The photon‐to‐photon upconversion efficiency can be calculated with ηp−p(%)=∫λupIup(λ)hcdλλincPinchc that describes the utilization rate of incoming infrared photons for visible photons.[21]h and c are the Planck constant and the speed of light in the vacuum. λup and Iup(λ) are the upconversion visible wavelength and its corresponding intensity. λinc and Pinc are the incident infrared wavelength and the related irradiation power of interest. If enough band bending is satisfied, the leakage current under the dark contributes to the upconversion luminance under the infrared stimulus. However, in principle, the device can no longer distinguish a low‐intensity signal with upconversion luminance comparable to the leakage luminance, i.e., a poor signal‐to‐noise ratio. In this regard, we reported the maximum upconversion efficiency at which the device maintained linear response (LDR region) and excluded the leakage luminance to conform to the upconversion luminance in response to the infrared stimulus. The same practice was adopted on the OUD managing single‐type charge carriers in the Supporting Information for consistency. The photoluminescence spectrum, transmittance spectrum, absorption spectrum, EQE spectrum, specific detectivity spectrum, brightness, and current density were recorded according to the previous practice.[26] The AVT can be calculated by the equation of AVT(%)=∫T(λ)P(λ)S(λ)dλ∫P(λ)S(λ)dλ where T, P, S, and λ represent the transmittance spectra, the human photopic response, solar photon flux, and the optical wavelength. A detailed description of the ultrafast pump‐probe spectroscopy can be found in Supporting Information. To evaluate the spatial resolution of the device, the system was set up on the operational stage of the optical microscope (Taiwan Microscope Enterprise). The infrared light source (M780L3, Thorlabs) was first collimated by the adapter (COP1‐B Olympus, Thorlabs) and propagated through the photomasks with a line‐shaped pattern (line spacing 3, 5, and 10 µm). The large‐area, semi‐transparent device revealed the infrared images, which were subsequently captured by the CCD camera for image resolution determination. Actual photos for the overall system setup can be found in the Supporting Information (Figure S13, Supporting Information).
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