## 3 Results and Discussion  
Quartz was used as the substrate for depositing the films. Figure 2 shows the absorption spectra of a variety of organic films. Examining the spectra reveals that the PbPc film displays strong NIR absorption due to \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) electron transitions, whereas the pentacene film exhibits weaker absorption in the NIR region, and the C\({}_{60}\) film shows no significant absorption in this wavelength range. The pentacene (30 nm)/PbPc (20 nm), C\({}_{60}\)(30 nm)/PbPc (20 nm), and pentacene (30 nm)/PbPc (20 nm)/C\({}_{60}\)(20 nm) films display desirable NIR absorption, which stems from the NIR absorption of PbPc film. It is observed that the NIR absorption of the pentacene/PbPc/C\({}_{60}\), pentacene/PbPc, and C\({}_{60}\)/PbPc films is stronger than that of the PbPc film, and that of the pentacene/PbPc/C\({}_{60}\), pentacene/PbPc films is higher than that of the C\({}_{60}\)/PbPc film. Generally, the evaporated PbPc film contains the monoclinic and triclinic phases, and the triclinic phase has higher NIR absorption than the monoclinic. Therefore, the higher the triclinic phase content in the PbPc film, the stronger the NIR absorption of the PbPc film, and thus, the more triclinic phase is expected to be produced in the evaporated PbPc film. In this place, the enhanced NIR absorption of the pentacene/PbPc/C\({}_{60}\), pentacene/PbPc, and C\({}_{60}\)/PbPc films can be attributed to the increased triclinic phase content and crystallinity of PbPc films due to the template  
Figure 1: Structure schematic diagrams of the NIR OPTs with different active layer structures. Devices (**a–d**) correspond to Dev-A, Dev-B, Dev-C, and Dev-D, respectively.  
inducing of C\({}_{60}\) or pentacene film, and pentacene has better template inducing for PbPc than C\({}_{60}\), and thus the pentacene/PbPc/C\({}_{60}\) and pentacene/PbPc films display stronger NIR absorption than C\({}_{60}\)/PbPc film. It is established that the high-mobility small-molecule materials have a good induction template to the low-mobility ones. The reason is that the high-mobility small-molecule materials always have a molecular orientation nearly perpendicular to the _ab_ plane, and such a molecular orientation will produce a nice carrier transport in the _ab_ plane, which is beneficial for field-effect transistors. Therefore, a better molecular orientation can be acquired in a low-mobility small-molecule material via template induction, leading to an enhancement in crystallinity, light absorption, etc.  
In the evaluation of an OPT, there are two crucial parameters for assessing its photosensitive performance: photoresponsivity (\(R\)) and photo-to-dark current ratio (\(P\)). These parameters are defined as follows:  
\[R=I_{ph}/P_{opt}=|I_{ill}-I_{dark}|/P_{inc}A \tag{1}\]  
\[P=I_{ph}/|I_{dark}| \tag{2}\]  
where \(I_{\text{ph}}=\mid I_{\text{ill}}-I_{\text{dark}}\mid\) is photocurrent, and \(I_{\text{dark}}\) and \(I_{\text{ill}}\) are the drain currents in the darkness and illumination, respectively. Where \(P_{\text{opt}}\) stands for the optical power incident on the device channel, \(P_{\text{inc}}\) represents the power density of incident light, and \(A\) denotes the effective irradiated area. The detectivity (\(D^{\bullet}\)) signifies the minimum optical power required to generate photocurrent, while the external quantum efficiency (\(EQE\)) indicates the capacity to produce photo-generated carriers per individual incident photon. These parameters are defined as follows [28]:  
\[D^{*}=\frac{RA^{1/2}}{\left(2 qI_{dark}\right)^{1/2}} \tag{3}\]  
where \(q=1.6\times 10^{-19}\) C represents the elementary charge. And  
\[EQE=\frac{I_{ph}/q}{P_{inc}\lambda/hc}=R\frac{hc}{q\lambda} \tag{4}\]  
Here, \(h=6.6\times 10^{-34}\) J-s is [PERSON]'s constant, \(\lambda\) is incident wavelength, and \(c=3.0\times 10^{8}\) m/s is velocity of light.  
Figure 2: Photo-absorption spectra of PbPc, pentacene, pentacene/PbPc, C\({}_{60}\), C\({}_{60}\)/PbPc, and pentacene/PbPc/C\({}_{60}\) films.