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Chemical bath deposited PbS thin films on ZnO nanowires forphotovoltaic applications
Ronen Gertman a,c, Anna Osherov b,c, Yuval Golan b,c, Iris Visoly-Fisher c,d,⁎a Dept of Chemistry, Ben Gurion University of the Negev, Be'er Sheva 84105, Israelb Dept of Materials Engineering, Ben Gurion University of the Negev, Be'er Sheva 84105, Israelc Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion University of the Negev, Be'er Sheva 84105, Israeld Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research,Ben Gurion University of the Negev, Sede Boqer Campus 84990, Israel
Article history:Received 5 June 2013Received in revised form 21 October 2013Accepted 22 October 2013Available online 5 November 2013
Keywords:PbSChemical bath depositionZnONanowiresSemiconductor sensitized solar cellsInfrared absorptionPhotovoltaicsBulk-like
Photovoltaic devices usually exploitmid-range band-gap semiconductorswhich absorb in the visible range of thesolar spectrum. However, much energy is lost in the IR and near-IR range. We combined the advantages of smallband-gap, bulk-like PbS deposited by facile, cheap and direct chemical bath deposition (CBD),with the good elec-tronic properties of ZnO and the large surface area of nanowires, towards low cost photovoltaic devices utilizingIR and near-IR light. Surprisingly, CBD of PbS on ZnO, and particularly on ZnO nanowires, was not studied hith-erto. Therefore, themechanism of PbS growth by chemical bath deposition on ZnO nanowires was studied in de-tails. A visible proof is shown for a growthmechanism starting from amorphous Pb(OH)2 layer, that evolved intothe ‘ion-by-ion’ growth mechanism. The growth mechanism and the resulting morphology at low temperatureswere controlled by the thiourea concentration. The grain size affected the magnitude of the band-gap and wascontrolled by the deposition temperatures. Deposition above 40 °C resulted in bulk-like PbS with an opticalband-gap of 0.4 eV. Methods were demonstrated for achieving complete PbS coverage of the complex ZnONW architecture, a crucial requirement in optoelectronic devices to prevent shorts. Measurements of photocur-rents under white and near-IR (784 nm) illumination showed that despite a 200 meV barrier for electron trans-fer at the PbS/ZnO interface, extraction of photo-electrons fromPbS to the ZnOwas feasible. The ability to harvestelectrons from a narrow band-gap semiconductor deposited on a large surface-area electrode can advance thefield towards high efficiency, low cost IR and near-IR sensors and third generation solar cells.
Narrow band-gap semiconductors are of interest for photovoltaic(PV) solar energy conversion as they can absorb the “IR tail” of thesolar spectrum, which is not absorbed by commonly used PV materials.The use of such absorbers in semiconductor sensitized solar cells allowsthe integration of low cost device configurations and broad spectral re-sponse, which may also be utilized in IR and near-IR (NIR) photodetec-tors. PbS is an attractivematerial in that sense, as it has a high extinctioncoefficient, band-gap tunability via quantum size effects, and multipleexciton generation which has been demonstrated both for quantumdots and bulk-like PbS thin films [1–6]. Bulk PbS has a narrow band-gap of 0.37–0.41 eV (300 K) [7,8] and an exciton Bohr radius of18–20 nm [9,10]. PbS has been employed in semiconductor sensitizedsolar cells in the form of nanoparticles (or quantum dots), rather thanas bulk-like, to afford a type-II band alignment between PbS and metaloxides such as TiO2 [11–16]. This configuration, however, limits the ab-sorption to wavelengths in the NIR and shorter. Absorption of longer
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wavelengths in the IR range requires the use of bulk-like films with asmaller band-gap. Towards that end, alternating PbS/CdS films havebeen deposited by successive ionic layer adsorption and reaction onmeso-porous SnO2 electrodes which have a lower conduction bandminimum [17].
Semiconductor thin films can be deposited using a variety of tech-niques; however, chemical bath deposition (CBD) is probably the sim-plest and most inexpensive one [18]. In CBD, film growth occurs in aone-bath solution and the morphology, size and optical properties ofthe deposited film can easily be controlled by varying growth parame-ters such as temperature, pH, reagent concentrations, and depositiontime [19–23]. CBD implies direct, surfactant-free deposition of the ab-sorber film on the electrode surface, thus eliminating electrode contam-ination and binding ligands. The intimate contact between the electrodesurface and the absorber film is expected to improve photo-inducedcharge transfer. PbS was deposited by CBD on a variety of substrates[20,21,23–28]. PbS was also deposited on metal oxide electrodes viamore complex and expensivemethods such as successive ionic layer ad-sorption and reaction [17,29–33] and chemical vapor deposition [34].Surprisingly, CBD of PbS on ZnO, and particularly on nanowire struc-tured ZnO electrodes, was not studied hitherto. Understanding the
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growth mechanisms and the effect of growth parameters can result inbetter control of the interface quality between the substrate and the de-posited PbS, and superior PbS coverage of the structured ZnOelectrodes.
ZnO nanowires (NWs) [35] are one dimensional n-type semicon-ductors with a large energy gap of 3.37 eV, relatively large surfacearea, high strength, and transparency to visible and IR light. They havea good electron mobility of 2.2 cm2/V·s [36], better than that of TiO2
or SnO2 previously used in PbS-sensitized solar cells. Such propertiesmake ZnONWs a promising candidate for electrodes in light harvestingdevices such as photodetectors [37], solar cells [38] and other optoelec-tronic devices.
In this paper we combine the advantages of IR absorption of bulk-like PbS, good electronic properties of ZnONWs, and facile, direct chem-ical bath deposition, towards devices utilized for IR and NIR light har-vesting. We demonstrate control over the growth mechanism of PbSdeposited on ZnO NWs by CBD and thus control the PbS grain size andfilm morphology. Understanding the growth parameters required inorder to achieve complete filling of PbS in ZnO NW voids, as shownhere, can pave the way towards utilizing bulk-like PbS in IR detectorsand third generation solar cells.
2. Experimental
150 nm thick indium tin oxide (ITO, 15–25 Ω/sq, Delta Technologies)films, deposited on SiO2 passivated polished float glass substrates, wereused. ZnO NW electrodes were fabricated using a previously reportedtwo-step CBD process [35]. ZnO seeds were formed on the substratesby spin casting 5 mMzinc acetate dehydrate (Alfa Aesar, 97%) in ethanol(Bio-Lab, AR) at 4000 rpm three times. The samples were then heatedbriefly to 400 °C in air and the process was repeated once more. Thesamples were then placed in the reaction vessel facing downward at45° angle using a Teflon stage, to prevent sediment accumulation onthe surface, and the NWswere grown in a solution of 25 mMzinc nitratehydrate (Acros, 98%) and 25 mMmethenamine (Alfa Aesar, 99%) in dis-tilled 18 MΩ water (Millipore Direct Q3) at 90 °C for 3 h. A followingheat treatment in air at 450 °C for 20 min was used to remove organicresidues.
PbS thin filmswere deposited on ZnONWs using a three componentsolution: Pb(NO3)2 as the lead source, CS(NH2)2 as the sulfur source,and NaOH to adjust the pH. The solutions typically contained 8.9 mMPb(NO3)2 (Aldrich, analytical 99.99+%), 135 mM of NaOH (Gadot,AR) and 51 mM of CS(NH2)2 (Aldrich, ACS 99.0%) in distilled 18 MΩwater. N2 gas was bubbled into the solution for 1 h to reduce the levelsof dissolved O2 and CO2 [20]. The samples were placed in the reactionvessel facing downward at 60° angle on a Teflon stage. The concentra-tion of NaOH was kept relatively low (pH ~ 12) compared to otherPbS CBD procedures as larger pH resulted in etching of the ZnO NWs.The Pb(NO3)2 concentration was relatively high (resulting in whitelead hydroxide particles in the solution) in order to increase the PbSgrain size [23].
The morphological characterization included studies using a JEOL7400F field emission gun high resolution scanning electron microscopy(HRSEM), with acceleration voltages ranging from 2 to 4 kV and beamcurrent of 10 μA. The samples were observed without further coatingand secondary electron contrast was used to obtain the topographyimages. High resolution transmission electronmicroscopy (HRTEM) im-aging was carried out using a JEOL JEM-2100F analytical TEM operatingat 200 keV. Energy-filtered TEM (EFTEM) experiments were performedusing a Gatan image filter. The sulfur L-edge (164.8 eV), oxygen K-edge(532 eV), zinc L-edge (1020 eV) and leadM-edge (2484 eV) were usedfor elemental mapping using the three-windowmethod. The phase andcrystallographic structure were determined by X-ray diffraction (XRD)using a PANalytical Empyrean powder diffractometer equipped withPIXcel linear detector and amonochromator on the diffracted beam, op-erating in the 2θ/θ geometry using Cu Kα radiation (λ = 1.5405 Å) at40 kV and 30 mA. Scans were running in a 2θ range of 20–65° in
0.039° steps for 8 min. The Scherrer formula was used to determinethe grain size from XRD:
bDN ¼ Kλ=β cos θ ð1Þ
where bDN is the average grain size, β is the full width at halfmaximum(FWHM) of the peak corrected for instrumental broadening (inradians), λ is the X-ray wavelength and θ is the diffraction angle. Dueto the isotropic shape of the PbS grains, the Scherrer shape parameter,K, used was K = 1.
Optical transmission and reflection data were measured using aBruker Vertex V70 FTIR, equipped with a deuterated triglycine sulfatedetector. The measurement range was 600–3500 cm−1. The correctedtransmission was calculated by:
Tr ¼Tm
1−Rmð2Þ
where Tm is the measured transmission and Rm is the measured reflec-tion. The Tr values were converted to absorption coefficient α valuesusing the Beer–Lambert equation [39].
Photoelectrochemical measurements were performed using aVersaSTAT 4 potentiostat (PAR) at room temperature in a two-electrodeelectrochemical glass cell. A 1 × 2 cm2 PbS-coated ZnO NW electrodewas employed as the working electrode, the potential of the workingelectrode was set to 0.0 V and the reference electrode was shorted witha 1 mm Pt wire counter electrode. The electrodes were placed 1.5 cmapart. Polysulfide electrolyte was prepared from 1 M Na2S (Aldrich60%), 0.1 M sulfur powder (Strem, 99%) and 0.1 M NaOH (Gadot, AR) in18 MΩ water. Illumination was carried out using a 150 W xenon arclamp light (ASB-XE-175, Spectral products) and a 784 nm laser (Toptica)at 35 mW intensity. The PbS in the samples used for photocurrent mea-surements was deposited at 50 °C for 24 h in a single deposition solutionas detailed above, with partial PbS coverage on the ZnO NWs.
3. Results and discussion
3.1. ZnO nanowire electrode
HRSEM images showed good coverage of a highly uniform, denselypacked array of ZnO NWs on the ITO substrate (Fig. 1a). A typicalsynthesis yielded wires perpendicular to the surface with diametersranging between 20 and 100 nm, lengths of 2–3 μm, and spacing be-tween the ZnO NWs ranging from 150 to 400 nm (Fig. 1b). The X-raydiffractogram of the ZnO NWs showed that most of them grow withthe b0001N axis parallel to the long axis of the wire, and are verticallyaligned, with some angular distribution, with respect to the substrate(Fig. 1c).
3.2. PbS growth mechanism
It was previously suggested that the formation of PbS from thioureaoccurs via decomposition involving Pb–CS(NH2)2 complexes [18]. TheNaOH concentration is known to affect the deposition rate via lead com-plexation as well as decomposition of thiourea [18,26,28]. Low concen-tration of NaOH, as used here, typically slows the deposition rate, whilethe increased CS(NH2)2 concentration is expected to accelerate thegrowth. The two main growth mechanisms in CBD are the ‘cluster’growth mechanism, where colloidal particles form in the solution andthen attach to the substrate, and the ‘ion-by-ion’ mechanism. In thelatter anions and cations adsorb onto the nuclei which grow by incorpo-ration of ions, and the resulting crystals are typically larger than thosegrown in the ‘cluster’ mechanism [19]. PbS grown at 50 °C is knownto form large bulk crystals via the ‘ion-by-ion’ mechanism [23], whileat low temperatures small grain sizes are expected [39].
Fig. 1. ZnO nanowires on ITO surface: (a) SEM image, top view, magnification of ×10,000. Scale bar is 1 μm. (b) SEM image, cross section view, magnification of ×40,000. Scale bar is0.1 μm. (c) XRD patterns of ITO, ZnO NWs and PbS deposited at 5 °C for 24 h.
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To investigate the growth mechanism of PbS on ZnO NWs at 50 °Cwe studied films deposited at short deposition times of 1, 1.2, and2 min (Figs. 2, 3). Brightfield TEMand EFTEM images after 1 minute de-position (Fig. 2a, b) showed the formation of PbS crystalline nuclei em-bedded in an amorphous layer on the ZnONWsurface. EFTEM indicatedthe presence of lead and oxygen in the amorphous layer. We postulatethe formation of an amorphous Pb(OH)2 film in an incubation step, fa-cilitated by the relatively high Pb precursor concentration. PbS particles
Fig. 2. PbS deposited on ZnO nanowires: (a) Brightfield TEM image after 1 minute growthat 50 °C. The green arrow points at the PbS nucleus, the black arrow shows the amorphouslayer; (b) EFTEM image of the area in (a): green— Zn, blue— O, red— Pb and yellow— S.The NW color is a superposition of signals from the ZnO NW and the Pb(OH)2 film.(c) Bright field TEM image after 1.2 minute growth at 50 °C; (d) EFTEM of the area in(c) (similar color coding as in b). Note that the ZnO NW color now reflects signals fromthe ZnO only. (e) Bright field TEM image after 1.2 minute growth at 50 °C, larger magni-fication. The white stripes mark the different grains' orientation.
are subsequently formed by anionic exchange or by decomposition of a(OH)2Pb·SC(NH2)2 complex, as was previously suggested for PbS andCdS growth from alkaline solutions [18,19,28,40]. This is a direct evi-dence for the nucleation of PbS from an amorphous Pb(OH)2film. Brightfield TEM and EFTEM of the sample deposited for 1.2 min showed thedisappearance of the amorphous layer and the formation of PbS parti-cles on the ZnO NWs (Fig. 2c, d). A TEM image at larger magnificationtaken after 1.2 min (Fig. 2e) showed several grainswithin a PbS particle,all with similar crystal orientations. A bright field TEM image takenafter 2 min (Fig. 3a) showed small PbS cubes, implying an ‘ion-by-ion’growth mechanism following the initial nucleation. The relativelyslow nucleation and fast growth, as expected at 50 °C, allowed orderedgrowth of large crystals on the well-separated nuclei, taking into ac-count the closely aligned crystals observed at the initial growth stage(Fig. 2e).
The PbS crystallites were attached mostly on the non-polar a-faceof the ZnO NWs (the longer prismatic crystal faces of the wurtzitestructure), and the particles grew from the a-face into the gaps betweenthe ZnO NWs. Few round particles, grown by solution ‘cluster’ growth,were attached to the polar ZnO c-face (top of the NW), but no ‘ion-by-ion’ growthwas observed on the c-face (Figs. 3, 4). The crystallographicrelationship between the ZnO NW surface and the deposited PbS cubeswas studied by selected area electron diffraction (SAED). While epitax-ial PbS growth on wurtzite CdS was previously demonstrated with(111)PbS/(00.2)CdS and (224)PbS/(11.0)CdS relationships [41,42], wedid not observe growth on the ZnO c-face. Fig. 3b shows a PbS 112
� �/
ZnO 12:0� �
orientation; yet, other orientations could also be found,such as PbS 11 6
� �/ZnO 12:0
� �(Fig. 3c), and the XRD diffractogram
of PbS deposited on ZnO NWs showed a powder pattern due to PbSgrowth at random orientations (Fig. 1c). Hence, we conclude thatunder the experimental conditions used, we cannot identify a uniquewell-defined orientation relationship between the PbS and the ZnONWs.
The effect of CS(NH2)2 concentrationwas studied at 5 °C since at lowtemperatures the growthmechanismwasmore sensitive to the solutioncomposition [26]. Both low and high concentrations of CS(NH2)2 are ex-pected to result in small particles [23]. However, the reason for this andthe effect on the growthmechanismare still unclear. Fig. 4 shows the ef-fect of different concentrations of CS(NH2)2 on the growth mechanismof PbS on ZnO NWs. Deposition from a solution of 25 mM CS(NH2)2yielded both round and faceted particles (Fig. 4a) with sizes rangingfrom 100 to 200 nm after 3 h growth. The low temperature implied alarge number of small nuclei, and the low CS(NH2)2 concentration im-plied slow growth. The slow growth enabled the growth of facetedcubes; however the dense, random nucleationmay have partially inter-fered with the lateral growth of ordered crystals. At later stages, whenthe slow growth dominated over nucleation due to CS(NH2)2 depletion,a larger percentage of the ordered cubic particles was observed, as ex-pected (not shown).
Fig. 3. (a) Bright field TEM image of PbS deposition on a ZnO nanowire at 50 °C after 2 minute growth. (b, c) SAED of PbS and ZnO in areas indicated in image (a).
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Doubling the concentration of CS(NH2)2 to 51 mM yielded largerfaceted cubes after a similar growth time (3 h) with sizes rangingfrom 100 to 300 nm (Fig. 4b). Such faceted shapes are typical of growthvia the ‘ion-by-ion’ mechanism. A faster transition from disordered toordered crystal growth was induced by the faster growth compared tothe 25 mMCS(NH2)2 case. Deposition of PbS by the ‘ion-by-ion’ growthmechanism is expected to improve the contact at the interface betweenthe PbS and the ZnO NWs and to result in a smoother, denser film withless voids.
Further increasing the concentration of CS(NH2)2 to 97 mM yieldedmostly round particles with sizes ranging from 50 to 200 nm (Fig. 4c),resulting from fast nucleation and growth allowing continuous and ho-mogeneous nucleation. The latter can be responsible for the predomi-nance of the ‘cluster’ growth mechanism in solution, typical of CBD atlow temperatures due to a smaller critical nucleus size [19], whichyielded mostly round particles.
3.3. Controlling grain sizes
At low temperatures small PbS grain sizes are expected [23], due tostabilization of smaller nuclei resulting in a large number of smaller par-ticles [19], while at high temperatures large particles or evenmonocrys-talline films are typically formed [20]. However, the PbS grown hereboth at 5 °C and at 50 °C exhibited faceted cubes of 100 to 300 nm, asobserved by HRSEM images (Figs. 4b, 5a). Therefore, XRD experimentswere performed to estimate the coherence lengths and to evaluate thepossibility of a sub-grain microstructure.
Fitting the PbS (grown at 5 °C) XRD (220) peak to the Scherrermodel yielded a coherence length of 30 nm, suggesting the presenceof small grains within the larger cubes. An average size of 90 nm wasobtained for PbS deposited at 50 °C (Fig. 5c). In both cases, the averagegrain size found byXRDwas considerably smaller than that observed byHRSEM imaging.We suggest that the large cubes may consist of several
Fig. 4. Top viewHRSEM images of PbS deposited on ZnOnanowires for 3 h at 5 °C: (a) 25 mMCSof ×45,000. Scale bar is 0.3 μm. (c) 97 mM CS(NH2)2, magnification of ×90,000. Scale bar is 0.
smaller crystals, as a result of the incubation/nucleation mechanismin the Pb(OH)2 layer. However, deposition at higher temperatures(50 °C) resulted in better aligned and less dense nuclei (Fig. 2e) whichcan allow the formation of larger crystals by epitaxial growth. DensePbS nucleation at low temperatures (5 °C) limited the ‘ion-by-ion’ crys-tal growth to smaller sizes, as the growth was terminated when adja-cent crystals touch each other [19]. Attributing the XRD results tosmall PbS particles deposited on the uncovered area between the ZnONWs is unlikely, since the ZnO NWs are very dense and the free areais very small compared to the ZnO NW surface area. Therefore, we as-sume that the XRD signal originates almost entirely from the PbS depos-ited on the ZnO NWs.
PbS exciton Bohr radius is ~18 nm, hence PbS particles with diame-ters smaller than 40 nm can be considered quantum dots. The relation-ship between PbS thin film grain size and the PbS band-gap wasconfirmed by Osherov et al. [39]. They showed that the band-gap of PbScan be shifted from 0.41 eV (bulk PbS grown at 30 °C) to 0.48 eV (PbSgrown at 0 °C). To determine the threshold temperature needed toachieve bulk PbS particles on ZnO NWs, with grain sizes above the Bohrradius, XRD measurements of PbS deposited at 20, 35, 40 and 50 °C for24 h were conducted and grain sizes were determined (Fig. 5c). Growthof PbS on ZnO NWs at temperatures≥40 °C resulted in grain sizes above40 nm, hence bulk-like PbS. Optical transmission measurements werecarried out to estimate the band-gap of PbS deposited at 50 °C (Fig. 5d).Analysis of the absorption of a direct band-gap [43] yielded a band-gapof 0.4 eV, in close agreement with the previously published value of0.41 eV for the direct band-gap in bulk PbS thin films [7,8].
3.4. Controlling the surface coverage
The homogeneity of a CBD film is typically good even on irregularsurfaces since growth is often controlled by kinetics via surface reac-tions [19,44]. PbS void filling in complex architectures using CBD is a
(NH2)2,magnification of ×45,000. Scale bar is 0.3 μm. (b) 51 mMCS(NH2)2,magnification3 μm.
Fig. 5. (a) HRSEM cross section image of PbS deposited on ZnO nanowires for 3 h at 50 °C at magnification of ×20,000. Scale bar is 0.5 μm. (b) HRSEM cross section image of PbS depositedon ZnO nanowires for 3 h at 5 °C atmagnification of ×33,000. Scale bar is 0.5 μm. (c) PbS grain size, determined fromXRD, as a function of the CBD deposition temperature (deposition for24 h). (d) The absorption coefficient squared of PbS, deposited at 50 °C for 6 h, as a function of the illuminatingwavelength. The dashed line is an extrapolation of the linear section of thecurve to intercept with the x axis to determine the band-gap.
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challenge and is rarely reported in the literature (for two recent exam-ples on filling Si pores with CBD PbS nanoparticles for fabrication of hy-brid photonic crystals see refs. [45,46]). Achieving complete surfacecoverage and void filling can increase the absorption and prevent elec-trical shorts in a PV device. In order to achieve full deposition of PbSfilm and filling the spaces between the ZnO NWs the deposition timeat 5 °C was increased to 24 h. The ‘cluster’ mechanism dominated atlow temperatures due to a smaller critical nucleus size allowing homo-geneous nucleation. The increased number of nucleation sites also in-creased the surface coverage, albeit by small particles. Indeed, after24 h deposition the PbS completely filled the ZnO NW voids (Fig. 6a).Full coverage of the ZnO NWs was achieved in all CS(NH2)2 concentra-tions studied at 5 °C.
Similar experiments of 24 h deposition were conducted at highertemperatures (50 °C), however, the increased deposition time did notresult in a complete coverage of PbS onto the ZnO NWs. A possible ex-planation can be the increased growth rate at 50 °C [20], resulting inrapid depletion of the Pb2+ ions that attach to the glass vessel wallsrather than to the ZnO electrode [25]. Renewing the growth solutionby replacementwith a fresh solutionwas found to increase the coverage
Fig. 6. HRSEM images of complete PbS coverage on ZnO nanowires: (a) 5 °C, 24 h, cross sectionview, magnification of ×10,000. Scale bar is 1 μm. (c) Three step deposition at 50 °C, cross sec
of PbS on the ZnONWs at 50 °C. Complete coveragewas achievedwhenthe growth solution was replaced after 1 h and again after 4 h and thedeposition was resumed for a total deposition time of 6 h. Fig. 6b, cshows a complete coverage of PbS deposited on the ZnO NWs at 50 °Cusing this sequential deposition method.
3.5. Photocurrent measurements
Fig. 7a, b shows photo-induced short circuit currents of the ZnO/PbSdeposited at 50 °C electrode in a photo-electrochemical cell. The ZnONW/PbS electrode exhibited photocurrents ca. 6 times larger than inthe ZnO NW electrode control sample (without PbS) under whitexenon arc lamp light (Fig. 7a). Anodic and cathodic photocurrent spikesappeared only with the PbS samples upon light turn on and off, respec-tively, and were attributed to recombination processes of chargestrapped at the ZnO/PbS and/or the PbS/electrolyte interfaces [47].Fig. 7b shows the photo-response of the ZnO/PbS sample upon NIR(784 nm laser) illumination. No photo-response was observed withZnO NWs only under NIR laser illumination, indicating the PbS sensiti-zation to NIR illumination. The PbS samples showed a photocurrent
view, magnification of ×30,000. Scale bar is 1 μm. (b) Three step deposition at 50 °C, toption view, magnification of ×20,000. Scale bar is 1 μm.
Fig. 7. Short circuit photocurrents in ZnONW/PbS/PS electrolyte/Pt photoelectrochemical cells: (a) underwhite light illumination, (b) under 784 nm laser illumination (35 mW). The insetis a HRSEM cross section image of the sample used in these measurements. (c) Schematic architecture of the photoelectrochemical cell with PbS as the absorber material, ZnONWs as theelectron acceptor and PS electrolyte as the hole acceptor. ITO and Pt serve as front and back contacts, respectively. (d) Schematic energy band diagram according to published values: va-lence band maximum and conduction band minimum in ZnO [16] and PbS [37], PS redox level [16], work functions of ITO [21] and Pt [35].
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density of 3.824 ± 0.005 mA/cm2 and similar photocurrent spikes.Controlmeasurements showed that the contribution of direct photocur-rent injection to the ITO (rather than to the ZnO NWs) was smaller bymore than an order of magnitude, verifying the PbS sensitization ofthe ZnO NW electrode.
Photocurrent is generated when photons are absorbed by the PbSlayer and photo-excited electrons are injected into the conductionband of the ZnO NWs, while holes are collected by the electrolyte(Fig. 7c). The energy diagram presented in Fig. 7d shows an expected200 meV barrier for electron transfer between the conduction bandminima of ZnO and bulk PbS (type I junction). It is evident, from thephotocurrent measurements, that the bulk-like PbS film was responsi-ble for the photocurrent generation, and that charge transfer occurreddespite the energy barrier. Several arguments can be raised forexplaining this: Fermi level pinning may cause band bending which re-sults in depletion and accumulation areas at the surfaces of the PbS andthe ZnO NWs, respectively, aligning the band edges. Band bending canalso be attributed to uncontrolled surface modification of the ZnONWS during CBD PbS growth at relatively high pH. Another possibilityis that the ZnO energy band edges change during the heat treatmentof the ZnO NW electrode in air [48]. Charge accumulation at the inter-face can “push” the carriers across the depletion/accumulation layers.It is also possible that the photo-current resulted from collection ofhot electrons. Further work is required to elucidate the charge transfermechanism. Interfacial layers may be used for tailoring the ZnO/PbS in-terface towards more efficient transfer of photogenerated charges [17].
4. Conclusions
PbS was deposited on ZnO NWs using CBD. Deposition at high tem-peratures (50 °C) for short deposition times revealed that the growthmechanism started from an amorphous Pb(OH)2 layer and evolved
with time into the ‘ion-by-ion’ mechanism. At low temperatures(5 °C) the growthmechanism of PbS can be controlled by thiourea con-centrations, and PbS was deposited onto ZnO NWs predominantly viathe ‘cluster’ mechanism. Growth via the ‘ion-by-ion’ mechanism couldbe induced by decreasing thiourea concentration. Deposition at temper-atures higher than 40 °C yielded grain sizes above the Bohr exciton ra-dius, namely bulk-like PbS. PbS deposited at 50 °C exhibited an opticalband-gap of 0.4 eV, in accordance with published band-gap values forbulk PbS. Complete coverage of the ZnO NWs by PbS was achieved at5 °C by using long deposition time (24 h). At high temperatures(50 °C) complete deposition was achieved by applying three consecu-tive deposition stages, each using a fresh solution.
Photoelectrochemical measurements showed photocurrents underwhite light illumination as well as under NIR (784 nm laser) illumina-tion. The photocurrent was shown to result from PbS absorption andsensitization. These findings indicated that extraction of photo-electrons from PbS to ZnO was feasible despite a 200 meV barrier ex-pected from the energy band alignment. The ability to harness photo-excited electrons from a narrow band-gap semiconductor depositedon large surface-area 3-dimensional structures can pave the way to-wards high efficiency IR and NIR sensors or third generation solar cells.
Acknowledgments
The authors are grateful to Prof. Amir Sa'ar and Neta Arad-Vosk(Hebrew University Jerusalem) for the absorption measurements,Dr. Vladimir Ezersky (BGU) for the TEM measurements and Dr. LeilaZeiri (BGU) for the critical reading of themanuscript. Thisworkwaspar-tially supported by the Israel Science Foundation, Grant # 1298/07 (YG)and by the Focal Technological Area (FTA) programof the Israel NationalNanotechnology Initiative.
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