Introduction

Photodetectors (PDs) with the ability to capture optical signals and convert them into electrical signals, are significance in optoelectronic applications1,2,3,4. Particularly, uncooled broadband PDs capable of covering the near infrared (NIR) region with low-cost and high-performance are vital components for optical communication, national security, military monitoring, and bioimaging5,6,7. Currently, the commercial broadband PDs were fabricated by mature materials with narrow bandgap, such as silicon (Si), indium gallium arsenide (InGaAs), and mercury cadmium telluride (HgCdTe) etc. With the rapid development of detection technology, they achieved excellent response within visible to short-wave-NIR (eg., Si PDs)8 or the second near-infrared (NIR-II) region (eg., InGaAs PDs)9. However, to our knowledge, few of them are targeted to realize high performance in the whole region spanning from deep ultraviolet (DUV) to NIR-II. Previously, to construct fully spectral responsive PDs, two or more PDs should be integrated into one system, inevitably bringing about large size, high price, and especially for incomparable responsiveness among different PDs. Meanwhile, these PDs usually encounter with complex fabrication process and low temperature working conditions etc. All of these cause huge difficulties in practical applications.

All inorganic lead halide (CsPbX3, X = Cl, Br or I) perovskite quantum dots (PQDs) have attracted numerous attention in solar cells, light-emitting diodes (LEDs), lasers and PDs, owing to their excellent light sensitivity, tunable bandgap, large absorption coefficient, and high collection efficiency of carriers10,11,12,13,14,15,16. Especially, iodine based PQDs process narrow bandgap (∼1.73 eV) with a wide spectral response, exhibiting a great potential for photoactive layer to constructing broadband PDs17,18,19. Nevertheless, the current PDs based on CsPbI3 PQDs still suffer from low sensitivity, slow response time, and poor stability, due to the high trap density and susceptibility to UV light and oxygen20,21,22. In addition, its responsive wavelength was concentrated in UV–Visible region (350 nm–750 nm), uncapable of covering the DUV–UV (200 nm–350 nm) and NIR (>750 nm) because of the insensitivity for DUV–UV light and limitation of bandgap23,24. Therefore, how to overcome the above issues as the main challenges for obtaining broadband and high performance PDs.

Doping with lanthanide ions (eg.,Ce3+, Eu3+, Eu2+, Sm3+, Nd3+, Er3+) has emerged to boost the optical and electrical performance of PQDs, such as improving the photoluminescence quantum yields (PLQYs), enhancing UV- and long term- stability, and decreasing the trap density25,26,27,28,29. It is mainly assigned to their unique spectroscopic properties and atomic radii suitable for the tolerance factor of PQDs13,27,30,31,32. Meanwhile, a hybrid structure combining perovskites with the organic or inorganic narrower bandgap semiconductor materials (e.g.,BTP-4Cl:PBDB-TF, PbSe, HgTe, PbS) is one of a promising strategy to achieve high performance broadband PDs with NIR spectral response33,34,35. Among them, lead sulfide (PbS) quantum dots (QDs) with easily adjustable absorption range to the NIR-II region via the size dependent quantum confinement36,37. Besides, it has large absorption coefficient (∼106  M-1 cm-1) and better chemical stability38.

Aiming at the low DUV–UV response of PDs, the downshifting luminescence materials provide an effective route to convert DUV–UV light into visible or NIR photons, and subsequently being recaptured by the integrated PDs8,23. Various luminescent conversion materials including the quantum cutting materials with the PLQYs of ~200% have been implemented to significantly improve the UV response of solar cells or PDs (eg., Si and perovskites)39,40. But the huge photon energy loss by directly integrating with photoelectric devices always happens, stemming from the random emission direction of luminescent conversion materials. Luminescent concentrator (LC) devices with directional emission consisting of transparent polymer sheets doped with luminescent species can collect and concentrate photon energy to minimize the photons loss through waveguide effect.

In this work, high PLQYs (93.5%) and stability, and less defects of CsPbI3:Ho3+ PQDs were synthesized, served as the UV–Visible photoresponsivity layer for broadband PDs. To broaden the NIR-II response of broadband PDs, the PbS QDs with Visible-NIR absorption region were directly assembled on the surface of CsPbI3:Ho3+ PQDs to form a heterojunction composite. Meanwhile, the Cr3+, Ce3+, Yb3+, Er3+ doped CsPbCl3 PQDs with the high PLQYs (~179%) were incorporated into the polymethyl methacrylate to prepare a quantum cutting LC (QC–LC), which can convert DUV–UV light into NIR photons and subsequently being recaptured by the integrated PDs. This broadband PDs consists of QC–LC and CsPbI3:Ho3+ - PbS QDs heterojunction to realize the full spectrum response spanning from 200 nm–1700 nm, showing the detectivity of 3.19 × 1012 Jones at 260 nm, 1.05 × 1013 Jones at 460 nm, and 2.23 × 1012 Jones at 1550 nm. Furthermore, this broadband PDs demonstrate outstanding air- and UV- stability and high-contrast imaging applications.

Results

In general, the commercial broadband PDs (UV–NIR) usually by integrating two or more PDs (eg., GaN, Si, and InGaAs) for detecting DUV to UV, Visible to short-wave—NIR, and NIR light (Fig. 1a), respectively, which can bring complex fabrication process and higher costs. Herein, we successfully fabricated a broadband responsive PD from DUV to NIR-II (200 nm–1700 nm) with the device structures of CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs doped polymethyl methacrylate QC–LC / FTO / SnO2 / CsPbI3:Ho3+ - PbS QDs heterojunction / MoO3 / Au, as shown in Fig. 1b. The corresponding cross-sectional scanning electron microscope (SEM) image of the broadband PD is displayed in Fig. S1. In such devices, the CsPbI3:Ho3+ PQDs with outstanding optoelectronic performance is responsible for UV–Visible light (350 nm–700 nm). The PbS QDs were prepared through hot injection method exhibits broadband absorption from 500 nm to 1700 nm (Fig. 1c). Then, they were assembled on the surface of CsPbI3:Ho3+ PQDs via mechanical stirring, serving as the NIR-II absorption material. Finally, QC–LC with the strong absorption from 200 nm to 400 nm and efficiently converts them into typical NIR emission of Yb3+ and Er3+ through quantum cutting process (Fig. 1d). Consequently, these NIR photons can be further reabsorbed by the CsPbI3:Ho3+—PbS QDs heterojunction, thereby expanding the response in DUV to UV region (Fig. 1e).

Fig. 1: DUV—NIR-II broadband PD.
figure 1

a The detection range for the commercial GaN PDs, Si PDs, InGaAs PDs, and this work. b Schematic of a broadband PD configuration. c Absorption and emission spectra of PbS QDs. Inset is the image of PbS QDs. d Energy level diagram of QC–LC. e Absorption spectra of QC–LC and CsPbI3:Ho3+—PbS QDs, and emission spectra of QC–LC

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To obtain the high performance and broadband response in a single PD, a series of optical and electrical experiments were conducted. Firstly, to improve the performance of PQDs, lanthanide ions (Ho3+) was selected as dopant to incorporate into CsPbI3 PQDs prepared by modified hot injection method41,42,43. As revealed in the transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images (Fig. 2a and S2–S4), the undoped CsPbI3 and CsPbI3:Ho3+ PQDs with the similar cubic shape were obtained. The average size of CsPbI3 PQDs is ~11.6 nm, which gradually decreases to ~11.4 nm, ~11.1 nm, ~10.8 nm and ~10.5 nm with increasing Ho3+ concentrations from 1.4% to 8.3%, respectively (Fig. S5). The practical Ho3+ doping concentration was identified by the inductively coupled plasma optical emission spectrometry (ICP–OES) and the X-ray photoelectron spectroscopy (XPS) (Table S1). The lattice distances of the (100) plane of undoped CsPbI3 and CsPbI3:Ho3+ PQDs are determined to be ~6.3 Å and ~6.2 Å. The lattice shrinkage is ascribed to the replacement of the larger radius of Pb2+ (~119 pm) by smaller size of Ho3+ (~90.1 pm)44,45,46. The energy dispersive X-ray (EDX) mapping images demonstrate that all of the elements (cesium, lead, iodine, and holmium) exist in CsPbI3:Ho3+ PQDs (Fig. S6). The X-ray diffraction (XRD) patterns in Fig. S7 shows that undoped CsPbI3 and CsPbI3:Ho3+ PQDs have the same cubic structure without impurity peak, and the (100) peaks of PQDs gradually shift to higher diffraction angle after Ho3+ doping. Meanwhile, the XPS survey spectra evidence that the appearance of Ho3+ 4d peaks in CsPbI3:Ho3+ PQDs, and the peaks of Cs+ 3d, Pb2+ 4 f, and I- 3d are presented in undoped CsPbI3 and CsPbI3:Ho3+ PQDs (Fig. 2b and S8). Compared with the undoped CsPbI3 PQDs, the binding energy of Pb2+ 4f5/2 and 4f7/2 moves to lower energy after Ho3+ doping, while Cs+ 3d and I- 3d shows slight changes. Based on the above observations, we confirm that Ho3+ ions are successfully doped into the CsPbI3 PQDs and mainly replace the Pb2+ sites.

Fig. 2: Ho3+ doped CsPbI3 PQDs.
figure 2

a TEM and HR-TEM (right) of undoped CsPbI3 and CsPbI3:Ho3+ (6.4%) PQDs. b XPS spectra of Ho3+ (4d) and Pb2+ (4 f) of undoped CsPbI3 and CsPbI3:Ho3+ (6.4%) PQDs. Absorption and emission spectra (c), and PLQYs and PL lifetime (d) of undoped CsPbI3 and CsPbI3:Ho3+ PQDs with different Ho3+ concentrations. e Mott–Schottky of CsPbI3 and CsPbI3:Ho3+ (6.4%) PQDs. f I-V curves with electron-only devices based on CsPbI3 and CsPbI3:Ho3+ (6.4%) PQDs

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Figure 2c shows the absorption and emission spectra of pristine CsPbI3 and CsPbI3:Ho3+ PQDs with various Ho3+ doping concentration. The absorption peak of CsPbI3 PQDs gradually blue shifts from 683 nm to 678 nm with increasing Ho3+ concentration. In line with the absorption peak variation, the exciton bandgap increases from 1.76 eV of pristine CsPbI3 to 1.82 eV of CsPbI3:Ho3+ (6.4%) (Fig. S9). It can be mainly due to the lattice contraction by substituting Pb2+ with Ho3+ (Supplementary Note 1)47,48. Accordingly, the exciton emission peak of CsPbI3 PQDs moves to high energy from 693 nm to 685 nm with Ho3+ doping (Fig. S10). Significantly, after Ho3+ doping, the emission intensity of CsPbI3 PQDs rapidly enhances with the optimal Ho3+ concentration of 6.4%. The PLQYs of CsPbI3:Ho3+ PQDs reaches remarkable 93.5% (Fig. 2d). In addition, the exciton decay lifetimes gradually decrease from 183.5 ns of pristine CsPbI3 PQDs to 82.6 ns of CsPbI3:Ho3+ (8.3%) PQDs (Fig. S12). Then, the radiative rates (kr) and nonradiative rates (knr) of CsPbI3 and CsPbI3:Ho3+ PQDs were calculated according to their PLQYs and PL decay lifetimes. As displayed in Table S2, the kr of PQDs increases about 3.44 folds from 2.79 × 106 S-1 of undoped CsPbI3 PQDs to 9.59 × 106 S-1 of CsPbI3:Ho3+ (6.4%) PQDs. The knr of CsPbI3 PQDs is 2.66 × 106 S-1, which decreases to 0.67 × 106 S-1 of CsPbI3:Ho3+ (6.4%) PQDs. Compared to the undoped CsPbI3 PQDs, the kr process becomes dominant for CsPbI3:Ho3+ PQDs, leading to the high emission efficiency of CsPbI3:Ho3+ (6.4%) PQDs. Based to the formula44:

$${k}_{r}=\frac{n{e}^{2}{{\rm{\omega }}}^{2}}{3{{\rm{\varepsilon }}}_{0}{m}_{0}{c}^{3}}{f}_{0}{\left(\frac{{\rm{\mu }}}{M}{E}_{b}\right)}^{1.5}\frac{1-{e}^{\frac{-\Delta E}{{kT}}}}{\Delta E}$$
(1)

where μ, k, m0, ε0, ΔE, ω, and n are the equivalent mass of exciton, the Boltzmann’s constant, the mass of exciton, the vacuum permittivity, the energy line width of PQDs, the optical transition frequency and the refractive index. The kr can be mainly related to the exciton binding energy (Eb) of PQDs. The Eb is achieved from the temperature-dependent PL intensity of PQDs by21:

$${I}_{(T)}=\frac{{I}_{0}}{1+A\exp \left(\frac{{-E}_{b}}{{kT}}\right)}$$
(2)

where the I0 is the PL intensity of PQDs at 0 k, A is the proportional constant. The Eb of CsPbI3:Ho3+ (6.4%) PQDs is 32.8 meV (Fig. S13), and 20 meV for pristine CsPbI3 PQDs49. On the basis of Eq. (1), it suggests that the increasing kr with Ho3+ doping is mainly originates from the increase of the Eb of PQDs.

The role of Ho3+ on the electrical performance of PQDs films were investigated using I-V curves. As shown in Fig. S14 and Supplementary Note 2, the conductivity (σ) was determined to be 1.18 × 10-6 S cm-1 for the pristine CsPbI3 PQDs, which increases to 3.54 × 10-6 S cm-1 in CsPbI3:Ho3+ PQDs. The Mott–Schottky analysis was used to obtain the built-in potential (Vbi) of the devices through capacitance-voltage (C–V) measurements (Fig. 2e and Supplementary Note 3). The CsPbI3:Ho3+ PQDs exhibits higher Vbi (~0.89 V) than that of the undoped CsPbI3 PQDs (~0.81 V), indicating the decreased carrier accumulation and improved carrier transports in PDs50. Meanwhile, according to the space charge limited current (SCLC) method, the trap density of undoped CsPbI3 and CsPbI3:Ho3+ PQDs were deduced to be 1.05 × 1017 cm-3 and 4.08 × 1016  cm-3 (Fig. 2f and Supplementary Note 4). It reduces 2.57 folds of CsPbI3 PQDs with Ho3+ doping, dominating the decrease of knr for CsPbI3:Ho3+ PQDs. The charge carrier mobility increases from 6.35 × 10-4 cm2 V-1 S-1 of pristine CsPbI3 PQDs to 4.94 × 10-3 cm2 V-1 S-1 of CsPbI3:Ho3+ PQDs. Furthermore, the air- and UV- stability of CsPbI3 PQDs significantly improves after Ho3+ doping (Fig. S15). It can be seen that the emission intensity of undoped CsPbI3 PQDs degrades rapidly, almost disappears after 8 days storage and 12 h UV irradiation. In contrast, the emission intensity of CsPbI3:Ho3+ PQDs maintains about 95% and 90% of the original intensity after 50 days storage and 24 h UV radiation. The tolerance factor of CsPbI3:Ho3+ PQDs was estimated to be 0.813, larger than that 0.807 of pristine CsPbI3 PQDs (Fig. S16). Therefore, reduced trap density, boosted the carrier mobility and conductivity, enhanced the stability, and increased tolerance factor of CsPbI3 PQDs with Ho3+ doping account for the improved performance for broadband PDs based on CsPbI3:Ho3+ PQDs.

We next performed the optical and electrical analysis of CsPbI3:Ho3+ PQDs by combining with PbS QDs. As shown in Fig. 3a and S17–S18, the PbS QDs exhibits the cubic shape with an average diameter of ~4.4 nm were obtained by typical hot injection method51. Then, the smaller sized PbS QDs would attach on the CsPbI3:Ho3+ PQDs surface to formation of a heterojunction composite by electrostatic interactions (Fig. 3b and S19). We further revealed the HR-TEM of CsPbI3:Ho3+—PbS QDs heterojunction, in which the lattice plane of CsPbI3:Ho3+ (100) and PbS (200) can be clearly identified (Fig. 3c). The CsPbI3:Ho3+—PbS QDs heterojunction exhibits a complementary absorption in the visible to NIR-II region (400 nm–1700 nm), where the enhanced absorption within 400 nm–681 nm is corresponding to exciton absorption of CsPbI3:Ho3+ PQDs (Fig. S20). The PL intensity of CsPbI3:Ho3+ PQDs dramatically reduces after integrating with PbS QDs (Fig. 3d), accompanied by the shortening of exciton lifetime of CsPbI3:Ho3+ PQDs from 97.5 ns to 62.3 ns for CsPbI3:Ho3+—PbS QDs heterojunction (Fig. 3e). Those results indicate that the CsPbI3:Ho3+—PbS QDs heterojunction can effective improve the carrier extraction and transfer. Meanwhile, the NIR emission of PbS QDs slightly increases for CsPbI3:Ho3+—PbS QDs heterojunction under 808 nm excitation (Fig. S21), enabling the high responsivity of PDs in NIR region.

Fig. 3: Photoelectric characteristics of CsPbI3:Ho3+- PbS QDs heterojunction.
figure 3

TEM images of PbS QDs (a) and CsPbI3:Ho3+—PbS QDs (b). c HR-TEM image of CsPbI3:Ho3+—PbS QDs. d Emission spectra of CsPbI3:Ho3+ PQDs and CsPbI3:Ho3+ - PbS QDs. e PL lifetime of CsPbI3:Ho3+ PQDs and CsPbI3:Ho3+—PbS QDs at 692 nm. f I-V curves of CsPbI3:Ho3+ PQDs and CsPbI3:Ho3+—PbS QDs. UPS curves of CsPbI3:Ho3+ PQDs (g) and PbS QDs (h). The Fermi level of CsPbI3:Ho3+ PQDs and PbS QDs are -4.29 eV and -5.18 eV, respectively. i Schematic illustration of the each layer in PDs

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To further investigate the charge recombination dynamic of CsPbI3:Ho3+ PQDs after combining with the PbS QDs, the I-V curves of CsPbI3:Ho3+ PQDs and CsPbI3:Ho3+—PbS QDs heterojunction were measured (Fig. 3f). It can be seen that CsPbI3:Ho3+ - PbS QDs heterojunction obtains higher σ (5.11×10-6 S cm-1), compared with the CsPbI3:Ho3+ PQDs device (3.54 × 10-6 S cm-1). The ultraviolet photoelectron spectroscopy (UPS) was employed to determine the energy levels of the CsPbI3:Ho3+ PQDs and PbS QDs (Fig. 3g, h). The valence band maximum (VBM) and the conduction band minimum (CBM) of CsPbI3:Ho3+ PQDs and PbS QDs are deduced to be -5.65 eV and -5.31 eV, and -3.83 eV and -4.43 eV, respectively. As illustrated in Fig. 3i, the band alignment between CsPbI3:Ho3+ PQDs and PbS QDs enables them to form a heterojunction. Under visible light illumination, the generated electrons in CsPbI3:Ho3+ PQDs tends to enter electron transport layer (ETL, SnO2). While the holes move to PbS QDs, subsequently transfer to hole transport layer (HTL, MoO3). Meanwhile, the PbS QDs captures the NIR light to produce electrons and holes, the electrons transfer to ETL by CsPbI3:Ho3+ PQDs, and the holes directly enter into HTL. Thus, the PDs combining CsPbI3:Ho3+ PQDs with PbS QDs brings about the broadband response from visible to NIR-II.

The structural and photophysical interaction between of CsPbI3:Ho3+ PQDs and PbS QDs were calculated through density functional theory (DFT) (Supplementary Note 5). Figure 4a, b shows the optimized CsPbI3:Ho3+—PbS QDs heterojunction with the PbI2 / PbS and CsI / PbS interfaces. The adhesive energies of PbI2 / PbS interface (-3.22 eV) is smaller than that of the CsI / PbS interface (-2.07 eV), indicating that the PbI2 / PbS is more stable interface structure52. The strong electron interface coupling between PbS and CsPbI3:Ho3+ is revealed by the density of states (DOS) functional (Fig. 4c), where the significant overlap between the Pb 6p and I 5p states in CsPbI3:Ho3+ PQDs with the S 2p orbital in PbS QDs. Furthermore, the bandgap of CsPbI3:Ho3+—PbS QDs heterojunction shows a slight smaller than that of the CsPbI3:Ho3+ PQDs, benefiting for the effectively transition of electrons from VBM to CBM. In order to understand the charge distribution of the CsPbI3:Ho3+—PbS QDs heterojunction interface, the charge density difference Δρ is obtained by53:

$$\varDelta {\rho }={{\rho }}_{{hete}}-{{\rho }}_{{PbS}}-{{\rho }}_{P{QDs}}$$
(3)

where ρhete is the charge density of the fully relaxed CsPbI3:Ho3+—PbS QDs heterojunction, ρPbSand ρPQDs are the isolated PbS QDs and CsPbI3:Ho3+ PQDs slab. As shown in Fig. 4d, the charge depletions underneath Pb / I atoms for PbI2 / PbS interface, which is favorable for efficient charge migration from CsPbI3:Ho3+ PQDs to PbS QDs. Meanwhile, the x–y plane average charge difference Δq is calculated by53:

$$\varDelta q={\int }_{\!-{{\infty }}}^{{{\infty }}}{dy}{\int }_{\!-{{\infty }}}^{{{\infty }}}\Delta {\rm{\rho }}\,d{\chi }$$
(4)
Fig. 4: DFT calculations of CsPbI3:Ho3+- PbS QDs heterojunction.
figure 4

Optimized CsPbI3:Ho3+-PbS heterojunction with the (a) CsI / PbS and (b) PbI2 / PbS interfaces from side view. c Calculated DOS of the PbS, CsPbI3:Ho3+ and CsPbI3:Ho3+-PbS heterojunction. Charge density difference (d) x–y plane average charge difference and charge displacement curves along the z-direction of the PbI2 / PbS interface (e). f The Pb vacancies (VPb) and I vacancies (VI) defect formation energies for the CsPbI3:Ho3+ and CsPbI3:Ho3+ - PbS heterojunction with respect to the chemical potential of the I atom

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In general, the charge accumulation and depletion can be obtained based on the positive and negative Δq, respectively54. Fig. 4e displays the Δq of PbI2 / PbS interface, where a dipole can be produced between the CsPbI3:Ho3+ PQDs surface and PbS QDs because of the charge accumulation and depletion, facilitating the charge separation and transport. Moreover, the charge displacement (CD) functional (ΔQ) can be achieved through integrating the x–y plane average charge difference Δq along the z direction52:

$$\varDelta Q={\int }_{\!-{{\infty }}}^{z}\varDelta q\,{dz}$$
(5)

where the positive ΔQ values indicates the charge transfer from the right to the left across the perpendicular plane through this direction and vice versa. The CD plot of PbI2 / PbS interface along z direction present that the whole negative for CsPbI3:Ho3+ PQDs side, which means that the charge transport from CsPbI3:Ho3+ PQDs to PbS QDs. As shown in Fig. 4f, we calculated the defect formation energies of Pb2+ vacancies (VPb) and I- vacancies (VI) in CsPbI3:Ho3+ PQDs and CsPbI3:Ho3+—PbS QDs heterojunction, where the VPb and VI exhibit higher defect formation energies after PbS QDs coupling, benefiting for the responsivity of PDs23.

To fabricate QC–LC, the cubic and uniform Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs with the average diameter of ~8.4 nm were prepared through the hot-injection method (Fig. S22)55. The Cr3+ and Ce3+ were selected to improve emission intensity and boost DUV absorption of CsPbCl3 PQDs. The emission of Yb3+ and Er3+ ions locate within the absorption of CsPbI3:Ho3+—PbS QDs heterojunction, and has high efficient quantum cutting behavior in CsPbCl3 PQDs. All the elements (Cs, Pb, Cl, Cr, Ce, Yb, and Er) exist in Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs (Fig. S23). Figure 5a displays the UV–Visible absorption spectra of undoped, Cr3+ doped CsPbCl3 PQDs, Cr3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs, and Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs. It can be seen that the absorption peak of CsPbCl3 PQDs locates at 405 nm, and blue shifts to 391 nm after Cr3+ / Ce3+ / Yb3+ / Er3+ doping, ascribed to the lattice contraction of PQDs by substituting Pb2+ with the smaller size doping ions47,56. Moreover, the absorption spectra of Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs present a significant enhancement within 200 nm–280 nm after Ce3+ doping, attributed to absorption contribution of 4 f - 5d for Ce3+42. Fig. 5b shows the emission spectra of undoped CsPbCl3 PQDs, CsPbCl3:Cr3+ PQDs, CsPbCl3:Cr3+, Yb3+, Er3+ PQDs, and CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs. It is found that an obvious exciton emission band centering at 410 nm for CsPbCl3 PQDs was observed under 365 nm excitation, which significant improvement after Cr3+ doping. Except for exciton emission, two additional emission peaks in CsPbCl3:Cr3+, Yb3+, Er3+ and CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs were identified, appearing at 980 nm and 1540 nm, assigned to Yb3+ (2F5/2 - 2F7/2) and Er3+ (4I13/2 - 4I15/2). Similarly to the previous reported, they originate from the quantum cutting energy transfer from CsPbCl3 PQDs to Yb3+, and further Yb3+ to Er3+57,58. Furthermore, the introduction of Ce3+ regards as an intermediate bridge to match the energy between CsPbCl3 PQDs and NIR quantum cutting emission of Yb3+59. Meanwhile, the PLQYs is estimated by23:

$${PLQYs}=\frac{{N}_{{em}}}{{N}_{{abs}}}=\frac{\int {I}_{{sample}}\left({\rm{\lambda }}\right)-{I}_{{ref}}\left({\rm{\lambda }}\right)d{\rm{\lambda }}}{\int {E}_{{ref}}\left({\rm{\lambda }}\right)-{E}_{{sample}}\left({\rm{\lambda }}\right)d{\rm{\lambda }}}$$
(6)

where Nem and Nabs are the number of emission and absorption photons of PQDs. Isample and Esample present the spectral intensity of the emitted light and excitation light of samples, and Iref and Eref are the spectral intensity of the emitted light and excitation light for a reference cuvette containing toluene. It should be highlighted that the PLQYs of CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs reaches to 179% (Fig. 5c). The higher PLQYs are concluded in the following three aspects: (1) reduced knr of CsPbCl3 PQDs after Cr3+ doping, (2) enhanced energy transfer from CsPbCl3 PQDs to Yb3+ and improved DUV absorption after Ce3+, (3) NIR quantum cutting emission (2F5/2 - 2F7/2) after Yb3+ doping. The energy transfer process for Cr3+, Ce3+, Yb3+, Er3+ doped CsPbCl3 PQDs was represented in Fig. 5d.

Fig. 5: Optical characteristics of QC-LC.
figure 5

Absorption spectra (a), emission spectra (b), and PLQYs (c) of undoped, Cr3+, Cr3+ / Yb3+ / Er3+, and Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs. d Energy level diagram of Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs. e The emission spectra of QC–LC using an integrating sphere method. To accuracy obtained the face emission spectra of QC–LC, the black tapes with high light absorption was used to the edges of QC–LC. The inset is the picture of QC-LC. f Normalized PL intensity of QC–LC and Cr3+ / Ce3+ / Yb3+ / Er3+ doped CsPbCl3 PQDs films as a function of storage time

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Then, the CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs were incorporated into a PMMA polymer matrix to form QC–LC. The performance of the QC–LC was achieved using an integrating sphere. As seen in Fig. 5e and S24, the QC–LC presents the similar emission spectra with CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs and high transparency. Considering directly integrating one of edge of QC–LC with the PDs, the emission intensity of the edge, face, and total of LC were recorded (Fig. 5e). The emission intensity of the edge is higher than that of face, attributed to the waveguide effect of QC–LC. The edge and face emission intensity ratio of the QC-LC was 73.61%, which is nearly to the ideal light trapping efficiency of 75%13,60. Moreover, the stability of CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs were also remarkably enhanced by incorporating them into LC, in which their PL intensity remains 97.5% after 100 days storage (Fig. 5f). Based on the high luminescence performance and stability and transparency, the QC–LC can be severed as a converter to improve the DUV–UV responsivity for broadband PDs.

Figures 6a and S25 shows the photocurrent-time (Ip–t) response curves of FTO / SnO2 / CsPbI3 PQDs / MoO3 / Au (PD1), FTO / SnO2 / CsPbI3:Ho3+ PQDs / MoO3 / Au (PD2), FTO / SnO2 / CsPbI3:Ho3+-PbS QDs heterojunction / MoO3 / Au (PD3), and QC-LC / FTO / SnO2 / CsPbI3:Ho3+-PbS QDs heterojunction / MoO3 / Au (PD4) in dark and 260 nm, 460 nm, and 1550 nm illumination at a bias of 0.5 V, respectively. It can be seen that the Ip reach to 3.09 µA of PD4, whereas only 0.84 µA for PD1 under 460 nm illumination. Compared with the PD1, the Ip of PD4 increase 3.68 folds for 460 nm, due to the reduce density of defects and boost carriers mobility after Ho3+ doping, and passivized surface vacancy by PbS QDs. Besides, the dark current (Id) of PD1–PD4 were recorded in Fig. S26. The value of Id was about 2.44 pA for PD1, which decreases to 0.26 pA for PD4. Furthermore, the light to dark current ratio of PD4 was 1.19 × 107, which further suggest the formation of uniform CsPbI3:Ho3+—PbS QDs heterojunction film35,61. Interestingly, the high Ip signals of DUV (0.91 μA of 260 nm) and NIR-II (0.64 μA of 1550 nm) were observed for PD4. The enhancement is mainly due to the strong absorption in the DUV region and extremely efficient NIR quantum cutting emission of QC–LC, and high performance NIR-II broadband absorption of PbS QDs. Figure 6b, c and S27–S28 shows the responsivity (R), detectivity (D*), and external quantum efficiency (EQE) of PD1–PD4 (Supplementary Note 6). Compared to the PD1 with lower R (119.04 mA W-1) and D* (5.43 × 1011 Jones) and EQE (16.9%), the maximum R and D* and EQE of the PD4 were calculated to be 596.62 mA W-1 and 1.05 × 1013 Jones and 84.6% under 460 nm illumination. Furthermore, these values of PD4 were largely enhanced in the DUV–UV and NIR-II region, realizing a broadband spectral response ranging from 200 nm to 1700 nm, where the R and D* and EQE of PD4 were 180.6 mA W-1 and 164.5 mA W-1, 3.19 × 1012 Jones and 2.23 × 1012 Jones, and 45.3% and 7.2% under 260 nm and 1550 nm, respectively. The statistical analysis on 25 devices for PD4 under 260 nm, 460 nm, and 1550 nm illumination, respectively, showing the excellent repeatability of this broadband PDs (Fig. S29). Importantly, the D* of PD4 improves two orders that of commercial GaN PDs and Si PDs and InGaAs PDs under the same detection condition. Meanwhile, the noise current (In) of PD4 (2.34 × 10-14 A Hz-1/2) was lower than that of PD1 (8.33 × 10-14 A Hz-1/2) at 1 Hz in Fig. S30. According to the noise-equivalent power (NEP) equation, the D* can also be calculation by ref. 6:

$$D* =\frac{\sqrt{{SB}}}{{NEP}}({cm}{{Hz}}^{-\frac{1}{2}}{W}^{-1}{or\; Jones})$$
(7)
$${NEP}=\,\frac{{I}_{n}}{R}$$
(8)

where S and B are the active layer area and the electrical bandwidth. The D* of PD4 was obtained to be 7.71 × 1011 Jones of 260 nm, 2.55 × 1012 Jones of 460 nm, and 5.23 × 1011 Jones of 1550 nm, respectively (Fig. S31). Compared with the PD1, the D* of PD2–PD4 shows the consistent increases trend for it calculated by the dark current, indicating a great potential for weak light detection.

Fig. 6: Performances of PDs.
figure 6

a Ip–t curves of PD4 under the 260 nm, 460 nm, and 1550 nm illumination, respectively. b Responsivity of PD1–PD4. c Detectivity of this work (PD4), GaN PDs, Si PDs and InGaAs PDs, respectively. d Response time of the PD4 under 460 nm illumination. e, f The evolution of responsivity under 25% RH at room temperature and UV radiation. g Schematic diagram of imaging arrays. DUV–Visible-NIR-II mode imaging output for “U, E, H” mask under 260 nm (h), 460 nm (i) and 1550 nm (j), respectively

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The photon-response times of PDs are exhibited in Fig. 6d and S33–S34. The rise (τr) and decay times (τd) of the PD4 were measured to be ~82 µs and ~83 µs under 460 nm illumination, much faster than that of the PD1 (~202 µs and ~223 µs). The fast response time can be attributed to the restrained defects after Ho3+ doping, and the enhanced charge extraction and transport by integrating with PbS QDs. Figure S35 represent the relative response trend versus the frequency of 460 nm light. The -3dB bandwidth of PD4 was calculated as 4.04 KHz, indicating a rapid response to 460 nm light signal. According to the law: f-3dB = 0.35/τr62 the response time of PD4 is estimated to be 86.6 µs, similar to the response time measured in Fig. 6d. Furthermore, compared to the recent works of broadband PDs (Table 1 and Fig. S36), this broadband PDs shows the higher D* and faster response time from DUV to NIR-II regions. As shown in Fig. S37, the R as a function of light power density (P) can be fitted by the formula: R ∝ Pα63 where α were achieved to be 0.67, 0.56, and 0.66 for PD4 under 260 nm, 460 nm, and 1550 nm, respectively. The α value of the PD4 is not closer to ideal result (α = 1), suggesting that defects still exist in the heterojunction interface64,65,66. Figure 6e, f displays the normalized R variation curves of PD1–PD4 at humidity of 25% and under UV irradiation at room temperature. The R of the PD1 declines to 0% after 100 h storage and 10 h UV irradiation, while the R of the PD4 remained 89% and 99.3% for the original R-value after 100 h storage and 10 h UV irradiation. The enhanced air- and UV- stability of PDs would be attributed to the Ho3+ doping, PbS QDs hybridization, and the buffer layer of QC–LC in the devices.

Table 1 Recent works of broadband PDs
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Based on the outstanding DUV–Visible-NIR-II detection performance, the imaging application of this broadband PD was further explored. As presented in Fig. 6g, the imaging schematic is composed by different light source (DUV of 260 nm, Visible of 460 nm, and NIR-II of 1550 nm), lens, object letters (“U”, “E”, and “H”), broadband PDs and signal collector. We fabricated a 10 × 16 pixel detector array as the each photograph shown in Fig. 6h–j. The thickness of the detectors is 1 mm and the electrode area of each pixel is 1 × 1 mm2. It can be seen that the letters were clearly achieved with high light to dark current ratios under different illuminations, highlighting that the excellent performance of broadband detection and imaging capability for the QC–LC / FTO / SnO2 / CsPbI3:Ho3+-PbS QDs heterojunction / MoO3 / Au PDs. Moreover, these results strongly indicate that the remarkable adaptability of the PDs and great promising for broadband imaging without low temperature working conditions.

Discussion

In this work, an excellent performance DUV–Visible-NIR-II broadband PD was successfully fabricated based on QC–LC and CsPbI3:Ho3+—PbS QDs heterojunction, exhibiting the high detectivity of 3.19 × 1012 Jones at 260 nm, 1.05 × 1013 Jones at 460 nm, and 2.23 × 1012 Jones at 1550 nm, respectively. The optimized CsPbI3:Ho3+ PQDs demonstrated low trap density and high charge mobility compared with the undoped CsPbI3 PQDs, enabling superb responsivity and detectivity of visible region. Then, the hybridization of CsPbI3: Ho3+—PbS QDs heterojunction successfully expand the response of broadband PDs to the NIR-II region, due to the narrow bandgap and large absorption coefficient of PbS QDs. Meanwhile, high efficiency NIR quantum cutting emission (PQLYs ~ 179%) and strong DUV–UV absorption of Cr3+, Ce3+, Yb3+, Er3+ doped CsPbCl3 PQDs were embedded to form QC–LC, which can be integrated with the FTO side to improve the responsivity of PDs for DUV–UV light. Our work shows a promising path for fabricated the high performancebroadband PDs and imaging capability.

Materials and methods

Materials

Cs2CO3 (99.9%), 1-octadecene (ODE, 90%), oleic acid (OA, 85%), oleylamine (OAm, 70%), PbO (99.99%), PbI2 (99.99%), HoI3 (99.99%), PbCl2 (99.9%), CrCl3 (99.99%), CeCl3 (99.99%), YbCl3 (99.99%), ErCl3 (99.99%), bis(trimethylsilyl) sulfide, toluene, cyclohexane and ethyl acetate (99%) were purchased from Mackline and were used without further purification.

Synthesis of CsPbI3 PQDs

0.8 g Cs2CO3 was added into a mixture of 30 mL of ODE and OA (2.5 mL) and then heated to 150 °C until the white powder was completely dissolved. The mixture was then kept at 120 °C to obtained the Cs-oleate. Then, PbI2 (0.3 mmol), OAm (1.5 mL), OA (1.5 mL), and ODE (10 mL) were added to a 50-mL 3-neck round-bottomed flask and were evacuated with N2, by heating the solution to 120 °C for 1 h. The temperature of the solution was then increased to 180 °C for 10 min. Finally, the Cs-oleate (1 mL) was swiftly injected into the solution. After 10 s, the solution was cooled in an ice bath. The CsPbI3 PQDs were precipitated and then centrifuged, followed by dissolution in toluene.

Synthesis of CsPbI3:Ho3+ PQDs

HoI3 (x mmol), PbI2 (0.3 mmol), OAm (1.5 mL), OA (1.5 mL), and ODE (10 mL) were adequately dissolved at 120 °Cfor 1 h under purging N2 gas. The following steps were the same with synthesis of CsPbI3 PQDs.

Synthesis of Cr3+,Ce3+,Yb3+,Er3+ doped CsPbCl3 PQDs

For the CsPbCl3: Cr3+, Ce3+, Yb3+, Er3+ PQDs, PbCl2 (0.5 mmol), CrCl3 (0.3 mmol), CeCl3(0.2 mmol), YbCl3 (0.3 mmol), ErCl3 (0.2 mmol) were loaded into round-bottom flask with ODE (10 mL), OAm (1.5 mL) and OA (1.5 mL). The following steps were the same with the synthesis of the Ho3+ doped CsPbI3 PQDs.

Fabrication of QC-LC

0.8 g PMMA (MW ~ 350000) was dispersed in 5 mL toluene by sonication, where toluene solution (3 mL) of CsPbCl3: Cr3+, Ce3+, Yb3+, Er3+ PQDs were added. The mixture was sealed and stirred overnight to obtain homogenous slurry. The slurry was centrifuged at 2000 rpm and the supernatants were used for LC fabrication. The above supernatants were spin-coating onto borosilicate glass substrates.

Syntheses of PbS QDs

Firstly, PbO (0.36 g), OA (1 mL), and ODE (15 mL) were stirred and heated to 145 °C under nitrogen atmosphere. Then, ODE (4.0 mL) containing bis (trimethylsilyl) sulfide (168 µL) was quickly injected into the reaction flask, and meanwhile the heater was switched off. The hot solution was naturally cooled down, and the formed oil-soluble PbS QDs were precipitated. The purification was carried by isopropyl alcohol and acetone. Finally, the PbS CQDs were dispersed in toluene to produce a 50 mg mL-1 solution.

Preparation of the CsPbI3:Ho3+-PbS QDs heterojunction composite

In a typical procedure, a 200 μL CsPbI3:Ho3+ solution (10 mg mL-1, toluene) was mixed with 2 mL of PbS solution (1 mg mL-1, toluene), and the total volume remained at 5 mL. Under dark conditions, the suspension was ultrasonicated for 10 min and stirred for 0.5 h. Finally, the CsPbI3:Ho3+—PbS CQDs heterojunction composites were collected by centrifuging at 8000 rpm for 5 min.

Device fabrication

FTO coated glass substrates were etched by zinc powder and HCl to define the electrode patterns and washed in deionized water, acetone, and ethanol for 20 min, respectively. The ultraviolet-ozone was used to remove the organic residues of FTO surface. To fabricate the compact SnO2 layer, the SnO2 colloid solution by water to the concentration of 2.14 wt% and was spin-coated on FTO substrates at 5000 rpm for 30 s and then annealed at 150 °C for 30 min. The CsPbI3:Ho3+—PbS CQDs heterojunction film was fabricated on the SnO2 layer by spin-coating at 2000 rpm for 40 s. For broadband PDs, MoO3 and Au was evaporated on CsPbI3:Ho3+—PbS CQD film layer. Then, the edge surface of QC–LC with the edge size of 1 × 0.4 mm2 was attached and fixed to the ITO layer of PD with area of 1 × 1 mm2. When UV light radiation on the face of QC–LC, then the emitted 400–1700 nm light is coupled out of the edge surface into the FTO of PDs. The visible and NIR lights directly pass through the FTO and reach the PD. Because the QC–LC only occupies a part of surface of the FTO layer and has high transparency for photons with the longer wavelength (>400nm), which would not affect the light collection of PDs.

Characterization

UV/vis-NIR absorption spectra were measured with a Shimadzu UV-3600PCscanning spectrophotometer in the range from 200 to 2500 nm. Patterns were recorded in thin film mode on a Bruker AXS D8 diffractmeter by Cu Kαradiation (λ = 1.54178 Å). Atomic Force Microscope was texted using a DI Innova AFM (Bruker) in light tapping mode. The morphology of the products was recorded with a Hitachi H-8100IV transmission electron microscope (TEM) under an acceleration voltage of 200 kV. A Visible-NIR photomultiplier combined with a double-grating monochromator were used for spectral collection. The X-ray photoelectron spectroscopy (XPS) was carried out in a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) operated at 150 W with a multichannel plate, and a delay line detector under 1.0 × 10-9 Torr vacuum. Nanosecond fluorescence lifetime experiments were performed by the time correlated single-photon counting system. The Mott–Schottky curves via capacitance-voltage measurements of CsPbI3:Ho3+ PQDs were obtained by a Princeton electrochemical workstation. The Xe lamp with the spectral range from 200 nm to 2500 nm equipped with a monochromator was used to generate the monochromatic light to conduct the spectral response measurements. Actually, the intensity of Xe lamp is weak in the region of 200 nm–300 nm, thus we must correct it before the measurement.