Boosting the Efficiency of Quantum Dot-Sensitized Solar Cells through

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Boosting the Efficiency of Quantum Dot Sensitized Solar Cells through Formation of Cation Exchanged Hole Transporting Layer Sourav Maiti, Farazuddin Azlan, Pranav Anand, Yogesh Jadhav, Jayanta Dana, Santosh K. Haram, and Hirendra N. Ghosh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02659 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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Boosting the Efficiency of Quantum Dot Sensitized Solar Cells through Formation of Cation Exchanged Hole Transporting Layer Sourav Maiti†‡, Farazuddin Azlan†, Pranav Anand†, Yogesh Jadhav‡, Jayanta Dana¶†, Santosh K. Haram‡ and Hirendra N. Ghosh*†§ †

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India.

§

Institute of Nano Science and Technology, Mohali, Punjab 160062, India.

‡

Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India.

¶

Homi Bhabha National Institute, Mumbai 400094, India.

*E-mail: [email protected], [email protected]; Tel: +91-22-25593873.

ACS Paragon Plus Environment

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Abstract: In search of a viable way to enhance the power conversion efficiency (PCE) of quantum dot sensitized solar cells (QDSSC) we have designed a method by introducing a hole transporting layer (HTL) of p-type CuS through partial cation exchange process in post synthetic ligand assisted assembly of nanocrystals (NCs). High quality CdSe and CdSSe gradient alloy NCs were synthesized through colloidal method and the charge carrier dynamics was monitored through ultrafast transient absorption (TA) measurements. Notable increase in the short circuit current concomitant with the increase in open circuit voltage and fill factor lead to 45% increment in PCE for CdSe based solar cell upon formation of the CuS HTL. Electrochemical impedance spectroscopy (EIS) further revealed that the CuS layer formation increases recombination resistance at the TiO2/NC/electrolyte interface implying interfacial recombination gets drastically reduced due to smooth hole transfer to the redox electrolyte. Utilizing the same approach for CdSSe alloy NCs highest 4.03% PCE was obtained upon CuS layer formation compared to 3.26% PCE for the untreated one and 3.61% PCE with the conventional ZnS coating. Therefore, such strategies will help to cross the kinetic barriers for hole transfer to electrolyte which is one of the major obstacle to have high performance devices.


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1. Introduction Quantum dot sensitized solar cells (QDSSC) are promising candidates for 3rd generation solar cells due to easy solution processability and low cost fabrication technology, broad absorption spectra, high extinction coefficient, size, shape and composition tunable band-gap, high photostability and possibility of multiple exciton generation.1-12 However, till date QDSSC has reached about 13% efficiency which way beyond the theoretical limits.13 In QDSSC exciton trapping and recombination are the major two processes that inhibit the charge carrier extraction thus reducing the efficiency.14,15 Moreover, proper choice of counter electrode and electrolyte is also very important and use of Cu2S or CuxSe electrode along with polysulfide electrolyte is well accepted in literature for QDSSC.10,15-20 As for QD sensitizers alloy nanocrystals (NCs) having excellent opto-electronic properties and photostability compared to their individual counterparts results in improved solar cell performance. The alloy structure helps to absorb in a broader range and exhibits better charge separation.

21-23

Therefore, the biggest challenge in QDSSC is to

control and reduce the unwanted exciton trapping and recombination at the TiO2/QD/electrolyte interface, which is the focus of many recent investigations.22,24-29 In this regard a passivating layer of wide band gap semiconductor such as ZnS24,25,30,31 together with SiO2 has proved to be advantageous in improving the efficiency for TiO2/QD/ZnS/SiO2 assembly.22 As discussed in literature one of the main limiting factors for the performance of QDSSC is the hole transfer to the redox electrolyte which is orders of magnitude slower than the electron injection to the TiO2.26,32,33 Previously, inorganic capping layer has been used to facilitate hole transfer to electrolyte from NC valence band (VB).34-36 One of the limitations was the use of NCs grown by successive ionic layer adsorption and reaction (SILAR) technique which suffers from corrosion of the device and limited control over NC size distribution resulting uncontrolled


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surface passivation and trap state density leading to poor solar cell performance.

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7,14,37

To the

contrary, in post-synthesis assembly high quality presynthesized colloidal NCs with well defined size, morphology and surface properties are deposited on the TiO2 film.13,22,37-40 So far linker assisted deposition using short chain thiols as linkers results in the best efficiencies reported for QDSSC due to controlled opto-electronic properties of the NCs and excellent loading on TiO2.21,37,41 Although, wide band-gap materials have been used as a passivating layer to enhance PCE, the effect of a hole transporting layer after linker assisted deposition of NCs have not been elucidated in detail. In this paper, we have obtained significant improvement in PCE after depositing partial cation exchanged CuS p-type layer on CdSe and CdSSe alloy QDSSC in the liquid junction solar cell assembly FTO/TiO2/QDs/CdS/CuS/CuxSe using polysulfide electrolyte. Recently, organic hole transporting materials have also been used to enhance the PCE of QDSSCs where the organic layer facilitates hole extraction and prevents electron recombination between the QD (PbS) and metal electrode (Au).42-44 However, organic hole transporting materials are costly and requires rigorous synthetic protocols whereas inorganic hole transporting CuS is easy to deposit, cost effective and efficient. Interestingly, after formation of the CuS at the surface the PCE increased to 3.57% compared to the initial 2.56% PCE for CdSe NCs. The highest PCE obtained was for CdSSe NC of 4.03% after CuS deposition, which is about 25% higher, compared to the untreated one and ~12% higher than ZnS coating. Electrochemical impedance spectroscopic (EIS) measurements unveil that the CuS layer increases the charge collection efficiency by augmenting the recombination resistance at the TiO2/QD/electrolyte interface acting as a hole transporting buffer layer indicating viability of this methodology to design highly efficient QDSSC.


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2. Experimental Section 2. A. Chemicals. Cadmium oxide (CdO, 99.5%), selenium powder (Se, 99.99%), sulfur powder (S, 99.99%), oleic acid (OA, 90%), trioctyl phosphine (TOP, 90%), octadecene (ODE, 90%), Cadmium nitrate tetrahydrate (Cd(NO3)2. 4H2O), sodium sulfide (Na2S), zinc nitrate hexahydrate (Zn(NO3)2,·6H2O), potassium chloride (KCl), potassium hydroxide (KOH), mercaptopropionic acid (MPA) were purchased from Sigma-Aldrich and used as received. Copper chloride dihydrate (CuCl2. 2H2O, 99%) was purchased from Alfa Aesar. AR-grade methanol, ethanol, acetone and chloroform were used for cleaning. De-ionized (DI) water was used for phase transfer and electrolyte preparation. Tetrabutyl-ammonium-perchlorate (TBAP), dichloro methane (DCM), and Ferrocene (Sigma-Aldrich) were used for CV measurements. 2. B. Synthetic Procedure. The CdSe and CdSSe NCs were synthesized through high temperature hot injection method following previously reported procedures with amendments as necessary and described in detail in the supporting information.45-47 2. C. Solar Cell Fabrication. For solar cell fabrication, QD sensitized TiO2, polysulfide solution and copper selenide (CuxSe) were used as photoanode, electrolyte and counter electrode, respectively. The device fabrication protocols were adopted from previous reports with required modifications as described in detail in the supporting information.16,22,23,38,39 2. D. Ultrafast Transient Absorption Spectroscopy. The TA measurements were carried out in films either NCs deposited on glass or on loaded on TiO2. The NCs were excited with a 400 nm pump beam and the resulting carrier dynamics was probed in the 450-750 nm regions. The pump fluence was kept low enough so that number of exciton per NC is less than 0.5 to avoid multiexciton related complications. Details of the experimental setup is described elsewhere.48


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2.E. IPCE-JV Measurement. For the measurements of photovoltaic performance and power conversion efficiency current density vs. voltage (J-V) curves were obtained for solar cell assembly under 1 sun illumination (100mW/cm2) using G-short arc Xe lamp solar simulator (Peccell, Model PEC-L01) with a Keithley 2400 source meter. Using Action Spectrum Measurement System (Peccell, model: PEC-S20) with a xenon lamp (150 W) as the light source IPCE measurements were carried out in the region 300-800 nm.

2. F. Impedance Measurement. The electrochemical Impedance spectroscopic measurements were performed on Biologic SP 300. For impedance measurements, the solar cell device was kept at a voltage close to VOC (0.5V) and 10 mV AC perturbation signal with frequency 100 KHz to 0.1Hz under dark conditions were applied. 3. Results and Discussion Oleic acid capped CdSe and CdSSe alloy NCs were synthesized through high temperature hot injection method through required modifications of the reported procedure.45-47 Figure 1(A) shows the absorption and photoluminescence spectra of the CdSe, CdSSe alloy NCs. The sharp first excitonic band ~564nm, 14 nm stokes shift and symmetric photoluminescence Exciton

Table 1. Optical properties in terms of first excitonic absorption maxima (λabs Exciton

photoluminescence maxima (λem

),

), stokes shift (∆λ), quantum yield (QY) and

photoluminescence lifetime (τPL) for CdSe, CdSSe_a and CdSSe_b NCs.

Material

λabsExciton

λemExciton

∆λ (nm)

QY (%)

τPL (ns)

CdSe CdSSe_a CdSSe_b

564 552 586

578 572 604

14 20 18

25 70 73

16.4 22.9 21.5


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spectra clearly ensures the good size distribution and high quality of the CdSe NCs. The size of the CdSe NCs was determined to be ~ 3.36 nm from the sizing curve provided by Peng et al.49 For CdSSe alloy NCs two different sizes were obtained as confirmed from transmission electron micrographs (figure S1 (A-B), SI). Energy dispersive X-ray (EDS) analysis confirms presence of all three elements as shown in figure S1 (C-D), SI.

Figure 1. (A) UV-vis absorption and PL spectra, (B) Transient absorption spectra after 400 nm pump excitation at different delay times and (C) Ground state bleach dynamics of (a) CdSe, (b) CdSSe_a and (c) CdSSe_b NCs. Both the CdSSe alloy NCs show high photo-luminescence quantum yield and improved photoluminescence lifetime (figure S1, SI (E)) compared to CdSe due to minimized trap states consistent with the literature.46,47,50 All the optical parameters are summarized in table 1. The Xray diffraction (XRD) measurements as depicted in figure S1 (F), SI further suggests zinc blend structure for all the NCs and confirms formation of alloy NCs as all the three major diffraction peaks (111), (220) and (311) shift towards higher angle due to incorporation of smaller sized sulphur in the CdSe lattice. The CdSSe_a of 3.20±0.05 nm size has Se:S ~39:61 compared to


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CdSSe_b NCs of 4.54±0.08 nm size and Se:S ~29:71. Furthermore, we have performed cyclic voltammetric experiments on CdSSe_a and CdSSe_b NCs to determine their band positions (figure S1 (G), SI). For CdSSe_a and CdSSe_b NCs the conduction band (CB)-edge was found to be ~-3.44 eV and ~-3.48 eV (vs. vacuum), respectively which are well above the CB of TiO2 (~-4.0 eV) suitable for electron injection. Transient absorption spectroscopy (TAS) was utilized to understand the carrier dynamics in the NCs which is one of the crucial factors influencing PCE. The NCs deposited on glass were excited with 400 nm pump beam (number of exciton/NC was kept 400 ps component is much slower compared to pure CdSe in both the alloy NCs. Therefore, alloy NCs are superior to their binary counterparts having long lived electrons, both hot and thermalized available for PCE.23,56 To obtain significant increments in PCE the recombination of electrons between TiO2/QD interfaces with holes in the NC VB (process 3, scheme 1) also needs to be minimized

Figure 2. (A) Absorption spectra of CdSe NCs deposited on transparent TiO2 (a), after 6 cycles of SILAR with CdS (b) and after cation exchange with CuCl2 solution (c) (Inset) SEM of the solar cell device (TiO2/QD/CdS/CuS). (B) J-V plot for CdSe NCs without (d) and with (e) the CuS coating. (Inset) Optimization of PCE for CdSe QDSSC with respect to cation exchange time. which was obtained by putting p-type hole transporting CuS through cation exchange. At first the conditions for cation exchange was optimized for CdSe NCs. After transferring to the


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aqueous phase through ligand exchange with MPA CdSe NCs were dropcasted on TiO2 photoanode for 1 hour followed by washing with DI water to remove the excess NCs. The CdSe NCs deposited TiO2 photo-anodes were coated with 6 cycles of CdS through SILAR technique. The CdS quasi-shell also helps to passivate the NCs and also the TiO2 surface defects.27 During the SILAR process at first Cd2+ gets adsorbed on the NC surface and on bare TiO2 as the TiO2 nanoparticles will not be completely covered by the NCs. The unbound Cd2+ is removed through washing step followed by dipping in S2- solution. The S2- reacts with adsorbed Cd2+ to form CdS crystallites.57,58 The TiO2/CdSe/CdS photo-anodes were further dipped in CuCl2. 2H2O solution (details in supporting information) for varying amounts of time (5 seconds to 30 seconds) resulting cation exchange of Cd2+ with Cu2+. This cation exchange will proceed to form CuS by replacing Cd2+ adsorbed on both NC surface and TiO2 surface (scheme 1). We studied the absorption of the QD deposited TiO2 films as shown in figure 2A. After completing six SILAR cycles with CdS the absorption on the blue side increases considerably due to absorption from CdS (figure 2A). Also, the first excitonic absorption broadens after CdS SILAR. Upon CuCl2 treatment the absorption on blue region decreases a bit as Cu replaces some of the Cd ions through cation exchange. The incorporation of Cu was also confirmed through X-ray photoelectron spectroscopy (XPS) analysis as shown in figure S3 (A), SI with varying cation exchange time. The XPS data shows decrease in Cd:Cu ratio as the cation exchange time was increased and finally upon prolonged cation exchange the Cd on the surface is scarcely detected. Raman analysis was performed on the cation exchanged films which shows the CdS band ~ 302cm-1 disappears after prolonged cation exchange as Cd gets replaced by Cu, with emergence of a band ~474 cm-1which can be attributed to the hexagonal CuS phase (figure S3 (B), SI). From the cross sectional scanning electron micrographs (SEM) of TiO2 photoanode (Figure 2A,


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inset) the thickness of the TiO2 layer is ~2 µm. The EDS X-ray mapping of the elements as shown in figure S3 (C), SI shows the NCs and the passivation layers get absorbed within the mesoporous structure of TiO2 and we mostly observe the TiO2 layer on top of the conducting glass. The surface mapping of Cu through X-ray suggests homogeneous distribution of Cu throughout the surface (figure S3 (D), SI).The atomic force microscopic (AFM) images (figure S3 (E), SI) depicts the morphology of mesoporous TiO2 having ~20nm of nanoparticles. Major morphological changes were not observed after sensitization of QDs followed by CdS deposition and subsequent cation exchange of Cd with Cu suggesting the thickness of CdS (and CuS) are on the nm level. Presumably due to very short reaction time an alloy structure is forming due to partial replacement of Cd with Cu Cd depositing a mixed layer of (Cd, Cu)S. The IPCE spectra shown below in figure 3(B) before and after CuS deposition remains more or less similar suggesting formation of an alloy layer. For simplicity we are mentioning it as CuS layer afterwards. These TiO2/CdSe/CdS/CuS photo-anodes were assembled with CuxSe counter electrode using polysulfide electrolyte. The CuxSe electrodes were fabricated following procedures developed by Zhang et al.16 The device fabrication process is represented in scheme 1 (A) and discussed in detail in the supporting information. Moreover, scheme 1(B) depicts the band energy diagram showing feasibility of electron transfer from CB of NC to TiO2 and hole transfer from VB of NC to the CuS layer. The photo-voltaic performance of the cells in terms of current density-voltage (J-V) measurements under 1 sun (AM 1.5G, 100mW/cm2) illumination are shown in figure 2(B) and the relevant parameters are tabulated in table 2.


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Scheme 1. (A) Schematic of solar cell fabrication process. After depositing MPA capped QDs into TiO2 CdS was coated through SILAR followed by partial cation exchange (CE) with Cu2+ to form the hole transporting CuS layer. Prolonged cation exchange decreases the PCE as shown in figure 2(B). The major three recombination processes are from QD CB to redox electrolyte (process 1); from TiO2 CB to redox electrolyte (process 2) and from TiO2 CB to QD VB (process 3). The CdS passivation can prevent process 1 and 2 by passivating the QD and TiO2 nanoparticles. The CuS layer facilitates hole transfer to redox electrolyte thus prevents process 3 also. (B) Band energy diagram for TiO2/CdSSe_a/CdS/CuS/electrolyte to demonstrate facile electron injection from QD to the CB of TiO2 and hole transfer from VB to electrolyte throgh CuS. The band energies of CdSSe_a was determined from CV, whereas for CdS and CuS values reported in literature were taken.59 For CdSe the PCE was 2.56% with current density (JSC) 12.03 mA/cm2, open circuit voltage (VOC) 0.54 V and fill factor (FF) of 38%. After dipping in copper chloride solution the JSC increases significantly with concomitant increase in VOC and FF. The best PCE of 3.57% is found for the photo-anode that is dipped for 5s in the copper chloride solution with JSC 14.11 mA/cm2, VOC 0.57 V and FF 43%. Beyond this the efficiency decreases a bit and saturates


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Table 2. Photovoltaic parameters in terms of short circuit current (JSC, mA/cm2), open circuit voltage (VOC, V) and fill factor (FF, %) for CdSe NC solar cells with and without CuS layer. QD System

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Champion Cell PCE

CdSe/CdS

12.03

0.54

38

2.46±0.04

2.56

CdSe/CdS/CuS

14.11

0.57

43

3.47±0.06

3.57

(figure S4, SI and table S2, SI). Therefore, we find short 5s dipping is sufficient to enhance the PCE after forming cation exchanged CuS on top of CdS. The JSC and FF depend on the carrier mobility, lifetime and diffusion length along with trap density whereas VOC depends on the trap density.7 The increase in all three parameters suggest the p-type cation exchanged CuS acts a hole transporting layer facilitating smooth transfer of hole from the VB of QD to the redox electrolyte without introducing additional defect states due to Cd to Cu cation exchange. Thus, the passivation effect of CdS is retained upon cation exchange with Cu. When the cation exchange time is increased the CuS layer becomes thicker (as shown in the XPS data, figure S3, SI) hindering the hole transfer to the counter electrode decreasing the PCE. The incident photonto-electron conversion efficiency (IPCE) of CdSe NCs with and without the CuS layer is shown in figure S4, SI. The IPCE spectrum matches well with the absorption spectra of CdSe NCs with photocurrent onset ~620 nm. However, the IPCE spectra is broader compared to the absorption spectra due to light scattering by the large TiO2 particles in the scattering layer.21 The increase in JSC after CuS layer formation is in accord with the IPCE spectra where the quantum efficiency (QE) increased all over the spectral region spanning from 370-700 nm. Due to very short


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reaction time (5s) an alloy composite consisting of (Cu, Cd)S was presumably formed as discussed earlier on the CdS surface as the onset of light absorption spectra (figure 2A) and IPCE spectra (figure 3B) remains similar before and after cation exchange. Table 3. Photovoltaic parameters for CdSSe NC solar cells with and without the CuS layer formation. QD System

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Champion Cell PCE

CdSSe_a/CdS

14.11

0.57

39

3.22±0.02

3.26

CdSSe_a/CdS/CuS

16.58

0.60

40

3.98±0.04

4.03

CdSSe_b/CdS

14.06

0.59

37

3.10±0.07

3.31

CdSSe_b/CdS/CuS

16.14

0.60

38

3.67±0.02

3.70

After devising a suitable condition for cation exchange, CdSSe NCs were assembled in the solar cell utilizing a methodology similar to CdSe NCs utilizing CuxSe counter electrode and polysulfide electrolyte (details are in the experimental section and supporting information). The p-type CuS layer was put in a similar fashion with 6 cycles of CdS followed by cation exchange with CuCl2 solution. The resulting photovoltaic performance with and without the CuS layer are shown in figure 3 in terms of IPCE and J-V measurements. The JSC (mA/cm2), VOC (V) and FF (%) are summarized in table 3. Both the alloy NCs showed higher efficiency than binary CdSe


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Figure 3. (A) J-V curves under 1 sun illumination and (B) IPCE spectra of CdSSe_a NCs without (a) and with (a') the CuS hole transporting layer. NCs because of less defect states and slow electron cooling. Interestingly, after depositing the hole transporting layer of CuS the PCE improved ~25% with 4.03% PCE for CdSSe_a and ~15% with 3.70% PCE for CdSSe_b. The J-V plots are provided in figure S5, SI for CdSSe_b NCs. Moreover, similar to CdSe for each system increase in Voc following CuS layer formation has been observed. For all the NCs increase in PCE was found implying the applicability of this strategy to a wide range of NCs. TA measurements were performed on CdSSe_a NC sensitized TiO2 photoanodes with and without the CuS layer as shown in figure 4 and the multi-exponential fitting parameters are tabulated in table S3, SI. For photoanodes with only the NC sensitizer the bleach recovery becomes much faster due to fast electron transfer to TiO2 which is a thermodynamically viable process (scheme 1B). After adding CdS quasi-shell through SILAR the electron injection to TiO2 becomes sluggish due to an additional barrier. Passivation of ZnS on NCs was found have similar effect in the TA measurements.39 However, this is compromised as CdS also act as a passivation removing defects on TiO2 and preventing the recombination of e-(TiO2)-h+(QD or


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electrolyte). After cation exchange with Cu2+ there is no effect on the bleach dynamics confirming CuS does not additionally hinder the electron injection to TiO2. To understand the internal mechanism and justify the increment in PCE upon formation of the CuS layer, electrochemical impedance spectroscopy (EIS) was performed under dark condition for the device with and without the CuS layer.14,60,61 The cells were kept at forward bias close to their VOC (-0.5 V) followed by applying a 10 mV ac voltage having frequency ranging from 100 KHz to 0.1Hz. Resultant Nyquist plot for CdSSe_a NCs is shown in figure 5. The most important parameters obtained after fitting the data with standard equations for the

Figure 4. Kinetics of ground state bleach recovery for CdSSe_a NCs on glass (a), on TiO2 (b), after SILAR with CdS (c) and after cation exchange with Cu2+(d). equivalent circuit as shown in figure 5 are the chemical capacitance (Cµ) and the recombination resistance (Rrec). Cµ provides information about the density of trap states on TiO2 along with the change in CB position whereas Rrec reflects the recombination of charge carriers at the TiO2/QD/electrolyte interface.22,27,39 The similar Cµ values (~5 mF cm-2) before and after the CuS coating indicates TiO2 conduction band edge or density of states have not changed.60 The larger


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radius of the middle semicircle suggests that the Rrec which is inversely proportional to the recombination rate has decreased upon formation of CuS on top of CdS. The significant increase in Rrec for QD/CdS/CuS solar cells (table 4) compared to QD/CdS solar cells indicates that the CuS layer prevents the electron recombination in the TiO2/QD/electrolyte interface by facilitating the hole transport from QD to electrolyte thus improving both charge collection efficiency and VOC.22 The recombination lifetime (τrec) as determined from Bode plots (figure S6, SI) also implies presence of long lived electrons due to reduced interfacial recombination process. The Nyquist plots along with fitting parameters for CdSe and CdSeS_b NCs as shown in

Figure 5. Nyquist plots for CdSSe_a NCs using (a) TiO2/QD/CdS and (b) TiO2/QD/CdS/CuS assembly for solar cell fabrication. The solid line is the fit to the data points. (Inset) The equivalent circuit used for fitting the Nyquist plots. figure S7, SI and table S4, SI also strengthen aforementioned arguments as the Rrec increased for all the NCs upon CuS coating. The IPCE is a combination of light harvesting efficiency (LHE), electron injection efficiency (Φinj) and charge collection efficiency (ηc).62 There is not much change in the LHE as onset of IPCE spectra remains similar after CuS deposition whereas there


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Table 4. Parameters obtained from electrochemical impedance spectroscopy for CdSSe_a NCs with and without the CuS deposition. QD System

RS (Ω )

Rrec (Ω )

τrec (s)

CdSSe_a/CdS

30.1

147.2

0.07

CdSSe_a/CdS/CuS

28.2

483.1

0.28

is no hindrance to the electron transfer process as evident from the TA measurements. From EIS measurements we have also confirmed increase in ηc as ηc is directly proportional to Rrec which increased almost threefold once CuS was formed on the surface of CdS through cation exchange. We

compared

CdSSe_a

NC

solar

cell

with

standard

ZnS

coating

in

TiO2/CdSSe_a/CdS/ZnS photo-anode assembly (figure S8, SI) and obtained 3.61% PCE which is less than the 4.03% PCE with CuS coating. In case of CdS/ZnS passivation layer, process 1 and 2 in Scheme 1 are blocked leading to higher efficiency. However, due to slow hole transfer to redox electrolyte holes accumulate at the VB of QDs and process 3 can still happen. In case of CdS/CuS layer the CuS mediates smooth hole transfer to the redox electrolyte also preventing the process 3. To confirm our arguments we have performed EIS on the ZnS device which is included in figure S8, SI. The Rrec and τrec (obtained from Bode plot) is ~278.6 ohm and 0.17 s, respectively which is better than pure CdS layer as ZnS provides additional surface passivation. However, both Rrec and τrec are smaller than CdS/CuS layer confirming recombination resistance is higher for CdS/CuS. This means formation of CuS through partial cation exchange does not create additional defect states that can deteriorate the efficiency. When Cu is replacing Cd, the


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newly formed CuS retains the original passivating effect of CdS. The added benefit of CuS is it acts as a hole transporting material increasing the PCE. 4. Conclusion In conclusion, to improve the efficiency of QDSSCs designed from post synthetically assembled NCs on TiO2 p-type CuS was deposited through partial cation exchange with CdS in the assembly FTO/TiO2/QDs/CdS/CuS/CuxSe to catalyze the transfer of hole to the redox electrolyte. After deposition of this CuS through cation exchange the PCE improved to 3.57% from 2.56% for CdSe NCs. Interestingly, for CdSSe alloy NCs maximum PCE of 4.03% was achieved compared to 3.26% for the uncoated one. Ultrafast TA measurements further confirm that CuS coating does not additionally hinder the electron injection process to TiO2. Moreover, recombination resistance (Rrec) which is inversely proportional to the recombination rate increases implying the CuS layer formation prevents the unwanted recombination between electron (TiO2) and hole (VB of NC). Therefore, the CuS coating acts as a hole transporting material and better than the conventional ZnS coating which only acts as a passivation layer. The data presented here pinups new strategies for high performance QDSSC by putting the hole transporting layer for boosting the hole transfer at the QD−electrolyte interface. Moreover, for the first time we have used this strategy for gradient CdSSe alloy NCs and obtained 4.03% PCE with detail study through EIS and TA. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected]. Tel: +91-22-25593873, Fax: (+) 91-2225505331/25505151. Notes


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The authors declare no competing financial interests. Acknowledgement: We thank Dr. R. S. Dutta, MSD, BARC for XPS measurements. S.M. and J.D. acknowledge CSIR for research fellowship. Y.J. thanks to DST INSPIRE PhD program for providing research fellowship (Grant 2013/606). This work was supported by “DAE-SRC Outstanding Research Investigator Award” (Project/Scheme No.: DAE-SRC/2012/ 21/13-BRNS) granted to H.N.G. Supporting Information: Experimental details of NC synthesis and solar cell assembly, EDX and CV data, carrier quenching plots, XPS data, Additional IV-IPCE data, Bode plot and additional Nyquist plots, IPCE-IV after ZnS coating. This material is available free of charge via the Internet at http://pubs.acs.org/. References: (1)

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