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Quantum Dot Sensitized Solar Cells ByAbhinav Gupta IIT Kharagpur Tutor Prof P BhargavaIIT Bombay 1 List of Topics Factors limiting efficiency of Silicon Solar Cells Dye Sensitized Solar Cells Why Quantum Dot Solar Cells Quantum Dot sensitized Solar Cells Materials of QDSSCs. Sensitization Of QDSSCs Properties of QDSSCs Configurations Of QDSCs 2 Key Terms External Quantum efficiency- IPCE, which stands for Incident-Photon-to-electron Conversion Efficiency i.e. The ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy of incident photons. Internal Quantum Efficiency (IQE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy that are absorbed by the cell. absorbance A (also called optical density) is defined as The fill factor is defined as the ratio of the actual maximum obtainable power, to the product of the open circuit voltage and short circuit current First generation solar cells-which are solar cells made of semiconducting p-n junctions Second generation solar cells-based on reducing the cost of first generation cells by employing thin film technologies 3 Factors limiting efficiency of Traditional Solar Cells When a photon hits a piece of silicon, one of three things can happen: the photon can pass straight through the silicon this (generally) happens for lower energy photons, the photon can reflect off the surface, the photon can be absorbed by the silicon,4 Queisser Shockley Limit The maximum thermodynamic efficiency for the conversion of unconcentrated solar irradiance into electrical free energy in the radiative limit assuming detailed balance and a single threshold absorber was calculated by Shockley and Queisser in 1961to be about 31%;The solar spectrum contains photons with energies ranging from about 0.5 to 3:5 eV.Leads to generation of hot electrons and hot holes 5 Working of Dye Sensitized Solar Cells Composed of a porous layer of titanium dioxide nanoparticles, covered with a molecular dye that absorbs sunlight The titanium dioxide is immersed under an electrolyte solution, above which is platinum-based catalyst. anode -the titanium cathode the platinum conductor -the electrolyte. Sunlight passes through the transparent electrode into the dye layer where it can excite electrons that then flow into the titanium dioxide. 6 The electrons flow toward the transparent electrode where they are collected for powering a load. After flowing through the external circuit, they are re-introduced into the cell on a metal electrode on the back, flowing into the electrolyte. The electrolyte then transports the electrons back to the dye molecules. Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte. Working of Dye Sensitized Solar Cells 7 Quantum Dot Solar Cells Quantum dot solar cells are an emerging field in solar cell research Uses quantum dots as the photovoltaic material, as opposed to better-known bulk materials such as silicon, copper indium gallium selenide (CIGS) or CdTe(as in 2nd generation solar cells). bandgapsare tunable across a wide range of energy levels by changing the quantum dot size in contrast to bulk materials, where the bandgap is fixed Here we will discuss the use of Quantum Dot as a sensitizer Metal Chalcogenides are generally used as the bandgap matches the required level. 8 Utilize the hot carriers before they relax to the band edge via phonon emission. two fundamental ways 1. produce an enhanced photo voltage-requires that the carriers be extracted from the photo converter before they cool 2. produces an enhanced photocurrent.- requires the energetic hot carriers to produce a second (or more) electronhole pair through impact ionization Is the inverse of an Auger process where by two electronhole pairs recombine to produce a single highly-energetic electronhole pair Why Quantum Dot solar Cells 9 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Auger Effect The Auger effect -transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron This second ejected electron is called an Auger electron after one of its discoverers, Pierre Victor Auger. 10 Why Quantum Dot solar Cells In order to achieve the former, the rates of photogeneratedcarriers separation, transport, and interfacial transfer across the contacts to the semiconductor must all be fast compared to the rate of carrier cooling . The latter requires that the rate of impact ionization (i.e. inverse Auger effect) be greater than the rate of carrier cooling and other relaxation processes for hot carriers 11 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Points to Note Hot electrons and hot holes generally cool at different rates Electrons have effective masses that are significantly lighter than holes and consequently cool more slowly. Cooling rates are dependent upon the density of the photogenerated hot carriers Dynamical effects we will discuss are dominated by electrons rather than holes 12 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Hot electron gives its excess kinetic energy to a thermalized hole via an Auger process The hole can then cool quickly because of its higher effective mass and more closely spaced quantized states. Hole is removed from the QD core by a fast hole trap at the surface, then the Auger process is blocked leading to slow electron cooling.Effect first shown for CdSe QDs Now also been shown for InP QDs, Slows the electron cooling to about 7 ps compared to 0.3 ps 13 Why Quantum Dot solar Cells Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Tunability of optical properties with size Better hetero junction formation with solid hole conductors. Production of quantum yields by impact ionization (inverse Auger effect) . Efficient inverse Auger effects in QD-sensitized solar cellscan result in much higher conversion efficiencies than dye-sensitized solar cells Dyes are more susceptible to U.V. Degradation . Advantages over DSSCs 14 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 QDSSCs : Working Materials Sensitization 15 Working Of QDSSCs This configuration is a variation of a recent promising new type of photovoltaic cell that is based on dye-sensitization of nanocrystalline TiO2 layers. For the QD-sensitized cell, QDs are substituted for the dye molecules; they can be adsorbed from a colloidal QD solutionor produced in situ . . 16 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Materials-WideBandgap Nanostructures Mesoporous films, nanorods, nanowires and nanotubes are the most common nanostructures for providing a large surface area for QD deposition. TiO2has been used as mesoporous film as well as nanotubes. ZnO has been used as mesoporous films and as nanorods. 17 Scanning electron micrographs of (A) TiO2 particulate film cast on OTE and (B, C, and D) TiO2 nanotubes prepared by electrochemical etching of titanium foil. The side view (B), top view(C), and magnified view (D) illustrate the tubular morphology of the film. Sven Rhle, Menny Shalom, Arie Zaban, ChemPhysChem 2010, 11, 2290 2304 Materials-Quantum Dots Electronic characteristics are closely related to the size and shape of the individual crystal. The smaller the size of the crystal, the larger the band gap, the greater the difference in energy between the highest valence band and the lowest conduction band Hence more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. Because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material Especially CdS, CdSe and PbS have been used to investigate the operating principles of QDSCs. 18 Sven Rhle, Menny Shalom, Arie Zaban, ChemPhysChem 2010, 11, 2290 2304 Materials-Electrolytes/ Hole conductors The most common electrolytes used in QDSCs are aqueous polysulfide and organic electrolyte with I/I3redox couple , commonly used in DSCs. The electrolyte should not affect the stability of the QDs. Unlike DSSCs an ideal redox system has not been found. Solidstate hole conductors such as spiroOMeDATand CuSCNhave been used to replace the liquid electrolyte. 19 Sven Rhle, Menny Shalom, Arie Zaban, ChemPhysChem 2010, 11, 2290 2304 Materials-Counter Electrodes Platinized TCO layers usually used. Long term stability is an important criterion. It affects the fill factor and hence efficiency of the cell. 20 Sven Rhle, Menny Shalom, Arie Zaban, ChemPhysChem 2010, 11, 2290 2304 Sensitization-In Situ Sensitization of a wide gap nanostructured semiconductor electrode (TiO2, ZnO) with QDs is done by: 1. Direct growth of the semiconductor QDs on the electrode surface by chemical reaction of ionic species using the methods of chemical bath deposition (CBD) or successive ionic layer adsorption and reaction (SILAR ) anucleation and growth mechanism leading to a high coverage of the effective TiO2 surface but rendering rather difficult the control of the size distribution of the deposited QDs. 21 Mora-Sero 1848-1857 ACCOUNTS OF CHEMICAL RESEARCH Sensitization-Ex-Situ Metal precursor heated before organometallic precursor is added. Capping agents dissolve the precursors and prevent aggregation of QDs. Cooling terminates reaction, used to synthesize any size. Bifunctionalmolecular linkers (usually (COOH)RSH) used to attach QDs to metal oxide electrode. Different linkers decrease charge separation efficiencies to different extents. the attachment of colloidal QDs through molecular wires leads to precise morphological characteristics (shape and size) of the semiconductor nanocrystalssubsequently leading to potentially enhanced properties compared with CBD. Mora-Sero 1848-1857 ACCOUNTS OF CHEMICAL RESEARCH 22 Sensitization-Ex-Situ Alternative methodto attach the colloidal QDs onto the TiO2 surface without the need for molecular linkers is termed direct adsorption (DA). This procedure has already been employed to sensitize TiO2 with CdSe ,InAs, and InP although the obtained photocurrents at 1 sun intensity were low. Use of presynthesized colloidal QDs leads to less efficient solar cell devices compared with directly grown QDs as it provides a low surface coverage of about 14%. 23 Mora-Sero 1848-1857 ACCOUNTS OF CHEMICAL RESEARCH Figureshows the photocurrent spectra of Ti02 electrodes which were sensitized byvarious sulfide Q-particles. All spectra weremeasuredafteronecoating process withtherespective material.Q-Particles of Various Materials and Sizes on Ti02 R. Vogel, P. Hoyer, and H. Weller-J. Phys. Chem. 1994,98, 3183-3188 24 The absorption spectra of a Ti02 electrode after different numbers of coatings with PbS Photocurrent Spectra (in IPCE units) of a TiO2 electrode after various coatings of PbS. 25 R. Vogel, P. Hoyer, and H. Weller-J. Phys. Chem. 1994,98, 3183-3188 Different Substrate Materials. 26 R. Vogel, P. Hoyer, and H. Weller-J. Phys. Chem. 1994,98, 3183-3188 27 Conduction band and valence band positions (vs NHE, pH =7) R. Vogel, P. Hoyer, and H. Weller-J. Phys. Chem. 1994,98, 3183-3188 Surface Modification andPhoto stability Photocurrent Quantum yields for differently treated PbS-TiO2 electrodes as a function of illumination time 28 R. Vogel, P. Hoyer, and H. Weller-J. Phys. Chem. 1994,98, 3183-3188 CdSe Vs. CdTe Absorption and emission spectra ofCdSe and CdTe nanocrystal suspension in toluene. The excitation wavelength was 450 nm. Absorption spectra of CdSe-TiO2 and CdTe-TiO2 lms deposited on OTE (optically transparent electrode). Ho Bang and Prashant V. Kamat-ACS Nano 2009 29 Charge Injection into TiO2 Nanoparticles Fluorescence emission decay of (A) CdSe and (B)CdTe QDs: (a) deposited on glass (OTE) and (b) linked ontoTiO2 film cast on OTE slide. The measurements were carried out in the absence of electrolyte. 30 Ho Bang and Prashant V. Kamat-ACS Nano 2009 Photo electrochemical Measurements Photocurrent action spectra. Electrolyte, 0.1 M Na2S (total illuminated area was 0.3 cm2) CdSe-TiO2CdTe-TiO2 31 Ho Bang and Prashant V. Kamat-ACS Nano 2009 32 Ho Bang and Prashant V. Kamat-ACS Nano 2009 The reactivity of photo generated holes with the sulfide electrolyte, determines their suitability in QDSCs. CdSe remains regenerative during the operation of QDSC as the photo generated holes are scavenged by S2- ions. A maximum IPCE of 70%shows the suitability of this system in a photo electrochemical solar cell. However inCdTe-based QDSC the formation of a CdS shell and the inability to scavenge photo generated holes make CdTe a poor candidate for QDSC No other redox couples seems to provide the required photo stability for the CdTe QD electrodes; in fact, most of them immediately corrode CdTe QDs even under ambient conditions. CdSe Vs. CdTe 33 Ho Bang and Prashant V. Kamat-ACS Nano 2009 CdS vs. CdSe UV-vis absorption spectra of CdSe-sensitized TiO2 films prepared by various cycles of CBD process in the absence (a) or in the presence (b)of a self-assembled layer of CdS-QD. The number on each curve corresponds to the CBD cycle introduced to assemble the CdSe. 34 Chem. Mater. 2008, 20, 69036905-Yuh-Lang Lee Incident photon to current conversion efficiencies (IPCE) of various electrodes measured as a function of wavelength. 35 CdS vs. CdSe The growth of CdSe on a mesoporous TiO2 film can be induced by a pre-self-assembled layer of CdS-QDs, leading to a higher deposition rate and an interfacial structure with superior ability in inhibiting the charge recombination at the electrode/electrolyte interface. The presence of the SAM CdS-QD layer significantly increases the Voc and FF of the CdSe-SSC, and an efficiency as high as 2.9% was achieved for the TiO2/SAM-CdS-QD/CdSe/ZnS device. Chem. Mater. 2008, 20, 69036905-Yuh-Lang Lee MAJOR FACTORS LIMITING EFFICIENCY The poor performance of the QD-SSCs may be ascribed to the difficulty of assembling the QDs into the mesoporous TiO2 matrix to obtain a well-covered QD layer on the TiO2 crystalline surface. the selection of an efficient electrolyte in which the metal chalcogenide can run stably without serious degradation. 36 Chem. Mater. 2008, 20, 69036905-Yuh-Lang Lee Other Quantum Dot Based Solar Cell Devices 37 Quantum dot solar cell configurations Quantum dot arrays in pi-n cells ordered 3-D array with inter-QD spacing sufficiently small such that strong electronic coupling occurs and minibands are formed to allow long-range electron transport the QD array is placed in the intrinsic region of a p+in+ structure .38 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Quantum dot arrays inp-i-n cells The delocalized quantized 3-D miniband states could be expected to slow the carrier cooling and permit the transport and collection of hot carriers at the respective p and n contacts to produce a higher photo potential Impact ionization might be expected to occur in the QD arrays, enhancing the photocurrent However, hot electron transport-collection and impact ionization cannot occur simultaneously; they are mutually exclusive. 39 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Quantum dots dispersed in organicsemiconductor polymer matrices .In one configuration, a disordered array of CdSe QDs is formed in a hole-conducting polymerMEH-PPV (poly(2-methoxy,5-(2-ethyl)-hexyloxy-pphenylenevinylene). Upon photo excitation of the QDs, the photogenerated holes are injected into the MEH-PPV polymer phase, and are collected via an electrical contact to the polymer phase.40 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 The electrons remain in the CdSe QDs and are collected through diffusion and percolation in the nanocrystalline phase to an electrical contact to the QD network. Initial results show relatively low conversion efficienciesbut improvements have been reported with rod-like CdSe QDshapesembedded in poly(3hexylthiophene) (the rod-like shape enhances electron transport through the nanocrystalline QD phase). In another configuration, a polycrystalline TiO2 layer is used as the electron conducting phase, and MEH-PPV is used to conduct the holes; the electron and holes are injected into their respective t transport mediums upon photoexcitation of the QDs. Quantum dots dispersed in organicsemiconductor polymer matrices 41 Quantum dot solar cells A.J. Nozik-Physica E 14 (2002) 115 120 Use of Nanotubes Random versus Directed Electron Transport through Support Architectures, (a) TiO2 Particle and (b) TiO2 Nanotube Films Modified with CdSe Quantum Dots42 Anusorn Kongkanand, Kevin Tvrdy, Kensuke Takechi, Masaru Kuno,and Prashant V. Kamat Schematic Diagram Illustrating the Energy Levels of DifferentSized CdSe Quantum Dots and TiO2 43 Artistic Impression of a Rainbow Solar Cell Assembled with DifferentSized CdSe Quantum Dots on a TiO2 Nanotube Array Anusorn Kongkanand, Kevin Tvrdy, Kensuke Takechi, Masaru Kuno,and Prashant V. Kamat Summary Breakthroughs will come from (i) materials, (ii) surface treatments and (iii) nanocomposite absorbers. The semiconductor lightabsorber properties dictate the requirements that the other components of the device need to satisfy. In the long term it is desirable to replace the liquid electrolyte by solidstate hole conductors New methods for energylevel alignment should boost the QDSC efficiency to compete with the conversion efficiency of 1520 %, typical for polycrystalline silicon, but at a significant lower cost. Starting from quite low conversion efficiencies, these semiconductor sensitized solar cells have grown very rapidly to values around 45% 44 Sven Rhle, Menny Shalom, Arie Zaban, ChemPhysChem 2010, 11, 2290 2304 Bibliography 1. All pages without citation courtesy wikipedia 2. http://en.wikipedia.org/wiki/Solar_cells 3. Quantum Dot Sensitized Solar Cells. ATale of Two Semiconductor Nanocrystals: CdSe and CdTe Jin Ho Bang and Prashant V. KamatKamat-ACS Nano 2009 4. R. Vogel, P. Hoyer, and H. Weller-J. Phys. Chem. 1994,98, 3183-3188 5. Mora-Sero 1848-1857 ACCOUNTS OF CHEMICAL RESEARCH 6. Chem. Mater. 2008, 20, 69036905-Yuh-Lang Lee 7. Quantum dot solar cells A.J.Nozik-Physica E 14 (2002) 115 120 8. Sven Rhle, Menny Shalom, Arie Zaban, ChemPhysChem 2010, 11, 2290 2304 9. Anusorn Kongkanand, Kevin Tvrdy, Kensuke Takechi, Masaru Kuno,and Prashant V. Kamat 45 THANK YOU !!! 46