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(Left) Band diagram for a 0.9 eV bandgap PbS CQD heterojunction device. The device energy band diagram was drawn assuming no interface effects (i.e., band bending, interface dipoles, etc.) due to an uncertainty as to the Fermi levels of materials involved. The energy levels are labeled relative to vacuum and are obtained from a number of references. The PbS quantum dot energy levels are from Ref. 14. The fullerene levels are from Ref. 15. The MoO3 and bathocuproine (BCP) levels are from Ref. 16. (Right) Depiction of the CQD-fullerene heterojunction device structure.
Cross-section SEM images that demonstrate abrupt interfaces between the CQD and C60 layers and reduced CQD layer thicknesses for films (a) before and (b) after formic acid treatment.
Current-voltage (I-V) behavior for PbS-C60 devices with (a) 0, 1, and 2 and (b) 3 nm MoO3 EBL. Inset (a): AFM image of the surface morphology of a 1 nm MoO3 film on ITO showing the discontinuous nature of the EBL. Inset (b): Relative change in photoluminescence intensity for PbS films on ITO as a function of MoO3 thickness relative to a control sample without MoO3.
(a) The current-voltage characteristics of the PbS/C60 device using a 1 nm MoO3 blocking layer with the highest PCE. Under 100 mW/cm2 AM1.5 illumination the device exhibited a short circuit current density of 26.7 mA/cm2, Voc of 329 mV, and fill factor of 59% for an overall power conversion efficiency of 5.18%. (b) External quantum efficiency and absorption as a function of wavelength for a PbS-C60 device with a 1 nm MoO3 blocking layer. Correcting for increasing absorption by ITO in the infrared as well as for interference effects results in a calculated internal quantum efficiency in the range of ∼50%–70% across the region of the CQD absorption.
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