Origin of Unusual Excitonic Absorption and Emission from ColloidalAg2S Nanocrystals: Ultrafast Photophysics and Solar CellWasim J. Mir,† Abhishek Swarnkar,† Rituraj Sharma,§ Aditya Katti,† K. V. Adarsh,§

and Angshuman Nag*,†

†Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India§Department of Physics, Indian Institute of Science Education and Research, Bhopal 462023, India

*S Supporting Information

ABSTRACT: Colloidal Ag2S nanocrystals (NCs) typically do not exhibitsharp excitonic absorption and emission. We first elucidate the reason behindthis problem by preparing Ag2S NCs from nearly monodisperse CdS NCsemploying cation exchange reaction. It was found that the defect-relatedmidgap transitions overlap with excitonic transition, blurring the absorptionspectrum. On the basis of this observation, we prepared nearly defect-free Ag2SNCs using molecular precursors. These defect-free Ag2S NCs exhibit sharpexcitonic absorption, emission (quantum yield 20%) in near-infrared (853 nm)region, and improved performance of Ag2S quantum-dot-sensitized solar cells(QDSSCs). Samples with lower defects exhibit photoconversion efficiencies>1% and open circuit voltage of ∼0.3 V, which are better compared with prior reports of Ag2S QDSSCs. Femtosecond transientabsorption shows pump−probe two-photon absorption above 630 nm and slow-decaying excited state absorption below 600 nm.Concomitantly, open-aperture z-scan shows strong two-photon absorption at 532 nm (coefficient 55 ± 3 cm/GW).

Monoclinic bulk Ag2S with a direct bandgap 1170 nm(1.06 eV) has been used as infrared photodetectors,

thermopower cells, and luminescence-based applications.1

Recently, colloidal Ag2S NCs have been synthesized and usedas a biocompatible near-infrared (NIR) fluorescent probe.2−7

Ag2S NCs are nontoxic; therefore, unlike Cd-, In-, and As-basedNCs, a polymer coating is not required for Ag2S NCs tomitigate the toxicity.8 However, Ag2S NCs typically do notexhibit sharp excitonic absorption and emission unlike othersystems such as CdSe, CdS, InP, and PbS NCs.2,5,7,9−14 Earlystudies of Brelle et al. suggested Ag2S NCs exhibit indirectbandgap and therefore do not exhibit excitonic absorptionfeatures.15 Recently, Wang and coworkers14 mentioned “specialelectronic property” is responsible for the absence of excitonictransitions in Ag2S NCs. Neither the details of this specialbehavior nor the origin of formation of unexpected (sincemonoclinic bulk Ag2S is a direct bandgap semiconductor)indirect bandgap semiconductor was proposed. In a rareoccasion, Jiang et al.16 showed both excitonic absorption andemission from Ag2S NCs without explaining its origin. Clearly,the reason behind the presence or absence of excitonicabsorption and emission from Ag2S NCs is far from beingunderstood.There can be many possible reasons for not observing

excitonic features in absorption spectrum, for example, (i) smallexcitonic binding energy, (ii) the ensemble spectrum may getbroadened because of polydispersity in NC size, and (iii) sub-bandgap transitions involving defect states or cappingmolecules or impurity phase overlapping with the excitonictransition. All of these possibilities are detrimental for various

applications including NIR luminescence for biologicalapplications and optical limiters. Other less exploredapplications such as photovoltaics and photodetectors usingAg2S NCs also require defect-free NCs with narrow sizedistribution. Therefore, understanding the reason behind theabsence of both excitonic absorption and emission in Ag2S NCsis important for both fundamental studies and applications.To address the problem, we prepared Ag2S NCs using two

different pathways. First, Ag2S NCs were prepared from CdSNCs employing postsynthesis cation exchange reaction. Thesecation-exchanged Ag2S NCs exhibit quantum-confinementeffect and narrow size distribution but still do not exhibitsharp excitonic absorption because of overlapping defect-related midgap absorption. On the basis of this learning, weemployed a second pathway of directly preparing Ag2S NCsfree from midgap defect-related absorption and therebyobserved sharp excitonic absorption and emission from Ag2SNCs. The excitonic emission shows a small radiative lifetime of11 ns and high photoluminescence (PL) efficiency of ∼20%.Transient absorption and open aperture z-scan data show bothexcited-state absorption (ESA) and strong two-photonabsorption (TPA) from our Ag2S NCs. Finally, our Ag2SQDSSC exhibits one of highest open-circuit voltages andefficiencies for any Ag2S QDSSC.Ag2S NCs f rom CdS NCs Employing Cation Exchange. To

understand the effect of size distribution and quantum

Received: August 5, 2015Accepted: September 14, 2015Published: September 14, 2015

Letter

pubs.acs.org/JPCL

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confinement effect on the excitonic transitions of Ag2S NCs, weemployed cation exchange reaction (schematically representedin Figure 1a) to synthesize Ag2S NCs from CdS NCs following

ref 17. Such cation-exchange reactions are known to retainalmost the same size of original NCs with minor adjust-ment.17,18 Figure 1b shows a UV−visible-NIR absorptionspectrum of CdS NCs exhibiting four sharp excitonic featuresconfirming narrow size distribution; however, Ag2S NCsobtained after cation exchange of these CdS NCs exhibit abroad absorption spectrum, without any excitonic feature.Powder X-ray diffraction (XRD) patterns in the inset of Figure1b confirm the success of this cation exchange reaction.Interestingly, the Ag2S NCs formed can be reversibly cation-exchanged back to CdS, and the final CdS NCs obtained alsoexhibit sharp excitonic features in the absorption spectrum, as

shown by the red-colored spectrum in Figure 1b. Thisreversible cation exchange (CdS to Ag2S to CdS) experimentconfirms that the size and size distribution of NCs remainedintact throughout the entire cation-exchange processes. There-fore, the Ag2S NCs obtained after cation exchange, despitehaving narrow size distribution, did not exhibit any excitonicfeature.After ruling out broad size-distribution as a possible cause for

the absence of excitonic absorption features, let us study theeffect of quantum confinement of charge carriers in Ag2S NCs.For this purpose, different sizes (1.6, 2.6, and 4.1 nm) of CdSNCs were prepared (Figure S1−S3 in the SupportingInformation (SI)), followed by cation-exchange reactionforming different sizes of Ag2S NCs. Ag2S NCs preparedafter cation exchange of 1.6, 2.6, and 4.1 nm CdS NCs aretermed here as Ag2S-1.6, Ag2S-2.6, and Ag2S-4.1 for theremaining part of the paper. The transmission electronmicroscopy (TEM) image of Ag2S-4.1 in Figure S4 shows anaverage diameter of ∼4 nm. Absorption spectra for Ag2S NCobtained after cation exchange reactions for all sizes of CdSNCs were broad and featureless similar to that in Figure 1b(black spectrum). PL spectra of CdS (Figure S5 in SI) andAg2S NCs (Figure 1c) exhibit the systematic quantumconfinement effect with NC size. All cation-exchanged Ag2SNC samples exhibit broad defect-related peak in the NIRregion with PL efficiency <0.5%.Cation-exchanged Ag2S NCs exhibit both quantum-confine-

ment effect and narrow size distribution, so it is unclear whyexcitonic transition is not observed in absorption spectrum.There may be other transitions that overlap with excitonictransition. In an attempt to remove any such noncrystallinedefects contributing to the absorption spectrum, we annealedthe colloidal Ag2S NCs dispersed in ODE under an inertatmosphere. Interestingly, Figure 1d shows that the excitonicpeak starts appearing at ∼2 eV after annealing at 140 °C. Thismild annealing is not expected to improve the size distribution,nor should it increase the excitonic binding energy. Elementalanalysis using energy-dispersive X-ray analysis (EDAX) showsAg/S ratio remains unchanged (1.9:1) before and after theannealing. Therefore, the only possible explanation for thispartial appearance of excitonic feature after this mild annealingcan be because of improvement of crystallinity through removalof defects.19,20 Further increase in annealing temperature to 200°C leads to the segregation of Ag impurity phase, similar to aprevious report.21 Results of cation exchange reaction infer thatunwanted defect-related broad absorption blurs the features ofexcitonic absorption and quenches the excitonic emission.Therefore, a model synthesis of high-quality Ag2S NCs shouldmainly target to remove such defects.High-Quality Ag2S NCs with Sharp Excitonic Absorption and

Emission. There are some inherent differences of Ag2S NCsystem compared with well-known II−VI NCs such as CdSNC. For example, (i) Ag2S has very low solubility product (6 ×10−51) compared with CdS (8 × 10−28) and thereforenucleation of Ag2S NCs can become uncontrollably favorable,even at lower temperature; (ii) Ag+ ion in Ag2S crystal has highionic mobility unlike Cd2+ in CdS;22 and (iii) Ag+ precursors inthe reaction mixture have a propensity to form Ag metal(standard reduction potential, EAg

+/Ag

o = +0.80 V), whereassynthesis of CdS NCs is not known to suffer from suchimpurity phase (ECd2+/Cd

o = −0.40 V).23 All of these factors canlead to the formation of detrimental defects in Ag2S NCs.Keeping in mind the highly reactive formation of Ag2S and high

Figure 1. (a) Schematic illustration of reversible cation exchangeprocess, CdS to Ag2S, and back to CdS NCs. (b) UV−visible-NIRabsorption spectra of initial CdS NCs (blue) and Ag2S NC (black)obtained after forward cation exchange and regenerated CdS NCs(red) obtained after reverse cation exchange. Blue and red spectrawere normalized at the peak of lowest energy excitonic transitions. Theinset shows XRD patterns of initial CdS NCs and Ag2S NCssynthesized via cation exchange. (c) PL spectra of Ag2S NCs showingsize-dependent change in emission wavelength. (d) Optical absorptionspectra of Ag2S-2.6 NCs (synthesized via cation exchange of 2.6 nmCdS NCs) and subjected to annealing under inert atmosphere using 1-octadecene as solvent. Spectrum for 140 °C annealing (red color) wasshifted vertically upward for clarity.

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mobility of Ag+, a lower reaction temperature and less reactiveprecursors for both Ag+ and S2− appear to be a rational designfor improving the crystallinity of Ag2S NCs. These are probablythe reasons why the popular “hot injection”24 synthesis ofcolloidal NCs at temperatures ∼300 °C has largely failed toproduce high-quality Ag2S NCs. Additionally, because there isthe possibility of formation of Ag metal, excess of sulfurprecursor will help in the formation of Ag2S NCs. Weemployed these reaction conditions for the direct synthesis ofAg2S NCs, as discussed later in the Methods section.Figure 2a shows UV−visible-NIR absorption spectra for Ag2S

NCs prepared at 120 °C at different reaction times. A broad

absorption shoulder appears at ∼650 nm after 10 min ofreaction, followed by emergence of excitonic peak at 800 nmafter 20 min. Importantly, significant absorption is observed atwavelengths longer than 850 nm, that is, at energies lower thanthe optical gap. This low-energy absorption may arise fromdefect-related midgap transitions.25 Corresponding PL data in

Figure 2b show defect-related broad emissions after 10 min ofreaction. Excitonic PL at 858 nm is observed only after 20 minof reaction, along with a more intense defect-related emission at1088 nm (Figure 2b). After 60 min, relative intensity ofexcitonic emission increased compared with defect-relatedemission. PL excitation (PLE) spectra (not shown here) forboth 858 and 1088 nm emission suggest that both emissionsoriginate via excitation of Ag2S NCs, without involving anyseparate impurity phase. These results suggest that the reactionwas slow at 120 °C, where midgap defects dominate theabsorption and PL data until 20 min of reaction, after which theexcitonic peak starts appearing but the midgap defect states arepresent even after 60 min of reaction.Consequently, we increased the reaction temperature to 145

°C. Figure 2c,d shows that the excitonic absorption at 800 nmand emission at 853 nm appear as early as 10 min for thereaction carried out at 145 °C. TEM image (Figure S6 of SI)shows 4.5 nm average diameter of these Ag2S NCs. The XRDpattern shown in Figure S7 confirms the monoclinic phase ofAg2S NCs. Importantly, defect-related absorption tail at longerwavelength (>850 nm) is absent, which leads to the rareobservation of sharp excitonic absorption and emission fromAg2S NCs. A high PL efficiency of 20% is also observed for thisoptimized sample. Please note that it is more difficult to achievehigh PL efficiency in NIR region than in the visible region. Asthe reaction time increases, the PL efficiency decreases, asshown in the inset of Figure 2d. A reaction temperature higherthan 145 °C is again not suitable for both excitonic absorptionand emission, as shown in Figure S8 of SI. Therefore, 145 °C isthe optimized reaction temperature for the synthesis of thesecolloidal Ag2S NCs. Please note that Figure 1d also suggested∼140 °C is the optimum temperature to anneal out defectsfrom cation-exchanged Ag2S NCs. Higher temperatures maylead to the starting of partial segregation of Ag phase.21

We also note that a very high molar ratio of 3-mercaptopropionic acid (3-MPA) to Ag+ ion has beenmaintained to obtain high-quality Ag2S NCs. 3-MPA acts asboth S-precursor and capping ligand. A higher concentration of3-MPA inhibits the formation of Ag impurities. Anotherimportant chemical used in the synthesis is ethylene glycol(EG). When reaction was carried out in the absence of EG bymixing AgNO3 and 3-MPA in dimethyl sulfoxide (DMSO), theformation of Ag2S NCs was too fast even at a low temperatureof 70 °C. (See Figure S9 in the SI.) Such fast and low-temperature reaction conditions lead to Ag2S NCs with a highdensity of midgap defects.Figure 2e shows the PL decay dynamics of optimized Ag2S

NCs (reaction temperature = 145 °C, reaction time = 10 min).The decay can be fitted with biexponential decay with lifetimes∼1 and 11 ns. The shorter lifetime (∼1 ns or less) is typicallyattributed to nonradiative decay, and we attribute the lifetime∼11 ns to radiative excitonic recombination. This is a uniqueobservation because all prior reports of Ag2S NCs showeddecay with a significant contribution from longer lifetimecomponent >50 ns and which can even extend to hundreds ofnanoseconds.12,14 This faster decay of our Ag2S NCs is thecharacteristic of excitonic emission, whereas prior reportsmeasured lifetime of defect-related emission. To study thelonger wavelength tail of the asymmetric PL from optimizedAg2S NCs prepared at 145 °C, PL decay dynamics wasmeasured at different emission wavelengths, as shown in FigureS10 in SI. The decay profile remain fast and similar for theemission wavelength range of 820 to 950 nm, suggesting the

Figure 2. Temporal evolution of UV−visible-NIR absorption andcorresponding PL spectra of Ag2S NCs synthesized directly frommolecular precursors at different reaction temperatures. (a,b) Forreaction carried out at 120 °C and (c,d) reaction carried out at 145 °C.(e) PL decay dynamics of Ag2S NCs, prepared at 145 °C for 10 min.(f) Schematic illustration of possible electronic absorption andemission pathways in Ag2S NCs. Midgap defects in Ag2S blursexcitonic absorption and quenches excitonic emission. Removal ofsuch midgap defects resulted in excitonic absorption and emission forour optimized Ag2S NCs.

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domination of excitonic emission; however, further increase inthe emission wavelength systematically slows down decay.These observations suggest contribution of defect-relatedemission toward the longer wavelength tail of the PL spectrum.In Figure S11 of the SI, we present lifetime data of one of oursamples that exhibit both excitonic emission and defect-relatedemission. Clearly, defect-related emission has the longerlifetime component, whereas PL decay of excitonic emissionof the same sample is significantly faster.Figure 2f schematically summarizes the discussion regarding

excitonic transition and defects in Ag2S NCs. Unlike II−VIsemiconductor NCs, midgap defects in Ag2S NCs exhibitsignificant absorption. The exact nature of these defect statesthat obliterate excitonic feature from absorption spectrum ofAg2S NCs is not yet known. Surface capping of our Ag2S NCsremains the same for all samples (Figure S12 in SI), but someoptimized samples do not exhibit defect-related absorption,whereas other samples show significant defect-related absorp-tion. These results suggest that defects exhibiting midgapabsorption might be located inside the lattice itself. Comparedwith surface defects, a lattice defect is expected to have a betterspatial overlap with the delocalized valence and conductionband electronic wave functions and therefore can exhibit ahigher transition probability involving localized lattice defectand delocalized valence or conduction band.19,20,42 Pointdefects in the lattice of Ag2S NCs can arise from Ag+

vacancy/interstitial because of high ionic diffusion capabilityof Ag+.32,26 The possibility of any other kinds of crystal defectcannot be ignored at present. So, we speculate that the latticedefect is the primary cause of not observing excitonicabsorption and emission in Ag2S NCs, and our optimizedreaction conditions lead to better crystallinity of Ag2S NCsremoving unwanted lattice-defects, which, in turn, exhibit sharpexcitonic absorption and emission.Ag2S QDSSC. The previously described results showed that

modification of synthesis protocols can minimize the defectdensity in colloidal Ag2S NCs, which leads to the observation ofexcitonic absorption and emission. We will now correlate thisinfluence of defect levels on the performance of Ag2S QDSSC.Colloidal Ag2S NCs with broad absorption spectra and low-toxicity can be a useful QDSSC material. A recent ultrafastspectroscopic study also suggested multiple exciton generationvia absorption of a single high-energy photon.27 A survey ofliterature shows no report of QDSSC starting from colloidalAg2S NCs exhibiting excitonic feature. There are reports wherethe Ag2S sensitization layer was grown by successive ionic layeradsorption and reaction (SILAR), but broad bulk-likeabsorption spectra were observed for all such cases.28,29

Figure S13a in the SI shows current density (J) versusvoltage (V) characteristics of QDSSC prepared using cation-exchanged Ag2S-4.1 NCs exhibit an open-circuit voltage (VOC)= 0.23 V with power conversion efficiency (η) = 0.17%. Wethen used colloidal Ag2S NCs prepared by direct synthesisusing molecular precursors as sensitizers in QDSSCs. As shownin Figure 3 (red curve), QDSSC prepared using Ag2S NCssynthesized at 145 °C exhibits J−V characteristics with VOC =0.25 V, short-circuit current density (JSC) = 4.7 mA·m−2, and fillfactor (FF) = 0.37, resulting in η = 0.43%. These solar cellparameters are significantly better compared with those (FigureS13b) obtained by using Ag2S NCs prepared at 120 and 155°C. This higher VOC and JSC for Ag2S NCs synthesized at 145°C is attributed to lower defects in this sample compared withthe other two samples. Optical data shown in Figure 2 and

Figure S8 suggested that Ag2S NCs prepared at 145 °C exhibitfewer defects compared with NCs prepared at both 120 and155 °C. This agreement among results obtained fromindependent experiments using UV−visible-NIR absorption,emission, and QDSSC data, suggesting that Ag2S NCs preparedat 145 °C from molecular precursors have lesser defect, isnoteworthy.It was observed that the performance of a QDSSC improves

significantly by suppressing electron−hole recombination atboth FTO/electrolyte and TiO2/electrolyte interfaces.30,31 Weused compact TiO2 layer on FTO to suppress recombinationsacross the FTO/electrolyte. To suppress recombination acrossthe TiO2/electrolyte, we have grown one monolayer (ML) ofAg2S using SILAR on top of Ag2S-sensitized TiO2 photoanode.Such SILAR deposition is expected to remove any pinholesfrom sensitization layer, thereby inhibiting interaction betweenTiO2 and electrolyte. Expectedly, the black curve in Figure 3shows the JSC and VOC increased to 8.7 mA·cm−2 and 0.30 V,respectively, after depositing 1 ML Ag2S on top of Ag2S-sensitized TiO2 photoanode, resulting into η = 1.04%.Our control experiment with sensitization of mesoporous

TiO2 via depositing 1 ML Ag2S using SILAR, in the absence ofAg2S NCs, did not yield any solar cell characteristic; however,we got poor efficiency (η = 0.17%) and lower VOC = 0.2 V for 3ML Ag2S solar cell, as shown in Figure 3 (blue curve). We notethat the prior reports suggests similar VOC (<0.25 V) for Ag2S-based solar cells obtained using chemical bath deposition(CBD),32 sonochemical method,29 or Ag2S-polymer hybridstructure.33 There is one report based on SILAR growth Ag2SQDSSC showing VOC > 0.3 V, but the obtained JSC was less,yielding η < 1%.28 These enhanced VOC and JSC for our Ag2SQDSSCs again suggest the defect-free nature of synthesizedAg2S NCs.34 To the best of our knowledge, the obtained η =1.04% is the highest reported so far using Ag2S QDSSC. Toimprove η of Ag2S QDSSC, VOC needs to be further increased.The search for better electrolyte or hole transport layer and

Figure 3. Current density (J) versus voltage (V) plot for QDSSCsprepared using optimized Ag2S NCs synthesized at 145 °C, with 10min reaction time. In one case (black), 1 monolayer (ML) Ag2S wasgrown on the Ag2S NC-sensitized photoanode to fill up any pin-holesin the sensitization layer, which, in turn, reduce the loss due tointeraction of electrolyte with TiO2. Also, comparison of data is shownwith solar cell sensitized by 3 ML of Ag2S (blue) grown by SILARmethod but without having colloidal Ag2S NCs. Effective cell area wastypically kept in the range of 0.24 to 0.28 cm2 for all cells.

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optimization of device fabrication minimizing electron−holerecombinations and enhancement of photoabsorption isrequired to improve performance of Ag2S QDSSC.Ultrafast Spectroscopy of High-Quality Ag2S NCs. After

optimizing the excitonic absorption, emission, and solar-cellproperties of colloidal Ag2S NCs, by minimizing the defects,here we show the ultrafast spectroscopy of the optimizedsample. To the best of our knowledge, there are only two priorreports on femtosecond (fs) ultrafast optical absorption studiesof colloidal Ag2S NCs, and both were probed in the NIRregion.15,27 Also, there are two reports on nanosecondnonlinear optical absorption of Ag2S NCs.21,35 Out of thesespectroscopic investigations, three (refs 15, 21, and 35) studieswere carried out on Ag2S NCs that do not exhibit any excitonicabsorption. Clearly, ultrafast spectroscopy of high-quality Ag2SNCs has remained largely unexplored, which motivated us tostudy the femtosecond transient absorption (TA) of Ag2S NCsprobing the visible region (450 to 730 nm) and nanosecondTPA at 532 nm.Figure 4a shows contour plot of the TA of the Ag2S NCs

prepared at 145 °C after 10 min of reaction. For better

understanding, several cross sections of the contour (Figure 4a)are plotted in Figure 4b. Pump beam induces a broad TA thatcontains two distinct regions >630 and <600 nm. TA can arisebecause of various reasons such as excited-state absorption(ESA) or pump−probe TPA (or multiphoton absorption).Pump excites electrons from valence band to conduction band,which thermalizes to the lowest energy excitonic state (orbottom of conduction band) and even trap states. This excitedstate can absorb probe light giving ESA. On the contrary,pump−probe TPA occurs because of simultaneous absorptionof two photons of different wavelength, and transition occurs toa level whose energy is equal to hυpump+ hυprobe. Suchsimultaneous TPA occurs only when the pump and theprobe beams overlap temporally and therefore have faster decaytime compared with ESA that involves sequential absorption of(1 + 1) photons without requiring a temporal overlap.

Strikingly, strong and fast-decaying TA was observed inspectral range >630 nm as compared with <600 nm. At thispoint, we conjecture that TA in <600 nm may be due to theESA because of slower decay time, and fast decaying TA in>630 nm is mostly originating from the pump−probe TPA.ESA (<600 nm) may originate from both lowest energyexcitonic state and midgap defect states that trapped the excitedelectron; however, it has been previously observed that ESAoriginating from deep-trap states appears on a longer time scale(more than a few tens of picoseconds) and ESA originatingfrom conduction band or lowest energy excitonic statedominates on shorter time scales.15 Therefore, the observedESA can be attributed mostly to the lowest energy excitonicstate (or bottom of conduction band). These ESA results agreewith steady-state absorption data (Figure 2c) showing defect-free high-quality Ag2S NCs exhibiting sharp excitonic features.To develop a better understanding of TA in the spectral

region >630 nm, which has been assigned as pump−probeTPA, we have shown in Figure 4c the temporal evolution of TAat 730 nm. The temporal full width at half-maximum (fwhm) isfound to be ∼530 fs, and the asymmetry in width is due toinduced phase modulation and some contribution from long-lived absorption. Because the growth and decay of TA (fwhm)is in femtoseconds, it suggests the pump−probe TPA; however,the fwhm (cross-correlation) is slightly increased, whichindicates that the pulses are broadened in our experiments.Furthermore, we probed >630 nm region using different

excitation pump energy of 560 nm. This excitation is still higherthan the bandgap of Ag2S NCs but does not satisfy theconditions for pump−probe two-photon absorption previouslydiscussed because {hυ560 nm + hυ(640−740 nm)} < {hυ400 nm +hυ(640−740 nm)}. So if the mechanism is two-photon absorption,then we should not see any TA in the probe range of 640−740nm for 560 nm excitation. Figure S14 in the SI indeed showsthe absence of TA with 560 nm excitation, as opposed to TAobserved in Figure 4 with 400 nm excitation. We note that ourexperimental setup is not suitable to probe the lowest energyexcitonic transition at 800 nm mainly because the white-lightprobe is generated from the 800 nm.After demonstrating the pump−probe TPA in the wave-

length region 630−740 nm, it is of natural interest to seeksingle wavelength TPA to confirm the credibility of our claim.In this context, we have performed open-aperture Z-scanexperiment. The excited beam in this experiment was selectedas 532 nm (2.33 eV) such that its energy is approximately(hυ400 + hυ(640−740 nm))/2. Figure 4d shows the normalizedtransmittance recorded at three different peak intensities. Thepeak intensities written in Figure 4d are measured at z = 0 (atthe focal point), and the sample is moved in both the + veand−ve directions of z (symmetric points), where the intensityvalues decreases systematically. Figure 4d shows a clear dip innormalized transmittance when z approaches zero, implyingthat a new absorption channel arises at higher input intensitynear z = 0. This intensity-dependent absorption establishesstrong nonlinear optical absorption from our Ag2S NCs. Theexperimental data could be fitted well considering TPA. Theobtained TPA coefficient is reasonably high (55 ± 3 cm/GW),therefore making our Ag2S NCs a suitable candidate forapplications such as optical limiter.In conclusion, apart from narrow size distribution, the

removal of midgap defects is an important criterion forobserving sharp excitonic absorption in colloidal Ag2S NCs,which are more prone to form lattice defects compared with

Figure 4. (a) Contour plot of femtosecond time-resolved TA spectraof optimized Ag2S NCs prepared at 145 °C with 400 nm excitation.(b) TA at various time delays between pump and probe beams. (c)Temporal evolution of TA at 730 nm. (d) Normalized transmittanceas a function of sample position in open aperture z-scan. Symbols andsolid line represent experimental data and theoretical fit, respectively.

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other semiconductor NCs such as CdS, CdSe, and PbS. Thesedefects in Ag2S NCs lead to broad absorption that overlap withexcitonic transitions blurring the excitonic absorption andquenches the excitonic PL. After realizing that the midgapstates arising from lattice defects are the major problem, defect-free colloidal Ag2S NCs (4.5 nm diameter) were preparedstarting from molecular precursors. These defect-free Ag2S NCsexhibit sharp NIR excitonic absorption at 800 nm and excitonicPL at 853 nm. High efficiency (20%) was observed for the NIRemission along with fastest PL decay (two lifetimes, non-radiative ∼1 ns and radiative 11 ns) reported so far for Ag2SNCs. Similar to optical properties, the performance of QDSSCsalso exhibits a strong dependence on defects. The optimized(with less defect) Ag2S NCs exhibiting excitonic absorption andintense excitonic emission also exhibit the best solar-cellperformance compared with other samples with more defects.This defect-free nature of our Ag2S NCs leads to animprovement of VOC (0.3 V) and JSC (8.7 mA·cm−2) yieldingη = 1.04%, which is the highest compared with prior reports ofAg2S QDSSC prepared via SILAR, CBD, or sonochemicalmethods. The efficiency needs to be increased further for real-life applications. Femtosecond TA study on the optimized Ag2SNCs reveals a fast decaying pump−probe TPA at wavelength>630 nm and a slow decaying ESA from lowest energyexcitonic state at wavelength <600 nm. A high value (55 ± 3cm/GW) of single-wavelength (532 nm) TPA coefficient isobserved in open-aperture z-scan data.

■ METHODSNC Synthesis. CdS NCs with average diameters in the range of1.6 to 4.1 nm were prepared following ref 36. Cation exchangereactions (CdS NCs to Ag2S NCs) were carried out by treatingdilute dispersion of CdS NCs in toluene with concentratedmethanolic solution of AgNO3 (keeping Cd

2+/Ag+ ratio of 1:4)at room temperature, following ref 17, whereas reverse cationexchange from Ag2S to CdS was done at elevated temperaturearound 80−90 °C. A dilute dispersion of Ag2S NCs in tolueneand 0.2 mL of trioctylphosphine (TOP) was heated to 80 °Cunder an inert atmosphere, followed by injection of excessCd((NO)3)2 in acetonitrile. Details of both synthesis of CdSNCs and subsequent cation exchange reactions are given in theSI.Direct synthesis of 3-MPA capped Ag2S NCs was carried out

after modifying the synthesis reported in ref 16. 0.05 mmolAgNO3 and 10 mL of EG were taken in a three-necked round-bottomed flask and subjected to vacuum and an inertatmosphere alternatively two to three times along withmagnetic stirring at room temperature. The flask was thenheated to 100 °C under an inert atmosphere with continuousstirring, followed by the injection of 1 mL of 3-MPA. A whiteprecipitate was observed, which subsequently became yellowprecipitate followed by a clear solution. Reaction times reportedthroughout this paper considered the color change of whiteprecipitate as the reaction start-time. The reaction temperaturewas set to different temperatures and time, and the optimizedreaction condition was found to be 145 °C for a reaction timeof 10 min exhibiting sharp excitonic absorption and emission.Photographs of the reaction mixture in different stages ofreaction are given in Figure S15 of the SI. The product NCswere precipitated and washed using methanol/ethanol andredispersed in a polar solvent such as DMSO.Femtosecond (fs) Transient Absorption (TA). In our femto-

second time-resolved pump−probe TA measurement, the

output of Ti:sapphire amplifier system (4 mJ/pulse, pulsewidth 120 fs, centered at 800 nm with a repetition rate of 1kHz) is split into two beams to generate pump and probepulses. Second harmonics of first beam (400 nm, using β-barium borate crystal) with an average fluence of 500 μJ/cm2

was used as the pump beam. The second beam was delayedwith a computer-controlled motion controller and then focusedinto a CaF2 plate to generate a white-light continuum (500−850 nm) and was used as the probe beam. The pump andprobe beams were overlapped on the sample surface andmeasured the change in absorbance of the probe beam. Thechange in absorbance of the probe beam (mOD) was calculatedusing the equation mOD = log[Iex(s)/I0(s)] − log[Iex(r)/I0(r)].The symbols s and r correspond to the sample and reference,respectively. Iex and I0 are the transmitted intensities ofsequential probe pulses after a delay time τ following excitationby pump beam and in ground states, respectively. Transmittedintensities were measured using a spectrometer (MS 2004, 600lines/mm diffraction gratings blazed at 600 nm) and Si linearphotodiode arrays. Kinetic traces were obtained by varying thepath lengths of the probe beam using a micrometer resolutiontranslation stage and chirp-corrected using a procedurereported elsewhere.37 To avoid heating effects, we rotated thesample continuously.Open-Aperture z-Scan Studies. To study TPA, we have used a

standard open aperture z-scan setup.38 In our experiments, 5 nspulses centered at 532 nm from the Nd:YAG laser were used toexcite the sample.Ag2S QDSSC. TiO2-coated FTO (F-doped SnO2) electrode

was prepared following a standard protocol and discussed inSI.39 Hot TiO2 electrode (at 60 °C) was dipped in 3-MPA-capped Ag2S NCs dispersed in DMSO (∼5 mg mL−1) and keptin the dark for 60 h at room temperature. 3-MPA-capped Ag2SNC-sensitized TiO2 electrode was then kept in a sulfidesolution (0.8 M Na2S in 1:1 methanol/water solution) for 48 hat room temperature in the dark. This sulfide treatmentimproves the charge transport by modifying the surface-cappinglayer of Ag2S NCs.40 The obtained photoanode was cleanedusing water and then ethanol and dried at 80 °C under N2environment for 30 min. In some cases, the obtainedphotoanode was coated with 1 ML of Ag2S grown by SILARto improve the current density. For SILAR, photoanode wassequentially dipped in solution of Ag+ (0.1 M AgNO3 inethanol) and S2− (0.1 M Na2S in methanol). In between thetwo dippings the electrode was rinsed with ethanol and finallywith methanol; then, the resultant photoanode was dried atroom temperature.QDSSCs were prepared by assembling the photoanode and

Pt counter electrode using parafilm as spacer (∼80 μm) alongwith a 10 μL electrolyte solution in between. Two types ofelectrolyte solutions, A and B, were used in our work.Electrolyte solution A was prepared by mixing Na2S (0.5 M),sulfur (2 M), KCl (0.2 M), and NaOH (0.5 M) in methanol/water in 7:3 v/v following refs 29 and 41. Methanol-freeelectrolyte solution B was prepared by mixing Na2S (2 M) andsulfur (2 M) in water following ref 39; however, powerconversion efficiency decreases with electrolyte B comparedwith electrolyte A. Current−voltage (I−V) measurement wasperformed using a Keithley 2611 source meter. All of the solarcells were illuminated using a Xe lamp of 150 W, fitted withAM 1.5 filter and adjusted at 1 Sun intensity (100 mW/cm2) byusing an NREL-calibrated silicon solar cell. The active areas forthe measurement were kept in the range of 0.24 to 0.28 cm2.

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpclett.5b01692.

Synthesis of CdS NCs, z-scan data analysis, preparationof TiO2-coated FTO electrode, photographs showingprogress of synthesis of Ag2S NC, UV−visible-NIRabsorption, PL, PL decay, XRD, FTIR, TEM, and J−Vplot for different Ag2S QDSSCs. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: angshuman@iiserpune.ac.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge Dr. Shouvik Datta and Mr. Sreejit P. Nandanof IISER Pune for solar cell setup and participation in a fewsyntheses of Ag2S NCs, respectively. We thank Director, IISERPune for financial support and encouragement. We acknowl-edge Mr. Shyamashis Das of Indian Institute of ScienceBangalore for useful discussion. A.N. acknowledges Science andEngineering Research Board (SERB) for Ramanujan Fellow-ship (SR/S2/RJN-61/2012) and fast-track research grant (SB/FT/CS-151/2012) Govt. of India. W.J.M. acknowledges CSIR,Govt. of India, for a junior research fellowship.

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