Improving Loading Amount and Performance of Quantum Dot

Subscriber access provided by Vanderbilt Libraries

Article

Improving Loading Amount and Performance of Quantum Dot Sensitized Solar Cells through Metal Salt Solutions Treatment on Photoanode Wenran Wang, Jun Du, Zhenwei Ren, Wenxiang Peng, Zhenxiao Pan, and Xinhua Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11122 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

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

ACS Applied Materials & Interfaces

Improving Loading Amount and Performance of Quantum Dot Sensitized Solar Cells through Metal Salt Solutions Treatment on Photoanode Wenran Wang,† Jun Du,† Zhenwei Ren,† Wenxiang Peng,† Zhenxiao Pan*,† and Xinhua Zhong*,†,‡ †

Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering,

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

College of Materials and Energy, South China Agricultural University, 483 Wushan Road,

Guangzhou 510642, China Email: [email protected] (for Z.P.); [email protected] (for X. Z.) Tel/Fax: +86 21 6425 0281

1

ACS Paragon Plus Environment


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

ABSTRACT: Increasing QD loading amount on photoanode and suppressing charge recombination are prerequisite for high efficiency quantum dot sensitized solar cells (QDSCs). Herein, a facile technique for enhancing loading amount of QDs on photoanode and therefore improving the photovoltaic performance of the resultant cell devices is developed by pipetting metal salt aqueous solutions on TiO2 film electrode and then evaporating at elevated temperature. The effect of different metal salt solutions were investigated and experimental results indicated that the isoelectric point (IEP) of metal ions influenced the loading amount of QDs and consequently the photovoltaic performance of the resultant cell devices. The influence of anions was also investigated, and the results indicated that anions of strong acid made no difference, while acetate anion hampered the performance of solar cells. Infrared spectroscopy confirmed the formation of oxyhydroxides, whose behavior was responsible for QD loading amount and thus solar cell performance. Suppressed charge recombination based on Mg2+ treatment under optimal conditions was confirmed by impedance spectroscopy as well as transient photovoltage decay measurement. Combined with high QD loading amount and retarded charge recombination, the champion cell based on Mg2+ treatment exhibited an efficiency of 9.73% (Jsc = 27.28 mA/cm2, Voc = 0.609 V, FF = 0.585) under AM 1.5 G full one sun irradiation. The obtained efficiency was one of the best performances for liquid-junction QDSCs, which exhibited a 10% improvement over the untreated cells with the highest efficiency of 8.85%.

KEYWORDS: photovoltaics, quantum dot sensitized solar cells, metal oxyhydroxide, loading amount, charge recombination

2


Page 2 of 29

Page 3 of 29

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


INTRODUCTION Colloidal quantum dots (QDs) have been conceived as promising light harvesting materials for the application in photovoltaic solar cells due to their prominent advantages, such as tunable light absorption range, high extinction coefficient, multiple excition generation (MEG) possibility, and solution processability.1−3 Quantum dot sensitized solar cells (QDSCs) are perceived to be a promising candidate for the third generation photovoltaic cells. Benefiting from the design and adoption of new kind of QD sensitizers with light absorption range extending to near infrared window, together with the interface engineering strategy to suppress charge recombination dynamics, photovoltaic performance of QDSCs experiences a rapid evolution in the last decade with power conversion efficiency (PCE) increasing from less than 1% to beyond 10%.4−11 However, it is still an urgent task to further improve the photovoltaic performance of QDSCs, making it competitive to other kinds of emerging solar cells. In QDSCs, it is emphasized that improving QD loading amount to form a full monolayer on the metal oxide (mainly TiO2) substrate is a “must” so as to increase light-harvesting capacity, reduce charge recombination at photoanode/electrolyte interfaces, and consequently improve photocurrent, and photovoltaic performance of the resultant cell devices.12 The capping ligand-induced self-assembly (CLIS) deposition method, developed by Zhong’s group, has been proven to achieve a QD surface coverage as high as 34%,12,13 and PCE beyond 11%.4,5 However, the relative low extinction coefficient of QD sensitizer in long wavelength window results in an insufficient harvesting of sunlight, and consequently a relatively low incident photon-to electron conversion efficiency (IPCE) value in the corresponding spectral region as observed in previous reports.5,14 This indicates that it is still necessary and urgent to enhance furthermore the loading amount of QD sensitizers on photoanode in order to harvest more incoming solar photon and achieve higher photocurrent. In addition, the enhanced surface coverage of QD on photoanode means the reduction of the fraction of uncovered surface of TiO2 substrate contacting directly with electrolyte, which

3



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

Page 4 of 29

serves as recombination centers for photogenerated electrons in TiO2 with oxidized species in electrolyte,

and

therefore

diminishes

electron

recombination

rate

occurring

at

photoanode/electrolyte interface and improves the photovoltaic performance of cell devices, especially the photovoltage.12,13 Insulator or wide band gap metal compound layers, such as Al2O3, MgO, ZrO2, SiO2, Mg(OH)2, BaTiO3 et al., were extensively applied as barrier layers around electron conductor TiO2 surface for restraining charge recombination or enhancing sensitizer loading in both dye sensitized solar cells (DSCs) and QDSCs.15−36 In previously reported DSCs, the target barrier layers were usually overcoated on plain TiO2 film electrode prior to the sensitization procedure via solution-based deposition methods, followed by sintering at high temperature in aerobic condition to form a metal oxide layer.15−24 However, in the case of QDSCs, the formed crystalline metal oxide layer around plain film electrode prior to QD sensitization can block the nanometer scaled channels inside the film electrode and is unfavorable for the uploading of QD sensitizers on photoanode, and thus deteriorates photovoltaic performance.6,9 To avoid this limitation, a series of metal oxyhydroxide gels have been overcoated around QD sensitized photoanode via a hydrolysis and condensation process at 40 oC from the respective metal salt aqueous solutions without any further high temperature sintering treatment.6 Experimental results indicated that the formed amorphous Zr or Nb oxyhydroxide layer can suppress charge recombination process and lead to a remarkable improvement in photovoltaic performance, especially in photovoltage. It is noted that the main effect of the previously reported crystalline metal oxide and amorphous oxyhydroxide layer is to diminish the density of trap state in photoanode and therefore suppress the charge recombination at the photoanode/electrolyte interfaces.23 Herein, we present a facile procedure for the formation of oxyhydroxide overcoating layer around plain TiO2 film electrode to improve QD loading and reduce the exposure fraction of photoanode, consequently improve photocurrent and photovoltaic performance of the resultant cell devices. Zn-Cu-In-Se (ZCISe) QDs was chosen as a model QD sensitizer in this work because: (1) the previously reported best photovoltaic performance was based on this 4


Page 5 of 29

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


kind of QD sensitizer;5 (2) a highly reproducible and facile synthetic approach has been developed for this kind of QD; (3) there is no highly toxic Cd, and Pb component in it. The metal oxyhydroxide layers were deposited before QD sensitization by pipetting corresponding meal ionic aqueous solution on TiO2 film electrode, followed by drying at 50 oC. Absorption spectral results indicated that the loading amount of ZCISe QDs is positively relevant to the isoelectric point (IEP) of oxyhydroxides. However, high QD loading amount alone is not equal to good photovoltaic performance. Photovoltaic measurement results indicate that the modification with use of alkaline earth metal ion (Mg2+, Ca2+, Sr2+ and Ba2+) gave the best performance, while transition and heavy metal ion (Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+ and Pb2+) presented a negative result. The representative best result with use of Mg2+ treatment was chosen as a model to perform further characterization. Infrared spectroscopy (IR) confirmed the formation of oxyhydroxide after photoanode treatment, and electrochemical impedance spectroscopy (EIS) as well as open circuit decay (OCVD) analysis results confirmed the retarded charge recombination process. Upon optimization of Mg2+ deposition amount, ZCISe QDSCs with efficiency of 9.73% is obtained, representing a 10% improvement over the untreated cells with the highest efficiency of 8.85%.

5



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

Page 6 of 29

EXPERIMENTAL SECTION Chemicals. All metal salts used for TiO2 film treatment are analytical purity and purchased from Aldrich, including magnesium chloride (MgCl2·6H2O, 99.99%), magnesium nitrate (Mg(NO3)2·6H2O, 99.99%), magnesium sulfate (MgSO4, 99.99%), magnesium acetate (Mg(CH3COO)2·4H2O, 99.98%), calcium chloride (CaCl2, 99.99%), strontium chloride (SrCl2·6H2O,

99.99%),

barium

chloride

(BaCl2·2H2O,

99.99%),

iron(II)

chloride

(FeCl2·4H2O, 99.99%), cobalt(II) chloride (CoCl2·6H2O, 99.99%), nickel chloride (NiCl2·6H2O, 99.9%), copric chloride (CuCl2·2H2O, 99.99%), zinc nitrate (Zn(NO3)2·6H2O, 99.99%), cadmium chloride (CdCl2, 99.99%), silver nitrate (AgNO3, 99.99%), and lead nitrate (PbCl2, 99.99%). Oleylamine (OAm, 97%), 1-octadecene (ODE, 90%), sodium sulfide (Na2S·9H2O, 99.99%), indium acetate (In(OAc)3, 99.99%), zinc acetate (Zn(OAc)2, 99.99%), and selenium powder (200 mesh, 99.99%) were purchased from Aldrich. Copper iodide (CuI, 99.998%) and 3-mercaptopropionic acid (MPA, 97%) were obtained from Alfa Aesar. Diphenylphosphine (DPP, 98%) was obtained from J&K. Synthesis and Water-solubilization of (CISe)0.7(ZnSe)0.3 (ZCISe) QDs. OAm-capped oil-soluble ZCISe QDs were firstly synthesized according to literature method.5 Briefly, CuI (19.0 mg, 0.1 mmol) and In(OAc)3 (29.0 mg, 0.1 mmol) were mixed with 0.4 mL of Zn(OAc)2 stock solution (prepared by dissolving Zn(OAc)2 into OAm and ODE mixture with a volume ratio of 1:4 to form a 0.1 mmol/mL solution), 2.0 mL OAm and 1.5 mL ODE in a 50 mL three-necked flask. The reactant was heated to 110 oC with stirring under vacuum for 5 min. The system was then heated to 180 oC, followed by injection of Se-DPP stock solution (obtained by dissolving 0.024 g of selenium powder in 0.3 mL DPP and 0.5 mL OAm). After injection, the reaction temperature was maintained at 180 oC for 5 min, then cooled to 90 oC, followed by addition of 10 mL hexane. Further purification was carried out by precipitation and centrifugation procedure with the use of ethanol and acetone. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) confirmed the chemical composition of the obtained QD is (CuInSe2)0.7(ZnSe)0.3. For convenience, ZCISe QDs are referred specially to the QD with this composition henceforth. 6


Page 7 of 29

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


Phase transfer procedure was carried out by re-dissolving the precipitate in CH2Cl2, followed by ligand exchange process with the use of MPA, as demonstrated in our previous work.5,12,13 After another precipitation and centrifugation cycle, the water-soluble QDs were dissolved in 1.5 mL deionized water, and pH of the solution was adjusted to 10 by addition of NaOH solution. Metal Ion Deposition on TiO2 Film Electrode. The preparation of TiO2 mesoporous film electrode (20.0 µm transparent layer together with 5.0 µm light scattering layer) was the same as our previous work.5,12,13 Metal ion stock solutions with concentration of 0.02 M were prepared by dissolving corresponding metal salts in deionized water. For metal ion deposition, certain amount of corresponding metal ion stock solution was pipetted onto TiO2 film with area of 0.235 cm2 at room temperature, followed by evaporation of water at 50 oC for 60 min. While in infrared (IR) and UV-vis absorption characterization, the adopted TiO2 film electrodes possess only 6.0 µm transparent layer with active area of 1.6 cm2 (2.0 cm × 0.8 cm). For the purpose of convenient representation, abbreviations are used without anion, physical unit or concentration, henceforth. For instance, for each TiO2 film electrode with photoactive active area of 0.235 cm2, the treatment using 10 µL of 0.02 M BaCl2 aqueous solution is abbreviated as “Ba2+-10”, and 20 µL 0.02 M CaCl2 aqueous solution is abbreviated as “Ca2+-20”. Immobilization of QDs onto TiO2 Film Electrodes and Construction of Solar Cells. QD sensitizers were immobilized on metal ions modified TiO2 mesoporous film electrodes and plain film electrodes without metal ion treatment as reference by pipetting MPA-capped ZCISe QD aqueous solution onto the film and leaving it stationary until saturated loading amount reached (from 1 to 5 h depending on the metal ion solution volume used for the treatment), followed by rinsing with water and ethanol sequentially. After QD deposition, a ZnS barrier layer was overcoated on the sensitized photoanode by alternately dipping the QD sensitized electrode into 0.1 M Zn(OAc)2 methanol solution and 0.1 M Na2S aqueous solution for 6 cycles.5,11 Sandwich-structured solar cells were fabricated by assembling Cu2S/brass counter electrode and QD-sensitized TiO2 film electrode using a 60-µm thick Scotch spacer 7



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

with a binder clip and filled with polysulfide electrolyte aqueous solution (2.0 M Na2S, 2.0 M S, and 0.2 M KCl). The Cu2S/brass counter electrodes were prepared by immersing brass foil in 1.0 M HCl solution at 75 °C for 30 min, followed by reacting with polysulfide aqueous solution (2.0 M Na2S, 2.0 M S). Characterization. The FTIR spectra was recorded by using a NICOLET 6700 spectrometer equipped with a DTGS KBr detector. UV/vis absorption spectra was recorded based on a UV-visible spectrophotometer (Shimadzu UV-3101 PC). The photovoltaic properties and J-V curves of the QDSCs were measured by Keithley 2400 source meter equipped with a 150 W AM 1.5 G solar simulator (Oriel, model no. 94022A). Calibration was taken by an NREL standard Si solar cell to set the power of the simulated solar light to 100 mW/cm2. During the measurement, the photoactive area was defined by a shading mask of 0.235 cm2. Incident photon-to-current conversion efficiency (IPCE) spectrum was measured on a Keithley 2000 multimeter with illumination of a 300 W tungsten lamp with a Spectral Product DK240 monochromator. Electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD) measurements were carried out on an electrochemical workstation (Zahner, Zennium) at 298 K. EIS was measured under dark conditions at different forward bias ranging from 0.3 V to 0.55 V, applying a 20 mV AC sinusoidal signal over the constant applied bias with the frequency ranging from 1 MHz to 0.1 Hz. The samples for OCVD measurements were illuminated by a white LED with intensity of 100 mW/cm2, and the changes of photovoltage with time were recorded after switching off the illumination.

8


Page 8 of 29

Page 9 of 29

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


RESULTS AND DISCUSSIONS Zn-Cu-In-Se (ZCISe) alloyed QDs with an absorption onset at ~1000 nm were employed as the sensitizer according to our recent report.5 Before QD sensitization, the TiO2 film electrodes were treated by pipetting certain amount of corresponding metal ion aqueous solution onto them, followed by drying at 50 oC for 60 min. During drying of the film electrode loaded with metal ion aqueous solution, metal ion underwent partial hydrolysis, for simplicity reason we refer to these hydrolysis product as oxyhydroxides. Since the active area of the studied photoanode film is fixed, the absolute quantity of oxyhydroxide retained onto the film is responsible for final result. The effects of a series of treatments on TiO2 electrodes were systematically investigated. Subsequent experiments showed that the cell devices based on Mg2+ treatment exhibited the best photovoltaic performance. Hereafter, Mg2+-treated cells were investigated as a model for infrared (IR) spectroscopy, electrochemical impedance spectroscopy (EIS) and open circuit decay (OCVD) characterization, while only photovoltaic performances were recorded for other ion treated cells. Characterization of Formed Oxyhydroxide by Infrared Spectroscopy. Infrared spectroscopy (IR) was employed to probe the structure information of TiO2 film electrodes treated by Mg(NO3)2 aqueous solution. Figure 1 shows the IR spectra of untreated and Mg(NO3)2 treated TiO2 films. The wide and strong peak at ∼3400 cm−1 is assigned to OH stretching and the peak at ∼1600 cm−1 is assigned to OH scissoring.31 This observation indicates the formation of magnesium oxyhydroxide. Furthermore, with the increase of the amount of Mg(NO3)2, the intensity of the peaks increases gradually. This behavior can be attributed to the increase of magnesium oxyhydroxide formed on the surface of TiO2 film electrode as the amount of Mg(NO3)2 solution increases.

9



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

Figure 1. FTIR spectra of TiO2 films (active area of 1.6 cm2) without and with treatment by different volumes of 0.02 M Mg2+ aqueous solution.

Dependence of QDs Loading Amount on Metal Ion Treatment. Figure 2a illustrated the absorption spectra of QD sensitized films (with active area of 1.6 cm2 and transparent layer thickness of 6.0 µm) based on TiO2 film treated by 50 µL of corresponding metal ion aqueous solution with identical concentration of 0.02 M together with the reference film (Ref) without metal ion treatment. Since the thickness of the films are identical, according to Lambert-Beer’s Law, the QD loading amount can be represented by the absorbance at the first excitonic peak. It is found that for the film undergone different ions treatment, except the change of absorbance, there is no other variation in the absorption profile (including peak shape and position), indicating no formation of alloyed/core-shell structure or QD aggregation. It can be observed that the absorbance of the resulting QD sensitized film is dependent on the IEP of the formed oxyhydroxides on the TiO2 film after metal ion treatment. The higher the IEP, the larger the absorbance. IEPs of corresponding hydroxides are listed in Table 1,37 and the deduction of IEPs of semi-soluble Ca(OH)2, Sr(OH)2 and Ba(OH)2 is available in Supporting Information (SI). From these results we deduce that the oxyhydroxides with higher IEPs enhance significantly the loading amount of QDs. This phenomenon is clear, since IEP reflects the chemical composition of the solid and the electrolyte in which it is immersed.37 In aqueous solution, water molecules interact with the surface of solid to form 10


Page 10 of 29

Page 11 of 29

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


oxyhydroxides, whose behavior accounts for surface charge and electrostatic force. The oxyhydroxides with higher IEPs are more negatively charged (or less positively charged) under the same condition. The isoelectric point of P25 (mainly composed of anatase) is ~6, while the pH of QD aqueous solutions is in the range of 10−11,5,38 under this condition the surface of TiO2 is fully negatively charged. However, in basic aqueous solution, the terminal carboxyl groups on the surface of QDs are also negatively charged. As a consequence, an unwanted repulsive force is generated, inhibiting QD loading onto TiO2. The deposition of metal oxyhydroxides with high IEPs, however, neutralizes the surface negative charge, thus promoting QD loading. Our results showed that only when the IEPs of hydroxides exceed 11 (including Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2, Ni(OH)2, Ag2O and Pb(OH)2), could the absorbance enhanced significantly. Note that the pH of QD aqueous solution is in the range of 10−11, so it is concluded that the IEPs of hydroxides should exceed the pH of QD aqueous solution so as to bring forward a significant beneficial effect. Fe(OH)2 was an exception. This might be due to its strong reducibility in air, and the ultimate oxidized product of high valence, Fe(OH)3, exhibits much lower IEPs, and consequently, no apparent alteration of absorbance was observed in the resultant film electrode.

Table 1. Isoelectric Points of Hydroxides and Hydrous Oxides.37 Hydroxides and oxides

IEP

Hydroxides and oxides

IEP

TiO2 (anatase)

6.2

Mg(OH)2

∼12

Ag2O (hydrous)

∼12

Ca(OH)2

>12

Cd(OH)2

>10.5

Sr(OH)2

>12

Co(OH)2

11.4

Ba(OH)2

>12

Cu(OH)2

9-10

Ni(OH)2

>11

Fe(OH)2

∼12

Pb(OH)2

11

Fe(OH)3

6-7

ZnO (hydrous)

10.3

11



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

a)

b)

Figure 2. UV-vis absorption spectra of ZCISe sensitized TiO2 film electrodes with active area of 1.6 cm2 undergone: (a) 50 µL of 0.02 M metal ion solution treatment; (b) various volumes of Mg2+ treatment. Inset, comparative photographs of sensitized electrodes based on various volumes of Mg2+ treatment. Absorption spectra of ZCISe sensitized films (with active area of 1.6 cm2) based on various volumes of Mg2+ treatment are shown in Figure 2b, and the corresponding photographs are shown in the inset. As the treating volume increased systematically, the absorbance increased as well, which was also verified by the incremental dark color of TiO2 films. However, further increasing the treating volume (over 150 µL) led to troublesome in QD loading. For overtreated film, the QDs could not distribute evenly in the film as shown in the inset of Figure 2b. It should be stemmed from the amorphous oxyhydroxides formed in situ blocking the pore of TiO2 films and inhibiting QD diffusion and penetration.6 This indicates that the deposition amount of metal oxyhydroxides should be controlled and optimized.

12


Page 12 of 29

Page 13 of 29

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


a)

b)

c)

d)

Figure 3. Photovoltaic parameter analysis of (a) PCE, (b) Voc, (c) Jsc and (d) FF for ZCISe based QDSCs treated with different metal ion at the treating volume of 10 µL. The dashed lines indicate the average value obtained from five cells for reference and modified samples.

Dependence of Photovoltaic Performance on Metal Ion Treatment. After deposition of MPA-capped water-soluble ZCISe QDs on TiO2 film electrodes with active area of 0.235 cm2, sandwich structured solar cell was fabricated according to our previous method.5,12,13 Assuming the coating of metal oxyhydroxides on TiO2 is uniform and the composition of coating layer is pure oxyhydroxide, the equivalent thickness of a series oxyhydroxides was estimated with results and detailed calculations available in Table S1. For each condition, five cells were prepared and tested in parallel to ensure the validity of the results. The corresponding average photovoltaic performances (short-circuit current density Jsc, open-circuit voltage Voc, fill factor FF, and power conversion efficiency PCE) based on five 13



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

solar cells in each condition were shown in Figure 3 with their detailed information available in Table S2-S6, and the current density-voltage (J-V) curves of corresponding champion cells are shown in Figure S1-S5. The calculated equivalent thickness of deposited metal oxyhydroxide layers shown in Table S1 was only a few angstroms, which is thin enough to allow efficient electron injection from QDs to TiO2 matrix. The influence on photovoltaic performance is classified into two categories. In the case of Mg2+, Ca2+, Sr2+ and Ba2+ treatment, beneficial effect was observed with PCE increased from 8.80% to 9.11%, 8.95%, 8.88% and 8.94%, respectively. The enhanced PCE is mainly ascribed to the enhanced Jsc, from 24.86 to 25.45, 25.17, 25.14, and 25.17 mA/cm2, respectively. Slight increase of Voc could be observed as well, while FF values remained almost unchanged. It is evident that so high PCE and Jsc should be ascribed to increased loading amount of ZCISe QDs, as has been demonstrated in absorption spectra of Figure 2. Higher Jsc is also partially responsible for increased Voc according to the following equation:

Where kB is the Bolzmann constant, T is the absolute temperature, e is the electron charge, β is a parameter related to the nonlinear recombination and j0 is the diode dark current.39 Provided that the other conditions remain unchanged, higher Jsc values signify larger charge accumulation, which accounts for higher Voc as well. It is worth mentioning that the enhanced Voc could be also explained by blocking effect of oxyhydroxides and increased QDs loading amount, under this condition more surface of TiO2 are shielded, leading to retardation of charge recombination with electrolyte.12,13 This will further be characterized by electrochemical impedance spectroscopy (EIS) and open circuit decay (OCVD) analysis below. In the cases of Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+, and Pb2+, the treatment led to negative or even disastrous results with both Voc and Jsc values decreased dramatically, even if some of them (Ag+, Pb2+ and Ni2+) could indeed significantly improve QD loading amount.

14


Page 14 of 29

Page 15 of 29

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


They are all transition elements (Fe, Co, Ni, Cu, Zn, Cd, and Ag) and long-periodic elements (Pb). As has been pointed out by Grätzel et al., Fe should be avoided during the entire solar cell fabrication process, since Fe compounds could dramatically enhance charge recombination in solar cells.40 This is also confirmed by density function theory (DFT) calculation in our recent report.6 Analogous to Fe, most of the transition elements, containing unsaturated d-electrons and/or unoccupied d-orbitals, could serve as severe recombination centers due to their intraband energy levels, leading to serious loss of charge collection efficiency.41 As a consequence, they should also be carefully avoided during solar cell fabrication (at least in the form of free ions). In contrast, Mg, Ca, Sr and Ba are alkaline earth elements. The easily donated s electrons determine the shallow intraband energy levels, and therefore their benign effect on cell performance is observed. It should be noted that for QDSCs containing Cd, Pb or Cu, once they were exposed in air or irradiated by light for a long time, decomposition would occur and free ions would be released, leading to similar effects to those caused by the treatments investigated here. Our results, on one hand, account for the reason why QDSCs usually show particularly poor performance after long time placement, on the other hand, indicate that considerable enthusiasm about long time stability of QD materials should be emphasized.14,42 In the cases of Mg2+, Ca2+, Sr2+ and Ba2+ treatment where beneficial effect appeared, we tuned the treating volume so as to obtain the best photovoltaic performance, and the dependence of average photovoltaic performance on treating volume are displayed in Figure 4. As expected, PCE and Jsc keep increasing as treating volume increased systematically, while Voc and FF remained almost constant. However, subsequently increasing the volume over 30 µL resulted in an invalid or even side effect. When the volume increased to 50 µL, uneven distribution of QD loading occurred, and we gave up measuring these cells since the parallelism of testing results could not be warranted. The champion cells exhibited efficiencies of 9.73%, 9.35%, 9.25% and 9.37% for Mg2+, Ca2+, Sr2+ and Ba2+ treated cells, respectively, and the peak value of Jsc based on Mg2+-40 treatment is 27.28 mA/cm2, which is one of the highest Jsc values in the entire field of QDSCs. We attribute the best performance of 15



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

Mg2+ based solar cells to the thinner oxyhydroxide layer formed on TiO2 as indicated in Table S1, which introduces minor obstacle for QD loading and/or electron injection at large treating volume.

a)

b)

c)

d)

Figure 4. Evolution of photovoltaic parameters for ZCISe QDSCs based on treatment by different volumes of Mg2+, Ca2+, Sr2+, or Ba2+ solution on TiO2 film with active area of 0.235 cm2. (a) PCE, (b) Voc, (c) Jsc, and (d) FF. The incident photon conversion efficiency (IPCE), also termed as external quantum efficiency (EQE), was investigated for well-performed Mg2+, Ca2+, Sr2+ and Ba2+ treated QDSCs, and the results are illustrated in Figure 5 and Figure S6. The photocurrent responses matched the absorption profile well, in which a fairly wide response wavelength range of all cells were displayed with absorption onset at more than 1000 nm. Moreover, all of the cells exhibited relative high IPCE values close to the theoretical up limit of ~80% in consideration

16


Page 16 of 29

Page 17 of 29

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


of loss from the reflection of glass substrate,43 indicating a perfect work state of our cell configurations. After treatment by Mg2+, Ca2+, Sr2+ or Ba2+, the IPCE values increased but did not simply follow proportional relationship. To be specific, more apparent increase at longer wavelength region (550−1000 nm) was observed compared with that at shorter wavelength region (300−550 nm). This observation can be ascribed to the greater absorbance of QD light absorber at short wavelength spectral region in comparison with that at long wavelength window. This means that less QD loading can also harvests nearly all incoming solar photons. As a result, there is little room for further increase of IPCE and Jsc values at short wavelength region and the improved IPCE and Jsc is mainly observed in the long wavelength window. Figure 5a indicated that the IPCE of Mg2+ treated solar cells exhibited the most prominent increase, and Figure 5b verified the trend of Jsc variation as the treating volume increased, both of which verified the results drawn above. In order to improve photocurrent, recent investigations have been focusing on synthesizing QDs with wider light absorption range, however, the non-flat IPCE curves in previous experiments combined with our results5,11,14,44−46 indicated that great potential could still be explored in current QD based solar cells, especially at longer wavelength region where light absorption and IPCE response are intrinsically weak. It has been established that IPCE is influenced by a series of device parameters: IPCE = LHE × φinj × ηcc,47 where LHE is the light harvesting efficiency, φinj is the electron-injection efficiency, and ηcc is the charge-collection efficiency. It is evident that the enhanced QD loading with the introduction of metal oxyhydroxides contributes to an enhanced LHE and therefore improves the IPCE. The insulator metal oxyhydroxide layer between QD and TiO2 substrate acts as an energetic barrier layer and retards the electron injection rate from QD to TiO2 substrate, therefore a reduced φinj value should be expected.48 It is noted that the exact determination of the φinj value can rely on transient absorption (TA) characterization. In the subsequent electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD) characterizations, ηcc is proven to increase with the introduction of metal oxyhydroxide layer. 17



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

a)

b)

Figure 5. (a) Comparison of IPCE of QDSCs based on Mg2+, Ca2+, Sr2+ and Ba2+ treatments. (b) Detailed IPCE of QDSCs based on incremental volume of Mg2+ treatment.

Influence of Anions. As anions might affect the cell performance, their influences were also investigated with a series of 0.02 M Mg2+ ionic aqueous solutions. The results in Table 2, Table S7 and Figure S7 showed that anions of strong acids, such as Cl−, NO3−, and SO42−, makes no difference, while acetate anion (OAc−) diminishes PCE. This should be further ascribed to the decreased loading amount of QDs, and subsequently decreased Jsc and FF values. Having studied microscopic mechanism of QD loading, we propose the reason with reference of early investigation regarding protonation of TiO2 film electrodes in DSCs.49 Both MPA-capped QDs and anions from metal salt solutions are negatively charged, hence competitive absorption on TiO2 film electrodes would be conceivable. Although Cl−, NO3− and SO42− groups are able to attach to TiO2 surface to some extent, the anchoring group COO− of MPA-capped QDs has stronger coordination capability and therefore can replace them easily. Mg(OAc)2, however, contains COO− group as well, and would eventually occupy part of TiO2 surface, giving rise to reduction of QD absorption and resulting in poor performance compared with MgCl2, Mg(NO3)2 and MgSO4 treated solar cells.

18


Page 18 of 29

Page 19 of 29

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


Table 2. Average Photovoltaic Parameters of Solar Cells based on a Series of 10 µL of 0.02 M Mg2+ aqueous solutions. Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

MgCl2

0.608 ± 0.008

25.45 ± 0.09

0.589 ± 0.008

9.11 ± 0.12

Mg(NO3)2

0.608 ± 0.003

25.38 ± 0.19

0.591 ± 0.006

9.11 ± 0.06

MgSO4

0.605 ± 0.005

25.43 ± 0.20

0.588 ± 0.005

9.10 ± 0.05

Mg(OAc)2

0.604 ± 0.004

24.97 ± 0.17

0.568 ± 0.008

8.55 ± 0.07

Mg salts

Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) was employed to elucidate the effect of Mg2+ treatment on the performance of devices with utilization of standard models developed.4−10 The EIS was recorded at different Vapp ranging from −0.3 V to −0.55 V by applying a 20 mV AC sinusoidal signal with the frequency ranging from 100 kHz to 0.1 Hz under dark conditions. The Vapp dependent Cµ (chemical capacitance representing the change of electron density as a function of Fermi level) and Rrec (the charge recombination resistance at the photoanode/electrolyte interfaces) extracted from corresponding Nyquist curves are illustrated in Figure 6. The deposited magnesium oxyhydroxide layer might exert an influence on Voc in two manners. On the one hand, it could trigger a change in the Fermi level of TiO2 by establishing a new charge equilibrium; on the other hand, the insulator magnesium oxyhydroxide deposited on TiO2 could serve as energetic barrier layer to inhibit charge recombination. The results in Figure 6a indicated that the treatment of Mg2+ did not contribute to significant changes in Cµ. This indicated that the magnesium oxyhydroxide coatings do not induce a displacement of the TiO2 conduction band edge. However, a systematical enhancement of Rrec was observed in Figure 6b as Mg2+ treating volume increased steadily. Further comparison of Nyquist plots and Bode plots of solar cells based on different Mg2+ treating volume at forward bias of −0.55 V was illustrated in Figure S8. It is known that Rrec is inversely proportional to the charge recombination rate at

19



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

Page 20 of 29

the interface between TiO2/electrolyte and QD/electrolyte.50,51 Since Mg2+ treatment was applied before QD loading, charge recombination at QD/electrolyte interface should not change dramatically. Through integrated into account, the observed greater recombination resistance after Mg2+ treatment should be ascribed to the blockage of electron recombination at TiO2/QD interface and TiO2/electrolyte interface. Meanwhile, the calculated electron lifetime (τn = Rrec × Cµ) at the open-circuit condition as shown in Table 3 is enhanced systematically from 906 ms to 1487 ms with the increase of Mg2+ amount in the process of photoanode treatment. The enhance electron lifetime favors the improvement of charge collection efficiency, ηcc. The systematically increased τn can also be indicated by Bodes plot as shown in Figure S8b (τn = 1/2πfmax, where fmax stands for the characteristic peak frequency), wherein the calculated electron lifetime increase from 454 to 557, 632, 686 and 717 ms as Mg2+ treating volume increases steadily. Having comprehensively analyzed the configuration of solar cells after Mg2+ treatment, the suppression of charge recombination should be ascribed to the blocking effect, which is contributed by both the oxyhydroxide coating layers and the increased QD loading amount. Table 3. Simulated Values of Recombination Resistance (Rrec), Chemical Capacitance and Calculated Electron Lifetime (τn) of ZCISe Sensitized Solar Cells with and without Mg2+ Treatment at the Forward Bias of −0.55 V. Cells

Cµ (mF/cm2)

Rrec (Ω cm2)

τn (ms)

No Treatment

3.000

302.0

905.9

Mg2+-10

2.984

366.9

1095

Mg2+-20

2.991

385.0

1152

Mg2+-30

3.084

420.3

1296

Mg2+-40

3.133

474.5

1487

20


Page 21 of 29

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


a)

b)

Figure 6. EIS characterization of QDSCs with and without Mg2+ treatment. (a) chemical capacitance Cµ, (b) recombination resistance Rrec.

Open Circuit Decay. Open circuit decay (OCVD) analysis was employed to investigate the different electron recombination processes furthermore.6−9,52 The cells were illuminated by a white LED with intensity of 100 mW/cm2 initially, followed by switching the light off after reaching a steady stage and recording the open circuit voltage values versus time. The OCVD results were shown in Figure 7. We could observe from Figure 7a that the transient Voc values of the cells with and without Mg2+ treatment maintained at corresponding open circuit voltages under illumination and continued to decay according to the exponential formula until close to 0 V in the dark. However, the decay rate decreases as the Mg2+ treatment volume increases. The rate of decay (slope of the decay curve) is a direct reflection of change recombination and electron lifetime. The electron lifetime, τn, is calculated by the equation:

where kB is the Bolzmann constant, T is the absolute temperature (298K) and e is the electron charge.51 The plot of τn versus Voc is shown in Figure 7b. It is concluded that the calculated electron lifetime (τn) is longer at the same Voc value as Mg2+ treatment volume increases. The trend is consistent with the EIS results, both of which verified the conclusion

21



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

drawn above.

a)

b)

Figure 7. (a) Voc decay curves of QDSCs with and without Mg2+ treatments recorded during the relaxation from illuminated quasi-equilibrium to the dark, (b) electron lifetime derived from OCVD measurements.

CONCLUSIONS In this work, we have examined the effect of various metal ion deposition on TiO2 films without sintering procedure for ZCISe based QDSCs. Our investigation revealed that oxyhydroxides were formed in alkaline environment, which accounted for surface isoelectric properties and carrier recombination behavior. The oxyhydroxides with higher IEP promoted the loading amount of MPA-capped ZCISe QDs onto TiO2 film electrodes, while high QD loading amount is not equivalent to high photocurrent and better photovoltaic performance, the influence on charge recombination has to be considered as well. Our work concluded that the treatment based on alkaline earth metal ion (Mg2+, Ca2+, Sr2+ and Ba2+) was beneficial due to their strong affinity with MPA-capped QDs (hydroxides with high IEPs) and suppression of electron recombination via TiO2 surface. The amount of metal ions applied for treatment was also investigated, which indicated that the performance of solar cells increased continuously as the amount increased until reaching saturated loading amount. However, further increasing the amount resulted in trouble for QD penetration in porous TiO2. Benefiting from the treatment, the champion cells exhibit efficiencies of 9.73%, 9.35%, 9.25%, and 9.37% for 22


Page 22 of 29

Page 23 of 29

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


Mg2+, Ca2+, Sr2+ and Ba2+ treated cells, respectively, which are among the highest in the entire field of QD based solar cells. Finally, the influence of anions on the performance of solar cells was investigated. To be specific, anion of strong acid, such as Cl−, NO3−, and SO42−, makes no difference, while acetate anion (OAc−) deteriorates PCE, which are related to the affinity of corresponding anion with TiO2.

ASSOCIATED CONTENT Supporting Information Deduction of IEPs of Ca(OH)2, Sr(OH)2 and Ba(OH)2, calculations of equivalent thickness of coating layers on TiO2 surface, calculated equivalent thickness of a series of coating layers, J-V curves of champion ZCISe QDSCs based on a series of treatments, details of photovoltaic performance of solar cells based on a series of treatments, detailed IPCE of QDSCs based on Ca2+, Sr2+ and Ba2+ treatment, J-V curves of champion cells based on MgCl2, Mg(NO3)2, MgSO4 and Mg(OAc)2 treatment, details of photovoltaic performance of solar cells based on MgCl2, Mg(NO3)2, MgSO4 and Mg(OAc)2 treatment, Nyquist plots and Bode plots of Mg2+ treated cells at the forward bias of −0.55 V.

AUTHOR INFORMATION Corresponding Author

[email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (No. 21421004, 91433106), and the Fundamental Research Funds for the Central Universities in China.

23



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

REFERENCES (1) Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells. Chem. Rev. 2010, 110, 6664‒6688. (2) Albero, J.; Clifford, J. N.; Palomares, E. Quantum dot Based Molecular Solar Cells. Coord. Chem. Rev. 2014, 263, 53–64. (3) Nozik, A. J.; Beard, M. C.; Luther, J. M.; Law, M.; Ellingson, R. J.; Johnson, J. C. Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells. Chem. Rev. 2010, 110, 6873‒6890. (4) Du, Z.; Pan, Z.; Fabregat-Santiago, F. ; Zhao, K.; Long, D.; Zhang, H.; Zhao, Y.; Zhong, X.; Yu, J.-S.; Bisquert, J. Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%. J. Phys. Chem. Lett. 2016, 7, 3103‒3111. (5) Du, J.; Du, Z.; Hu, J.-S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong X.; Wan, L.-J. Zn−Cu−In−Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201‒4209. (6) Ren, Z.; Wang, Z.; Wang, R.; Pan, Z.; Gong, X.; Zhong, X. Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum Dot Sensitized Solar Cells. Chem. Mater. 2016, 28, 2323‒2330. (7) Wang, J.; Li, Y.; Shen, Q.; Izuishi, T.; Pan, Z.; Zhong, X. Mn Doped Quantum Dot Sensitized Solar Cells with Power Conversion Efficiency Exceeding 9%. J. Mater. Chem. A 2016, 4, 877‒886. (8) Yang, J.; Wang, J.; Zhao, K.; Izuishi, T.; Li, Y.; Shen, Q.; Zhong, X. CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9%. J. Phys. Chem. C 2015, 119, 28880‒28808. (9) Ren, Z.; Wang, J.; Pan, Z.; Zhao, K.; Zhang, H.; Li, Y.; Zhao, Y.; Mora-Seró, I.; Bisquert, J.; Zhong, X. Amorphous TiO2 Buffer Layer Boosts Efficiency of Quantum Dot Sensitized Solar Cells to over 9%. Chem. Mater. 2015, 27, 8398‒8405. (10) Zhao, K.; Pan, Z.; Mora-Seró, I.; Canovas, E.; Wang, H.; Song, Y.; Gong, X.; Wang, J.; Bonn, M.; Bisquert. J.; Zhong, X. Boosting Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137, 5602‒5609. (11) Kim, J.-Y.; Yang, J.; Yu, J. H.; Baek, W.; Lee, C.-H.; Son, H. J.; Hyeon T.; Ko, M. J. Highly Efficient Copper-Indium-Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers. ACS Nano 2015, 9, 11286‒11295. (12) Li, W.; Zhong, Capping Ligand-Induced Self-Assembly for Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 796‒806.

24


Page 24 of 29

Page 25 of 29

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


(13) Zhao, K.; Pan, Z.; Zhong, X. Charge Recombination Control for High Efficiency Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2016, 7, 406‒417. (14) McDaniel, N.; Fuke, N.; Makarov, N. S.; Pietryga, J. M.; Klimov, V. I. An Integrated Approach to Realizing High-performance Liquid-junction Quantum Dot Sensitized Solar Cells. Nat. Commun. 2013, 4, 2887‒2896. (15) Zaban, A.; Chen, S. G.; Chappel, S.; Gregg, B. A. Bilayer Nanoporous Electrodes for Dye Sensitized Solar Cells. Chem. Commun. 2000, 2231‒2232. (16) Kay, A.; Grätzel, M. Dye-Sensitized Core-Shell Nanocrystals: Improved Efficiency of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an Insulating Oxide. Chem. Mater. 2002, 14, 2930‒2935. (17) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Control of Charge Recombination Dynamics in Dye Sensitized Solar Cells by the Use of Conformally Deposited Metal Oxide Blocking Layers. J. Am. Chem. Soc. 2003, 125, 475‒482. (18) O’Regan, B. C.; Scully, S.; Mayer. A. C.; Palomares, E.; Durrant, J. The Effect of Al2O3 Barrier Layers in TiO2/Dye/CuSCN Photovoltaic Cells Explored by Recombination and DOS Characterization Using Transient Photovoltage Measurements. J. Phys. Chem. B 2005, 109, 4616‒4623. (19) Barea, E.; Xu, X.; González-Pedro, V.; Ripollés-Sanchis, T.; Fabregat-Santiago, F.; Bisquert, J. Origin of Efficiency Enhancement in Nb2O5 Coated Titanium Dioxide Nanorod Based Dye Sensitized Solar Cells. Energy Environ. Sci. 2011, 4, 3414‒3419. (20) Chandiran, A. K.; Tetreault, N.; Humphry-Baker, R.; Kessler, F.; Baranoff, E.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Subnanometer Ga2O3 Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells. Nano Lett. 2012, 12, 3941‒3947. (21) Son, H.-J.; Wang, X.; Prasittichai, C.; Jeong, N. C.; Aaltonen, T.; Gordon R. G.; Hupp, J. T. Glass-Encapsulated Light Harvesters: More Efficient Dye-Sensitized Solar Cells by Deposition of Self-Aligned, Conformal, and Self Limited Silica Layers. J. Am. Chem. Soc. 2012, 134, 9537‒9540. (22) Hanson, K.; Losego, M. D.; Kalanyan, B.; Ashford, D. L.; Parsons, G. N.; Meyer, T. J. Stabilization of [Ru(bpy)2(4,4′-(PO3H2)bpy)]2+ on Mesoporous TiO2 with Atomic Layer Deposition of Al2O3. Chem. Mater. 2013, 25, 3‒5. (23) Chandiran, A. K.; Nazeeruddin, M. K.; Grätzel, M. The Role of Insulating Oxides in Blocking the Charge Carrier Recombination in Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2013, 24, 1615‒1623. (24) Brennan, T. P.; Tanskanen, J. T.; Roelofs, K. E.; To, J. W. F.; Nguyen, W. H.; Bakke, J. R.; Ding, I-K.; Hardin, B. E.; Sellinger, A.; McGehee, M. D.; Bent, S. F. TiO2 Conduction Band Modulation

25



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

with In2O3 Recombination Barrier Layers in Solid-State Dye-Sensitized Solar Cells. J. Phys. Chem. C 2013, 117, 24138‒24149. (25) Tachan, Z.; Hod, I.; Shalom, M.; Grinis, L.; Zaban, A. The Importance of the TiO2/quantum Dots Interface in the Recombination Processes of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 3841‒3845. (26) Prasittichai, C.; Avila, J. R.; Farha, O. K.; Hupp, J. T. Systematic Modulation of Quantum (Electron) Tunneling Behavior by Atomic Layer Deposition on Nanoparticulate SnO2 and TiO2 Photoanodes. J. Am. Chem. Soc. 2013, 135, 16328‒16331. (27) Brennan, T. P.; Trejo, O.; Roelofs, K. E.; Xu, J.; Prinz, F. B.; Bent, S. F. Efficiency Enhancement of Solid-state PbS Quantum Dot-sensitized Solar Cells with Al2O3 Barrier Layer. J. Mater. Chem. A 2013, 1, 7566‒7571. (28) Roelofs, K. E.; Brennan, T. P.; Dominguez, J. C.; Bailie, C. D.; Margulis, G. Y.; Hoke, E. T.; McGehee, M. D.; Bent, S. F. Effect of Al2O3 Recombination Barrier Layers Deposited by Atomic Layer Deposition in Solid-State CdS Quantum Dot-Sensitized Solar Cells. J. Phys. Chem. C 2013, 117, 5584‒5592. (29) Roelofs, K. E.; Brennan, T. P.; Bent, S. F. Interface Engineering in Inorganic-Absorber Nanostructured Solar Cells. J. Phys. Chem. Lett. 2014, 5, 348‒360. (30) Yum, J.-H.; Nakade, S.; Kim, D.-Y.; Yanagida, S. Improved Performance in Dye-Sensitized Solar Cells Employing TiO2 Photoelectrodes Coated with Metal Hydroxides. J. Phys. Chem. B 2006, 110, 3215‒3219. (31) Peiris, T. A. N.; Senthilarasu, S.; Wijayantha, K. G. U. Enhanced Performance of Flexible Dye-Sensitized Solar Cells: Electrodeposition of Mg(OH)2 on a Nanocrystalline TiO2 Electrode. J. Phys. Chem. C 2012, 116, 1211‒1218. (32) Peiris, T. A. N.; Wijayantha, K. G. U.; García-Canadas, Insights into Mechanical Compression and the Enhancement in Performance by Mg(OH)2 Coating in Flexible Dye Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 2912‒2919. (33) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. Core-Shell Nanoporous Electrode for Dye Sensitized Solar Cells: the Effect of the SrTiO3 Shell on the Electronic Properties of the TiO2 Core. J. Phys. Chem. B 2003, 107, 1977‒1981. (34) Wang, Z.-S.; Yanagida, M.; Sayama, K.; Sugihara, H.; Electronic-Insulating Coating of CaCO3 on TiO2 Electrode in Dye-Sensitized Solar Cells: Improvement of Electron Lifetime and Efficiency. Chem. Mater. 2006, 18, 2912‒2916. (35) Zhang, L.; Shi, Y.; Peng, S.; Liang, J.; Tao, Z.; Chen, J. Dye-sensitized Solar Cells Made from BaTiO3-coated TiO2 Nanoporous Electrodes. J. Photochem. Photobiol., A 2008, 197, 260‒265.

26


Page 26 of 29

Page 27 of 29

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


(36) Kim, S.-K.; Son, M.-K.; Park, S.; Jeong, M.-S.; Prabakar, K.; Kim, H.-J. Surface modification on TiO2 nanoparticles in CdS/CdSe Quantum Dot-sensitized Solar Cell. Electrochim. Acta 2014, 118, 118‒123. (37) Parks, G. A. The Isoelectric Points of Solid Oxides, Solid Hydroxides, and Aqueous Hydroxo Complex Systems. Chem. Rev. 1965, 65, 177‒198. (38) Meng, X.; Du, J.; Zhang, H.; Zhong. X. Optimizing the Deposition of CdSe Colloidal Quantum Dots on TiO2 Film Electrode via Capping Ligand Induced Self-assembly Approach. RSC Adv. 2015, 5, 86023‒86030. (39) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Seró, I.; Bisquert, J. Characterization of Nanostructured Hybrid and Organic Solar Cells by Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 9083‒9118. (40) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Grätzel, C.; Nazeeruddin, M. K.; Grätzel, M. Fabrication of Thin Film Dye Sensitized Solar Cells with Solar to Electric Power Conversion Efficiency over 10%. Thin Solid Films 2008, 516, 4613‒4619. (41) Cherepy, N. J.; Liston, D. B.; Lovejoy, J. A.; Deng, H.; Zhang, J. Z. Ultrafast Studies of Photoexcited Electron Dynamics in γ- and α- Fe2O3 Semiconductor Nanoparticles. J. Phys. Chem. B 1998, 102, 770‒776. (42) Aldana, J.; Wang, Y. A.; Peng, X. Photochemical Instability of CdSe Nanocrystals Coated by Hydrophilic Thiols. J. Am. Chem. Soc. 2001, 123, 8844‒8850. (43) Wang, J.; Mora-Seró, I.; Pan, Z.; Zhao, K.; Zhang, H.; Feng, Y.; Yang, G.; Zhong, X.; Bisquert, J. Core/Shell Colloidal Quantum Dot Exciplex States for the Development of Highly Efficient Quantum Dot Sensitized Solar Cells. J. Am. Chem. Soc. 2013, 135, 15913‒15922. (44) Jara, D. H.; Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. Size-Dependent Photovoltaic Performance of CuInS2 Quantum Dot Sensitized Solar Cells. Chem. Mater. 2014, 26, 7221‒7228. (45) Chang, J.-Y.; Lin, J.-M.; Su, L.-F.; Chang, C.-F. Improved Performance of CuInS2 Quantum Dot-Sensitized Solar Cells Based on a Multilayered Architecture. ACS Appl. Mater. Interfaces 2013, 5, 8740‒8752. (46) Chang, C.-C.; Chen, J.-K.; Chen, C.-P.; Yang, C.-H.; Chang, J.-Y. Synthesis of Eco-Friendly CuInS2 Quantum Dot-Sensitized Solar Cells by a Combined Ex Situ/in Situ Growth Approach. ACS Appl. Mater. Interfaces 2013, 5, 11296‒11306. (47) Fillinger, A.; Parkinson, B. A. The Adsorption Behavior of a Rutheninm-based Sensitizing Dye to Nanocrystalline TiO2 Coverage Effects on the External and Internal Sensitization Quantum Yields. J. Electrochem. Soc. 1999, 146, 4559‒4564. (48) Yang, J.; Oshima, T.; Yindeesuk, W.; Pan, Z.; Zhong, X.; Shen, Q. Influence of Linker

27



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

Molecules on Interfacial Electron Transfer and Photovoltaic Performance of Quantum Dot Sensitized Solar Cells, J. Mater. Chem. A, 2014, 2, 20882–20888. (49) Wang, Z.-S.; Yamaguchi, T.; Sugihara, H.; Arakawa, H. Significant Efficiency Improvement of the Black Dye-Sensitized Solar Cell through Protonation of TiO2 Films. Langmuir 2005, 21, 4272‒4276. (50) Mora-Seró, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848‒1857. (51) González-Pedro, V.; Xu, X.; Mora-Seró, I.; Bisquert, J. Modeling High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 4, 5783‒5790. (52) Zhang, Q.; Guo, X.; Huang, X.; Huang, S.; Li, D.; Luo, Y.; Shen, Q.; Toyoda, T.; Meng, Q. Highly Efficient CdS/CdSe-sensitized Solar Cells Controlled by the Structural Properties of Compact Porous TiO2 Photoelectrodes. Phys. Chem. Chem. Phys. 2011, 13, 4659‒4667.

28


Page 28 of 29

Page 29 of 29

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


Table of Contents (TOC)

Improving Loading Amount and Performance of Quantum Dot Sensitized Solar Cells through Metal Salt Solutions Treatment on Photoanode Wenran Wang, Jun Du, Zhenwei Ren, Wenxiang Peng, Zhenxiao Pan* and Xinhua Zhong*

29


Improving Loading Amount and Performance of Quantum Dot

Recommend Documents