Fabrication NIR-sensitive OPD.
Discrete OPDs and OPD arrays were made using the same fabrication method. Printed Cu grids were deposited on glass substrates (1.1 mm, EAGLE XG®) by Asahi Kasei’s high-resolution printing technology52 to form the pixelated transparent bottom electrode. Next, a thin film (16 nm) of a-IGZO was sputtered and patterned by wet-etch on the bottom electrode as an electron transport layer (ETL) as well as a hole blocking layer (HBL). An edge cover layer of a SU8 resist was deposited and photolithographically structured, preventing shorts between bottom and top electrodes as well as defining an OPD subpixel active area. The 300 nm thick photoactive layer is based on a bulk-heterojunction (BHJ) structure consisting of a p-type donor polymer, PCE-10 (PTB7-Th, purchased from 1-Material), and n-type non-fullerene acceptor small molecule, IEICO-4F (purchased from 1-Material) in a 2:3 weight ratio. The optical band gap, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of these materials are 1.6 eV, − 5.20 eV and − 3.59 eV for PCE-1035 and 1.24 eV, − 5.44 eV and − 4.19 eV for IEICO-4F36, respectively. The effective band gap, i.e. the difference between HOMO of the donor and LUMO of the acceptor, is 1.01 eV. PCE-10 and IEICO-4F were dissolved in chlorobenzene (CB) at 20 mg mL− 1 with 40 µL chloronaphthalene (CN), added to optimize the nanomorphology of the photoactive layer53. The PCE-10:IEICO-4F OPD blend was slot-die coated under ambient conditions, and subsequently annealed at 60 ºC for 5 minutes in ambient. The photoactive layer was patterned using photolithography in a similar way to ref.37,38 to make the parallel OPD subpixel array structure. As a hole transport layer (HTL) and an electron blocking layer (EBL), a 60-nm thick MoOx layer was deposited by thermal evaporation, followed by a sputtered ITO thin transparent electrode (~ 100-nm thickness, sheet resistance of ~ 40 Ω □−1). The low Jdark of the OPDs is attributed to the charge blocking properties of a-IGZO54,55 and MoOx under reverse bias, whereas the low effective band gap (1.01 eV) enhances bulk thermal charge generation and charge injection56. The OPD devices were protected from oxygen and moisture by an optically transparent laminated barrier film. The laminated barrier film was a multi-layer stack that has a low-temperature plasma-deposited amorphous hydrogenated silicon nitride (a-SiNX:H) layer and an organic layer between a PEN substrate (125µm, Melinex® ST504™) and a barrier adhesive layer. Total thickness of the laminated barrier film was 160 µm.
Characterization discrete OPD.
Discrete OPDs were characterized in a glovebox under a N2 atmosphere and at ambient condition. Current density versus voltage (J-V) characteristics in dark and under NIR light conditions were measured using a semiconductor parameter analyzer (Agilent 4155C) with a manual probe station and in-house OPD measurement setup. The OPDs were illuminated from the bottom through the Cu grid TCE with a NIR LED light source (850 nm wavelength, light intensity of ca. 0.28 mW cm− 2, 15414185BA210 from Wurth Elektronik). The voltage was swept from − 3 V to 2 V using a scan speed of 5 mV s− 1. Static J-V characteristics were measured using the same set-up by setting a fixed voltage and measuring the current in time. The EQE was measured using a custom-made setup consisting of the following: a tungsten-halogen lamp, a chopper, a monochromator (Oriel, Cornerstone 130), a pre-amplifier (Stanford Research Systems SR570) and a lock-in amplifier (Stanford Research Systems SR830 DSP). Although the setup was in ambient air, the devices were constantly kept sealed in a N2-filled box equipped with a quartz window. For this measurement, a circular aperture with a diameter of 1 mm was used to define the active area. To convert the current signal from the device into an EQE value, a comparison was made with a reference calibrated silicon solar cell. In the range of wavelengths from 350 nm to 1050 nm, the standard deviation of this setup is less than 0.005 electron/photon. Noise measurements were performed at room temperature and in dark conditions, exploiting a battery-powered current to voltage conversion readout circuit developed with off-the-shelf components. The OPD is first connected by means of two probes and triaxial cables to a Trans-Impedance Amplifier (TIA) implemented with the operation amplifier (Analog Devices ADA4530). An adjustable DC voltage source is applied to the non-inverting terminal of the TIA to modify the biasing of the DUT. The feedback network of the TIA is designed with 1 GΩ resistor and 1 pF compensation capacitor, required for the stability of the circuit. The input-referred noise of the TIA is dominated by the Johnson–Nyquist noise component associated to the feedback resistor. Next, the output of the TIA is fed to an active bandpass amplifier, realized with the operational amplifier (Analog Devices AD8065) in closed loop configuration, which exhibits an in-band voltage gain of 100V/V. The 3 dB bandwidth of the conditioning chain is approximately limited to the frequency range 0.1 Hz – 100 Hz. Finally, the output of the readout chain is connected to the HP35670A Dynamic Signal Analyzer, to extract the noise spectral density of the OPD. Optical characteristics of the discrete OPDs were measured by UV-vis-NIR spectroscopy (Agilent Cary 5000) at the wavelength range from 300 nm to 1200 nm with a step of 1 nm.
EQE simulations for NIR-sensitive OPDs with a printed Cu grid TCE.
EQE simulations for NIR-sensitive OPDs with a printed Cu grid TCE were carried out by coupling input of simulated J-V curves of the NIR-sensitive OPD stacks on the Cu line and in a gap area between the Cu lines based on a numerical electro-optical simulation, and output of surface potential and current density distributions derived from the printed Cu grid geometrical structure solved by 2D FEM.
Firstly, the numerical electro-optical simulations for the photogenerated J-V curves of our NIR-sensitive OPDs were performed by using Setfos 5.2 (FLUXiM AG, Switzerland). The numerical electro-optical simulations are performed by coupling between input of optical properties based on the transfer matrix method57 and output of electric characteristics calculated by the numerical drift-diffusion simulation58. Specifically, a simulated photon absorption profile in the OPD stack is used to determine charge generation (electron-hole pair) in absorbing layers and the generated charges are distributed across the OPD stack, leading to electric current between both electrodes. For the numerical electro-optical simulations of the NIR-sensitive OPDs, we modeled two types of OPD stacks, one is for the OPD stack on the printed Cu line, and another is for the OPD stack in the gap area, shown in Supplementary Fig. 12. For optical simulations of the OPD stacks, wavelength-dependent refractive index (n) and extinction coefficient (k) of each layer and their thickness were used as the input parameters. The n-k profiles as a function of wavelength of each layer were determined by spectroscopic ellipsometry (shown in Supplementary Fig. 13), except for the printed Cu lines and the PEN substrate of the transparent laminated barrier film. For the Cu lines, the n-k profile was derived from the printed Cu line’s layer structure. For the PEN substrate, the n-k profile was retrieved from an available database59. The thicknesses of all layers were identical to those used during the fabrication of the NIR-sensitive OPD. For numerical drift-diffusion simulations of the OPD stacks, we modeled them with 4 elements: the printed Cu grid TCF for the OPD stack on the printed Cu line or a quasi-transparent electrode for the OPD stack in the gap area (Supplementary Fig. 12), a-IGZO, the photoactive layer and MoOx/ITO thin transparent electrode. The simulations were carried out using input parameters and boundary conditions of both electrodes shown in Supplementary Table 2. Work function of the quasi-transparent electrode was set to the same value as the printed Cu grid TCE. In the numerical electro-optical simulations, the OPD stacks were modeled upside down compared to the experimental inverted NIR-sensitive OPDs, and polarity of an applied voltage is also inverted for implementing simulated J-V curves for each OPD stack in subsequence 2D FEM simulations. The applied voltage was swept from − 1 V to 2.5 V with a step size of 5 mV and 850-nm NIR light with a light intensity of 1.05 mW cm–2, which was matched with the light intensity at 850 nm in the EQE measurement, was illuminated from the printed Cu grid TCE in the simulations. The resulting simulated J-V curves that are coupled with simulated absorption rate profiles (Supplementary Figs. 14a-b) for each OPD stack are shown in Supplementary Figs. 14c-d.
Next, the 2D FEM simulations for the surface potential and the current density distributions derived from the printed Cu grid structure for our NIR-sensitive OPDs were performed by LAOSS 4.0 (FLUXiM AG, Switzerland). The NIR-sensitive OPDs were approximated as a 2D:1D:2D system which combined 2D FEM electrodes (“top electrode in the simulation software”: a-IGZO/printed Cu grid TCE and “bottom electrode in the simulation software”: ITO thin transparent electrode) with the aforementioned 1D OPD stack models’ J-V curves simulated by Setfos. The simulated J-V curves were set on the corresponding area on the 2D FEM electrodes. The 2D FEM solved Ohm’s law for the local current density in the electrodes which were coupled with a given small area wavelength-dependent J-V curve at a given point for charge conservation. Input parameters for 2D FEM simulations are presented in Supplementary Table 3. The simulations were done for 3 × 3 unit-cells of the printed Cu grid TCE. Meshing conditions were set so that mesh edge size was half or less than half of the Cu line width. Boundary conditions were set so that the voltage on the four edges of the top electrode (a-IGZO/printed Cu grid TCE) was at the applied voltage 2 V (corresponding with − 2 V for the experimental inverted OPD stack) and the voltage on the bottom electrode (ITO thin transparent electrode) was applied 0 V. The solving parameters were set using a Newton solver with convergence parameters of relative residual convergence type, L2 norm, a tolerance of 1 × 10–7, and maximum iteration count of 10. These were kept constant in all simulations in this work. An example of the simulated surface potential and current density distributions of the system are shown in Supplementary Figs. 1a-b. Based on the calculated current density distribution, the EQE is calculated with the following formula:
$$EQE\left(\lambda \right)=\frac{{J}_{s}\left(\lambda \right)}{{I}_{P}\left(\lambda \right)}\bullet \frac{hc}{\lambda q}$$
where Js (λ) is global current density derived from the current density distribution across the whole studied system. IP (λ) is illuminated light intensity, that was set to the same value used in J-V curve simulations performed in Setfos. h, c, λ and q are the Plank constant, the speed of light, wavelength (850 nm), and the elementary charge, respectively.
Optical transmittance prediction model for patterned OPDs with a printed Cu grid TCE.
A parallel OPD subpixel array was modeled by dividing three simplified components: 1. a photoactive stack without the Cu lines, 2. a non-photoactive stack (i.e., an open area of the parallel array) without the Cu lines, and 3. the printed Cu line (Supplementary Fig. 2). Overall optical transmittance (T(λ)) of the parallel OPD subpixel array was calculated by the following equation:
$$T\left(\lambda \right)=\left\{\frac{{\left({W}_{OPD}+2{w}_{OPD}\right)}^{2}}{{P}_{OPD}^{2}}\bullet {T}_{OPD}\left(\lambda \right)+(1-\frac{{\left({W}_{OPD}+2{w}_{OPD}\right)}^{2}}{{P}_{OPD}^{2}})\bullet {T}_{open}\left(\lambda \right)\right\}\times \left\{1-\frac{{\left({P}_{Cu}-{W}_{Cu}\right)}^{2}}{{P}_{Cu}^{2}})\bullet {T}_{Cu}\left(\lambda \right)+\frac{{\left({P}_{Cu}-{W}_{Cu}\right)}^{2}}{{P}_{Cu}^{2}}\right\}$$
where WOPD, wOPD and POPD represent an OPD subpixel width, an OPD subpixel overlap with the edge cover layer for alignment tolerance, and an OPD subpixel pitch, respectively (Supplementary Fig. 2b). WCu and PCu are a line width and a pitch of the printed Cu grid TCE, respectively. These geometric variables are used for calculating fill factors of the photoactive stack and the printed Cu lines. TOPD(λ), Topen(λ) and TCu(λ) are the simulated optical transmittance of the photoactive stack without the Cu lines, non-photoactive stack without the Cu lines and the printed Cu line, respectively (Supplementary Fig. 3a). These optical simulations were performed by Setfos 5.2 (FLUXiM AG, Switzerland) with the same manner described in “electro-optical simulation”. For the SU8 edge cover layer, the n-k profile was retrieved from the datasheet60.
Touchless user interface demo characterization.
Photocurrent measurements as function of light intensity were performed while illuminating the 16 × 16 OPD array with a 10 × 10 cm2 LED tile emitting at a wavelength of 850 ± 15 nm (Phlox). The LED tile was driven using a voltage source (TTi EL302R Power Supply). The light intensity was measured using a calibrated photodiode (FDS1010-CAL, Thorlabs). The OPD response was measured using a custom-made electronic system and software (LabVIEW based). A silicon readout IC (Analog Devices AD71124) collects the data of all 256 pixels. The image sensor is biased at − 2 V using a custom-made board and connected to an FPGA digital interface that reads the data. The FPGA interface is connected to a PC through a USB connection.
For the NIR penlight demo, a custom made battery driven NIR penlight was built using a LED with a peak emission at 850-nm wavelength (TSHG6200, Vishay). The NIR penlight has two buttons by which two different frequency periodic pulse train signals of 800 Hz and 960 Hz are generated when the respective buttons are pressed to activate a right and a left click, respectively. An idle frequency of 1200 Hz is generated when no buttons are active. The left click is used to pan a screen. The right click is used to initiate zooming so that moving the penlight closer towards or away from the screen during the right click results in a zoom-in or zoom-out.
SNR and position accuracy of the touchless user interface were determined by illuminating the centre of the image sensor with the penlight at a disctance of 60 mm inside a light-tight cabinet. The bandpass filtered pixel signals have been averaged and 1024 samples were saved. A subset of 100 pixel signals (Supplementary Fig. 9a), representing 2.5 s, was used to determine the standard deviation on the amplitude, resulting in the SNR of the image sensor.
The x-position resulting from the gaussian fit used to calculate the position in the software is plotted vs. time. The bandpass filtered pixel signals have been averaged and 1024 samples were saved. A subset of 100 pixels signals (Supplementary Fig. 9b), representing 2.5 s, was used to calculate the peak to peak error and position accuracy. The range of the position signal is from 0 to 1 and corresponds to a distance of 100 mm.
For the gesture recognition demo, a PCB was made containing in total 40 LEDs with a peak emission of 850 nm (15411085A4570, Würth Elektronik) that are equally arranged at four sides of the PCB. A LED modulation frequency of 1 kHz was used to filter background noise.
Data availability
The datasets analyzed in this study are available from the corresponding authors upon reasonable request.