Solution-Processed Ultrahigh Detectivity Photodetectors by Hybrid Perovskite Incorporated with Heterovalent Neodymium Cations.

Hybrid perovskite materials have drawn a remarkable attention for approaching high-performance photovoltaics owing to their superior optoelectronic properties. But most of research studies focused on the pristine hybrid perovskite CH3NH3PbI3. In this study, we utilize a newly developed CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film, where Pb2+ is partially substituted by a heterovalent Nd3+ cation, as the photoactive layer for solution-processed perovskite photodetectors. It is found that the resultant CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film possesses superior thin film morphology, enhanced and balanced charge carrier mobilities, and suppressed trap density, resulting in enhanced photocurrent and reduced dark current for perovskite photodetectors by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film. Thus, operated at room temperature, solution-processed perovskite photodetectors exhibit over 1014 cm Hz1/2 W-1 photodetectivity in a spectrum range from 350 to 800 nm, a linear dynamic range over 100 dB, and fast response time. All these results indicate that high-performance solution-processed perovskite photodetectors can be realized by novel hybrid perovskite materials, where Pb2+ is partially substituted by heterovalent Nd3+ cations.

Introduction

In the past 10 years, studies found that hybrid perovskite materials possess high absorption coefficient, long charge carrier diffusion length, and low density of defects and traps.[1−4] Such superior optoelectronic properties contributed perovskite solar cells with an efficiency over 23% from 3.8% rapidly.[5−11] Impressive photocurrent and restricted dark current were realized in perovskite photovoltaics, which strongly suggested that hybrid perovskite materials are promising alternatives for fabrication ultrasensitive photodetectors (PDs). In 2014, Dou et al. reported perovskite PDs with an inverted device structure.[11] In early 2015, we reported ultrasensitive solution-processed perovskite PDs.[12] Further studies found that perovskite PDs exhibited a wide spectrum response range, high detectivity, and fast response.[11−16] However, most of studies were focused on the pristine perovskite, methylammonium lead triiodide (CH3NH3PbI3).[11−16] Recently, we reported a novel CH3NH3PbI3:xNd3+ (where x is the nominal ratio) thin film, where Pb2+ was partially substituted by a heterovalent neodymium (Nd3+) cation, and found out that the CH3NH3PbI3:xNd3+ thin film exhibited superior film quality with significantly improved charge carrier mobilities and highly suppressed trap states.[17] We further demonstrated highly reproducible power conversion efficiency from inverted planar heterojunction perovskite solar cells by the Nd3+-doped perovskite as the photoactive layer.[17]

In this study, we report ultrahigh detectivity solution-processed perovskite PDs by CH3NH3PbI3:xNd3+, where Pb2+ is partially substituted by the heterovalent Nd3+ cation. It is found that by partially substitution of Pb2+ by the heterovalent Nd3+ cation, the resultant CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film possesses superior film morphology, enhanced and balanced charge carrier mobilities, and suppressed trap density, resulting in enhanced photocurrent and reduced dark current for perovskite PDs by the CH3NH3PbI3:xNd3+ thin film. Thus, operated at room temperature, perovskite PDs exhibit over 1014 cm Hz1/2 W–1 photodetectivity in a spectrum range from 350 to 800 nm, a linear dynamic range over 100 dB, and fast response time.

Results and Discussion

For the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film, the partial substitution of Pb2+ by Nd3+ was confirmed by the binding energy shifts of N 1s, I 3d, and Pb 4f in X-ray photoelectron spectroscopies (XPS) of pristine CH3NH3PbI3 and CH3NH3PbI3:xNd3+ thin films.[17] Thin film characteristics including the crystal structure, film morphology, charge carrier mobility, and trap density were reported in our previous publication.[17]

Scheme a displays perovskite PDs with an inverted device structure, where the PEO-doped poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer is used as the hole extraction layer (HEL),[20] and a 4-lithium styrenesulfonic acid/styrene copolymer (LiSPS) ionomer layer serves as the interfacial layer,[21] where PEO is poly(ethylene oxide). Scheme b presents the molecular structures of PEDOT:PSS, PEO, LiSPS, and PC61BM, where PC61BM is [6,6]-phenyl-C61-butyric acid methyl ester. The PEO-doped PEDOT:PSS layer was selected as the HEL since it possesses a dramatically enhanced electrical conductivity, which would facilitate the hole extraction, transport, and collection from the perovskite photoactive layer to the ITO anode, where ITO is indium tin oxide.[20] The LiSPS is selected as an interfacial layer because it can fill in the pinholes of the solution-processed perovskite thin film and benefit the electron collection.[21] Thus, high photocurrent and low dark current are anticipated from perovskite PDs. Scheme c displays the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of CH3NH3PbI3:xNd3+, LiSPS, and PC61BM and the work function of ITO, PEO-doped PEDOT:PSS, and aluminum (Al) electrodes. The deep work-function PEO-doped PEDOT:PSS layer can facilitate separated holes to be efficiently collected by the ITO anode. The high LUMO energy level and the deep HOMO energy level of the LiSPS thin layer could prevent the back transfer of electrons to the perovskite and hole injection from the cathode, resulting in suppressed interfacial charge carrier recombination and leakage current, consequently improved photocurrent and reduced dark current.[21]

Scheme 1

Device Structure, Molecular Structures, and LUMO and HOMO Energies and Work Function

(a) Device structure of perovskite photodetectors, (b) molecular structures of PEDOT:PSS, PEO, LiSPS, and PC61BM, and (c) LUMO and HOMO energies of CH3NH3PbI3:xNd3+, LiSPS, and PC61BM and work function of ITO, PEO-doped PEDOT:PSS, and Al.

Figure a presents the current density versus voltage (J–V) characteristics of perovskite PDs fabricated by either the pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film, measured in dark and under monochromatic light at a wavelength of 500 nm with a light intensity of 0.28 mW cm–2 at room temperature. The dark current density (Jd) observed from perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film is 4.4 × 10–8 mA cm–2 at a bias of −1.5 V, which is nearly two orders of magnitude smaller than that (2.7 × 10–6 mA cm–2) by the pristine CH3NH3PbI3 thin film. Moreover, the photocurrent density/dark current density (Jph/Jd) ratios versus the applied voltages for perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film are over two orders of magnitude higher than those of perovskite PDs fabricated by the pristine CH3NH3PbI3 thin film at the same reverse bias, as shown in Figure b, indicating that perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film possess high sensitivity.

Figure 1

(a) J–V characteristics of the perovskite photodetectors fabricated by either the pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film, measured both in dark and under monochromatic light at a wavelength of 500 nm with a light intensity of 0.28 mW cm–2 at room temperature; (b) Jon/Joff ratio versus biases of the perovskite photodetectors fabricated by either pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film.

In PDs, the saturated dark current density, J0, determining the detectivity of PDs,[22] is directly related to the band-to-band thermal emission and charge carrier recombination in semiconductors.[23,24] The J0 can be extracted from the equation of J = J0[exp (q(V – JRs)/nkbT) – 1] – Jph (where J is the total current density, V is the applied voltage, q is the elementary electron charge, RS is the series resistance, n is the idea factor, kb is the Boltzmann constant, T is the absolute temperature, and Jph is the photocurrent.).[23,24] The versus the (Jph + J)−1 is shown in Figure a. Thus, Rs values for perovskite PDs fabricated by either the pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film are 0.90 and 0.20 Ohm cm2, respectively. Smaller R indicates that perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film possess high Jph. Figure b presents the ln(Jph + J) versus the (V – RsJ) and the linear fittings, which are based on the J–V characteristics of perovskite PDs. Thus, J0 values are estimated to be 1.02 × 10–11 mA cm–2 for perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film and 4.01 × 10–10 mA cm–2 for perovskite PDs fabricated by the pristine CH3NH3PbI3 thin film. Both J0 values are lower than that from the copper indium gallium selenide PDs (6 × 10–7 mA cm–2),[23] which indicates that perovskite PDs possess high sensitivity.[22]

Figure 2

(a) Plot of versus (Jph + J)−1 and linear fitting of the perovskite photodetectors by either pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film and (b) plot of ln(Jph + J) versus (V – RsT) and the linear fitting.

At a reverse bias of −100 mV and under monochromatic illumination at a wavelength (λ) of 500 nm with a light intensity of 0.28 mW cm–2, Jph values of perovskite PDs fabricated by the pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film are 3.0 × 10–4 and 8.7 × 10–4 A cm–2, respectively. Thus, for perovskite PDs fabricated by the pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film, R (responsibility) values, which (where Llight is the incident light intensity), are 1.1 and 3.1 A W–1, respectively. The projected detectivities (D*), described by ,[22] are estimated to be 2.2 × 1013 cm Hz1/2 W–1 for perovskite PDs fabricated by the pristine CH3NH3PbI3 thin film and 5.2 × 1014 cm Hz1/2 W–1 for perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film. Noted that projected detectivity, D*, is estimated solely based on measured dark current density rather than measured noise current density. The real detectivity of perovskite PDs is probably lower than D* due to the omission of thermal noise and 1/f noise, where f is the frequency.[25,26] However, the estimated detectivity is acceptable since the noise current is typically frequency-independent and is in the same magnitude of shot noise (from dark current) for PDs with a “vertical” photodiode device structure.[27−29]

Based on measured external quantum efficiency (EQE) spectra shown in Figure a and D* at 500 nm, R and D* over the spectral response region are derived. Figure b presents R and D* over the spectral region ranging from 350 to 800 nm. The perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film possess D* over 1014 cm Hz1/2 W–1 at room temperature, which is over 20 times higher than those by the pristine CH3NH3PbI3 thin film. Moreover, as indicated in Table , the perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film is one of the highest detectivities reported for perovskite PDs.[11−15,26,28,30−33] Moreover, this value is about two orders of magnitude higher than that of silicon-based PDs in the spectral response from 350 to 700 nm also. Such high detectivity and responsibility at room temperature are synergistically originated from high Jph and extremely low Jd.[22]

Figure 3

(a) EQE spectrum of the perovskite photodetectors fabricated by either pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film. (Source obtained from ref (17). Copyright from 2019 Elsevier.) (b) The detectivities and responsivities versus wavelength of the perovskite photodetectors fabricated by either pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film.

Table 1

Summary of Device Performance of PDs by CH3NH3PbI3 Thin Filma

active materials	device structure	applied bias (V)	incident light (nm)	responsivity (A W–1)	detectivity (cm Hz1/2 W–1)	refs
CH3NH3PbI3	photodiode	0.1	550		8 × 1013 (C)	(11)
CH3NH3PbI3	photodiode	0.1	680		7.4 × 1012 (M)	(28)
CH3NH3PbI3	photodiode		500 (0.53 mW cm–2)	0.339	4.8 × 1012 (C)	(12)
CH3NH3Pb0.9Co0.1I3	photodiode	0.1	500	1.8	2.1 × 1013 (C)	(15)
PbS QD/CH3NH3PbI3	photodiode		500 (0.80 mW cm–2)	0.302	1.2 × 1013 (C)	(13)
CH3NH3PbI3	phototransistor	5	830	275	1 × 1013 (M)	(30)
graphene/CH3NH3PbI3	phototransistor		515	115	3 × 1012 (C)	(31)
CH3NH3PbI3/PDPP3T	phototransistor	1	835 (0.5 mW cm–2)	0.154	8.8 × 1010 (M)	(32)
CH3NH3PbI3:C8BTBT bulk	photoconductor	3	532 (0.0075 mW cm–2)	8.1	1.65 × 1013 (C) 2.17 × 1012 (M)	(26)
CH3NH3PbI3	photoconductor	10	532	8.95	2.9 × 1012 (M)	(33)
CH3NH3PbI3:xNd3+ (x = 0.5 mol %)	photodiode	0.1	500 (0.28 mW cm–2)	3.1	5.2 × 1014 (C)	this work

(C) corresponds for the calculated noise current regarding shot noise only. (M) corresponds for the measured noise current.

A linear dynamic range (LDR) or the photosensitivity linearity (typically quoted in dB), one typical figure of merit that used to evaluate PD device performance, is also investigated. The LDR can be estimated according to the equation of LDR = 20 log (Jph*/Jd) (where the Jph* is the photocurrent measured at a light intensity of 1 mW cm–2).[22,23]Figure a displays the Jph versus the light intensities for perovskite PDs fabricated by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film. At room temperature, the LDR is over 100 dB, which is higher than that (47 dB) of perovskite PDs by the pristine CH3NH3PbI3 thin film.[15] This large LDR is comparable to that of Si-based PDs (120 dB, at 77 K) and is significantly higher than that (66 dB, at 4.2 K) of InGaAs-based PDs.[34]

Figure 4

(a) Linear dynamic range of the perovskite photodetectors by the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin film and (b) transient photocurrent of the perovskite photodetectors measured with an optical chopper (frequency: 2 kHz) controlled 532 nm laser pulse.

The temporal response time of perovskite PDs, an important parameter reflecting PD performance, is also characterized. The response time of PDs is strongly related to the charge transport and collection. Rise and fall times are defined as the time to increase from 10 to 90% of the peak photocurrent and decrease from 90 to 10% of the peak photocurrent, respectively. As shown in Figure b, perovskite PDs fabricated by the pristine CH3NH3PbI3 thin film show values of 32.8 and 28.4 μs for rise and fall times, respectively, whereas the fast rise time of 19.6 μs and fall time 12.8 μs are observed from perovskite PDs fabricated by CH3NH3PbI3:xNd3+(x = 0.5 mol %). The faster response time of perovskite PDs fabricated by CH3NH3PbI3:xNd3+(x = 0.5 mol %) is attributed to the suppressed defects and enhanced charge carrier mobilities of the CH3NH3PbI3:xNd3+(x = 0.5 mol %) thin film.

Conclusions

In summary, novel hybrid perovskite materials, where Pb2+ cations were partially substituted by heterovalent neodymium cations (Nd3+), were utilized for solution-processed perovskite photodetectors (PDs). Operated at room temperature, an ultrahigh detectivity over 1014 cm Hz1/2 W–1 in a spectral region from 350 to 800 nm with a linear dynamic range over 100 dB was observed from perovskite PDs by the CH3NH3PbI3:xNd3+(x = 0.5 mol %) thin film. Such boosted device performance is resulted from superior film morphological and optoelectronic properties in the CH3NH3PbI3:xNd3+(x = 0.5 mol %) thin film, which brings enhanced photocurrent but reduced dark current. These findings open a new window for tuning the electronic properties of hybrid perovskite materials via heterovalent substitution for boosting device performance of perovskite PDs.

Experimental Section

Materials

Nd2O3 (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. PEDOT:PSS (Clevios PH1000) was purchased from Heraeus Precious Metals North America. PEO (with a molecular weight (Mw) of 500 g mol–1) was purchased from Scientific Polymer Inc. PC61BM (99.5%) was purchased from Solenne BV. Lead iodide (PbI2, 99.999%, beads), anhydrous N,N-dimethylformamide (DMF, 99.8%), ethanol (CH3CH2OH, 99.5%), anhydrous chlorobenzene (CB, 99.8%), and anhydrous toluene (99.8%) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Methylammonium iodide (CH3NH3I) and LiSPS were synthesized in our labortary.[18,19] NdCl3 precursor solution was prepared by adding Nd2O3 into HCl (37% w) with an accurate stoichiometric ratio and then accompany with the addition of DMF solvent. The Nd-doped PbI2 precursor solution was prepared by mixing NdCl3 precursor solution with 0.87 M PbI2 in DMF with the fixing molar ratio of Nd to Pb equal to 0.5%.

Thin Film Preparation

Either the pristine CH3NH3PbI3 or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin films were prepared via a two-step method: First, the lead precursors (pristine PbI2 in DMF and Nd-doped PbI2 in DMF) and the substrates were preheated at 70 °C for 5 min; different lead precursors were spin-coated on the top of substrates, followed with thermal annealing at 70 °C for 10 min and then cooling down to room temperature. Second, CH3NH3I solution (0.25 M in ethanol) was then casted on the top of lead precursor layers. Afterward, above layers were thermally annealed at 108 °C for 2 h for converting either PbI2 or Nd-doped PbI2 with CH3NH3I into the pristine CH3NH3PbI3 thin film or the CH3NH3PbI3:xNd3+ (x = 0.5 mol %) thin films.

Fabrication and Characterization of Photodetectors

The ITO-coated glasses were precleaned by detergent, deionized water, acetone, and isopropanol sequentially. After oven drying, the ITO-coated glasses were treated with UV–ozone for 40 min under an ambient atmosphere. Then, a ∼40 nm PEO-doped PEDOT:PSS layer was spin-coated on the top of the ITO surface from PEO-doped PEDOT:PSS solution, followed with thermal annealing at 150 °C for 10 min in an ambient atmosphere. Afterward, the PEO-doped PEDOT:PSS-coated substrates were immediately transferred into a nitrogen-filled glovebox and ready for deposition of either the pristine CH3NH3PbI3 or the CH3NH3PbI3:xNd3+ layers via a two-step method. After the predevices were cooled down to room temperature, a ∼15 nm LiSPS layer was spin-coated from toluene solution on the top of the perovskite active layer. A ∼50 nm PC61BM layer was then coated on the top of the LiSPS layer from chlorobenzene solution. Last, a ∼100 nm-thick Al was thermally deposited through a shadow mask under a pressure of 6 × 10–6 mbar in a vacuum chamber on the top of the PC61BM layer. The device area was measured to be 0.16 cm2. The J–V characteristics were measured by using a Keithley 2400 source-power unit. A Newport Air Mass 1.5 Global (AM1.5G) full-spectrum solar simulator was applied as the light source, calibrated by a monosilicon detector from NREL. A specific wavelength was obtained by a spectrum filter for giving a monochromic light at 500 nm with a light intensity of 0.28 mW cm–2. The transient photocurrent measurements were performed by using an optical chopper controlled at λ = 532 nm laser pulse at a frequency of 2 kHz.