Air-processed depleted bulk heterojunction solar cells basedon PbS/CdS core–shell quantum dots and TiO2 nanorod arrays

Belete Atomsa Gonfa a, Haiguang Zhao a, Jiangtian Li b, Jingxia Qiu c, Menouer Saidani d,Shanqing Zhang c, Ricardo Izquierdo d, Nianqiang Wu b, My Ali El Khakani a, Dongling Ma a,n

a Institut National de la Recherche Scientifique (INRS), Centre-Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes,QC, Canada J3X 1S2b Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, USAc Environmental Futures Centre, Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australiad Département d’informatique, University of Quebec at Montreal (UQAM), Case postale 8888, succursale Centre-ville, Montréal, QC, Canada H3C 3P8

a r t i c l e i n f o

Article history:Received 29 June 2013Received in revised form13 January 2014Accepted 23 January 2014Available online 14 February 2014

Keywords:Wet chemical synthesisTiO2 nanorod arraysCore–shell QDsDepleted bulk heterojunctionSolar cells

a b s t r a c t

All solution processed depleted bulk heterojunction (DBH) solar cell devices based on near infrared (NIR)PbS/CdS core–shell quantum dots (QDs) and films of rutile TiO2 nanorod arrays have been investigated.The device fabrication was achieved through the layer-by-layer spin coating of PbS/CdS QDs, in ambientatmosphere, onto hydrothermally grown TiO2 nanorod arrays film leading to the general devicearchitecture consisting of fluorine doped tin oxide (FTO)/TiO2/QDs/interfacial layer/Au. The performanceof these devices fabricated under different processing conditions was tested and compared with that ofsimilar devices where the PbS/CdS QDs were replaced by a spin-coated layer of colloidal PbS QDs(processed under inert atmosphere). It was found that the maximum power conversion efficiency of theformer devices is about 40% higher when MoO3 was used as an interfacial layer (2.02%70.15 vs 1.40%70.11). The stability and ease of processing in air together with the higher performance of the PbS/CdScore–shell QDs, as compared to the PbS QDs, strongly suggest their high potential in solar cellapplications. This work represents the first demonstration of the use of NIR PbS/CdS core–shell QDs insolar cells.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Colloidal QDs have emerged as good candidates for the third-generation solar cells, mainly due to their easy synthesis, solutionprocessability and wide absorption tunability. They also offerpossibilities of multiple exciton generation [1–3] and fast extrac-tion of hot carriers [4,5]. Especially, NIR QDs are attractive forapplication in solar cells [6,7] as they can absorb photons ofwavelength ranging from NIR to UV leading to the absorption ofthe major part of the solar spectrum, which is hard to achieve withcommercially available Si solar cells and typical organic solar cells.NIR QDs such as PbS, PbSe, InAs and HgTe have so far been appliedinto solar cells with different device architectures, such as liquidheterojunction [8] (e.g., QD sensitized solar cells), hybrid bulkheterojunction [9,10], Schottky junction [11], depleted heterojunc-tion [12–14] and DBH [15–17].

The overall power conversion efficiency (η) of solar cell devicesdepends on their open circuit voltage (Voc), short circuit current

(Jsc) and fill factor (FF), which in turn largely depend on thearchitecture of the device and the energetics of the materialsinvolved. Among QD-based solar cells, liquid heterojunction solarcells show the highest Voc and FF. However, their efficiency isnormally limited by low Jsc due to lower light absorption of the QDmonolayer as compared to a dye layer [8]. Corrosion of QDs in aliquid electrolyte represents another major concern [18]. HybridQD-based bulk heterojunction solar cells usually involve at leastone type of organic semiconducting polymers. Their low chargecarrier mobility limits the thickness of the photoactive layer toabout 100 nm [19], eventually limiting the efficiency due tolimited light absorption capacity. In addition, although the bulkheterojunction in theory solves the problem of a very short excitondiffusion length of �10 nm in polymers, the actual realization ofphase segregation on the 10 nm length scale is quite challenging.Different from the above QD/polymer excitonic solar cells,Schottky junction QD solar cells provide a built-in potential forefficient charge separation and transport at a much larger lengthscale (�100–200 nm [14]). Nonetheless, they still suffer fromsome inherent problems. The Schottky contact is built up at theback contact between the QD film and low-work-function metalelectrode; the poor diffusion (on the order of 10 nm) of minority

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0927-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.solmat.2014.01.037

n Corresponding author. Tel.: þ1 514 228 6920; fax: þ1 450 929 8102.E-mail address: ma@emt.inrs.ca (D. Ma).

Solar Energy Materials & Solar Cells 124 (2014) 67–74

carriers generated on the illumination side in the quasi-neutralregion of the QD film limits the thickness of light-absorbing QDlayer to about the width of the depletion region and therebyresulting in limited light absorption and photocurrent generation.Voc limitation by Fermi level pinning at the QD–metal interfaceand back recombination at the metal electrode are two othermajor concerns [14]. Depleted heterojunction solar cells based onthe construction of the heterojunction between a p-type QD filmand an n-type transparent electrode (such as TiO2 and ZnO)promise to overcome some of the above-mentioned limitations.Such cells have led to an efficiency of over 5% [14]. However, thethickness of the active QD layer is still limited to a value similar tothe depletion width. It is thus crucial to maximize absorption ofsolar photons while maintaining efficient charge extraction. Intheory it can be achieved by combining the concepts of depletedand bulk heterojunction solar cells to build DBH cells. Veryrecently, significant progress has been made in this new direction[15–17]. The devices involving interpenetration of p-type QDs andn-type wide-band-gap semiconductors such as TiO2 and ZnOincrease the interfacial area and thus allow the depletion regionspreading in three dimensions in the photoactive layer. As a result,the photoactive layer can be made thicker so as to absorb moresolar photons yet enabling efficient charge carrier collection. Thehighest efficiency of �5.5% has been reported for the cells of thisconfiguration [15]. Up to date, the effort has been centered on theintegration of QDs with a porous network of semiconductornanoparticles (such as TiO2), in which the structural inhomogene-ity like voids can deteriorate photovoltaic performance. The use ofvertically oriented n-type semiconductor nanorods, that canprovide carriers a direct pathway to electrodes, is anticipated tohelp realizing better charge transport. Very recently, a novelanatase TiO2 nanopillar electrode was designed and combinedwith PbS QDs into a DBH solar cell, yielding a record highefficiency of �5.6% [16]. However, the fabrication of the anataseTiO2 nanopillars requires a template prepared by lithographytechnique, which may not be easily accessible in every laboratory.Alternatively, it is interesting to synthesize vertically aligned TiO2

nanorods by low cost template-free wet chemical approach andthen use them in DBH solar cells.

On the other hand, for most QD-based solar cells at least partialsteps of fabrication and even testing need to be performed underinert atmosphere [20,21], since QDs are sensitive to moisture andcan get easily oxidized in air. Such sensitivity to ambient atmo-sphere could cause problems of batch-to-batch reproducibility indevice performance. In addition, the operation of experiment in aglove box is not convenient and tedious, not to mention that itmay not be readily available in many laboratories. This sensitivityissue may be addressed by coating QDs with a robust inorganicshell to prevent their easy oxidation. Very recently we havedeveloped a two-step route for the synthesis of PbS/CdS core–shell QDs with tunable shell thicknesses and absorption properties[22,23]. These core–shell QDs have been found to possess higherphotoluminescence (PL) quantum yield and significantly improvedphoto- and thermal stability compared to uncoated PbS QDs onlycapped with ligands. The CdS shell provides protection to the PbScore and also passivates its surface defects, that may otherwiseserve as charge carrier trap sites. It is thus quite interesting toexplore the use of these high-quality core–shell QDs in solar cells.

In this work, we present DBH solar cell devices based oncolloidal PbS/CdS core–shell QDs deposition, via layer-by-layer spincoating in ambient atmosphere, onto hydrothermally-grown rutileTiO2 nanorod arrays. To the best of our knowledge, this kind of solarcells involving PbS/CdS core–shell QDs has not been reported so far.The devices show about 40% higher power conversion efficiencythan similar devices involving colloidal PbS QDs processed in inertatmosphere. The effects of multiple parameters on photovoltaic

performance were also investigated. These include the length of theTiO2 nanorods, the interfacial layer between the QD film and backelectrode, the atmosphere under which spin coating and ligandexchange were carried out, the number of QD deposition cycles, andthe short chain ligands used to be exchanged with the long chainligands capping the QDs.

2. Material and methods

2.1. Synthesis of TiO2 nanorod arrays

Vertically aligned TiO2 nanorod arrays with 2 mm length weregrown on cleaned FTO glass substrate (MTI corporation, TEC 15,R¼12–14 Ω/sq) by hydrothermal technique with a modified pro-cedure from references [24,25]. In this process, cleaned FTO glasssubstrates were placed at an angle in a 23 ml teflon cup, with theconducting side facing the wall. Then, 10 ml of a precursorsolution, which was prepared by mixing 20 ml of pure water and20 ml of concentrated HCl (ACS reagent, ca. 37%) and adding 1 mlof titanium (IV) butoxide (reagent grade, 97%) drop wise to it whilestirring, was added to it. The Teflon cup was loaded into anautoclave, which was then sealed tightly and placed in an oven.It was heated at 180 1C for 2 h. Finally, the autoclave was taken outfrom the oven and cooled. The substrates were then taken out andrinsed thoroughly with dilute HCl acid and then with pure waterand dried in air. The nanorods were obtained as white film on thesubstrate.

TiO2 nanorod arrays with 4 mm length were directly grown onthe FTO substrate via a previously reported hydrothermal method[26]. Typically, 0.75 ml of tetra-tertbutoxy titanate was dissolvedin 60 ml of 6 M HCl, and the solution was transferred into a steellined Teflon autoclave with a volume of 120 ml, where FTOsubstrates were placed in a Teflon holder. The autoclave wasmaintained at 150 1C for 20 h and the coated FTO substrate waswashed several times with deionized water and ethanol and driedin air.

2.2. Synthesis and purification of PbS QDs and PbS/CdScore–shell QDs

PbS QDs were synthesized using a modified procedure fromreferences [27,28]. In the typical procedure 760 mg of Pb(OAc)2 �3H2O (Z99.99%), 2.4 ml of OA (Z99% (GC)) and 15 ml ofODE (technical grade, 90%) were added to a three neck roundbottom flask. The mixture was heated to 150 1C for 1 h whilestirring and purging with N2 flow. It was then cooled undervacuum to 130 1C and the N2 flow was recovered. Two millilitersof mixture of bis(trimethylsilyl) sulfide ((TMS)2S) (synthesis grade)and trioctylphosphine (TOP) (technical grade, 90%) (1:10 ratio byvolume) was quickly injected into the flask, cooled to 100 1Cquickly and quenched with cold water after keeping the reactionfor 5 min. The QDs were precipitated by centrifugation and thenre-dispersed in cold hexane (certified ACS). After keeping the QDdispersion at 4 1C for two days, the QD dispersion was centrifugedat 8000 rpm for 30 min and the sediment was discarded. Follow-ing methanol (anhydrous, 99.8%) addition, it was centrifuged at3000 rpm for 5 min. After removing the supernatant the QDssediment was dispersed in toluene (certified ACS). This purifica-tion step was repeated one more time. Finally, the QDs weredispersed in mixture of octane (puriss. p.a., Z99.0% (GC)) anddecane (anhydrous, Z99%) (3:1 ratio by volume) at concentrationof 37.5 mg/ml for making solar cells.

PbS/CdS core–shell QDs have been synthesized following amodified procedure from previous work in our group [22] byheating a mixture of 5 mg CdO (Z99.99% trace metals basis),

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OA (6 ml) and ODE (10 ml) to 255 1C under N2 for 20 min whilestirring. The clear solution obtained was cooled to 155 1C undervacuum for 15 min. The flask was then reopened and the N2 fluxwas restored. PbS QDs solution in toluene (3 ml, absorbance¼2.5at the first excitonic absorption peak) was diluted in 10 ml bytoluene, bubbled with N2 for 30 min and then heated to 90 1Cquickly. The Cd/OA mixture was then injected. The reaction flaskwas quenched with cold water after the growth reaction wasconducted at 90 1C for about 30 min. Finally, the QDs were purifiedusing methanol and dispersed in mixture of octane and decane(3:1 ratio by volume) with QD concentration adjusted to 37.5 mg/mlfor making solar cells.

2.3. Fabrication of DBH solar cell devices

DBH solar cells were fabricated by spin coating QD solution layerby layer on the film of TiO2 nanorod arrays. Each layer was spincoated by dispensing 200 ml of the QDs solution on the 1 in. by 1 in.substrate and rotating at 2500 rpm for 10 s. Ligand exchange wasperformed on each layer by treating it with a solution of thioglycolicacid (TGA) (Z98%) or 3-mercaptopropionic acid (3-MPA) (Z99%)in methanol (1:9) for 1 min followed by spinning at 2500 rpm for10 s. The layer was then washed with methanol and hexane byspinning at 2500 rpm for 10 s. Such ligand exchange procedure wasrepeated three times for each layer. On some devices a final layer ofpoly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevios™, OLED grade) was deposited by dispensing 500 ml ofits solution (1.3% by weight in water) and spin coating at 2500 rpmfor 1 min. On some others MoO3 (ACS reagent, Z99.5%) layer wasdeposited by thermal evaporation. Finally gold electrodes (Kurt J.Lesker, 99.9%) were deposited by thermal evaporation as 3 by3 circular arrays through a shadow mask with circular apertures

of 4 mm diameter making photoactive area of the devices ca.0.13 cm2.

2.4. Characterizations

Scanning electron microscope (SEM) images of TiO2 nanorodarrays were taken with JEOL JSM-6300F SEM equipped with EDS.TEM, HRTEM images and SAED were taken by a JEOL 2100F TEM.The XRD study of TiO2 nanorod arrays was carried out with aPhilips X’pert diffractometer using Cu-Kα radiation source.

The electronic resistance of the rutile TiO2 nanorod arrayelectrode was measured by a photoelectrochemical (PEC) methodat 25 1C in 0.10 M NaNO3 solution in a three-electrode cell. Inparticular, the TiO2 nanorod working electrode was mounted in aspecial holder with an area of 0.78 cm2 exposed to UV illuminationvia a quartz window. A scanning potentiostat (PAR 362, Princeton)was used to conduct the linear sweep voltammetry (LSV) mea-surements. The light source was a 150 W xenon lamp (Trusttech,Beijing, China) with regulated optical output and an UV-band-passfilter (UG-5, Schott). The output UV light intensity was measuredat 365 nm wavelength using a UV irradiance meter (UV-A, Instru-ments of Beijing Normal University, China).

Absorption spectra were acquired using a Cary 5000 UV–vis–NIRspectrophotometer (Varian). Fluorescence spectra were recordedwith a Fluorologs-3 system (Horiba Jobin Yvon) with excitationwavelength of 636 nm.

The solar cells were characterized by performing current-voltagemeasurement using a Keithley 2400 programmable voltage sourcemeter in dark and under illuminationwith an AM1.5 solar simulatorcomprised of a 150W xenon lamp with a filter (Oriel) andcalibrated to an intensity of 100 mW cm�2.

Fig. 1. SEM images of TiO2 nanorod arrays on FTO glass substrate: (a) top view; (b) cross-sectional view. HRTEM image (c) and SAED pattern (d) of TiO2 nanorods.

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3. Results and discussion

3.1. TiO2 nanorod arrays

TiO2 nanorod arrays were grown on FTO glass substrate byhydrothermal technique, which involves controlled hydrolysis oftitanium precursor in acidic solution at elevated temperature in asealed vessel. SEM images of the TiO2 nanorod arrays are shown inFig. 1a and b. The as-grown TiO2 nanorods film is seen to have athickness of about 2 mm (Fig. 1b). The nanorods are relativelyaligned perpendicular to the substrate with average diameter ofabout 120 nm and the average inter-nanorod distance, at the topof the nanorods, of about 300 nm. The nanorods were found tohave a rutile TiO2 crystal structure, as revealed by X-ray diffraction(XRD) analysis (Fig. S1). High resolution transmission electronmicroscopy (HRTEM) image (Fig. 1c) and selected area electrondiffraction (SAED) pattern (Fig. 1d) of the nanorods further suggestthat the nanorods are quite likely single crystalline. The latticefringes with the lattice separation of �2.95 Å, corresponding tothe (0 0 1) inter-planar distance of rutile TiO2 (JCPDS No. 11292), inthe direction perpendicular to the nanorod axis confirm thegrowth of the nanorods in the [0 0 1] direction. The morphologyand crystalline structure of the TiO2 nanorod arrays are consistentwith what was reported in the literature [24,25,29].

Rutile TiO2 has a potential to be applied in solar cells withperformance comparable to the anatase phase as demonstrated indye sensitized solar cells [30]. Rutile TiO2 have their advantages ofhigher chemical stability and higher refractive index [31]. It alsohas a deeper conduction band level than anatase [32], which couldincrease the driving force for electron transfer from PbS QDs toTiO2 [14], while maintaining the same hole injection barrier fromQDs to TiO2 due to the similarity in the valence band edge of rutileand anatase TiO2. In general, oriented 1D-nanostructures, such asrutile TiO2 nanorods, are believed to be able to enhance theperformance of solar cells as they provide quick charge separationand continuous confined pathways for charge carriers transporttowards electrodes with increased mobility [33]. These advan-tages, further complemented by the low cost template-free wetchemical synthesis, make it very interesting to investigate the useof these rutile TiO2 nanorod arrays in solar cells.

The electronic conductivity of the as-prepared TiO2 rutilenanorod arrays was measured using a PEC method [34]. In thismethod, the TiO2 nanorod electrode was subject to the LSVmeasurements under various UV intensities (Fig. 2a). It can be

observed that each voltammogram (J–V), under a given lightintensity, consists of two stages. For the potential bias range of�0.3 V to 0.0 V, the photocurrent response increases monotoni-cally with the applied potential bias. In contrast, when potentialbias is greater than ca. 0.0 V, the photocurrent levels off, yieldingnamely saturation photocurrent (Jsph). Most importantly, excellentlinearity of J–V relationship can be observed in the potential rangefrom �0.20 V to �0.15 V (Fig. 2a). Ohm's law was used to calculatethe total resistance, i.e., R¼(V2�V1)/(J2� J1)¼ΔV/ΔJ. Using com-puter linear regression fitting, the obtained R values were plottedagainst the reciprocal of the saturation current (i.e. 1/Jsph) at 0.8 V,giving an excellent linear regression equation according to Eq. (1)(Fig. 2b),

R¼ k=JsphþR0 ¼ R1þR0 ð1Þin which k and R0 are the slope and Y-intercept for a givenelectrode, respectively.

The physical meaning of Eq. (1) is that R is the sum of a variableresistance (R1¼k/Jsph) and a constant resistance (R0). In this case,R0 is equal to 4200Ω, which is the sum of ohmic resistance at theTiO2/FTO interface and resistance along the TiO2 nanorods. R0value is ca. 10 times that of commercial porous TiO2 film [34]. Thefact that the film has very low photocurrent and very largeresistance compared with the conventional porous TiO2 filmsuggests that the conductivity of the rutile nanorod arrays needsto be improved. On the other hand, it implies that the rutilenanorod array electrode has low photocatalytic activity, whichshall be beneficial to the stability of the QDs.

3.2. PbS/CdS core–shell and PbS QDs

The UV–vis–NIR absorption spectrum and PL emission spec-trum of PbS/CdS core–shell QDs used to fabricate solar cell devicesin this work are shown in Fig. 3a. The distinct excitonic absorptionpeak and narrow PL peak with full width at half maximum(FWHM) of �112 nm indicate that the PbS core QDs have arelatively narrow size distribution. Based on the first excitonicabsorption peak in the UV–vis–NIR absorption spectrum, the coreQDs were estimated to have a band gap of 1.38 eV, correspondingto the average core size of 2.9 nm in diameter [22,35]. Since theaverage size of parent QDs (i.e., PbS QDs used to synthesize thecore–shell QDs) can also be deduced from their absorptionspectrum and the overall size of the QDs remains unchangedduring the cation exchange process for the formation of the

Fig. 2. (a) Voltammograms of the rutile nanorod array photoanode under various UV light intensities in 0.1 M NaNO3 solution. (b) Relationship between calculated resistanceand inversed saturation photocurrent of the photoanode.

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core–shell structure [22], the CdS shell thickness was estimated tobe about 0.1 nm in this case.

The TEM images of the PbS/CdS core–shell QDs used forfabrication of solar cell devices in this work are shown in Fig. 4.The average diameter of the QDs is ca. 3.1 nm and it is in goodagreement with the size estimated from the UV–vis–NIR absorp-tion spectrum of the parent PbS QDs used for the synthesis ofthese core–shell QDs. It is also clear from the TEM images that thecore–shell QDs have uniform size distribution.

Since the CdS shell can passivate the surface defects of the PbScore [23], which could act as charge recombination sites, it isexpected that the presence of the CdS shell will improve theperformance of QD-based photovoltaic devices. Furthermore, theshell can help to prevent the PbS core from oxidation, and thusavoiding its negative effect on the photovoltaic performance ofQD-based solar cells. Motivated by these expectations, we appliedthe PbS/CdS core–shell QDs into DBH solar cells. For comparison,pure PbS QDs of a similar band gap of 1.32 eV and an average sizeof about 3.1 nm in diameter were also synthesized and integratedinto the solar cells of the same architecture. Their UV–vis–NIRabsorption and PL spectra are shown in Fig. 3b. Same as the PbS/CdS core–shell QDs, these PbS QDs show a distinct excitonic

absorption peak and a relatively narrow PL emission peak(FWHM¼127 nm), confirming their narrow size distribution.

3.3. DBH solar cell devices

Taking several advantages (including the solution processabil-ity of the TiO2 nanorod arrays and the QDs) into consideration,DBH solar cells were fabricated and investigated. The QDs weredeposited onto the rutile TiO2 film by sequential spin coating.Fig. 5a shows the cross-sectional SEM image of a typical FTO/TiO2/QD sample. The QDs were found to infiltrate into the gaps betweenthe TiO2 nanorods and continue as a film over the TiO2 nanorodfilm as evidenced by the line-by-line lead and sulfur energy-dispersive X-ray spectroscopy (EDS) mapping (Fig. 5b and c).

With the deposition of interfacial layer and Au back electrode,such QD/TiO2 combination results in a solar cell design recentlydescribed as DBH [15,16], which involves at least partial inter-penetration of p-type (QDs in this case) and n-type (TiO2 in thiscase) materials. With the increased interfacial area between theQDs and the TiO2, the DBH solar cell architecture combines theconcept of traditional inorganic p–n junction solar cells and bulkheterojunction organic solar cells and hold high potential to breakthe absorption and charge extraction compromise. In addition tothe built-in electric field developed at the TiO2/QD interface, thehigher conduction band edge of the QDs with respect to that ofTiO2 also acts as a driving force for photogenerated chargeseparation and transport [15]. The conduction band level of QDsessentially depends on their size; the smaller the PbS QDs (or PbScore dots) are, the higher the conduction band edge is. As theconduction band edge of the PbS QDs (or PbS core dots in thecore–shell QDs) investigated herein is higher than that of TiO2

(�3.7 to �3.8 eV for QDs [36] versus �4.6 eV for rutile TiO2 [32]),the photo excited electron transfer from the PbS QDs (or PbS coredots) to TiO2 is aided by this potential gradient, with the electronback transfer being prevented.

Fig. 6 schematically illustrates the general device structure ofFTO/TiO2 nanorods/QDs/interfacial layer/Au and the electronicband structure of the different components involved.

DBH solar cell devices fabricated under different conditions havebeen investigated in this work. Fig. 7 shows the current density–voltage (J–V) characteristics of PbS/CdS QD solar cells fabricated andtested in air with a MoO3 layer being used as an interfacial layer. Sixspin coating cycles were performed to deposit the QDs, yielding theQD film of �300 nm in thickness. Following each cycle, the QDswere treated with 3-MPA in order to replace the longer ligand ofoleic acid (OA) on the QD surface with shorter molecules to reducethe inter-dot spacing. The specific device in Fig. 7 shows a power

Fig. 3. UV–vis–NIR absorption (black curve) and PL (blue curve) spectra of (a) PbS/CdS core–shell QD solution and (b) PbS QD solution. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. TEM image of PbS/CdS core–shell QDs. The inset shows highmagnification image.

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conversion efficiency of 2.14%. It is about 40% higher than that ofsimilar PbS QD solar cells fabricated in a glove box.

Closer examination of the J–V characteristics (Fig. 7) of thesedevices shows that the Voc of the core–shell QD and PbS QD

devices is the same, which is believed to originate from similarband edges of the PbS QDs and the PbS core of the core–shell QDs.The Jsc is also similar, likely due to the similarity in the lightabsorption and charge transport of the core–shell QD and PbS QDlayers. The FF is, however, much higher in the device involving thecore–shell QDs, suggesting the reduction of charge carrier recom-bination rate, which could be attributed to the passivation of thesurface defect states of the PbS core by the CdS shell. Thesignificant improvement in the FF leads to the considerableincrease in the overall power conversion efficiency. These resultsdemonstrate that the PbS/CdS core–shell QDs offer dual advan-tages—easy processing in air and better performance as comparedwith the PbS QD solar cells. The details of J–V characteristics ofspecific devices studied herein are summarized in Table 1.Although, there is data fluctuation among devices (possibly relatedto the degree of uniformity of the QD layer), the core–shell QDdevices processed in air are found to always achieve considerablybetter performance than PbS QD devices (without the CdS shellstructure) processed in the glovebox when MoO3 was used as aninterfacial layer.

Similar PbS/CdS QD solar cell devices have also been madeusing 4 mm long TiO2 nanorod arrays and MoO3 as the interfaciallayer. These devices show the maximum AM 1.5 power conversionefficiency of 1.32%, lower than that achieved by the 2 mm-long-TiO2-nanorod devices. It indicates that the 4 mm long TiO2 nanorodarrays are less efficient in photoegenerated charge carrier trans-portation. It is usually believed that longer nanorods help intrapping more light by scattering it into the QD film so as toenhance absorption [37,38]. However, in the current case, this

Fig. 7. J–V curves of DBH devices: (blue) FTO glass/TiO2 nanorod arrays/PbS/CdSQDs/MoO3/Au, the spin coating and ligand exchange of the PbS/CdS QDs were donein ambient atmosphere; (red) FTO glass/TiO2 nanorod arrays/PbS QDs/MoO3/Au,the spin coating and ligand exchange of the PbS QDs were done in a glove box. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

Fig. 5. (a) Cross-sectional SEM image of a typical PbS QD/TiO2 sample made by 6 cycles of spin coating of PbS QDs onto TiO2 nanorod arrays; line-by-line EDS mapping of (b)Pb and (c) S.

Fig. 6. (a) Schematic diagram of the cross-section of the FTO glass/TiO2 nanorod arrays/PbS/CdS core–shell QDs/MoO3/Au solar cell; (b) the energy band diagram of thesolar cell.

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could be outweighed by other factors such as enhanced recombi-nation occurring at the defects of the longer nanorods or at theinterface between the nanorods and the FTO substrate due to theweakening of interface between the longer TiO2 nanorod arraysand FTO [24,39]. The longer nanorod arrays could also insteaddecrease the intensity of the light reaching the QD film as thegrowth of the nanorod arrays in this case leads to denser and fusednanorods at the base [39].

The effect of multiple process parameters such as interfaciallayer on the photovoltaic performance of QD solar cells was alsoinvestigated. For PbS/CdS core–shell QD solar cells, when MoO3 isreplaced with PEDOT:PSS, the power conversion efficiency isdecreased to 1.05%, indicating that MoO3 is more favorable inour case. Similar beneficial effect of MoO3 has also been demon-strated in PbS QDs/ZnO devices [20,40] and PbS QDs/TiO2 devices[41]. It is attributed to the fact that the thin MoO3 layer helps theformation of Ohmic contact between the QD film and the goldback electrode, inhibiting the formation of a Schottky junction thatwould generate diode current opposing the photocurrent. Accord-ingly, Voc, Jsc and FF of the devices are all improved by using MoO3

as the interfacial layer. The interfacial layer (mainly MoO3) alsoplays a role in PbS QD solar cells processed in the glove box. Eventhough PEDOT:PSS layer is believed to facilitate the collection ofholes by the back electrode as reported in polymer-based solarcells [42], the efficiency achieved by PbS QDs with PEDOT:PSSinterfacial layer is in close range with those devices without anyinterfacial layer. Same as the core–shell QD solar cells, the powerconversion efficiency is largely increased to 1.53% when PEDOT:PSS is replaced with MoO3, again revealing the positive impact ofMoO3. But in contrast to the considerable difference in theefficiency (1.53% vs 2.14%, Table 1) observed between the PbSand PbS/CdS core–shell QD solar cells when MoO3 is used, thesetwo devices show similar efficiency (1.16% vs 1.05%, Table 1) withthe use of PEDOT:PSS as an interfacial layer. Quite different fromthe case of MoO3, FF is similar herein; it perhaps indicates thatPEDOT:PSS provides certain passivation to some QDs in top layerand may even penetrate into the QD layer, enhancing the competi-tion of charge transport with respect to charge trapping at thedefect sites on the PbS QDs.

The effect of atmosphere was studied on PbS QD solar cells.When spin coating and ligand exchange were performed in air, themaximum efficiency is lower than 1% no matter with or withoutthe interfacial layer (PEDOT:PSS) (Table 1). The poor performancemust arise from the oxidation of the PbS QDs. It suggests thesignificance of protecting the shell-free QDs against oxidativeatmosphere during processing.

With regard to the number of deposition cycles of the QD film,the best performance was attained with devices comprising6 layers of spin coated PbS QDs. This thickness could be thecompromise between the light absorption and charge carriercollection in these devices. The Voc, Jsc, FF and power conversionefficiencies of the devices comprising 3, 6 or 10 layers of PbS QDsare listed in Table S1.

In contrast to the above mentioned parameters that largelyaffect the photovoltaic performance of the QD solar cells, the useof two different short chain ligands (3-MPA and TGA) for displa-cing the long chain ligands capping the QDs does not lead to anysignificant difference in the efficiency of the devices involvingeither PbS or PbS/CdS core–shell QDs (Table S2).

Although the efficiency of the TiO2-nanorod-array/PbS/CdScore–shell-QD solar cells is lower than the record-efficiency of5.6% of the DBH solar cells based on PbS QDs and anatase TiO2

nanopillars, they have their own advantages. Fabricated in air, theyshow about 40% higher efficiency than similar PbS QD solar cellsfabricated under inert atmosphere. In addition, their performancecan be further improved by several strategies. For example,improving QD loading by increasing the spacing among the rutilenanorods will allow more QDs to penetrate deeper in the gapbetween the nanorods. According to Kramer et al. [16], the optimaldistance between one-dimensional TiO2 nanostructures and QDsshould be �275 nm, which permits to maximize the volumefiltration of the QDs yet retaining efficient charge collection.Controlled deposition of a seed layer before the hydrothermalreaction might address this issue [43]. It may simultaneouslyimprove the TiO2/FTO interface, and thus enhancing the conduc-tivity. As presented above, the resistance of current TiO2 nanorodarrays measured by the PEC method is ca. 10 times higher thanthat of conventional TiO2 film, appearing as one of the majorhurdles in achieving high efficiency. Doping the TiO2 nanorodarrays is another strategy to improve the conductivity and it canalso contribute to the absorption of photons in the visible range ofthe solar spectrum [44–46].

4. Conclusions

In summary, we report air-processed DBH solar cell devicesbased on colloidal PbS/CdS core–shell QDs and rutile TiO2 nanorodarrays, both of which were synthesized by low cost, wet chemicalapproaches. After optimization of several processing conditions,the power conversion efficiency of 42% have been attained,which represents �40% increase in the efficiency that can beachieved by similar devices involving PbS QDs processed in inertatmosphere. It is clear that the core–shell QDs approach offerseasy solution processability in air and better performance, promis-ing for solar cell applications. In addition, the current workdemonstrates that for QD-based solar cell devices investigatedherein the processing conditions such as atmosphere and inter-facial layer between the QD film and the back electrode playsignificant roles in the ultimate performance of the devices.Finally, the efficiency of these novel core–shell based PV devicescan be further improved by optimizing the device structure via, forexample, doping the TiO2 nanorod arrays and controlling thedensity and length of the nanorod arrays. The work is underwayto address these challenging issues.

Table 1J–V characteristics of devices fabricated under different conditions.

Sample no. TiO2 nanorod length QDs Interfacial layer Jsc (mA/cm2) Voc (mV) FF (%) η (%) Spin coating environment

1 2 mm PbS/CdS MoO3 7.58 501 56 2.14 Air2 2 mm PbS/CdS PEDOT:PSS 5.61 423 44 1.05 Air3 4 mm PbS/CdS MoO3 7.31 450 40 1.32 Air4 2 mm PbS MoO3 7.94 497 39 1.53 Inert atmosphere (N2)5 2 mm PbS PEDOT:PSS 6.14 425 44 1.16 Inert atmosphere (N2)6 2 mm PbS PEDOT:PSS 4.50 427 36 0.70 Air7 2 mm PbS none 2.84 420 35 0.42 Air8 2 mm PbS none 5.80 390 39 0.90 Inert atmosphere (N2)

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Acknowledgements

Financial support from the Natural Sciences and EngineeringResearch Council (NSREC) of Canada, Canadian Solar Inc. and OLADisplay Corp. in the context of a NSREC-Strategic grant is greatlyappreciated. The authors also acknowledge the financial supportfrom Nano-Québec.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.solmat.2014.01.037.

References

[1] R.J. Ellingson, M.C. Beard, J.C. Johnson, P.R. Yu, O.I. Micic, A.J. Nozik, A. Shabaev,A.L. Efros, Highly efficient multiple exciton generation in colloidal PbSe andPbS quantum dots, Nano Lett. 5 (2005) 865–871.

[2] V.I. Klimov, J.A. Mcguire, J. Joo, J.M. Pietryga, R.D. Schaller, New aspects ofcarrier multiplication in semiconductor nanocrystals, Acc. Chem. Res. 41(2008) 1810–1819.

[3] A.J. Nozik, M.C. Beard, J.M. Luther, M. Law, R.J. Ellingson, J.C. Johnson,Semiconductor quantum dots and quantum dot arrays and applications ofmultiple exciton generation to third-generation photovoltaic solar cells, Chem.Rev. 110 (2010) 6873–6890.

[4] Y.N. Gao, E. Talgorn, M. Aerts, M.T. Trinh, J.M. Schins, A.J. Houtepen, L.D.A. Siebbeles, Enhanced hot-carrier cooling and ultrafast spectral diffusion instrongly coupled PbSe quantum-dot solids, Nano Lett. 11 (2011) 5471–5476.

[5] R. Patterson, M. Kirkengen, B.P. Veettil, D. Konig, M.A. Green, G. Conibeer,Phonon lifetimes in model quantum dot superlattice systems with applica-tions to the hot carrier solar cell, Sol. Energy Mater. Sol. Cells 94 (2010)1931–1935.

[6] E.H. Sargent, Colloidal quantum dot solar cells, Nat. Photonics 6 (2012)133–135.

[7] J.A. Tang, E.H. Sargent, Infrared colloidal quantum dots for photovoltaics:fundamentals and recent progress, Adv. Mater. 23 (2011) 12–29.

[8] S. Ruhle, M. Shalom, A. Zaban, Quantum-dot-sensitized solar cells, Chem-physchem 11 (2010) 2290–2304.

[9] S. Gunes, H. Neugebauer, N.S. Sariciftci, H. Roither, M. Kovalenko, G. Pillwein,W. Heiss, Hybrid solar cells using HgTe nanocrystals and nanoporous TiO2

electrodes, Adv. Funct. Mater. 16 (2006) 1095–1099.[10] D. Wang, J.K. Baral, H. Zhao, B.A. Gonfa, V.V. Truong, M.A. El Khakani,

R. Izquierdo, D. Ma, Controlled fabrication of PbS quantum-dot/carbon-nanotube nanoarchitecture and its significant contribution to near-infraredphoton-to-current conversion, Adv. Funct. Mater. 21 (2011) 4010–4018.

[11] R. Debnath, J. Tang, D.A. Barkhouse, X.H. Wang, A.G. Pattantyus-Abraham,L. Brzozowski, L. Levina, E.H. Sargent, Ambient-processed colloidal quantumdot solar cells via individual pre-encapsulation of nanoparticles, J. Am. Chem.Soc. 132 (2010) 5952–5953.

[12] J.J. Choi, Y.F. Lim, M.B. Santiago-Berrios, M. Oh, B.R. Hyun, L.F. Sung, A.C. Bartnik, A. Goedhart, G.G. Malliaras, H.D. Abruna, F.W. Wise, T. Hanrath,PbSe nanocrystal excitonic solar cells, Nano Lett. 9 (2009) 3749–3755.

[13] K.S. Leschkies, T.J. Beatty, M.S. Kang, D.J. Norris, E.S. Aydil, Solar cells based onjunctions between colloidal PbSe nanocrystals and thin ZnO films, ACS Nano 3(2009) 3638–3648.

[14] A.G. Pattantyus-Abraham, I.J. Kramer, A.R. Barkhouse, X.H. Wang,G. Konstantatos, R. Debnath, L. Levina, I. Raabe, M.K. Nazeeruddin,M. Gratzel, E.H. Sargent, Depleted-heterojunction colloidal quantum dot solarcells, ACS Nano 4 (2010) 3374–3380.

[15] D.A.R. Barkhouse, R. Debnath, I.J. Kramer, D. Zhitomirsky, A.G. Pattantyus-Abraham, L. Levina, L. Etgar, M. Gratzel, E.H. Sargent, Depleted bulk hetero-junction colloidal quantum dot photovoltaics, Adv. Mater. 23 (2011)3134–3138.

[16] I.J. Kramer, D. Zhitomirsky, J.D. Bass, P.M. Rice, T. Topuria, L. Krupp, S.M. Thon,A.H. Ip, R. Debnath, H.C. Kim, E.H. Sargent, Ordered nanopillar structuredelectrodes for depleted bulk heterojunction colloidal quantum dot solar cells,Adv. Mater. 24 (2012) 2315–2319.

[17] K.S. Leschkies, A.G. Jacobs, D.J. Norris, E.S. Aydil, Nanowire-quantum-dot solarcells and the influence of nanowire length on the charge collection efficiency,Appl. Phys. Lett. 95 (2009).

[18] P.V. Kamat, Boosting the efficiency of quantum dot sensitized solar cellsthrough modulation of interfacial charge transfer, Acc. Chem. Res. 45 (2012)1906–1915.

[19] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, High-efficiency solution processable polymer photovoltaic cells by self-organizationof polymer blends, Nat. Mater. 4 (2005) 864–868.

[20] P.R. Brown, R.R. Lunt, N. Zhao, T.P. Osedach, D.D. Wanger, L.Y. Chang, M.G. Bawendi, V. Bulovic, Improved current extraction from ZnO/PbS quantum

dot heterojunction photovoltaics using a MoO3 interfacial layer, Nano Lett. 11(2011) 2955–2961.

[21] J.M. Luther, M. Law, M.C. Beard, Q. Song, M.O. Reese, R.J. Ellingson, A.J. Nozik,Schottky solar cells based on colloidal nanocrystal films, Nano Lett. 8 (2008)3488–3492.

[22] H. Zhao, M. Chaker, D. Ma, Effect of CdS shell thickness on the opticalproperties of water-soluble, amphiphilic polymer-encapsulated PbS/CdScore/shell quantum dots, J. Mater. Chem. 21 (2011) 17483–17491.

[23] H. Zhao, M. Chaker, N.Q. Wu, D. Ma, Towards controlled synthesis and betterunderstanding of highly luminescent PbS/CdS core/shell quantum dots, J.Mater. Chem. 21 (2011) 8898–8904.

[24] E.S. Aydil, B. Liu, Growth of oriented single-crystalline rutile TiO2 nanorods ontransparent conducting substrates for dye-sensitized solar cells, JACS 131(2009) 3985–3990.

[25] C.W. Zhou, A. Kumar, A.R. Madaria, Growth of aligned single-crystalline rutileTiO2 nanowires on arbitrary substrates and their application in dye-sensitizedsolar cells, J. Phys. Chem. C 114 (2010) 7787–7792.

[26] J.T. Li, M.W.G. Hoffmann, H. Shen, C. Fabrega, J.D. Prades, T. Andreu,F. Hernandez-Ramirez, S. Mathur, Enhanced photoelectrochemical activity ofan excitonic staircase in CdS@TiO2 and CdS@anatase@rutile TiO2 heterostruc-tures, J. Mater. Chem. 22 (2012) 20472–20476.

[27] M.A. Hines, G.D. Scholes, Colloidal PbS nanocrystals with size-tunable near-infrared emission: observation of post-synthesis self-narrowing of the particlesize distribution, Adv. Mater. 15 (2003) 1844–1849.

[28] T. Zhang, H. Zhao, D. Riabinina, M. Chaker, D. Ma, Concentration-dependentphotoinduced photoluminescence enhancement in colloidal PbS quantum dotsolution, J. Phys. Chem. C 114 (2010) 10153–10159.

[29] X.J. Feng, K. Shankar, O.K. Varghese, M. Paulose, T.J. Latempa, C.A. Grimes,Vertically aligned single crystal TiO2 nanowire arrays grown directly ontransparent conducting oxide coated glass: synthesis details and applications,Nano Lett. 8 (2008) 3781–3786.

[30] N.G. Park, J. van de Lagemaat, A.J. Frank, Comparison of dye-sensitized rutile-and anatase-based TiO2 solar cells, J. Phys. Chem. B 104 (2000) 8989–8994.

[31] K.J. Kim, K.D. Benkstein, J. van de Lagemaat, A.J. Frank, Characteristics of low-temperature annealed TiO2 films deposited by precipitation from hydrolyzedTiCl4 solutions, Chem. Mater. 14 (2002) 1042–1047.

[32] L. Kavan, M. Gratzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, Electrochemical andphotoelectrochemical investigation of single-crystal anatase, JACS 118 (1996)6716–6723.

[33] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455–459.

[34] H. Yu, S.Q. Zhang, H.J. Zhao, H.M. Zhang, Photoelectrochemical quantificationof electron transport resistance of TiO2 photoanodes for dye-sensitized solarcells, Phys. Chem. Chem. Phys. 12 (2010) 6625–6631.

[35] I. Moreels, K. Lambert, D. Smeets, D. De Muynck, T. Nollet, J.C. Martins,F. Vanhaecke, A. Vantomme, C. Delerue, G. Allan, Z. Hens, Size-dependentoptical properties of colloidal PbS quantum dots, ACS Nano 3 (2009)3023–3030.

[36] B.R. Hyun, Y.W. Zhong, A.C. Bartnik, L.F. Sun, H.D. Abruna, F.W. Wise, J.D. Goodreau, J.R. Matthews, T.M. Leslie, N.F. Borrelli, Electron injection fromcolloidal PbS quantum dots into titanium dioxide nanoparticles, ACS Nano 2(2008) 2206–2212.

[37] A.I. Hochbaum, P.D. Yang, Semiconductor nanowires for energy conversion,Chem. Rev. 110 (2010) 527–546.

[38] Y.J. Lee, D.S. Ruby, D.W. Peters, B.B. McKenzie, J.W.P. Hsu, ZnO nanostructuresas efficient antireflection layers in solar cells, Nano Lett. 8 (2008) 1501–1505.

[39] Z.J. Zhou, J.Q. Fan, X. Wang, W.H. Zhou, Z.L. Du, S.X. Wu, Effect of highlyordered single-crystalline TiO2 nanowire length on the photovoltaic perfor-mance of dye-sensitized solar cells, ACS Appl. Mater. Interfaces 3 (2011)4349–4353.

[40] J.B. Gao, C.L. Perkins, J.M. Luther, M.C. Hanna, H.Y. Chen, O.E. Semonin, A.J. Nozik, R.J. Ellingson, M.C. Beard, n-Type transition metal oxide as a holeextraction layer in PbS quantum dot solar cells, Nano Lett. 11 (2011)3263–3266.

[41] X.H. Wang, G.I. Koleilat, A. Fischer, J. Tang, R. Debnath, L. Levina, E.H. Sargent,Enhanced open-circuit voltage in visible quantum dot photovoltaics byengineering of carrier-collecting electrodes, ACS Appl. Mater. Interfaces 3(2011) 3792–3795.

[42] S. Gunes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organicsolar cells, Chem. Rev. 107 (2007) 1324–1338.

[43] P.P. Sun, X.T. Zhang, X.P. Liu, L.L. Wang, C.H. Wang, J.K. Yang, Y.C. Liu, Growth ofsingle-crystalline rutile TiO2 nanowire array on titanate nanosheet film fordye-sensitized solar cells, J. Mater. Chem. 22 (2012) 6389–6393.

[44] X.B. Chen, C. Burda, The electronic origin of the visible-light absorptionproperties of C-, N- and S-doped TiO2 nanomaterials, JACS 130 (2008)5018–5019.

[45] X.J. Lu, X.L. Mou, J.J. Wu, D.W. Zhang, L.L. Zhang, F.Q. Huang, F.F. Xu, S.M. Huang, Improved-performance dye-sensitized solar cells using Nb-dopedTiO2 electrodes: efficient electron injection and transfer, Adv. Funct. Mater. 20(2010) 509–515.

[46] J. Wang, D.N. Tafen, J.P. Lewis, Z.L. Hong, A. Manivannan, M.J. Zhi, M. Li, N.Q. Wu, Origin of photocatalytic activity of nitrogen-doped TiO2 nanobelts,JACS 131 (2009) 12290–12297.

B. Atomsa Gonfa et al. / Solar Energy Materials & Solar Cells 124 (2014) 67–7474