B. Si/ITO infrared photodetectors

The structure and performance of the Si-based PD are optimized as an essential part of the upconverter. ITO with a good conductivity and light transmission is adopted to construct a Si/ITO Schottky diode.3434. X. Li, Z. Deng, J. Li, Y. Li, L. Guo, Y. Jiang, Z. Ma, L. Wang, C. Du, Y. Wang, Q. Meng, H. Jia, W. Wang, W. Liu, and H. Chen, “Hybrid nano-scale Au with ITO structure for a high-performance near-infrared silicon-based photodetector with ultralow dark current,” Photonics Res. 8, 1662 (2020). https://doi.org/10.1364/prj.398450 The effects of p- and n-type Si substrates on the Si/ITO junction performance are explored. The n-type Si/ITO PD has a stronger built-in potential than the p-type Si/ITO ones, as shown in Fig. 2(a). In addition, the dark current of n-type Si/ITO PDs is nearly 10−4 A cm−2 at a bias of 1.5 V, which is an order of magnitude lower than that of the p-type Si/ITO ones [Fig. 2(a)]. Lower dark currents are critical for upconverters to remain off under dark conditions. The responsivity (R, units of A W−1) is an essential performance indicator for PDs. The calculation process of responsivity is as follows:where Iph is the photocurrent and P is the input optical power. The responsivity of n-type Si/ITO PDs is about 0.33 A W−1 at 5 V, as presented in Fig. S2. The external quantum efficiency (EQE, units of %) of the PD is the ratio of the number of carriers collected by the PD to the number of incident photons, as given by the following equation:where λ is the wavelength of the infrared light and R is the responsivity. The trend of EQE of n-type Si/ITO PDs at the NIR region at the bias of 5 V is shown in Fig. S3. The n-type Si/ITO PDs possess a high EQE of 85% at 5 V at 980 nm as shown in Figs. 2(b) and S3. The EQE of n-type Si/ITO PDs peaks at 970 nm with a high EQE of 88% (Fig. S3). An excellent EQE of PDs is vital to obtain the high upconversion efficiency of PD and OLED integrated upconverters.1818. W. Zhou, Y. Shang, F. P. García de Arquer, K. Xu, R. Wang, S. Luo, X. Xiao, X. Zhou, R. Huang, E. H. Sargent, and Z. Ning, “Solution-processed upconversion photodetectors based on quantum dots,” Nat. Electron. 3, 251 (2020). https://doi.org/10.1038/s41928-020-0388-x The specific detectivity given by Eq. (3) is a figure of merit of the signal-to-noise ratio for the detectors. The detectivity (D*, units of Jones) is calculated bywhere A is the area of the detector, Δf is the noise-equivalent electrical bandwidth, In is the spectral noise density, and ℜ is the responsivity. The n-type Si/ITO PDs possess a low noise current of 1.13 × 10−11 A Hz−1/2 at 5 V, resulting in a high detectivity of above 1010 Jones, which is shown in Fig. 2(b). In addition, n-type Si/ITO with an n–p junction favors the OLED with p–n polarity to form an n–p–n back-to-back photodiode configuration. Thus, high-performance n-type Si/ITO is selected to integrate with the OLED to construct Si-OLED upconverters.The response speeds of n-type Si/ITO PDs are acquired by measuring with modulated laser pulses having a wavelength of 808 nm, as shown in Fig. 2(c). The rise time (the time interval from 10% to 90% of the maximum response) of n-type Si/ITO PDs is about 50 µs, indicating that the response speed is fast enough to image moving objects in real-time. The fast response speed of n-type Si/ITO PDs is probably due to the generated strong built-in potential caused by the high-performance Si/ITO Schottky junction, leading to the photogenerated charge carriers swiftly drifting to produce the photocurrent. The infrared imaging of n-type Si/ITO PDs is conducted to give a visual demonstration that Si/ITO PDs could acquire information beyond the naked eye. The single-pixel scanning system for infrared imaging consists of a scanning lens, a detector, an amplifier, a signal collection board, and software, as illustrated in Fig. 2(d). The pattern of the “BIT” school logo printed on a white A4 paper cannot be observed in a dark environment from visible images, but it can be clearly captured by Si/ITO PDs [Fig. 2(e)]. The four chemical solvents of methanol, IPA, toluene, and water in glass vials are transparent under natural light. In contrast, the transparent solvents appear differently and can be distinguished based on their chemical composition when captured by Si/ITO PDs in the NIR spectral range [Fig. 2(e)]. The capability of the Si/ITO imager to capture NIR images provides additional valuable information that human eyes cannot obtain.