Introduction

Photodetectors (PDs) are the technical functional components for capturing and converting ultraviolet (UV) to near-infrared (NIR) photons into electronic outputs1,2,3,4,5. The broadband optical detection ability, especially from UV to NIR range, is critical for applications including medical monitoring, video imaging, optical communication, and civil engineering6,7,8,9,10,11,12. Generally, the commercial silicon PDs present the relatively broad wavelength response range from 400–1100 nm13,14, but usually suffer from high cost and low detectivity, especially in the UV region. Solution-processable broadband PDs based on soluble materials have numerous advantages of low cost, simple preparation, and high sensitivity, which has become the next generation of new detectors15,16,17.

Encouragingly, solution-processable metal halide perovskites process outstanding characteristics of large absorption coefficient, long diffusion length, low trapping density, and high photoluminescent quantum efficiency (PLQY), which have shown unprecedented radical progress for various optoelectronic devices, including solar cells (SCs), light-emitting diodes (LEDs), and photodetectors (PDs)11,18,19. Among them, all-inorganic perovskite quantum dots (ABX3, A = Cs; B = Pb, Ge, Sn; X = Cl, Br, I) (PQDs) have attracted extensive interest in broadband PDs, owing to their wide-range tunability of bandgap, large absorption cross-section, high carrier mobility, etc18,20,21,22,23. Especially, CsPbI3 PQDs process narrow bandgap of 1.73 eV, becoming a candidate for broadband PDs24. For example, Tian et al. fabricated 2-aminoethanethiol (AET)/CsPbI3 PQDs composite-based PDs device, exhibiting a high responsivity of 105 mA W−1 and the detection wavelength covering the visible light22. However, its spectrum covers mainly the blue to visible light range (400–700 nm), short of UV response and NIR absorption, due to the insensitivity to UV light and limitation of the bandgap. In addition, they also encounter relatively high trap density, poor carrier mobility, and high susceptibility to moisture and UV light, generating phase transition from cubic to orthorhombic phase25,26,27. The above issues severely limit its photodetection of broadband response spectrum with high stability and responsivity.

To overcome the challenges mentioned above, much efforts have been made to improve the stability and responsivity, and to expand the spectral response range of perovskite-based PDs. A number of metal ions (eg., Zn2+, Cr3+, Nd3+, Er3+, Ce3+) doping have been proved to be a promising way to boost the optical and electrical performance of perovskite materials28,29,30,31, including the decrease of trap density and the improvements of carrier mobility, stability, and photoluminescence quantum yield (PLQY). Meanwhile, the strategy of integrating perovskite with NIR absorption materials (e.g., organic bulk heterojunction (BHJ), lead sulfide quantum dots, etc.) was attempted to expand the spectral response range of PDs to the NIR region32,33,34. For example, Chen et al. achieved broadband photodetectors with high NIR external quantum efficiency of over 70% in organic-inorganic perovskite/BHJ hybrid35. Nevertheless, such PD has low responsivity in the UV region and relatively poor stability of organic-inorganic perovskite.

The scheme of luminescent conversion was proven to be an effective route to enlarge the response to the UV by absorbing and converting UV to visible photons and further being captured by PD. The luminescent conversion films consisting of Cr3+, Ce3+, Yb3+ tridoped CsPbCl3 PQDs or carbon dots were explored to boost the UV response of silicon PDs36,37,38. As a class of excellent luminescent conversion materials, luminescent concentrator (LC) consists of transparent polymer sheets doped with luminescent species that can be employed as a nonimaging optical device that collects and concentrates light energy39,40. It has been widely applied in photovoltaic cells or optical communications to largely improve the power conversion efficiency41,42.

In this work, we represent the design and fabrication of a novel type of hybrid PDs based on ultraviolet (UV) luminescent concentrators (LC) and doped PQDs and an organic bulk heterojunction, which can realize efficient photodetection in the whole range of 200–1000 nm. The device integrates a tridoped PQDs (CsPbCl3:Cr3+,Ce3+,Mn2+) photoluminescent layer to harvest and converts UV light to visible, a CsPbI3:Er3+ layer to realize the photoelectric conversion of visible light, and an organic bulk heterojunction to extend photoelectric response to NIR light. In such a device, CsPI3:Er3+ PQDs was explored as the visible photoelectric layer. Doping of Er3+ largely improved the radiative transition rate of the perovskite excitons and structure stability, altered charge carrier transport of CsPbI3 QDs, thus leading to performance enhancement considerably. In CsPbCl3:Cr3+,Ce3+,Mn2+ based LC, Mn2+ ions convert the UV to red lights, locating within the optimum regions of CsPbI3:Er3+ based PD, and simultaneously, Cr3+ and Ce3+ doping significantly improve PLQY of the PQDs and enhance light-harvesting of UV light for PQDs due to the coupling of 5d states of Ce3+ with the PQDs, extremely in the deep UV (DUV) region. In addition, an organic BHJ (BTP-4Cl: PBDB-TF) with an absorption extending 1000 nm was adopted as NIR photoelectric layer to integrate with CsPbI3:Er3+ layer. Taking all advantages above, the present PDs realize the spectral response spanning from 200 to 1000 nm and demonstrated detectivity reaching 1012 Jones.

Results

Figure 1a and S1 illustrates the structure and the cross-sectional scanning electron microscopy (SEM) images of the broadband PDs, which consists of CsPbCl3:Cr3+,Ce3+,Mn2+ PQDs doped polymethyl methacrylate LC (Cr/Ce/Mn-LC)/ITO/SnO2:Ti3C2/CsPbI3:Er3+ PQDs/PBDB-TF:BTP-4Cl (BHJ)/Ag. Firstly, the SnO2:Ti3C2 ETL (~50 nm) was spin-coated on ITO substrate and then annealed at 150 °C for 15 min. The CsPbI3:Er3+ PQDs with a thickness of 450 nm were fabricated on the ETL modified ITO glass by spin-coating. Then the organic BHJ of PBDB-TF: BTP-4Cl as an NIR photosensitive layer was deposited on top of the CsPbI3:Er3+ PQDs film. Finally, the Cr/Ce/Mn-LC was positioned at the ITO side to construct the UV-Visible-NIR PD. The mixture of BHJ can absorb the low-energy NIR photons and effectively passivate the defects in the perovskite film and improve carrier transport and collection (Fig. 1b)43. As demonstrated in Fig. 1c, CsPbI3:Er3+ PQDs has good absorption in the range of 350–700 nm, BHJ exhibits light absorption in the range of 700–1000 nm, and the CsPbI3:Er3+ PQDs/BHJ film shows the absorption band from 350 to 1000 nm. Differently, the detected lights with a wavelength within 200–400 nm, are completely absorbed by Cr/Ce/Mn-LC and converted to 400–700 nm, which are further absorbed by CsPbI3:Er3+ PQDs. Finally, combining UV-Visible-NIR absorption of Cr/Ce/Mn-LC and CsPbI3:Er3+ PQDs and BHJ, the fabricated PDs can exhibit a wide photodetection range from 200 to 1000 nm.

Fig. 1: Broadband photodetection based on Cr/Ce/Mn-LC and BHJ and CsPbI3:Er3+ PQDs.
figure 1

a Schematic device structure and 2D view of a broadband PD. b The molecule structures of BTP-4Cl:PBDB-TF (BHJ). c Absorption spectra of Cr/Ce/Mn-LC and BHJ and CsPbI3:Er3+ PQDs and CsPbI3:Er3+ PQDs/BHJ film, and photoluminescence spectra of Cr/Ce/Mn-LC

Full size image

Firstly, the visible sensitive materials of CsPbI3 and CsPbI3:Er3+ PQDs with different Er3+ doping concentrations (0–9.7%) were synthesized through the modified hot-injection method31,44. As revealed in a transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images (Fig. 2a and S2), the cubic-shaped and uniform CsPbI3 PQDs are successfully obtained with and without Er3+ doping. The average diameter of pristine CsPbI3 PQDs is determined to be 11.4 nm, which gradually decreases to 10.9, 10.7, 10.3, and 9.9 nm with different Er3+ doping concentrations (1.2, 4.6, 7.7, and 9.7%), respectively (Fig. S3). The doping amounts of Er3+ in the CsPbI3 PQDs were ascertained by the inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S1). Meanwhile, the lattice constant of the (100) plane of the CsPbI3:Er3+ (7.7%) PQDs is (6.1 Å), smaller than that of CsPbI3 PQDs (6.3 Å). This can be attributed to the lattice contraction of PQDs induced by partly replacing the Pb2+ (~119 pm) with the doping Er3+ ions (~88.1 pm). The X-ray diffraction (XRD) patterns evidence that CsPbI3:Er3+ PQDs has the same cubic structure as CsPbI3 PQDs, and the diffraction peaks of (100) and (200) planes shift toward higher diffraction angle after Er3+ doping (Fig. S4). The energy-dispersive X-ray (EDX) mapping images reveal that all the elements (Cs, Pb, I, and Er) exist in CsPbI3:Er3+ (7.7%) PQDs (Fig. S5). The X-ray photoelectron spectra (XPS) of pristine CsPbI3 and CsPbI3:Er3+ PQDs in Fig. 2b and S6 demonstrate that the peaks of Cs 3d, Pb 4f, and I 3d are identified in both two samples, and two additional peaks are observed appearing at 167.6 and 172.1 eV in CsPbI3:Er3+ PQDs, which can be assigned to the 4d signal of Er3+. Compared with CsPbI3 PQDs, the binding energy of Pb2+ 4f5/2 and 4f7/2 in CsPbI3:Er3+ PQDs shifts to higher energy, while remains unchanged for I− 3d and Cs+ 3d. When doping with Er3+, the lattice contraction of CsPbI3 PQDs happens induced by partly replacing the Pb2+ (~119 pm) with the smaller sized Er3+ (~88.1 pm), leading to the shrinking of PbI6 octahedron with shortening of Pb-I bond and enhancing the interaction45,46. Thus, the binding energy of Pb2+ 4f in PQDs shifts with Er3+ doping, while remains unchanged for I− 3d and Cs+ 3d. Combined with the HRTEM (Fig. 2a) and XRD results (Fig. S5), we can deduce that Er3+ ions are successfully incorporated into the CsPbI3 PQDs and occupy the lattice position of Pb2+ ions. Similar results were reported in the previous literatures19,47,48.

Fig. 2: Er3+ doped CsPbI3 PQDs.
figure 2

a TEM and HR-TEM images of CsPbI3 and CsPbI3: Er3+ (7.7%) PQDs. b XPS spectra of Er3+ (4d) and Pb2+ (4f) of CsPbI3 and CsPbI3: Er3+ (7.7%) PQDs. Absorption and emission spectra (c), trap density and PLQY (d), and PL decay lifetimes and transition rates (e) of CsPbI3 and CsPbI3:Er3+ PQDs with different Er3+ concentrations of 1.2%, 4.6%, 7.7%, 9.7%. f I–V curves of CsPbI3 and CsPbI3:Er3+ PQDs as a function of different Er3+ concentrations in ITO/ETL/ PQDs /Ag devices. g Mott–Schottky measurement of CsPbI3 and CsPbI3: Er3+ (7.7%) PQDs. h Normalized PL intensity of CsPbI3 and CsPbI3: Er3+ (7.7%) PQDs as a function of storage time and temperature; Inset is PL images of CsPbI3 and CsPbI3: Er3+ (7.7%) PQDs after 30 days and 390 K under 365 nm UV lamp

Full size image

Figure 2c shows the absorption and emission spectra of CsPbI3 and CsPbI3:Er3+ PQDs. The absorption peak at 681 nm can be observed, attributed to the intrinsic exciton transition of CsPbI3 PQDs. With increasing Er3+ doping concentration, the absorption peak of CsPbI3 PQDs gradually shifts from 681 to 672 nm, and the bandgap of PQDs increases from 1.73 eV (CsPbI3 PQDs) to 1.81 eV (CsPbI3: 9.7% Er3+ PQDs), as represented in Fig. S7. This can be mainly attributed to the lattice contraction by substituting Pb2+ ions with Er3+ ions (Supplementary Note 1), as observed in Fig. 2a. Similarly, the emission peak of CsPbI3 PQDs gradually blue shifts from 697 to 687 nm with increasing Er3+ doping. Interestingly, the PLQY of PQDs rapidly increases after Er3+ doping, from 52% for CsPbI3 PQDs to 93% for CsPbI3: Er3+ (7.7%) PQDs (Fig. 2d). Correspondingly, the trap density of CsPbI3 PQDs is estimated to be 1.18 × 1017 cm−3, which quickly decreases to 1.6 × 1016 cm−3 for CsPbI3: Er3+ (7.7%) PQDs (Fig. S8 and Supplementary Note 2). Meanwhile, the carrier densities are evaluated as 1.404 × 1019 cm−3 for CsPbI3 PQDs and 1.621 × 1019 cm−3 for CsPbI3: Er3+ (7.7%) PQDs. It is found that the effect reduces the trap density and increases the carrier densities of PQDs after Er3+ doping, similar to the previous literatures49,50.

The Er3+ doping concentration dependence of PL lifetimes (τ) and transition rates of CsPbI3 and CsPbI3:Er3+ PQDs were measured (Fig. 2e and S9). It can be observed that the PL lifetimes gradually decrease from 168 ns of CsPbI3 PQDs to 69 ns of CsPbI3:Er3+ (9.7%) PQDs. The radiative rates (kr) and nonradiative rates (knr) of CsPbI3 and CsPbI3:Er3+ PQDs were calculated in Table S2 according to the following equations: kr = PLQY/τ, and knr = (1-PLQY)/τ. Compared to the CsPbI3 PQDs, the kr of CsPbI3:Er3+ (7.7%) PQDs increases about 3.6-folds and the knr of CsPbI3:Er3+ (7.7%) PQDs decreases about 3.5-folds. It suggests that the Er3+ doping can boost the radiative decay rate, reduce the trap density and enhance the PLQY of PQDs. The increase of the radiative decay rate in CsPbI3:Er3+ (7.7%) PQDs can be proved by the decreased power index as a function of the excitation power density (Fig. S10)51.

The role of Er3+ in the electrical conductivity of PQDs films were studied using I–V curves of the ITO/PQDs /Ag devices (Fig. 2f and Supplementary Note 3). It can be seen that the conductivity (σ) of pristine CsPbI3 PQDs is 1.2 × 10–6 S cm−1, which increases to 5.2 × 10−6 S cm−1 for CsPbI3:Er3+ (7.7%) PQDs. The built-in potential (Vb) values of CsPbI3 and CsPbI3:Er3+ (7.7%) PQDs based devices are estimated to be 0.85 and 0.93 V by the Mott–Schottky analysis (Fig. 2g and Supplementary Note 4). A larger built-in potential value means an enhanced driving force for the separation of photogenerated carriers as well as an extended depletion region for efficient suppression of electron-hole recombination, which is favorable for carrier separation, transport, and extraction52,53,54. The built-in potential (Vb) values of CsPbI3:Er3+ PQDs is larger than that of CsPbI3 PQDs, realizing the more effective separation of photogenerated carriers after Er3+ doping.

It should be highlighted that incorporating Er3+ ions into CsPbI3 PQDs demonstrates significantly enhanced air- and UV- stability, which is of great importance for practical applications35,55,56. The PL intensity of CsPbI3:Er3+ (7.7%) PQDs still maintains above 97 and 67% of its initial value after 30 days of storage and 10 h UV light radiation, but the PL of CsPbI3 PQDs almost disappears after 5 days storage and 8 h UV light radiation (Fig. 2h and Figs. S11, S12). The thermal stability of CsPbI3 PQDs is also improved remarkably after Er3+ doping, which PL intensity remains 87% after annealing at 390 K, and no PL is recorded for CsPbI3 PQDs after annealing at 350 K. Considering the better conductivity, the CsPbI3 and CsPbI3:Er3+ (7.7%) PQDs are further treated with ethyl acetate to remove the original long-chain ligands of oleic acid (OA) and oleylamine (OAm) of PQDs57,58. The CsPbI3:Er3+ PQDs demonstrate outstanding air- and UV- stability (Fig. S13).

We next performed density functional theory (DFT) calculations aimed at understanding the origin of Er3+ doping-induced changes in structural and photophysical properties of CsPbI3 PQDs. Electronic structures and formation energy of intrinsic vacancies with different charge states (qi) at Pb rich and I poor condition in pure cubic CsPbI3 and Er3+ doped CsPbI3 were calculated using Perdew–Burke–Ernzerhof (PBEsol) functional without considering the spin-orbit coupling effect (Supplementary Note 5)59. Fig. 3a illustrate the electronic structure and density of states (DOS) of pure cubic CsPbI3 (2 × 2 × 2 supercell, 8fu/cell) with direct bandgap 1.72 eV and internal symmetry breaking. The I 5p and Pb 6 s orbital mainly contribute to the valance band maximum (VBM), and Pb 6p and I 5p dominate the conduction bands minimum (CBM). As shown in Fig. 3b, the bandgap of CsPbI3:Er3+ is larger than pristine CsPbI3, and the band edge states of CsPbI3:Er3+ (8 fu/cell) does change (i.e., VBM and CBM), presenting the bandgap increase to 1.82 eV. We also theoretically calculated the formation energy of intrinsic vacancy defects of Cs, Pb, and I (labeled as VCs, VPb, and VI) in pure cubic CsPbI3 and CsPbI3:Er3+ (64 fu/cell) by using PBEsol functional and 2 × 2 × 2 k-grid. As illustrated in Fig. 3c, d and Tables S3, S4, the intrinsic vacancies in CsPbI3 are shallow defects with relatively smaller formation energy, which shows larger formation energy in CsPbI3:Er3+, leading to the reduced trap density after Er3+ doping, similar to the experimental results in Fig. 2d. Generally, the defects can act as carrier traps, resulting in nonradiative recombination, whereas fewer defects largely preserve the bulk electronic band structure and can improve the optoelectronic properties of PQDs31,60. Table 1 lists the formation energy of ternary compounds to the corresponding binary compounds by DFT. The calculated formation energy ΔHf of CsPbI3:Er3+ reveals that the reaction spontaneously occurs starting from binary precursors because of the exothermic reaction. The reaction path is: CsI + Er3+I2 + e + PbI2 -> Cs8Pb7ErI24 + e. The calculated formation energy ΔHf (defined in Table 1) of Er3+ doped CsPbI3 with respect to binary precursors CsI, Er3+I2, and PbI2 is positive, referring to the exothermic reaction61. This indicates that the reaction CsI + 0.077(ErI3) + 0.923(PbI2) - >CsPb0.923Er0.077I3 spontaneously can occur. The formation energy of CsPbI3:Er3+ PQDs is larger than prsitine CsPbI3 PQDs, which demonstrate the CsPbI3:Er3+ PQDs are more energetically stable than CsPbI3 PQDs. Moreover, the chemical potential in 2 × 2 × 2 supercell of cubic CsPbI3 was calculated by considering the binary competing phase CsI and PbI2. As illustrated in Fig. 3e, f, the chemically stable range for cubic CsPbI3 is smaller than CsErI3. Meanwhile, the formation energy and tolerance factors of CsPbI3 become larger after Er3+ doping (Table S1 and Fig. S14), implying the more energetically stable structure of CsPbI3:Er3+ (Fig. 2h). Experimentally, the reduced defect density, the enhanced carrier densities and the conductivity, and the increased built-in potential (Vb) values of CsPbI3 PQDs after Er3+ doping were observed. Therefore, experimental and theoretical results demonstrate that the Er3+ doping can reduce trap density, improve the density and mobility of carriers, accelerate carriers' separation, and enhance the stability of CsPbI3 PQDs.

Fig. 3: Density functional theory calculations of CsPbI3 and CsPbI3:Er3+ PQDs.
figure 3

a, b Band structure and DOS of CsPbI3 and CsPbI3:Er3+ PQDs. c, d Calculated formation energy of intrinsic vacancy defects for Cs, Pb and I (labeled as VCs, VPb and VI) of CsPbI3 and CsPbI3:Er3+ PQDs. e, f Calculated phase diagrams of cubic CsPbI3 (8 fu/cell) and CsErI3 (8 fu/cell) with internal symmetry breaking. The pink polygons indicate the stable regions of the single-phase of cubic CsPbI3 and CsErI3

Full size image
Table 1 Formation energy of ternary compounds (CsPbI3, and Er3+ doped CsPbI3) to binary compounds calculated by using PBEsol functional
Full size table

Figures 4a and S15 display the SEM and atomic force microscopy (AFM) images of CsPbI3:Er3+ (7.7%) PQDs film and the CsPbI3:Er3+ (7.7%) PQDs/BHJ film. It is found that the CsPbI3:Er3+ PQDs / BHJ film exhibited smaller roughness with root mean square 6.75 than 8.5 nm for the CsPbI3:Er3+ (7.7%) PQDs film. When combining CsPbI3:Er3+ (7.7%) PQDs with BHJ to form heterogeneous conjunctiva, the hybrid film becomes more smooth, which provides a high-speed transport channel for carriers and promotes the dissociation of excitons58,62,63,64. As expected in Fig. 4b, the CsPbI3:Er3+ (7.7%) PQDs film shows a strong red emission band located at 685 nm. The PL intensity of CsPbI3:Er3+ (7.7%) PQDs significantly decreases after integrated ETL and BHJ, attributed to the enhanced charge extraction and suppressed carrier recombination in CsPbI3:Er3+ (7.7%) PQDs film. The ultraviolet photoemission spectroscopy (UPS) was performed to confirm the energy level locations for CsPbI3:Er3+ (7.7%) PQDs (Fig. 4c), in which the Fermi level shifts to −4.37 eV, and the CBM and VBM values were determined to be −3.90 and −5.69 eV after Er3+ doping. As revealed in Fig. S16a, b, the alignment of the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) of BHJ were −5.50 and −3.46 eV, which presents the secondary electron cutoffs and valence band edges of the PBDB-TF (donor) and BTP-4Cl (acceptor), respectively. Figures 4d and S16c illustrate the energy diagram of the PD device with the structure of ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/BHJ/Ag. The CsPbI3:Er3+ (7.7%) PQDs film absorbs visible photons, and the generating carriers separate at the interface of ETL/CsPbI3:Er3+ (7.7%) PQDs film and CsPbI3:Er3+ (7.7%) PQDs/BHJ film. Meanwhile, the BHJ captures the low-energy photons (e.g., NIR light) to produce electrons and holes, and the electrons transported to ETL through CsPbI3:Er3+ (7.7%) PQDs film, and the holes move to Ag electrode.

Fig. 4: Photoelectric characteristics of BHJ and Cr/Ce/Mn-LC.
figure 4

a SEM (top) and AFM (bottom) images of the CsPbI3:Er3+ (7.7%) PQDs film and the CsPbI3:Er3+ (7.7%) PQDs /BHJ film. b Emission spectra of CsPbI3:Er3+ (7.7%) PQDs film, ETL/CsPbI3:Er3+ (7.7%) PQDs film, and ETL/CsPbI3:Er3+ (7.7%) PQDs/BHJ film. c UPS curves of CsPbI3:Er3+ (7.7%) PQDs film. d Energy band diagram of each layer in the PDs. e, f Absorption spectra, emission spectra, and PLQY of undoped, Cr3+, Cr3+/Mn2+, and Cr3+/Mn2+/Ce3+ doped CsPbCl3 PQDs. g Energy transfer mechanism of CsPbCl3: Cr3+, Ce3+, Mn2+ PQDs. h Total, face, and edge emissions measured for a Cr/Ce/Mn-LC using Cr3+,Mn2+,Ce3+ doped CsPbCl3 PQDs. The inset is the picture of Cr/Ce/Mn-LC under sunlight and UV illumination

Full size image

To obtain Cr/Ce/Mn-LC, the homogeneous and cubic Cr3+, Cr3+/Mn2+, and Cr3+/Mn2+/Ce3+ doped CsPbCl3 PQDs were successfully prepared by the modified hot-injection method37,65 as revealed in TEM images, XRD patterns, and XPS spectra in Figs. S17–19. Figure 4e shows the UV-Vis absorption spectra of pristine and doped CsPbCl3 PQDs. It presents that the absorption peak of CsPbCl3 PQDs locates at 400 nm ascribed to the excitonic transition, and slightly shifts to a shorter wavelength after Cr3+,Mn2+,Ce3+ doping into CsPbCl3 PQDs. Interestingly, the absorption of Cr3+,Mn2+,Ce3+ doped CsPbCl3 PQDs increases remarkably at the range of 200–300 nm, which can be assigned to the high states of 5d of Ce3+, similarly to the previous literature37,66. As displayed in Fig. 4f, one emission band (417 nm) in CsPbCl3 and CsPbCl3:Cr3+ PQDs is observed, while two emission peaks in CsPbCl3:Cr3+, Mn2+, and CsPbCl3:Cr3+, Mn2+, Ce3+ PQDs are identified, appearing at 415 and 590 nm, respectively, corresponding to exciton emission of PQDs and 4T1 - 6A1 transition of Mn2+67. The Mn2+ emission originates form the energy transfer from PQDs to Mn2+ ions, evidenced by the decreased decay lifetimes of excitation emission in CsPbCl3:Cr3+ PQDs after Mn2+ doping (Fig. S20). Surprisingly, the PLQY of CsPbCl3:Cr3+, Mn2+, Ce3+ PQDs is measured to be 93.5%. According to the previous literatures68,69, such high PLQY (Fig. 4f) are mainly due to the reduced nonradiative decay rate after Cr3+ doping, the boosted energy transfer from PQDs to Mn2+ ions and the enhanced UV absorption after Ce3+ doping. The energy transfer mechanism of CsPbCl3:Cr3+, Mn2+, Ce3+ PQDs is presented in Fig. 4g. Importantly, the doping with Cr3+,Mn2+,Ce3+ can also largely improves the stability of CsPbCl3 PQDs, in which the PL intensity remains around 93% after 30 days (Fig. S21). In virtue of high conversion efficiency from UV to visible lights, the CsPbCl3:Cr3+, Mn2+, Ce3+ PQDs are embedded into a PMMA polymer matrix to form a Cr/Ce/Mn-LC (inset of Fig. 4h). The Cr/Ce/Mn-LC demonstrates the similar emission spectra with CsPbCl3:Cr3+,Mn2+,Ce3+ PQDs (Fig. 4h and S22) and high transparency to visible lights (Fig. S23). The face emission of the PLQY Cr/Ce/Mn-LC is 20.03%, which is lower than the edge emissions (61.47%) due to the total internal reflection to the edges in the LC (Fig. S24). Such Cr/Ce/Mn-LC processes the high efficiency and transparency, can be severed as a photoluminescent converter to boost the UV response of PD.

Figure 5a shows the charge generation and transport mechanism of the broadband PDs with the response range spanning from UV to NIR lights, in which PQDs straightly absorb the visible lights, the BHJ layer captures the NIR lights, Cr/Ce/Mn-LC converts the UV lights (200–400 nm) to visible lights (400–700 nm) further absorbing by PQDs. The photocurrent– time (I–t) response curves based on the ITO/ETL/CsPbI3 PQDs/Ag (S1), ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/Ag (S2), ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/BHJ/Ag (S3) and Cr/Ce/Mn-LC/ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/BHJ/Ag (S4) under the 260, 460, and 860 nm with an incident light intensity of 0.5 mW cm−2 are recorded in Fig. 5b. The photocurrents reach 0.89, 0.68, and 0.32 mA of S4 PD, while they are only 0.098 mA, 1.4 µA, and 0.01 µA of S1 PD under 460, 860, and 260 nm, respectively. Compared with S1 PD, the photocurrents of S4 PD enhance ninefold for 460 nm, 485-folds for 860 nm, and 3.2 × 104 folds for 260 nm. The improvement of photocurrent at 460 nm mainly originates from that the Er3+ doping into CsPbI3 PQDs resulting in increased carriers mobility, and accelerated carriers transport (Fig. 2). BHJ hybridization not only significantly boosts the photocurrent of 860 nm owing to the direct NIR absorption, but also contributes to the slight increase of photocurrent at 460 nm (S3) due to the inhibited electron-hole pair recombination at the interfaces of CsPbI3:Er3+ (7.7%) PQDs/BHJ film. The huge enhancement of photocurrent at 260 nm (S4) is on account of Cr/Ce/Mn-LC. In order to further demonstrate highly light emission efficiency of the Cr/Ce/Mn-LC on the side edge, the photocurrent–time (I–t) response curves based on the Cr/Ce/Mn-LC face and the edge of S4 PDs at 260 nm illumination with an incident light intensity of 0.5 mW cm−2 are recorded in Figs. S25, 26. It can be seen that the photocurrent of Cr/Ce/Mn-LC surface is 0.092 mA, which increases to 0.32 mA for the side edge of S4 PDs. The improvement of photocurrent at 260 nm mainly originates from the waveguide structure and the highly improved light collecting efficiency70. Moreover, the photocurrent response of PDs using PMMA: CsPbCl3: Cr3+, Mn2+, Ce3+ luminescent conversion layer (0.105 mA) is also lower than that of Cr/Ce/Mn-LC (0.32 mA). In the luminescent conversion layer, the direction of visible emissions are random under UV illumination, and only the emission photons which irradiate on the PD can be utilized. But in the Cr/Ce/Mn-LC, most of the emission photons can be traveled to the edge side attached to PD. Thus, the improvement of photocurrent in Cr/Ce/Mn-LC mainly originates from the waveguide structure and the highly improved light (400–700 nm) concentrate efficiency. The dark currents of the S2–S4 PDs are lower than that of the pristine PD device by one to two orders of magnitude, suggesting the defect sites of PQDs are effectively passivized by Er3+ doping and BHJ hybridization (Fig. S27), consistently with the results in Figs. 2, 4a–d46,71. In line with the decreased dark current of S3 and S4 PDs, the θ coefficients in the S3 and S4 PDs (θ = 0.763 and 0.767) have better linearity than the pristine PD (θ = 0.698), following the power-law I ∼ Pθ, where I and P represent the photocurrent and the incident light power intensity (Fig. S28)72. Figs. 5c, d and S29 display the detectivity (D*), external quantum efficiency (EQE), and photoresponsivity (R) of S1–S4 PDs. These three parameters satisfy the following equations20,37:

$${{{\mathrm{R}}}} = \frac{{I_{ph} - I_d}}{{PS}}$$
(1)
$${{{\mathrm{D}}}} \ast = \sqrt {\frac{S}{{2eI_d}}} {{{\mathrm{R}}}}$$
(2)
$${{{\mathrm{EQE}}}} = {{{\mathrm{R}}}}\frac{{hc}}{{\lambda e}}$$
(3)

where Iph and Id are the photocurrents under the illumination of light and in the dark, P and S are the input light power density and the effective irradiated area, h and c are the Planck’s constant and the speed of light, λ and e are the incident light wavelength and the elementary charge. The pristine S1 PD presents the low D*, EQE, and R in the whole region. Those values are largely boosted in the visible region in S2–S4 PDs, and simultaneously, its responses expand to NIR (S3 PD) and UV (S3 and S4 PD) regions. The R, EQE, and D* of S4 PDs are 266.2, 439.8, and 326 mA/W; 89.13, 91.84, and 47.12%; 1.14 × 1012 Jones, 2.46 × 1012 Jones, 1.82 × 1012 Jones at 260, 460, and 860 nm light detection, respectively. Compared with S1 PD, owing to the contribution of Er3+ doping, the BHJ film, and Cr/Ce/Mn-LC, the D* of S4 PDs improves 1.2 × 104 folds, 1.24 × 102 folds, and 5.7 × 103 folds at 260, 460, and 860 nm lights, which increases two to four orders than that of the pristine PDs. Moreover, the S4 PDs exhibits a fast response time (Rt) of hundreds of microseconds (Fig. S30). The long-term stability of the S1–S4 PDs was further studied. As displayed in Fig. 5e, the S2–S4 PDs maintain about 82% of the initial photocurrent, while the photocurrent of the S1 device dropped to 0% after 60 h, owing to the outstanding stability of PQDs by Er3+ doping. The UV stability of the S1–S4 devices in Fig. 5f illustrates that the stability of S2–S4 PDs are large improved, especially, S4 PD represents the best UV light stability, maintaining above 86% of the initial photocurrent after 10 h UV illumination, but it degrades to 0% for pristine PD within 7 h. The reason for those improvements in the air- and UV- stability are mainly attributed to the role of Er3+ doping, BHJ hybridization, and the buffer layer of Cr/Ce/Mn-LC in the device. Notably, the photocurrent of the PD is repeatable even after five thousand cycles (Fig. S31), confirming the excellent reversibility of this photodetector. Compared to the previous broadband perovskite PDs (Table 2), our device exhibits excellent performance with a relatively wide response, high responsivity, and detectivity, especially in UV and NIR regions, and good stability, which exceeds the results of the previous reports.

Fig. 5: Performance of broadband PDs.
figure 5

a Charge generation and transport mechanism of a broadband PD. b Photocurrents of the ITO/ETL/CsPbI3/Ag (S1), ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/Ag (S2), ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/BHJ/Ag (S3) and Cr/Ce/Mn-LC/ITO/ETL/CsPbI3:Er3+ (7.7%) PQDs/BHJ/Ag (S4) devices under the 260 nm, 460 nm, and 860 nm, respectively. c D* of the S1–S4 devices and commercial Silicon PD. d EQE of the S1–S4 devices. e, f Stability of the S1–S4 devices under 30% RH and UV light radiation

Full size image
Table 2 Figures-of-merit in typical perovskite PDs
Full size table

Discussion

In this work, unique broadband PDs with the response range of 200–1000 nm and the D* value reaching of 1.14 × 1012 at 260 nm and 2.46 × 1012 at 460 nm, and 1.85 × 1012 at 860 nm based on doped PQDs and an organic bulk heterojunction and Cr/Ce/Mn-LC were reported. Several new contributions for developing broadband PDs in this work should be highlighted. Firstly, CsPbI3:Er3+ PQDs serve as a visible photosensitive layer of PD, and the performance improves two orders that of pristine PDs at the wavelength of 400–700 nm. Experimental and theoretical results demonstrate that the highly improved optoelectronic properties, such as low trap density, high charge mobility, and PLQY, excellent stability through Er3+ doping, lead to the great performance of PD. Secondly, the hybridization of CsPbI3:Er3+ PQDs / BHJ successfully expands the response of PDs to the NIR region, due to the NIR light-absorbing of low bandgap organic BHJ film. Thirdly, the high efficiency (PQLY = 93.5%) and strong absorption ability of Cr3+, Mn2+, Ce3+ doped CsPbCl3 PQDs are incorporated into PMMA to form a Cr/Ce/Mn-LC. The Cr/Ce/Mn-LC is employed as a UV photosensitive layer of PDs to highly enhanced UV response of PDs. Furthermore, the air- and UV- stability of broadband PDs are also significantly boosted. The performance of our PD is much better than that of other perovskite PDs and commercial Si PDs. Overall, this work presents a novel strategy to obtain a broadband PD with excellent performance.

Materials and methods

Materials

Cs2CO3 (99.9%), 1-octadecene (ODE, 90%), oleic acid (OA, 85%), oleylamine (OAm, 70%), PbI2 (99.99%), ErI3 (99.99%), PbCl2 (99.9%), CrCl3 (99%), MnCl2 (99%), toluene, and ethyl acetate (99%) were purchased from Sigma-Aldrich and were used as starting materials without further purification.

Synthesis of Cs-oleate

About 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 and the white powder was completely dissolved. The mixture was then kept at 120 °C.

Synthesis of CsPbI3 PQDs

PbI2 (0.3 mmol), OAm (1.5 mL), OA (1.5 mL), and ODE (10 mL) were added to a 50-mL three-neck round-bottomed flask and were evacuated and refilled with N2, followed by heating the solution to 120 °C for 1 h. The temperature of the solution was then increased to 180 °C for 10 min. Then, 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 Er3+ doped CsPbI3 PQDs

PbI2 (0.3 mmol) and ErI3 (0.15 mmol) were loaded into round-bottom flask with OAm (1.5 mL), OA (1.5 mL), and ODE (10 mL). It was continued heated at 120 °C for 2 h and refilled with N2. Then the solution was increased to 230 °C. Then, the Cs-oleate (1 mL) was swiftly injected into the solution. After 10 s, the solution was cooled in an ice bath. Finally, the CsPbI3:Er3+ (7.7 %) PQDs were precipitated and then centrifuged, followed by dissolution in toluene.

Synthesis of Cr3+,Ce3+,Mn2+ doped CsPbCl3 PQDs

PbCl2 (0.5 mmol), CrCl3 (0.3 mmol), CeCl3(0.2 mmol), and MnCl2 (0.2 mmol) were loaded into round-bottom flask with OAm (1.5 mL), OA (1.5 mL), and ODE (15 mL). The following steps were the same with the synthesis of the Er3+ doped CsPbI3 PQDs.

Fabrication of Cr/ Ce /Mn –LC

About 0.8 g PMMA (MW ~350000) was dispersed in 5 mL toluene by sonication, to which 2.5 mL toluene solution of CsPbCl3: Cr3+(8.3%),Ce3+(3.2%),Mn2+(9.3%) PQDs were added. The mixture was sealed and stirred overnight to obtain a homogenous slurry. The slurry was centrifuged at 2000 rpm and the supernatants were used for LC fabrication. The above supernatants were dropped onto borosilicate glass substrates and LC was fabricated by spin-coating.

Syntheses of BHJ film

The PBDB-TF:BTP-4Cl (1:1.2) was dissolved in chloroform. The mixture was heated and stirred at 60 °C for 5 h to obtain an organic active layer solution. The solvent additive of 1-chloronaphthalene (CN) (0.5%) was added half an hour before the organic active layer solution deposition. For the hybrid PDs, the PBDB-TF:BTP-4Cl solution was spin-coated on a perovskite layer at 1800 rpm for 60 s and subsequently annealed at 80 °C for 10 min.

Device fabrication

ITO-coated glass substrates were etched with 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 the ITO surface. To fabricate the compact SnO2:Ti3C2 layer, the SnO2:Ti3C2 colloid solution by water to the concentration of 2.14 wt% was spin-coated on ITO substrates at 5000 rpm for 30 s and then annealed at 150 °C for 30 min. The Er3+ doped CsPbI3 PQDs film was fabricated on the SnO2:Ti3C2 layer by spin-coating at 600 rpm for 6 s and 4000 rpm for 40 s, respectively. The PBDB-TF in chlorobenzene (CB) (400 μL) at various concentrations was dropped on a substrate at 20 s before the end of the spinning process. After that, the PBDB-TF:BTP-4Cl solution was spin-coated on a perovskite layer at 1800 rpm for 60 s and subsequently annealed at 80 °C for 10 min. The Ag electrode was deposited by thermal evaporation to complete the device fabrication. Then, the edge surface of Cr/Ce/Mn-LC with an edge size of 0.1 × 0.04 cm was attached and fixed to the ITO layer of PD with an area of 0.1 × 0.1 cm. When UV light radiation on the face of Cr/Ce/Mn-LC, then the emitted 400–700 nm light is coupled out of the edge surface into the ITO of PDs. The visible and NIR lights directly pass through the ITO and reach the PD. Because the Cr/Ce/Mn-LC only occupies a part of the surface of the ITO layer and has high transparency for photons with a longer wavelength (>410 nm), which would not affect the light collection of PD.

Characterization

UV/vis-NIR absorption spectra were measured with a Shimadzu UV-3600PC UV/vis-NIR scanning spectrophotometer in the range from 200 to 2500 nm. Patterns were recorded in thin-film mode on a Bruker AXS D8 diffractometer using Cu Kαradiation(λ = 1.54178 Å). Atomic Force Microscope (AFM) was performed 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. The samples were pumped using a laser system consisting of a tunable optical parameter oscillator (OPO, Continuum Precision II 8000) with a pulse duration of 10 ns, a repetition frequency of 10 Hz, and a line width of 4–7 cm−1. A visible photomultiplier (350–850 nm) 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. A photomultiplier combined with a monochromator was used for dynamics signal collection of samples from 350 to 850 nm. Nanosecond fluorescence lifetime experiments were performed by the time-correlated single-photon counting system (HORIBA Scientific iHR 320). Absolute photoluminescence quantum yield measurements were performed on colloidal CsPbCl3:Cr3+,Ce3+,Mn2+ and CsPbI3:Er3+ PQDs (dispersed in toluene placed in a sealed 1 cm path length quartz cuvette) and Cr/Ce/Mn-LC. They were positioned in a Teflon-based integrating sphere using a custom cuvette holder and directly excited with a 365 nm Xe lamp. The typical PLQY in such a system is estimated as follows:

$${{{\mathrm{PLQY}}}} = \frac{{N_{{\mathrm{em}}}}}{{N_{{\mathrm{abs}}}}} = \frac{{{\int} {I_{{\mathrm{sample}}}\left( \lambda \right) - I_{{\mathrm{ref}}}\left( \lambda \right)d\lambda } }}{{{\int} {E_{{\mathrm{ref}}}\left( \lambda \right) - E_{sample}\left( \lambda \right)d\lambda } }}$$

where Nem and Nabs are the numbers of emission and absorption photons of samples. Isample and Esample present the spectral intensity of the emitted light and excitation light of samples, and Iref and Eref is the spectral intensity of the emitted light and excitation light for a reference cuvette containing neat toluene. The monochromatic light was from a Newport Oriel 200TM. The Mott–Schottky curves via capacitance-voltage measurements of CsPbI3: Er3+ PQDs are obtained by a Princeton electrochemical workstation (Parstat Mc Princeton Instruments Co., Ltd., USA). The Xe lamp (LSB-X150AUV 200–2500 nm, Zolix) with the spectral range from 200 to 2500 nm equipped with a monochromator (Omni-λ3007i, Zolix) was used to generate the monochromatic light to conduct the spectral response measurements. Actually, the intensity of the Xe lamp is weak in the region of 200–300 nm, thus we must correct it before the measurement.