Influence of annealing treatments on solution-processed ZnO film deposited on ITO substrate as electron transport layer for inverted polymer solar cells

In thisworkwestudiedtheinfluence oftheannealingtreatmentsofasol–gel derivedZnOelectron transport layerdepositedonITOsubstrate,ontheperformancesofinvertedbulkheterojunctionpolymer solar cellsusingablendofpoly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt- (4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl] and[6,6]-phenylC71 butyric acidmethylester. Since theannealingtreatmentsneededtocompletetheformationofthesolution-processedZnO film can modify theunderlyingITOelectrode,weanalyzedtheperformanceofthefabricatedcellsintermsofthe properties ofITOandZnO films. WefoundalinearrelationshipbetweenthesheetresistanceoftheITO layerandtheseriesresistanceofthecorrespondingdevice,whichstronglyinfluences the fill factor.The best powerconversionefficiency (7%)undersimulatedAM1.5Gilluminationof100mW/cm2 was achieved forthepolymersolarcellfabricatedusingaZnO film annealedat150 °C foronly5min.Higher annealing temperaturesandtimesincreasethesheetresistanceoftheITOworseningthedeviceper- formances.