The Dependence of Device Dark Current on the Active-Layer Morphology of Solution-Processed Organic Photodetectors

Organic photodiodes are presented that utilize solution-processed perylene diimide bulk heterojunctions as the device photoactive layer. The polymer (9,9'-dioctylfluorene-co-benzothiadiazole; F8BT) is used as the electron donor and the N,N'-bis(1-ethylpropyl)-3,4,9,10-perylene tetracarboxylic diimicle (PDI) derivative is used as the electron acceptor. The thickness-dependent study of the main device parameters, namely of the external quantum efficiency (EQE), the short-circuit current (I-SC), the open-circuit voltage (V-OC), the fill factor (FF), and the dark current (I-D) is presented. In as-spun F8BT:PDI devices the short-circuit EQE reaches the maximum of 17% and the Voc value is as high as 0.8 V. Device I-D is in the nA mm(-2) regime and it correlates with the topography of the F8BT:PDI layer. For a range of annealing temperatures I-D is monitored as the morphology of the photoactive layer changes. The changes in the morphology of the photoactive layer are monitored via atomic force microscopy. The thermally induced coalescence of the PDI domains assists the dark conductivity of the device. I-D values as low as 80 pA mm(-2) are achieved with a corresponding EQE of 9%, when an electron-blocking layer (EB) is used in bilayer EB/F8BT:PDI devices. Electron injection from the hole-collecting electrode to the F8BT:PDI medium is hindered by the use of the EB layer. The temperature dependence of the I-D value of the as-spun F8BT:PDI device is studied in the range of 296-216 K. In combination with the thickness and the composition dependence of to, the determined activation energy E-a suggests a two-step mechanism of I-D generation; a temperature-independent step of electric-field-assisted carrier injection from the device contacts to the active-layer medium and a thermally activated step of carrier transport across the device electrodes, via the PDI domains of the photoactive layer. Moreover, device I-D is found to be sensitive to environmental factors.