Biexciton Dissociation Dynamics in Nanohybrid AuâCuInS2

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Bi-Exciton Dissociation Dynamics in Nano-Hybrid Au-CuInS Nanocrystals Somen Mondal, Radhamanohar Aepuru, Jayanta Dana, Nandan Ghorai, and Hirendra N. Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09261 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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The Journal of Physical Chemistry

Bi-Exciton Dissociation Dynamics in NanoHybrid Au-CuInS2 Nanocrystals Somen Mondal,a Radhamanohar Aepuru, a Jayanta Dana,b Nandan Ghorai,a Hirendra. N. Ghosh,a,b* a

Institute of Nano Science and Technology, Mohali, Punjab 160064

bRadiation

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

India.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected],in, [email protected]

ACS Paragon Plus Environment

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

ABSTRACT: Multiexciton harvesting from semiconductor quantum dot has been a new approach for improving the solar cell efficiency in Quantum Dot Sensitized Solar Cells (QDSC). Till date, relation between multiexciton dissociation in metal−semiconductor nanohybrid system and boosting the power conversion efficiency (PCE) of QDSC were never discussed. Herein we report a detailed spectroscopic investigation of biexciton dissociation dynamics in copper indium sulfide (CuInS2, also referred as CIS) and Au-CIS nanohybrid, utilizing both timeresolved PL and ultrafast transient absorption (TA) techniques. Ultrafast transient absorption suggests the formation of bi-exciton in CIS NCs which efficiently dissociates in Au-CIS nanohybrids. Maximum multiexciton dissociation (MED) efficiency is determined to be ~ 80% at higher laser fluency, however it was observed to be 100% at lower laser fluency. Prior to exciton dissociation electrons are captured by Au NP in the nanohybrid from the conduction band of CIS NCs which is energetically higher than Fermi level of Au. Here we demonstrate the proof-of-concept in multi-electron dissociation which may provide a new approach for improving the efficiency in QDSSCs, where we found power conversion efficiency (PCE) of Au-CIS nanohybrids up to 2.49% as compared to ~1.06% ~for pure CIS NCs in similar condition. This finding can be an efficient approach towards the design and development of efficient solar cell and optoelectronic devices using the principles of multiexciton generation and extracting multiexcitons in metal-semiconductor nanohybrid system.


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1. INTRODUCTION Copper chalcogenide nanocrystals (NCs)and their derivatives has attracted great attentions for their unique properties such as high molar extinction coefficient (105 cm2),1,2 tunable band gap,3 facile multiple exciton generation (MEG),4 easy synthesis and low cost.5Being a ternary nanocrystal, CuInS2 (CIS) has been found

as an alternative for

conventional semiconductor quantum dots due to its non-toxic properties.6It has different applications in various field such as light-emitting diodes,7–9 photovoltaics,5,10,11 luminescent solar concentrators,9,10 and bio-imaging6 etc. These environment friendly materials CIS NCs has a suitable band gap ~1.5 eV which can absorb a broad range of solar spectrum.12Among these applications, CIS NCs photovoltaic cells have emerged as new material for the third generation solution-processed photovoltaic cells with the power conversation efficiencies (PCE) of CIS NCs have been reported to be nearly 1.14 %.10 Generation of exciton (electron hole pair) by absorbing photon in the bulk semiconductor is a general property and the exceeding band gap energy released as heat energy when hot electron and hole are relaxed to its minimum energy of band-edge state. MEG is a process in which more than one electron-hole pair is generated by absorbing single photon in semiconductor quantum dot materials. The MEG, i.e., carrier multiplication (CM) is only possible when the band gap of the semiconductor is low and the photon energy is more than twice the band gap of the semiconductor. During the MEG, hot carriers are cooled via the generation of additional excitons rather than the generation of heat. The efficient multi- or biexciton generation using single photon is useful for potential application in optoelectronic devices, sensitive photodetectors, high power light emitting diodes (LED) and efficient photovoltaic devices or catalytic processes. 13,14 In this regard, low band gap materials such as the I−III−VI ternary CIS and AgInS2 (AIS) NCs are efficient materials for CM processes.15,16 Recently, we have demonstrated probe-induced bi-exciton generation in CIS NCs where



antisite defect states (donor states) are found to be responsible for the process. 16 CM efficiency in CuInSe2 NCs is comparable to other types of nanocrystals due to longer Auger lifetime and lower threshold energy.17 Thus one of the possible way for the utilization of short-lived multiexcitons in solar energy conversion is the ultrafast bi-exciton dissociation by interfacial charge transfer from NCs to any acceptors. Recently, the demand of functional nanomaterials in photovoltaic applications is greater than traditional quantum dots due to its significant functional properties aiming to achieve desired output performance.5,18 These hybrid nanomaterials or nanocomposites are fulfilling the wider range of application by changing its compositions and architectures. In these hybrid systems, interfacial charge separation processes play an important role to control their performance towards solar energy conversion, bio-sensing and photocatalysis. 19,20In this contest, hybrid system of metal and semiconductor are formed directly by the growth of metal NPs on the semiconductor surface by tuning the electronic states of both metal and semiconductor. As a result, semiconductor domain transfers the electron very efficiently to the Fermi level of metal domain. The electron transfer process is thermodynamically favourable as in those cases the Fermi level metal lies below the CB of semiconductor QD. A number of research groups have focused on understanding the exciton-plasmon interaction in Ausemiconductor nano-hybrid system. 21-25 Kobayashi et al.reported that spectral properties of AuPbS is strongly depend on the nanohybridation but changing of excited state dynamics as well as the time constant of electron−phonon coupling remain unaltered with bear Au NP.21The excited state dynamics in Au-CdSe nanorod suggests that electrons could rapidly escape from the CdSe rods to Au tips with lower activation energy. 22 Recently we have also demonstrated ultrafast charge transfer dynamics of CdSe-Au and CdSe@CdS-Au hetero-structure where photo-excited electrons are transferred from semiconductor domain to metal domain 23–25and shown that hot electron from CdSe@CdS core-shell can be transferred to Au NP which depends


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on the size of the metal particle.23 We have also demonstrated that CdSe-Au hybrid materials give higher PCE (ƞ =4.39%) as compared to pure CdSe QD (ƞ = 3.37%) due to facilitated electron

transfer

reaction

to

the

photo-anode

from

the

photo-excited

hybrid

materials.25Increment of PCE efficiency is rather marginal (~30%) due to generation of single electron-hole pair on photo-excitation of the nano-hybrid. A drawback of hybrid or heterostructure in solar technologies is significant loss of solar power to heat, caused by the release of excess energy when hot carriers relax via electron–phonon scattering. It would be interesting if by some means using MEG materials where bi/multi exciton could be dissociated by which heat-related energy losses can be avoided with drastic increment of PCE in QD solar cell. With an aim to extract the biexciton from CIS NCs to enhance PCE, herein, we have modified the CIS NCs with Au NP through the formation of hybrid Au-CIS. Till date the process remains unclear, how the multi-excitons are broken and charge carriers are extracted before the ultrafast exciton-exciton annihilation process. Herein, steady-state and timeresolved photoluminescence studies coupled with femtosecond transient absorption spectroscopy were employed to understand the charge carrier dynamics in both CIS NCs AuCIS nanohybrid. Ultrafast transient absorption studies are employed for the determination of percentage of biexciton breaking from CIS NCs in Au-CIS nano-hybrid system. For the first time, in this report we have demonstrated the role of metal NP in biexciton dissociation of nano-hybrid system by monitoring the charge carrier dynamics and its direct influence on the PCE of the photovoltaic device is been addressed in detail. 2. EXPERIMENTAL SECTION (a) Materials. Cu (II) acetylacetonate (>99.99%), In (III) acetylacetonate,1-dodecanethiol (1DDT), oleic acid (OA), 1-octadecene (ODE), and olylamine (OAm),gold-(III) bromide (AuBr3), 1,2-ethane dithiol (1-DDT)were purchased from Sigma-Aldrich. For purification of the synthesized materials AR grade methanol and chloroform were used.



(b) Preparation of CIS NCs. Multinary Quantum dot CIS NCs were prepared by hot injection method via a reported protocol with minor modification.3 Briefly, 26 mg of Cu (II) acetylacetonate (0.1 mmol), 41 mg of In (III) acetylacetonate (0.1 mmol), 1 mL of 1-DDT, 0.2 ml of OA and 0.5 mL of OAm were first heated at 120°C under Argon atmosphere follow by 1 hour vacuum in a three-necked flask. The solution temperature was fixed at 120°C until the clear yellow colour solution was formed. After complete dissolution, the temperature was increased to 230°C and the solution progressively changes the colour yellow to dark red, indicating the nucleation and subsequent growth of CIS NCs. Then the solution temperature was allowed to cool room temperature to obtain CIS NCs. After the synthesis of CIS NCs, purification was carried out using methanol and acetone mixture (1:1). The CIS NCs were precipitated with addition of methanol and acetone mixture and washed by repeated redissolution in toluene and precipitation with methanol and acetone mixture. Finally, CIS NCs were dissolved in toluene for further study. (c) Preparation of Au/CIS nanohybrids: For formation of Au/CIS nanohybrids a modified synthetic procedure was followed.26,27 For preparation of a gold stock solution, 43 mg of gold (III) bromide (0.1 mmol) was takenwith1 ml OAm in a two-necked flask. Then the mixture was stirred under Argon atmosphere at room temperature until getting a clear solution. All of the CIS NCs Precursors (26 mg of Cu (II) acetylacetonate (0.1 mmol), 41 mg of In (III) acetylacetonate (0.1 mmol), 1 mL of 1-DDT, 0.2 ml of OA and 0.5 mL of OAm) were taken in another three-neck round-bottom flask. Similarly, we followed the above mention procedure for preparation of CIS NCs. When the precursor stock solution of CIS NCs reached at 230 °C, the gold stock solution was added drop wise into the CIS NCs over a time period under vigorous shaking in Argon atmospheres. With the addition of gold stock solution the colour of the solution turned pink which is indicating the formation of Au (0) particle. The reaction was annealed for a further 10 min at the same temperature to obtain a dark brown coloured solution


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which indicating the formation of Au-CIS nanohybrid. After the synthesis of Au-CIS nanohybrid, we followed the reported protocol for purification using the methanol and acetone mixture (1:1) and then repeated the washing for 3-4 times. Finally, the product was re-dispersed in toluene for further analysis. (d) Spectral Measurements. The morphology of the synthesized nanostructures were obtained by using Transmission Electron Microscope (JEOL JEM 2100) at an accelerating voltage of 200 kV.

The absorption spectra were recorded using a Shimadzu UV-2600 UV-vis

spectrophotometer. Fluorescence measurements were made using an Edinburge FS5 spectrofluorimeter. The fluorescence lifetimes were measured by the method of time-correlated single-photon counting using a Deltaflex Modular Fluorescence Lifetime System (HORIBA Scientific). Either a laser head or a nano-LED pulsed diode powered by a pulsed diode controller (IBH) was used as the excitation light source. The excitation wavelengths used were 402 nm per the cases. The typical detection time of these was 100 ps. To calculate the lifetime, the fluorescence decay curves were analyzed by an iterative fitting program provided by IBH. Femtosecond pump–probe transient absorption studies were performed using Ultrafast Systems HELIOS transient absorption spectrometer. The laser system is a Coherent regenerative amplifier (Astrella Ultrafast Ti:Sapphire Amplifier) (800 nm with repetition rate of 1 kHz and pulse duration of 2 ns (5%). Moreover, bleach recovery is faster in case of Au-CIS nano-hybrids as compared to CIS NCs suggesting photo-excited electron transfer from CIS NCs (semiconductor domain) to Au (metal domain) within the nano-hybrid.22,24,30,37 The main aim of the present investigation is to monitor bi-exciton dissociation of photoexcited CIS in the Au-CIS nano-hybrid. Figure 4B shows the transient absorption kinetics at 580 nm where the bi-excitonic features appears10,16for both CIS and Au-CIS. The observed transient kinetics looks completely different for Au-CIS as compared to CIS. The kinetics in both the systems rather very complicated and fitted multi-exponentially. The time constants contain the growth time for biexciton in CIS NCs is 𝜏 = 2 ns(29%) (Table 1).However, the transient kinetics at 580 nm looks completely different (Figure 4Bb). The time constant contain the growth time for biexciton in Au-CIS NCs is 𝜏 = 2 ns (17%) (Table 1). It’s interesting to see that the signal due to bi-exciton decreases drastically which clearly suggests that bi-exciton generated out of photo-excitation of Au-CIS nano-hybrid gets dissociated due to transfer of electrons from semiconductor domain to metal domain. It’s clear from transient kinetics that formation of bi-exciton in both CIS NCs and Au-CIS nanohybrid is similar (2ns (6%) =0.80 0.110 1.60ps (89%), 83.2ps (7%), >2ns (4%) =0.35 aThe values in the parentheses indicate the percentage of amplitude of the particular time constant. The ultrafast study clearly suggests the breaking of bi-exciton is more efficient in hybrid Au-CIS NCs and photoexcitation charge separation has taken place in Au-CIS NCs where holes are localized in CIS NCs and electrons are localized in the Fermi level of Au NP. For a proof of principle in MED efficiency we have realized that it’s important correlate this observation with photo-conversion efficiency of quantum dot solar cell using these hybrid materials. To do



so we have fabricated quantum dot sensitized solar cell (QDSC) measured power conversion efficiency (PCE), current density−voltage (J−V) and IPCE for both CIS NCs and Au-CIS nanohybrids. The efficiency of PV cell depends on short circuit current (JSC), open circuit voltage (VOC), and fill factor (FF). Herein we have used CIS NCs and hybrid Au-CIS NCs as photoanode after loading them on the mesoporous TiO2 film through electrophoretic deposition method. Cu2S and polysulfide were employed as photocathode and electrolyte materials respectively.38The photovoltaic device is constructed by combination of photo-anode, electrolyte, and Cu2S counter electrode.39 15

(A) 10

50

5

(B)

b

40

b

IPCE

J (mA/ Cm2)

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

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a

30 20

a

10 0 0.0

0.1

0.2

0.3

0.4

Voltage (V)

0.5

0 300

400

500

600

700

Wavelength (nm)

Figure 6. (A) Jsc−V curves (B) IPCE spectra of (a) CIS NCs and (b) Au-CIS nano-hybrids material under 1 sun illumination. Figure 6A shows the J−V curves of QDSCs for CIS NCs and Au-CIS nano-hybrids after illuminating by 1 sun light (AM 1.5G, 100 mW/cm2) and measured the solar parameter such as short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are shown in Table 2. Here FF in both the cases is almost same but the open circuit voltage (Voc) for Au-CIS nanohybrids is slightly higher as compared to CIS NC. Short circuit current (Jsc= 13.128 mA/Cm2) for Au-CIS nanohybrids is found to be


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85% higher as compared to CIS NCs which might be due to higher generation mobility of photo-injected electron from Au-CIS nanohybrid system to TiO2 electrode.

Table 2: Measured Solar Cell Parameters for CIS and Au-CIS nano-hybrids

JSC (mA/Cm2)

VOC (V)

FF

PCE (%)

CIS

7.046

0.386

0.389

1.06

Au-CIS

13.128

0.479

0.395

2.49

Here AuNP plays an important role for enhancement of solar cell efficiency in Au-CIS nanaohybrid system. After the photoexcitation of Au-CIS nanohybrid, when multiexciton are generated in CIS NCs, they readily get dissociated through immediate (Sub-picosecond) transfer the electrons to Au NP which are finally transported to TiO2 electrode while the holes are localized in the VBs of CIS NCs which finally transferred to the counter electrode. This multi-exciton dissociation, charge separation and also slow recombination are responsible for the huge increment of Jsc in Au-CIS nanohybrid system as compared to pure CIS NCs.Incident photon-to-carrier conversion efficiency (IPCE) represents the percentage of the conversion of incident photon to charge carrier and collection at the electrode. In IPCE spectrum of Au-CIS nano-hybrids have an enhancement at the range of 350-610 nm compare to CIS NCs which are shown in Figure 6B. The measured PCE for CIS and Au-CIS nanohybrids were recorded to be 1.06% and 2.49%respectively.To explain the enhancement of PCE in Au-CIS nanohybrid system than CIS NCs, it is very important to understand the excited state dynamics of both these materials. Hence, the transient absorption spectroscopic measurements show that MED efficiency is directly correlated with the enhancement of PCE in Au-CIS nano-hybrids as compared to CIS NCs. In the present investigation we did not find very high value of PCE in the Au-CIS nano-hybrid system, however for the proof of principal that MED is one of the



important approaches towards achieving higher PCE in QDSC is clearly demonstrated. To the best our knowledge, we are reporting for the first time dissociation of multiple exciton in semiconductor-metal nano-hybrid system through ultrafast spectroscopic studies and for proof of principle we have demonstrated noticeable increment of photovoltaic performance in the nano-hybrid solar cell. 4. CONCLUSION In summary, CIS NCs and hybrid Au-CIS NCs have been synthesized, and detailed spectroscopic investigations have been carried out using steady state and fast and ultrafast timeresolved absorption and emission techniques. Steady state and time-resolved luminescence spectroscopy suggests photo-excited electrons are transferred from CIS NCs to Au NP in nanohybrid. Ultrafast transient absorption studies confirm the formation of multi-exciton in the photo-excited CIS NCs. Most interesting observation in the current studies indicate that multi/bi-exciton of CIS NCs can be efficiently dissociated with the help of Au NP in Au-CIS nanohybrid. Breaking of bi/multi exciton and related dynamics has been demonstrated with the help of femto-second transient absorption spectroscopy. Our calculation suggests the efficiency of MED in Au-CIS nano-hybrid is approximately 78% at higher laser fluency (= 3.46).However at lower laser fluency ( =

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