Optical Whispering Gallery-Enabled Enhanced Photovoltaic Efficiency

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C: Energy Conversion and Storage; Energy and Charge Transport

Optical Whispering Gallery Enabled Enhanced Photovoltaic Efficiency of CdS-CuInS Thin Film Sensitized Whisperonic Solar Cells 2

Perumal Ilaiyaraja, Tapan Kumar Das, Pavana S.V. Mocherla, and Chandran Sudakar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09292 • Publication Date (Web): 01 Jan 2019 Downloaded from http://pubs.acs.org on January 1, 2019

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The Journal of Physical Chemistry

Optical Whispering Gallery Enabled Enhanced Photovoltaic Efficiency of CdS-CuInS2 Thin Film Sensitized Whisperonic Solar Cells P. Ilaiyaraja, Tapan Kumar Das, Pavana S.V. Mocherla and C. Sudakar* Multifunctional Materials Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai-600036, India

Abstract: Composite photoanode comprising of mesoporous microspheres with smooth spherical morphology exhibiting high light scattering due to optical whispering gallery modes (WGM) is used to fabricate whisperonic solar cell (WSC) devices. The photoanode is sensitized with CdSCuInS2 thin films (CdS-CIS-TF) of ~5 nm thickness. CdS-CIS-TF sensitized photoanodes exhibit strong coupling with WGM. These WSC devices show an average (avg) efficiency () of  3.2% in comparison to avg  1.9% for nanoparticulate-based photoanode. The observed efficiency is the highest for CdS-CIS-TF sensitized solar cells made using I −/I3− electrolyte. This remarkable increase in avg (~ 60%) is attributed to increased photon absorption by the sensitizer films due to the presence of WGM scattering prevailing in SS microspheres. Thus, WSC photoanode configuration is a promising approach to enhance the efficiency of thin film sensitized solar cells.

* Corresponding Author: [email protected]: Ph: +91-44-22574895

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1. INTRODUCTION CuInS2 (CIS), a less-toxic chalcopyrite semiconductor, is an ideal sensitizer for capturing sunlight owing to its high absorption coefficient (105 cm−1) and appropriate direct bandgap (Eg ~ 1.53 eV) required for solar cell application. Tunable bandgap of CIS also offers the maximum achievable efficiency as predicted by the Shockley-Queisser limit.1-3 CIS semiconductor is used either in quantum dot or thin film form for fabricating CIS based solar cell.1-4 Thin film absorbers are most commonly used in solar cells for their high efficiencies.5 In particular, CIS thin film (CIS-TF) solar cells with pn-junction architecture are studied extensively.6-7 For example, CIS-TF solar cell in the configuration Glass/Mo/CIS/CdS/ZnO/Au has shown efficiency of 11.4%.8 Thin film based solar cells can be fabricated by adopting different CIS-TF synthesis methods such as chemical vapor deposition,9 spray pyrolysis technique,7 rapid thermal processing,8 atomic layer deposition,10 sulfurization of metallic precursors,11 co-evaporation,5 electro deposition,12-13 ion layer gas reaction,14 etc. However, these methods of preparing the CIS-TF solar cell are expensive and difficult to make in large areas. Similar techniques can be used to fabricate CIS-TF sensitizer on TiO2 photoanode used in Grätzel type solar cells.15 However, much cheaper chemical routes are better and cheaper for fabricating Grätzel type solar cells. During the last two decades, Grätzel type based CIS-TF sensitized solar cell configurations are being explored by depositing extremely thin absorber (ETA), 10 to 30 nm thick CIS, on a morphologically different TiO2/ZnO photoanodes. Grätzel type structure of FTO/dense ZnO layer/CIS/Au solar cell reported by Lee et al. has shown efficiency close to 7%.6 However, it should be noted that such superstrate photovoltaic devices used interpenetrated radial p-n junctions across nanostructured CdS/ZnO nanorod arrays.6 Synthesis techniques such as chemical bath deposition,16 successive ionic layer adsorption and reaction (SILAR)17 and mild 2 ACS Paragon Plus Environment

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solvothermal routes18-19 have been adopted to reduce the cost in preparing these sensitized solar cells. Yet, the efficiency of CIS-TF sensitized Grätzel type solar cell is far less (only 40%) than that of the pn-junction solar cell. Therefore, in addition to developing approaches for low cost synthesis of CIS-TF absorber, the main challenge remains to be addressed is achieving highly efficient TiO2/ZnO photoanode film. The phase purity, size and morphology of TiO2/ZnO photoanode highly influence the efficiency of solar cells.20 Photoanode films with large surface area, effective light scattering and good connectivity are also found to yield better efficiency in sensitized solar cells.21 Tailoring light scattering is a very crucial component in the strategies used to enhance the efficiency of sensitized solar cells.22 Monolayers of sensitizer molecules or quantum dots or thin layers on the surface of TiO2 photoanode should frequently encounter the incident light in order to effectively capture the photons and generate electron–hole pairs. However, thin sensitizer and transparent or highly absorbing photoanode with negligible light scattering and low optical extinction coefficient, would significantly limit the power conversion efficiency of sensitized solar cells (SSC). In order to enhance the efficiency of SSC, large particles are incorporated into the nanocrystalline film either in double layer or composite structure which enhances efficient light harvesting by scattering effects. Larger sized microstructures, with the size comparable to the wavelength of incident light, are preferred as it exhibit Mie scattering and thus enhances the light scattering in SSCs. Whispering gallery modes (WGM) exhibiting optical cavity resonators (like the spherical TiO2 particles in the present study) is a special kind of Mie scatterer where the total internal reflection in the cavity modulates the light so that resonant modes are emitted in the scattered light. TiO2 smooth microsphere (SS-TiO2) act as dielectric resonator with multiple total internal reflection giving rise to resonant WGMs. Thus, photoanode with WGM show more



light scattering. In photoanode comprising of micron-sized oxide particles, the incident light is coupled to the sensitizer through this WGM light scattering. This coupling, which is shown for the first time to be present in our system is a direct evidence that WGM scattered light are absorbed by the sensitizer. WGM resonators with strong light confinement and long photon lifetime enhances effective energy exchange between WGM scattered light and the surrounding sensitizer as compared to normal light scattering. Further, WGM enhances light scattering effects of most intense red light in the solar spectrum thus enhancing the photoresponse. Such WGM scattering is generally seen in the case of microspheres,26 nano-rods,27 nano-tubes,28 and photonic crystals.29 The power conversion efficiency of solar cells with WGM structure incorporated photoanodes are being explored and are shown to significantly enhance when photoanodes are made of WGM structures. In the manuscript by Yang Wang, et al,30 an increase in PCE of 29.4 % was shown when PSC is made with light trapping structures which exhibit whispering gallery modes. The trapping WG structures were made directly on the perovskite layer by a simple imprint process using a robust microstructure stamp. In another report by Yang Wang, et al,31 PSC were shown to exhibit ~18% higher PCE compared to the pristine solar cells when light trapping periodic micronanodiffraction-grating structures were used on the active perovskite layer. Similarly, incorporating a bio-inspired light-trapping structure mimicking the moth-eye into the metal back electrode via soft nanoimprint lithography has been shown to remarkably enhance the light harvesting in PSCs.32 In our earlier studies we reported composite TiO2 photoanode films made of well-connected micron-sized smooth sphere (SS) and P25 nanoparticles (80:20 ratio of SS and P25) that exhibit WGMs. TiO2 smooth microsphere (SS-TiO2) act as dielectric resonator with multiple total internal reflection giving rise to resonant WGMs in composite photoanode.23-


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25

The enhanced light scattering from such structures improves the absorption of the sensitizers.

Enhanced power conversion efficiency (PCE) in DSSC and QDSSC devices made from SμSTiO2 microspheres are shown to mainly arise by WGM assisted efficient light absorption.23-24 Such whisperonic solar cell configuration can also be extended to extremely thin absorber layer coated photoanode based photovoltaic devices. In the present work, CdS-CIS thin film sensitized whisperonic solar cells were fabricated by loading CdS-CIS-TF on (SS+P25)-TiO2 composite photoanode using nanocrystal layer deposition (NCLD)33 and successive ion layer adsorption reaction (SILAR)34 methods. Photovoltaic properties of CdS-CIS-TF sensitized on SS+P25 (80%+20%) composite photoanode film are studied and compared with the efficiency obtained from P25-TiO2 based photoanode. Whispering gallery modes in SμS-TiO2 being a prominent Mie scattering phenomenon, lead to higher light scattering and couple strongly with the CdS-CIS-TF sensitizer. A maximum efficiency of 4.3% for CdS-CIS-TF based thin film sensitized whisperonic solar cells is achieved. This measured efficiency is higher than the values reported earlier for CdSCIS-TF sensitized solar cells using I−/I3− liquid electrolyte.

2. EXPERIMENTAL SECTION Synthesis of SS TiO2 microsphere Anatase TiO2 microsphere with smooth morphology (SS) is synthesized by solvothermal following the procedure given in our previous work.21 The synthesis details of SS-TiO2 microspheres and characterization techniques used are given in the supplementary material.



Fabrication of TiO2 photoanode TiO2 paste was prepared by mixing P25 nanocrystallite TiO2 powder, ethanol solvent and TritonX-100 binder with a mortar and pestle. TiO2 films were fabricated by applying the paste on FTO/glass substrate using doctor blade technique and then annealed at 450 C for 3 h in air. These annealed TiO2 films were immersed in 40 mM TiCl4 solution at 70 C for 30 min to form tiny TiO2 nanocrystals on TiO2 films and again annealed at 450 C for 30 min. 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 following the same procedure mentioned above. The thickness of fabricated photoanode is ~ 20 m with the area of ~ 0.25 cm2. The coating thickness and area were kept constant for all the solar cells fabricated. CdS-CIS thin film sensitization on TiO2 photoanode The CdS buffer layer was coated on composite TiO2 photoanode by adopting the reported nanocrystal layer deposition method (NCLD).33,

35

TiO2 photoanodes were placed in an equi-

molar (20 mM) mixture of CdCl2 and thioacetamide aqueous precursor solution for 75 min at room temperature. After the NCLD process CdS-TiO2 photoanode was washed several times with deionized water and dried in air. A molecular precursor solution comprising of CuI (0.1429 g), In(OAc)3 (0.2428 g), triethyl amine (4 ml) and acetic acid (2 ml) in 20 ml of ethanol was used to deposit a CIS thin film by SILAR method.34 This ethanol-based precursor yielded homogeneously dissolved metal precursor solution. Cu:In:S ratio was adjusted to 0.9:1:2.5 to obtain Cu deficient CIS films. CdS-TiO2 photoanode was immersed in the metal precursor for 30 s followed by an additional 30 s immersion in thioacetamide solution (0.156 g in 10 mL of ethanol). Between each immersion the photoanode was dipped in ethanol solution to avoid multilayer adsorption. The samples were then placed on a 150 °C preheated hot plate kept in air 6 ACS Paragon Plus Environment

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ambient for 10 min. An additional heat-treatment at 250 °C in air for 10 min using another preheated hot plate is made immediately after the initial heating. Finally, the samples were allowed to naturally cool to room temperature in the same air ambient. In order to obtain an entirely infiltrated uniform coating the above-mentioned coating-drying-annealing process was repeated three times. Fabrication of CdS-CIS-TF sensitized solar cell Modified Grätzel type whisperonic solar cells were fabricated by assembling CdS-CISTF loaded composite photoanode (SS+P25)-TiO2 with the Pt coated FTO counter electrode. The gap between two electrodes was filled with iodide/tri-iodide (I-/I3-) electrolyte (0.5 M of LiI, 0.05 M of I2 and 0.5M 4-TBP).

3. RESULTS AND DISCUSSION We fabricated (SS+P25)-TiO2 composite photoanode with optimized weight ratio of 80:20 (SS:P25).21 Figure 1a shows the XRD patterns of (i) (SS+P25)-TiO2, and (ii) CdS-TF and (iii) CdS-CIS-TF sensitized (SS+P25)-TiO2. The XRD pattern of SS+P25 clearly shows the anatase form of TiO2. The diffraction peaks of CdS and CIS are not discerned in sensitized photoanodes due to very thin layer on the TiO2 surface (Figure 1a). Only diffraction peaks corresponding to TiO2 and FTO substrates are seen in all the patterns. Raman spectra of (SS+P25)-TiO2 photoanode and the same sensitized with CdS-TF and CdS-CIS-TF are shown in Figure 1b. The vibrational modes at 144 (Eg), 195 (Eg), 395 (B1g), 516 (A1g) and 639 (Eg) cm-1 observed in the composite photoanode are characteristic of anatase TiO2. These modes are seen in all the three samples. When CdS-TF alone was coated on the composite photoanode, we could see the LO1 and LO2 phonon peaks positioned at 305 cm-1 and 608 cm-1, respectively. With CIS7 ACS Paragon Plus Environment


CdS-TF sensitized photoanode, the Raman peaks of CIS (~ 282 corresponding to symmetric A1g modes) are not discerned clearly. The Raman spectrum of CIS is known to have very poor scattering cross section. Thus the modes merge with peaks from CdS (305 and 608 cm-1 corresponding to LO modes and overtone of LO mode) and TiO2 (395, 516 and 639 cm-1) leading to broad indistinguishable Raman modes (Figure 1b).

Figure 1. (a) XRD and (b) Raman spectra, (c) Absorption spectra and (d) Tauc plots of (i) (SS+P25)-TiO2 composite (ii) (SS+P25)/CdS-TF and (iii) (SS+P25)/CdS-CIS-TF. Representative photographic images of (i), (ii) and (iii) are shown in (e)


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Panel C

Figure 2. Electron microscopy characterization: Panel A is FE-SEM image of (a) CdS-CIS-TF loaded (SS+P25) composite photoanode. The panel A from (b) to (h) correspond to the EDS elemental mapping of (b) Cu Kα, (c) Cd Lα, (d) In Lα, (e) Ti Kα, (f) S Kα and (g) O Kα from CdS-CIS-TF loaded composite (SS+P25)-TiO2 photoanode from the same region. Panel B is bright field TEM micrographs of (h) CdS-CIS-TF loaded (SS+P25)-TiO2 composite and (i) shows a magnified region from (h) depicting the CdS-CIS coating on TiO2 nanoparticles. Panel C shows a representative HRTEM lattice image depicting the polycrystalline CdS-CIS thin film coating. Representative photographic pictures of pristine TiO2 photoanode and the same coated with CdS-TF and CdS-CIS-TF are shown in Figure 1e. In spite of the coating rendering visibly clear color changes to the TiO2 photoanode, discerning the structural information of these coatings from both XRD and Raman is very challenging. Figure 1c shows the absorption spectra of CdS-TF and CdS-CIS-TF sensitized composite photoanodes along with the pristine (SS+P25)-TiO2 photoanode. Corresponding diffuse reflectance spectra are shown in supplementary Figure S1. Distinct absorption edges due to CdS and TiO2, and the red shift of absorption edge in the case of CdS-CIS-TF coated TiO2 clearly indicate the formation of sensitized layer over photonaode. The band gap of CdS buffer layer and CdS-CIS-TF sensitizer 9 ACS Paragon Plus Environment


were estimated from the Tauc plots obtained from the spectra (Figure 1d). The bandgaps (Eg) are estimated by extrapolating the linear region of the absorption edge in the Tauc plot. The Eg values estimated for TiO2, CdS and CIS are found to be 3.2, 2.4 and 1.75 eV, respectively. These values are consistent with reported values.20, 36-37 The microstructural studies and elemental mapping of CdS-CIS-TF loaded (SS+P25)TiO2 photoanode were carried out using field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). CdS-CIS-TF coating on (SS+P25)-TiO2 photoanode could not be discerned from SEM micrograph. However, EDS elemental mapping carried out using nanoprobe electron beam on these microstructures indicate uniform distribution of CdS-CIS on composite photoanode (Figure 2, Panel A). The cross section SEM images of bare and CdS-CIS-TF coated SS+P25 composite photoanode is shown in Figure S2. The well connected compact packing of SS TiO2 microsphere and P25 in clearly discerned from cross section SEM images of bare SS+P25 composite photoanode (Figure S2a). The CdS-CIS-TF coating uniformly covers the surface of SS+P25 composite photoanode (Figure S2b). The EDS mapping confirm the presence CdS-CIS-TF layer on SS+P25 composite photoanode (Figure S2 c-h). Further, Panel B of Figure 2 shows the HRTEM images of CdSCIS-TF loaded particles in the composite photoanode. The surface regions clearly depict distinct contrast from the CdS-CIS-TF coating over TiO2 nanoparticles in the composite. CdS-CIS form a uniform coating with an average thickness of ~ 2 to 5 nm over the surface of TiO2 nanoparticles in the (SS+P25)-TiO2. A representative high resolution lattice image of CdS-CIS thin film coating on TiO2 is shown in Figure 2, Panel C. As can be discerned from the image the coatings exhibit lattice fringes indicating the polycrystalline nature of the CdS-CIS layers.


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The composite photoanode films exhibit efficient electron transport due to wellconnected nature of the microspheres and better light harvesting efficiency due to the whispering gallery modes.23-24 FESEM studies show the (SS+P25)-TiO2 photoanode with clearly distinct microstructural features of dispersed nanocrystalline P25-TiO2 particle along with average size of 2.61 micron-sized spherical SS-TiO2 particles (Figure S3a). These SS-TiO2 microspheres themselves are made of nanoparticles with average size ~ 25 nm (Figure S3d) and connected to each other by a broad strong neck of width ranging between 0.5 and 2 µm (Figure 3 and S3). The P25 nanoparticles cover the microsphere uniformly, filling up the voids between the microspheres, thereby increasing the packing fraction of the film (Figure 3c). This further improves the connectivity of the microspheres and enhances the electron transport significantly. In addition, SS-TiO2 microspherical particles in (SS+P25)-TiO2 photoanode film possess whispering gallery modes, which enhance the light absorption of sensitizer materials (Figure 3d). Light scattered by the micron-sized spheres resonate with the incident wave when they are in phase, giving rise to sharp peaks in the photoluminescence (PL) spectra of photoanode (Figure 3d). These multiple sharp peaks are collectively referred to as whispering gallery modes (WGM). A schematic depicting the formation of WGM in an optically resonating TiO2 microsphere is shown in Figure 3d (inset). PL spectra of SS-TiO2 measured using 325 nm He-Cd laser sources consists of two major broad peaks, one ~ 425 nm and another ~ 520 nm, similar to that found in PL spectra of P25-TiO2 (Figure 3d). These broad PL spectral responses are attributed to nearband emission, self-trapped excitons, oxygen vacancies and surface defect states.36, 38 In addition to these features, PL spectra of SS-TiO2 exhibit multiple sharp emissions overlapped on the broad peaks. The occurrence of WGM in microspheres and other microstructures is known in the literature.23-24,

26, 39

We observed WGM in micron-sized smooth spheres made of compactly 11 ACS Paragon Plus Environment


packed TiO2 nanoparticles. These WGM resonances in SS-TiO2 were shown to enhance the light absorption in the sensitizer with associated increase in PCE of dye/quantum dots (CdSe and CIS QDs) sensitized whisperonic composite photoanode.23-24, 36 The PL emission characteristics of individual micron sized sphere of SS-TiO2 are found to be several fold stronger than the PL emission from nanoparticular TiO2. Further when these microspheres are coated with sensitizers, it strongly couples with the emission characteristics of the sensitizer. PL spectra of CIS sensitizer loaded photoanode were collected using 488 nm Ar-ion laser source. CIS-TF coated P25 and SS-TiO2 photoanodes show the PL emission at ~740 nm corresponding to the near-band-edge emission (NBE) of CIS-TF. We observed WGM very distinctly on the top of NBE of CIS-TF (Figure 3e) for CIS-TF/SSTiO2 compared to CIS-TF/P25-TiO2 photoanode. The presence of WGM in the CIS-TF sensitizer coated SS-TiO2 based photoanode indicates an efficient coupling between sensitizer and SS-TiO2 WGM resonance.23 These PL spectra were collected from well-separated microsphere dispersed on the Si-substrate (see inset, Figure 3e). Figure 3f shows the PL emission from CIS-TF, CdS-TF and CdS-CIS-TF coated on composite photoanode used in the fabrication of solar cell. In spite of densely packed TiO2 microsphere-nanoparticle layer, the PL emission from (SS+P25)-TiO2 photoanode coated with sensitizers shows WGM overlapping with the regular broad PL emission (Figure 3f). The coupling of WGM with the sensitizers is seen to exist over the wide range of visible spectrum. These observations from PL studies on thin film sensitized whisperonic solar cell (WSC) photoanode suggest efficient absorption and charge separation in the solar cell devices.


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Figure 3. FESEM images of (a) SS, (b) CIS thin film (CIS-TF) loaded SS and (c) CIS-CdSTF loaded (SS+P25) composite photoanode. PL spectrum of (d) smooth microsphere SμS-TiO2 and P25-TiO2 nanoparticles excited using a 325 nm UV laser source. Schematic shown as inset in (d) depicts the formation of whispering gallery modes (WGM) in TiO2 microsphere photoanode. (e) PL spectra of isolated microspheres of SμS-TiO2 loaded with CIS-TF. For comparison PL spectra obtained from P25-TiO2 loaded with CIS-TF is shown. (f) PL spectra of bare SμS/P25-TiO2 (i) and CdS-TF (ii), CIS-TF (iii), CdS-CIS-TF (iv) loaded (SμS+P25)-TiO2 composite photoanode. PL spectra given in (e) and (f) were measured by exciting the samples with 488 nm Ar-ion laser source. PL spectra obtained from isolated microspheres, given in (d) for SμS and in (e) for SμS/CIS-TF, were collected by dispersing the microspheres on Si substrate. Inset in (e) shows one such isolated microspherical particle.



The photovoltaic properties of CdS-CIS-TF sensitized on P25 and (SS+P25)-TiO2 photoanodes are studied under 1 sun illumination. The maximum photovoltaic efficiencies of CdS-CIS-TF sensitized solar cells made using P25 and composite photoanodes are found to be 2.9% and 4.3% (Figure 4). The open circuit voltages VOC for these devices are found to be 689 and 604 mV respectively (Table 1). Higher VOC ~ 689 mV is obtained for CdS-CIS-TF sensitized on P25 photoanodes. Despite this, a large increase of JSC (to ~11.4 mA/cm2) in CdSCIS-TF sensitized (SS+P25)-TiO2 composite microsphere photoanode leads to higher  (%), compared to CdS-CIS-TF sensitized on P25 (Figure 4a). The photovoltaic parameters measured from several solar cells tested using CdS-CIS-TF sensitized on P25 and (SS+P25)-TiO2 photoanodes are summarized in Figure 5a to 5d. The average efficiency (avg) values of the P25 and (SS+P25)-TiO2 photoanodes are 1.9% and 3.2% respectively. While the fill factor and open circuit voltage show similar range of values for both sets of devices, a large increase in the short circuit current is witnessed for the photoanodes comprising of microspheres. The amount of CdS-CIS sensitizer loading on Degussa P25 and (SS+P25)-TiO2 photoanode from weight gain measurements is estimated to be 0.68 mg and 0.40 mg/cm2 (Table 1), respectively. The CdS-CISTF loading on (SS+P25)-TiO2 is found to be lesser compared to that on P25. Nevertheless, the solar cells made from composite photoanode exhibited enhanced efficiency (avg) of ~ 60 %. Such an enhancement in the efficiency can be attributed to presence of WGM and better electron transport in the (SS+P25)-TiO2 composite photoanode. WGM increases the optical path length of the incident light in the photoanode thereby increasing the light interaction with CdS-CIS-TF, which in turn increases the light harvesting efficiency. Also, SS particles remain well connected even after photoanode fabrication, which helps in directed charge transport in photoanode.


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In order to better understand the photovoltaic performances the incident photon to current conversion efficiency (IPCE) spectra were measured for P25/CdS-CIS-TF and SS+P25/CdSCIS-TF devices are shown in Figure 4b. The IPCE spectra display a sharp maximum around 350 nm, a characteristic feature of TiO2, and a broad feature between 390-750 nm from the sensitizer CdS-CIS-TF (Figure 4b). Both P25/CdS-CIS-TF and SS+P25/CdS-CIS-TF devices show similar IPCE profiles, however, significant difference in the intensity between 390 and 750 nm are evidenced. The composites SS+P25/CdS-CIS-TF devices exhibit higher IPCE than the P25/CdS-CIS-TF device in this range of 390 to750 nm. The higher IPCEs in the composite structures could be attributed to the efficient light absorption due to WGM in SS+P25 TiO2 photoanode. The increased IPCEs, especially in the wavelength ranging from 390 nm to 750 nm, and the corresponding enhancement in the photovoltaic performance of SS+P25/CdS-CIS-TF system also indicate that the generated electrons in the overlayer are collected efficiently. Thus the improved IPCE in the SS+P25 TiO2 photoanode are attributed to the combined effect of increase light absorption and effective charge collection in the photoanode. We also calculated Jsc by integration IPCE curve and we find that Jsc estimated from IPCE compares with the data from J-V curves (Figure S4). The stability of solar cell fabricated using TiO2 composite photoanode sensitized with CdS-CIS thin film, FTO/SS+P25/CdS-CIS-TF device, was also studied for period of 10 days with the photovoltaic measurements recorded for every 2 days (Figure S5). Solar cells with proper sealing made using a small piece of hot-melt polymer was used for this study. We observed that SS+P25/CdS-CIS-TF device efficiency is almost constant with small deviation around 0.2 % when measured over a period of ten days. Thus, we strongly believe that CdS-CIS-TF devices will exhibit longer stability.



Electronic and ionic conduction process of sensitized solar cell is studied using electrochemical impedance spectroscopy (EIS). The Nyquist plots of CdS-CIS-TF sensitized solar cells made from P25 and composite (P25+SS) photoanodes are given in Figure 4c. The Nyquist curves could be fitted to two semicircles one in the high-frequency (>1 kHz) and the other in the low-frequency region (10−100 Hz). The semicircles reveal the interfacial chargetransfer resistances of the cells. The semicircles are modeled with RC circuit with the electrical elements resistance (R) and constant phase element (CPE) describing the interfacial properties, internal resistance, and charge-transfer kinetics. The experimental data were fit with simulated curves using the equivalent circuits shown in inset of Figure 4c. From the fit the contact resistances (RS), resistances at the counter electrode/electrolyte interfaces (RCT) and at the TiO2/sensitizer-TF/electrolyte interface (RCE) are obtained. The contact impedance between FTO/Pt (RS) is 19 Ω and 32 Ω (Table 1) due to use of identical counter electrodes. The charge transfer resistance (RCT) between the counter electrode-electrolyte interface (mostly observed in the high frequency regime, i.e. ~104 Hz) shows that the interfacial resistances are 428 Ω and 810 Ω for the composite and P25 photoanode.


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Figure 4. (a) J-V curves, (b) Incident photon to current conversion efficiency for devices made of CdS-CIS-TF loaded P25 and (SS+P25)-TiO2 composite photoanode, and (c) Electrochemical impedance spectroscopy (EIS) of CdS-CIS-TF loaded P25 and (SS+P25)-TiO2 composite photoanode.


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

Figure 5. The J-V statistics parameters such as (a) efficiency, (b) JSC, (c) FF and (d) VOC for five measured devices. 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. (a) The photovoltaic parameters such as short-circuit current density (JSC), open circuit voltage (VOC), fill factor (FF) and efficiency (), extracted from the best performing cells, and (b) EIS fitting parameters extracted from the impedance spectra of CdS-CIS-TF loaded on P25 and (SS+P25) composite TiO2 photoanodes. The interfacial capacitance Cint corresponds to the interfacial résistance Rct. Electron life time is calculated from Bode phase plot. The two rows of data given in each column refer to the average values with standard deviation (top row) and champion data (bottom row) measured over five sets of devices tested. (a) J-V characteristic of CdS-CIS-TF loaded on different TiO2 photoanode Amount of CdSCIS loading on JSC TiO2 VOC (mV) FF (%) ɳ (%) (mA/cm2) Photoanode (mg/cm2) P25 Avg. 5.61.5 60367 5712 1.90.5 0.530.1 Best 5.9 689 71 2.9 0.68 SS+P25 Avg. 9.21.9 63432 582 3.20.8 0.340.05 Best 11.4 604 64 4.3 0.40 (b) EIS characteristic of CdS-CIS-TF loaded on different TiO2 photoanode CdS-CIS-TF Sensitized Photoanode

CdS-CIS-TF Sensitized Photoanode P25

Avg. Best SS+P25 Avg. Best

RS (Ὠ)

RCE(Ὠ)

RCT(Ὠ)

Cint (×10-6F)

e (ms)

266 19 273 32

52171 527 12151 108

792176 810 429146 428

2.81.9 3.5 7740 63

43.56 41.9 155 12.5

Thus, RCT is found to be half the magnitude lesser in the composite samples. The complex impedance at the interface of TiO2/CdS-CIS/electrolyte redox couples (RCE) shows the values of 527 and 108 Ω for P25-TiO2 and composite samples. The resistances of composite photoanodes are smaller than that of the individual solar cell. This indicates that the composite photoanode films help in easy electron transfer at this interface (Figure 4d). A decrease in the interfacial resistance is understood from the significant change in RCE for composite photoanodes due to the better connectivity between microspheres through nanoparticles. This clearly implicates the facilitation of efficient electron transport in composite samples. The Bode phase



plot (Figure S6) presents the characteristic frequency values during the charge transfer processes. The lifetime of the electrons, τe=1/(2πfmax) where fmax is the maximum frequency of the peaks, is estimated from the characteristic frequencies of the impedance semicircle at medium frequencies. The fmax of the composite photoanode films is found to be slightly lower when compared to the fmax observed for the anode made of P25-TiO2. The shift in the characteristic frequency from a low to high value and the lower electron life time reveals a much faster carrier transport process in the composite photoanode. 4. CONCLUSION We fabricated whisperonic solar cells, a modified Grätzel type sensitized solar cell by loading CdS-CIS-TF sensitizer on photoanode made of P25 nanocrystallites and composite (SS+P25)-TiO2 microspheres. The photovoltaic properties of solar cells made using CdS-CISTF sensitized SS+P25 composite film yielded maximum efficiency of 4.3 %. On the other hand, ~ 2.9 % was obtained for P25-TiO2 based photoanode. The presence of WGMs in the photoanode strongly couple with the sensitizer and significantly increases light absorption in the thin film sensitized solar cells. Thus, WGM in the microsphere resonator enable increased light absorption in the sensitizer and good electron transfer in well-connected TiO2 microsphere enhances the power conversion efficiency (η %). Electrochemical impedance studies suggest efficient charge separation and transportation in the composite mixtures in addition to the decrease in the interfacial resistance facilitating an efficient electron transport. Thus, we conclude that composite photoanodes exhibiting whispering gallery modes are highly suitable for sensitized solar cells compared to bare nanocrystaline P25-TiO2.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details on the synthesis of TiO2 microsphere (SS-TiO2), characterization techniques, diffuse reflectance spectra, cross-sectional SEM images of bare SS+P25 photoanode and SS+P25/CdS-CIS-TF device. FE-SEM and TEM images of SS-TiO2 photoanode and Jsc estimated from the IPCE plots and Bode plots of the solar cell device are given in supporting information. AUTHOR INFORMATION Corresponding Author * Email: [email protected] ORCID: C. Sudakar: https://orcid.org/0000-0003-2863-338X Notes The authors declare no competing financial interest.

Acknowledgement C.S.

acknowledges

the

support

by

DST-SERI

through

the

Grant

no.

DST/TM/SERI/2K11/113. PI acknowledges the DST-National postdoctoral fellowship through the Grant no. PDF/2016/000461. Author’s thank Mr. Vikas Sharma for helping in doing the cross sectional SEM images.



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Graphical Abstract


Optical Whispering Gallery-Enabled Enhanced Photovoltaic Efficiency

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