Energy &EnvironmentalScience

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Tailoring of the P

aDepartment of Materials Science and Engine

and Technology (KAIST), 291 Daehak-ro, Y

Korea. E-mail: ysjung@kaist.ac.krbGraduate School of EEWS (Energy, Enviro

Advanced Institute of Science and Technolo

Daejeon 305-701, Republic of Korea

† Electronic supplementary informationcharacterization of the QDSC devices, Usolutions, XPS depth proling analysis da

Cite this: Energy Environ. Sci., 2014, 7,3052

Received 13th February 2014Accepted 16th June 2014

DOI: 10.1039/c4ee00502c

www.rsc.org/ees

3052 | Energy Environ. Sci., 2014, 7, 30

bS/metal interface in colloidalquantum dot solar cells for improvements ofperformance and air stability†

Min-Jae Choi,a Jihun Oh,b Jung-Keun Yoo,a Jaesuk Choi,a Dong Min Sima

and Yeon Sik Jung*a

Despite the outstanding advantages of a simple structure and cost-effectiveness of solution-based

fabrication, Schottky junction quantum dot solar cells (QDSCs) often demonstrate low open-circuit

voltage and power conversion efficiency (PCE) due to insufficient band bending at the QD/metal

Schottky junction. Generally, this undesirable result stems from the presence of many defects at the QD/

metal interface and the consequent Fermi-level pinning effect. Here, we show how the simple oxidation

of PbS QDs at the PbS/metal interface can greatly improve the open-circuit voltage, fill factor, and PCE

of Schottky junction QDSCs. On the basis of systematic analysis results using current–voltage

characterization, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and light-

soaking tests, we reveal that this enhancement originates from reduced interface states at the PbS/metal

Schottky junction. Moreover, a significant enhancement of stability of the device is confirmed by the

maintenance of >55% of its initial PCE even after 500 hours exposure in air without additional passivation.

Broader context

The increasing demand of a low-cost and high-efficiency solar cell has attracted many researchers toward exploration of third-generation photovoltaic tech-nology. Colloidal quantum dot solar cells (QDSCs) have been suggested as one of the promising candidates due to exceptional cost-effectiveness and solution-based processability, and extensive tunability based on the quantum connement effect. However, some issues still remain for the realization of QDSCs that canoutperform other solar cells. One of them is the strong Fermi-level pinning occurring at the QD/metal interface, which disturbs the formation of a high-qualitySchottky junction that can efficiently collect generated charge carriers in a Schottky-type QDSC. In this paper, we systematically tailor the PbS QD/metal interfacethrough the formation of an ultrathin interfacial layer. By controlling the degree of oxidation at the top surface of the QD lm, we reveal that the oxidizedinterfacial layer formed at the QD/metal interface improves the quality of the Schottky barrier and achieves signicant enhancements of both the cell efficiencyand air stability. On the basis of systematic analysis results using current–voltage characterization, X-ray photoelectron spectroscopy, ultraviolet photoelectronspectroscopy, and light-soaking tests, we reveal that these enhancements originate from reduced interface states at the QD/metal Schottky junction. This study isexpected to provide better understanding on the critical role of metal/semiconductor junctions in various QD-based optoelectronic devices.

1. Introduction

Colloidal quantum dots (QDs) have been extensively studied foruse in optical, electronic, and optoelectronic devices due totheir size-dependent optical,1–3 electronic,4,5 optoelectronicproperties,6,7 as well as their exceptional cost-effectiveness andsolution-based processability.8,9 In particular, lead sulde (PbS)and lead selenide (PbSe) QDs have large Bohr radii, providing a

ering, Korea Advanced Institute of Science

useong-gu, Daejeon 305-701, Republic of

nment, Water and Sustainability), Korea

gy (KAIST), 291 Daehak-ro, Yuseong-gu,

(ESI) available: The current–voltageV/visible spectra of the QD lms andta. See DOI: 10.1039/c4ee00502c

52–3060

strong quantum connement effect and a facile tunability ofband gap energy (Eg).10,11 These advantages also offer bettercarrier transport characteristics and wide spectral responses forphotovoltaic applications.12,13 With these advantages, theperformance of PbS and PbSe quantum dot solar cells (QDSCs)has shown rapid improvements over the last few years. Forexample, pioneering researchers recently reported impressiveenergy-conversion efficiencies of PbS QDSCs of up to 7% (ref.14) by employing short organic thiols,15–19 new device struc-tures,20–27 and effective surface passivation of individualQDs.28–32

While rapid increase of efficiencies has been achievedthrough engineering the optoelectronic properties of the QDlm33,34 and the device architectures,35,36 relatively little attentionhas been paid to the understanding of the interface properties inQDSCs. To realize the further improvement in efficiency, theability to engineer and optimize their interfaces with high

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precision is necessary. For CIGS,37,38 dye-sensitized solarcells,39–41 and organic solar cells,42,43 it is well known that inter-facialmodication signicantly affects the device performances.In the case of QD/metal interfaces, previous studies have showedthat the strong Fermi-level (EF) pinning occurs at the interfacedue to the surface states of QDs,16,44 which disturbs the forma-tion of the Schottky barrier between QDs and metal. As a result,the QD/metal Schottky junction exhibits insufficient bandbending, regardless of the work function of metals,16 whichleads to low open-circuit voltages (VOC) and hence to low powerconversion efficiencies (PCEs) in various types of QDSCs.45–47

Recent studies have shown that modication of interface prop-erties effectively improves their device performances.46–51 Forexample, introducing a thin LiF layer at the PbS/metal interfacesuccessfully passivated localized traps in Schottky-junctionQDSCs.47,50 In addition, air annealing and post-annealing thioltreatment on QD lms successfully modied the interfaceproperties in PbS QD/organic bilayer solar cells.49 However, tothe best of our knowledge, no systematic study for under-standing of the oxidation of QDs, which improves interfaceproperties at the QD/metal interface, has been performed.

Here, we demonstrate that the simple oxidation of PbS QDsat the PbS/metal interface signicantly improves the deviceperformance and air stability in Schottky junction QDSCs.Instead of introducing an additional oxide material, we con-trollably produce a thin oxidized layer at the PbS/metal interfacethrough mild-oxidation treatments of the PbS QD lmincluding air annealing at 60 �C, UV/ozone treatment, and airexposure at room temperature (RT): these treatments convertthe top-most PbS QDs to a thin wide-band-gap oxidized layer.We show how to tailor the interface properties of PbS/metalthrough these processes, allowing the minimization of interfacestates at the PbS/metal junction. As a result, a device with anoxidized interfacial layer shows a signicantly larger degree ofband bending at the Schottky junction and a higher value ofshunt resistance (RSH), leading to a much improved PCE of3.39% (under AM1.5G illumination), which is 670% higher thanthat of a device without oxidized interfacial layer. Besides this,the oxidized interfacial layer signicantly enhances the airstability of Schottky junction QDSCs to more than 500 hours,with a retention rate >55%, by acting as robust protection forthe PbS/metal interface from air.

2. Experimental section2.1 Chemicals

Lead(II) oxide powder (PbO, 99%), 1-octadecene (ODE, technicalgrade90%), oleic acid (OA, technical grade 90%),oleylamine (OLA,technical grade70%),hexamethyl-disilathiane ((TMS)2S, synthesisgrade), and 1,2-ethanedithiol (EDT, 97%) were purchased fromSigma-Aldrich and used without further purication.

2.2 Synthesis of PbS quantum dots

PbS QDs with a rst exciton peak at 910 nm (1.36 eV) weresynthesized with a standard Schlenk line under an Ar atmo-sphere. PbO (0.9 g), OA (3mL), and ODE (33mL) were mixed in a

This journal is © The Royal Society of Chemistry 2014

three-neck ask and heated to 120 �C under a vacuum for 2 hand then lled with Ar. A stock solution, 0.36 mL of (TMS)2Sdissolved in 10 mL of ODE, was swily injected into the ask forPbS QD synthesis. Then, the QD solution was slowly cooled toroom temperature. Aer washing the QDs three times withacetone and toluene, PbS QDs were dissolved in 6 mL toluene,and 2 mL of OLA was added to the solution. The QD solutionwas stored in a glove box for 3 days and precipitated usingmethanol. The OLA process was repeated and the QDs powderwas stored in a glove box until use.

2.3 Device fabrication

All fabrication steps were done in a fume hood. The PbS QD thinlms were fabricated via layer-by-layer (LbL) deposition.52 PbSQDs were prepared in octane solution with a concentration of60mgmL�1 and were spin-coated on cleaned ITO glasses. Then,5 drops of 1% v/v EDT in acetonitrile solution were cast on thesamples, followed by 10 drops of acetonitrile and octane to washaway residual EDT and detached ligands. All of the spin coatingprocess was done at a speed of 2500 rpm and the solutions werecast while the substrate was rotating. As-fabricated devices werekept in a glove box for 1 day and air-annealed at 60 �C in a dryoven for various times prior tometal electrode deposition. Usinga thermal evaporator at a pressure of 4 � 10�6 Torr, electrodeswere deposited sequentially: 0.9 nm of LiF, 100 nm of Al, and120 nm of Ag. An active device area (3.14 mm2) was determinedby the overlapped area between the ITO and themetal electrode.

2.4 Characterizations

The surface morphologies and cross-section images of PbS QDlms were observed using a eld emission scanning electronmicroscope (FE-SEM, Hitachi, S-4800). UV-visible absorptionspectra of PbS QD lms were measured using a UV/visiblespectrophotometer (Mecasys, Optizen POP, Korea). Chemicalbonding of PbS QD lms were analyzed by X-ray photoelectronspectroscopy (VG Scientic, ESCALAB 200i). The XPS depthproling was done with 1 keV Ar+ beam at a current of 1 mA.Ultraviolet photoemission spectroscopy (UPS) data wereobtained using a He lamp at 21.2 eV incorporated into an ultra-high vacuum (UHV) chamber with a base pressure of 10�10 Torr.

2.5 Device measurement

The current–voltage characteristics were obtained using an I–Vmeasurement system (WonATech,WBCS3000) in air without anyencapsulation. The I–V sweeps were done between �1 and +1 V,with a step size of 0.015 V. The devices were illuminated throughthe glass substrate using a 150 W Xe lamp with an AM1.5G lter(LS-150-Xe, Abet Technologies). The light intensity was adjustedto 100 mW cm�2 using a Si reference cell (BS-520, Bunko Keiki).

3. Results and discussion3.1 Fabrication of QDSC devices embedded with an oxidizedinterfacial layer

Fig. 1a schematically describes the procedure for the formationof an oxidized interfacial layer and the fabrication of QDSC

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devices. Oleic-acid-capped PbS QDs with an average diameter of3 nm were synthesized (see the methods section for moredetails on synthesis), and then the ligands were pre-exchangedto oleylamine,52 a less strongly bound ligand. The PbS QD lmswere produced via a layer-by-layer (LbL) technique that con-sisted of the spin-casting of PbS colloidal QD solution andligand exchange with an ethanedithiol (EDT) step.52 The LbLdeposition steps were repeated ve times to obtain a 160 nm-thick PbS QD lm with an estimated optical band gap energy of1.26 eV (Fig. 1b). Aer being stored in an Ar-lled glove box for 1day, the PbS QD lms were processed with various mild-oxida-tion treatments, including thermal annealing in air at 60 �C (airannealing), UV/ozone treatment (UV/ozone), and air exposure atroom temperature (RT air exposure). Then a metal electrodecomposed of LiF/Al/Ag was deposited on the treated QD lmsusing thermal evaporation to form Schottky junction solar cells.The scanning electronmicroscope (SEM) image in Fig. 1b showsthe device structure of our PbS QDSCs.

3.2 Characterization of device performance

We rst measured the current density–voltage (J–V) character-istic of the pristine-QD (without any oxidation treatment) device(Fig. 2a). Under light illumination of the device, partial satura-tion of current density at V > VOC was observed, while the darkJ–V curve shows normal diode behavior. This effect can beoriginated from EF pinning at the interface,53,54 due to a largenumber of trap states at the PbS/metal interface (further anal-ysis will be provided in Fig. 6). However, as can be seen inFig. 2b, a short air-annealing process prior to metal depositionin a Schottky junction QDSC completely eliminated the partialsaturation of current, leading to signicant increases of all the

Fig. 1 Device structure of the PbS Schottky QDSCs. (a) Schematicillustration of the device fabrication process. (b) False-color cross-section scanning electron microscopy (SEM) image of the 30 min-air-annealed-QD device. Scale bar ¼ 100 nm.

Fig. 2 Change of device performances of the PbS Schottky QDSCswith mild air-annealing (60 �C). (a) J–V characteristics of the pristine-QD device in the dark (black) and under AM1.5G illumination (red)conditions. (b) J–V characteristics of the devices with different air-annealing times under AM1.5G illumination. The performanceparameters for the devices are listed in Table 1. (c) Changes of VOC, JSC,FF, and PCE depending on air annealing time. Empty circles representthe best values, and filled circles and error bars represent the averageand standard deviation across four to six devices, respectively.

3054 | Energy Environ. Sci., 2014, 7, 3052–3060

solar cell performance parameters such as short-circuit currentdensity (JSC), VOC, ll factor (FF), and PCE. Moreover, the J–Vcharacteristics of the air-annealed-QD devices were stronglydependent on the annealing time. Detailed performanceparameters of the representative devices are listed in Table 1:

This journal is © The Royal Society of Chemistry 2014

Table 1 Performance parameters of PbS QDSCs under AM1.5G illumination. RS and RSH were obtained from vV/vJ for the dark J–V curve at V ¼0.6 V and V ¼ �0.1 V, respectively

Air-annealing time at 60 �C JSC (mA cm�2) VOC (V) Vbi (V) FF (%) RS (U cm2) RSH (U cm2) PCE (%)

Pristine (0 min) 8.98 0.156 0.184 31.4 5.9 2.9 � 102 0.445 min 12.16 0.450 0.465 50.3 7.4 7.5 � 103 2.7530 min 11.07 0.487 0.497 62.9 10.9 3.6 � 104 3.3960 min 7.66 0.407 0.432 52.9 12.8 1.9 � 104 1.6590 min 2.73 0.205 0.207 35.7 38.4 1.6 � 104 0.20

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our best device (treated for 30 min at 60 �C) exhibited a JSC of11.07 mA cm�2, a VOC of 0.487 V, an FF of 62.9%, and a PCE of3.39%, which values correspond to a 1.2-fold increase in JSC, a3.1-fold increase in VOC, a 2.0-fold increase in FF, and a 7.7-foldincrease in PCE compared to those values for the pristine-QDdevice. Notably, our method increased not only the VOC, FF, andPCE, but also the JSC of the device, which is in contrast to theprevious work that employed air annealing in QD/organicbilayer solar cells.49 This improvement of the JSC can be attrib-uted to the well-controlled formation of the ultrathin oxidizedlayer conned at the lm surface (Fig. 3c and d), which processwill be discussed in detail. Similar enhancement of deviceperformance was also obtained by UV/ozone treatment for ashorter time (0–5 min) and by RT air exposure treatment for alonger time (30–360 min) (see Fig. S2 and S3 in ESI†), whileannealing in the Ar-lled glove box had almost no effect on theJ–V characteristics of the devices (Fig. S4 in ESI†). These resultssuggest that the enhancement of device performance stemsfrom the controlled oxidation of the QD lm.

It is important to note that an appropriate degree of oxida-tion is crucial for the maximization of device performance. Ascan be seen in Fig. 2c, the performance parameters of thedevices show a volcano-shaped curve as a function of air-annealing time. The VOC and FF values increased with anneal-ing time up to 30 min and decreased for longer annealing,whereas JSC increased up to 5 min. To understand these trends,we extracted the values of the shunt resistance (RSH) and theseries resistance (RS) from the dark J–V curve (Table 1). Notably,the air annealing increased RSH up to 30 min, which resulted inan almost two orders of magnitude higher value for RSH for the30 min-air-annealed-QD device compared to that of the pristine-QD device. It implies that the controlled oxidation minimizesnon-radiative recombination at the interface, consequentlyenhancing the device performance. In contrast, RS monoto-nously increased with the annealing time due to the formationof oxides. For longer oxidation treatment, the high RS of thedevice suppresses the electron transport, leading to a reductionof device performance.

We also estimated the built-in voltages (Vbi) of each device toquantify the effect of oxidation on band bending of the Schottkybarrier. In a typical Schottky diode, Vbi can be calculated byextrapolating from the voltages at which the photocurrent (JLight� JDark) becomes zero.16,55 Table 1 shows the Vbi values; thesevalues were calculated from the light J–V curves shown in Fig. 2band the dark J–V curves shown in Fig. S5.† The value of Vbi withrespect to air-annealing time showed almost the same trend

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with the value of VOC, which suggests that the air-annealed-QDdevices exhibited a large amount of band bending compared tothat of the pristine-QD device. This large band-bending maylead to the increase of the hole-injection barrier and theenhancement of VOC and FF. This result is consistent with adecrease in the forward dark current density for the air-annealed-QD devices, indicating that undesirable hole-injectionto the metal electrode can be signicantly reduced with theincrease of the Schottky barrier height (Fig. S5 in ESI†).

3.3 Spectroscopic analyses on the oxidized interfacial layerand modeling of the energy band diagram

To investigate the origin of oxidation-induced enhancement, weanalyzed the air-annealed-QD lms in detail. SEM images of thelms (Fig. 3a) show that air annealing did not induce anychanges in the thin lm morphology. Although there are someminor intrinsic cracks resulting from the LbL depositionprocesses, no noticeable difference was observed between thesamples before and aer air annealing. While as-synthesizedPbS QD solutions were highly stable in air for a long storagetime (see Fig. S6 in ESI†), the top-surface of the PbS QD lmswas effectively oxidized during the air annealing treatment aerligand exchange with a short-chain ligand (EDT) to replace theelectrically insulating long-chain oleate ligands.27,56,57 X-rayphotoemission spectroscopy (XPS) analysis reveals that airannealing of the PbS QD lm produces a few oxygen-containingcompounds (PbO, PbSO3, and PbSO4), as can be seen in Fig. 3b.While the pristine-QD lm showed almost negligible oxygencontent, the air-annealed-QD lms contained proportionallyincreasing amounts of oxygen with air-annealing time, i.e., 5.85and 15.41 at% of oxygen for annealing times of 30 min and 90min, respectively. The S2p XPS spectra for the air-annealed PbSQD lms (see Fig. S7 in ESI†) are consistent with the O1s spectrain Fig. 2b in that both sets of data indicate partial oxidation ofthe top-most QD layer. The atomic ratio of the three compoundsfor the 30 min-annealed sample was measured to be 13.87% forPbO, 43.70% for PbSO3, and 42.43% for PbSO4.

Because XPS is a very surface-sensitive analysis method(typical probe depth � a few nm), we also performed XPS depthproling analysis in order to identify whether the oxidecompounds are produced deep inside the lm. The 30 min-air-annealed-QD lm, which showed the best device performanceas mentioned above, was used for the depth proling analysis.Fig. 3c indicates that the interior of the air-annealed lm con-tained surprisingly little elemental oxygen, to the degree that its

Energy Environ. Sci., 2014, 7, 3052–3060 | 3055

Fig. 3 Characterizations of pristine and air-annealed PbS QD films. (a) The top-view SEM image of the PbS QD films: pristine, air-annealed for 30min, and air-annealed for 90 min. Scale bar ¼ 200 nm. (b) XPS O1s spectra of the air-annealed PbS QD films. Dashed lines indicate the differentchemical bonding states of oxygen (529.3 eV for PbO, 530.9 eV for PbSO3, and 531.7 eV for PbSO4).45 (c) XPS depth profiling analysis of the 160nm-thick PbSQD film on ITO glass (air-annealed for 30min). (d) Transmittance spectra for the pristine and air-annealed (for 30min) PbSQD filmson the glass substrate.

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O1s peak intensity level is comparable with that of the back-ground noise. This result is consistent with a previous obser-vation involving air-exposed PbSe nanocrystal lms17 andsupports that most of the oxygen-containing compounds wereformed at the surface. The estimated average thickness of theoxidized layer is <5 nm (see Fig. S8 in ESI†). The overall degreeof oxidation in the QD lms can also be qualitatively estimatedby their optical absorption spectra because signicant oxidationof QDs should cause a substantial blue-shi in the absorptionpeaks due to the reduced diameter of unoxidized QDs comparedto that of the original QDs.17,49,57–59 However, the 30 min-air-annealed-QD lm showed an absorption spectra that wasalmost identical to that of the pristine-QD lm (Fig. 3d), whichsupports the idea of negligible oxidation inside the lm duringthe short, low-temperature air-annealing process. Furthermore,in order to verify that the improvement of device performancecomes from surface oxidation of the QD lm, we tested theoperation of a device which is annealed aer each of LbLdeposition steps. Such a device would be expected to have anoverall oxidized QD lm and it experimentally shows low Jsc, VOCand FF (see Fig. S9 in ESI†).

In order to investigate the role of the oxidized layer in moredetail, we measured the interfacial energy levels between the

3056 | Energy Environ. Sci., 2014, 7, 3052–3060

PbS QD layer and the oxidized interfacial layer using ultravioletphotoelectron spectroscopy (UPS) analysis of the pristine-QDlm and the air-annealed-QD lm (Fig. 4a). As shown in Fig. 3, ashort air-annealing produces an ultrathin oxidized-QD layer,and thus an underlying unoxidized-QD layer with much greaterthickness can also be detected during the measurement. Wehence prepared a 90 min-air-annealed-QD lm for the UPSmeasurement of the oxidized layer. The work function (4) wasderived from the high binding energy cutoff, where the spec-trum ends. The EVB (valence band maximum energy) withrespect to EF was extrapolated according to the linear portionfrom the proper valence band (VB) peak to the energy axis at thelow binding energy side,46 as indicated in Fig. 4a. The lightsource used for the UPS characterization was a He lamp emit-ting at 21.2 eV (He I radiation). From the measurement data, theestimated 4 of the pristine-QD lm is 4.69 eV (¼ 21.2–16.51 eV),and that of the oxidized-QD lm is 4.45 eV (¼ 21.2–16.75 eV).The vacuum level shi of 0.24 eV between the pristine PbS layerand the oxidized interfacial layer can be explained by theinterfacial dipole among them, as demonstrated at the PbS/MoOx interface.46 For the oxidized sample, the UPS measure-ment indicated an EF � EVB of 1.56 eV, and the UV/Vis spec-troscopy provided an optical band gap of 2.96 eV (see the details

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Fig. 4 UPS analysis of the PbS QD film surface. (a) UPS cutoff spectrafor the pristine-QD and air-annealed-QD (for 90 min) films. (b)Energy-level diagram deduced from the UPS data in panel (a) showingthe energy levels of the conduction bandminimum (ECB), valence bandmaximum (EVB), and Fermi energy (EF) of the PbS QD film and oxidizedQD layer.

Fig. 5 Schematic illustration of the energy band structure near theQD/electrode interface. The main effect of the oxidized interfaciallayer is the enhancement of the Schottky barrier height and of thehole-injection barrier. Jphoto and Jdark are the photocurrent and thedark current, respectively.

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in Fig. S10†). Using these values, energy band diagrams of thepristine and oxidized-QD layers were developed and are shown,respectively, in Fig. 4b. The thin wide-band-gap oxidized layer isexpected to act as a considerable hole-injection barrier and toeffectively suppress the dark current and thus increase VOC andFF, whereas a thick oxidized layer, formed in the case of pro-longed oxidation, hinders the photocurrent collection due tothe increased electron tunneling distance.

Using the results that have been presented so far, we suggestthe following model that can account for the enhancement ofdevice performance with the oxidized interfacial layer. Thepristine-QD device inherently has a large number of interfacestates that originated from the defects at the PbS/metal inter-face. As can be seen in Fig. 5, these interfacial states causesignicant EF pinning and reduce the Schottky barrier height,and thus this barrier cannot effectively block the dark current.For the air-annealed-QD device, the incorporation of anoxidized interfacial layer minimizes the defects at the interface,signicantly increasing the Schottky barrier height anddecreasing interface recombination. Moreover, the interfacialdipole between the pristine PbS layer and the oxidized interfa-cial layer can also enhance the degree of band bending.46,60

Therefore, the air-annealed-QD device achieves a signicantly

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larger Schottky barrier height, which results in a greatenhancement of the device performance. We believe that thesephenomena and mechanisms are analogous to those ofconventional silicon metal–oxide–semiconductor solar cells, inwhich the device performance can be signicantly improved byemploying a very thin oxide layer to separate the metal and thesemiconductor.61,62 The deposition of ultrathin metal oxides inthe form of particles or a dense lm on the QD lm would alsoprovide similar enhancement of device performance.

3.4 Light-soaking test and investigation of interface states

We now investigate the change of device performances uponlight-soaking of QDSC to gain further insight into the role of theoxidized interfacial layer. For this experiment, the devices werecontinuously exposed to AM1.5G illumination. As can be seen inFig. 6a, the pristine-QD device exhibited a low VOC of 0.154 Vand a JSC of 8.94 mA cm�2 on initial light exposure. However,continuous illumination of the device signicantly enhancedthe device performance. Aer 20 min of light-soaking, anenhanced VOC of 0.352 V and a JSC of 12.24 mA cm�2 wereobtained. These enhancements due to light-soaking can beattributed to the lling of the interface states by photoexcitedcarriers,63,64 leading to the increase of the quasi-Fermi leveldifferences.

In order to identify whether the light-soaking effect for thepristine-QD device is permanent or temporary, aer the rstlight-soaking test, we stored the devices under dark conditions

Energy Environ. Sci., 2014, 7, 3052–3060 | 3057

Fig. 6 Effect of light soaking on the device performance. (a) J–Vcharacteristics of the pristine-QD device with different light-soakingtimes and (b) the change of VOC for the pristine-QD device and the air-annealed-QD (for 30 min) device under continuous AM1.5Gillumination.

Fig. 7 Air-stability test of the QDSCs. Normalized power conversionefficiencies are shown as a function of the air-storage time. Eachsymbol represents the devices with different air-annealing times. Theinset data present the time decay exponents.

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for 24 hours and resumed the illumination experiment. Thisprocedure was repeated one more time. Fig. 6b shows the VOCvariations as a function of light-soaking time for the pristine-QDdevice and for the 30 min-air-annealed-QD device. The initialvalue of VOC during the second and the third light-soaking testswas signicantly lower than the last VOC values in the previoustests. However, similarly to the case of the rst light-illumina-tion, during the second and the third tests, the pristine-QDdevice exhibited a rapid increase of VOC with illumination time.These drops and recoveries of VOC support the lling andevacuation of the interface states at the PbS/metal uponphotoexcitation. The storage of the light-soaked device (pris-tine-QD device) under dark conditions signicantly reducedVOC, which can be understood by considering the release of thetrapped charges from the states and consequent elimination ofthe enhancement effect of VOC. Themaximum VOC values for thesecond and the third illumination tests were lower than that ofthe rst test, which phenomenon is related to the generalstability issue of QDSCs and will be discussed in detail in thenext part. These results show that light-soaking temporarilyreduced the effect of interface states on the value of VOC of thepristine-QD device. In contrast, the air-annealed-QD device

3058 | Energy Environ. Sci., 2014, 7, 3052–3060

exhibited an almost consistent VOC, which also supports theidea that the incorporation of an oxidized interfacial layermarkedly reduces the interface states at the PbS/metal interfaceand minimizes the density of trap states that can be lled withphotoexcited carriers, achieving higher and stable VOC regard-less of the light-soaking treatment.

3.5 Monitoring of air stability

Another challenging issue in QDSCs is their poor oxidativestability. In general, Schottky junction QDSCs tend to degraderapidly in air, which results in linear current–voltage charac-teristics aer the devices are exposed to air for severalminutes.16,27 To compare the oxidative stability of each air-annealed-QD device, we periodically measured their J–V char-acteristics for 1 month under storage in air. As can be seen inFig. 7, the performance of the pristine-QD device degradedrapidly in a few hours. In contrast, the air-annealed-QD lmspresented greatly improved air stability of the devices: our bestQDSCs with air annealing retained >55% of its initial PCE evenaer 500 hours of air storage time without any passivation. Inorder to quantitatively investigate the effect of the air-annealingprocess on air stability, we dene PCE decay with storage timeas

PCE(l, t) ¼ PCE0e�lt (1)

Here, PCE0 is the initial power conversion efficiency, l is acharacteristic power decay coefficient, and t is the air storagetime. Themeasured l values can be seen in the inset of Fig. 7. Ascan be seen in the inset of Fig. 7, the QDSC with the 30 min-air-annealing shows the lowest decay rate of 0.026 per day, whilethe pristine-QD device has the highest l value of 1.325 per day,indicating a much slower degradation of PCE for the air-annealed-QD device. Moreover, as can be seen in Fig. S11,† theair-annealed-QD devices exhibited quite stable values of VOCand FF. We believe that there are two possibilities regarding the

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role of the oxidized interfacial layer in the high oxidativestability. First, an oxidized layer might act as a diffusion barrier,impeding the oxidation at the PbS/metal interface. Second, itmay prevent reactions between ligands and the metal electrodedue to its placement at the PbS/metal interface. Because QDsare passivated with organic ligands, direct contact between theQD and the metal electrode can cause a reaction that will formorgano-metallic species65 and thus promote the degradation ofthe metal electrode.

In order to investigate the mechanism, we performed an XPSdepth proling analysis on the metal (Al) electrode. Fig. S12†shows a signicantly higher degree of electrode oxidation at theAl/PbS interface side for the pristine-QD cell, suggesting that thepenetration of oxygen into the Al electrode can be relativelyretarded in the air-annealed-QD device. It should be noted thatQD lms contain numerous nanopores, which serve as chan-nels for the supply of oxygen. With the oxidation of Al near themetal/QD interface, the Schottky junction quality will be grad-ually degraded, leading to a decreasing PCE, as shown in Fig. 7.The depth proling analysis data strongly suggest that the thinoxidized layer at the interface can serve as an effective oxygendiffusion barrier against oxidation of the Al electrode, analo-gous to native oxides that prevent further oxidation of metalsand semiconductors.

4. Conclusion

We systematically tailored the PbS/metal interface through theoxidized interfacial layer in Schottky junction QDSCs and reportsignicant enhancements of JSC (by 23%), VOC (by 212%), FF (by100%), and PCE (by 670%) compared to values of a conventionaldevice without an oxidized layer. The controlled conversion ofthe top-most PbS QD layer into a thin oxidized layer, which wasperformed by low-temperature (60 �C) oxidation and which wasconrmed by XPS analyses, considerably increased the values ofVbi and RSH for the improvement of charge collection efficiency.The signicant reduction of interface states due to thin oxidizedlayer formation is well substantiated by the two-orders-of-magnitude higher value of RSH and the stable value of VOC undercontinuous light-illumination, while the pristine-QD deviceshows considerable uctuations of VOC under the same envi-ronment due to the temporary occupation of the trap states byphotoexcited carriers. Another important effect of the oxidizedinterfacial layer that we showed is the substantially improvedoxidative stability of the QDSCs. We believe that this study willprovide better understanding of the effect of interface quality invarious QD-based applications including photovoltaic devices,light-emitting devices, sensors, and so on.

Acknowledgements

This work was supported by the Center for Inorganic Photo-voltaic Materials (no. 2012-0001173) grant funded by the Koreagovernment (MEST).

This journal is © The Royal Society of Chemistry 2014

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