## 3 Results and Discussion
### Tri-Layer Planar Heterojunction  
The presence of dark current can significantly affect various aspects of a photodetector's performance, including its photosensitive properties and power consumption. Therefore, in order to improve device performance and reduce power consumption, the dark current should be minimized in the design of the device. In an effort to further diminish the dark current of Dev-B (Si/SiO\({}_{2}\)/pentacene (30 nm)/PbPc (20 nm)/Au), a C\({}_{60}\) hole-blocking layer was deposited onto the surface of the PbPc/pentacene film. Consequently, an OPT (denoted as Dev-D) with the structure of Si/SiO\({}_{2}\)/pentacene (30 nm)/PbPc (20 nm)/C\({}_{60}\) (20 nm)/Au was successfully fabricated. Figure 4e,f display the output and transfer characteristics of Dev-D. The most noticeable difference lies in the dark current, which has been significantly reduced to 81% (\(-\)562.41 nA) compared to that of Dev-B (\(-\)693.78 nA). At the same time, the photocurrent of Dev-D has increased by 1015% and reached 3651.24 nA. As can be seen from Table 1, the \(R\) of Dev-D rises to 1415 mA/W, which is 10 times bigger than that of Dev-B. The \(D^{\star}\) and \(EQE\) reach 7 \(\times\) 10\({}^{11}\) Jones and 206.55%, which are 12 and 9 times larger than those of Dev-B, respectively. And it is noteworthy that the \(P_{\text{max}}\) of Dev-D increases to 1.2 \(\times\) 10\({}^{4}\) substantially, surpassing that of Dev-B by 2.3 times. The decrease in dark current can primarily be attributed to the presence of the C\({}_{60}\) acceptor layer, which can effectively block the holes from flowing towards the drain electrode. And the effective blocking is due to the relatively large energy barriers between the HOMO levels of PbPc and C\({}_{60}\) and the HOMO level of C\({}_{60}\) and the Fermi level of Au, as visually depicted in Figure 6. It can be seen that the energy barriers between the HOMO levels of PbPc and C\({}_{60}\) and between the HOMO level of C\({}_{60}\) and the Fermi level of Au are 1.0 eV and 1.1 eV, respectively, which are both much bigger than those of Dev-B (for Dev-B, the energy barriers between the HOMO levels of PbPc and pentacene and between the HOMO level of PbPc and the Fermi level of Au are 0.1 eV and 0.1 eV, respectively, as shown in Figure 6). In Dev-D, fewer holes can be injected into the C\({}_{60}\) layer due to a big energy barrier lying between the HOMO level of C\({}_{60}\) and the Fermi level of Au, and the holes injected into C\({}_{60}\) layer may transport along the HOMO levels of active layer, and some of them might be blocked at the PbPc/C\({}_{60}\) interface, and only the rest of them could be collected with the drain electrode successfully, thus generating a small drain current. Additionally, the \(\upmu\) of Dev-D is 2.4 \(\times\) 10\({}^{-2}\) cm\({}^{2}\)/V\(\cdot\)s, smaller than that of Dev-B (3.0 \(\times\) 10\({}^{-2}\) cm\({}^{2}\)/V\(\cdot\)s, shown in Table 1). This is also due to the presence of the C\({}_{60}\) hole-blocking layer. Under the equal electric field effect, the hole drift velocity of C\({}_{60}\) is far less than that of pentacene or PbPc. Therefore, the addition of the C\({}_{60}\) layer gives rise to a decrement in both hole drift velocity and mobility. And the increase in photocurrent and photoresponsivity is thanks to the increased exciton dissociation efficiency owing to the newly established exciton dissociation centers located at the PbPc (donor)/C\({}_{60}\) (acceptor) interface. Here, the excitons can be dissociated into free electrons and holes under an interfacial electric field. From the above, we can see that creating exciton dissociation centers in some ways is particularly beneficial for the performance of OPTs [28]. In summary, the overall performance of Dev-D, featuring a tri-layer PHJ structure, is better than that of Dev-A or Dev-B and surpasses that of the NIR OPTs reported in Refs. [26; 27; 31; 38; 39] (see Table 2).