Shell Quantum Dot Sensitized Solar Cells

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CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9% Junwei Yang, Jin Wang, Ke Zhao, Takuya Izuishi, Yan Li, Qing Shen, and Xinhua Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10546 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 10, 2015

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The Journal of Physical Chemistry

CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9% Junwei Yang†, Jin Wang†, Ke Zhao†, Takuya Izuishi‡, Yan Li†,*, Qing Shen‡,*, and Xinhua Zhong†,* †

Key

Laboratory

for

Advanced

Materials,

Institute

of

Applied

Chemistry,

East China University of Science and Technology, Shanghai 200237, China ‡

Department of Engineering Science, the University of Electro-Communications, 1-4-1

Chofugaoka, Chofu, Tokyo 182-8585, Japan

Email: [email protected] (for Y. L.); [email protected] (for Q. S.); [email protected] (for X. Z.)

ABSTRACT: Surface trap defects are the limited factor for quantum dots (QDs) application in solar cells. The trapping-states can be efficiently suppressed by coating a shell of wider band gap material around the core QDs. We choose CdSe0.65Te0.35 (simplified as CdSeTe) as a model core material, and CdS shell was then overcoated around the CdSeTe core QD to decrease surface defect density and increase the stability of the core QDs. By optimizing the thickness of CdS shell, the power conversion efficiency (PCE) of the CdSeTe/CdS QDSCs is enhanced by 13% in comparison with that of plain CdSeTe QDSCs. Transient absorption (TA), incident-photo-to-carrier conversion efficiency (IPCE), open circuit voltage decay (OVCD) and electrochemical impedance spectroscopy (EIS) measurements confirmed the suppressed charge recombination process in internal QDs and QD/TiO2/electrolyte interfaces with the overcoating of CdS shell around CdSeTe core QDs. With the further overcoating of a-TiO2 and SiO2 barrier layers around the QD-sensitized photoanode, the PCE of champion CdSeTe QDSCs achieved 9.48 % (Jsc = 20.82 mA/cm2, Voc = 0.713 V, FF = 0.639) with average PCE 9.39±0.09 % under AM 1.5 G one full sun illumination.

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INTRODUCTION As a promising light-harvesting material, semiconductor quantum dots (QDs) are attracting increasing attention to constitute low-cost third-generation solar cells for dealing with global warming concern. QDs possess superb features including easy tunable absorption edge, high absorption coefficient, and the multiple exciton generation and hot electron extraction possibilities expecting to break the Shockley−Queisser limit (31%).1-4 Up to date, the quantum dots sensitized solar cells (QDSCs) achieved 8.21% certified efficiency,7 still less than dye-sensitized solar cells with 12.3% efficiency.5 The moderate performance in QDSCs is partly ascribe to the undesired photo-generated electron recombination in both internal QDs and interfaces of QD/TiO2/electrolyte due to high density of trapping states on QD surface.6-10 Trapping states are induced by the dangling bonds associated with unsaturated atoms on QDs surface in the synthesis process due to imperfect coverage of capping ligands.11 The existence of surface defects is a crucial limiting factor for the application of QDs in photovoltaics. It leads to photo-generated electron recombination in both internal QDs and interface of QDs/metal oxide matrix/electrolyte,6-8 and thereby limits power conversion efficiency (PCE) of the resulting solar cells. Hence, surface passivation of QDs to reduce the trapping-state density is a prerequisite to obtain high efficiency QDSC devices. One solution to passivate surface defects is the use of small molecular and atomic ligands.12-17 Page et al. applied chloride ions to treat CdTe QDs in a post-synthetic process, and achieved higher photoluminescence quantum yield (PL QY) and air-stability.12 Fuente et al. passivated the PbS/CdS/ZnS sensitized photoanode using various small molecules and obtained a significant increased cell efficiency (4.65%).13 A certified efficiency of 8.55% for QD depleted heterojunction solar cells was reported by different atomic-ligand passivation and band alignment engineering.14 Another solution to surface passivation can be accomplished by introducing a wider band gap inorganic shell around the core QDs to from the so-called type-I core/shell structure, which has been proved to be an effective method to lessen trap-state defects and improve PL QY and chemical stability of QDs.18-20 Recently, ZnS or

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CdS layer is popularly post-deposited over QD sensitized photoanode via the conventional successive ionic layer adsorption and reaction (SILAR) or chemical bath deposition (CBD) technique. Even though wider band gap layer of ZnS or CdS as a barrier can suppress the interfacial recombination and increase the cell efficiency,21-24 this route suffers from unsatisfactory passivation on exposed surface of QD sensitizer and cannot form a uniform shell around the surface of QD. Recently, CuInS2/ZnS type-I core/shell structured QDs were designed to suppress the surface defects of plain CuInS2 QDs, and dramatically improved PCE has been obtained in the resultant cell devices.9 Lai et al synthesized PbS/CdS core/shell QDs, and the enhanced PCE was ascribed to the reduced trapping states by the passivating CdS shell.25 Herein, we focused on the synthesis of type-I core/shell structured CdSe0.65Te0.35/CdS (CdSeTe/CdS) QDs and its application in QDSCs to improve the PCE furthermore. The reasons for choosing CdSeTe as the model core QDs are that: (i) the plain CdSeTe QDs based solar cells have achieved the highest PCE of 8.21%,7,26 and the further improvement on this basis will be more meaningful; (ii) compared with binary QDs (such as, CdSe, CdS), ternary CdSeTe QDs suffer from higher density of surface defects on account of nonstoichiometric compositions;27 (iii) element Te is susceptible to oxidation,28 and this results in a gradual degradation of the QDs. In this work, CdSeTe/CdS core/shell QDs was prepared by overcoating a CdS thin shell layer around the CdSeTe core QDs via SILAR strategy at a moderate temperature. After ligand exchange process, thioglycolic acid (TGA) capped CdSeTe/CdS QDs were tethered on TiO2 film electrode through capping ligand induced self-assembly deposition technique.29-31 The PCE of the CdSeTe/CdS QDSCs was enhanced by 13% compared to that of plain CdSeTe QDSCs. To further boost cell efficiency, SiO2 and a-TiO2 passivation layers were employed on the sensitized photoaondes, and the champion CdSeTe/CdS QDSCs achieved a PCE of 9.48 % (with average PCE of 9.39±0.09 %), which is among the best performances for liquid-junction QDSCs.

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EXPERIMENTAL SECTION Chemicals. Oleic acid (90%) and thioglycolic acid (TGA, 97%) were obtained from Alfa Aesar. 1-octadecene (ODE, 90%), trioctylphosphine (TOP, 90%), selenium powder (200 mesh, 99.99%), cadmium oxide (CdO, 99.99%), tellurium powder (200 mesh, 99.99%), and oleylamine (OAm, 95%) were purchased from Aldrich. Paraffin liquid (chemical grade) was purchased from Shanghai chemical reagents company, China. Didodecyldimethylammonium bromide (DDAB) and cadmium acetate dehydrate (Cd(OAc)2· 2H2O, 98.5%) were purchased from Aladdin. All reagents were used as received without any further purification. Synthesis of CdSeTe/CdS core/shell QDs. The synthesis of CdSeTe alloyed QDs with particle size of 5.2 ± 0.4 nm and absorption edge at 800 nm was according to literature method.32 Briefly, a mixture of 0.1 M Te and Se stock solutions (obtained by dissolving Se or Te powder in TOP and paraffin (v/v, 1:3)), and 0.1 M Cd precursor solution (obtained by dissolving CdO in oleic acid and paraffin (v/v, 1:3)) with Cd:Te:Se molar ratio of 5:0.65:0.35 was heated to 320 oC under N2 atmosphere and stayed for 10 min, and another 2 mL of OAm was injected into reaction system when reaction temperature reduced to 260 oC and kept for another 8 min. The obtained CdSeTe QDs were purified by centrifugation and decantation with the addition of ethanol and acetone. CdS thin shell was overgrown on the preformed CdSeTe core QDs by a SILAR strategy at moderate temperature according to a literature method with slight modification.33 In a typical process, the S and Cd stocks were primarily prepared for further use. The S precursor solution (DDAB-S2-) was prepared by mixing 3 mL of toluene containing 0.15 mmol of DDAB and 3 mL of 0.05 M aqueous Na2S solution at room temperature. The mixture kept stirring for 1 h, and then the S2- anions was extracted to toluene phase as sulfur precursor used in the next experiment. The Cd precursor solution (OA-Cd2+) was prepared by dissolving 0.15 mmol of cadmium acetate in 1 mL of OA and 2 mL of ODE. Then the solution containing 0.1 mmol of purified CdSeTe, 1 mL of OA and to 5 mL of ODE was loaded in a 50 mL 3-neck round-bottomed reaction flask, and the reaction system was heated to 80 oC under N2 atmosphere. Equimolar amount of S and Cd precursor solutions (0.02 mmol) were 4


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added into the reaction system alternately at a 20 min interval. After 2 cycles of S/Cd overgrowth, 5.9 ± 0.4 nm sized CdSeTe/CdS core/shell QDs were obtained. The reaction mixture was dispersed in hexane then purified by centrifugation and decantation with the addition of acetone and ethanol. Fabrication of QD-Sensitized Photoanodes and Solar Cells. The double layer TiO2 mesoporous film consists of a transparent layer (9.0 ± 0.5 µm) and a light scattering layer (6.0 ± 0.5µm) was prepared by successive screen-printing method according to a literature method.34 The ex-situ ligand exchange technique was employed to prepare TGA-capped water-soluble QDs from initial oil-soluble QDs by using bifunctional molecule TGA as phase transfer agent.31 To immobilize the QD sensitizers on TiO2 mesoporous films, 30 µL of TGA-capped QD aqueous dispersion was pipetted on the film and staying stationary for 8 h. After the deposition, a ZnS barrier layer was coated on the QD sensitized TiO2 films to further inhibiting the interfacial charge recombination according to a literature method.7 The quantum dots sensitized solar cell devices were constructed by assembling Cu2S counter electrode and sensitized TiO2 photoanode and separated by a 60-µm Scotch spacer. A droplet (10 µL) of polysulfide electrolyte (composed of 2.0 M Na2S, 2.0 M S powder and 0.2 M KCl) was then injected into the cell device. Under each condition, 5 cells were prepared and tested in parallel. Characterization. The UV/vis absorption spectra and the PL emission spectra were measured using a UV-visible spectrophotometer (Shimadzu UV-3101 PC) and a fluorescence spectrophotometer (Cary Eclipse Varian), respectively. Transition electron microscopy (TEM) images were obtained on a JEOL JEM-2100 microscope. J−V curves of cell devices were recorded on a Keithley 2400 source meter under illumination by an AM 1.5 G solar simulator. The incident photon-to-current conversion efficiency (IPCE) curves were recorded by a Keithley 2000 multimeter. IS (impedance spectroscopy) were measured on an impedance analyzer (Zahner, Zennium) at forward bias ranging from 0 V to -0.6 V in dark conditions. OCVD (open circuit voltage decay) curves were also recorded using the same Zahner workstation. In measurements, the circuit voltage of cells reached steady state under

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illumination by a white LED with intensity of 100 mW/cm2, then recorded after switching off the light. The X-ray photoelectron spectroscopy (XPS) were obtained using an ESCALAB 250Xi spectrometer with focused monochromatized Al Kα radiation (hν = 1486.7 eV). Transient absorption (TA) measurements were carried out to detect the samples in the femtosecond (fs) range, and the details have been explained in reported literatures.35-36 The titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) was used as laser source with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs. The light was separated into two parts. One part of it was used as a probe pulse. The other half of the light was used to pump an optical parametric amplifier (OPA) (a TOAPS from Quantronix) to generate light pulses with a wavelength tunable from 290 nm to 3 µm, which was used as a pump beam. The other part of the light was used for the probe beam. A white light continuum probe beam was generated by focusing the light in a 1 mm Sapphire plate. In this study, the experiment was carried out under N2 environment, and a pump light wavelength of 470 nm and a probe beam wavelength of 700 nm were used. The samples showed no apparent photo-damage during the TA measurements. RESULTS AND DISCUSSION Synthesis and Optical Properties of CdSeTe/CdS QDs. The CdSe0.65Te0.35 (simplified as CdSeTe) alloyed QDs with absorption edge at 800 nm was synthesized using previously reported method.26 The reasons for choosing CdSeTe QDs as a host material has been elaborated above, simply, high basis cell efficiency, high concentration of surface defects and easily oxidized surface. In order to overcome the latter two disadvantages, CdS shell was then overgrown around the CdSeTe QDs to decrease surface defects and shield the core from oxidation by alternately adding S/Cd precursor to a dispersion of the purified CdSeTe QDs at moderate temperature. The relatively low temperature for growing CdS shell can restrain the effect of Ostwald ripening, and narrow the size distribution.37 Detailed procedure for preparation of CdSeTe core and CdSeTe /CdS core/shell QDs are described in Experimental Section. The corresponding absorption and photoluminescence (PL) spectra for both CdSeTe and derivative CdSeTe/CdS QDs are given in Figures S1a, b of Supporting Information (SI).

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We found that the PL intensity increased gradually during the first two cycles of CdS shell growth (noted as CdSeTe/1CdS and CdSeTe/2CdS, respectively), and then remained constantly after the third cycle (CdSeTe/3CdS). Representative absorption and PL spectra before and after 2CdS shell overgrowth are shown in Figure 1a, b. The profile of both the absorption and PL spectra kept unchanged and no new peak was observed after CdS overgrowth. This observation indicates no CdS nucleation separately. Figures 1e, f show the wide-field TEM image of CdSeTe QDs (5.2 ± 0.4 nm) and the derivative CdSeTe/2CdS core/shell QDs (5.9 ± 0.4 nm), respectively. The increased particle size is derived from the formation of CdS shell with thickness of 0.35 nm, corresponding to nearly 1 monolayer of CdS. The X-ray photoelectron spectroscopy (XPS) was used to further confirm the epitaxial growth of CdS shell around the CdSeTe core QDs (Figure S1c, d). Most notably, the overgrowth of CdS also results in a dramatic increase of PL QY by more than 2-fold. The increased PL QY demonstrates that nonradiative recombination has been remarkably suppressed and therefore surface trap density has been effectively reduced. Furthermore, a significant decrease in PL QY (Figure 1c) was observed for plain CdSeTe QDs when the samples were exposed to atmosphere after 30 days, due to its susceptible oxidized surface. While, for CdSeTe/2CdS QDs, PL intensity remained constant during the 30 days’ period. This observation indicates that the CdS shell around CdSeTe QDs can improve the optical stability of the resultant QDs. The enhanced optical stability of QD light absorber is beneficial for the stability of the resultant solar cell devices. The stability of both CdSeTe/2CdS and CdSeTe QDSCs in work state under the illumination by a full sun intensity from an AM 1.5G is shown in Figure S3. The results indicated that CdSeTe/CdS cells kept initial value for 30 h under continuous 1 sun illumination, while the PCE value of plain CdSeTe cell decayed after 18 h.

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Normalized absorbance

a)

400

CdSeTe CdSeTe/CdS

500

600

700

800

b)

500

700

1.2

d)

900

CdSeTe CdSeTe/CdS

1.0

Absorbance

After 30 days

800

Wavelength(nm)

c) CdSeTe CdSeTe/CdS

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CdSeTe CdSeTe/CdS

600

Wavelength (nm)

Rerative PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Relative PL intensity (a.u.)


0.8 0.6 0.4 0.2 0.0

500

600

700

800

900

Wavelength(nm)

400

500

600

700

800

900

Wavelength(nm)

e)

f)

20nm

20nm

Figure 1. (a)-(b) UV−vis absorption and PL emission spectra (λex = 400 nm) of QD dispersions, respectively. (c) PL emission spectra (λex = 400 nm) of QD dispersions after 30 days, respectively. (d) UV−vis absorption spectra of QD-sensitized TiO2 film electrodes. (e)-(f) TEM images of CdSeTe (left) and CdSeTe/CdS (right) QD dispersions, respectively. To assemble plain CdSeTe and CdSeTe/CdS QD sensitizers in cell devices and evaluate the effect of CdS shell around CdSeTe QDs, the ex-situ ligand exchange post-synthesis assembly approach was adopted.30 Figure 1d shown the absorption spectra of both CdSeTe and derivative CdSeTe/2CdS QDs sensitized TiO2 film electrodes, which show the similar spectral profile and nearly identical absorbance. This result indicates a nearly equal loading

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amount between the CdSeTe and CdSeTe/2CdS sensitized TiO2 film electrode, and the CdS shell overgrowth on CdSeTe QDs has no detrimental effect on the QD loading. The similar coverage of QDs sensitizer on TiO2 film electrode means similar light-harvesting capacity

1.0

a)

Normarlized intensity (a.u.)

between the two kinds of photoanodes. Normarlized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CdSeTe

0.8 0.6 0.4 0.2 0.0 0

100

200

300

400

500

1.0

b)

CdSeTe/CdS

0.8 0.6 0.4 0.2 0.0 0

100

200

Time (ps)

300

400

500

Time (ps)

Figure 2. Transient absorption characterization in picosecond time scale. Kinetic traces of the excitonic decay of (a) CdSeTe, (b) CdSeTe/2CdS QDs deposited on SiO2 (blue fine lines) and TiO2 substrates (red fine lines). The full lines represent the corresponding biexponential fit. Ultrafast Carrier Dynamics Study. To evaluate the electron injection kinetics from QDs to TiO2 for both CdSeTe and CdSeTe/CdS sensitized photoanodes, transient absorption (TA) responses were measured for QDs deposited on insulating SiO2 or TiO2 substrate, respectively. 35-36 As shown in Figure 2a, in the case of the QDs on insulating SiO2, the recovery of the bleaching is relatively slow where the recovery of the bleaching process is controlled by radiative and nonradiative recombination of electron in conduction band with holes in valance band and surface traps respectively. In this case, the photo-generated electron cannot transfer from QDs to SiO2 substrate due to the much higher conduction band edge of SiO2. When QDs deposited on TiO2 (Figure 2b), the recovery of the bleaching process got significantly faster since photo-generated electrons can inject into TiO2 matrix as an additional pathway for carrier relaxation in QDs. Biexponential decay function (eq 1) can fit well with the TA response and average lifetime (τav) was then estimated based on the fitting results by using eq 2.39

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y = A1e − t/τ1 + A2 e − t/τ 2 + y0

(

2

2

(1)

)

τ av = A1τ1 + A2 τ 2 / ( A1τ1 + A2 τ 2 )

(2)

where A1 and A2 are weight coefficients and τ1 and τ2 are the decay time for the two exponential components. τav represents the average lifetime of photo-generated electrons in QDs on SiO2 or TiO2, respectively. The fitting results and calculated average lifetime (τav) are listed in Table 1. In comparison, the values of both τ1, τ2 and τav for the CdSeTe on SiO2 substrate (7.48, 65.8, 63.43 ps) are increased by ~two-fold for the CdSeTe/2CdS system (13.15, 130, 120.1 ps, respectively). The increased lifetime of τ1, τ2 and τav are derived from the fact that the recombination of photo-generated electron through surface defects was suppressed by CdS shell passivation. This observation demonstrates that CdS shell coating on the CdSeTe core can effectively reduce surface trapping state density. This observation is consistent with the result of PL enhancement with overcoating of CdS around CdSeTe core QDs as mentioned above. The same variation trend of τ1, τ2 and τav were also obtained for the case of TiO2 substrate, but with a remarkable decreased lifetime. Table 1. Fitting Results of the TA Responses of CdSeTe and CdSeTe/2CdS QDs Sensitized TiO2 Films Samples

τ1 (ps)

τ2 (ps)

A1

A2

y0

τav (ps)

CdSeTe-SiO2

7.48

65.8

0.196

0.525

0.20

63.43

CdSeTe-TiO2

2.25

12.3

0.337

0.598

0.04

11.36

CdSeTe/2CdS -SiO2

13.15

130

0.357

0.391

0.25

120.1

CdSeTe/2CdS -TiO2

3.71

33.4

0.580

0.336

0.06

28.62

ket (×1010 s-1) 7.23

2.66

The rate constants of electron transfer (ket) from the QDs to TiO2 can be calculated through comparing the TA decay processes of the QDs on TiO2 to those of QDs on SiO2, following eq 3:38

ket = 1 / τ av (TiO2 ) − 1 / τ av ( SiO2 )

10


(3)

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where τav(TiO2 ) represents the average lifetime of QDs on TiO2 film, and τav( SiO2 ) is the average lifetime of QDs on SiO2 substrate. According to eq 3, we obtained ket of 7.2×1010, 2.7×1010 s-1 for CdSeTe and CdSeTe/2CdS QDs, respectively (Table 1). It is found that the electron injection rate is reduced with overgrowth of CdS shell. Nonetheless, the reduced ket of 2.7×1010 s-1 has not a heavy impact on electron transferring to TiO2 matrix, because the competing electron-hole recombination rate is much slower at the level of 108-106 s-1.6,39 Since electron extractions play an important role in PCE of cells, it cannot be ignored that CdS shell acts as a barrier for electron injection, meanwhile reduces internal recombination.40 So it is crucial to balance the two conflicting effects by finding out optimum thickness of CdS shell as discussed below. 20

80

a)

12 8

CdSeTe CdSeTe/CdS

b)

70

16

IPCE (%)

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40

CdSeTe CdSeTe/CdS ∆IPCE

30 20

4

10

0 0.0

0.2

0.4

0 300

0.6

Potential(V)

400

500

600

700

800

900

Wavelength (nm)

Figure 3. (a) J−V curves of CdSeTe (black) and CdSeTe/CdS cells (red) representative cells under the irradiation of 1 full sun, (b) Incident photon to current efficiency (IPCE) curves.

Table 2. Photovoltaic Parameters Extracted from J−V Measurements QDs

Jsc (mA·cm-2)

Voc (V)

FF (%)

PCE (%)

CdSeTe

19.48

0.589

0.612

7.02±0.05 (7.10)

CdSeTe/1CdS

20.01

0.610

0.611

7.47±0.10 (7.64)

CdSeTe/2CdS

20.50

0.626

0.619

7.93±0.07 (8.02)

CdSeTe/3CdS

20.28

0.624

0.609

7.69±0.18 (7.85)

Photovoltaic Performance. To further passivate the QDs sensitized TiO2 film electrode and suppress charge recombination occurring at photoanode/electrolyte interface, a thin ZnS

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layer was overcoated onto QDs sensitized TiO2 electrode via the successive ionic layer absorption and reaction (SILAR) route according to standard literature method.7 The cell devices were assembled using QD-sensitized TiO2 film electrode with ZnS coating as photoanode, Cu2S/brass foil counter electrode, and polysulfide electrolyte (2.0 M Na2S, 2.0 M S and 0.2 M KCl in aqueous solution). For each type of cell, five cells in parallel were prepared and measured under standard conditions (100 mW/cm2, AM 1.5G). To highlight the effect of CdS shell around CdSeTe QDs, only the ZnS layer was used to passivate QDs sensitized TiO2 electrode, considering that further passivation layer of SiO2 or TiO2 will weaken the photovoltaic performance difference between CdSeTe and CdSeTe/CdS QDSCs devices. The extracted average photovoltaic parameters together with the values for the champion cell are shown in Table 2, and detailed parameters for individual cell are given in Table S1. As shown in Table 2, it is noted that the cell devices based on CdSeTe/CdS QDs show an enhanced open-circuit voltage (Voc) and short-circuit current density (Jsc) values compared to those based on plain CdSeTe QDs. It is found that the PCE was increased by ~6% with use of CdSeTe/1CdS QD sensitizer, and a further PCE enhancement of ~13% was observed in the CdSeTe/2CdS sensitizer. With the further enhancement of CdS shell thickness around CdSeTe QD sensitizer (for the case of CdSeTe/3CdS), the efficiency of the resultant cells decreases gradually. This observation is ascribed to the not enough effective passivation of trapping-states for the case of 1CdS shell thickness, and excessive suppression of photogenerated electron injection into TiO2 substrate and hole transfer from the QD to electrolyte for the case of 3CdS shell thickness.40 Hereafter, CdSeTe/2CdS based solar cells were selected as the studied subject. Figure 3a shows the J-V curves of the QDSCs based on plain CdSeTe and CdSeTe/2CdS sensitizers with intermediate performance in their corresponding groups. The Jsc and Voc of CdSeTe/2CdS QDSC devices significantly increases from 19.48 to 20.50 mA/cm2 and 0.589 to 0.626 V compared with those of plain CdSeTe QDSC devices, and the champion PCE are boosted from 7.10% to 8.02%. The observed higher PCE values of CdSeTe/2CdS QDSC devices are mainly attributed to the reduced surface defects of the QDs and accordingly the suppression of charge recombination both from internal of QDs and the interface of QDs/TiO2/electrolyte. For convenience, CdSeTe/2CdS (simplified as CdSeTe/CdS henceforth) based solar cell was analyzed as 12


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example in the next section. To further investigate the enhancement of Jsc value by CdS shell overcoating, we measured the incident-photo-to-carrier conversion efficiency (IPCE, i.e. external quantum efficiency, EQE), and the result is illustrated in Figure 3b. The ∆IPCE curve shows the difference in IPCE spectra between the CdSeTe/CdS and the CdSeTe based devices, and the ∆IPCE curve clearly shows that IPCE was enhanced for CdSeTe/CdS QDSCs in the wavelength region of 500−850 nm. As reported, IPCE = [1-10-Abs(λ)] × φinj × ηcc, where Abs(λ) is the absorbance of photoanode at wavelength λ, φinj is the electron injection efficiency, and ηcc is charge collection efficiency. Since CdSeTe and CdSeTe/CdS sensitized TiO2 film shown a similar spectral profile with equal absorbance (Figure 1d), the increased IPCE value signified that φinj and/or ηcc was improved, and the improvement were mainly in long-wavelength region after 500 nm. It is probably ascribed to the fact that the lower-energy photogenerated electrons excited by the longer-wavelength light could be more easily trapped by surface defects prior to injecting into TiO2 substrate.41 In QD system, energy edge for trap states is generally lower than the excitonic band gap edge. Therefore, these trap states can only capture lower energy photon and have no influence on the high energy photon. Therefore shell coating treatment can only take effect to the long wavelength light can enhance the IPCE in the long wavelength side consequently. While the overgrowth of CdS shell around CdSeTe QDs can effectively reduce surface trap defects, this can explain the enhancement of IPCE between CdSeTe/CdS and CdSeTe QDSC devices. CdSeTe CdSeTe/CdS

b)

2

Rrec (Ω /cm )

2

4

-3

10

0.3

0.4

0.5

Vapp (V)

10

3

0.3

0.4

CdSeTe CdSeTe/CdS

c) Dark Current (A)

10

CdSeTe CdSeTe/CdS

a) Cµ (F/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1E-4

1E-5

1E-6

0.5

Vapp (V)

0.1

0.2

0.3

0.4

0.5

0.6

Vapp (V)

Figure 4. EIS characterization of the CdSeTe/CdS cells (red) compared with plain CdSeTe cells (black), (a) chemical capacitance Cµ, (b) recombination resistance Rrec, (c) dark current on corrected voltage Vapp.

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Table 3. Simulated Values of Resistance (R) and Capacitance (C) of CdSeTe and CdSeTe/CdS cells under the Forward Bias of −0.55 V Cells

Rs (Ω cm2)

Cµ (mF cm−2)

Rrec (Ω cm2)

τn (ms)

CdSeTe

19.48

2.28

296.6

676.2

CdSeTe/CdS

20.34

2.37

469.3

1112

Impedance Spectroscopy. To clarify the intrinsic mechanism of the enhanced performance in the CdSeTe/CdS QDSCs, the electrochemical impedance spectroscopy (EIS) was applied at forward bias ranging from 0 V to -0.55 V under dark conditions, and the EIS parameters were extracted using the standard fitting models for QDSCs.42-43 The extracted chemical capacitance Cµ, recombination resistance Rrec, and dark current for both CdSeTe/CdS and CdSeTe QDSC devices from the corresponding Nyquist curves are shown in Figure 4, and the detailed Nyquist curves under different bias are available in Figure S4. As shown in Figure 4a, the chemical capacitances Cµ for both CdSeTe and CdSeTe/CdS QDSC devices are similar with the variation of the potential. The similar Cµ values reveal that the position of the conduction band edge of TiO2 does not be varied by the nature of the different sensitizers.44 Nevertheless the nature of QD sensitizer has a remarkable effect on recombination resistance, Rrec, reflecting the charge recombination process between QD sensitizers/TiO2/electrolyte interfaces,42,44 as shown in Figure 4b. The improved Rrec values in CdSeTe/CdS devices indicates that the overgrowth of CdS shell around CdSeTe QDs suppresses the electron back transfer from TiO2 to QD sensitizer and/or recombination reaction from QD to electrolyte. For clarity, Table 3 shows the extracted EIS parameters at forward bias of -0.55V of CdSeTe and CdSeTe/CdS QDSC devices. Noted that Rrec value of CdSeTe/CdS cell is nearly two-fold to that of CdSeTe cells. The observed dark current of CdSeTe/CdS QDSC devices is weaker than that of the CdSeTe QDSC devices at the same potential, which agrees well with the Rrec result, as illustrated in Figure 4c. The reduced dark current also confirms the prevention of the charge recombination by CdS shell. Furthermore, the electron lifetime τn was calculated following the equation τn = Rrec × Cµ, and the τn value for CdSeTe/CdS QDSC devices is also ∼two-fold longer than that of the CdSeTe cell. The

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longer electron lifetime will benefit the charge collection efficiency, ηcc, of the cell device.

a)

CdSeTe CdSeTe/CdS

b)

CdSeTe CdSeTe/CdS

Electron Lifetime(s)

0.6

Potential(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 0.4 0.3 0.2 0.1

3

2

1

0

0.0 0

20

40

60

80

100

120

Time(s)

0.3

0.4

0.5

0.6

Potential(V)

Figure 5. (a) Voc decay curves of the CdSeTe (black) and CdSeTe/CdS (red) QDSC devices recorded during the relaxation from illuminated quasi-equilibrium to the dark, (b) Electron lifetime derived from OCVD measurements. Open Circuit Voltage Decay. To further confirm the suppression of charge recombination process and improvement of electron lifetime in the CdSeTe/CdS cell devices, an open circuit voltage decay (OCVD) analysis was implemented by monitoring the decay of photovoltage (Voc).45-47 Figure 5a shows the representative Voc decay curves of CdSeTe and CdSeTe/CdS QDSC devices, which recorded the procedures of transient Voc values gradually decaying to near 0 V from the value of Voc at stable state after switching off the illumination. It can effectively reflect the charge recombination rate in cell devices. Obviously, the Voc decay rate of CdSeTe/CdS cells is much slower than that of CdSeTe cells. Moreover, there is direct connection between Voc decay rate and the electron lifetime, and the electron lifetime, τn, can be calculated by the eq 4:45 τn = −(kBT/e)(dVoc/dt)−1

(4)

Where kB is the Boltzmann constant, T is the absolute temperature, and e is the electronic charge. Following eq 4, we obtained the calculated electron lifetime (τn) curves of both CdSeTe and CdSeTe/CdS QDSC devices, as plotted in Figure 5b. We found that the electron lifetime of CdSeTe/CdS cells was significantly improved compared to that of CdSeTe cells at the same potential. The enhanced electron lifetime is ascribed to suppression of charge recombination at TiO2/CdSeTe/CdS/electrolyte interfaces by passivating the QD surface trap defects with overcoating of CdS shell in comparison with plain CdSeTe QDs.

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2

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20 16 12 8 4 0 0.0

0.2

0.4

0.6

0.8

Potential (V)

Figure 6. J−V curves of CdSeTe/CdS champion cells with the a-TiO2/ZnS/SiO2 barrier layers under the irradiation of 1 full sun. Table 4. Photovoltaic Parameters Extracted from J−V Measurements

a

Post-treatment

Jsc (mA·cm-2)

Voc (V)

FF (%)

PCE (%)

ZnSa

20.50

0.626

0.619

7.93±0.07

ZnSb

20.34

0.620

0.636

8.02

a-TiO2/ZnS/SiO2a

20.74

0.708

0.643

9.39±0.09

a-TiO2/ZnS/SiO2b

20.82

0.713

0.639

9.48

Average value of five devices in parallel. bPerformance of champion cells.

To boost the efficiency of CdSeTe/CdS QDSC devices furthermore, amorphous TiO2 (a-TiO2) and SiO2 barrier layers were overcoated around the QD sensitized TiO2 film electrode to further reduce interfacial recombination and increase charge collection efficiency.7 Figure 6 shows the J-V curves of CdSeTe/CdS champion cells with standard ZnS barrier layer, and with the a-TiO2/ZnS/SiO2 barrier layers around the QD sensitized photoanodes. The extracted average photovoltaic performance as well as the performance for the champion cells are shown in Table 4. It is found that with the furthermore a-TiO2 and SiO2 overcoating around photoanode, the average PCE of CdSeTe/CdS QDSCs was improved to 9.39% from the original 7.93%, and the champion cell exhibited an efficiency of 9.48% (Jsc = 20.82 mA/cm2, Voc = 0.713 V, FF = 0.639). As far as we know, the obtained efficiency is among the best performances for all kinds of QD based solar cells.7,14-15,48-49

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CONCLUSION In summary, due to the merit and demerit of CdSeTe QDs, we designed and synthesized type-I CdSeTe/CdS core/shell QDs. By optimizing the thickness of CdS shell, the PCE of resulting CdSeTe/CdS QD based solar cells was increased by ~13 % compared to that of plain CdSeTe QDs. The enhanced PL QY in the obtained CdSeTe/CdS QDs indicated the reduced trapping state defects in QDs. The suppressed charge recombination rate in internal QDs and QD/TiO2/electrolyte interface were also confirmed by TA, IPCE, OVCD and EIS measurements, respectively. With the further coating of barrier layers a-TiO2 and SiO2 around the sensitized photoanode, the PCE of CdSeTe QDSC champion cell achieved 9.48 % with average PCE 9.39±0.09 % under AM 1.5 G one full sun illumination, which is among the best performances in all kinds of quantum dots based solar cells. These results indicate that QDs surface passivation using type-I core–shell structure can effectively boost performance of QDSCs by suppressing recombination occurring at internal QD and QD/TiO2/electrolyte interfaces.

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AUTHOR INFORMATION Corresponding Author [email protected]; [email protected] (for Q. S.); [email protected] (for X. Z.) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research is supported by the Natural Science Foundation of China (nos. 21421004, 91433106), and the Fundamental Research Funds for the Central Universities in China. Q.S thanks for the supporting of MEXT KAKENHI Grant Number 26286013 and the CREST program, Japan Science and Technology Agency (JST).

ASSOCIATED CONTENT Supporting Information PL emission spectra (λex= 400 nm) and UV−vis absorption of CdSeTe and CdSeTe/CdS QDs corresponding to different cycles of CdS shells. X-photoelectron spectra of CdSeTe and CdSeTe/CdS QDs. Photovoltaic Parameters for 5 QDSCs in parallel based on CdSeTe and CdSeTe/CdS QDs corresponding to different cycles of CdS shells. Nyquist curves under different bias voltages for CdSeTe and CdSeTe/CdS QDSC devices. Photovoltaic Parameters for 5 QDSCs in parallel based on CdSeTe and CdSeTe/CdS QDs with the further passivation layers of a-TiO2 and SiO2. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table of Content (TOC) CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9% Junwei Yang†, Jin Wang†, Ke Zhao†, Takuya Izuishi‡, Yan Li†,*, Qing Shen‡,§,*, and Xinhua Zhong†,*

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20

PCE = 9.48 % 16 CdSeTe

12 8

Trap state Eredox

4 TiO2

0 0.0

0.1

CdS

0.2

0.3

0.4

0.5

0.6

0.7

Potential (V)

23


Shell Quantum Dot Sensitized Solar Cells

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