A synergetic approach of employing smooth mesoporous TiO2 microsphere (SμS-TiO2)-nanoparticulate TiO2 (np-TiO2) composite photoanode, and size and defect controlled CdSe quantum dots (QD) to achieve high efficiency (η) in a modified Grätzel solar cell, quantum dot sensitized whisperonic solar cells (QDSWSC), is reported. SμS-TiO2 exhibits whispering gallery modes (WGM) and assists in enhancing the light scattering. SμS-TiO2 and np-TiO2 provide conductive path for efficient photocurrent charge transport and sensitizer loading. The sensitizer strongly couples with the WGM and significantly enhances the photon absorption to electron conversion. The efficiency of QDSWSC is shown to strongly depend on the size and defect characteristics of CdSe QD. Detailed structural, optical, microstructural and Raman spectral studies on CdSe QD suggest that surface defects are prominent for size ~2.5 nm, while the QD with size &gt; 4.5 nm are well crystalline with lower surface defects. QDSWSC devices exhibit an increase in η from ≈0.46% to η ≈ 2.74% with increasing CdSe QD size. The reported efficiency (2.74%) is the highest compared to other CdSe based QDSSC made using TiO2 photoanode and I-/I3 - liquid electrolyte. The concept of using whispering gallery for enhanced scattering is very promising for sensitized whisperonic solar cells. © 2018 The Author(s).

Sudakar Chandran

Department Of Physics

Tapan Kumar Das

Perumal Ilaiyaraja

Fulltext

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Scientific Reports

Composite photoanode comprising mesoporous microspheres with a smooth spherical morphology exhibiting high light scattering due to optical whispering gallery modes (WGMs) is used to fabricate whisperonic solar cell (WSC) devices. The photoanode is sensitized with CdS-CuInS 2 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 (η avg ) efficiency (η) of ≈3.2% in comparison with η avg ≈ 1.9% for the nanoparticulate-based photoanode. The observed efficiency is the highest for CdS-CIS-TF-sensitized solar cells made using I - /I 3 - electrolyte. This remarkable increase in η avg (∼60%) is attributed to increased photon absorption by the sensitizer films because of the presence of WGM scattering prevailing in smooth microspheres. Thus, WSC photoanode configuration is a promising approach to enhance the efficiency of TF-sensitized solar cells. © 2019 American Chemical Society.

Journal of Physical Chemistry C

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

Environmental deterioration and depletion in conventional energy resources greatly demand the need for photovoltaic devices, which use solar radiation to meet future energy demands. Efficient light management plays a pivotal role in improving the performance of photovoltaic devices. Various avenues have been explored to address light management in solar cells. Employing whispering gallery mode (WGM) microresonators in solar cell device is one such strategy. Using resonating structures for light scattering is recently gaining momentum as they exhibit great potential to enhance the efficiency through light trapping. Functional material-based microresonators further provide an added advantage as they combine inherent optical resonance with the material properties suitable for photovoltaics like efficient charge separation and transport in one platform. “Whisperonic solar cell” is a broadly classified device in which resonating cavities are used in the cell architecture to effectively scatter the light, resulting in enhanced light absorption and thus efficiency. Recent studies reveal that WGM-enabled optical microcavities can effectively get coupled to the light absorber in a sensitized solar cell (SSC) and improve the performance of SSC significantly. In this short review, we briefly present the idea of enhancing the efficiency of solar cell using WGMs. Several case studies available from the literature for realizing the concept of WGM for light trapping are highlighted. Particular focus is given to the quantum dot sensitized whisperonic solar cells. The concept is much more universal and will be useful in both thin-film and sensitized solar cells. © Copyright © 2019 Cholasettyhalli Dakshinamurthy, Das, Ilaiyaraja and Sudakar.

Frontiers in Materials

Quantum Dot Sensitized Whisperonic Solar Cells—Improving Efficiency Through Whispering Gallery Modes

Whispering Gallery Mode Enabled Efficiency Enhancement: Defect and Size Controlled CdSe Quantum Dot Sensitized Whisperonic Solar Cells

Mesoporous TiO 2 nanoparticles are excellent photoanodes for sensitized solar cells (SSC). However, a 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 a TiO 2 mesoporous microsphere with smooth surface (SμS-TiO 2 ) exhibiting whispering gallery modes (WGM) and a commercial TiO 2 mesoporous nanoparticle (Degussa P25) with an optimum ratio (80:20 wt %) shows significant enhancement in PCE of DSSC and QDSSC devices. SμS-TiO 2 exhibits strong WGM as evidenced from the photoluminescence studies carried out using a laser with an excitation wavelength λ exc = 325 nm. These microspheres sensitized with N719 dye or CdSe/CuInS 2 QDs are shown to exhibit emission characteristics strongly coupled to WGM resulting in an enhanced PCE in SSC. Increases in PCE of ∼24% in DSSC devices and 80-95% in QDSSC devices of 0.25 cm 2 area are demonstrated. Thus, TiO 2 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 "whisperonic solar cells". © 2018 American Chemical Society.

ACS Applied Energy Materials

Whispering Gallery Mode Assisted Enhancement in the Power Conversion Efficiency of DSSC and QDSSC Devices Using TiO 2 Microsphere Photoanodes

We demonstrate white light emission (WLE) from (Cd,Zn)Se system, which is a composite of Zn alloyed CdSe quantum dot and ZnSe-amorphous (ZnSe-a) phase. Detailed structural and photoluminescence emission studies on pure CdSe and (Cd,Zn)Se show cubic zinc blende structure in the size range of 2.5 to 5 nm. (Cd,Zn)Se quantum dots (QDs) also have a significant fraction of ZnSe-a phase. The near-band-edge green-emission in crystalline CdSe and (Cd,Zn)Se is tunable between 500 to 600 nm. The (Cd,Zn)Se system also exhibits a broad, deep defect level (DL) red-emission in the range 600 to 750 nm and a sharp ZnSe near-band-edge blue-emission (ZS-NBE) between 445 to 465 nm. While DL and CdSe near-band-edge (CS-NBE) emissions significantly shift with the size of QD due to strong confinement effect, the ZS-NBE show minimal change in peak position indicating a weak confinement effect. The intensities of ZS-NBE and DL emissions also exhibit a strong dependence on the QD size. A gamut of emission colors is obtained by combining the CS-NBE with the ZS-NBE emission and broad DL emission in (Cd,Zn)Se system. Interestingly, we find the convergence of Commission Internationale de l'Eclairage (CIE) coordinates towards the white light with increasing Zn concentration in CdSe. We demonstrate by combining these three emissions in a proper weight ratio WLE can be achieved. Cd1-yZnySe (y = 0. 5; QD size ∼4.9 nm) alloy with a maximum quantum yield of 57% exhibits CIE coordinates of (0.39, 0.4), color rendering index (CRI) of 82, correlated color temperature (CCT) of 3922 K, and Duv of 0.0078 which is very promising for white light applications. © 2017 Author(s).

Journal of Applied Physics

Coexistence of strongly and weakly confined energy levels in (Cd,Zn)Se quantum dots: Tailoring the near-band-edge and defect-levels for white light emission

Anatase TiO2 microspheres with smooth and fibrous morphology (SμS and FμS) resembling laddu and dandelion are synthesized by solvothermal and hydrothermal methods, respectively. A detailed analysis of these two microstructures using XRD, UV–vis spectroscopy, electron microscopy and surface area measurement techniques are presented. Photoanodes fabricated using these microspheres and their composites with nanoparticles (Degussa P25) are tested for photovoltaic (PV) performance using a standard Grätzel-type dye-sensitized solar cell (DSSC) configuration. The DSSC made up of SμS and FμS microspheres exhibit an efficiency (η) of 8.4% and 6.4% respectively, in comparison to η≈7.1% for nanoparticulate P25. Further enhancement in η is realized in the composite photoanode films made of porous SμS/FμS microspheres filled with nanoparticulate P25 TiO2. High power conversion efficiency of 8.9% (for cell area of 0.5 cm2 and thickness of ~25 µm) was achieved in composite photoanode film consisting of 80 wt% SμS and 20 wt% P25. Electrochemical impedance spectroscopy studies reveal a low interfacial resistance in composite photoanodes, which is desired for efficient electron generation and transport. Composite microspheres filled with P25 nanoparticles in their voids show enhanced efficiency than the mesoporous TiO2 microspheres. Thus, SμS-nanoparticle composite TiO2 film possesses essential attributes necessary for an efficient photoanode viz. large surface area for dye adsorption, good connectivity between nanocrystallites for the efficient electron transport, and higher scattering properties for better light harvesting efficiency. © 2017 Elsevier B.V.

Solar Energy Materials and Solar Cells

Well-connected microsphere-nanoparticulate TiO2 composites as high performance photoanode for dye sensitized solar cell

We present a detailed study on the microstructural, structural and optical properties of hydrogen treated (HT) diverse TiO2 nanostructures (TNS) in comparison to the as-prepared (AP) samples. The effect of oxygen vacancies (VO), introduced in the anatase TiO2 by hydrogenation, on the photovoltaic (PV) characteristics is discussed. Raman spectroscopy shows A1g second order scattering modes due to VO-influenced surface structural changes. EELS confirm the presence of Ti4+/Ti3+ mixed states and oxygen deficiency in all TNS-HT. Bandgap (Eg) of TNS can be tuned by controlling the temperature and/or duration of annealing. The indirect bandgap is found to be larger (~3.2-3.4 eV) for TNS derived from Ti-foil, whereas TNS prepared using wet-chemical methods exhibit Eg in the range of 3.1-3.3 eV. The introduction of VO red-shifts the band-edge by ~0.25 eV. Hydrogenation decreases the PV efficiency (η) by 2-4 times compared to η=6-7% observed in DSSCs of diverse nanostructures. Despite the reduction in interfacial resistance enhancing electron generation and transport in TNS-HT samples, EIS studies indicate that the drop in η (%) is mainly due to the recapture of conduction band electrons via defect states shortening the electron lifetime. © 2015 Elsevier B.V. Allrightsreserved.

Journal	Scientific Reports
Publisher	Nature Publishing Group
ISSN	20452322
Open Access	No