The photovoltaic photodetector structure is Al₂O₃/ITO/HgTe/Ag₂Te/Au. Fig. 2(a) shows the structure diagram and cross-sectional SEM of high-mobility photovoltaic photodetectors. In ambient conditions, a layer of about 50 nm ITO electrode is deposited on the Al₂O₃ substrate. The ITO serves as electron contact, and then a layer of HgTe CQDs film with high carrier mobility obtained by mixed ligand exchange is deposited by drop-coating. The CQDs film is an intrinsic type regulated by surface doping, which helps increase photocurrent. The HgTe CQDs surface is treated with EDT, HCl, and IPA (1∶1∶20 by volume), with the CQDs thickness of around 400 nm. The Ag₂Te nanoparticle solution (Ag⁺ as P-doping) is prepared on the HgTe CQDs film by spin-coating. It is then exposed to 10 mmol/L HgCl₂/methanol solution, which is helpful to diffuse Ag⁺ into the CQDs film. Finally, a layer of gold electrode is evaporated on top with 50 nm thickness. The energy diagram is shown in Fig. 2(b).

Short-wave infrared (SWIR) and mid-wave infrared (MWIR) bands catch much attention because of matching the atmosphere window. In this spectral range, solution-based zinc-blend HgTe colloidal quantum dots (CQDs) become a potential alternative to traditional epitaxial materials for photodetection. However, the performance of CQD photodetectors should be improved, and controlling the transport properties like doping and mobility would be the key to high-performance photodetectors. We employ the mixed phase ligand exchange method to achieve high carrier mobility in HgTe CQDs films, which is more than 1 cm²/(V·s). Meanwhile, different doping types in HgTe CQD solid are realized, such as N, intrinsic, and P types. We also demonstrate that the high carrier mobility improves CQD photovoltaic performance. For example, SWIR and MWIR photovoltaic photodetectors are achieved with intrinsic high mobility HgTe CQDs solid, where the external quantum efficiency (EQE) is 61% for SWIR photovoltaic photodetectors and 30% for MWIR photovoltaic photodetectors. Additionally, the detectivity (D^*) is 4×10¹¹ Jones at 300 K for SWIR photovoltaic photodetectors and 1.2×10¹¹ Jones at 110 K for MWIR photovoltaic photodetectors.

The mixed-phase ligand exchange process involves liquid-phase ligand exchange and solid-phase ligand exchange. In the liquid phase ligand exchange, 4 mL HgTe CQDs in n-hexane would mix with 160 μL β-ME and 8 mg DDAB in DMF, which is stewed for 10 s to accelerate separation. Then the solution is centrifuged, and after decanting the supernatant, 60 μL DMF is adopted to dissolve the CQD solids in centrifuge tubes to obtain stable CQD ink. In this method, β-ME replaces the long-chain ligand on the CQD surface in the liquid phase, and DDAB is a catalyst to assist CQDs transfer from n-hexane to the polar solvent DMF. The CQD films are prepared by spin or drop coating, and then solid-state ligand exchange with EDT/HCl/IPA (1∶1∶50 by volume) solution is performed for 10 s, rinsed with IPA, and dried with N₂. Solid-phase ligand exchange can both remove the additional hybrid ligands on the film surface and stabilize the Fermi level of CQD films. For controllable CQDs doping, as Hg²⁺ can stabilize electrons in CQDs by surface dipoles, we choose mercury salts such as HgCl₂ to regulate CQDs to intrinsic or N types, and in liquid phase ligand exchange, 10 mg HgCl₂ is added to obtain intrinsic CQDs, and 20 mg HgCl₂ is added to obtain N-type CQDs.

Mixed-phase ligand exchange includes liquid-phase and solid-state ligand exchange, which can improve carrier mobility and control the doping density of HgTe CQDs by surface dipole regulation. The TEM image of the MWIR CQDs before and after the liquid phase ligand exchange is shown in Fig. 1(a). The spacing between CQDs is reduced with tight arrangement, which can improve the light absorption by HgTe CQDs and is conducive to improving device performance. Field effect transistor (FET) is adopted to measure the mobility and doping level of carriers in the film, and the structure is shown in Fig. 1(c). The FET transfer curves of N-type, intrinsic, and P-type MWIR HgTe CQDs are shown in Fig. 1(d). The slope of the FET transfer curve is utilized to calculate the carrier mobility. The carrier mobility of N-type, intrinsic type, and P-type SWIR and MWIR HgTe CQDs all exceeds 1 cm²/(V·s). The I-V characteristic curves of high carrier mobility photovoltaic photodetectors on SWIR and MWIR are shown in Figs. 3(a) and (b), and the open circuit voltages on SWIR and MWIR photodetectors are 140 mV and 80 mV, which indicates a strong internal electric field. At zero bias, the photocurrents on high mobility SWIR and MWIR devices are 0.27 μA and 5.5 μA respectively. The input optical signal power of SWIR and MWIR at the blackbody temperature of 874 K is 0.29 μW and 5.46 μW respectively, and the responsivity (ℜ) is obtained. At zero bias, ℜ reaches 0.9 A/W (at 300 K) and 1.0 A/W (at 80 K) for SWIR and MWIR devices respectively. The D^* of high carrier mobility SWIR photovoltaic photodetectors is 4×10¹¹ Jones in all temperature ranges, and that of MWIR photovoltaic photodetectors is 1.2×10¹¹ Jones at 110 K. Additionally, the EQE increases several-fold in high mobility photovoltaic photodetectors, where it is 61% for SWIR devices and 30% for MWIR devices.

The carrier mobility in HgTe CQD films is increased to 1 cm²/(V·s) by the mixed phase ligand exchange method. By adding salt, the doping control of P-type, intrinsic type, and N-type CQD films is realized. Meanwhile, photovoltaic photodetectors in SWIR and MWIR are prepared based on intrinsic high mobility CQD solid. For the 1.9 μm SWIR photodetectors, ℜ is 0.9 A/W and D^* is 4×10¹¹ Jones at 300 K. For the 4.2 μm MWIR photodetectors, ℜ is 1.1 A/W and D^* is 1.2×10¹¹ Jones at 110 K. In addition, the EQE would be improved to 61% for SWIR photodetectors at 300 K and 30% for MWIR at 110 K, without applied bias. The test results show that the transport property control of CQDs can improve the core performance of photodetectors, such as ℜ and D^*. Our study can promote the development of low-cost and high-performance CQDs infrared photodetectors.