Nano-Structures & Nano-Objects Synthesis and

Nano-Structures & Nano-Objects 14 (2018) 118–124

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Synthesis and characterization of Mn:ZnS quantum dots for photovoltaic applications Vadiraj K.T. *, Shiddappa L. Belagali Environmental Chemistry Laboratory, Department of Studies in Environmental Science, University of Mysore, Mysore 570 006, India

highlights

graphical abstract

• The hydrothermally synthesized QD

•

•

•

•

is optically active with absorption at 269 nm showing optical band gap of 3.86 eV. The absorbed radiation was emitted at 587 nm which is visible range showing a red shift. This was confirmed by FESEM, TEM, and SEAD. For elemental analysis and active orbital identification in the compound, EDS and XPS were carried out. The synthesized material is nondispersive but showed an excellent photo-activity when combined with P3HT as a solar cell. The properties of VOC (310 mV) and ISC (0.002 mA) were clearly identified with a fill factor of 24% and 0.006% efficiency.

article

info

Article history: Received 31 May 2017 Received in revised form 1 February 2018 Accepted 9 February 2018 Keywords: Zinc sulfide Dopant Hydrothermal synthesis Hybrid solar cell P3HT

a b s t r a c t The field of photovoltaics has evolved over the past few decades and introduction of quantum dots (QDs) have further improved their ability to absorb shorter wavelength radiations, which further attracted investigation on quantum dots. Among the QDs, zinc sulfide has shown remarkable results as a semiconductor. It is a low cost and a non-toxic material. Mn:ZnS is synthesized by a hydrothermal process and is characterized by UV–Visible spectroscopy to calculate the optical band gap (3.86 eV) and by X-ray diffractometer (XRD), particle size was calculated (±40 nm) which was confirmed by scanning electron microscopy (SEM). Elemental analysis was carried out by energy dispersion spectroscopy (EDS) and Xray photoemission spectroscopy (XPS). Here manganese is used as dopant to increase the luminescence property by red shift (587 nm), and thus, making a better candidate for photovoltaic applications. The ability of this material for photovoltaic application is evaluated by constructing a hybrid solar cell with P3HT and calculating the efficiency. © 2018 Elsevier B.V. All rights reserved.

1. Introduction

* Corresponding author. E-mail address: [email protected] (Vadiraj K.T). https://doi.org/10.1016/j.nanoso.2018.02.001 2352-507X/© 2018 Elsevier B.V. All rights reserved.

So for most of the energy demand have been met by fossil fuels all over the globe. In the recent decade the increase in demand and fuel price have lead to explore alternative fuel source such as

Vadiraj K.T, S.L. Belagali / Nano-Structures & Nano-Objects 14 (2018) 118–124

solar energy which is also a clean energy source with abundant availability [1]. Crystalline silicon-based solar cells have dominated the market for the past 5 decades. These required a lot of processing of silicon for construction and production of solar cells which eventually increased the cost. Hence, search for the alternative materials started which lead to the finding of nanomaterials in the field of solar cells. Nanomaterials have caught attention of many researchers because of their peculiar physical and chemical properties. The increase in surface area to volume ratio and change in electronic structure due to quantum confinement has been significantly utilized by the scientific community [2]. Nano semiconductors in the field of electronics have provided greater scope. These miraculous tiny particles have changed the world by reducing the size of electronic appliances, increasing the storage capacity, and decreasing the processing time of several components. The nanoscale quantum dots (QDs) have increased the ability to absorb high energy side of the solar spectrum [3,4]. Their ability of quantization show a change in electronic state, which lead them to absorb blue light. These absorptions depend mainly on the size of QDs, hence band gap can be tuned easily [5]. This ability has increased the effective absorption of all wavelengths and in turn improves the efficiency. But these QDs are constructed with transition elements, such as cadmium, lead, mercury and selenium which are heavy metals [6,7]. Thus, focus has shifted towards ZnS which is a non-toxic, easily synthesizable and lowcost luminescent material [8]. Zinc sulfide has demonstrated promising results in the field of semiconductors. It is a II–VI direct gap semiconductor having a better chemical stability than chalcogenides. Accordingly, manganese-doped zinc sulfide is referred to as diluted magnetic semiconductor and has attractive functions [9]. Doping of transition and rare metals like iron, copper, nickel, manganese, yttrium with ZnS have proven to be of greater advantage [10–12]. The presence of manganese is known to change both the optical and physical properties of the ZnS in a recordable scale. Greater research can be carried out by transporting and controlling various spin states by the presence of manganese. Manganese tweaks band gap and other luminance centers by different mixing energy levels of the 3d electrons with the s-p electronic state of the host. The Mn2+ ions exhibit a broad change in the crystal field strength with the host. The emission of color may vary accordingly with 4 T1 6 A1 transition [13–15]. These properties of the materials find application in optical coatings, electro-optical modulators, photoconductors, sensors, phosphors, reflectors, dielectric filters, and other light emitting materials [16]. In synthesis, hydrothermal method is a promising technique for synthesizing nanoparticles. The process occurs at high temperature and vapor pressure is maintained to get crystallized QDs. This process is less time-consuming and most suitable for zinc sulfide synthesis where a good yield is obtained. The synthesized material can be easily separated by simple filtration. This paper concentrates on the synthesis and study of various physical and optical properties of manganese-doped zinc sulfide. Application of Mn:ZnS, as a n-type semiconductor against organic semiconductor P3HT in a hybrid solar cell has been discussed. The uniqueness is, synthesis of Mn:ZnS as a non-dispersive material. It is first reported where nondispersive Mn:ZnS has photovoltaic applications [8] (see Fig. 1). 2. Experimentation 2.1. Mn:ZnS quantum dot synthesis and characterization For the experiment, chemicals such as zinc acetate (Zn(CH3 COO)2 · 2H2 O), manganese acetate (Mn(CH3 COO)2 · 4H2 O)

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Fig. 1. Energy level diagram of P3HT/Mn:ZnS device in vacuum, showing the work function of electrodes, CB and VB of Mn:ZnS and HOMO, LUMO of polymer P3HT.

and thiourea (NH2 CSNH2 ) were procured from Merck India, Mumbai. Double distilled and deionized water was used for the preparing solutions and washing the obtained material. In a typical synthesis, 0.25 M zinc acetate and manganese acetate solutions were prepared and kept for 3 h to settle down. A 0.75 M thiourea solution was prepared 30 min prior to the synthesis. Then 12.5 ml of zinc acetate solution was mixed with 12.5 ml of manganese acetate solution with constant stirring, followed by addition of 25 ml of thiourea solution dropwise to seed the reaction. After the addition, the mixture was stirred for 30 min. The mixture was then taken in a sealed Teflon-lined autoclave of 50 ml capacity and placed in a box furnace for 24 h at 200 ◦ C; later the mixture was allowed to cool naturally [17]. The obtained product was a white amorphous material, which was washed copiously with deionized water several times and finally with acetone. The mixture was then dried at 100 ◦ C for 12 h and stored in an airtight container. Synthesized QDs were characterized by UV–visible spectroscopy, photoluminescence, XRD, FESEM-EDS, TEM-SEAD and XPS. 2.2. Construction of cell P3HT/Mn:ZnS For construction of solar cells, transparent conducting oxide (TCO) glass slides coated with indium tin oxide (ITO) of surface resistance 8–13 /cm2 , poly(3,4-ethylene dioxythiophene)poly(styrene sulfonate) (PEDOT:PSS) and the semiconducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) were used along with the synthesized Mn:ZnS. These were procured from Sigma-Aldrich India and used without further processing. The ITO plates were etched using concentrated HCl and washed with NaOH solution, water and then sonicated in isopropyl alcohol (IPA) for 15 min. These slides were wiped with lint-free tissue papers and stored in vacuum. The fabrication process started with deposition of PEDOT:PSS on the prepared glass slides. Vacuum spin coating unit was employed for this process. Slide was set up on the stage of the instrument and vacuum was created underneath the slide, this prevented the slide from slipping during deposition. 3–4 drop of PEDOT:PSS was sufficient to cover the complete surface. It was spin coated at 700 RPM for 30 s and deposited slides were annealed at 115 ◦ C for 15 min in vacuum. Previously equiweight (10 mg/ml) P3HT and Mn:ZnS were dispersed in dichlorobenzene and sonicated for 24 h. This blend was spin coated on pre-deposited slides at 500 rpm for 30 s. These slides were vacuum annealed at 70 ◦ C for 15 min for proper adhesion and were stored in vacuum till further process was carried out [18].

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Fig. 2. The expected cell shown in schematic diagram, here active layer is sandwiched between transparent electrode (ITO), hole acceptor (PEDOT:PSS) and back electrode. Fig. 4. Photoluminescence spectrum of Mn:ZnS QDs.

determine the nature and optical characteristics of optical band gap of the synthesized material. For characterization of ZnS QDs, the absorption coefficient ‘α ’ is calculated using Beer–Lambert’s equation (Eq. (1))

α=

1 d

( ) ln

1

T

(1)

where d is the path length and T is the transmittance. The absorption coefficient α and the incident energy of photons hν is obtained from the following relationship which is used for optical band gap calculation

( ) (α hϑ)1/m = c hϑ − Eg

Fig. 3. UV–Vis absorption spectrum of the synthesized Mn:ZnS.

Aluminum was used as back electrode and was deposited using thermal deposition method. The pressure was maintained to 2 × 10−6 torrs. A current of 23 A was supplied to the tungsten boat to heat and evaporate aluminum wire. The whole process of deposition was carried out at room temperature. The current was passed for 10–15 s to obtain 200–250 nm of aluminum layer and area of the cell was 0.25 mm2 . Then, it was transported in anhydrous condition to avoid moisture contamination. Schematic of the cell is shown in Fig. 2. 3. Results and discussion 3.1. Optical properties, band gap, and particle size Optical properties of the material were characterized by UV– Visible and near infrared (NIR) spectroscopic technique, which gave a clear idea for absorbance of light and band gap of the material. The synthesized material was a white colored nano-size powder. At first, the synthesized nanomaterial was dispersed in methanol and the optical property was measured in a quartz cuvette. The light absorption peak ranged from 250 to 350 nm with a maximum at 269 nm. The peak range reflects the band gap of the particles (Fig. 3). In this experiment, the basic phenomenon is the excitation of electrons from valence band to the conduction band, which will

(2)

where C is a constant and Eg is optical band gap of the material. The value of m depends on the type of transitions. For direct and allowed transition, m = 1/2; for indirect transition, m = 2 and for direct forbidden transition m = 3/2. In this experiment, the band gap (Eg ) is calculated using Tauc plot taking m = 2 in Eq. (3), and by extrapolating the linear portion of the (α hν )1/2 versus (hν ) curve to the energy axis at a value of α = 0. The optical band gap was calculated which is found to be 3.86 eV. This band gap shows that the synthesized material is a semiconductor. Here Mn2+ doping reduces the conduction band of ZnS and thus, increases the lightto-electricity conversion efficiency [19]. 3.2. Photoluminescence properties Photoluminescence (PL) spectrum is analyzed for Mn:ZnS and shown in Fig. 4. An emission peak between 580 and 590 nm is a characteristic of 4 T1 -6 A1 transition in Mn2+ and the highest intensity of emission is observed at 587 nm. This can be compared with the red-shift of bulk Mn:ZnS [20,21]. The emission shows that the Mn2+ ion is either in the lattice or linked outside. It is known that the presence of Mn2+ in different centers results in various luminescence properties of Mn:ZnS [22]. Here, the emission of orange light in UV atmosphere shows that the Mn2+ ion is incorporated in the lattice of host ZnS which has resulted in the colored emission. In general synthesis, Mn2+ ion is distributed on the surface and forms non-radiative recombination routes from surface Mn2+ ions to surface active quenching centers, which usually reduces photoluminescence intensity. In the case of hydrothermal synthesis, Mn2+ ions have been distributed equally into ZnS host resulting in reduction of non radiative recombination on the surface. Hence, the photoluminescence intensity increases as the Mn2+ ion


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Fig. 5. X-ray diffractogram for Mn:ZnS.

Table 1 Inter planar spacing from XRD and JCPDS data and corresponding (hkl) values of ZnS. dXRD (Å)

dJCPDS (Å)

hkl

3.135 1.911 1.630

3.123 1.912 1.633

111 220 311

transition is enhanced through the increase of radiative transition through the host ZnS [23]. 3.3. Morphology, crystallinity, and elemental analysis The sample was exposed to X-ray for this study using Rigaku Miniflex II desktop X-ray diffractometer with Cu Kα source, operating at 30 mA and 40 kV. The crystallinity and crystal size of Mn:ZnS nanoparticles is determined on the basis of X-ray line broadening and calculated by Debye–Scherrer formula (Eq. (3)): D=

kλ

β cosθ

(3)

2 where β = (βM − βI2 ), βM is the full width at half maximum, and βI is the correction factor for the instrument broadening, θ is the angle of maximum peak, and λ is the wavelength of the source (Cu Kα = 1.5406 Å). The pattern of XRD (Fig. 5) for the synthesized Mn:ZnS was recorded 10◦ /min and peaks are found between 2θ = 20◦ to 60◦ . In the pattern, three prominent peaks are found at 28.46◦ , 47.44◦ , and 56.28◦ . These peaks correspond to the plane of cubic ZnS (JCPDS No.05-0566). The first peak at 28.46◦ is indexed to (111) plane, the other two peaks at 47.44◦ and 56.28◦ correspond to (220) and (311), respectively. The pattern indicates that material is having face-centered cubic lattice. Hence, the inter-planer spacing or the d value of the peaks and the lattice constant of the face-centered cubic lattice are tabulated in Table 1. To calculate the crystallinity using Debye–Scherrer formula (Eq. (1)), the obtained reading of XRD from the instrument is deconvoluted using standard software ‘‘Origin 8.5’’. From this, full width at half maximum is calculated and the obtained crystalline size of the Mn:ZnS is 52 nm. The morphology and elemental analysis of the synthesized Mn:ZnS was carried out by FESEM. The imaging was done at 4 KV, with magnification up to 300 nm and vacuum pressure of 10−6 Torr. The images show that the nanoparticles are aggregated in a spherical structure. This spherical agglomerate is about 1 µ

Fig. 6. (a) Scanning electron microgram (b) Energy dispersion spectrogram of synthesized Mn:ZnS.

Table 2 Elemental analysis by EDAX spectroscopy. Element

Weight (in %)

Atoms (in %)

C O S Mn Zn Total

1.56 1.09 25.77 2.86 68.72 100.00

19.3 5.00 35.92 1.30 38.47 100.00

in size and nanoparticles in the sphere have sizes ranging from 40 to 90 nm. In hydrothermal synthesis, internal pressure and higher temperature leads to the formation of nanoparticles but due to the cooling effect these nanoparticles aggregates and form spherical structure (Fig. 6(a)). This can be reversed by sonicating the synthesized material for 20 min in a bath sonicator. The elemental analysis was carried out by EDAX analysis. The peaks showed the presence of zinc, sulfur, manganese with traces of carbon and oxygen (Fig. 6(b)). The elemental composition is displayed in Table 2. EDAX of quantum dot gives an atomic ratio of sulfur to zinc as 0.93:1.00 which is close to the theoretical value. The Mn2+ is 2.86% and the ratio between Zn to Mn is 1.00:0.03. This clearly shows that the dopant (Mn) is around 3% compared to the central zinc atom. 3.4. Transmission electron microscopy TEM images of Mn:ZnS quantum dots are shown in Fig. 7. The images were taken in different resolutions. TEM micrograph gives direct information about particle size and morphology. QDs were

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size is about 20–25 nm, which is smaller than the values obtained from SEM image, and XRD. Inset of Fig. 7 shows SAED pattern of Mn:ZnS QDs. The bright patches along the ring, indicate (111), (220) and (311) planes, similar to XRD and confirms its polycrystalline. Interplanar distance is 2.5 nm. 3.5. X-ray photoelectron spectroscopy

Fig. 7. Transmission electron micrograph of Mn:ZnS quantum dots at 20 nm and 2 nm (upper left inset) and selective area energy dispersion pattern(lower right inset)

first dispersed in methanol and sonicated for 24 hrs to break the agglomerate and get uniform distribution on Cu grid. The particle

X-ray photoelectron spectroscopy has been used to determine the dopant content and confirm the valance state of zinc, sulfur, and manganese atoms in ZnS matrix along with corresponding binding energy. Fig. 8(a–d) display the measured XPS survey spectra of Mn:ZnS and core level spectra of Zn2p, S2p and Mn2p. Fig. 8(a) shows the survey spectra of all the present elements, which confirms doping in ZnS host. Orbitals are shown along with the peaks. Fig. 8(b) shows an enlarged view of Fig. 8(a) for observation of orbitals of Zn2+ ions. The figure shows two peaks; first peak around 1020.5 eV corresponding to 2p(3/2) and the second at 1043.8 eV representing 2p(1/2) . These two peaks match exactly with the binding energy of Zn-S bonding [24,25]. In Fig. 8(c), the signal corresponding to S2p binding energy is observed. The peak at 160 eV confirms both the Zn-S bonding and surface contamination [26], which further confirms the results of PL where the Mn2+ ion are spread throughout the nanoparticles. The presence of Mn is evident from the Mn2p as shown in Fig. 8(d) in the range of 630 to 660 eV. As Mn is doped in small quantity, the peak of Mn2p is weak. The peaks at 642.2 eV and 654.8 eV represent Mn 2p(3/2) and Mn2p(1/2) , respectively, which clearly shows that the presence of Mn in ZnS and Mn2+ ion acts as a substitute for Zn atom and interstitial one [27].

Fig. 8. X-ray photoelectron spectrogram of Mn:ZnS (a) Wide survey spectrum (b) Zn 2p3/2 spectrum (c) S 2p spectrum (d) Mn 2p.


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Table 3 Various cell Parameters compared with the fabricated cell. Sample

VOC (V)

JSC (mA/cm2 )

FF (%)

Efficiency (%)

Ref.

P3HT/ZnS P3HT/Mn:ZnS

0.64 0.31

0.001 0.002

20 24

0.0023 0.0061

[28,29] Present work

short circuit in the devices. Hence before the deposition, thorough scanning of active layer is carried out under a simple microscope. Then aluminum deposition is carried out to have optimum results for conductivity. In order to improve the efficiency and connectivity, the active layers with quantum dots of spiral shape, nanowire, nanorods or nanobelts, nanocomposites can be used for easy transfer of electrons. These nanostructured materials are more interesting, help in easy charge separation and have a high charge carrying capacity [30–32]. However, the structures like combs, conducting filament etc. can improve the structure of quantum dots and hold them for proper transfer of hole–electron pairs. Other way to improve the conductivity of ZnS quantum dots based solar cells is by utilizing carbon nanotubes and graphene. This helps in restricting the movement of electrons through the dimensions of ZnS [33,34]. 4. Conclusion Fig. 9. IV curve of device with Mn:ZnS QDs (P3HT/Mn:ZnS).

3.6. Characterization of P3HT/Mn:ZnS The Fig. 9 shows the I–V characteristic of the device. P3HT/Mn:ZnS accounted for 50% of weight together with generated electrical power. The curve clearly shows that value of open circuit voltage is 310 mV and short circuit current density is 0.002 mA. When compared to each other, VOC has around 60% of the maximum value. After drawing power from the device the output voltage is 310 mV with power density of 0.0024 mW/cm2 . Fill factor of the constructed cell is 24% with efficiency of 0.006%. There is a room for improvement by increasing the area of active layer and development of conductivity inside the subsystems. To understand the significance of Mn dopant, in P3HT/ZnS layer, we have compared it with pristine/undoped P3HT/ZnS hybrid solar cells. Such structures were studied by Mall et al., and Karim et al., as shown in Table 3. The doping with Mn results in a 3 times increase in efficiency. In the present system, short circuit current is higher with optimum voltage which increases the efficiency to 0.006% with a fill factor of 24% and utilizable power is 0.002 mW/cm2 . The obtained results of P3HT/Mn:ZnS cell are superior compared with previous reported similar devices and with optimization, higher efficiency can be expected. Another critical aspect is the junction between active layer and back electrode (Al)which is affected during deposition. During spin coating, the irregularities such as roughness, particle contamination, pin hole formation lead to a reduction of ISC . In some cases, VOC is found to reduce in the worst case of these defects. During the deposition of active materials by spin coat, the governing factor is layer thickness. This is because in hybrid solar cells a thinlayer of active material will have a higher band gap leading to higher efficiency. This can be determined by rate of spin coating and adhesive nature of material. Fortunately, ZnS QDs with P3HT has a very good adhesive property with ITO glass, hence there is no serious pin-hole formation problem. After the active material deposition, electrodes are deposited by thermal evaporation. Electrode preparation is an important aspect as the formation of pin holes in the active layer can lead to

From the results, it can be concluded that the hydrothermally synthesized QD is optically active with absorption at 269 nm and shows an optical band gap of 3.86 eV, which is advantageous for photovoltaic applications. The absorbed radiation was emitted at 587 nm which is in visible range showing a red shift. The size of material was calculated by XRD (52 nm)and was further confirmed by FESEM, TEM, and SAED. For elemental analysis and active orbital identification in the compound, EDS and XPS were carried out. The synthesized material showed an excellent photo-activity when combined with P3HT as a solar cell. The properties of VOC (310 mV) and ISC (0.002 mA) were clearly identified. The fill factor of the cell was 24% with a power of 0.0024 mW/cm2 and moderate efficiency of 0.006%. This demonstrated that P3HT/Mn:ZnS formed an active material for novel hybrid bulk heterojunction solar cell with cheap and readily available materials. Acknowledgments The authors thank University of Mysore (DV9/192/NON-NETFS/ 2013-14, dated: 03.09.2013) and University Grants Commission (D.O.No.F.87-1-2012(SU-1) 2 Dated: 06.06.2013), New Delhi for providing financial support for this research. For the Instrumentation facility INUP, facilities CeNSE, Indian Institute of Science, Bengaluru, Funded by Department of Electronics and Information Technology (DietY), Government of India. References [1] A.L. Donne, S.K. Jana, S. Banerjeen, et al., Optimized luminescence properties of Mn doped ZnS nanoparticles for photovoltaic applications, J. Appl. Phys. 113 (2013) 014903. [2] R. Sharma, D.P. Bisen, S.J. Dhoble, et al., Mechanoluminescence and thermoluminescence of Mn doped ZnS nanocrystals, J. Lumin. 131 (10) (2011) 2089. [3] C.Y. Huang, D.Y. Wang, C.H. Wang, et al., Efficient light harvesting by photon downconversion and light trapping in hybrid ZnS nanoparticles/Si nanotips solar cells, ACS Nano 4 (10) (2010) 5849. [4] N. Tsolekile, S. Parani, M.C. Matoetoe, et al., Evolution of ternary I–III–VI QDs: Synthesis, characterization and application, Nano-Struct. Nano-Objects 12 (2017) 46. [5] V.I. Klimov, S.A. Ivanov, J. Nanda, Single-exciton optical gain in semiconductor nanocrystals, Nat. Phys. 1 (2005) 189. [6] R.D. Schaller, M.A. Petruska, V.I. Klimov, Effect of electronic structure on carrier multiplication efficiency: Comparative study of PbSe and CdSe nanocrystals, Appl. Phys. Lett. 87 (2005) 253102.

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