Intraband Electron Cooling Mediated Unprecedented Photocurrent

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Intraband Electron Cooling Mediated Unprecedented Photocurrent Conversion Efficiency of CdSSe Alloy QDs: Direct Correlation between Electron Cooling and Efficiency x

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Partha Maity, Sourav Maiti, Tushar Debnath, Jayanta Dana, Saurav K. Guin, and Hirendra N. Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07876 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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The Journal of Physical Chemistry

Intraband Electron Cooling Mediated Unprecedented Photocurrent Conversion Efficiency of CdSxSe1-x Alloy QDs: Direct Correlation between Electron Cooling and Efficiency Partha Maity†, Sourav Maiti†, $, Tushar Debnath†, Jayanta Dana†, Saurav K. Guin‡ and Hirendra N. Ghosh†* †

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

India. $ ‡

Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India.

* E-mail: [email protected]. Tel: +91-22-25593873, Fax: (+) 91-22-25505331/25505151.

Abstract: Composition and size dependent band gap engineering with longer excited state charge carrier life time assist CdSxSe1−x alloy semiconductor quantum dots (QDs) as a promising candidate for quantum dot solar cell (QDSC). Colloidal CdSxSe1-x alloy QDs were synthesized using hot injection method where stoichiometric mixture of S-TOP and Se-TOP were injected at 270°C in a mixture of Cd-oleate. The electron decoupled from hole in the alloyed structure due to delocalization of electron in electronically quasi type-II graded CdSxSe1-x alloyed structure. As a result, intraband electron cooling time increases from 100s of fs to sub 10-ps time scale in the alloyed graded structure. Extremely slow electron cooling time (~8 ps) and less charge recombination (~50% in > 2 ns) as compared to both CdS and CdSe QDs are found to be beneficial for charge carrier extraction in QD solar cells. Using polysulfide electrolyte and Cu 2S deposited ITO glass plates as photocathode, the efficiency of the QD solar cell was measured to be 1.1 (± 0.07)% for CdS, 3.36 (± 0.1)% for CdSe, and 3.95 (± 0.12)% for CdS0.7Se0.3 QDs. An additional non-epitaxial CdS quasi-shell and followed by ZnS passivation layer (TiO2/ CdS0.7Se0.3 /quasi-CdS/ZnS) was deposited on top of CdS0.7Se0.3 film which showed photo current conversion efficiency (PCE) 4.5 (± 0.18) %. The overall 14% increase of PCE is due to

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the quasi CdS shell helps to separate more electrons through passivating the surface states of TiO2. 1. Introduction: Last few years research on quantum dot (QD) materials has been surged tremendously due to their exciting opto-electronic properties such as higher extinction coefficient, optical tunability, slow carrier relaxation process, large excited state charge carrier life time, formation of multiple exciton generation.1-6 Especially, lower band gap II-VI7,8 and IV-VI9-11 semiconductor materials, which absorb light in the visible and near infrared regions are widely used as light harvesters in quantum dot solar cell (QDSC). Apart from this, some narrow bandgap ternary semiconductor materials like CuInS212,13, CuInSe214, CuInGaSe215and of course organic inorganic perovskite16,17 have been extensively used as sensitizer in the low-cost third generation solar cell. In addition to that it has been observed that excited state charge carriers life time of the QDs can also be increased through metal ion doping (usually Mn2+) which boost up the efficiency of the QDSC.18,19 Moreover, utilizing the concept of band gap engineering in heterostructure core/shell nanocrystals,20 such as ZnTe/CdSe,21 CdS/CdSe,22 CdTe/CdS,23 CdSeTe/CdS,24 PbS/CdS/ZnS,25 CuInS2/CdS26 core/shell/quasi shell27 and ternary alloy QDs, CdSexTe1-x,28 PbSxSe1-x,29 CdSe/CdSxSe1–x/CdS30 are used as a photo anode for the development of higher efficient QDSC. It has been observed that in ternary alloy QDs, the optical tunability not only depends on the size of the QD but also on the composition of the constituents. Therefore, composition plays an extra degree of freedom towards the optical and photo physical properties of the ternary QDs. However, to optimize photocurrent efficiency it is necessary to optimize all the processes after characterization and measure the photovoltaic performance of a real cell. Even though the QD materials have all the exciting properties, till date the efficiency of the QDSC has not superseded dye-sensitized solar cell (DSSC).31 However, uninterrupted research on QDSC by several researchers helps to reach the PCE in double numerical figure.32,33 One of the major reasons for poor performance in QDSC is due to lesser charge separation, where in QD material most of the photo-generated electron are trapped into the internal defect states and/or recombines with electrolyte at QD interface 2,12,34 etc. Besides, efficiency of QDSC depends on extraction of photogenerated electron from CB of sensitizer to another


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semiconductor. The electron injection efficiency is directly related to the intraband electron cooling time. So it is expected that increasing intraband electron cooling time can enhance the electron injection efficiency resulting increasing PCE. In QDSC lower band gap sensitizer QDs are deposited on the wide band gap semiconductor such as TiO2, ZnO NCs. Deposition of QD materials on the TiO2 film involve two major techniques; like direct growth of QDs on TiO2 electrode through chemical bath deposition technique (CBD)23,35 or successive ion layer absorption and reaction (SILAR),8 and post synthesis deposition technique which includes direct deposition,35 electrophoretic technique13 or by ligand assisted assembly.36 Although the direct growth technique is beneficial for high coverage, easy growth and nucleation on the TiO2 mesoporus surface, however size of the NCs and the size quantized quantum confinement of the QDs are difficult to control. In direct deposition technique surface defect states within the QDs are easily generated which annihilates the charge carrier through non-radiative trapping process.36,37 As a result, efficiency decreases drastically in QDSC. On the other hand, in post synthesis deposition process size quantization, high crystallinity, optical properties, band gap tunability and surface passivation of the QDs can easily be controlled. Post synthesis deposition through electrophoretic techniques prevents good loading of QDs on the TiO2 surface due to presence of long chain ligand molecules. Both high quality of the NCs and homogeneous deposition can be maintained through ligand assisted deposition processes.36 Utilizing the above post synthesis ligand assisted technique Zhong et al. tremendously ameliorate the photocurrent efficiency in QDSC.3,24 CdSxSe1-x alloy QDs offer double benefits in terms of band gap tunability and slower carrier cooling with changing the composition. In addition to that, CdSxSe1x

alloy QDs can be formed in all possible composition owing to less than 3.5% lattice

mismatched in the crystals structure of CdS and CdSe.38 The CdSxSe1-x alloy QDs show better coverage of the solar spectrum,39-41 high emission quantum yield,42 long excited state life,42 higher photostability 43 as compare to their corresponding constituent. Thus CdSxSe1-x alloy can be directly used as sensitizer in the QDSC.44 It will be interesting to compare the photo-current conversion efficiency of CdSxSe1-x alloy QDs with CdS and CdSe QDs and make a correlation with the electron cooling dynamics after measuring in ultrafast time scale which never been discussed in literature.



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Slow electron cooling mediated unprecedented photocurrent conversion efficiency of CdSxSe1-x alloy QDs as compared to CdS as well as CdSe QDs has been performed in terms of IPCE and IV measurements. High quality non aqueous CdS0.7Se0.3alloy QDs were deposited on the sintered TiO2 film after phase transfer with MPA at pH 12 which act as photo anode of the working QDSC. Using polysulfide electrolyte and Cu2S deposited ITO glass plates as photocathode the efficiency of the QD solar cell was measured to be 1.1 (±0.07)% for CdS QDs, 3.36 (± 0.1)% for CdSe and 3.95 (± 0.12)% for CdS0.7Se0.3 QDs under 1 sun illumination. However, a non-epitaxial CdS quasi shell was coated to passivate the TiO2 surface states and followed by passivated through ZnS layer that increased PCE to 4.5 (± 0.18) % which is the highest reported value for CdSxSe1-x alloy QDs.

2. Experimental Section: (A) Chemicals Used: Cadmium oxide (CdO, 99.5%), sulfur powder (S, 99.99%), selenium powder (Se, 99.99%), oleic acid (90%), trioctyl phosphine (TOP, 90%) and octadecene (ODE, 90%) were purchased from Sigma-Aldrich and used as it is. 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), copper sulphate, sulfuric acid (H2SO4). AR-grade methanol, ethanol, acetone and chloroform were used for cleaning the QDs. Deionised (DI) water was used for phase transfer and solar cell preparation.

(B) Synthesis of oleic acid capped QDs and Phase Transfer: (a) Synthesis of OA capped QDs High quality, monodispersed non-aqueous CdS, CdSe and CdSxSe1-x alloy QDs were synthesized using hot injection method.40 In brief, a stock cadmium oleate and the chalcogen solution were prepared separately in different three neck volumetric flask. The Cd-oleate solution was prepared by heating a mixture of 1 mmol (0.128 g) CdO, 4 mmol OA in 15 ml ODE at 200°C in an inert atmosphere. The colorless solution indicates the formation Cd-oleate. Previously prepared total 0.25 mmol of the chalcogen in 1 mmol TOP was directly injected to the Cd-oleate mixture at 270 °C. The composition of the different alloy QDs reported in our


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previous report. The desire size of the QDs was achieved by heating the reaction mixture for 5 min at 250 °C. Finally the crude product was washed with methanol and kept in chloroform solution. (b) Ligand Exchange: The OA capped QDs were solubalized in water through phase transfer technique using MPA as a surfactant following literature techniques.36 The pH of the MPA solution in methanolwater mixture (3:1) was adjusted to 12 using a 40% aq. KOH solution. This solution was dropwise added to the QD solution in chloroform until precipitation and stirred for 15 min. 5 ml of water was added followed by 15 min further stirring. The pH of the aqueous phase was maintained at 12. The QDs phase transferred to the aqueous phase which was collected and precipitated with acetone. After centrifugation, the water soluble MPA capped QDs was dispersed in required amount of water for further use.

(C) Steady State UV-Vis and Emission Spectrometer: UV-Vis optical absorption spectra were recorded on a JASCO-640 spectrophotometer. Steady state photo luminescence (PL) spectra were recorded using Hitachi model 4010 Spectrofluorimeter. (D) Time Resolved Emission Spectrometer: Time resolved fluorescence measurements were carried out using a diode laser based spectrofluorimeter from IBH (U.K). The instrument works on the principle of time-correlated single photon counting (TCSPC). In the present investigation 406 nm laser pulses were used as the excitation light sources and a TBX4 detection module (IBH) coupled with a special Hamamatsu PMT was used for fluorescence detection. All the decay traces are fitted using the software and by using the equation remaining at time t.

i

and

i

where, I (t) is the total intensity

are the amplitude and decay time of ith component respectively. The

average de-excitation life time of the sample is measured using the equation

.

(E) Transient Absorption Measurements: The experimental setup of the transient absorption measurements is described in our earlier publication.45 The excited state dynamics in the 450-750 nm spectral regions were carried



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out from 0.1 ps to 2 ns time span after 400 nm laser excitation. The mode lock ultrashort pulses (seed pulses) of 800 nm, 50 fs, ~4 nJ energy, and 86 MHz repetition rate, is generated from Ti: sapphire oscillator (CDP, Moscow). The seed pulses are amplified in a multi-pass amplifier system pumped by a 20W DPSS laser (Jade-II, Thales Laser, France) to generate 1.8 ± 0.1 ns (49%) in chloroform and τ1 = 50 ± 2.2 ps (36%), τ2 = 250 ± 10.5 ps (17%), and τ3 > 1.8 ± 0.1 ns (47%) in water, respectively (Table 1). It is interesting to see that the bleach



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recovery kinetics of the CdS0.7Se0.3 alloy before and after ligand exchange does not vary significantly. The 1S bleach recovery kinetics of CdS0.7Se0.3 alloy QDs on ITO glass (nonreactive surface) can be fitted bi-exponential growth τ1g = 150 ± 8 fs (-78%), τ2g = 8 ± 0.3 ps (22%), and recovers multi-exponentially with time constant τ1 = 40 ± 2 ps (35%), τ2 = 200 ± 10 ps (24%), and τ3 > 1.8 ± 0.1 ns (41%) (Table 1). Relatively faster bleach recovery of the alloy QDs in solid film as compare to solution phase is due to the intra particle non radiative energy transfer on ITO film surface where the concentration of QD particles is much higher as compared to solution phase.49

Table 1: Multi-exponential Fitting Parameters of CdS0.7Se0.3 alloy QD in Different Experimental Conditions at 550 nm. The Percentages at the Parenthesis Represent Amplitude of the Corresponding Exponential Functions.

System

1g

2g

1

2

3

CdS0.7Se0.3 non-aq.

150 ± 8 fs (-78%)

8 ± 0.3 ps (-22%)

60 ± 2.5 ps (40%)

300 ± 12 ps (11%)

> 1.8 ± 0.1 ns (49%)

CdS0.7Se0.3 aq.

150 ± 8 fs (-78%)

8 ± 0.3 ps (-22%)

50 ± 2.2 ps (36%)

250 ± 10.5ps (17%)

> 1.8 ± 0.1 ns (47%)

CdS0.7Se0.3_film

150 ± 8 fs (-78%) 1.8 ± 0.1 ns (41%) 300 ± 12 ps (16%)

CdS0.7Se0.3 _TiO2 film

Intriguingly, the bleach kinetics of CdS0.7Se0.3 alloy on TiO2 film can be fitted with pulse width limited (

Intraband Electron Cooling Mediated Unprecedented Photocurrent

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