Optimizing the Photovoltaic Properties of CdTe Quantum Dot

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Optimizing the Photovoltaic Properties of CdTe Quantum Dot-Porphyrin Nanocomposites - A Theoretical Study Biplab Rajbanshi, and Pranab Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04662 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 3, 2016

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The Journal of Physical Chemistry

Optimizing the Photovoltaic Properties of CdTe Quantum Dot-Porphyrin Nanocomposites - A Theoretical Study Biplab Rajbanshi and Pranab Sarkar∗ Department of Chemistry, Visva-Bharati University, Santiniketan- 731235, India E-mail: [email protected]

Abstract By using the self-consistent charge density functional tight binding method, we explore the photovoltaic properties of CdTe quantum dot (CdTeQD)-porphyrin nanocomposites. It is well known that for dye sensitized solar cell (DSSCs) applications, the composite system should have a type-II band alignment that hinders the recombination of charge carriers, thereby improving the photovoltaic performance. The emphasis of our tight-binding calculations is to find the suitable CdTeQD-porphyrin nanocomposites that can offer better performance for solar energy conversion. By analyzing the electronic energy levels of the composite systems we have shown that the axially coordinated Zn-tetraphenylporphyrin (ZnTPP) functionalized CdTe quantum dot (ZnTPP-CdTeQD) nanocomposites are really promising material for application in DSSCs, as they produce type-II band alignment for all the composites irrespective of the size of CdTeQDs. The TDDFT calculations demonstrate that the size of CdTeQD plays a crucial role in improving the charge transfer properties from the ZnTPP to the QD. We also ∗ To

whom correspondence should be addressed

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tried to give a qualitative prediction about the performance of ZnTPP-CdTeQD nanocomposites in light harvesting device using some empirical relations coupled with TDDFT calculations. We suggest that the ZnTPP-CdTeQD nanocomposites with moderate size of QD will show high performance for light harvesting rather than too big or too small one.

1. INTRODUCTION Over the past couple of decades Dye-sensitized solar cells (DSSCs) have witnessed extensive development as a promising clean and renewable energy source. Due to the potential advantages of low cost, easy production, structural flexibility and transparency, DSSCs have attracted great attention in scientific research for practical applications as compared to the conventional crystalline silicon-based solar cells. 1–5 It is now well established that the nanohybrid materials are particularly suitable for use in designing solar cell, because of the capability of efficient photo electron transfer from one material to other, as compared to the individual components, they are made of. To this end, the interaction of luminescent organic photosensitizers with the II-VI metal chalcogenide quantum dots (QDs) can lead to the construction of new light-harvesting photochemical and photoelectronic devices such as DSSCs. The inorganic-organic nanohybrids composed of inorganic semiconductor quantum dots and organic components such as carbon nanotube, fullerene, graphene have been studied extensively from both experimental 6–16 and theoretical 17–22 point of view, which confirmed the greater photovoltaic performance of hybrid systems as compared to individual semiconductor materials. Among different semiconductor quantum dots, cadmium chalcogenides semiconductor nanocrystals have received great attention to design the next generation solar cells due to their unique physical properties such as large extinction coefficient, multiple exciton generation, and size-dependent tunability of the optical and electronic properties. 23–27 In the recent past, thiol-capped CdTe quantum dot (CdTeQD) is the subject of extensive study because of its diverse application from biological labelling to third-generation solar cells. This particular material can serve as an ideal model system for building various functional nanostructures and devices. 28–33 Porphyrins and their 2 ACS Paragon Plus Environment

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derivatives, on the other hand, have been investigated as efficient sensitizers for DSSCs due to their vital roles in photosynthesis, strong absorption in the visible region, and the ease of adjusting their chemical structures for light harvest. 34–38 These tetrapyrrole macrocycles, in their free-base form or with a redox inactive metal such as zinc inside the macrocycle cavity, reveal strong light absorption with a Soret band in the 400-450 nm range and visible bands in the 500-650 nm range, 39 which enforces their use in the fabrication of DSSCs. However, due to the tendency to form dye aggregates, which occurs because of nearly planar structure, porphyrin alone suffers from poor device performance. 40 The problem can be solved by implementation of cosensitization with other dye or light absorbing materials as implemented in DSSCs. Since the pioneering work of Officer and co-workers in 2007, reporting a power conversion efficiency (PCE) of 7.1%, 41 a number of porphyrin-based DSSCs have been reported. 42–48 One of the major focus of the study of such systems is to understand the interfacial charge transfer dynamics between the two components of the nonocomposite and their role in controlling the overall performance of the system. The choice of a porphyrin sensitizing agent is advantageous as the UV/Vis spectra of porphyrins can be tuned by appropriate choice of the central metal and are generally quite sensitive to binding interactions. 49,50 So, nanohybrids composed of porphyrin and CdTeQDs, afford the possibility for the construction of efficient light-harvesting devices. Using Rehm-Weller calculation and lifetime measurements, Jhonsi et al. 51 have shown that the positively charged porphyrin involves electron transfer from thioglycolic acid (TGA) capped CdTeQDs to porphyrin, whereas negatively charged porphyrins were involved in the energy transfer mechanism. Recent work by Amelia and Credi demonstrates the energy transfer from a zinc phenylporphyrin non-covalently bound to 5.6 nm CdTeQD in chloroform and is evident by the presence of the Soret band in the excitation spectrum of CdTeQD. 52 Keane et al. 53 have reported that the donor-acceptor composite of mesotetrakis(4-N-methylpyridyl) zinc porphyrin (ZnTMPyP4) and TGA coated CdTeQD, exhibits very rapid photoinduced electron transfer. Very recently Aly et al. 54 have reported the experimental observations of controlled on/off ultra-fast electron transfer (ET) at cationic porphyrin-CdTeQD interfaces using femto- and nanosecond broadband transient absorption (TA) spectroscopy. In an-



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other very recent work Ahmed et al. 55 have explored the excited state deactivation processes of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra( p-toluene-sulfonate) (TMPyP) upon the addition of water-soluble TGA-capped CdTeQDs. They have demonstrated that the intersystem crossing (ISC) and the triplet state lifetime of porphyrin can be dramatically tuned by changing the size of the QDs in the nanoassembly. In spite of the fact that the porphyrin-CdTeQD nanocomposites have drawm great attention to the experimentalists as a potential material for solar energy conversion, theoretical investigation concerning its photovoltaic performance is rather limited. However, the photovoltaic performance is essentially depends on electronic energy levels of the nanocomposites. So for optimizing the photovoltaic performance, the calculation of electronic structure of these nanocomposites is very crucial. In view of that we herein perform the electronic structure calculation of porphyrinCdTeQD nanocomposites as a function of size of the CdTeQDs, with the goal to optimize the photovoltaic properties. We have also tried to draw a qualitative prediction to achieve the maximum photoelectric conversion efficiency (PCE) by varying the QD size. In section 2 of this article, we briefly outlined the computational method used in this work. The results of our calculation on tetraphenylporphyrin (TPP) functionalized CdTe quantum dot (TPP-CdTeQD) and Zn-tetraphenylporphyrin functionalized CdTe quantum dot (ZnTPP-CdTeQD) nanocomposites are discussed in section 3. Finally, conclusions are drawn in Section 4.

2. DETAILS OF COMPUTATION To study the electronic structure of TPP-CdTeQDs and ZnTPP-CdTeQDs, we have employed the self-consistent charge density-functional tight-binding (SCC-DFTB) method. This method has been described in detail elsewhere. 56–59 In one of our previous study 60 we have established the accuracy of our SCC-DFTB parameter set for Cd-chalcogenides and their interaction with several different systems. For the details of the parametrization the readers are referred to our earlier paper. 60


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The porphyrin-Cdn Ten QD nanocomposites are modeled by four different thiol-capped zincblende QDs (n = 17, 34, 65, 98). The radius of the QDs are 0.91, 1.08, 1.28, 1.45 nm, respectively. One Cd atom of the QDs is functionalized with a long chain thiol molecule, 3-aminopropane-1¨ 2 ). Now using the concept of bio-conjugation, we attached the functionthiol ( –S–(CH2 )3 – NH alized CdTeQDs to the -COOH group at the para− position of one phenyl ring of the porphyrin molecule (TPP) through amide linkage. The geometry optimizations have been performed with the conjugated gradient algorithm, until all forces became smaller than 0.0001 eV/Å. We employed the Slater type orbitals (STOs) as basis sets and Perdew-Burke-Ernzerhof (PBE) 61 exchange correlation energy functional in this study. All the electronic structure calculations have been performed with the DFTB+ program package. 62 The SCC-DFTB optimized geometry of ZnTPP-CdTeQD nanocomposites are used for the calculation of excitation energies. The calculations are performed by time-dependent density functional theory (TDDFT) method as implemented in the Gaussian 09 software package. 63 using B3LYP with LANL2DZ/6-31G(d) mixed basis set. The LANL2DZ basis set is applied to heavy Cd and Se atoms and the 6-31G(d) all-electron basis set is assigned to the rest of the atoms. 20,64–67 The vertical excitation energies for the 40 lowest spin-allowed singlet transitions are investigated using TDDFT on the B3LYP hybrid functional.

3. RESULTS AND DISCUSSION Although there are other factors, the efficiency of a photovoltaic cell depends mainly on two factors; one is the efficient electron transfer between the components and the other is spatial separation of the charge carriers that lowers the electron-hole recombination rate. Conventionally, the type-II band alignment between the components is a crucial phenomenon for solar energy conversion, as it lowers the chance of electron-hole recombination through the spatial separation of the charge carriers. The emphasis of our present electronic structure calculation will be on the qualitative prediction of the way in which porphyrin functionalized CdTe QDs show greater electron transfer rate



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and lesser electron-hole recombination rate, so that they become an eligible candidate for solar energy conversion. In the present work, to achieve the desired spatial charge separation for porphyrin functionalized CdTe quantum dots, we have considered three different strategies; the first one is the covalent linking of pristine TPP with CdTeQDs of different size, next is the covalent linking of para− substituted TPP with CdTeQDs, and finally, introduction of a Zn atom into the macrocycle cavity of TPP, or in other words, ZnTPP functionalized CdTeQDs with different modes of attachment. In the following sections, we will unravel the effects of functionalization, by porphyrin, on the electronic properties of the CdTe QDs.

3.1. TPP-CdTeQD nanocomposites The optimized structure of one representative of TPP-CdTeQD nanocomposites (TPP-Cd65 Te65 QD) is shown in Figure 1a. For detail understanding of electronic energy levels of the TPP-CdTeQDs, we have shown in Figure 1(b-d) the density of states (DOS) plot for three CdTeQDs of different sizes. The projected density of states (PDOS) of individual QD and TPP are also shown in the same figure. From the figure it can be seen that, the band gap region of the nanocomposites for very small sized QD (Cd17 Te17 QD, 0.91 nm of radius) is dominated by porphyrin, no states from QD contribute in between them. Thus, these composites show type-I band alignment. On the other hand, when size of the QD is relatively large (Cd98 Te98 QD), LUMO of QDs appears below that of porphyrin indicating type-II band alignment. That is also reflected from the spatial charge separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as shown in Figure 2. So, one way to achieve the intended type-II band alignment is the variation of size of the CdTeQDs. It is here worth mentioning that, achieving type-II band alignment by changing the size of QD is not a good strategy. This is so because in our previous studies 17,21 we have shown that with increasing the size of CdTeQD, the HOMO of QDs shifted to the higher energy, whereas the LUMO shifts towards lower energy. Consequently, beyond Cd98 Te98 QD we again get type-I band alignment for the composites where the band gap region is dominated by QD. 6 ACS Paragon Plus Environment

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Now the question is, whether is it possible or not to attain the spatial charge separation by some chemical modification to porphyrin. As the passivation with organic functionalizing groups can tune the electronic structure of nanoparticles, we set our second strategy through substitution at para− position of phenyl ring of the porphyrin molecule by electron donating (–NH2 and –CH3 ) groups and electron withdrawing groups (–NO2 and –COOH) to track the changes in the electronic structure of the TPP-CdTeQD nanocomposites. The transition from type-I to type-II band alignment of TPP-Cd17 Te17 QD nanocomposites with the substitution, can easily be understood from the calculated DOS, PDOS of the corresponding porphyrin and Cd17 Te17 QD and the corresponding HOMO and LUMO charge densities, as shown in Figure 3. From the figure it is evident that, the substitution with electron donating group (–NH2 in Figure 3b) rises the LUMO energy of porphyrin above to that of Cd17 Te17 QD. Thus, the para− substitution of porphyrin with strong electron donating groups results in spatial charge separation, where the HOMO charge density is located on porphyrin and the LUMO charge density is on CdTeQD. On the other hand, the substitution with electron withdrawing group (–NO2 in Figure 3d) causes the downwards shifting of HOMO of porphyrin to such an extent that it appears deeper than the HOMO of QD. So, the para− substitution of porphyrin with strong electron withdrawing groups also yield spatial charge separation, where the HOMO charge density is located on CdTeQD and the LUMO charge density is on porphyrin. Such a band alignment gives rise to a type-II system where QD acts as sensitizer to the porphyrin, which is in good agreement with the previous experimental observation. 51,68 However, for the substitution with moderate or weak electron donating groups (–CH3 (Figure 3a)/–COOH (Figure 3c)), both the HOMO and LUMO of TPP-Cd17 Te17 QD nanocomposites are controlled by porphyrin molecule. Hence, we can predict that either very strong donor or very strong acceptor substituted porphyrins are suitable for DSSC applications of TPP-CdTeQD composites with very small size QD. It is here worth to mention that, TPP-CdTe QDs of larger size remain type-II even after substitution with mild to strong electron withdrawing/donating groups at para position of TPP. So, we suggest that TPP-CdTe QDs of any size with proper functionalization will show spatial charge separation and consequently the chance of electron-hole recombination rate will be



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much slower. So these nanohybrids may find application in solar cell designing.

3.2. ZnTPP-CdTeQD nanocomposites All the discussions we have made so far, are about the nanocomposites of CdTeQD with pristine TPP and its derivatives. Now, at this stage we are interested to know the electronic structure of the nanocomposites where Zn porphyrin (ZnTPP) is used instead of TPP. So, there are two possibilities for the formation of ZnTPP-CdTeQD nanocomposites; first one is the amide linkage as before and the another one is the nanocomposites where CdTeQD uses its long chain passivating tail (–S– ¨ 2 ) for axial coordination to the Zn atom of ZnTPP. The optimized structures of one (CH2 )3 – NH representative for each possible nanocomposites are shown in the Figure 4. We have calculated the binding energy (Eb ) as,

Eb = [ECdTeQD + EZnT PP − nEH2 O ] − Ecomposite

(1)

where, Ecomposite is total energy of the ZnTPP-CdTeQD nanocomposites, ECdTeQD , EZnT PP and EH2 O are the total energy of corresponding isolated CdTeQD, ZnTPP and H2 O molecule, respectively. The value of n = 1 for covalent functionalization through amide linkage and n = 0 for coordination linkage. While comparing the binding energies of ZnTPP-Cd17 Te17 QD for both types of linkages, following Eq. (1), we find that the axially coordinated nanocomposite (Eb = 1.97 eV) is energetically very much favored over amide linkage (Eb = 0.13 eV). Thus, we set our focus on axially coordinated nanocomposites. The metal ion of the porphyrin moiety is tilted towards the long chain thiol molecule functionalized QDs (Figure 4b), clearly indicating the formation of a new Zn–N bond. 69 This observation is further supported by the comparable value of newly formed Zn–N distance (2.106 Å) to the Zn–N distance of ZnTPP (2.10 Å), which in turn explains the energetic preference for the axially coordinated supramolecule. Figure 5(a,b) depicts the DOS picture for ZnTPP-CdTeQD nanocomposites. The corresponding PDOS for ZnTPP and QDs are also shown in the same figure. The relative position of the


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PDOS for ZnTPP and QDs reveals that, the nanocomposite of very small sized QD shows type-II band alignment, where HOMO comes from ZnTPP and LUMO from CdTeQDs, clearly indicating the possibility of porphyrin sensitized charge separation As a result, for composites of ZnTPP with large QD also show type-II band alignment, where HOMO is controlled by porphyrin and LUMO is dominated by CdTeQDs, implying a clear charge separation. Hence, one can achieve type-II band alignment for the coordination complex of CdTeQDs with zinc porphyrin without any worry about the size of the QDs, where porphyrin plays the role of sensitizer. So, the photoinduced electron transfer could be expected from ZnTPP to CdTeQDs. Infact, such kind of photoinduced energy transfer has recently been observed by Amelia et al. 52 Using steady-state and time-resolved luminescence experiments they have shown that, CdTe nanocrystals of diameter 5.6 nm afford self-assembled complexes with ZnTPP in chloroform solvent and the fluorescence of the ZnTPP is quenched. The band gap luminescence of the QD is sensitized, implying the occurrence of photoinduced energy transfer from the ZnTPP donors to the noncovalently bound CdTe acceptors. Hence, our theoretical results are in good agreement with experimental observation of Amelia et al. 52 For better understanding the spatial charge separation in ZnTPP-CdTeQD nanocomposites we plotted the HOMO and LUMO densities of nanocomposites in Figure 5(c,d) for two different QD sizes. The figure clearly indicates that for nanocomposites, irrespective of the size of CdTeQDs, the HOMO is localized on porphyrin and LUMO density is localized on QD. The semiclassical model of Marcus electron-transfer theory has given the rate of charge recombination processes as 70 2 kab = Vab

r

π (λ + ∆G)2 ] exp[− 4λ kB T λ kB T h¯ 2

(2)

where kab is the recombination rate constant, Vab is the electronic coupling between the initial and final states, which in turn depends on the overlap of HOMO and LUMO wave functions of the composite system, λ is the total reorganization energy, and ∆G is the Gibbs free energy change for the electron transfer process. The T, kB and h¯ denote the temperature, Boltzmann constant and the reduced Planck constant, respectively. From this expression it is obvious that the electron-hole recombination rate largely depends on Vab the value of which depends on the overlap HOMO and 9 ACS Paragon Plus Environment


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LUMO densities. For our case, the electron and hole wave functions are spatially well separated as can be found from Figure 5, implying negligible overlap. Therefore, we can conclude at least qualitatively, the rate of charge recombination is quite low for the nanocomposites we have considered here and are thus suitable for application in solar cell. Another very interesting feature that can be predicted from the energy offset is the dynamics of electron transfer. As the driving force for the electron transfer is the difference in energy between the donor LUMO and the acceptor LUMO, one could expect a faster electron transfer rate for a larger CdTeQD as compared to smaller QDs when they are coupled with the donor ZnTPP. This observation is quite contrary to our previous calculations, 17,21 where smaller QDs were supposed to show faster electron transfer than the larger. In that case, the situation was different, the QDs were serving as the sensitizer. However, it should be kept in mind that this fast electron transfer rate for large CdTeQDs does not necessarily mean that larger the size of CdTeQD, higher is the photoelectric conversion efficiency (PCE). Apart from the energy offset there are other factors for determining the PCE of photovoltaic devices. In this context, we have tried to offer an idea, at least qualitatively, to get higher efficiency with ZnTPP-CdTeQD nanocomposites. It is well known that the PCE, η , of DSSCs can be predicted based on the short-circuit current density, Jsc , the open-circuit photovoltage, Voc , the fill factor, ff, and the intensity of the incident light (Pinc ), and is given by Eq. (3) 71,72

η=

JscVoc f f Pinc

(3)

The short circuit current density (Jsc ) of photovoltaic cell can be expressed as 73

Jsc =

Z

LHE(λ )φin j ηcollect d λ

(4)

where LHE(λ ) is the light harvesting efficiency at a given wavelength (λ ), φin j and ηcollect are the electron injection efficiency to conduction band of the acceptor and the electron collection efficiency at the transparent conductive electrode, respectively. The LHE(λ ) is mainly determined 10 ACS Paragon Plus Environment

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from the oscillator strength of absorption (f) as 74 LHE(λ ) = 1 − 10− f

(5)

In order to calculate the oscillator strength of absorption for the ZnTPP-CdTeQD nanocomposites, as well as to investigate the mechanism of charge transfer from ZnTPP to CdTeQDs, we have performed TD-DFT calculations as implemented in the Gaussian 09 program. 63 The calculations reveal that the nanocomposites show absorption maxima in the visible region. The excitation energies and the associated electronic transitions involving different molecular orbitals are summarized in Table 1. Figure 6 shows the pictorial representations of the corresponding molecular orbitals. From the table it is seen that the values of oscillator strengths are small implying low absorption intensities. For better photovoltaic performance, the nanocomposites should show high absorption intensities as well as separation of charge carriers. So we have to find out a suitable system where these two contrasting events show optimal behavior i.e. the oscillator strength of the nanocomposite should not be too low and at the same time there is charge separation. Now, from Table 1 we found that the oscillator strength of absorption increases as the size of CdTeQD increases, so does LHE (using Eq. (5)). This is also reflected from the higher intensity of the absorption band for the nanocomposite with bigger sized QD (picture is not shown here). The phenomena could be understood from the fact that, for the smaller composite, ZnTPP-Cd17 Te17 QD, the transition is of π → π ∗ type involving different bonding and antibonding MOs of ZnTPP counterpart with a little contribution from ZnTPP → QD charge transfer characteristic. On the other hand, for the nanocomposite with bigger QD (ZnTPP-Cd34 Te34 QD), the absorption is mainly characterized by the intermolecular charge transfer from ZnTPP to QD with a major contribution (83%) of HOMO → LUMO+4 transition. These results strongly indicate that the size of CdTeQD plays a crucial role in improving the charge transfer properties from the ZnTPP to the QD, which strengthen our SCC-DFTB observation. For DSSC systems we studied, there is no change in the electrode, only the difference is in



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the size of the QDs, the ηcollect term of Eq. (4) can be treated as constant. Hence, Jsc is mainly guided by LHE and φin j . The φin j is related to the driving force (∆G) of electrons injection from the excited state of donor molecule to the acceptor substrate. 75 Among the different contributions to ∆G (such as ∆Gcharging , ∆Gcoulomb and ∆Gelectronic , for the details please see Ref. 76 and 77), the major one ∆Gelectronic is determined by the energy difference between the LUMO of donor A 76,77 (ED LUMO ) and LUMO of acceptor (ELUMO ) as below

D A ∆Gelectronic = ELUMO − ELUMO

(6)

For type-II ZnTPP-CdTeQD nanocomposites the energy difference between the LUMO of ZnTPP (donor) and the LUMO of CdTeQD (acceptor) continuously increases with increasing the size of the QD (see Figure 5). So, for ZnTPP-CdTeQD nanocomposites, ∆Gelectronic is continuously increasing with the size of QDs and so φin j . Hence, according to Eq. (4), the rises of both LHE and

φin j result in larger value of Jsc for the larger QD. The open-circuit voltage (Voc ) of a bulk-heterojunction solar cell can be estimated by 71 1 D A Voc = (|EHOMO | − |ELUMO |) − 0.3V e

(7)

D A where EHOMO and ELUMO denote the HOMO energy of donor and LUMO energy of acceptor, D A respectively. Basically, the term |EHOMO | − |ELUMO | represents the system band gap for a type-II

nanocomposite. The Figure 5 clearly shows that, as the size of the QD increases the band gap of type-II ZnTPP-CdTeQD nanocomposites gradually reduces and consequently it produces smaller value of the Voc . Hence, there are two main factors that controls the PCE of ZnTPP-CdTeQD nanocomposites and they are affecting in the opposite direction with the size of QDs. As the size of the QD increases, Jsc starts to increase, while Voc get reduced. So, size of the QD takes crucial role to the PCE of ZnTPP-CdTeQD nanocomposites. Now, if the considered QD size is very large then using Eq. (4) and Eq. (6) one may get high value of Jsc . But at the same time for that case Voc will be 12 ACS Paragon Plus Environment

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very small as can found from Eq. (7). Considering this two adversary factors and Eq. (3) one can conclude, that ZnTPP-CdTeQD nanocomposites with large size QD will show low PCE. On the other hand, for nanocomposites with smaller QD, will again show low PCE, because for that case Voc will be large, whereas, Jsc become very little. Finally, in qualitative sense we can conclude that nanocomposites with moderate sized CdTeQDs, having moderate values of Jsc and Voc , will produce maximum PCE for ZnTPP-CdTeQD nanocomposites compared to the very small and very large one and these are therefore promising candidates for application in dye-sensitized solar cell.

4. CONCLUSION In this paper, by using density-functional method an exhaustive theoretical study of a novel configuration for dye-sensitized solar cell (DSSC) based on TPP-CdTeQD and ZnTPP-CdTeQD nanocomposites, is presented. The results indicate that the nanocomposites composed of simple TPP and CdTeQD are not so suitable for DSSCs application as they devoid of any charge separation, except for few particular sized QDs. The substitution at para− position of phenyl ring of the porphyrin molecule by organic functionalizing groups can overcome such unpleasant situation. The substitution with either strong electron donating (NH2 ) or withdrawing groups (-NO2 ) can yield charge separation even for the very small QD. Whereas, weak/mild functionalizing groups (-OCH3 or COOH) can do so for larger QDs. On the other hand, if the starting porphyrin is ZnTPP instead of TPP then, whatever may be the size of the QD, there will be charge separation between ZnTPP and CdTeQD in the axially coordinated nanocomposites. Moreover, as the size of the QD in ZnTPPCdTeQD nanocomposites increases, the difference between the LUMO of the QDs and LUMO of the ZnTPP, which is key for dictating the dynamics of electron transfer, starts to increase continuously. So, one could expect a faster electron transfer rate for a larger CdTeQD as compared to smaller one. Such observation is quite contrary to our previous studies, where smaller QDs are supposed to show faster electron transfer. Hence, ZnTPP-CdTeQD nanocomposites open a scope of ease to the experimentalists for the application in DSSCs. The TDDFT calculation for ZnTPP-



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Cd17 Te17 QD shows that, the contribution of charge transfer process from ZnTPP to QD is only about 15% to electronic excitation with highest oscillator strength. Whereas, ZnTPP-Cd34 Te34 QD shows solely the charge transfer process with major contribution of 83%. So, the size of CdTeQD plays a crucial role in improving the charge transfer properties from the ZnTPP to the QD. We have also tried to give a qualitative idea about the performance of ZnTPP-CdTeQD nanocomposites as an applicant in light harvesting device using some empirical relations coupled with TDDFT calculations. Based on that qualitative idea we have ultimately conclude that the ZnTPP-CdTeQD nanocomposites will show higher performance for light harvesting devices with moderate size of QD, rather than too big or too small one. We hope our theoretical studies will encourage more experimental work on porphyrin-CdTeQD nanocomposites to enrich the era of DSSCs. ACKNOWLEDGMENTS The authors would like to thank the UGC, New Delhi, for financial support through research grant [Ref. No. 43-174/2014(SR)]. One of the authors BR is grateful to UGC, New Delhi for awarding him RGNSRF.


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Figure 1: (a) Optimized structures of one representative TPP-CdTeQD (TPP-Cd65 Te65 QD) and functionalized TPP-CdTeQD (NO2 –TPP-Cd65 Te65 QD) nanocomposite, respectively, and Density of states (DOS) of TPP-CdTeQD nanocomposite with QDs of three different size: (b) Cd17 Te17 (c) Cd65 Te65 (d) Cd98 Te98 . The zero of energy is set to the top of the valence band.


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Figure 2: HOMO and LUMO densities of TPP-CdTeQD nanocomposites for three QDs of different size: (a) Cd17 Te17 (b) Cd65 Te65 (c) Cd98 Te98 . The red surface represent the HOMO whereas blue surface represent the LUMO.



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Figure 3: Density of states (DOS) of functionalized-TPP-CdTeQD nanocomposites with corresponding HOMO (red surface) and LUMO (blue surface) densities: (a) –CH3 (b)–NH2 (c) –COOH (d) –NO2 . In the inset of (b) and (d), zoomed views of LUMO and HOMO, respectively, of corresponding composites have shown. The zero of energy is set to the top of the valence band.


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Figure 4: The optimized structure of one representative for (a) ZnTPP-CdTeQD (ZnTPPCd65 Te65 QD) nanocomposites attached through amide linkage and (b) the axial coordination of CdTeQD to the Zn atom of ZnTPP (Cd65 Te65 QD to ZnTPP) [views from two different angel].



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Figure 5: Density of states (DOS) of axially coordinated ZnTPP-CdTeQD nanocomposites with two different sized QDs and their corresponding HOMO (red surface) and LUMO (blue surface) densities: (a,c) ZnTPP-Cd17 Te17 QD (b,d) ZnTPP-Cd65 Te65 QD. The zero of energy is set to the top of the valence band.


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Table 1: Calculated excitation energies (eV), oscillator strengths (f), and electronic configuration for the ZnTPP-CdTeQD nanocomposites by B3LYP/6-31(d). Energy Oscillator (eV, nm) strength (f) ZnTPP-Cd17 Te17 QD 2.2371 (556.7) 0.023

ZnTPP-Cd34 Te34 QD 1.8835 (658.2)

0.068

Composition (%) H-1 → L+10 (15.46) H-1 → L+21 (14.87) H-1 → L+22 (13.83) H → L+21 (26.5) H → L+22 (28) H → L+2 (2.9) H → L+3 (2.67) H → L+4 (82.56) H → L+8 (6.8)


Type

π π π π

CT → π∗ → π∗ → π∗ → π∗ CT CT CT CT


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Figure 6: The molecular orbitals involved in the electronic excitations of (a) ZnTPP-Cd17 Te17 QD and (b) ZnTPP-Cd34 Te34 QD by B3LYP/6-31G(d).


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110x112mm (96 x 96 DPI)

ACS Paragon Plus Environment

Optimizing the Photovoltaic Properties of CdTe Quantum Dot

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