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Whispering-gallery mode assisted enhancement in the power conversion efficiency of DSSC and QDSSC devices using TiO microsphere photoanodes 2

Tapan Kumar Das, Perumal Ilaiyaraja, and Chandran Sudakar ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00207 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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ACS Applied Energy Materials

Whispering-Gallery Mode Assisted Enhancement in the Power Conversion Efficiency of DSSC and QDSSC Devices Using TiO2 Microsphere Photoanodes Tapan Kumar Das, Perumal Ilaiyaraja and Chandran Sudakar* Multifunctional Materials Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai-600036, India *corresponding author email - [email protected]

ABSTRACT: Mesoporous TiO2 nanoparticles are excellent photoanodes for sensitized solar cells. However, significant loss in the photoconversion efficiency (PCE) occurs due to the inefficient light absorption. Large Mie scattering from mesoporous micron-sized spheres could be used to enhance the absorption and hence the PCE. Here, we show that a composite comprising of TiO2 mesoporous microsphere with smooth surface (SµS-TiO2) exhibiting whispering gallery modes (WGM) and a commercial TiO2 mesoporous nanoparticle (Degussa P25) with an optimum ratio (80:20 wt. %) show significant enhancement in PCE of DSSC and QDSSC devices. SµS-TiO2 exhibit strong WGM as evidenced from the photoluminescence studies carried out using a laser with an excitation wavelength λexc=325 nm. These microspheres sensitized with N719 dye or CdSe/CuInS2 QDs are shown to exhibit emission characteristics strongly coupled to WGM resulting in an enhanced PCE in SSC. Increase in PCE of ~24 % in DSSC devices and 80 to 95 % in QDSSC devices of 0.25 cm2 area are demonstrated. Thus, TiO2 composite exhibiting WGM can be used as a generic photoanode to achieve maximum PCE for Grätzel type sensitized solar cells. This study suggests special class of solar cells called "whispheronic solar cells". Keywords: Microspheres, TiO2, Whispering gallery modes, DSSC, QDSSC, Photoluminescence * Corresponding Author: [email protected]: Ph: +91-44-22574895 ORCID Chandran Sudakar: https://orcid.org/0000-0003-2863-338X 1 ACS Paragon Plus Environment

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INTRODUCTION Solar cells employing dye and semiconductor quantum dots (QDs) as sensitizers are evolving rapidly over last two decades with a significant focus on improving the power conversion efficiency (PCE).1-6 Photoanode play important role in deciding the performance of the sensitized solar cells (SSCs). Various photoanodes are being employed for both dye and quantum dot sensitized solar cells. These include mostly the wide bandgap semiconductor photoanodes such as TiO2, ZnO and SnO2.6-8 Anatase form of TiO2 is the most extensively studied photoanode material in SSC devices due to high photocatalytic activity and appropriate energy level matching with the sensitizer, electrolyte and the conducting electrode.9-10 Further the choice is also due to its abundancy and low cost. Other photoanodes like ZnO and SnO2 show much higher electron mobility than TiO2.11-13 However, due to poor sensitizer adsorption, aggravation of sensitizer degradation and faster electron recombination reaction, the performance of the devices based on these photoanodes are found to be inferior than the anatase TiO2 based device.14-15 Anatase TiO2 photoanode is one of the major component in SSCs which also dictate the performance of the device. Mesoporous TiO2 nanoparticle based photoanodes are reported to produce very high photocurrent due to its high surface area and is the most commonly used material for sensitizer adsorption.16-17 However combining photoanode of highly active surface area with suitable morphology and superior light scattering effect will make TiO2 much more promising for solar cell applications.18-21 Several approaches have been made to improve the performance of SSCs using different morphology of TiO2 photoanode. Some of the morphologies that have been reported include nano-embossed hollow sphere,22 multi-shell porous hollow nanoparticles,23 urchin-like hollow spheres,24 hierarchical microsphere25-26 and hierarchical nanotubes.27 In all these microstructures scattering play a key role in deciding the

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performance of the devices. However, the type of scattering that prevails in these microstructures resulting in higher PCE is not discussed.18-20 Other factor that limit the PCE in solar cell include the energy loss due to charge recombination.28-29 At the interface of the nanoparticle boundaries the charge carriers easily get trapped resulting in the decrease of mobility and carrier lifetime.30 Thus, essential factors that influence PCE include efficient light absorption and charge transfer ability.31-32 Reducing the specular reflection of light through anti-reflection coating and texturing the surface to increase diffuse reflection which result in multiple scattering and hence lead to higher absorption have been employed to enhance the harvesting of light.33-34 Alternatively, the light trapping schemes such as whispering gallery modes (WGMs) in oxide materials are found to be an efficient way to improve light harvesting. Normally large sized microstructures with the size comparable to the wavelength of incident light are preferred as it enhances the light scattering in SSCs.35-36 Light scattered by micron sized spheres resonate with incident wave due to phase matching and give rise to sharp peaks in photoluminescence spectra of photoanode.37 Such scattering is generally seen in the case of microspheres,37 nano-rods,38 nano-tubes,39 and photonic crystals and are called whispering gallery modes (WGM).40 In an effort to increase the signal from the sources, composite resonators including coated spheres and hybrid metallodielectric resonators are used recently.41-43 In micron-sized oxide particles, the incident light is coupled to the sensitizer through this WGM light scattering.44-45 Recently, double layer and composite photoanode film structures consisting of different sized TiO2 particles are being explored to avail the benefits of both large specific surface area and light scattering effect to enhance the PCE of SSCs.46 Plasmonic nanoparticles, which work as the source for light scattering, were also demonstrated to enhance power conversion efficiency (PCE) above 8% in



single junction polymer solar cells.47 Such solar cells are generally called as "plasmonic solar cells".48 The photoanode microstructural configuration of TiO2 microsphere with unique hierarchical structure provide better scattering.49 All these studies on TiO2 microstructure suggest the effective coupling could originate from the whispering-gallery modes (WGM) in the sphere.45 In this manuscript, we report on the whispering-gallery modes (WGMs) observed in TiO2 smooth microsphere (SµS-TiO2) and its effect on the photo conversion efficiency (PCE) of SSCs. SµS-TiO2 act as dielectric resonator with multiple total internal reflection giving rise to resonant WGMs. Studies on porous fibrous microsphere (FµS-TiO2) and their composites are also presented for comparison. While SµS-TiO2 and its composites exhibit WGM modes, FµSTiO2 and its composite do not act as spherical optical resonator due to its fibrous microstructure. We find enhanced PCE in DSSC and QDSSC devices made from SµS-TiO2 mainly assisted by WGM for the efficient light absorption. The efficiency is shown to improve further in a composite photoanodes (80:20 wt.% ratio of SµS-TiO2 and P25-TiO2) where better electrical connectivity between microspheres enables a continuous path for the transport of charge carriers. The observation of WGMs in the dye and QD sensitized TiO2 microsphere suggest that the underlying mechanism for better performance of the solar cell device is due to the enhanced light absorption in sensitizers assisted by the WGM scattering effect. We show enhancement in the PCE of ~24 % and 80 to 95 % in DSSC and QDSSC devices, respectively. RESULTS AND DISCUSSION Various TiO2 photoanodes are used in the dye/QD sensitized solar cells. These include P25-TiO2 (from Degussa), smooth mesoporous microsphere (SµS-TiO2), fibrous microsphere (FµS-TiO2) and their composite with P25-TiO2. All the TiO2 we have used in the current study 4 ACS Paragon Plus Environment

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are in the anatase form as inferred from XRD (Figure S1 of the Supporting Information). A small fraction of rutile phase is seen in commercial TiO2 sample (P25-TiO2). However, we do not decipher any such phase from XRD for SµS-TiO2 and FµS-TiO2. The morphological and microstructural details of TiO2 microspheres (SµS-TiO2 and FµS-TiO2), P25-TiO2 and their composites analyzed using FESEM are shown in Figure 1a-d, and the corresponding bright field TEM and HRTEM images are presented in Figure 1e-h. The mesoporous film formed by P25TiO2 exhibit uniform morphology (Figure 1a) with the average nanocrystallite size found to be ~25 nm. The nanocrystallites in P25-TiO2 are highly crystalline as evidenced from the lattice images corresponding to (101) set of planes (Figure 1e, inset). The SµS-TiO2 films comprise of spherical shaped microspheres with size ranging from 1 to 6 µm (Figure 1b). The histogram for spherical particle size distribution shows an average diameter ~ 2.6 µm with standard deviation of ~ 1.4 (Figure S1 in the Supporting Information). The microspheres are connected by the formation of large neck between the micron sized spheres (Figure 1b, inset). The bright field TEM images of SµS-TiO2 show a typical pair of adjoined microspheres (Figure 1f). These microspheres are in fact made of nanoparticles with average size ~ 25 nm. HRTEM image of nanoparticles from SµS-TiO2 show most commonly seen (101) lattice fringes (0.35 nm) of high crystalline nature (Figure 1f, inset). The SµS-TiO2 has large unfilled space in between the microspheres. These gaps are filled up by the smaller P25-TiO2 nanoparticles in the composite (SµS+P25)-TiO2 microstructure (Figure 1c). The magnified FESEM image shows that the P25TiO2 nanoparticles uniformly coat the microsphere. The packing density and the connectivity of the particles significantly increase due to the filling of void region by the nanoparticles. The uniform coating of P25-TiO2 particles on the surface of microsphere in the composite (SµS+P25)-TiO2 microstructure can be clearly seen from the bright field TEM images (Figure 5 ACS Paragon Plus Environment


1g). Microsphere composed of fibrous strands of TiO2 nanoparticles (FµS-TiO2) and their composites with P25-TiO2 nanoparticles (FµS+P25)-TiO2 are also studied to compare the properties with SµS-TiO2 and (SµS+P25)-TiO2 composite. FESEM and bright field TEM images of FµS-TiO2 and its composite with P25-TiO2 are shown in Figure 1d & 1h. The size estimated from FESEM and TEM is found to be in the range of 1 to 3 µm. This FµS-TiO2 comprise of fibrous network with the connected nanoparticles forming fibrous strand originating from the center of the sphere and growing radially outwards. FµS-TiO2 exhibit low dense fibrous feature and unlike SµS-TiO2 they do not form a compact microstructure. The composite of these low dense fibrous features filled with P25-TiO2 nanoparticles show an increase in the packing density (see Figure 1d & 1h, inset). The uniform coating of nanoparticles on the FµS-TiO2 microsphere make compact layer, which can still act as a cavity for multiple scattering. Further, increase in packing fraction of (FµS+P25)-TiO2 composite photoanode enables a continuous path for the transport of charge carriers. Thus, addition of P25-TiO2 nanocrystallites to the microsphere not only increases the effective area for sensitizer adsorption but also gives a conducting channel for charge carrier transport. To understand the light scattering mechanism, photoluminescence (PL) studies on the TiO2 were carried out. Figure 2a and 2b show the PL spectra of P25-TiO2, FµS-TiO2, SµS-TiO2 acquired at room temperature using a 325 nm UV excitation line from He-Cd laser source and a 488 nm visible light excitation from Ar-ion laser source, respectively. PL spectra of P25-TiO2 and FµS-TiO2 consists of two major broad peaks, one ~ 425 nm and another ~ 520 nm (Figure 2a). In general, PL spectral responses in TiO2 are attributed to self-trapped excitons, oxygen vacancies and surface defect states.50-52 The green emission in visible region mainly originates from the oxygen vacancies created in TiO2 due to mixed valence states of Ti4+ and Ti3+ in 6 ACS Paragon Plus Environment

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anatase.53 The presence of trace quantities of Ti3+ and oxygen vacancy (VO) can be inferred from the resonant absorption of unpaired spins using electron paramagnetic resonance (EPR) and xray photoelectron spectroscopy (XPS) studies (Figure S3 in the Supporting information). The presence Ti3+ at lattice sites and interstitial sites in TiO2 give rise to EPR signals at g-values of 1.972 and 1.942, respectively.

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On the other hand formation of oxygen radicals similar to

Ti4+−O– species results in EPR absorption with a g-value of ~ 2.02 and are typically related to oxygen vacancies (VO).57 These signals assigned to VO species are very likely to be located at surface sites. Both spectra exhibit very weak signals corresponding to the unpaired electrons. The signal strength is more than an order of magnitude smaller than what is observed in our previous work in VO rich TiO2 nanoparticles.

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The oxidation state of Ti is also studied using

XPS analysis, however this technique is sensitive to the chemical information from the surface. The XPS study on P25 and SµS indicate the presence of minimal concentration of Ti3+ in TiO2 (~ 6 %) (Figure S4, Supporting Information). The oxygen vacancies inferred from the secondary peaks of O1s XPS spectral lines mainly arise from the under coordinated lattice oxygen. The peak centered at ~ 425 nm in the PL spectra is ascribed to self-trapped excitons located at TiO6 octahedra, while second broad peak with maxima at ~520 nm is ascribed to defect related trap states.53 In PL spectra, scattering related spectral activities can be inferred in addition to the defect or near-band emission characteristics. For example, the morphology of TiO2 may play a major role in modifying the intensity of PL spectra but it does not influence the peak positions.59 The PL spectra of SµS TiO2, in stark contrast to P25-TiO2 or FµS-TiO2, exhibit multiple sharp emissions on top of the broad peaks observed in these nanostructured TiO2 samples. Figure 2b show the PL spectra of P25-TiO2, FµS-TiO2, SµS-TiO2 acquired with a 488 nm excitation source. The spectrum mainly originates from the oxygen vacancies created in TiO2 due to mixed



valence states. The sharp emissions are still seen in SµS-TiO2 samples in the emission region corresponding to the defect trap states. The multiple sharp emissions seen ~ 425 nm and ~ 520 nm in SµS-TiO2 (Figure 2a) fade out at longer wavelength (Figure 2b). The prevalence of WGM in microspheres and other microstructures is known in the literature.37, 42, 60-62 When the photon is made to incident on the microspheres, they get trapped inside the spheres by scattering and produce a resonance due to phase matching with the incident waves. The total internal reflection of light at the oxide-air interface in microspheres leads to the confinement of light, when a particular wavelength (λ) couples into the microsphere-resonator, leading to the propagation and circulation of scattered light in a ring like path through the surface of microsphere.63 A schematic diagram depicting the formation of WGM in an optically resonating TiO2 microsphere is shown in Figure 3c. When the wavelength of emitted light satisfies the constructive interference condition ߨ‫ߣ݉ = ܦ‬ൗ݊ (where D is the diameter of the sphere, m is the integer mode number, λ is the resonant wavelength and n is the refractive index of the microsphere), sharp resonating light emissions, WGMs, in the form of multiple peaks are observed on top of the regular PL spectra.42 WGMs are strongly dependent on the size parameter of the morphology and refractive-index contrast between the dielectric sphere and the surrounding medium.64 In our study such existence of WGM are shown in micron-sized smooth spheres made of compactly packed TiO2 nanoparticles. Such modes do not exist in the nanoparticular TiO2 as shown contrastingly in Figure 3a. A comparative PL spectrum of P25-TiO2 along with SµS-TiO2 clearly show the presence of resonant modes from WGMs in the form of spikes on top of two major broad peaks characteristic of nanocrystalline TiO2 seen at ~ 425 nm and ~ 520 nm. WGM modes simulated using MiePlot program65 considering a microsphere size R= 2.426 µm and refractive index n = 2.65 for SµS-TiO2 matches with the experimentally observed modes obtained from the 8 ACS Paragon Plus Environment

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background subtracted PL spectra (Figure 3b). Broader WGMs peaks are observed in SµS-TiO2 samples due to more absorption, which leads to the reduction in the scattering amplitude.66 The multi-peak spectrum (i.e. WGMs) covers almost the whole visible range (Figure 3a), with the spacing between the resonating peaks increasing slightly in the long wavelength region indicating that this microsphere can be used as a high quality optical resonator. As these multipeak sharp resonances cover the whole visible region extending up to 640 nm, which form the major component of sun’s energy spectrum, the light scattered by this process can be used to enhance the light absorption in the sensitizer and hence the PCE. The WGM scattering from microspherical oxide can increase the intensity of PL spectra in visible region due to the change in the dielectric function of the surrounding nanoparticles or refractive index contrast between particle and surrounding medium.42 In order to investigate the presence of WGM mode after coating sensitizers on different photoanode, we acquired PL spectra of sensitizer (N719 dye, CdSe and CuInS2 QD) coated TiO2 photoanode (Figure 4). The PL spectra were collected from well-separated microspheres dispersed on the Si substrate. The optical microscopic images of such dispersed microspheres are shown in the insets of Figure 4. The PL spectra of dye coated P25-TiO2, FµS-TiO2 and SµS-TiO2 nanostructures acquired at room temperature using 325 nm and 488 nm excitation sources are shown in Figure 4a and Figure 4b respectively. The PL emission centered at 425 nm from pure TiO2 microstructure shows weak emission, however the emission centered at 520 nm from the defect level of TiO2 get enhanced due to the radiative energy transfer from the former to the latter mode.60 In contrast, the PL emissions from SµS-TiO2 samples show weak WGMs on the top of PL background in this region (Figure 4a). Figure 4b shows the PL emission from dye loaded TiO2 photoanode along with bare dye excited with a 488 nm laser source. An enhanced PL emission



centered between 700 -750 nm is observed in dye loaded photoanodes. The dye emission from the coated photoanodes is found to exhibit significant blue shift (~ 20 to 30 nm) from actual dye emission. The presence of WGMs and the blue shift in dye emission region on the PL spectrum of dye-loaded SµS-TiO2 indicate an efficient coupling between microsphere and resonance modes. The PL emission centered between 700-750 nm gets blue shifted by ~ 30 nm in dyeloaded SµS-TiO2. Such a shift has been attributed to the coupling between PL band of dye and WGMs peaks.42, 67 Due to the strong coupling between PL band and WGMs peak, the gain is found to be more in dye loaded SµS-TiO2 due to the presence of WGMs. Thus, the intense WGMs in the dye emission region strongly suggest the radiative energy transfer to the dye molecules (Figure 4b). Interestingly, in addition to sustaining the WGM modes in TiO2, WGMs are formed in dye emission region suggesting strong energy transfer between these two modes. Similar studies were carried out on photoanodes sensitized with inorganic sensitizers such as CdSe QDs and CuInS2 (CIS) QDs. Figure 4c and 4d shows the PL spectra of CdSe QDs and CIS QDs sensitized SµS-TiO2 respectively. The PL emission spectra from the quantum dots alone are also shown in these figures. The size of quantum dot employed in this study is ~ 4 to 5 nm. PL spectra of CdSe QD show the PL emission peak corresponding to the near-band-edge emission (NBE) at ~ 600 nm (Figure 4c). This NBE is also seen in CdSe loaded SµS-TiO2, however with the coexistence of sharp spikes from the WGM. Similarly, the CIS QDs, exhibiting NBE ~ 675 µm, show WGM on top of this emission in the CIS sensitized SµS-TiO2 photoanode (Figure 4d). Prominent WGM with broad emission ~ 575 nm are also seen in the CIS sensitized SµS-TiO2 photoanode, which could arise due to radiative energy transfer.60 This radiative energy transfer could happen by the WGMs present in SµS-TiO2 and the energy transfer from WGMs to the sensitizer coated on it. The interesting observations from all these studies (Figure 4) on 10 ACS Paragon Plus Environment

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sensitized photoanodes are due to the prevalence of WGM modes on the PL emission related to the sensitizer. We demonstrate the effect of the enhanced light scattering on the current density-voltage (J-V) characteristics curves of both DSSC and QDSSC devices (Figure 5). The photovoltaic properties of DSSCs fabricated using P25, FµS, SµS, and their composite (SµS+P25 & FµS+P25) as photoanodes are shown in Figure 5a and the characteristics parameters are summarized in Table 1. The maximum measured power conversion efficiency (η) of DSSC made of P25-TiO2 is found to be η~ 7.2 % and SµS-TiO2 is η~ 8.4 %. The photoanode made of SµS-TiO2 thus show an enhancement in PCE of ~ 17 % compared to the P25-TiO2 based devices. The device made of FµS show relatively small efficiency (η= 6.3 %) compared to P25-TiO2 based devices. The lack of connectivity and compact packing of nanoparticles could lead to poor electron transport in FµS-TiO2 (Figure S3 Supporting Information). For the case of SµS-TiO2 in spite of large micrometer sized particles the fabricated devices exhibited higher PCE (η=8.4 %) on DSSCs devices. Such an enhancement could only result from the resonance scattering due to WGM modes, on top of the benefit gained due to the well-connected microspheres made of compact TiO2 nanoparticles. Further, increase in efficiency can be achieved by employing the composite structure of photoanode such as (SµS+P25)-TiO2. Efficiency values close to η=8.9 % is achieved with this configuration which amounts to an increase of 24 % compared to P25-TiO2 based devices. But in the case of photoanode made from (FµS+P25)-TiO2 composite, the PCE is only just around the same (η~7.1 %) as that of PCE of P25-TiO2. It should be noted that the WGM modes are not seen in FµS-TiO2 or P25-TiO2. Further, it is noteworthy that the composites structure always show enhanced PCE at an optimized ratio of nanoparticle to microsphere ratio of 20:80 (P25:SµS/P25:FµS). While we report the J-V curves corresponding to 11 ACS Paragon Plus Environment


the highest PCE cells in Figure 5, the distribution of the cell performance measured over 5 to 10 cells are presented in Figure 6. The WGM effect leading to an enhanced PCE is not only observed in the case of DSSC devices but also found to be very consistent and effective in QDSSC devices (Figure 5b and Figure 5c). The CdSe QDSSC device made of SµS-TiO2 as photoanode shows PCE of η ~ 1.8 %, which is ~ 27 % higher than the PCE of P25-TiO2 based QDSSC devices. The PCE of QDSSC devices made of composite film (SµS+P25-TiO2) is found to be η~ 2.7 %, which is ~ 94 % more compared to the P25-TiO2 based devices. Thus, PCE is found to show two-fold increase in QDSSCs made of composite photoanodes. The QDSSC cells made with CuInS2 also show similar trend with the highest PCE of η= 3.8 % observed for composite structure, where SµS-TiO2 form the major component. Interestingly, CIS-QD based QDSSC devices show high VOC in the range of 840 mV and 926 mV when sensitized on SµSTiO2 and (SµS+P25)-TiO2 photoanodes respectively. Such a high VOC could be possible due to the combined effects of better properties of TiO2 (low defects in SµS TiO2, effective light scattering, minimum recombination and efficient electron transport) and band-alignment in CIS QDs. We measured several SSC devices and the observed generic trend is summarized in Figure 6. While the increase in η (%) has been attributed to various factors, our results strongly suggest that the enhancement mainly arise due to the additional resonant scattering existing in the photoanodes made of SµS-TiO2 microsphere resonator. Diffuse reflectance study also showed increased reflection in the microsphere and its composite photoanode compared to the nanostructured TiO2 photoanode (Figure S4 Supporting information). It is interesting to note that in FµS-TiO2 and its composites, where these WGM modes are absent the efficiency is found to be much lower than SµS-TiO2 and its composite photoanode devices. We also note that the sensitizer loading is almost similar in all the three structures and cannot account for the large 12 ACS Paragon Plus Environment

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increment in η (%). The observation of enhancement in the PCE even in other sensitized photoanode structures, including QDSSCs made of CdSe QDs and CuInS2 QDs, unequivocally suggest the role of WGMs from the microsphere resonator in enabling increased light absorption in the sensitizer and thus enhancing the power conversion efficiency (η %). Our study also suggests possible new class of Grätzel type solar cell called "whispheronic solar cells" which uses resonant Mie scattering from micron-sized particle in the photoanode to enhance power conversion efficiency, similar to the "plasmonic solar cells"48 which uses localized plasmon resonance scattering from metal nanoparticles. CONCLUSION In summary, we demonstrate the presence of whispering gallery modes (WGM), the waves within the microspheres due to Mie resonant scattering, in the mesoporous smooth microspheres (SµS-TiO2) from the PL studies. We demonstrate the effect of WGMs on the power conversion efficiency (PCE, η) of Grätzel type sensitized solar cell (SSC) devices. The presence of WGMs in the photoanode significantly increase light absorption in SSCs. Composite of SµS-TiO2 with nanocrystalline P25-TiO2, with a composite ratio of 80:20, exhibit highest PEC. The intense WGMs in the sensitizer emission region strongly suggest the radiative energy transfer to the sensitizer energy levels. Thus, WGMs in the microsphere resonator enable increased light absorption in the sensitizer and enhances the power conversion efficiency (η %). Increase in PCE of ~24 % in DSSC devices and 80 to 95 % in QDSSC devices of 0.25 cm2 area are demonstrated in comparison to the nanocrystalline TiO2 based photoanode. Thus, TiO2 composite exhibiting WGM can be used as a generic photoanode to achieve maximum PCE for large area sensitized solar cells. Such modified Grätzel type solar cells can be classified as "whispheronic solar cells".



MATERIALS AND METHODS Materials: Titanium (IV) iso-propoxide -Alfa Aesar (97 %), glacial acetic acid – Vetec (AR > 99.8 %), ethanol (AR-99.9 %), poly ethylene glycol 4-tert-octyl phenyl ether- Triton X100 (SRL), Degussa P25 TiO2 nanocrystallites, Cadmium oxide (CdO), Se powder, 1-octadecene (ODE; 90 %), oleic acid (OA), and trioctylphosphine (TOP), copper iodide (99.99 %; Alfa Aesar), indium acetate- (99.99 %; Alfa Aesar), docanethiol (98 %; Alfa Aesar), octadecene (90 %; Alfa Aesar), Acetonitrile (HPLC grade 99.8 %), butanol (> 99 %; Emplura), lithium iodide (LiI), Iodine, 4-tertiary butyl pyridine (TBP, 96 %; Aldrich), Poly ethylene glycol 4-tert-octyl phenyl ether (Triton X-100; SRL), and fluorine doped tin oxide (FTO) coated transparent glass (8 ohm/Sq, 2.2 mm thick), Ru N719 dye -Solaronix, acetonitrile (HPLC grade-99.8%) toluene (99.8 %; Sigma Aldrich). Synthesis of smooth TiO2 microsphere (Sµ µS) by solvothermal method:35 About 200 mL of ethanol was taken in a beaker and adjusted to pH 3 using glacial acetic acid. 0.25 moles of titanium (IV) iso-propoxide was added into the beaker and stirred vigorously to obtain a homogeneous solution. This solution was transferred to an autoclave and heated at 200 oC for 2 h. After the solvothermal treatment, the residual solvent in the reaction mixture was slowly evaporated to obtain a white powder. The white powder was washed with de-ionized water until it reached pH 7. The powder was then calcined at 450 oC for 3 h to obtain crystalline anatase TiO2 nanoparticles. Synthesis of fibrous TiO2 microsphere (Fµ µS) by hydrothermal method:26 0.1 g of Degussa P25 particle added into 60 ml of 10 M NaOH solution was ultrasonicated and stirred for 5 min each alternatively for 3 times. The solution was transferred to Teflon autoclave and 8 mL of hydrogen peroxide was added. The autoclave was maintained at 160 oC for 2 h. The white


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powder obtained was filtered and washed several times till the pH reached ~7. The powder was finally calcinated at 450 oC for 3 h to obtain crystalline TiO2 microsphere. Synthesis of CdSe quantum dots:68 CdSe QDs of different size with zinc blende structure were prepared using hot-injection technique

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and a larger size was chosen for device

fabrication. CdO of 0.51 g, 6.19 ml OA and 70 ml of ODE were taken for cadmium precursor solution. Then solution was heated to 250 °C under N2 atmosphere until a clear solution was obtained. Solution of Se was prepared separately by dissolving selenium powder (2 mmol) in 5 ml of TOP and 10 ml of ODE at room temperature. Se solution was injected to the hot Cd precursor solution and the temperature was maintained at 200 °C throughout the reaction. Synthesis of CuInS2 (CIS) quantum dots:70 In a typical synthesis of CIS QDs, 1 mmol (0.2919 g) of In(OAC)3, 1 mmol (0.1904 g) of CuI, 15 mL of 1-octadecene (ODE) and 5 mL of 1-dodecanethiol (DDT) were added to three-neck flask (100 mL) and stirred under N2 atmosphere for 30 min at 100 °C. The temperature of reaction mixture was kept at 210 °C, where the reaction of metal precursors with DDT leads to the formation of CIS and the color of reaction mixture gradually turned from colorless to yellow, orange, red, dark red and brown in that order as the reaction proceeded. A typical size of ~ 4.7 nm QDs was selected for device fabrication. The collected CIS-QDs samples were cooled down to room temperature and purified using acetone to remove excess solvent and unreacted precursor. The purified CIS QDs were dispersed in chloroform for further characterization. Fabrication of solar cell device: TiO2 paste of P25, FµS and SµS were prepared by mixing individual TiO2 powder, ethanol solvent and TritonX-100 binder with a mortar and pestle. TiO2 layers were coated on FTO/glass substrate by applying the paste using the doctor blade technique. For the fabrication of composite photoanode, TiO2 microsphere and P25



nanocrystallites in an optimized weight ratio were mixed together initially and then films were prepared by the above-mentioned method. These TiO2 layer was annealed at 450 °C for 3 h in air. Tiny TiO2 nanocrystals were also nucleated on these annealed TiO2 photoanode by immersing the films in 40 mM TiCl4 solution at 70 °C for 30 min and washing three times with water before annealing again at 450 °C for 30 min. The thickness of fabricated photoanode is ~ 20 µm and the area is ~ 0.25 cm2. The coating thickness and area were kept constant for all the solar cells fabricated. The solar cell devices were fabricated and the fabrication details of these devices are presented elsewhere.16, 35 Loading of sensitizer: After annealed TiO2 photoanode cooling to the room temperature dye loading is carried out by immersing the TiO2 layer immediately in a N719 dye (0.3 mM acetonitrile and butanol) for 20 h in dark for DSCs fabrication. To load CdSe QDs, electrophoretic bath method was adopted,71 where FTO/TiO2 was connected to positive terminal and a Platinum wire (Pt) was connected to negative terminal of DC source. The distance between electrodes was maintained at1 cm. A solution of QDs was prepared by dispersing the CdSe QDs in 40 ml of 1:1 toluene: acetonitrile solution. A DC voltage of 100 V was applied for 5 min followed with a rinse in toluene solution. This procedure was repeated for 5 cycles. Ligand exchange was performed by dipping the CdSe loaded photoanode in a MPA in methanol solution (10 mg/mL) for 1 h. CIS QDs were loaded using dip coating procedure. A turbid solution was obtained by the addition of purified DDT capped CIS-QDs in methanol (20 mL) solvent containing 0.5 mL of MPA. The solution becomes clear after continuous stirring for 12 h as the DDT replaces MPA on CIS QDs. CIS loading was carried out by dipping the TiO2 photoanode in a methanol dispersion comprising of MPA capped CIS quantum dots. The Pt is coated on FTO by DC-magnetron sputtering was used as counter electrode in the SSC device. An electrolyte


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containing iodide/tri-iodide redox couple (0.5 M of LiI, 0.05 M of I2 and 0.5 M 4-TBP in 3Methoxy propionitrile) was used. Characterization: Scanning electron microscopy (SEM) was obtained from a FEI Quanta 400 FEG electron microscope at operating voltage 30 kV. High-resolution transmission electron microscopy images were recorded using FEI Tecnai G2 T20 with an operating voltage of 200 kV. The bandgap of our samples were studied using diffuse reflectance spectroscopy (DRS) using an integrated sphere configuration in a PVE 300 (Bentham) quantum efficiency measurement system. The PL measurement were done using Horiba Jobin-Yvon (HR 800 UV) micro-Raman spectrometer using a 488 nm from Ar ion laser with a holographic grating of 1800/600 lines/mm. For 325 nm excitation source, He-Cd laser source with Horiba Jobin Yvon LabRAM HR Evolution Raman spectrometer coupled with an Olympus metalo-graphic microscope in configuration was used. The photovoltaic characterizations of devices were measured using a solar simulator with filter A.M 1.5 from Photo Emission Tech (Model # CT50AAA) fitted with a Xenon source (300 W). J-V characterization was done under illumination conditions using Keithley 2400 source meter which is connected PET solar simulator.

ASSOCIATED CONTENT Supporting information Available: Figures S1−S6. XRD patterns of TiO2, Size distribution histogram plot for SµS-TiO2, EPR and XPS spectral studies of SµS-TiO2 and P25-TiO2 photoanode, EIS spectral studies of all DSSC and QDSSC device and DRS of all TiO2 nanostructures.



ACKNOWLEDGEMENTS Authors acknowledges the Central Electron Microscopy Facility, IIT Madras for SEM and TEM studies and Prof. A. Subrahmanyam and Deepak Kumar, Semiconductor Lab, Dept. of Physics, IIT Madras for PL studies using He-Cd laser. C.S. acknowledges the support by DST-SERI through the grant no. DST/TM/SERI/2K11/113.


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Figures

Figure 1. (a) to (d) Field emission scanning electron microscope (FESEM) images and (e) to (h) bright field TEM images of TiO2 nanostructures. (a) and (e) P25-TiO2; (b) and (f) SµS-TiO2; (c) and (g) (SµS+P25)-TiO2 composite; (d) and (h) FµS-TiO2; Inset in (d) and (h) (FµS+P25)-TiO2 composite.



Figure 2. Photoluminescence spectra of TiO2 nanostructures of P25-TiO2 (Degussa), fibrous microspheres (FµS-TiO2) and smooth microspheres (SµS-TiO2). PL emission obtained using (a) 325 nm UV laser and (b) 488 nm laser excitation sources are shown. Whispering gallery modes (WGM) is seen only in SµS-TiO2. The insets show a representative optical micrographic image of SµS-TiO2 loaded on Si substrate. The PL emission spectra are collected from an isolated microsphere.


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Figure 3. (a) PL spectrum of P25-TiO2 nanoparticle and smooth microsphere (SµS-TiO2) excited using a 325 nm UV laser source. The PL emission from SµS-TiO2 show the WGMs standing out on the broad emission otherwise seen from TiO2 nanoparticles. (b) Expanded region of PL spectra shown in the dashed box in (a) along with simulated spectra. The microsphere size of r = 2.426 µm, and refractive index n= 2.65 are used to simulate the WGMs modes from SµS-TiO2 microspheres. (c) Schematic picture depicts the formation of whispering gallery modes (WGMs) in TiO2 microsphere photoanode.


ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 4. PL spectra of N719 dye coated TiO2 nanostructure of P25-TiO2 (Degussa), fibrous microspheres (FµS-TiO2) and smooth microspheres (SµS-TiO2); PL emission obtained using (a) 325 nm UV laser excitation source, and (b) using 488 nm laser source are shown. The emission spectra of dye superimpose with the WGMs. The insets in Figure 4a & 4b show the optical image of dye loaded SµS-TiO2 on Si substrate. The PL emission spectra are collected from an isolated microsphere marked in white dot. Figure 4c & 4d are the PL spectra of CdSe QD and CIS QD loaded SµS-TiO2 dispersed on Si substrate respectively and the spectra were collected with source of 488 nm excitation line from Ar-ion laser. All these PL spectra were collected from the samples dispersed on Si substrate. 32 ACS Paragon Plus Environment

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Figure 5. J–V characteristic curves of (a) DSSC; (b) QDSSCs based on CdSe QDs and (c) QDSSCs based on CIS QDs made of TiO2 nanostructure of P25-TiO2 (Degussa), Fibrous microspheres (FµS-TiO2), Smooth microspheres (SµS-TiO2) along with their composite (FµS+P25)-TiO2 and (SµS+P25)-TiO2 under AM 1.5 G simulated full sunlight (100 mW/cm2) illumination.



Figure 6. The device statistics parameters such as JSC (a, e & i), FF (b, f & j), VOC (c, g & k) and efficiency (d, h & l) of DSSC (a, b, c & d), CdSe-QDSSC (e, f, g & h) and CuInS2 (i, j, k & l). The J-V characteristics of best performing devices are shown in Figure 5. The filled diamond in red color is the mean and the middle horizontal line in the box indicates the median of the data. The box range is selected as standard deviation and the vertical whisker line connected with maximum and minimum value of the data points. The cross symbols (⨉) on the whisker line are the number of devices tested for the respective photoanode.


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Table 1: The characteristics parameters such as current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (η) of best DSSCs and QDSSCs devices made of P25, FµS, SµS, FµS+P25 and SµS+P25 TiO2 photoanode.

DSSCs TiO2

Jsc

Voc 2

Photoanode (mA/cm )

CdSe-QDSSCs FF

PCE

Jsc

Voc 2

FF

CIS-QDSSCs PCE

(mV)

(%)

(%)

(mA/cm )

(mV)

(%)

(%)

Jsc

Voc

FF

PCE

(mA/cm ) (mV) (%)

(%)

2

P25

15

758

65

7.2

3.4

681

63

1.4

4.5

758

61

2.1

FµS

13.9

765

65

6.3

3

681

64

1.3

3.3

765

66

1.6

FµS+P25

16

728

63

7.1

3.5

667

66

1.5

4.8

773

68

2.5

SµS

17.4

766

65

8.4

4.1

669

65

1.8

4.6

840

85

3.2

SµS+P25

17.6

778

66

8.9

5.6

672

71

2.7

6.2

926

66

3.8



Graphical Abstract


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ACS Applied Energy Materials - ACS Publications - American

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