High Efficiency Quantum Dot Sensitized Solar Cells Based on Direct

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High Efficiency Quantum Dot Sensitized Solar Cells based on Direct Adsorption of Quantum Dots on Photoanodes Wenran Wang, Guocan Jiang, Juan Yu, Wei Wang, Zhenxiao Pan, Naoki Nakazawa, Qing Shen, and Xinhua Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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ACS Applied Materials & Interfaces

High Efficiency Quantum Dot Sensitized Solar Cells based on Direct Adsorption of Quantum Dots on Photoanodes Wenran Wang,†,‡ Guocan Jiang,† Juan Yu,† Wei Wang,† Zhenxiao Pan,*,‡ Naoki Nakazawa,§ Qing Shen,*,§,# 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 §

Department of Engineering Science, University of Electro-Communications, Tokyo

182-8585, Japan #

Japan Science and Technology Agency (JST), Saitama 332-0012, Japan

Email: [email protected] (for Z. P.); [email protected] (for Q. S.); [email protected] (for X. Z.) Tel/Fax: +86 21 6425 0281

KEYWORDS: photovoltaics, quantum dot sensitized solar cells, direct adsorption, loading amount, QD agglomeration

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ABSTRACT: Unambiguously direct adsorption (DA) of initial oil-soluble QDs on TiO2 film electrode is a convenient and simple approach in the construction of quantum dot sensitized solar cells (QDSCs). Regrettably, low QD loading amount and poor reproducibility shadow the advantages of DA route and constrain its practical application. Herein, the influence of experimental variables in DA process on QD loading amount as well as on the photovoltaic performance of the resultant QDSCs were investigated and optimized systematically, including the choice of solvent, purification of QDs, sensitization time, as well as QD concentration. Experimental results demonstrated that it is essential to choose appropriate solvent as well as control purification cycles of original QD suspensions so as to realize satisfactory QD loading amount and ensure the high reproducibility. In addition, DA mode renders efficient electron injection from QD to TiO2, yet low QD loading amount and adverse QD agglomeration in comparison with the well-developed capping ligand induced self-assembly (CLIS) deposition approach. Mg2+ treatment on TiO2 photoanodes can promote QD loading amount in DA mode. The optimized QDSCs based on DA mode exhibited efficiencies of 6.90% and 9.02% for CdSe and Zn−Cu−In−Se QDSCs, respectively, which were comparable to the best results based on CLIS mode (6.88% and 9.56%, respectively).

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1. INTRODUCTION Quantum dot sensitized solar cells (QDSCs) are promising candidates for the third generation solar cells due to the distinguished optoelectronic properties of quantum dot light-harvesting materials as well as the low cost of production.1−5 The photovoltaic performance of QDSCs has been undergoing a rapid evolution with power conversion efficiencies (PCEs) increasing from less than 5% in 2012 to the state-of-art 12.2%.6 However, it remains an urgent challenge to further optimize device configuration and improve the photovoltaic performance of QDSCs to make it competitive to the analogue dye sensitized solar cells (DSCs).7−12 QDSC shares similar structures and working principles to DSC, yet the size of a QD is larger than a dye molecule.13 In addition, the terminal anchoring groups of dye molecules can bond to Ti4+ while the presynthesized QDs in oil-phase solution exhibit no such interaction. As a consequence, the penetration and loading of QDs on porous TiO2 substrates encounter greater resistance, and one of the factors that limits the performance of QDSCs is the mode of QD sensitizers depositing on TiO2 film electrodes since this process determines the loading amount of QD as well as the electron injection rate from QD to TiO2 substrate. In recent years, capping ligand-induced QD self-assembly (CLIS) deposition method has been the main stream for immobilizing QD on TiO2 film electrode, and a series of new PCE records have been achieved based on this sensitization route.6,14−20 In this approach, the native organic ligands containing long hydrocarbon chain tails around the initial oil-soluble QDs are first replaced by bifunctional molecules, such as mercaptopropionic acid (MPA), via a phase transfer process, to get the MPA-capped water-soluble QDs. Then the MPA-capped QDs are immobilized on TiO2 film electrodes relying on the driving force from the formation of coordinating bonds between terminal carboxyl group and TiO2.21 However, completing this ligand exchange process is time-intensive, and sophisticated techniques are necessary to attain a colloidal stable MPA-QD aqueous dispersion to ensure high QD loading amount for the following step of QD self-assembly deposition. In addition, the introduction of the linker molecule MPA between QD and TiO2 substrate hinders photoexcited electron extraction rate 3



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as well as creates additional trapping state defects for QDs.22−25 Unambiguously, direct adsorption (DA) of initial oil-soluble QD on TiO2 electrode is a much simpler deposition approach, avoiding the tedious ligand exchange process and, more importantly, rendering an intimate contact between QD and TiO2 matrix, which favors electron injection from QD to TiO2.26−38 Unfortunately, low loading amount and poor reproducibility are still bottlenecks for the DA route, and therefore limits the photovoltaic performance of the resultant cell devices. In 1998, Zaban et al. demonstrated the sensitization of TiO2 through direct adsorption of presynthesized InP QDs and opened up this sensitization mode for the first time.26 Following this pioneering step, different QD sensitizer materials were successfully tethered on TiO2 film electrodes to construct QDSCs, and a series of encouraging PCEs have been obtained.27−34 Based on this route, Klimov et al. reported an appealing PCE of 5.5% based on CuInSexS1−x QD sensitizer in 2013.32 By precisely controlling the thickness of ZnS passivation layer, Ko et al. achieved a PCE of 8.10% with use of CuInSe2 QDs in 2015.34 However, the adsorption behavior of QDs on mesoporous TiO2 films seems to be sophisticated. This should be ascribed to the limited knowledge about adsorption mechanism. In contrast to CLIS mode where coordination bonds between linker molecule around QD and TiO2 provide strong driving force for QD loading, a weaker interaction without the formation of strong chemical bonds plays a vital role in DA process. As a consequence, DA process is influenced by various factors, such as nature of solvent,28−31 particle size distribution of TiO2 particle29, QD purification process,30,35−37 adsorption time,29,33,35,38 and concentration of QD suspension.38 In addition, there has been a consensus that the uncontrollable but detrimental QD agglomeration process is an outstanding feature in DA process.21,29,30,33,35,36,38 As a consequence, the performance of QDSCs associated with DA mode is sensitive to experimental conditions and lacks repeatability. Therefore, exploring the inherent mechanism of DA and creating a highly reproducible DA process for high efficiency QDSCs is meaningful. Herein, we systematically investigated and optimized the influence of experimental 4


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variables in DA process on the QD loading amount, and the photovoltaic performance of the resultant QDSCs, including the purification of QDs, deposition time, concentration of QD suspension, and photoanode pretreatment with Mg2+ etc. CdSe QDs were chosen as a model for this investigation due to the excellent photovoltaic performance of the resultant solar cells as well as their reproducible and facile synthetic nature. The acquired knowledge was further applied to Zn−Cu−In−Se (ZCISe) QDSCs with broader photo-response range and better environmental benignity.15 Experimental results demonstrated that the utilization of sole dichloromethane (CH2Cl2) as solvent and an optimized purification of crude QD suspensions are essential to achieve high QD loading amount for DA process. Meanwhile, QD agglomeration on TiO2 substrates in DA process could be avoided by control of sensitization time and the concentration of QD suspensions. Mg2+ pretreatment of TiO2 photoanodes could further promote QD loading amount and lead to better cell performance. Transient absorption (TA) measurement results confirmed that DA approach favors the electron transfer kinetics between QDs and TiO2 substrates related to CLIS route. The optimized performance in DA approach (PCE = 6.90% and 9.02% for CdSe and ZCISe based QDSCs, respectively) reached a comparable PCE level to the ones achieved by the well-developed CLIS process (PCE = 6.88% and 9.56% for CdSe and ZCISe based QDSCs, respectively).

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2. EXPERIMENTAL SECTION Chemicals. Sodium sulfide (Na2S·9H2O, 99.99%), zinc acetate (Zn(OAc)2, 99.99%), magnesium chloride (MgCl2·6H2O, 99.99%), and 3-mercaptopropionic acid (MPA, 98%) were received from Aldrich. Poly(vinyl pyrrolidone) (PVP) with molecular weight of 8000 was obtained from Aladdin China. All reagents were used as received without further purification. Synthesis and Purification of CdSe and Zn−Cu−In−Se (ZCISe) QDs. Oil-soluble OAm-capped CdSe,39 and Zn−Cu−In−Se (ZCISe)15 QDs were synthesized through a high temperature pyrolysis route according to our previous reports and all chemicals used were identical to literature methods. The oil-soluble CdSe and ZCISe QDs for direct adsorption (DA) were obtained from the as-prepared OAm-capped QDs after several cycles of purification. In each purification cycle, the QD suspensions were precipitated with addition of nonsolvent, followed by redispersion of pellet in dichloromethane (CH2Cl2) after centrifugation. For the first cycle of CdSe precipitation, a mixture of methanol/acetone (v/v = 4/1) served as nonsolvent, while sole acetone was used for the other precipitation cycles. In the case of ZCISe QDs, the ethanol/acetone mixture (v/v = 2/1) and sole acetone served as nonsolvent for the first and the second precipitation cycle, respectively. Water soluble MPA-capped CdSe and ZCISe QDs for CLIS were obtained from the original OAm-capped QD suspensions via ligand exchange procedure with use of MPA as the phase transfer agent.15,39 After purification, the water-soluble MPA-capped CdSe and ZCISe QDs were dispersed in water with pH of 10. Sensitization of TiO2 Photoanode and Construction of Solar Cells. The preparation of TiO2 mesoporous film electrodes were the same as our previous work.21 For CdSe QDSCs, the electrodes with an area of 0.235 cm2 were composed of 10 µm-thick transparent layer and 5 µm-thick light scattering layer, while for ZCISe QDSCs, the electrodes were composed of 20 µm-thick transparent layer and 5 µm-thick light scattering layer. In UV-vis absorption and transient adsorption characterization, the adopted large TiO2 film electrodes possess only a 4.0 6


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µm transparent layer with active area of 1.6 cm2 (2.0 cm × 0.8 cm). The sensitization process was realized by immersing TiO2 mesoporous film electrodes in QD-CH2Cl2 suspensions under dark condition for a demand period of time. For CdSe based QDSCs, the experimental variables (QD purification, sensitization time and suspension concentration) changed in different sections, while for optimized CLIS based QDSCs, one batch of QDs synthesized according to the approach in ref. 15 were eventually dissolved in 10 mL of CH2Cl2 and the sensitization time was 4 h. For solar cell fabrication, a ZnS barrier layer was overcoated on the sensitized photoanodes after QD deposition by alternately dipping the QD sensitized electrode into 0.1 M Zn(OAc)2 methanol solution and 0.1 M Na2S aqueous solution (four cycles for CdSe and six cycles for ZCISe based sensitized photoanode). The ZnS coated CdSe sensitized films were further coated with SiO2 layer by immersing them in 0.01 M tetraethyl orthosilicate ethanol solution at 35 oC for 2 h according to our modified method.40 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 with a binder clip and filled with polysulfide electrolyte aqueous solution (2.0 M Na2S, 2.0 M S, and 0.2 M KCl). Cu2S counter electrodes were prepared by immersing brass foils in 1.0 M HCl solution at 90 °C for 30 min, followed by reacting them with fresh polysulfide aqueous solution. For further optimization, Mg2+ treatment was applied before QD sensitization by pipetting 0.02 M MgCl2 aqueous solution onto TiO2 film with area of 0.235 cm2, followed by evaporation of water at 50 oC for 60 min.41 The volume of MgCl2 aqueous solution used are 10 and 40 µL for CdSe and ZCISe based QDSCs, respectively, and the volume are 50 and 200 µL for corresponding UV-vis absorption characterization with use of large TiO2 film electrodes. The modified polysulfide electrolytes with addition of PVP (20 wt%) were applied for further optimization, too.42 Characterization. The UV−vis absorption and photoluminescence (PL) emission spectra were recorded from a UV−visible spectrophotometer (Shimadzu UV-3101 PC) and a fluorescence spectrophotometer (Cary Eclipse Varian), respectively. The photovoltaic properties and J-V curves of the QDSCs were measured by Keithley 2400 source meter 7



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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 0.236 cm2. Incident photon-to-current conversion efficiency (IPCE) spectrum was measured using a Keithley 2000 multimeter with illumination of a 300 W tungsten lamp with a Spectral Product DK240 monochromator. Electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical workstation (Zahner, Zennium) at 298 K under dark conditions at different forward bias ranging from 0.3 V to 0.6 V, applying a 20 mV AC sinusoidal signal over the constant applied bias with the frequency ranging from 1 MHz to 0.1 Hz. Transition electron microscopy (TEM) images were obtained using a JEOL JEM-2100 instrument with accelerating voltage of 200 kV. For sample preparation, a few drops of diluted CdSe QD suspensions were taken and dipped on a TEM grid. The FTIR spectra were recorded on a NICOLET 6700 spectrometer equipped with a DTGS KBr detector. Samples for FTIR measurements were prepared by dipping QD suspensions on KBr crystal substrates followed by drying at 90 oC. Electrospray ionization mass spectrometry (ESI-MS) was performed using Agilent 1100 LC/MSD mass-spectrometer. Cyclic voltammetry (CV) was performed on an impedance analyzer (Zahner, Zennium) with a three-electrode configuration at 25 oC. All cyclic voltammograms were recorded at the rate of 20 mV/s under dark condition in polysulfide electrolyte using a calomel electrode as reference and a Pt wire as an auxiliary electrode. The transient absorption (TA) measurements were carried out in nitrogen atmosphere. The laser source was a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, repetition rate of 1000 Hz, and pulse width of 150 fs. The light was divided into two parts. One part was used to generate a white light as the probe pulse; the other as the pump light to pump an optical parametric amplifier (TOPAS from Quantronix) to generate light pulses with tunable wavelength from 290 nm to 2.3 µm. A pump light with wavelength of 470 nm was used to excite the CdSe QDs and a probe wavelength of 618 nm was used to monitor 8


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the intraband absorption of electrons from conduction band to higher excited states. The area of the laser beam on the sample was about 0.2 cm2. The samples showed no apparent photo-damage during the measurement processes.

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3. RESULTS AND DISCUSSIONS The fabrication process of QDSCs based on DA deposition route was optimized initially with use of 5.4 nm CdSe QDs (the excitionic absorption maximum is located at wavelength of 618 nm, and the TEM image is available in Figure S1) as a model due to the excellent photovoltaic performance of the resultant solar cells as well as their reproducible and facile synthetic method. The experimental variables in the optimization of DA process included the purification of QDs, sensitization time, QD concentration, and Mg2+ pretreatment of TiO2 photoanodes. The experimental results based on DA route were compared to those based on the well-developed CLIS approach. The acquired knowledge was further applied to Zn−Cu−In−Se (ZCISe) based QDSCs with broader photoresponse to gain better PCEs. To minimize the deviation, five cells were prepared and tested in parallel under each experimental condition. Purification of QD Suspensions. In DA route, the deposition of QDs was realized by immersing TiO2 film electrodes in as-prepared QD dispersions in nonpolar organic solvents (such as dichloromethane, hexane, or toluene etc.) for a certain period of time. Therefore, the nature of solvent plays a crucial effect on the deposition of QD on film electrodes. We adopted dichloromethane (CH2Cl2) as solvent for the following investigation because CH2Cl2 has been found to be the ideal choice in previous reports.28,29,33,34 Besides CH2Cl2, we discovered that some mixed solvents are also effective to realize high QD loading amount, yet the loading amount based on CH2Cl2 was still the greatest (see Figure S2 for detailed information). The purification process before QD deposition has been regarded as an efficient way to realize high QD loading in previous reports,32,35−37,43 in which the crude QD dispersions undergo precipitation and redispersion in fresh solvent, accompanied with the removal of original long-chain organic capping ligands around the surface of QDs. An overwhelming quantity of ligands can be removed based on alternate purification cycles, as proven in previous works by NMR, XPS and ICP-AES techniques,36,37,43 and the removal of surface 10


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ligand is believed to be the key factor that promote the performance of the resultant cell devices. In order to improve the photovoltaic performance of QDSCs, the influence of the number of purification cycles (PC number) on photovoltaic performance of QDSCs was investigated here. The sensitization of TiO2 photoanodes was implemented by immersing them in CdSe-CH2Cl2 dispersions (~25.0 µM) that had undergone a series of purification cycles. After 3 h of immersion, the photoanodes were passivated with ZnS and SiO2 sequentially according to literature procedure.40 Standard sandwich type QDSC devices were constructed using polysulfide as electrolyte redox couple, and Cu2S/brass as counter electrode. The corresponding average photovoltaic performances based on five solar cells under each condition are shown in Figure 1 with their detailed information available in Table S1, Figure S3, and Figure S4. It could be observed that under a series of PC number spanning from 1 to 5, the performance of QDSCs showed performance plateau at PC = 3 (PCE = 6.12%, Voc = 0.619 V, Jsc = 15.12 mA/cm2, and FF = 0.654). With overextending of PC number, both PCE, Voc and Jsc decreased, while FF remained almost unchanged.

a)

b)

Figure 1. Average photovoltaic parameter analysis for CdSe based QDSCs via DA mode based on a series of PC number. (a) PCE and Voc, (b) Jsc and FF.

The correlation between purification cycles and cell performance highlights the importance of controlled QD pretreatment for successful QD loading onto TiO2 surface. In order to find 11



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out the relation between the performance of QDSCs and PC number, the influence of PC number on optical properties of QD dispersions was investigated. It is found that the repeated purification cycle did not notably influence the excitonic absorption profile of QD dispersion (within 1 nm peak shift, Figure 2a), while PL intensity dramatically declined (Figure 2b). As PC number exceeds 3, almost no fluorescence could be detected. Meanwhile, it could be concluded from Figure 2c that QD loading amount increased with the enhancement of PC number and got a plateau value when the PC number approached 3. The mechanism of the effect of QD purification is intellectually clear.44,45 Adding excessive nonsolvent to QD dispersions increases the polarity of solvent, leading to flocculation of colloidal QDs due to the positive ligand-solvent mixing energy at the loss of surface-bound ligands. Further redispersion of QDs in fresh solvent reyields monodispersed QDs, accompanied with less surface-bound OAm and TOPO ligands. The quenching of PL intensity over PC number should be ascribed to the gradual removal of surface ligands such as OAm and TOPO, since these ligands can improve the PL quantum yield of semiconductor particles by remediating nonradiative recombination centers located on particle surface.23,24,43,46,47 The removal of OAm and TOPO ligands was further confirmed by mass (MS) and infrared (IR) spectroscopy. After 3 cycles of purification procedure, the dramatically decreased relative molecular ion peak intensity of OAm (Mw = 267) and TOPO (Mw = 386) in mass spectra (Figure S5a) as well as the weakened C−H vibration signals located around 2920 and 2850 cm−1 in FTIR spectra (Figure S5b) evidenced the successful removal of majority of surface organic ligands during the purification process.

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b)

a)

c)

Figure 2. (a) UV-vis absorption and (b) PL emission spectra of CdSe-CH2Cl2 suspensions based on a series of purification cycles. (c) Absorption spectra of CdSe QD sensitized TiO2 films based on a series of purification cycles. The results deducted the absorbance of blank TiO2 film. Inset: photographs of corresponding films.

Long chain organic ligands on QD surface, which control QD growth and stabilize QD in a colloidal suspension, are incompatible with TiO2 and significantly inhibits both QD loading and charge extraction, thus leading to inefficient solar cells in DA route. The removal of excess insulating ligands reduces electrostatic screen and increases the proportion of QD surface active sites, so the “exposed” QD surface increases its affinity with TiO2 matrix. On one hand, the QDs with less ligand coverage are relatively unstable with raised tendency to flocculate from suspension. This could be regarded as a driving force for QD loading. The decreased QD solubility after sufficient purification cycles can be evidenced by the following 13



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experiment. After loading on TiO2 films, the purified QDs could be dispersed in fresh CH2Cl2 conveniently at PC = 1, but they were no longer dispersible at PC = 3 (Figure S6). On the other hand, the removal of surface ligands is beneficial for photogenerated electrons injection into TiO2 substrate through reducing tunneling distance and energy barrier, thus promoting charge collection yield.36,48 The increased loading amount of CdSe QDs and the enhanced light harvesting capacity are the main factors that improve the performance of QDSCs by promoting Jsc when the PC number is less than 3 (Figure 1). The increased Voc should be ascribed to larger charge accumulation and retarded charge recombination at higher QD coverage.41 The deterioration of cell performance with PC number beyond 3 should be ascribed to the undesirable surface states derived from the excessive purification treatment.13,23 The undesired QD surface states provide additional charge recombination passway. As a consequence, severer charge loss occurs, leading to the decline of both Voc and Jsc. The variation of Jsc is in accord with the variation of corresponding incident photon conversion efficiency (IPCE) curves (Figure S4), and the variation of Voc can be analyzed by electrochemical impedance spectroscopy (EIS).49,50 The purification of QDs did not contribute to significant changes in chemical capacity Cµ (Table 1, Figure 3a, and Figure S7), indicating that QD purification process does not induce a displacement of the TiO2 conduction band edge notably. Meanwhile, the drop of Voc at PC number deviating from 3 is reflected by the decrease of recombination resistance Rrec and the calculated electron lifetime (τn = Rrec × Cµ, Table 1, Figure 3b, and Figure S7). Since Rrec is inversely proportional to the charge recombination rate at the interfaces between TiO2/electrolyte and QD/electrolyte, this trend indicated again that the lower QD coverage on TiO2 substrates (PC < 3) and the undesirable surface states derived from the excessive purification treatment (PC > 3) result in severer charge recombination and shorter electron lifetime. In the following investigation, the PC number was set at 3 to optimize the other experimental variables in CdSe based QDSCs.

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Table 1. Simulated Values of Recombination Resistance (Rrec), Chemical Capacitance and Calculated Values of Electron Lifetime (τn) of CdSe Sensitized Solar Cells via DA mode based on a Series of Purification Steps at the Forward Bias of −0.60 V. The Corresponding Nyquist Plots at Forward Bias of −0.60 V are Available in Figure S7. PC Number

Cµ (mF/cm2)

Rrec (Ω cm2)

τn (ms)

1

4.960

361.4

1793

2

4.822

425.1

2050

3

4.761

490.9

2337

4

4.677

480.2

2245

5

4.739

400.6

1898

a)

b)

Figure 3. EIS characterization of QDSCs via DA mode at a series of purification steps. (a) chemical capacity Cµ, (b) recombination resistance Rrec.

Influence of Sensitization Time and the Adverse Effects of QD Agglomeration. The influence of sensitization time on QD loading amount as well as on the photovoltaic 15



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performance of QDSCs was investigated hereafter. Temporal evolution of absorption spectra of sensitized film electrodes was recorded in both DA and CLIS routes. In DA route (Figure 4a), the absorbance at excitonic absorption peak showed a steady increase within 3 h sensitization time, after then the absorbance increased in a much slower mode. Correspondingly, the PCE of resultant QDSCs exhibited a peak value at sensitization time of 3 h. However, with extending sensitization time over 3 h, the PCE value exhibited a systematical drop (Figure 5 and detailed photovoltaic information in Table S2 Figure S8, and Figure S9). This observation suggests that QD loading amount is not the sole decisive factor for PCE in DA route. In CLIS route (Figure 4b), the absorbance reached saturation within 30 min. Contrast to the case of DA route, when the sensitization time was prolonged to 24 h, the photovoltaic performance did not exhibit notable degradation (Table S3 and Figure S10). It is also noted that even after 72 h of sensitization, the absorbance in DA route was still smaller compared with that in CLIS route at 30 min (0.47 vs. 0.58 at 618 nm, Figure 4a,b). This indicates that CLIS is a more efficient sensitization route and shares greater adsorption equilibrium constant.

a)

b)

Figure 4. Temporal evolution of absorption spectra of CdSe QDs sensitized TiO2 films via: (a) DA, (b) CLIS mode. The results deducted the absorbance of blank TiO2 film.

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Kamat et al. developed a kinetic adsorption model for CdSe QDs loading onto TiO2 via DA mode.36 The results in the report clarified that the total absorbance is the sum of two simultaneous processes: Langmuir-like submonolayer QD loading and QD agglomeration. The agglomeration is the fact that QDs from suspensions are loaded on the surface of TiO2 film without selectivity rather than on specific active sites of available TiO2 substrate. The nonselective loading behavior is a typical feature of physisorption, originating from the intrinsically weak driving force for QD loading.51 This renders QD unable to discriminate the unoccupied TiO2 surfaces from the QD loaded ones, and therefore results in undesirable QD multilayers.13 The absence of saturated absorbance in DA mode (Figure 4a) should therefore be attributed to the continuing occurred QD agglomeration. QD agglomeration as well as its deleterious effects in photovoltaic performance has been evidenced in previous works by CV and AFM techniques,30,33 and conceived as a principle factor that distinguishes DA and CLIS modes.21 The results from Figure 5 indicate that the increased light harvesting efficiency associated with high QD loading amount does not translate into improved photovoltaic performance when QDs agglomerate appreciable. Instead, the occurrence of QD agglomeration deteriorates all photovoltaic parameters. It is supposed that severe agglomeration leads to the blockage of mesoporous channels, thus inhibiting electrolyte penetration and leading to poor photovoltaic performance. This is also the reason for the poor reproducibility for the performance of the resultant solar cells via DA route.13 Moreover, the undesirable QD multilayers due to agglomeration mean that photogenerated excitions need to diffuse longer length and across more grain boundaries to encounter QD/TiO2 interface, leading to decreased electron injection yields due to remarkable increased competitive recombination passes.13 More importantly, the competition of QD loading and agglomeration in DA process suggests that it is imperative to control sensitization time within 3 h so as to achieve considerable QD loading amount while preventing severe QD agglomeration. In contrast, in CLIS mode, the formation of strong chemical bonds as well as the introduction of electrostatic repulsive force among individual MPA-capped QDs52,53 prevent effectively the unselective QD agglomeration during the QD 17



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loading process and contribute to higher QD loading amount. This is reflected by the appearance of saturated loading amount and the greater absorbance (Figure 4b) in the CLIS approach. EIS results further confirmed the increased charge recombination rates and shortened electron lifetime when severe QD agglomeration occurs (Table 2, Figure 6, and Figure S11). The nature of these two kinds of microscopic interactions determines that CLIS is a more efficient deposition route compared to DA route.

b)

a)

Figure 5. Average photovoltaic parameter analysis for CdSe based QDSCs via DA mode with different sensitization time intervals: (a) PCE and Voc, (b) Jsc and FF.

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Table 2. Simulated Values of Recombination Resistance (Rrec), Chemical Capacitance and Calculated Values of Electron Lifetime (τn) of CdSe Sensitized Solar Cells via DA mode based on a Series of Sensitization Times at the Forward Bias of −0.60 V. The Corresponding Nyquist Plots at Forward Bias of −0.60 V are Available in Figure S11. Sensitization Time

Cµ (mF/cm2)

Rrec (Ω cm2)

τn (ms)

2h

4.725

449.5

2124

3h

4.761

490.9

2337

5h

4.906

435.6

2137

24h

4.479

309.2

1385

a)

b)

Figure 6. EIS characterization of QDSCs via DA mode at a series of sensitization times. (a) chemical capacity Cµ, (b) recombination resistance Rrec.

To understand how QD loading affects the active surface area of photoanodes, cyclic voltammograms (CV) of photoanodes at a series of sensitization times were recorded in an 19



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aqueous polysulfide electrolyte. The photoanode serves as a capacitor and will be charged during the negative-going scan, simultaneously the accumulated electrons are scavenged by electrolyte and become the source of capacitive current. The capacitive current is proportional to the electrochemically active interfacial area of the electrode at each capacity.30,33 In Figure 7a, it could be observed that all sensitized photoanodes showed decreased capacitive current compared to blank TiO2. The gradual decrease over sensitization time for each deposition method is indicative of a partial loss of active surface area and this behavior is accounted by QD loading and agglomeration.30,33 After 24 h of QD sensitization, the capacitive current of CLIS-based photoanode exerted a neglected drop compared with that at 3 h, while further decrease could be observed in DA mode. Note that the QD loading amount did not increased distinctively after 3 h in DA mode (Figure 4a). Therefore, the observed decreased current and thus decreased active area after 3 h in DA mode is mainly induced by QD agglomeration. It could be concluded that the agglomeration is so remarkable in DA mode at 24 h that the current density was even smaller than that of CLIS mode in spite of higher QD loading amount for CLIS mode (Figure 7a). It is noted that the change of photoanode capacity should not be induced by the shift of TiO2 conduction band energy level after QD loading, because when a voltammetric curve of sensitized photoanode from Figure 7a is multiplied by a number beyond 1 (i.e. the curve obtained from the TiO2 film that has undergone 3 h of sensitization based on DA route is multiplied by 1.20), the resulted curve virtually coincides with the one obtained from blank TiO2 (Figure 7b). This is indicative of the same energetic location for the surface and conduction band states.30,33

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b)

a)

Figure 7. The negative-going scan branches of cyclic voltammograms of (a) TiO2 photoanodes with and without sensitization of CdSe QDs as a function of loading time. (b) multiplication of the curves from Figure 7a.

Influence of QD Concentration. Having noticed the effects of QD agglomeration at long sensitization time in DA mode, the influence of QD dispersion concentration on the performance of QDSCs was investigated to suppress QD agglomeration. In a series of experiments with sensitization time of 3 h, the half concentrated QD dispersion (i.e. 13.3 vs. 26.6 µM) exerted no notable influence on photovoltaic performance, while two-, and four-fold concentrated QD dispersions (53.2 and 106.4 µM) resulted in negative effect (Figure 8, Table S4, Figure S12 and Figure S13). This phenomenon is in accordance with a previous report.29 According to Langmuir isotherm, the increase of concentration promotes both the QD loading rate and the loading amount, but the increase of loading amount is negligible at already high concentration window.36,54 This is indeed reflected in Figure S14. As a consequence, the increased QD agglomeration rate, which is proportional to the concentration of QD dispersion,36 predominated in this process. We suppose that the agglomeration rate was so fast at high concentrations that its side effect had become notable before considerable QD loading amount was realized. This observation highlighted the simultaneous occurrence of QD 21



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loading and agglomeration in DA mode, so the concentration of QD dispersion in DA mode should be carefully controlled. In contrast, four-, and even eight-fold concentrated QD suspensions (i.e. 102.0 and 204.0 vs. 25.5 µM) have almost no negative effects on cell performance in CLIS mode (Table S5 and Figure S15). This indicates that QD agglomeration scarcely occur in CLIS mode.

a)

b)

Figure 8. Average photovoltaic parameter analysis for CdSe based QDSCs via DA mode based on different suspension concentration. (a) PCE and Voc, (b) Jsc and FF.

Kinetics of Electron Injection. To unravel electron transfer kinetics at QD/TiO2 interface in DA and CLIS modes, femtosecond (fs)-resolution transient absorption (TA) measurements were carried out for a series of samples with QD deposition on TiO2 substrate through DA or CLIS mode, or on SiO2 substrate obtained according to literature procedure.15,55 Three kinds of CdSe QDs were tested: OAm capped with and without 3 cycles of purification treatment (denoted as OAm-0 and OAm-3, respectively), and MPA capped CdSe (denoted as MPA). For OAm-0 and OAm-3, DA mode was applied to deposit QD on film substrates, while the loading of MPA-capped QDs were realized by CLIS mode. The TA samples underwent a sensitization time of 30 min in order to prevent the agglomeration of QD. In a typical TA measurement, the characteristic CdSe absorption maximum at 618 nm 22


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bleaches after photoexcitation. The TA decay of QDs on SiO2 and TiO2 substrates can be perfectly fitted (R2>0.97) by a biexponential function of eq 1: (1) each exponential component represents an electron recombination or transfer process. τ1, τ2 are lifetime constants, and A1, A2 are weighted coefficients for the two exponential components, respectively. The fitting results and further calculated parameters for all samples are shown in Figure 9, and Table 3. When QDs are loaded on SiO2 substrates, only two relaxation processes of photoexcited electrons occur. The faster time component (τ1) should be assigned to trap-mediated recombination, while the slower one (τ2) can be attributed to the intrinsic recombination of electrons in conduction band with holes in valence band. When TiO2 serves as substrate, an extra relaxation route, i.e. injection into TiO2 matrix, exists. Therefore, faster decay of TA signals was observed for samples of QDs tethering on TiO2 substrate. The average lifetime (τav) of photoexcited electrons in QDs can be calculated using eq 2. (2) Assuming that the only difference of QDs loaded on the two different substrates lies in an additional electron transfer route in the presence of TiO2, the corresponding electron injection rate constant ket can be calculated from eq 3. (3) It can be observed from Table 3 that the calculated ket values from TA measurement are at the same level as the ones obtained from previous reports.36,55–58 In the case of DA mode, the τ1 values for OAm-0 from both SiO2 and TiO2 substrates (16.2 and 12.5 ps) are larger than those of OAm-3 (13.0 and 10.8 ps). This indicates an accelerated trap-mediated process with the elimination of ligands. In addition, the calculated ket of OAm-3 (6.4×109 s−1) is only slightly greater than that of OAm-0 (5.3×109 s−1), which indicated that the influence of QD purification on electron transfer kinetics is rather limited. More importantly, in the case of CLIS mode, a substantially smaller ket value of 1.8×109 s−1 is observed. This indicates that DA 23



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mode brings forward an accelerated electron transfer process from QD to TiO2 substrates. It has been established that electron transfer rate at QD/TiO2 interface decreases exponentially over increased tunneling distance and the tunneling energy barrier.48,59,60 The increased ket in DA mode should be ascribed to both the intimate contact between QD and TiO2 and the relieved charge transfer energy barrier.

a)

b)

c)

Figure 9. Transient absorption response of a series of CdSe samples deposited on SiO2 (black scatters) and TiO2 (red scatters) substrates. (a) original OAm-capped CdSe (OAm-0), (b) OAm-capped CdSe based on 3 cycles of purification (OAm-3), (c) MPA-capped CdSe. The solid lines are the corresponding biexponential fitting curves.

Table 3. Transient Absorption Biexponential Fitting Results for CdSe QD Samples Loading on SiO2 and TiO2 Substrates. Samples

τ1 (ps)

τ2 (ps)

A1

A2

τav (ps)

SiO2/OAm-0

16.2±1.7 246.3 ± 38.3 0.47 ± 0.02 0.45 ± 0.02

231.3

TiO2/OAm-0

12.5±0.2

131.3 ± 8.9 0.87 ± 0.01 0.28 ± 0.01

103.8

SiO2/OAm-3

13.0±1.3 224.6 ± 34.6 0.44 ± 0.02 0.39 ± 0.02

211.5

TiO2/OAm-3

10.8±0.4 117.9 ± 17.6 0.84 ± 0.01 0.21 ± 0.01

89.4

SiO2/MPA

10.2±1.1 196.4 ± 24.9 0.39 ± 0.02 0.45 ± 0.02

188.3

TiO2/MPA

11.4±0.5

140.2

ket (×109 s−1)

5.3

6.4

1.8 151.5 ± 8.5 0.49 ± 0.01 0.42 ± 0.01

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Further Increasing QD Loading via Mg2+ Treatment on Photoanodes. After comparison of the two QD deposition modes, it is concluded that CLIS mode contributes to a greater QD loading amount, and better photovoltaic performance, especially Jsc (Table 4, Table S6, Figure S16, and Figure S17). With the initiative to further improve the performance of DA-based QDSCs, Mg2+ treatment on TiO2 photoanodes was applied to promote QD loading amount and thus to increase the light harvesting capacity to achieve higher photocurrent.41 Mg2+ treatment was applied before QD sensitization by pipetting 10 µL of 0.02 M MgCl2 aqueous solution onto a TiO2 film electrode with area of 0.235 cm2, followed by evaporation of water at 50 oC for 60 min. The deposition of Mg2+ on TiO2 photoanodes is believed to introduce additional electrostatic attraction due to its small ionic radius and large neat charge quantity, which is favorable for enhancing the loading amount of QD on TiO2 electrodes. As expected, with the assistance of Mg2+ treatment on photoanode, QD loading amount improved notably (Figure 10a), and the resulting Jsc can be improved from 14.97 to 15.82 mA/cm2, and the obtained PCE (6.90%) based on DA mode was comparable to CLIS based QDSCs (6.88%).

b)

a)

Figure 10. Absorption spectra of QD sensitized TiO2 films with and without Mg2+ treatment. (a) CdSe, and (b) ZCISe. The results deducted the absorbance of blank TiO2 film. Inset: photographs of corresponding films.

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We endeavored in applying the experience obtained from CdSe to Zn−Cu−In−Se (ZCISe) based QDSCs to get higher PCEs.15 The absorption spectrum of ZCISe QDs suspended in CH2Cl2 and the corresponding TEM image were illustrated in Figure S18. Limited by the relative instability, the original ZCISe QD suspensions could only withstand two purification cycles. Before QD loading, Mg2+ treatment was applied (40 µL of 0.02 M MgCl2 aqueous solution per TiO2 film electrode with area of 0.235 cm2), and the sensitization time was prolonged from 4 h to 6 h. It could be observed that QD loading amount increased and was close to the one realized by CLIS mode due to the presence of Mg2+ (Figure 10b). Due to the substantially increased Jsc (from 22.45 to 24.95 mA/cm2), the performance of ZCISe based QDSCs exhibited an optimized PCE of 9.02% (Voc = 0.620 V, Jsc = 24.95 mA/cm2, and FF = 0.584, see Figure S19 and Figure S20 for detailed information). The optimized PCE was close to the level of 9.56% achieved upon CLIS mode developed by ourselves (Voc = 0.632 V, Jsc = 26.01 mA/cm2, and FF = 0.581).

Table 4. Average Photovoltaic Parameters of a Series of Optimized QDSCs. Photoanodes

Voc (V)

DA-CdSe

0.646 ± 0.008

DA-Mg2+-CdSe

Jsc (mA/cm2)

FF

PCE (%)

14.97 ± 0.16

0.677 ± 0.010

6.54 ± 0.11

0.653 ± 0.006

15.85 ± 0.13

0.667 ± 0.006

6.90 ± 0.07

CLIS-CdSe

0.644 ± 0.006

15.82 ± 0.16

0.675 ± 0.008

6.88 ± 0.07

DA-ZCISe

0.603 ± 0.010

22.45 ± 0.24

0.574 ± 0.011

7.77 ± 0.10

DA-Mg2+/ZCISe

0.620 ± 0.009

24.95 ± 0.18

0.584 ± 0.011

9.02 ± 0.11

CLIS-ZCISe

0.632 ± 0.009

26.01 ± 0.22

0.581 ± 0.011

9.56 ± 0.12

It is highlighted that the small deviation for the obtained average photovoltaic parameters of five cells as shown in Table 4 demonstrates the excellent reproducibility for our adopted 26


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DA process in the construction of high efficiency QDSCs. Histograms of device performance from a batch of 100 cell devices based on DA mode (Figure S21) strengthens furthermore the high reproducibility of this QD deposition mode. The simplified operation procedure, the high reproducibility, and applicability to various QD sensitizers, all these features highlight the potential advantage for the DA mode in the construction of low-cost, high efficiency QDSCs.

4. CONCLUSIONS High efficiency QDSCs with high reproducibility could be constructed via the simple and facile DA QD deposition mode. Experimental results demonstrated that regardless of the nature of QDs, efficient DA could be realized by selecting suitable solvent and optimizing purification process. The realization of a well-covered QD layer relies on the control of sensitization time and the concentration of QD suspension. Transient absorption measurement confirmed a faster electron injection rate for DA mode in comparison with the state-of-art CLIS approach. However, due to the weak microscopic interaction, QD loading amount in DA mode is intrinsically less than that by CLIS method. While Mg2+ treatment on TiO2 photoanodes could further promote QD loading by applying an extra electrostatic attraction. After optimization of the experimental variables, the QDSC performance based on DA mode can reach a comparable level (PCE = 6.90% and 9.02% for CdSe and ZCISe based QDSCs, respectively) to the ones achieved by the well-developed CLIS method (PCE = 6.88% and 9.56% for CdSe and ZCISe based QDSCs, respectively).

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ASSOCIATED CONTENT Supporting Information Absorption spectra of CdSe and ZCISe QDs suspended in toluene, CdSe sensitized TiO2 films under various experimental conditions; TEM images of CdSe and ZCISe QDs; details of photovoltaic performance of QDSCs under various experimental conditions, including J-V and IPCE curves; FTIR and mass spectra of CdSe QDs based on a series of purification cycles; photographs of CdSe sensitized TiO2 photoanodes before and after soaking in 5 mL of fresh CH2Cl2; Nyquist plots of QDSCs via DA mode based on a series of purification steps and sensitization times at the forward bias of −0.60 V; histograms of device performance from a batch of 100 cell devices based on DA mode.

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AUTHOR INFORMATION Corresponding Author **Z.P.: e-mail, [email protected]. Q.S.: e-mail, [email protected]. *X.Z.: e-mail, [email protected]. Tel/Fax: +86 21 6425 0281. ORCID Qing Shen: 0000-0001-8359-3275 Xinhua Zhong: 0000-0002-2062-8773 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grants 91433106, 21573249), and the Fundamental Research Funds for the Central Universities in China. Q.S. thanks the support of the Japan Science and Technology Agency (JST) CREST program, MEXT KAKENHI Grant 26286013 and grant No. 17H02736.

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Table of Contents (TOC) High Efficiency Quantum Dot-Sensitized Solar Cells based on Direct Adsorption of Quantum Dots on Photoanodes Wenran Wang, Guocan Jiang, Juan Yu, Wei Wang, Zhenxiao Pan, Naoki Nakazawa, Qing Shen, and Xinhua Zhong

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High Efficiency Quantum Dot Sensitized Solar Cells Based on Direct

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