(HOMO–LUMO)islarge2–3eVandtheconcentrationofthermallyexcitedcarriersisextremelysmalllikeinaninsulator.Thebandbendingeffectisusuallyneglectedatinsulator/organicinterfaces.Fermilevelalignmentisacriticalproblem.Sinceitrequiresthermalequilibrium.InOrganicsemiconductors,moleculesareboundonlybyvanderWaalsforces,andwavefunctionandchargedensityarefairlylocalizedwithineachmolecule.Thecarrierexchangeprocessbetweenadjacentmoleculesisnoteffectiveincontrasttothecaseofinorganicsemiconductorswithgoodconductivity.TheunbalanceintheFermilevelcanbecompensatedbytheredistributionofcarriers
InOLEDs,theelectricfieldisoftenassumedtobeconstantandthepotentialisproportionaltothepositionasshowninFig.2(a).Incontrast,inthecaseoforganicsolarcells,bandbendinginthespacechargelayerhasbeenoftenassumedasshowninFig.2(b).Semiconductorwithasufficientcarrierconcentra