Nanoscale

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aCentre for Excellence in Solar Energy, Phy

National Chemical Laboratory (CSIR-NCL),bNetwork Institute of Solar Energy (CSIR-NI

ncl.res.in; satishogale@gmail.com; Fax: +91cDepartment of Physics, University of Pune, G

onkar.game@gmail.com

† Electronic supplementary informationmeasurements of self powered hybrid Znlaser, SI-2: Raman analysis of N:ZnOSpiro-MeOTAD SI-4: electrochemical MoSee DOI: 10.1039/c3nr04727j

Cite this: Nanoscale, 2014, 6, 503

Received 4th September 2013Accepted 10th October 2013

DOI: 10.1039/c3nr04727j

www.rsc.org/nanoscale

This journal is © The Royal Society of C

ZnO(N)–Spiro-MeOTAD hybrid photodiode: anefficient self-powered fast-response UV (visible)photosensor†

Onkar Game,*abc Upendra Singh,ab Tanya Kumari,ab Arun Banpurkarc

and Satishchandra Ogale*ab

Organic–inorganic hybrid photo-detectors with a self-sufficient mode of operation represent a research

area of great current interest. In most efficient photodetectors and optoelectronic devices compound

semiconductors containing toxic elements such as Cd, As, Te, S, Se etc. are used and these are also

expensive. Hence there is also a rapidly growing interest in replacing these with environmentally friendly

and earth-abundant materials. Herein, we report a facile solution-processed fabrication of a self-

powered organic–inorganic hybrid photodetector using n-type oriented ZnO nanorods and p-type

Spiro-MeOTAD semiconductor. ZnO is eco-friendly and earth-abundant, and Spiro-MeOTAD is non-

hazardous. We show that the latter has far less toxicity than the toxic elements stated above. This visible

blind UV photodetector shows high sensitivity (102) and a UV/visible rejection ratio of 300. It also exhibits

fast response times of srise � 200 ms and sfall � 950 ms. Importantly, with a small modification of nitrogen

incorporation in ZnO one can also realize a highly-sensitive self-powered visible light photodetector

with at least 1000% (or higher) improvements in quality factors (photocurrent/sensitivity/response time)

as compared to previously reported organic–inorganic hybrid photo-detectors based on metal-

chalcogenides (CdS–PANI or CuInSe2–P3HT). Interestingly, the broadband sensitivity of such N:ZnO–

Spiro-MeOTAD photodiode enables sensing of low intensity (�28 mW cm�2) ambient white light with a

high photocurrent density of 120 nA cm�2 making it an efficient ambient white light detector.

Introduction

Photodetectors with response in the UV and/or visible regionare used in a wide range of applications from everydayconsumer electronics (compact disc players, smoke detectors,remote control) to more sophisticated applications such asenvironmental monitoring, space research and opticalcommunication.1,2 Photodetectors operate on various principlessuch as photo-electrochemistry3 photoconduction or photo-diode action.4 In order to get reasonable detectivity, most of thephotodetectors need a certain applied potential (1–100 V) whichtherefore requires a constant use of batteries (power). Thismakes the overall circuitry heavy and uneconomical in the

sical and Materials Chemistry Division,

Pune 411 008, India

SE), New Delhi, India. E-mail: sb.ogale@

20 2590 2636

aneshkhind, Pune-411 007, India. E-mail:

(ESI) available: SI-1: response timeO–SPD under nanosecond pulsed UVand ZnO SI-3: cytotoxicity study ontt–Schottky plots of ZnO and N:ZnO.

hemistry 2014

current energy scenario. The ‘self-powered’ photo-detectorswhich are either integrated with an energy harvesting unitsuch as biofuel cell or pizotronic circuitry,5 or which cangenerate self-sufficient potential for their operation6 have latelyattracted considerable attention from scientists andtechnologists.

Photodiodes (Schottky or p–n junction type) operated in aphotovoltaic short-circuit or open-circuit modes form animportant class of such self-powered photodetectors.6,7 Mostsuch photo-diodes employ nanostructured inorganic semi-conductors8–10 which exhibit attractive functionalities such ashigh carrier mobility and high absorption coefficients; but theysuffer from drawbacks such as high temperature (>800 �C)synthesis, complex device fabrication methods and low exi-bility. On the other hand organic semiconductors are solutionprocessable, with good exibility and functional tunabilitythrough modications of involved monomers/molecules; butthey suffer from relatively low carrier mobility values. Therefore,hybrid organic–inorganic semiconductor junctions are envis-aged to complement each others’ functional advantages whileovercoming their respective drawbacks.

There is an impressive body of work reported in the literatureon organic–inorganic hybrid photodetectors11–16 with goodresponse both in the UV and/or visible region(s). However most

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of these hybrid photo-detectors employ compounds of rareelements such as indium (In), gallium (Ga) germanium (Ge) ortoxic elements such as cadmium (Cd), sulfur (S), selenium (Se)etc., thereby raising a concern over the possible health andenvironmental hazards when produced on an industrial scale.Recently, there have been a few interesting reports17 on hybridvisible light photo-detectors using earth abundant or nontoxicelements, but these lack the self-powered mode of operation.Therefore, organic–inorganic hybrid photo-detectors involvinguse of non-toxic components and operating under self-sufficient power mode are highly desired.

Zinc oxide (ZnO) is an environmentally friendly, non-toxicmetal–oxide semiconductor with a wide range of optoelec-tronic applications due to its several attractive physical prop-erties.18 In particular, this is an n-type semiconductor with veryhigh carrier mobility (>100 cm2 V�1 s�1 at RT, >2000 cm2 V�1 s�1

at 80 K) and crystal habits that allow growth of diverse nano-structured morphologies. Among the various morphologies ofZnO, quasi one dimensional (quasi-1D) nanorods are particu-larly interesting because their single crystalline nature offersefficient carrier transport and higher diffusion length. Lowtemperature (�90 �C) solution synthesis of single crystallineZnO nanorod arrays on conducting substrates makes ZnO themost preferred candidate for hybrid devices over many othersemiconductors used in photodetectors that require hightemperature (>800 �C) physical vapor deposition synthesis.15

Among various p-type organic semiconductors 2,20,7,70-tet-rakis (N,N-di-p-methoxyphenylamine)9,90-spirobiuorene(Spiro-MeOTAD) is an attractive hole-conductor which is widelyused in optoelectronic applications such as solid-state dyesensitized solar cells (DSSCs)19 or light emitting diodes.20 Effi-cient hole regeneration capability and high pore-lling fractionwithin mesoporous nanostructured lms make Spiro-MeOTADa preferred hole-conductor over other high-conductivity poly-mers such as poly-3-hexylthiophene-2,5-diyl (P3HT) or solutionprocessable but highly toxic inorganic hole-conductors such ascopper thiocyanide (CuSCN).21,22 To our best knowledge, this isthe rst report where Spiro-MeOTAD is used ias an organic–inorganic hybrid photodetector.

In this report, we rst prepared an organic–inorganic hybridself-powered UV photodiode using p-type Spiro-MeOTAD andsolution-processed n-type ZnO nanorods arrays. The UVphotodiode showed an impressive UV to visible rejection ratioof 102 and a high photocurrent density/photo-responsivity, atleast 250% higher than recently reported all inorganic ZnO-based photodiodes which employ highly toxic componentssuch as CuSCN.23 Equally importantly, the hybrid UV photode-tector showed a photo-response (sr � 200 ms and sd � 950 ms)faster than that of previously reported self-powered photo-electrochemical cell (PECC) type photodetectors3 or organic–inorganic hybrid UV photodetectors.24,25 Additionally, with aslight but important modication of in situ N doping in ZnO, wecould also obtain a highly-sensitive visible light photodiodewith a remarkable set of parameters. Our N:ZnO–Spiro-MeOTAD visible-light hybrid photodetector showed an absorp-tion onset at 650 nm which is further red-shied in the visibleregion as compared to absorption onset of common visible light

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sensitive semiconductors such as cadmium sulde or zincindium sulde. The various parameters of our self-poweredN:ZnO–SpiroMeOTAD visible-light photodetector are at least1000% superior as compared to previously reported ones onZnO26,27 or metal chalcogenide based organic–inorganic hybridvisible photodetectors such as CuInSe2–P3HT,14 Cu2ZnSnS4–P3HT17 or CdS–PANI.28 Most importantly, we show that N:ZnO–SpiroMeOTAD visible-light photodiode can be used to sense lowintensity (28 mW cm�2) of ambient room light with a highphotocurrent on/off ratio of 102. To our knowledge, this has notbeen demonstrated previously. All solution processing, use ofnon-toxic materials, impressive broadband performance, highsensitivity and fast response are the key highlights of theN:ZnO–SpiroMeOTAD photodetector system demonstratedhere.

ExperimentalMaterials and synthesis

The synthesis of ZnO nanorods on uorine doped tin oxide(FTO) substrates was performed using a reported protocol29,30

The seed solution was rst prepared by dissolving zinc acetate(Sigma Aldrich, 22.8 mM) in ethanol and then drop-cast on aclean substrate followed by heating it for 5 min at 350 �C. Thisprocess was repeated twice and then the substrate was annealedfor 30 min at 450 �C. For the growth of ZnO nanorods, separatesolutions of zinc nitrate hexahydrate (Merck, 25 mM) and hex-amethylene tetramine (HMTA) (Sigma Aldrich, 25 mM) wereprepared in D.I. water. Polyethylenimine (PEI) (Sigma Aldrich,Mw: 1800, 5 mM) was added to HMTA solution with continuousstirring, followed by addition of this solution to zinc nitratesolution. The seeded substrates were immersed into the growthsolution contained in a closed glass container with the seededside facing the bottom of the glass container. Further, thecontainer was heated to 95 �C on the magnetic heater-cum-stirrer for three hours. Aer the reaction was complete, thegrown ZnO NRs were thoroughly rinsed several times with D.I.water and nally with ethanol, and then dried under IR lamp.The obtained ZnO nanorod lms were annealed at 450 �C for 30min to remove the residual polymer and achieve better adhe-sion. The nal appearance of the lms was white in color.

The nitrogen doped ZnO (NZnO) nanorod arrays were grownon FTO using similar protocol as described for ZnO nanorodsynthesis with some modications. The growth solution waszinc nitrate (40 mM), HMTA (40 mM) and PEI (5 mM) solutions.In addition to this, ammonia solution (Merck, 25%, 3.8 mL) wasadded to the growth, which acts as a source of nitrogen. Further,these tightly closed glass vials containing seeded substrateswere heated to 100 �C in a silicon oil bath for 30 min. The NZnOnanorod lms were annealed at 450 �C for 30 min and the nalappearance of the lms changed to yellowish-orange due todefects created by nitrogen doping. X-Ray diffraction (XRD) wasrecorded on a Philips X’Pert PRO using Cu Ka radiation. UV-visible diffused reectance spectra were recorded using aJasco V-570 spectrophotometer. The Raman spectra for ZnOsamples were obtained using a Horiba Jobin Yvon Lab RAM HRSystem. The solution of Spiro-OMeTAD was prepared by

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dissolving Spiro-OMeTAD (68 mM), tert butylpyridine (55 mM)and lithium bis(triuoromethylsyfonyl)imide salt (9 mM) inchlorobenzene.

Device fabrication and characterization

25 mL of a Spiro-MeOTAD solution was drop cast on ZnO orNZnO nanorod lms and spun at a rate of 2000 RPM for 45 s. Agelectrode was thermally evaporated onto ZnO or N:ZnO lms asa top contact whereas FTO acted as bottom contact. The elec-trical characterization was done using Keithly 2420 sourcemeter and Lab Tracer 2.0 soware. Blue, green, red and whitelight LEDs (Luxeon batwing 1 W) were used as visible lightsources for different wavelengths. The power of LEDs atdifferent intensities was recorded using Coherent Laser PowerEnergy Meter, and Lab Max Top-0824C10R Photodiode. TES1332A Digital Lux-meter was used to measure the intensity ofwhite light. Response time measurements were made usingTetronix MSO 2024 Mixed Signal Oscilloscope.

Results and discussionZnO–SpiroMeOTAD (ZnO–SPD) UV photodiode

Fig. 1(a) shows the X-ray diffraction (XRD) of ZnO nanorod (NR)arrays grown on FTO. The presence of a sharp (002) peak at34.4� implies that the nanorods are oriented along the [0001]direction. The inset of Fig. 1(a) shows a characteristic ZnO bandedge absorbance at 385 nm in a normalized UV-Vis absorbancespectrum which corresponds to a band gap of 3.2 eV. Fig. 1(b)

Fig. 1 (a) XRD and UV-Vis absorbance (inset) of a ZnO nanorod film on Fof ZnO NR film infiltrated with Spiro-MeOTAD (c) device schematic and eZnO–SPD under dark and UV (390 nm) with intensity 3 mW cm�2, (e) Iof ZnO–SPD under 3 mW cm�2 UV (390 nm) illumination under alterna

This journal is © The Royal Society of Chemistry 2014

shows the SEM image of an oriented ZnO NR array on FTO witha length of �1 mm and width �200 to 300 nm. The tilted (45�)view of Spiro-MeOTAD–ZnO NR p–n junction shows well-percolated hole transport material within the gaps of ZnOnanorods. Good pore-lling of Spiro-MeOTAD within nano-structured ZnO nanorods gives a higher p–n interfacial arearesulting in a high rectication ratio and photocurrent density.

Fig. 1(c) shows the schematic of ‘ZnO–Spiro-MeOTADPhotodiode’ (ZnO–SPD) and the corresponding energy leveldiagram. It is clear that higher energy gaps (Eg $ 3 eV) andfavorable energy alignment of ZnO and Spiro-MeOTAD makethem suitable candidates for UV sensing application. The darkJ–V curve of ZnO–SPD in Fig. 1(d) shows a typical p–n junctionrectifying curve with a low turn-on voltage of 0.2 V and a fairlyhigh rectication ratio (RR) of 2.3 � 102 at a low bias of �0.5 V.The RR for ZnO–SPD is smaller by one order of magnitude thanthe recently reported ZnO nanorod based all inorganic photo-diodes.31 The low RR values in organic–inorganic hybrid p–njunctions as compared with their all inorganic counterparts canbe assigned to high diode ideality factors in organic–inorganichybrid systems.12,24 Using the dark IV curve for ZnO–SPD andthe diode equation,12,24 the ideality factor for ZnO–SPD wasfound to be 3.4. A high value of the ideality factor points towardsthe presence of recombination losses probably due to surfacetrap states at ZnO–Spiro-MeoTAD interface or a leakage currentat Spiro-MeOTAD–FTO interface. Nevertheless, the RR value of230 is one order of magnitude higher than the previouslyreported RR values12,23 for ZnO NR based organic–inorganichybrid photodiodes at a bias of�0.5 V. This can be attributed to

TO, (b) SEM of ZnO nanorod arrays with inset showing a 45� tilted viewnergy level diagram of ZnO–SpiroMeOTAD photodiode, (d) IV curve of–V curve in (d) plotted on absolute-semilog scale, (f) photo-responsete on and off cycles.

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appropriate energy alignment, fairly good pore-lling of Spi-roMeOTAD, and a low leakage current at the Spiro-MeOTAD–FTO interface. Therefore, ZnO–SPD heterojunction is a goodcandidate for low-voltage optoelectronic applications. Fig. 1(d)also shows the J–V curve of ZnO–SPD under low intensity (3 mWcm�2) UV (390 nm) illumination. The photo-response can beseen in both forward and reverse bias conditions. The inset ofFig. 1(d) shows a magnied J–V curve of ZnO–SPD near zero biaswhich clearly shows a small photovoltaic effect (Voc � 29 mV, Jsc� 50 mA cm�2) in the presence of UV (3 mW cm�2) illumination,and hence points towards the feasibility of using ZnO–SPDunder a self-power mode (i.e. at V � 0 V). When ZnO–SPD J–Vcurves under dark and illumination are plotted on a semi-logarithmic scale in Fig. 1(e), a maximum sensitivity (Ilight/Idark)of 3 � 102 is noted at a zero applied bias, reecting its self-powered mode of operation.

Fig. 1(f) shows the semilogarithmic plot of photocurrent vs.time of unbiased ZnO–SPD under chopped (10 s on and 10 s off)UV light (3 mW cm�2). A clear on/off ratio of 3 � 102 is observedon the semilog scale. The peak photocurrent density is observedto be 50 mA cm�2 at 3 mW cm�2 UV irradiation, which is a factorof two higher than recently reported values for solution-processed ZnO based self-powered UV photodetector.23,32

In order to further investigate the spectral response of ZnO–SPD we calculated wavelength dependant responsivity R(l) fromthe external quantum efficiency (EQE) measured using a stan-dard Newport quantum efficiency setup and the expressionR(l) ¼ EQE/hn. Fig. 2 shows the responsivity of ZnO–SPD withmaximum responsivity of �17 mA W�1 at 390 nm which closelymatches the band edge absorption of ZnO and Spiro-MeoTAD.The UV responsivity of 17 mA W�1 at 390 nm is at least afactor of two higher than previously reported self-powered23,32,33

UV photodetectors for 390 nm wavelength.Importantly, the UV to visible rejection ratio is found to be 3

� 102 making it suitable for visible blind UV detectionapplications.

Fig. 2 Spectral responsivity of ZnO–SPD showing a high UV to visiblerejection ratio.

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The response time of photodetectors is another signicantparameter for high speed optoelectronic applications. The timeresolution (typically 30–40 ms) of the measurement unit canlimit the response time of a photodetector. Thus to determinethe response times more accurately we used a pulsating sourceof UV (365 nm) light generated by keeping a mechanicalchopper rotating at a frequency of 230 Hz in front of a contin-uous UV source. An oscilloscope (Tetronix MSO 2024, 200 MHzbandwidth) was used to measure the transient voltage gener-ated across 1 MU resistance connected in series with ZnO–SPD.

Fig. 3(a) shows a schematic setup for measurement of theresponse time. The rise time (srise) and fall time (sfall) are thendened as the times taken by the signal to vary between 10 and90% of the saturation value. Fig. 3(b) shows the semilog plot ofvoltage (caused by photocurrent in ZnO–SPD) vs. timemeasuredacross a 1 MU resistance. A clear saturation is observed in thetransient signal with a rise time of 200 ms and a fall time of 950ms. The rise and fall times observed here are one order ofmagnitude faster than previously reported values for ZnO basedself-powered organic–inorganic hybrid photo-detectors.3,24,25,27

The faster response time can be attributed to efficient holeregeneration capability of Spiro-MeOTAD due to better pore-lling as compared to other organic semiconductors in hybridphotodiodes. Recently, some reports23,32,34 have also used ananosecond pulsed UV laser as an alternative to the mechan-ically chopped UV source for measurement of srise and sfall. Wepoint out in ESI-1† that although such measurements are usefulto get information about charge-transfer dynamics, they maygive inappropriate values of the response time. The parametersof self-powered ZnO–SPD reported here are superior to previ-ously reported ones for organic–inorganic hybrid UV photodi-odes. Importantly, the materials involved in fabrication of thephotodiode reported here are non toxic, and environmentfriendly in contrast to previous reports on self-poweredphotodetectors.23

N:ZnO–SpiroMeOTAD visible photodiode

Photodetectors with response in the visible region are alsoimportant for various applications such as environmentalmonitoring, optical communications, and conventional or

Fig. 3 (a) Schematic of the experimental arrangement used tomeasure the response time of the photodiode, (b) photovoltagegenerated across 1 MU resistor under pulsating light of frequency 230Hz.

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biomedical imaging.4 Recently, there has been a drive to replacethe visible-light-sensitive toxic compounds such as CdS, CdSe,PbS with non-toxic, environment-friendly and earth abundantmaterials for application in visible light photodetection.35 Mostof the recent efforts have been focused mainly on metalchalcogenides/nitrides/phosphides such as Bi2S3,36

Cu2ZnSnS4,17 InP,13 AgInS2 (ref. 11) etc. However, there havebeen only limited efforts on using the robust and eco-friendlymetal oxide systems37,38 for visible light detection. Recently wereported39 that concurrent nitrogen and defect incorporation inZnO can lead to a broadband absorption extending up to 650nm, which also yields enhanced performance in photo-electrochemical (PEC) water splitting. Although undoped/doped ZnO alone is capable of sensing UV/visible light, thesephotodetectors require some applied potential for their opera-tion and their response time is in few hundreds of seconds,1 asthe working principle mostly involves adsorption–desorption ofoxygen. Therefore, it is necessary to form a hybrid junction withN:ZnO which can sense visible light with very fast response. Wehave previously39 shown (in the context of PEC water splitting)that a simple addition of ammonia solution during the growth

Fig. 4 (a) UV-Vis absorbance and XRD of ZnO and N:ZnO nanorod arrapresence of N doping within ZnO, (c) photoluminescence (PL) spectrum osectional view of NZnO–SpiroMeOTAD photodiode.

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ZnO nanorods can lead to formation of Zn–nitrogen complexwhich, on heating in air leads to nitrogen doping within theZnO lattice. We followed a similar approach in this work to growthe oriented nitrogen doped ZnO nanorods on FTO.

The nitrogen doped ZnO (N:ZnO) grown in the presence ofammonia solution also showed oriented growth along [001]direction, as conrmed by XRD (inset Fig. 4(a)). The normalizedabsorbance data for ZnO and N:ZnO nanorod arrays in Fig. 4(a)explain the visual appearance of ZnO (white) and N:ZnO(orange-yellow). Interestingly, the N:ZnO lm shows absorptionbelow 650 nm, remarkably spanning more than half of thevisible spectrum. The N2p states introduced by nitrogen dopingand presence of other defects such as oxygen vacancies and zincinterstitials can explain the broadband absorption of N:ZnO.40

Nitrogen incorporation within ZnO lattice and other associateddefect complexes are further conrmed by Raman spectroscopywhich offers useful insights into the nature of defects andlattice disorder. Fig. 4(b) shows additional Raman modes at 279c and 511 cm�1 in the case of N:ZnO which point towardssuccessful incorporation of nitrogen within the ZnO latticealong with formation of a defect complex between the nitrogen

ys, (b) Raman comparison of ZnO and N:ZnO clearly pointing out thef ZnO and N:ZnO films, (d) SEM images of N:ZnO nanorods and cross-

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dopant and zinc interstitials.40�42 This concurrent incorporationof dopant and defects in ZnO results in broadband absorptionof N:ZnO. The Raman spectra of ZnO and N:ZnO are discussedin more details in the ESI-2.†

Fig. 4(c) shows the photoluminescence (PL) spectrum ofZnO and N:ZnO when excited at 350 nm (interband excitation).The ZnO case shows the near band edge emission at 380 nmwith deep-level green luminescence which is usually assignedto defects such as oxygen vacancies or zinc vacancies.Interestingly, the PL of N:ZnO shows suppressed band edgeluminescence and the defect-luminescence is unusually shif-ted to the orange region.43 The suppressed intensity ofN:ZnO band edge luminescence can be attributed to thepresence of nitrogen dopant levels near the valance band ofZnO which hinder the band to band transition and increasethe intensity of the conduction band to defect level transition.The orange luminescence in N:ZnO can be assigned to thetransition from the conduction band of N:ZnO to Oi or fromZi to Oi

43 whose presence is also suggested by Ramanspectroscopy.

Fig. 5 (a) J–V curves of ZnO–SPD and N:ZnO–SPD under white light Lcurve near zero bias, (b) dark and white light (18 mW cm�2) J–V curve ofof N:ZnO–SPD under 18 mW cm�2 white light illumination under altephotocurrent action under visible light illumination.

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Though various defects such as oxygen vacancies, zincvacancies44,45 etc. can lead to defect states within the energy gapof ZnO, it is important to identify the defects which can lead tobroadband absorption of N:ZnO. First principle electronicstructure calculations by Feng et al.40 on nitrogen doped ZnOhave shown that nitrogen substitution for oxygen (NO) alonecannot explain the reduction in band gap of ZnO. Their calcu-lations show that the defect complex between nitrogensubstituting oxygen (NO), interstitial zinc (Zni) and oxygenvacancy (VO) gives rise to a defect band near the valance bandwhich is formed by strongly coupled p-orbitals of anions (O andN) and d-states of Zni and host Zn atoms of the ZnO lattice.Importantly the defect band formed by 4NO–VO–Zni complexcan touch and resonate with the valance band maxima of ZnO,thereby raising the valance band edge which results in areduced band gap of ZnO. The presence of Raman mode at 279cm�1 in N:ZnO in Fig. 4(b) is assigned to the stretching vibra-tion of the defect complex between Zi and NO.42 The presence ofZi is further supported by the orange luminescence in N:ZnOwhich is assigned to the transition from Zi to oxygen

ED illumination of 18 mW cm�2, inset shows a magnified view of J–VN:ZnO–SPD plotted on the absolute semilog scale, (c) photo-responsernate on and off cycles, (d) energy diagram of N:ZnO–SPD showing

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Fig. 6 (a) Photo-response of NZnO–SPD under 13 mW cm�2 blue(460 nm), green (530 nm) and red (625 nm) illumination under alter-nate on and off cycles, (b) responsivity comparison of ZnO–SPD andN:ZnO–SPD.

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interstitials.43 Therefore both Raman and photoluminescencedata of N:ZnO point towards the presence of 4NO–VO–Zni

complex which results in broadband absorption of N:ZnO up to650 nm in the visible, thereby making it a good candidate forvisible light photo-detection.

Substitutional doping of nitrogen in ZnO lattice has longbeen predicted to provide p type conductivity to ZnO. But thesubject has remained controversial since no stable p-typenitrogen doped ZnO could be obtained or reproduced. Recentreport on nitrogen doped ZnO nano-crystals46 also points outthat air annealing at temperatures greater than 400 �C leads tooxygen vacancies and hence Zn-rich conditions, which gives n-type conductivity in ZnO despite nitrogen doping. Our N:ZnOsamples were heated at 450 �C post-synthesis which explainsthe n-type behaviour in our N:ZnO nanorod arrays. The n-typebehaviour in N:ZnO nanorods is also conrmed by the posi-tive slope of electrochemical Mott–Schottky plot of N:ZnO (nowadded as ESI-4†). n-Type behaviour of N:ZnO therefore pointstowards the possibility of formation of rectifying junction withp-type Spiro-MeOTAD.

Fig. 4(d) shows the SEM image of oriented N:ZnO nanorodarrays grown on FTO. We fabricated an organic–inorganichybrid photodiode for visible light detection using theseoriented N:ZnO nanorods and Spiro-MeOTAD with the samearchitecture as was used for the undoped ZnO–SpiroMeOTADphotodiode case discussed earlier. The inset to Fig. 4(d) showsthe 45� tilted view of N:ZnO–SpiroMeOTAD (N:ZnO–SPD)photodiode exhibiting good percolation of Spiro-MeOTADwithin N:ZnO array.

The opto-electronic characterization was performed underillumination of white light LEDs with intensity of 18 mW cm�2.Fig. 5(a) shows the comparative J–V curves of ZnO and N:ZnObased photodiodes under dark and white light illumination. Arectifying behaviour is observed in the case on N:ZnO–SPD witha turn-on voltage of 0.2 V which is similar to ZnO–SPD. n-Typebehaviour of N:ZnO therefore suggests that the Fermi energy ofN:ZnO should be near the conduction band of ZnO. This is alsoin accordance with the previous experimental results by Perkinset al.47 who carried out ultraviolet photo-electron spectroscopy(UPS) on nitrogen doped ZnO thin lms and observed that theFermi energy lies very close to the conduction band for nitrogendoped ZnO explaining its n-type behaviour. Thus, as empha-sized by Perkins et al., although nitrogen related defectcomplexes introduce a defect band near the valance band, theydo not cause any appreciable change in the Fermi energy ofZnO. A magnied view of the J–V curve near zero bias (inset toFig. 6(a)) clearly shows a small photovoltaic effect (Voc � 0.1 V;Jsc � 80 mA cm�2) for the case of N:ZnO–SPD, whereas no clearphotoresponse was observed in the case of ZnO–SPD. Thisclearly points towards applicability of N:ZnO–SPD as an effec-tive visible-light photodetector. Fig. 5(b) shows the dark andunder-illumination curves of N:ZnO–SPD plotted on the semi-log scale. The dark J–V curve shows a high rectication ratio of103 at �0.5 V. An order of magnitude increase in the rectica-tion ratio of N:ZnO–SPD in comparison with ZnO–SPD can beattributed to smaller reverse leakage current due to a denselayer of N:ZnO near FTO (evident in the inset to Fig. 4(d)) which

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prevents direct electrical contact between Spiro-MeOTAD andFTO. Therefore, such a blocking layer reduces recombination atthis interface, which also results in 300% improvement in Voc(0.1 V) of N:ZnO–SPD as compared to ZnO–SPD (0.026 V).

It can also be seen from Fig. 5(b) that the N:ZnO–SPD showsmaximum sensitivity (Iphoto/Idark) of 103 near zero bias, sup-porting its self-powered mode of operation. This is furtherveried by measuring the photocurrent of N:ZnO–SPD at zeroapplied bias under a pulsating white light source at an intervalof 10 s. The photocurrent density vs. time graph plotted on thesemilog scale (Fig. 5(c)) also shows high photosensitivity of 103

with a photocurrent density of 80 mA cm�2 under 18 mW cm�2

of white LED illumination. This visible light response of N:ZnOcan be explained on the basis of energy band diagram shown inFig. 5(d). Nitrogen doping in ZnO introduces a band due to N2plevels near the valance band of ZnO, which results in broadbandabsorption of N:ZnO. Importantly the absorption onset inN:ZnO starts from 650 nm which corresponds to a position 1.9eV below the conduction band whereas the Spiro-MeOTADHOMO lies 1 eV below the conduction band of ZnO. There-fore, the Spiro-MeOTAD HOMO lies more than 1 eV higher in

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Fig. 8 Photo-response of N:ZnO–SPD under ambient white light ofthe fluorescent tube.

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energy than the defect levels introduced by nitrogen doping inZnO. Thus, light with wavelength in the visible region (400 nm <l < 650 nm) can excite electrons from defect levels to theconduction band of ZnO and the photogenerated holes in defectlevels can be further transferred to Spiro-MeOTAD HOMObecause of favorable energy alignment. The visible light photo-response shown by N:ZnO–SPD proves the participation ofdefect levels in photocurrent generation.

In order to further investigate the visible photoresponse ofN:ZnO–SPD we measured the photo-response under blue (455nm), green (530 nm) and red (625 nm) LED illuminations.

From Fig. 6(a) it can be seen that the N:ZnO–SPD exhibitshigh photocurrent densities of 82, 50 and 10 mA cm�2 underblue, green and red illumination (intensity 13 mW cm�2),respectively. A reasonably high photoresponse even under redillumination agrees well with the broadband absorption spec-trum of N:ZnO nanorod arrays. The spectral response of N:ZnO–SPD over the wavelength range of 350 to 800 nm was thenplotted in terms of responsivity which is calculated using therelation R(l) ¼ EQE/hn. Fig. 6(b) shows the responsivity ofN:ZnO–SPD in comparison with ZnO–SPD. It can be clearly seenthat ZnO–SPD is active only in the UV region with UV to visiblerejection ratio of 102 whereas N:ZnO–SPD clearly shows a peakresponsivity of 6.5 mA W�1 at 470 nm, which extends deep intothe visible region up to 650 nm. Importantly, the N:ZnO–SPDshows less responsivity under UV illumination even thoughboth N:ZnO and Spiro-MeOTAD can absorb in the UV region.This can be understood by considering the device structure inwhich light (UV/Vis) enters from the FTO side. Firstly, strongabsorption of FTO between 350 and 370 nm leads to absorptionof the majority of photons of these wavelengths near thisinterface. Secondly, the presence of ammonia in the growthsolution of N:ZnO nanorod arrays leads to the formation of adense layer of N:ZnO NRs near FTO which can be as thick as 1–2mm. Such dense layer of N:ZnO near FTO allows less amount ofSpiro-MeOTAD to percolate through it thereby avoiding directcontact between Spiro-MeOTAD and FTO which results in the

Fig. 7 Photo-response of N:ZnO–SPD under light pulses of 40 Hz.Inset shows the rise time and decay time of N:ZnO–SPD.

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high rectication ratio. But the inadequate amount of holetransport material in this layer leads to higher recombination atthis interface where most of the UV light is absorbed. Thus, theresponsivity of N:ZnO–SPD in the spectral region from 370 to400 nm is lower as compared to undoped ZnO–SPD. Light withwavelengths longer than 400 nm requires higher absorptionlength, therefore these wavelengths can penetrate deep withinthe lm structure without being completely absorbed at thedense layer. Therefore, for l > 400 nm the photo-action takesplace in the region with adequate hole transport materialresulting in efficient electron hole separation and higher

Fig. 9 (a) Current of N:ZnO–SPD under dark, (b) photocurrentgenerated by N:ZnO–SPD under ambient room light equipped withfluorescent tubes giving white light of intensity �120 Lux (28 mWcm�2). A clear photocurrent response of two orders of magnitude isobserved at zero applied bias supporting its self-powered mode ofapplication for ambient light.

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Table 1 Comparative table of various parameters of hybrid visible photo-detectors

Photo-detector(ref.) Bias Wavelength and intensity

Dark current(density)

Photo-current(density) Iphoto/Idark ratio Rise time Decay time

N:ZnO–SpiroMeOTAD(this report)

0 V White light 18 mW cm�2 6 nA(0.05 mA cm�2)

9.6 mA(80 mA cm�2)

1600 4 ms 10 ms

CuInSe2:P3HT14 0.4 V White light 7.6 mW cm�2 0.15 pA(0.3 � 10�3 mA cm�2)

17 pA(0.38 mA cm�2)

113 — —

Cu2ZnSnS4:P3HT17 0.4 V White light 7.6 mW cm�2 0.012 pA(0.3 � 10�3 mA cm�2)

2.23 pA(0.05 mA cm�2)

150 — —

Ge:P3HT16 10 V White light 8 mW cm�2 0.05 pA 7.8 pA 156 — —AgInS2:P3HT11 2 V UV 365 nm 2 mW cm�2 4 nA 60 nA 15 — —CdS:polyaniline28 20 V 420 nm 5.2 mW cm�2 0.2 mA 1.2 mA 6 — —CdSe:P3HT(exible substrate)15

3 V White light 140 mW cm�2 30 pA 16 nA 500 10 ms 10 ms

InP:P3HT13 0 V AM 1.5 Sun light 100 mW cm�2 — 3.3 mA cm�2 — — —ZnO–rGO27 0 V White light 80 mW cm�2 0.2 nA 90 nA 430 200 ms 200 ms

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responsivity in the visible region. Broad band responsivity ofN:ZnO–SPD makes it an excellent candidate for white lightsensing.

Optoelectronic applications of visible photodetectors alsoneed fast response times. The response time of N:ZnO–SPD wasmeasured in the same manner as the ZnO–SPD using an oscil-loscope equipped with a pulsating source of light. Importantly,N:ZnO–SPD also shows a fast photoresponse in the millisecondtime region as shown in Fig. 7. More specically, we observed arise time of 4 ms and a decay time of 10 ms for the case ofN:ZnO–SPD. These values are slightly higher than those for theundoped ZnO–SPD which can be attributed to the trappingevents of carriers in the trap states introduced by doping whichcan lead to slower diffusion of carriers leading to higher riseand decay times. Also due to higher length of N:ZnO rods,carriers have to travel a larger distance to reach the respectiveelectrodes without getting recombined, thereby giving higherrise and decay times as compared to ZnO–SPD.

High responsivity of N:ZnO–SPD in the visible region makesit possible to use it for sensing very low intensity white light. Inorder to verify this we measured the photo-response of N:ZnO–SPD in a room equipped with uorescent tube lights emittingwhite light with intensity 28 mW cm�2. As shown in Fig. 8and Fig. 9 even under this low intensity white light N:ZnO–SPDshowed a photocurrent of 120 nA cm�2 with a high sensitivityof 102.

We have given a comparative chart of recently reportedorganic–inorganic hybrid visible light self-powered photode-tectors in Table 1. The N:ZnO–SPD also shows Iphoto/Idark ratio of1600 and faster photoresponse (srise �4 ms and sfall �10 ms)which represents at least one order magnitude improvement ascompared to other hybrid visible photodetectors. The overallenhancement in the photocurrent and sensitivity is dramaticconsidering that N:ZnO–SPD involves nitrogen-doped ZnO asthe only visible light sensing entity, whereas other (Table 1)hybrid photodetectors involve both organic and inorganiccomponents which respond in the visible region. Thisenhancement can be attributed to favorable energy alignmentbetween N:ZnO/Spiro-MeOTAD resulting in efficient carrier

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collection and transport by Spiro-MeOTAD. Most importantly,the performance of N:ZnO–SPD is better or comparable even tothe conventional visible light sensors such as CdS and CdSe,thereby eliminating the need for using these toxic elements forvisible light sensing.

According to the Materials Safety Data Sheet (MSDS) fromSigma Aldrich, Spiro-TAD is considered to be a substance withvery low levels of hazards as compared to other p-type semi-conductors such as CuSCN. Unfortunately, there are no detailedstudies on the toxicity of Spiro-MeOTAD. Therefore, in order toevaluate and compare the toxicity of Spiro-MeOTAD Vis a Viscadmium based p-type semiconductors such as CdTe we carriedout a comparative cyto-toxicity study on Spiro-MeoTAD andCdTe quantum dots with human hepatoma cell line (ESI-3†).The Spiro-MeOTAD is found to be 60–70% less toxic ascompared to CdTe. Although use of completely non-toxic p-typeorganic semiconductor will be ideal, we believe that ourdemonstration of ZnO–SpiroMeOTAD photodiode with excel-lent properties (fast response, broad wavelength range etc.) is animportant step towards environment friendly visible photo-detector designs.

Conclusion

In summary, we report a facile solution based fabrication of aself-powered organic–inorganic hybrid photodetector usingnanorods of environmentally-friendly and earth-abundantmetal oxide semiconductor n-type ZnO and an organic p-typesemiconductor Spiro-MeOTAD. Initially, we fabricated avisible blind UV photodiode using undoped ZnO nanorodarrays impregnated with Spiro-MeOTAD. Such UV photode-tector shows a UV to visible rejection ratio of 102 with a highphotocurrent density of 50 mA cm�2 under a low intensity (3 mWcm�2) UV illumination. A rapid photoresponse with rise time of200 ms and decay time 950 ms is observed in the case of undopedZnO–SpiroMeOTAD UV photodiode.

Importantly, we further show that a slight modication inthe reaction conditions (doping nitrogen in ZnO nanorods) cancovert the UV operative ZnO–Spiro-MeOTAD junction to be

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highly responsive even in the visible region (400–650 nm). Theincorporation of nitrogen within ZnO causing broadbandabsorption (up to 650 nm) of N:ZnO was conrmed usingRaman and UV-Vis absorption spectroscopy. The N:ZnO–Spi-roMeOTAD photodiode shows a rapid and strong photo-response under blue (460 nm), green (530 nm) and red (635 nm)LED illuminations, thereby making it capable of sensing whitelight. Interestingly, this self-powered visible photodetectorshows a high photocurrent density of 120 nA cm�2 under lowintensity (28 mW cm�2) white light. The photosensing parame-ters of N:ZnO–SpiroMeOTAD photodiode are much superior topreviously reported organic–inorganic hybrid photo-detectors.High visible light sensitivity of N:ZnO makes it an attractivealternative over toxic element (Cd, Pb) based current visiblelight photodetectors.

Acknowledgements

O.G. would like to thank the Council of Scientic and IndustrialResearch (CSIR) for Senior Research Fellowship (SRF). U.S .andT.K. would like to thank the Director, Indian Institute of ScienceEducation and Research (IISER), Pune for permission to doresearch internship at the National Chemical Laboratoty (CSIR-NCL), Pune. The authors also thank Pooja Singh for carrying outcytotoxicity studies on Spiro-MeOTAD.

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