8304 | J. Mater. Chem. C, 2016, 4, 8304--8312 This journal is©The Royal Society of Chemistry 2016

Cite this: J.Mater. Chem. C, 2016,

4, 8304

Schottky junctions on perovskite single crystals:light-modulated dielectric constant andself-biased photodetection†

Parvez A. Shaikh,a Dong Shi,b Jose Ramon Duran Retamal,c Arif D. Sheikh,a

Md. Azimul Haque,a Chen-Fang Kang,c Jr-Hau He,c Osman M. Bakr*b andTom Wu*a

Schottky junctions formed between semiconductors and metal contacts are ubiquitous in modern electronic

and optoelectronic devices. Here we report on the physical properties of Schottky-junctions formed on

hybrid perovskite CH3NH3PbBr3 single crystals. It is found that light illumination can significantly increase

the dielectric constant of perovskite junctions by 2300%. Furthermore, such Pt/perovskite junctions are

used to fabricate self-biased photodetectors. A photodetectivity of 1.4 � 1010 Jones is obtained at zero

bias, which increases to 7.1 � 1011 Jones at a bias of +3 V, and the photodetectivity remains almost

constant in a wide range of light intensity. These devices also exhibit fast responses with a rising time

of 70 ms and a falling time of 150 ms. As a result of the high crystal quality and low defect density, such

single-crystal photodetectors show stable performance after storage in air for over 45 days. Our results

suggest that hybrid perovskite single crystals provide a new platform to develop promising optoelectronic

applications.

1. Introduction

Recently, organic–inorganic hybrid perovskites, CH3NH3PbX3

(X = Cl�, Br�, I�, or their mixtures) have been intensivelyexplored for applications in solar cells and other optoelectronicdevices.1–19 As a new class of semiconductors, these materialsfeature extraordinary physical properties including high carriermobility, large light absorption coefficient, low exciton bindingenergy and long carrier diffusion length.15–23 Recently, hybridperovskite single crystals have been synthesized and they possessproperties superior to the polycrystalline counterparts as a resultof their structural integrity.21–23 Investigating basic electronicdevices based on perovskite single crystals could avoid someof the extrinsic effects in polycrystalline films related to struc-tural defects and grain boundaries. Schottky junctions formed

at metal/semiconductor interfaces are ubiquitous in modernelectronic and optoelectronic devices. The aim of this work is toinvestigate the physical properties and device functionalities ofSchottky junctions fabricated on perovskite single crystals.

As one of the key figures of merit, the high dielectric constantof lead halide perovskites directly leads to their low excitonbinding energy (Eb), which is a paramount requirement togenerate a strong photovoltaic effect. According to recent theore-tical and experimental studies, the exciton binding energy Eb ofCH3NH3PbI3 is between 20–50 meV,24–27 which is much smallerthan that of organic materials (Eb = 0.2 eV to 1 eV).28,29 This largedifference in binding energies is attributed to the difference oftheir dielectric constants,30 and the high dielectric constant isalso responsible for the high power conversion efficiency ofperovskite solar cells.31 Clearly, detailed study of photo-inducedmodulation of dielectric constant will provide a unique platformto explore these new photovoltaic materials with superior opto-electronic device performance.

In terms of optoelectronic applications, light-modulatedtransport properties of metal–semiconductor junctions under-lies the operation of photodetectors which are used in a widerange of technologies such as remote sensing, environmentalmonitoring, space exploration, optical communication.32–39 Mostphotodetectors require voltage bias to achieve practical photoresponsivity and photo detectivity.40 However, installation ofan external power source or battery makes the overall circuitry

a Materials Science and Engineering, King Abdullah University of Science and

Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.

E-mail: tao.wu@kaust.edu.sab Solar and Photovoltaic Engineering Research Center (SPERC), King Abdullah

University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.

E-mail: osman.bakr@kaust.edu.sac Computer, Electrical and Mathematical Sciences and Engineering (CEMSE)

Division, King Abdullah University of Science & Technology (KAUST),

Thuwal 23955–6900, Saudi Arabia

† Electronic supplementary information (ESI) available: Absorption for Pt thin layer,I–V curves for thin film device etc. See DOI: 10.1039/c6tc02828d

Received 6th July 2016,Accepted 16th August 2016

DOI: 10.1039/c6tc02828d

www.rsc.org/MaterialsC

Journal ofMaterials Chemistry C

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complex and expensive.41 Therefore, it is beneficial to develop‘‘self-biased’’ photodetectors which can produce self-sufficientpotential for operation. Photodiodes made of Schottky or p–njunctions are prominent examples in this desirable class of self-biased photodetectors.42,43

A significant amount of application-oriented efforts have beendevoted to developing photodetectors based on emerging materialssuch as quantum dots, ZnO nanorods, graphene, MoS2, andso on.33,34,36–44 Dou et al. reported hybrid perovskite photo-detectors with high detectivity using a solar-cell-type structurewith electron/hole transporting layers.45 In addition, there havebeen several recent works on photodetectors based on polycrystal-line perovskite thin films and nanowires.46–52 High-quality singlecrystals are known to possess advantages such as good crystal-linity, chemical purity and high-mobility transport, which aredesired for building photodetectors.53,54 There have been pioneeringworks on using hybrid perovskite single crystals in narrow-bandphotodetectors18 and UV photodetectors.17 However, to the bestof our knowledge, there has been no report on constructingself-biased photodetectors using Schottky junctions of hybridperovskite single crystals.

In this work, we report on the light-modulated dielectric andtransport properties of Schottky junctions built on CH3NH3PbBr3

perovskite single crystals. Among the well-studied hybrid perov-skites, CH3NH3PbCl3 does not absorb visible light due to its wideband gap (2.9 eV);55 therefore, it is not suitable for constructingdevices working in the visible regime. CH3NH3PbI3 has beenextensively used as light absorber, and its carrier mobility iscomparable to that of CH3NH3PbBr3,23 but it is much lessstable.56 Among the hybrid perovskites, CH3NH3PbBr3 is stablewith a cubic lattice structure at room temperature, and the growthof large CH3NH3PbBr3 single crystals with smooth flat surfacesfacilitates the electrode deposition in photodetector fabrication.23

Therefore, CH3NH3PbBr3 single crystals were chosen in this reportfor constructing a prototypical self-biased single-crystal perovskitephotodetector. Photodetectivity up to 1.4 � 1010 Jones andresponse time on the order of 100 microseconds were observed.Importantly, these single-crystal photodetectors presented stableperformance in air for over 45 days. Our results indicate that high-quality hybrid perovskite single crystals are promising to serve ashighly tunable dielectric materials as well as active components inoptoelectronic devices.

2. Experimental section2.1 Synthesis and device fabrication

The CH3NH3PbBr3 single crystals were prepared by the modifiedantisolvent vapor-assisted crystallization method reportedelsewhere.23 In order to reduce the parasitic resistance; thesesingle crystals were mechanically polished to a thickness ofapproximately 150 mm. A semitransparent Pt layer of 20 nm wasdeposited by sputtering on one surface of the single crystal asthe top electrode, and 100 nm Au was deposited on the oppositesurface of the crystal as the bottom electrode. As referencesamples, thin films of CH3NH3PbBr3 perovskite were prepared

by two-step sequential method. The glass substrate was coated with100 nm Au and 220 nm PbBr2 layers using thermal evaporation,and then the sample was dipped in solution containing CH3NH3Br,followed by heating at 70 1C to form the perovskite film with athickness of 300 nm.

2.2 Characterizations

X-ray diffraction (XRD) spectra were obtained for single crystaland thin film of CH3NH3PbBr3, using a Bruker D8 Advance X-raydiffractometer. A field emission scanning electron microscope(FESEM, Nova Nano-SEM) was used to acquire cross-section SEMimages of single crystal. Thicknesses of thin film were measuredusing a Veeco Dektak 150 surface profilometer. UV-Vis absorptionspectra of the CH3NH3PbBr3 single crystal were recorded on aVarian Cary 6000i spectrophotometer. Spectral dielectric constantand photo-response was measured using a Newport Oriel QE/IPCEmeasurement kit with a silicon photodiode detector. The dielectricconstant measurements were carried out on an Agilent PrecisionLCR Meter. During the measurements no external DC voltage (zerobias) applied to the photodetectors. The current–voltage (I–V)measurements were performed on a probe station connected toa Keithley 4200 semiconducting analyzer. The transient photoresponses were measured on an oscilloscope; light with wave-length of 532 nm and intensity of 1 mW cm�2 was used at achopping frequency of 400 Hz.

3. Results

Fig. 1(a) shows the energy band alignment between CH3NH3PbBr3

and metal electrodes in the heterojunction device.57–59 Thehigh work function of Pt is expected to lead to the formation ofa Schottky junction.60 As previously reported by Dong et al.,materials with high work functions such as MoO3 are effective torealize Schottky junctions when come in contact with perovskites.52

On the other hand, the work function of Au is much smaller thanthat of Pt, which might result in ohmic contact and good chargetransport. Recently, Huang et al. demonstrated ohmic transportwhen Au was coated on both sides of perovskite single crystals.22

Fig. 1(b) shows the basic operation mechanism of the Pt–CH3NH3PbBr3–Au photodetector under light illumination. At thePt–perovskite junction, the Schottky barrier is formed and thebuild-in bias is approximately 0.5 V. Under the light illumination,the photo-excited electron–hole pairs in the depletion layer areefficiently separated by the built-in field and subsequently col-lected by the electrodes. Herein, the top Pt electrode not only actsas the hole collector, but also allows the light to illuminatethrough as a result of its semitransparent nature.

The schematic device structure of the single-crystal photo-detector is shown in Fig. 1(c). A semitransparent Pt layer with athickness of 20 nm was deposited on the CH3NH3PbBr3 perov-skite single crystal as the top electrode, and its transmittance wasmeasured to be B20% in the visible range (ESI,† Fig. S1). A layer of100 nm Au was deposited on the opposite side of the single crystalas the bottom electrode. The photograph of CH3NH3PbBr3 singlecrystal based photodetector is shown in Fig. 1(d). The CH3NH3PbBr3

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single crystal has a bright orange colour. Fig. 1(e) shows theabsorption spectrum of the perovskite single crystal, and theabsorption edge at 565 nm is consistent with bandgap ofCH3NH3PbBr3 perovskite (2.2 eV). The crystal thickness wasestimated to be approximately 150 mm as shown in Fig. 1(f).Also shown in Fig. 1(f) is the XRD pattern of the CH3NH3PbBr3

single crystal, and the sharp peaks labelled by the respectivemiller indices confirm the high sample purity.

Current–voltage (I–V) characteristics of the single-crystalperovskite photodetector measured under dark and light condi-tions are shown in Fig. 2(a) and (b). A nonlinear and asymmetricbehavior with hysteresis was observed in the dark I–V curve. Thisrectifying behavior is consistent with the formation of Schottkyjunction at the Pt–perovskite interface. Fitting the I–V curves tothe Schottky diode model provides insights into the operation ofthese single-crystal photodetectors. Cheung et al. proposed thefollowing equation to extract the diode parameters:61

dV

dðln IÞ ¼ IRS þnkT

q; (1)

where, RS is the diode series resistance, n the ideality factor, T thetemperature and k the Boltzmann constant. Fig. 2(c) shows theplot of d(V)/d(ln I) vs. I measured in dark for the forward currentdirection. The ideality factor (n) is an important parameter that

can be used to understand our devices with reference to the idealSchottky diode behavior. The value of extracted ideality factor(n = 1.7) confirms that the Pt/CH3NH3PbBr3 junction roughlyfollows the ideal Schottky behavior, and the observed deviationof ideality factor from 1 could be caused by extrinsic factors likeleakage current and junction inhomogeneity.62,63 Furthermore,the ideality factor increases significantly up to 3.3 as shown inFig. 2(d) when the device is under light illumination, which canbe attributed to the generation and recombination of photo-induced carriers.64,65 Concurrently, the series resistance decreasesfrom 169.2 kO to 212.2 O due to the photoconductive nature of theperovskite single crystal.

Fig. 3(a) shows the dielectric constant (e0) of the perovskitesingle-crystal measured as a function of frequency under darkand white light conditions. The measurements were done atroom temperature in the frequency range of 150 Hz–1 MHz.When a 100 mW cm�2 white light was illuminated on the crystal, a10-fold enhancement of the dielectric constant at lower frequencywas observed. The percentage of change in dark dielectric constantafter illumination was found to be 2300% at 150 Hz. Thissignificant change in dielectric constant indicates the presenceof photo-dielectric effect in the CH3NH3PbBr3 single crystal.There have been different mechanisms proposed for the originof photo-dielectric effect, such as changes in dipole momentdue to photo-induced carriers,66 formation of photo-inducedpolar domains67 and modification of local unit cell equilibrium.20

Our result indicates that such a light-induced modulation ofdielectric properties exists in not only in thin films and compositesbut also in high-quality pure-phase perovskite single crystals.

As shown in Fig. 3(a), e0 under light illumination initially fallsquickly with increasing frequency and then becomes almostconstant at high frequencies (up to 1 MHz). This high frequencybehaviour arises due to the lagging of polarization with appliedoscillating field at high frequencies.68 In the present case, mostlikely, more than one type of ions, e.g., CH3NH3

+ and Pb+,

Fig. 1 (a) Energy band levels of each component with respect to thevacuum level. (b) Energy band alignment of the Pt–CH3NH3PbBr3–Audiode under light illumination. (c) Schematic structure of the single-crystalperovskite photodetector. (d) Photograph of the photodetector based onCH3NH3PbBr3 single crystal. (e) The absorption spectrum measured for theCH3NH3PbBr3 single crystal. (f) SEM image with false colors of the crosssection of the CH3NH3PbBr3 single crystal. Also shown is the XRD patternof the CH3NH3PbBr3 single crystal.

Fig. 2 I–V characteristics measured on the CH3NH3PbBr3 single-crystalphotodetector in (a) dark and (b) under white-light illumination. Plotsof d(V )/d ln(I) vs. the forward current I measured on the CH3NH3PbBr3

photodetector under (c) dark and (d) white light conditions.

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contribute to the relaxation process. The dispersion behavior ofthe dielectric constant can be described by the Debye model:69

e0 ¼ e10 þ e0

0 � e10

1þ ðotÞ2ð1�a0Þ (2)

where, e0 is the real part of dielectric constant, eN0 dielectricconstant at the high frequency limit, dielectric constant at the lowfrequency limit, o the angular frequency, i.e., 2pf, t the relaxationtime and a0 the spreading factor of actual relaxation time aboutthe mean position or the Debye’s distribution parameter. Thestraight line fit to a plot of log e0

0 � eN0/e0 � eN0 vs. logo shownin ESI,† Fig. S2 gives the slope, i.e., a0 and the intercept, i.e., t.Using these parameters the frequency-dependent dielectric con-stant was calculated and plotted along with the experimentaldata in Fig. 3(a). It was found that the experimentally obtainedrelaxation behavior of dielectric constant follows well the Debyemodel. Also, it is interesting to note that a quicker dielectricrelaxation time was observed after light illumination i.e. for darkt = 310 ms, and under light illumination t = 12 ms. This indicatesthat relaxation process reveals that illumination of light reducesthe delay of polarization switching with increasing frequency.70

Fig. 3(b) shows the AC conductivity of the single crystalheterojunction measured under dark and light conditions as afunction of frequency. The AC conductivity can be written as:71

sAC = oe0e0 tan d (3)

where, o is the angular frequency 2pf, e0 the permittivity of freespace and tan d the dielectric loss. When the measurementfrequency is high enough, the AC conductivity of a disorderedsolid can be written as:72

sAC ¼ sDCooc

� �n

for o4oc (4)

where sDC is the DC conductivity, oc is the critical frequency i.e.the frequency at which the conduction process switches tocapacitive behavior, and the parameter n is zero for a regularconductor and 1 for a perfect insulator. As rule of thumb, for adisordered solid or insulating material, 0.6 o n o 1.0, whereasfor ordered solid or conducting material, n o 0.6. A linear fit inthe high-frequency range can give the slope, i.e., the parametern and the intercept, i.e., sDC. Interestingly, the value of n = 0.70measured in the dark condition indicates the insulating natureof the perovskite single crystal. Under the illumination of light,it decreased to n = 0.34, indicating that the light-generatedcarriers significantly enhance the conduction of the sample.In addition, light illumination might lead to some kind ofordering, particularly improved alignment of dipoles, withinthe single crystal. In a recent report, a similar photo-inducedchange of dielectric constant was ascribed to the modificationof local unit cell equilibrium.20

In order to further quantify the dielectric properties of thesingle crystal photodetector, we performed wavelength and lightintensity dependent measurements. The photo-induced dielectricconstant measured as a function of wavelength at a light intensityof 10 mW cm�2 is shown in Fig. 3(c). This wavelength depen-dent result is consistent with the optical absorption data shown inFig. 1(c). A sharp absorption edge around 565 nm is characteristicof the band gap absorption for CH3NH3PbBr3 single crystal.73

Accordingly, the photo-dielectric effect disappears above thewavelength corresponding to the perovskite band gap.74 Toacquire a deeper understanding into the photo-induced dielectrictunability in perovskite single crystals, the dielectric constant at10 kHz was measured under the illumination of different lightintensities. Fig. 3(d) shows that the dielectric constant decreaseswith decreasing light intensity. Also, a power law can be appliedto describe the relationship between the dielectric constant andthe light intensity:75

Intensity = ke0y, (5)

where k is a proportionality constant. Furthermore, the powerlaw fitting revealed that the exponent y decreases with themeasurement frequency as a result of the dielectric dispersionbehavior.

Now we turn to the light response properties of the single-crystal perovskite heterojunction. The semi-log I–V plots for thesingle-crystal perovskite photodetector measured under dark andlight conditions are shown in Fig. 4(a). The slight hysteresis couldbe related to the voltage-induced drift of ions, as recentlyreported.76 When the device was illuminated with white light(intensity: 100 mW cm�2), photocurrent at zero bias was enhancedby more than three orders of magnitude, corresponding to anon/off ratio of B1� 104. The observed self-biased photosensitivitycan be attributed to the emergence of an open-circuit voltage ofapproximately 250 mV in the I–V curve under illumination,indicating that the photo-excited electron–hole pairs are effectivelyseparated by the built-in field at the Pt–perovskite Schottkyjunction. The ON–OFF characteristics of the photocurrent werealso measured at zero bias for the perovskite photodetector underalternating dark and light conditions. The data in Fig. 4(b) clearly

Fig. 3 (a) Dielectric constant measured on the CH3NH3PbBr3 single-crystal as a function of frequency in dark and under light illumination.The lines are the calculation result. (b) AC conductivity measured asfunction of frequency. (c) Dielectric constant at 10 kHz measured as afunction of wavelength. (d) Dielectric constant measured at differentfrequencies under the illumination with varying white light intensities.

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show that the zero-bias photocurrent remains stable and can bereliably switched.

Responsivity (R) and detectivity (D) are important performanceparameters of photodetectors. The responsivity indicates how

efficiently the devices convert light to photocurrent, and it can becalculated as

R ¼ ILight � IDark

P; (6)

where, ILight and IDark are the photocurrent and dark current ofthe photodetector, respectively, and P is the incident lightintensity.48 When the dark current is dominated by the shotnoise, which is often the case of Schottky-type photodetectors, thedetectivity can be calculated as

D ¼ R

2qJdð Þ12

; (7)

where q is the elementary charge and Jd is the dark currentdensity.45 Apparently, it is important to suppress the dark currentin order to increase the photodetectivity. Thus, the good crystal-line quality of perovskite single crystals is beneficial to reduce theleakage current and to improve the photodetectivity of fabricateddevices.

The responsivity and detectivity of our single-crystal perovskitephotodetectors were calculated as 2 mA W�1 and 1.4 � 1010

Jones at zero bias, respectively. At an applied bias of +3 V, theresponsivity and detectivity increase to 100 mA W�1 and 7.1 �1011 Jones, respectively. The performance of our devices iscomparable with the previously reported CH3NH3PbI3-basedphotodetector with electron/hole transporting layers.45 Using asolar-cell-type photodetector, Dou et al. reported a very highdetectivity of 1014 Jones,45 but their multilayer structure ismuch more complex than ours. In another recent study,50

CH3NH3PbI3/TiO2 photodetectors were reported to exhibit aresponsivity of 0.49 mA W�1. By using perovskite–graphene compo-sites in hybrid photodetectors, Lee et al. reported a photodetectivityof 107 Jones.49 Recently, self-powered photodetectors were alsorealized in CH3NH3PbI3/ZnO and CH3NH3PbI3–CH3NH3PbBr3

thin film heterojunctions,77,78 but their device architectures aredifferent from our metal/perovskite Schottky junctions. A detailedcomparison between the published results and ours is shown inESI,† Table S1.

We also fabricated a photodetector based on CH3NH3PbBr3

thin films with the device structure similar to the single-crystalcounterpart. It should be noted that the thickness of perovskitefilm affects the photodetector performance. A film too thinresults in poor light harvesting, while a film too thick causeshigh charge recombination resistance. The optimized thicknessof perovskite film is around 300 nm, which is consistent with thereports in literature. Fig. S3(a) (ESI†) shows the I–V characteristicof the thin-film device measured in dark and light conditions.The photodetectivity is about 2.7 � 107 Jones and the responsetime is on the scale of milliseconds (Fig. S3(b), ESI†). Thiscomparison indicates that a better performance can be obtainedusing grain-boundary-free single crystals, which possess advan-tages such as low trap density, long carrier diffusion length andhigh mobility. Furthermore, to determine the effect of chemicalcomposition, the performance of a photodetector based onCH3NH3PbI3 single crystal was examined and the result is shownin ESI,† Fig. S4. Its photodetectivity is approximately 2� 109 Jones,

Fig. 4 (a) Semi-log graph for I–V characteristics measured on theCH3NH3PbBr3 single-crystal photodetector in dark and under white-lightillumination. (b) Photocurrent response measured at zero bias underalternating dark and light conditions. (c) Transient photo response measuredon the single-crystal CH3NH3PbBr3 photodetector.

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comparable to the result of CH3NH3PbBr3 single crystal device, butit decreased by two orders of magnitude after being kept in air for3 days, which is a result of the lower stability of the CH3NH3PbI3

perovskite in comparison to the Br one.79,80

Furthermore, we measured the response time of the single-crystal perovskite photodetector, which is another key criterion ofdevice performance. The time required for the photocurrent toincrease from 10% to 90% in the alternating dark and lightconditions is generally defined as the rise time (ton) of photo-detectors, and vice versa for the definition of the decay time(toff).52 We performed temporal photoresponse measurementswith a light intensity of 1 mW cm�2 and a chopping frequency of400 Hz. As shown in Fig. 4(c), for the self-biased single-crystalphotodetector, the rise time ton is 70 ms and the decay time toff is150 ms. These photoresponse speeds are approximately 103 timeshigher than those recently reported for photodetectors based onCH3NH3PbBr3 nanowires (ton = 120 ms and toff = 86 ms)46 andCH3NH3PbI3–CH3NH3PbBr3 heterojunctions (ton = 120 ms andtoff = 94 ms).78 Using PEDOT:PSS and PCBM as charge trans-porting layers, Dou et al. reported that their perovskite photo-detectors could operate in the MHz regime.45 Thanks to theirhigh structural quality and low defect density, the photo responseof single-crystal photodetectors was faster than the control devicebased on Schottky junctions of CH3NH3PbBr3 thin films (ESI,†Fig. S3(b)). Compared with other perovskite-film-based photo-detectors without using electron/hole transporting layers,48,50

the much faster photoresponse of our devices could be attributedto the high carrier mobility and low defect traps of the perovskitesingle crystals.

It is well recognized that the configuration of photodetectorsdirectly influences the device performance. As an importantfactor influencing the detector speed, the transit time Ttr, whichdescribes how fast photocarriers drift across the photodetectors,is given by the following equation:81

Ttr ¼L2

Vmn; (8)

where L is the inter-electrode spacing (150 mm in the measureddevice), V is the built-in or applied bias and mn is the chargemobility. Clearly, the high mobility of CH3NH3PbBr3 singlecrystal (115 cm2 V�1 s�1) helps reduce the transit time.23 Forthe self-biased photodetector, the built-in bias Vbi can be calcu-lated as Vbi = FM� Fs, where FM and Fs are the work functions ofmetal and semiconductor, respectively. Using the reported workfunctions reported in literature for Pt and perovskite,57,58 Vbi canbe calculated as approximately 0.5 V. Then, according to eqn (8),the transit time is estimated as approximately 3.9 ms. The differencein the transit time and the actual measured decay time indicatesthat other effects like RC time constant, which is associated withthe capacitance (C) and the resistance (R) of the photodetector,significantly influence the photoresponse speed.81 Using a planardevice configuration may enhance the photodetector speed,82 but itis challenging to fabricate the interdigitated electrodes with twodifferent metals to retain the self-biased functionality of Schottkyjunctions. Further efforts such as thinning down the single crystalsto reduce Ttr and R are needed to improve the photoresponse speed

of our Schottky-type single-crystal photodetectors. Because of theirbrittle nature, conventional mechanical thinning approaches arenot suitable to process such perovskite single crystals. Clearly,developing new growth and processing strategies is necessary toachieve the optimal performance of such single-crystal-baseddevices.

We further measured the responsivity and detectivity of thesingle-crystal detector as a function of wavelengths. The spec-tral response from 350 nm to 800 nm is shown in Fig. 5(a). Thephotodetector exhibits the highest self-biased detectivity of3.1 � 1010 Jones under the illumination of UV light (350 nm),while weaker response was observed at longer wavelengths.

The increase in responsivity and detectivity below 400 nmmight be due to the enhanced electron transition from valenceband to conduction band under the excitation of high-energyphotons.83 A similar rise in responsivity in shorter wavelengthwas observed by Xin et al. in photodetectors based on perovskitethin films.48 Furthermore, the photodetectivity suddenly droppedby approximately one order of magnitude at 560 nm. This resultis consistent with the absorption data of the CH3NH3PbBr3 single

Fig. 5 (a) Responsivity and detectivity of the photodetector measured asa function of wavelength. (b) Photocurrent (circles) and detectivity(squares) measured at zero bias for the CH3NH3PbBr3 photodetectorunder different white light intensities. The straight line is the power lawfitting. Also shown are the photocurrent data (solid spheres) measured onthe same photodetector after being stored in air for 45 days.

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crystal shown in Fig. 1(e), indicating that light absorption dictatesthe operation of such photodetectors. This strong absorption-detectivity correlation also points out the perspective of leveragingon the different band gaps of perovskites to construct photo-detectors operating in selective wavelength ranges.

Dependence of detectivity on light intensity is another essentialfigure-of-merit for photodetectors. Fig. 5(b) shows the data ofintensity-dependent photocurrent measured on the single-crystalphotodetector. The variation of photocurrent with light intensityis usually described by a simple power law, I = aPy, with P and abeing light intensity and constant, respectively.84 The value ofy lies between 0.5 and 1, depending on the presence of trap statesand the associated charge recombination.85 In the present case,the intensity-dependent photocurrent data fall onto a straightline with y = 1, indicating that there exist only shallow (low-energy) traps, which is an essential condition for photodetectorswith good performance.23 As a result of this linear intensitydependence, the detectivity of the single-crystal photodetectorremained at approximately 1 � 1010 Jones within the range ofmeasured light intensity.

In addition, as shown in Fig. 5(b), the photodetector retainedits initial photocurrent after being kept in air for 45 days, givingfurther support to its stable performance. The mechanismbehind the observed high stability of single-crystal device istwo-fold. In one aspect, perovskite single crystals possess morestructural integrity than thin films. It is known that hybridperovskites tend to degenerate into their precursors due to thehygroscopic nature of amine salts, and degeneration starts atsites of structural defects and grain boundaries.79 Differentfrom the polycrystalline films, perovskite single crystals are freefrom grain boundaries and can remain stable in air for over onemonth. In the work of Wang and co-workers, good stability of aself-powered photodetector based on ZnO/CH3NH3PbI3 hetero-junctions was achieved by a top MoO3 layer which preventsmoisture penetration.77 In the other aspect, when I� ions(radius: 2.2 Å) in CH3NH3PbI3 is replaced with smaller Br�

ions (radius: 1.96 Å), cubic crystal structure can be stabilizedat room temperature and perovskite becomes less sensitive tomoisture.56,58

4. Conclusions

In summary, we have demonstrated a photo-dielectric effectand self-biased photo-detection via building Schottky junctionson high-quality perovskite CH3NH3PbBr3 single crystals. Highvalues of dielectric constant under light illumination supportsthe possibility of light induced ordering in the perovskite singlecrystal. These devices are different from the previous reportson using hybrid perovskite single crystals in narrowbandphotodetectors18 and UV detectors.17 Owing to the excellentabsorption and transport properties of hybrid perovskites, ourCH3NH3PbBr3-based devices exhibit promising properties suchas a detectivity of 1.4 � 1010 Jones at zero bias and responsetimes on the scale of 100 ms. Such an approach of constructingSchottky-type single-crystal CH3NH3PbBr3 detectors with high

performance and self-biased capability can be applied otherhybrid perovskites with different compositions and band gaps.

Acknowledgements

The research reported in this publication was supported by KingAbdullah University of Science and Technology (KAUST).

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