1 Introduction

Infrared photovoltaic cells (IRPCs) are attracting interests due to their potential applications in wireless optical power transfer (WOPT) systems, which converts the infrared laser light into electric energy (Fig. 1a and b) [1, 2]. With the increasing potential to realize long-range wireless power transfer [2,3,4,5], WOPT technology has shown great application prospects in chargers of portable electronic devices [6], sensors in the Internet of Things [7], as well as devices in industrial environments in which assembling or replacing cables is difficult [4.4 Device fabrication

The following process was conducted in a nitrogen atmosphere glove box. First, the control ligand was dissolved in 10 mL DMF solvent at the molar ratio of PbI2 (1.229 g):PbBr2 (0.428 g) (2.3:1). Then 10 mL PbS QD solution with a concentration of 10 mg/mL and n-octane as solvent was added to the centrifuge tube containing the above control ligand for ligand exchange. After three repetitions of solution-phase ligand exchange in the DMF and octane solvent system, the 380 mg/mL PbS-IBr (PbI2 and PbBr2 capped PbS QDs) in DMF:DMSO:BTA:4-AMPY (50:30:17:3) solvent was spin-coated onto ZnO film at 2500 r/min for 45 s and then annealed at 90 °C for 10 min. The absorber film cooled down naturally to below 40 °C. Then, two layers of EDT-treated PbS QDs (excitonic peak at 890 nm) were grown to act as the whole transport layer. Finally, the Au layer with a thickness of 60 nm was deposited by thermal evaporation, to act as the upper electrode. The effective area of the prepared QD solar cell was 0.0706 cm2.

4.5 Characterization of materials and devices

The optical absorption spectra of QDs were measured by a Shimadzu UV-3600 Plus spectrophotometer. The absorption spectra of PbS QDs film and the devices were collected using a spectrophotometer (PerkinElmer instrument, Lambda 950). The scanning electron microscopy (SEM) images were obtained using FEI Nova Nano SEM 450. The ZnO film crystallization was tested by X-ray diffractometer (XRD) with Cu Kα radiation (Philips, X pert pro-MRD, Netherlands). EQE of PbS QD PV cells was measured using a Quantum Efficiency Measurement Instrument QE-R (Enlitech Co., Ltd). The current density–voltage characteristics were recorded with a Keithley 2400 digital source meter under simulated solar light illumination (AM 1.5, 100 mW/cm2) or under 1550 nm laser illumination in the air at room temperature.

4.6 FEM simulation

Calculated absorption of the devices as a function of ZnO thickness at 1550 nm was simulated with commercial FEM software (COMSOL). The thickness of each layer in the simulation model was: Glass (1.1 mm), ITO (280 nm), ZnO (30–260 nm), EDT-PbS (46 nm), Au (60 nm), and PbS (450 nm). The incident light was a planar wave. The periodical boundary condition and perfectly matched layer were applied for the simulation of multi-layer films.