ZnS heterostructure

Electrochimica Acta 203 (2016) 74–83

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Quantum dot sensitized solar cell based on TiO2/CdS/CdSe/ZnS heterostructure Sachin A. Pawara , Dipali S. Patilb , Hyo Rim Junga , Ju Young Parka , Sawanta S. Malic , Chang K. Hongc , Jae-Cheol Shinc , Pramod S. Patild , Jin-Hyeok Kima,* a Department of Materials Science and Engineering and Optoelectronics Converging Research Centre, Chonnam National University, 300 Yongbong-Dong, Bukgu, Gwangju 500 757, South Korea b Department of Physics, Yeungnam University, Gyeonbuk 712 749, South Korea c School of Applied Chemical Engineering, Chonnam National University, Gwangju 500 757, South Korea d Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India

A R T I C L E I N F O

Article history: Received 13 January 2016 Received in revised form 4 April 2016 Accepted 6 April 2016 Available online 7 April 2016 Keywords: Quantum dot sensitized solar cell (QDSSC) Successive ionic layer adsorption and reaction (SILAR) Chemical bath deposition (CBD) Cascade structure TEM Efficiency

A B S T R A C T

A cascade structure of TiO2/CdS/CdSe/ZnS quantum dot sensitized solar cell (QDSSC) is achieved to boost the photoconversion efficiency of TiO2/CdSe system by incorporating inner buffer layer of CdS. A sodium dodecyl sulfate (SDS) surfactant mediated TiO2 nanorods assembly is prepared by a simple hydrothermal technique. The formation of CdS, CdSe and ZnS thin film over TiO2 nanorods assembly as a photoanode is carried out by successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) techniques. Thus synthesized electrode materials are characterized by XRD, XPS, field emission scanning electron microscopy (FE-SEM), TEM, High resolution-TEM (HR-TEM), STEM-EDS mapping, Optical and solar cell performances. The results designate that the thin film of CdS and CdSe have efficiently covered exterior surfaces of TiO2 nanorods assembly. The interfacial structure of TiO2/CdS/CdSe is also investigated and the growth interface is verified. A cautious evaluation between CdSe and CdS/CdSe sensitized cells tells that CdS helps to enhance the further growth of CdSe in the cascade electrodes and improves light harvesting. Under AM 1.5G illumination, the photoanodes show an improved power conversion efficiency of 2.56%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 4.80 mA cm 2 which is 18 times higher than that of a bare TiO2 nanorods assembly. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Dye sensitized solar cells (DSSCs) can be regarded as the heavily researched solar cells since the milestone work by O’Regan and Graetzel [1]. In order to enhance light harvesting ability of DSSCs in the visible regime, several exertions have been made which were in the directions of developing high performance sensitizers [2–5]. Low band gap semiconductor nanocrystals also known as quantum dots (QDs) of CdS, CdSe, PbS, PbSe and InAs are treated as promising materials in next-generation quantum dot sensitized solar cells (QDSSCs) due to their unique properties such as tunable band gap depending upon the QD size, high extinction coefficient, large intrinsic dipole moment etc. which helps to charge separation in solar cells [6–11]. One more additional advantage

* Corresponding author. E-mail addresses: [email protected] (S.A. Pawar), [email protected] (J.-H. Kim). http://dx.doi.org/10.1016/j.electacta.2016.04.029 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

of QDs is a phenomena of multiple exciton generation (MEG) which is observed in QDs elevating the theoretical efficiency of QDSSCs ceiling up to 44%. This is higher than the current Shockley-Queisser detailed balance limit of 33% for thin film solar cells [12]. Therefore, in coming days there is a vast opportunity to fabricate an efficient and affordable thin film based solar cells using QDSSC. Aligned TiO2 nanostructures can offer remarkably high surface area, which is vital to produce high photocurrent. Literature survey of TiO2 reveals that increased current density in TiO2 nanorods is observed which is credited to better injection or charge collection efficiency of oriented rutile TiO2 nanorods. Further, rutile TiO2 has advantages of higher chemical stability, higher refractive index and cheaper cost. Rutile TiO2 is also preferred for optical applications. Anatase pigmented coating films fail more quickly on outdoor exposure by chalking than do rutile. Therefore rutile TiO2 is preferred in most exterior applications in order to minimize chalking. However, although rutile is less photoreactive than anatase, it is still sufficiently reactive to reduce exterior durability. Rutile TiO2 has dielectric constant 114 which is higher than anatase

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with 31. Again, density of rutile TiO2 (4.13 g/cm3) is higher than anatase (3.79 g/cm3). The octahedron in rutile exhibits a slight ohrthorhombic distortion, while the octahedron in anatase is largely distorted, leading to lower symmetry compared to that of orthorhombic [13]. In general, oriented 1D-nanostructures, are thought to be able to enhance the performance of solar cells since they provide quick charge separation and continuous confined pathways for charge carriers transport towards electrodes with increased mobility. These advantages are further complemented by the low cost template-free one pot facile hydrothermal synthesis, which make it very interesting to make use of these rutile TiO2 nanorods assembly in solar cells. However, because of its wide band gap (3.2 eV) TiO2 harvests merely UV-part of the solar spectrum which constitutes only 6% of total solar radiations resulting in lower photoconversion efficiency. Therefore, in order to make use of versatile ability of TiO2 like excellent electrochemical stability and the efficient transfer of the photogenerated electrons, it is essential to increase its ability to harness visible part of solar spectrum. This can be achieved by sensitizing TiO2 nanorods by low band gap semiconductor NPs such as QDs that will harness UV to visible portion of solar spectrum producing greater photoconversion efficiency. Chemical routes such as Chemical bath deposition (CBD) and Successive ionic layer adsorption and reaction (SILAR) are alternative methods to colloidal QD preparation. There are various reports on solar cells prepared using TiO2 electrodes sensitized with semiconductor QDs which were deposited using CBD and SILAR [14–17]. These methods could produce high surface exposure on TiO2 films and direct linking between QDs and TiO2. Recently, Kamat et al. reported Mn-doped CdSe-sensitized solar cells with an efficiency of 5.4% where CdSe QDs were deposited by SILAR technique [18]. The best cell efficiency obtained till the date is of 9.2% for PbS Colloidal Quantum Dot Solar Cells with Hierarchical Structuring of TiO2 [19]. It was observed that CdSe was not easy to deposit directly on oxides, like TiO2 and ZnO. Therefore, alterations of oxide with CdS have been usually accepted to improve the adsorption of CdSe [20,21]. In order to widen the optical absorption of the solar cells it is essential to form CdS/CdSe cosensitization through CdS. Recently many layered semiconductor-sensitizers containing CdSe QDs or thin films of them with CdS and ZnS layers have shown promising results in polysulfide electrolytes [22–24]. These many layered CdS/CdSe/ZnS sensitizers have displayed short-circuit currents similar to those of DSSCs and are therefore being well-thought-out as an extremely important model system in QDSSCs. In this study, a cascade structure of TiO2/CdS/CdSe/ZnS QDSSC is achieved to boost the photoconversion efficiency for TiO2/CdSe system. A sodiumdodecyl sulfate (SDS) surfactant mediated TiO2 nanorods assembly is prepared by a modest hydrothermal technique. The construction of CdS, CdSe and ZnS thin films sensitized TiO2 nanorods photoanode is carried out by SILAR (for CdS, ZnS) and CBD (for CdSe) techniques. The synthesized electrode materials are characterized by XRD, XPS, FE-SEM, TEM, HR-TEM, STEM-EDS elemental mapping, Optical and PEC performances. 2. Experimental Details 2.1. Thin film deposition 2.1.1. Preparation of rutile TiO2 nanorods Rutile TiO2 nanorods are synthesized following our previous report [25]. Briefly, 0.1 M SDS is dissolved in 10 mL distilled water and subsequently added to the TiO2 sol which was prepared using titanium butoxide (TBT). The resulting mixture is stirred for 30 min. The clear transparent solution is obtained. Thus obtained

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solution is used for autoclaving and preparing TiO2 thin films following the procedure as mentioned for pristine TiO2 in our previous report. The obtained TiO2 thin film is marked as ‘TiO2’. Likewise 0.1 M EDTA, PVP and CTAB are added separately to pristine TiO2 sol and the reactions are carried out as mentioned for pristine TiO2 sample. Every time the temperature of the autoclave is kept constant at 180  C and a deposition is carried out for 3 h and the thin films are air annealed after deposition at 450  C for 30 min. Amongst all TiO2 samples synthesized above SDS mediated TiO2 has shown better solar cell performance than other TiO2 samples hence is used further for fabrication of QDSSC.The thickness obtained for TiO2 samples was 7 mm and is observed in the crosssectional FE-SEM of TiO2/CdSe/ZnS sample shown in electronic supplementary material. 2.1.2. Preparation of CdS sensitized TiO2 photoanode SILAR technique is used to deposit CdS over rutile TiO2 nanorods. For the growth of CdS thin film, initially, the TiO2 film is dipped into a 0.1 M cadmium nitrate (Cd (NO3)2) methanol solution for 1 min. After this, it is rinsed in a beaker containing methanol. Sequentially, the film is dipped into a 0.1 M Na2S methanol/water (7:3/v: v) solution for additional 1 min to allow S2 to react with the preadsorbed Cd2+, in order to remove the loosely bound anions, the film is again rinsed using methanol, leading to the formation of CdS. This procedure is called one SILAR cycle. Overall, five cycles are employed to obtain a suitable amount of CdS on the TiO2 film. Thus obtained photoanode is marked as TiO2/CdS. 2.1.3. Preparation of CdSe sensitized TiO2/CdS photoanode For the growth of CdSe thin film, CBD technique was used and followed the literature procedure [26]. In brief, 0.2 M sodium selenosulphate (Na2SeSO3) aqueous, 0.2 M cadmium acetate Cd (CH2COO)2 aqueous solution, and 0.3 M trisodium salt of nitrilotriacetic acid (N(CH2COONa)3) solution were mixed together with a volume ratio of 1:1:1. Then the CdS-coated TiO2 film was vertically immersed into the solution for the deposition of a CdSe layer under dark conditions at 24  C for 8 h. 2.1.4. ZnS passivation layer After formation of CdSe layer, a ZnS passivation layer is placed by SILAR with 2 cycles using an aqueous solution containing 0.1 M Zn(NO3)2 and 0.1 M Na2S, which act as Zn2+ and S2 sources, respectively. The samples were rinsed using double distilled water in the rinsing steps of SILAR. Lastly, the photoanodes are air annealed at 200  C for 30 min in a muffle furnace. These photoanodes are marked as TiO2/CdS/CdSe/ ZnS. 2.2. Characterizations of electrodes Surface morphology was examined using FESEM (JEOL JSM6500F). Transmission electron microscopy (TEM) micrographs, selected area electron diffraction (SAED) pattern and highresolution transmission electron microscopy (HRTEM) images were obtained by a TECNAI F20 Philips operated at 200 KV. TEM sample was prepared by drop casting ethanolic dispersion of sample onto carbon-coated Cu grid. Elemental mapping images were acquired by energy-dispersive X-ray spectroscopy (EDS), using a Tecnai G2 F30 installed in the scanning transmission electron microscopy (STEM). The elemental information of the electrodes was analyzed using an XPS Thermo K-Alpha with multichannel detector, which has high photonic energies from 0.1 to 3 keV. The XRD spectra of the films were recorded using X-ray powder diffractometer (Bruker AXS Analytical Instruments Pvt. Ltd., Germany, Model: D2 phaser). The room temperature optical

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3.2. X-ray photoelectron spectroscopy

absorption measurements were performed in the wavelength range over 200–1100 nm by using a UV–vis spectrophotometer (UV1800, Shimadzu, Japan). Electrochemical impedance study of all solar cell devices was performed using the electrochemical workstation Autolab PGSTAT302N in the frequency range of 0.1 to 10000 Hz with AC amplitude of 100 mV. The J-V curves were recorded on the Solar Simulator (model CT-150 AAA, Photoemission Tech, USA) under Air Mass 1.5G solar irradiations. For photoelectrochemical study the optimized photoanode (average area 0.30 cm2) and platinized FTO were employed as the working and counter electrodes, respectively.

Quantitative analysis of the electronic structures and chemical properties of TiO2 and TiO2/CdS/CdSe/ZnS were performed by XPS. Fig. 2 (a) and (b) illustrates the XPS survey scan spectra for samples TiO2 and TiO2/CdS/CdSe/ZnS, respectively. Presence of peaks corresponding to Ti(2p), O(1s), Cd(3d), Se(3d), Zn(2p) and S (2p) are clearly seen and can be attributed to the presence of TiO2, CdS, CdSe and ZnS. The high resolution spectra of Ti(2p), O(1s), Cd (3d), Se(3d), Zn(2p) and S(2p) are discussed in supporting information. The corresponding binding energy values are tabulated in Table 1. Total, these results approve that the synthesized photoanodes consists of TiO2, CdS, CdSe and ZnS.

3. Results and discussion 3.1. X-ray diffraction

3.3. Field Emission Scanning Electron Microscopy XRD measurement was performed to characterize the phase and crystallinity of both pristine TiO2 nanorods assembly and CdS, CdSe and CdS/CdSe/ZnS-decorated TiO2 electrodes. Fig. 1 shows the XRD patterns of samples TiO2, TiO2/CdS, TiO2/CdSe, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS electrodes deposited on FTO-coated glass substrates. Hydrothermal-grown TiO2 nanorods assembly have a rutile structure (JCPDS data 82-0514) [27] and the deposited CdS NPs show an alike hexagonal wurtzite structure (JCPDS data 41-1049), which is in agreement with the previous report [28,29]. The peaks corresponding to 2u values 26.5 and 54.5 in electrodes TiO2/CdS, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS are indexed to (002) and (112) crystal planes of hexagonal CdS. Similarly the peaks corresponding to 2u values 26.91, 52.15 and 63.06 in electrodes TiO2/CdSe, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ ZnS are indexed to (101), (004) and (211) crystal planes of hexagonal CdSe, respectively (JCPDS data 77-2307) [30,31]. This result affirms the successful CdS, CdSe and CdS/CdSe layer formation on the TiO2 electrode. It is notable that the XRD patterns show no peaks conforming to CdO or SeO2, which shows the thermal stability of CdSe nanocrystals and the absence of impurity in our sample [32]. No peak corresponding to ZnS was detected it is due to amorphous nature of ZnS in our case. In each samples the peaks denoted by ‘F’belongs to FTO substrate (JCPDS data 41-1445).

Fig. 3 (a) shows FE-SEM images of SDS mediated TiO2 electrode at X 25000 and TiO2/CdS/CdSe/ZnS electrodes at different magnifications. Compared with an image of TiO2 electrode (Fig. 3 (a)), a sum of CdS and CdSe thin film is observed to adhere on the surface of the TiO2 nanorods. The TiO2 nanorods were fully coated by uniform thin film of CdS, CdSe and ZnS in a vertical direction. After depositing 5 SILAR cycles of CdS and CdSe for 8 h of CBD, the ordered TiO2 nanorods assembly structure is retained and the TiO2/CdS/CdSe electrodes with uneven surfaces are witnessed as shown in Fig. 3 (a), which discloses that thin films of CdS and CdSe have enclosed the entire surfaces of TiO2 nanorods. The FE-SEM result confirms that CdS and CdSe thin film formation on the surface of the TiO2 nanorods via SILAR and CBD techniques. The different growth stages involved in TiO2/CdS/CdSe/ ZnS photoanode formation are summarized in Fig. 3 (b). 3.4. High Resolution Transmission Electron Microscopy The detailed interfacial behaviour of the TiO2/CdS/CdSe/ZnS electrode was analysed by using TEM in Fig. 4 (image (A)) and HRTEM (images (B), (C)). The typical TEM image of a sample TiO2/CdS/ CdSe/ZnS electrode is shown in Fig. 4.

CdSe

Intensity (A. U.)

F

CdS

TiO 2

(212)

F

TiO2/CdS/CdSe/ZnS

(301)

2

(211) (112)

TiO

TiO 2

(002)

CdSe F

(004) (112)

CdSe

(101) (002) (110)

CdS

20

(211)

(004)

TiO2/CdSe

TiO2/CdS

30

40

50

60

70

(212)

(301)

F

(112)

F

(002)

F

(220)

(112) F

(210)

TiO 2

(110)

CdS

(002)

CdSe

(101)

TiO2/CdS/CdSe

80

TiO2

90

2 θ (Degree) Fig. 1. X-ray Diffraction patterns of samples TiO2, TiO2/CdS, TiO2/CdSe, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS. ‘F’ denotes peaks belonging to substrate FTO.

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a

Ti2P

Ti3P O2s

Ti3s

C1s

Ti2s

O KLL

Ti LMM

Intensity (A.U.)

O1s

TiO2

1000

800

600

400

200

0

Binding Energy (eV)

Cd3d3/2 Cd3d5/2

1000

800

600

400

200

Zn3s Zn3P3/2 Se3d Cd4d

S2s S2p3/2

C1s

S2p1/2

Ti2P

O1s

Cd3p3/2

Zn2P1/2 Zn2P3/2

TiO2/CdS/CdSe/ZnS

Intensity (A.U.)

b

77

corresponding to the hexagonal plane (100) of CdS (JCPDS data 41-1049). Then CdSe with lattice spacing 0.255 nm corresponding to hexagonal plane (102) (JCPDS data 77-2307) is also observed. Further, HR-TEM analysis reveals that there occurs three leading interfaces: TiO2/CdS, TiO2/CdSe and TiO2/CdS/CdSe, between which the interfacial structures of the TiO2/CdS/CdSe/ZnS are recognized by HR-TEM and are as shown in Fig. 4 (images (B) and (C)). Fig. 4 (D) is a Selected area electron diffraction (SAED) pattern. SAED on the TiO2/CdS/CdSe/ZnS thin film shows the apparent ring diffraction patterns from the crystallinity of the wurtzite CdS and CdSe nanocrystals, as well as the spot diffraction patterns from the rutile TiO2 nanorods corresponding to (0-11) and (011) planes. The HR-TEM of a TiO2 nanorod with well-defined atomic arrangement along the zone axis [100] is seen in Fig. 4 (B). A fast fourier transformed (FFT) image of this nanorod gives a clear view of tetragonal crystal structure with rutile phase consisting of (0-11) and (011) refelections seen along [100] zone axis. In order to further approve the CdS/CdSe/ZnS enclose outer surface of TiO2 nanorods STEM images and corresponding STEMEDS elemental mapping of the sample (TiO2/CdS/CdSe/ZnS) is carried out. Fig. 5 shows the distribution of Cd, S, and Se elements over the whole TiO2 nanorods. These results confirm that CdS, CdSe, and ZnS nanocrystals have been successfully deposited on the surface of the TiO2 nanorods. It is difficult to determine exact thickness of each semiconductor layer over TiO2 nanorods as one can see each semiconductor layer thickness is in the order of few nm (in HR-TEM image (Fig. 4 (A))). Further efforts to determine the layer thickness of cascade structure is underway with respect to variation in SILAR cycles. 3.5. Optical absorbance

0

Binding Energy (eV) Fig. 2. (a) XPS Survey scan spectrum of sample TiO2. (b) XPS Survey scan spectrum of sampleTiO2/CdS/CdSe/ZnS.

TiO2 nanorods covered with CdS/CdSe/ZnS thin films can be seen in the TEM image of Fig. 4 (image (A)). The measured lattice spacings in Fig. 4 (images (B), and (C)) are in accordance with the d-spacings of TiO2, CdS and CdSe. We can see that the rutile TiO2 nanorods assembly has been covered with CdS, CdSe and ZnS thin films. HR-TEM images of TiO2/CdS/CdSe/ZnS heterojunction region have shown the good crystallinity of TiO2, CdS, CdSe but ZnS is amorphous hence there is absence of crystal planes belonging to the same in HR-TEM. The rutile plane of TiO2 is confirmed by measuring the lattice spacing of crystalline plane (011), which was found to be 0.249 nm (JCPDS data 21-1276). When we compared the lattice parameters and the JCPDS data we found that the crystallites connecting to TiO2 have lattice spacing of 0.360 nm,

The variation in optical spectra of the electrodes TiO2, TiO2/CdS, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS are shown in Fig. 6, from which improved absorption of visible light by TiO2/CdS/CdSe/ZnS heterostructure can be affirmed. The absorption edge of the TiO2 nanorods arises at about 400 nm and with little absorbance for visible light can be seen because of its large energy gap (3.2 eV). However, the optical spectrum of TiO2/CdS/CdSe/ZnS, exhibit wider bands appearing from 400 to 610 nm. This shows that the thin film deposition of CdS/CdSe has prominently stretched the photoresponse of TiO2 electrode into the visible light region. The improved ability to absorb visible light brings this type of heterostructure encouraging applications in photovoltaic devices. 3.6. QDSSC Device fabrication Fig. 7 (a) shows the final QDSSC device comprising TiO2/CdS/ CdSe/ZnS photoanode. The QDSSC device was formed using TiO2/ CdS/CdSe/ZnS as a photoanode and platinum (Pt)-coated FTO glass as cathode. A semitransparent Pt-FTO counter electrode is prepared by drop casting 0.5 mM chloroplatinic acid (H2PtCl6)/ isopropanol solution on FTO glass and later firing it at 500  C for 30 min. The TiO2/CdS/CdSe/ZnS (anode) and the Pt-FTO (cathode)

Table 1 Binding energy values of samples TiO2, CdSe, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS. Sample code

Binding energy (eV) Ti 2p

TiO2 CdSe TiO2/CdSe/ZnS TiO2/CdS/CdSe/ZnS

O 1s

Ti 2p3/2

Ti 2p1/2

457.66

463.38

528.66

458.44 458.57

464.32 463.60

530.91 531.34

Cd 3d

Se 3d

S2p

Cd 3d5/2

Cd 3d3/2

Se 3d5/2

Se 3d3/2

– 405.23 405.14 404.69

– 411.98 411.58 411.45

– 54.18 53.68 53.52

– 55.99 54.39 54.25

– – 161.05 161.45

Zn 2p Zn 2p3/2

Zn 2p1/2

– – 1021.12 1022.49

– – 1044.00 1045.84

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Fig. 3. (a) FE-SEM images of samples TiO2 and TiO2/CdS/CdSe/ZnS. (b) Stepwise cascade structure formation of optimized TiO2/CdS/CdSe/ZnS photoanode for QDSSC.

is sandwiched by applying a 25 mm thick non-tearing paper as the spacer. An electrolyte containing a mixture of 1 M NaOH, 1 M Sulphur and 1 M Na2S in distilled water (polysulfide) was injected between two electrodes driven by capillary force through holes on the non-tearing paper.

3.7. J-V measurements This work deals with the effect of CdS/CdSe/ZnS co-sensitization on TiO2 nanorods and it is found that the photoanode prepared by using 5 SILAR cycles of CdS plus 8 h CBD of CdSe with 2 SILAR

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Fig. 4. TEM (A) and HR-TEM (B) and (C) images of sample TiO2/CdS/CdSe/ZnS. Image (D) shows the corresponding SAED pattern.

cycles of ZnS as passivating agent (TiO2/CdS/CdSe/ZnS) displays the best performance. From the J–V curves (Fig. 7 (b)), photovoltaic parameters are estimated and are presented in Table 2. TiO2 has Jsc = 0.378 mA cm 2 with Voc = 615 mV while it increased for device TiO2/CdS with Jsc = 1.59 mA cm 2 with Voc = 487 mV. Device TiO2/CdSe posses Jsc = 2.10 mA cm 2 with Voc = 378 mV. While device TiO2/CdS/CdSe has Jsc = 3.18 mA cm 2 with Voc = 557 mV. Amongst all, TiO2/CdS/CdSe/ZnS exhibits highest solar cell parameters with Jsc = 4.80 mA cm 2, Voc = 573 mV, FF = 0.28 and h = 2.56%. The increase in the photocurrent from devices TiO2 to TiO2/CdS/ CdSe/ZnS can be attributed to its wider optical absorbance of the samples. As we move from TiO2 to TiO2/CdS/CdSe/ZnS there is an enhanced absorption in the photon of light is seen. Again, for materials like CdS and CdSe they posses very high absorption coefficient between 105 to 106 cm 1, ultimately the light is strongly absorbed in these materials. This has resulted into maximum photons of light utilised for the energy conversion leading to enhanced photocurrent. When we look at the solar cell parameters of devices TiO2 to TiO2/CdS/CdSe/ZnS we found that there is decrease in FF values from 41 to 28% except for TiO2/CdSe device it has increased to 42%. The current–voltage characteristics are largely dependent on the series (RS) and shunt (RSh) resistance. A lower RS means that higher current will flow through the device and high RSh corresponds to fewer shorts or leaks in the device. The ideal cell would have RS approaching zero and RSh approaching infinity. The reason for decrease in FF in our devices is due to poorer values of series and shunt resitances which lead to reduced values of FF. The decrease

in open ciruit voltage is due to following reasons-the intercolumnar space of TiO2 nanorods may become smaller with excessive CdS/CdSe and may thus cause an unfavorable clogging among TiO2 and CdS/CdSe thin films [33], which would reduce the diffusion of Sx2 /S2 ions into the TiO2 film; this reduced diffusion results in reduced Voc. It may be argued that higher quantity of CdS/CdSe may be associated with higher light harvesting capacity; it is at the same time true that the upper layer of a multilayered CdS/CdSe may not be in direct contact with TiO2 film and its electron injection rate to the TiO2 nanorods may be retarded; thus this can be another reason for the reduced Voc, because in this situation of CdS/CdSe there would be recombination of injected electrons with holes in the TiO2 nanorods or with the Sx2 ions of the electrolyte [34]. When light is fallen on photoanode, excitons were generated by CdSe NPs and charge separation occurred at the TiO2/CdSe interface which is shown in energy level diagram of TiO2/CdSe QDSSC in Fig. 7 (c). The subsequent electrons were quickly traveled to the FTO layer through the 1-D TiO2 nanorods and the holes were recollected by the polysulfide electrolyte. A thin ZnS passivation layer (2 cycles) was coated onto the sensitized electrode by SILAR technique to hamper the backward electron flow from sensitizers to the oxidized species in sulfide/polysulfide electrolyte [35–37]. The optimized SILAR cycles we found for TiO2/CdS was 5 for which the solar cell parameters obtained were highest i.e., it shown 1.59 mA cm 2 current density and 487 mV photovoltage with power conversion efficiency 0.77%. So we kept CdS underlayer of 5 SILAR cycles constant while preparing the device structure TiO2/CdS/CdSe/ZnS. CdSe-sensitized TiO2 nanorods electrodes is

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Fig. 5. STEM image and STEM-EDS elemental mapping of the sample TiO2/CdS/CdSe/ZnS depicting presence of each Ti, O, Cd, S, Se and Zn elements.

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1) The 1-D TiO2 nanorods have the ability of efficient charge separation and transport properties along with excellent light harvesting output. 2) When we compare absorbance ability of bare TiO2 nanorods assembly it is observed that TiO2/CdS/CdSe/ZnS photoanode have enhanced absorption in the visible region, this leads to increased solar energy consumption rate. 3) An efficient charge transfer channel is created due to a stepwise band-edge structure of TiO2/CdS/CdSe/ZnS which creates a higher resistance to backward flow of excited electrons to the electrolyte. This has resulted into a greater performance of multilayered photoanode like this. 4) The stepwise band-edge structure formed due to fine heterojunction between TiO2 and CdS helps CdS to collect electrons from CdSe to TiO2.

TiO2 TiO2/CdS TiO2/CdS/CdSe

Absorbance (A. U.)

TiO2/CdS/CdSe/ZnS

400

450

500

550

600

650

81

700

Wavelength (nm) Fig. 6. Optical absorbance spectra of samples TiO2, TiO2/CdS, TiO2/CdSe, TiO2/CdS/ CdSe and TiO2/CdS/CdSe/ZnS.

Finally, CdSe has wider solar energy absorption rate than CdS and additionally the interface formed due to TiO2/CdSe will increase further light absorption rate leading to increased photocurrent. Due to Fermi level realignment as shown in Fig. 6 (b) the upward shift of the band edges in CdSe helps the charge separation. 3.8. Electrochemical Impedance Spectroscopy

prepared by 8 h optimized CBD time. So we kept first CdSe deposition time of 8 h by CBD when we made-up TiO2/CdS/CdSe/ ZnS photoelectrodes and the corresponding photocurrent voltage (J-V) curves are shown in Fig. 7 (a). Hence, finally CdSe sensitized TiO2/CdS solar cells confirmed a better performance (2.56%) than the bare TiO2 (0.30%). There are several reasons to explain the increase in the solar cell parameters for device TiO2/CdS/CdSe/ZnS than bare TiO2.

EIS measurements of all solar cell devices was conducted in the dark using 0.1 M polysulfide electrolyte with applied forward bias voltage 0.5 V. The measuring AC frequency range was 0.1 Hz to 10000 Hz. Fig. 8 (a) shows the Nyquist plots of devices TiO2/CdS, TiO2/CdS/CdSe and inset shows TiO2/CdS/CdSe/ZnS (Nyquist plots of TiO2 electrodes is shown in supporting information). An equivalent circuit as shown in Fig. 8 (b) is proposed to fit the spectra. These plots are well fitted into two semi arcs using the Nova 9.1 software in terms of a suitable equivalent circuit

Fig. 7. (a) J-V curves of devices TiO2, TiO2/CdS, TiO2/CdSe, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS. (b) Energy levels of TiO2, CdS, CdSe and ZnS during bulk [1] and nano [39] regions. (c) QDSSC device architecture comprising TiO2/CdS/CdSe/ZnS photoanode.

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Table 2 Solar cell parameters for samples TiO2, TiO2/CdS, TiO2/CdSe, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS. Sample Code

Jsc (mA cm

TiO2 TiO2/CdS TiO2/CdSe TiO2/CdS/CdSe TiO2/CdS/CdSe/ZnS

0.378 1.59 2.10 3.18 4.80

2

)

Voc (mV)

Jmax (mA cm

615 487 378 557 573

0.265 0.94 1.19 0.164 0.262

2

)

Vmax (mV)

Rs (V)

Rsh (V)

FF (%)

h (%)

356 246 287 283 295

1012 1274 793 736 597

5249 28217 54003 23729 47624

41 30 42 26 28

0.30 0.77 1.11 1.53 2.56

Fig. 8. (a) Nyquist plots of devices TiO2/CdS, TiO2/CdS/CdSe and inset shows TiO2/CdS/CdSe/ZnS. (b) Equivalent circuit used to analyze EIS parameters. (c) Bode plots of devices TiO2/CdS, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS.

Table 3 Various EIS parameters obtained for the different electrode systems. Sample Code

Rs (V cm

TiO2 TiO2/CdS TiO2/CdS/CdSe TiO2/CdS/CdSe/ZnS

46.1 48.9 46 37.6

2

)

R1 (V cm

2

)

985 894 541 396

composed of constant phase elements (Q) and resistors (R) (Fig. 8 (b)). The first semi arc was attributed to Pt counter electrode/ electrolyte interface in high-frequency (HF) region and second semi arc to electron transfer at the TiO2/CdS/CdSe-electrolyte interface in low frequency (LF) region. The Rs, R1 and R2 in equivalent circuit are corresponding to the sheet resistance of the substrate, charge transfer resistance of the Pt counter and recombination resistance of the photogenerated electrons at the TiO2/CdS/CdSe- electrolyte interface [38]. Calculated resistance values from the fitted EIS spectra of QDSSCs are shown in Table 3. The device TiO2/CdS/CdSe/ZnS has R1 = 396 V cm 2 and R2 = 15432 V cm 2 which is much lower than TiO2, TiO2/CdS and TiO2/CdS/CdSe devices. A likely reason for this phenomenon is that added electrons were inserted into the conduction band of TiO2 nanorods i.e., the electrons from CdS and that of CdSe are being driven towards TiO2. This indicates that the device TiO2/CdS/CdSe/ZnS is more conducting than devices TiO2, TiO2/CdS and TiO2/CdS/CdSe which thereby produces the highest values of the photocurrent and photovoltage upon illumination amongst all the devices studied in this work. The electron lifetime of devices TiO2/CdS, TiO2/CdS/CdSe and TiO2/CdS/CdSe/ZnS is calculated using Bode plot which is obtained from EIS data. Following expression was used to calculate electron lifetime:

Q1 (mF)

R2 (V cm

0.103 5.59 38.98 58.13

454 48057 39808 15432

2

)

Q2 (mF)

te (ms)

349.8 21.9 211.9 349.87

0.102 5 21 23

Electron lifetime (te): te = 1/(2Bf), where f is the maximum frequencies from Bode plot. The electron lifetime is found to increase from 0.102 ms to 23 ms for devices TiO2 toTiO2/CdS/CdSe/ ZnS. From EIS, it is explicit that the charge recombination rate is the rate determining step of device performance, since decreased charge recombination rate from devices TiO2 to TiO2/CdS/CdSe/ZnS caused in an enhancement of the device performance, from TiO2 to TiO2/CdS/CdSe/ZnS. Therefore, we can conclude that charge recombination of TiO2/CdS/CdSe/ZnS is slower than that of other devices (TiO2, TiO2/CdS and TiO2/CdS/CdSe) leading to maximum photovoltaic parameters. 4. Conclusions The cascade structure of TiO2/CdS/CdSe/ZnS QDSSC has been successfully achieved through cost effective chemical routes such as hydrothermal, SILAR and CBD. The photoanodes have been characterized using different techniques and their PEC properties were examined judiciously. It is revealed that the CdS/CdSe/ZnS thin films enclosed outer surfaces of TiO2 nanorods assembly capably. An efficient charge transfer channel created due to a stepwise band-edge structure of TiO2/CdS/CdSe/ZnS forms a higher resistance to backward flow of excited electrons to the electrolyte

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at the same time interface formed due to TiO2/CdSe will increase further light absorption rate leading to increased photocurrent. PEC study of TiO2 decorated with CdS/CdSe thin film samples yielded an increase in the power conversion efficiency for TiO2, TiO2/CdS and TiO2/CdS/CdSe and produced highest current density of 4.80 mA cm 2 and the power conversion efficiency 2.56% under 100 mW cm 2 illuminations for TiO2/CdS/CdSe/ZnS heterojunction.

[20]

Acknowledgment

[21]

This work is supported by the Human Resources Development program (No. 20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

[22]

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