Fig. 1. (a) Schematic picture showing the device structure of quantum dot-based up-conversion photodetector. (b) The PL spectrum of ZnCdSe QDs and spectral absorbance of PbS QDs, the inset shows the TEM image of PbS QDs. (c) The J-V-L characteristics of the up-conversion photodetector in dark and under 18 mW/cm² 980 nm illumination. (d) η_pp of the up-conversion photodetector with its definition and physical interpretation shown in the inset. (e) The spectral IPCE of the up-conversion device under different applied bias. (f) The EQE of the up-conversion device; the inset shows the EL spectrum of the up-conversion device with 6 V applied bias and infrared illumination.

Fig. 2. (a) The J-V-L characteristics and (b) EQE of ZnCdSe-based QLEDs with TFB or PTAA as hole transport layer. (c) The J-V-L characteristics of the up-conversion device with TFB replaced by PTAA as interconnection layer. (d) The J-V characteristics of the PbS-based PDs in dark or under 18 mW/cm² 980 nm illumination. (e) The photocurrent vs. effective bias characteristics of PbS-based PD and up-conversion devices with TFB or PTAA as interconnection layers, solid line represents the fitted photocurrent with Hetch formula. (f) The effective biases across the PD unit and QLED unit in the up-conversion device with TFB or PTAA as interconnection layers.

Fig. 3. (a) Equivalent circuit explaining the voltage allocations in up-conversion devices. (b) The quantum efficiencies of independent PD and QLED under working conditions simulating those of the PD unit and QLED unit in up-conversion devices. (c−d) The comparisons between the products of effective IPCE × EQE_i and the measured η_pp of up-conversion device with TFB and PTAA as interconnection layers, the green curves show the gaps between IPCE × EQE_i product and η_pp, representing the integration losses.

Fig. 4. (a) EQE characteristics of the QLED based on large CdSe-based quantum dots. (b) J-V-L characteristics and (c) IPCE or EQE_i characteristics of the up-conversion devices integrating the QLED with less efficiency roll-offs. (d) The η_pp of (green) up-conversion devices with optimized PbS layers and improved matching between PD unit and QLED unit, (pale red) up-conversion devices before optimization.