Quantum Dot Solar Cells.Quantum Dot Solar Cells.Tuning PhotoresponseTuning PhotoresponsethroughthroughSize and Shape Control ofSize and Shape Control ofCdSeCdSe--TiO2 ArchitectureTiO2 ArchitectureYashvant RaoYashvant Rao
IntroductionIntroduction Sensitization of mesoscopic Tio2 with dyesSensitization of mesoscopic Tio2 with dyes(11% efficiency)(11% efficiency) Short band gap semi-conductors to transferShort band gap semi-conductors to transferelectrons to large band gap semi-conductorselectrons to large band gap semi-conductors Sensitizers: CdS, PbS, Bi2S3 CdSe, InP (shortgap) TiO2 , SnO2 ( large gap)
Short band gap semi-conductorsShort band gap semi-conductors Harvesting visible light energy.Harvesting visible light energy. Electron injection under visible lightElectron injection under visible light Fast charge recombinationFast charge recombination low efficiencylow efficiency
Semiconductor Quantum dotsSemiconductor Quantum dots Visible light harvesting assembliesVisible light harvesting assemblies Size quantizationSize quantization Tune visible responseTune visible response Vary band energiesVary band energies Open up ways utilize hot electrons and multipleOpen up ways utilize hot electrons and multiplecarriers with single photon.carriers with single photon.
Quantized CdSe Particles and TheirDeposition on TiO2Particulate Films and NanotubesRandom versus Directed Electron Transport throughSupport Architectures, (a) TiO2 Particle and (b) TiO2 NanotubeFilms Modified with CdSe Quantum Dots
- Absorption spectra of 3.7, 3.0, 2.6, and 2.3 nm diameter CdSequantum dots in toluene.- Shift due to quantization
Scanning electron micrographs of (A) TiO2 particulate film caston OTE and (B, C, and D) TiO2 nanotubes prepared by electrochemicaletching of titanium foil. The side view (B), top view(C), and magnifiedview (D) illustrate the tubular morphology of the filmDeposition of QD on Tio2 films
40-50 nm particles ( diameter)40-50 nm particles ( diameter) Electro chemical etching of Ti in fluorideElectro chemical etching of Ti in fluoride Tio2 nanotubesTio2 nanotubes 80-90 nm ( diameter) , 8 um long80-90 nm ( diameter) , 8 um longPhotograph of 2.3, 2.6, 3.0, and3.7 nm diameter CdSequantum dots(A) in toluene,(B) anchored on TiO2particulate films(OTE/TiO2(P)/CdSe,(C) attached to TiO2 nanotubearray (Ti/TiO2(NT)/CdSe).
Growth detailsGrowth details Constant absorption monolayer CdSe Linear increase in absorption with TiO2thickness CdSe quantum dots and TiO2 binding :bifunctional linker molecules (HOOC-CH2-CH2-SH)carboxylate and thiol functional groups
Absorption spectraAbsorption spectraAbsorption spectra of (a) 3.7, (b) 3.0, (c) 2.6, and (d) 2.3 nmdiameter CdSe quantum dots anchored on nanostructured TiO2 films (A)OTE/TiO2(NP)/CdSe (solid lines) and (B) (Ti/TiO2(NT)/CdSe (dashed lines).•Peaks due to the 1S exciton transitions•Binding of CdSe to TiO2
Selectively harvest lightSelectively harvest light CdSe maintains quantization properties afterCdSe maintains quantization properties afterbindingbinding Absorbance = 0.7Absorbance = 0.7 more than 80% absorptionmore than 80% absorptionof light below the onset.of light below the onset. Uniform coverage of CdSe is similar to modifiedUniform coverage of CdSe is similar to modifiedmesoscopic TiO2 with sensitizing dyes.mesoscopic TiO2 with sensitizing dyes.
Photoelectrochemistry of TiO2 FilmsModified with CdSeQuantum Dots Open circuit voltageOpen circuit voltage Short current circuitShort current circuit Open circuit voltage isOpen circuit voltage issame for all. (650+-20 mV)same for all. (650+-20 mV) Injected electrons relax toInjected electrons relax tolowest conduction bandlowest conduction bandconduction bandconduction bandlevel of TiO2+ redox = 600 mVlevel of TiO2+ redox = 600 mV
Photocurrent response depends on particle sizePhotocurrent response depends on particle sizePhotocurrent response of (A) OTE/TiO2(NP)/CdSe and (B) (Ti/TiO2(NT)/CdSeelectrodes. Individual traces correspond to (a) 3.7, (b) 3.0, (c) 2.6,and (d) 2.3 nm diameter CdSe quantum dots anchored on nanostructured TiO2films (excitation >420 nm, 100 mW/cm2; electrolyte, 0.1 M Na2S solution).
Maximum photocurrentMaximum photocurrent 3.0 nm CdSe3.0 nm CdSe Two opposing effects:Two opposing effects:1- decreasing size1- decreasing size shift of the conduction bad toshift of the conduction bad tomore negative potentialmore negative potential driving force fordriving force forcharge injectioncharge injection2- decreasing size2- decreasing size smaller response in visiblesmaller response in visibleregionregion less photocurrentless photocurrent
I-V characteristics of (A) OTE/TiO2(NP)/CdSe and (B) (Ti/TiO2(NT)/CdSe electrodes (excitation >420 nm; intensity 100 mW/cm2;electrolyte, 0.1 M Na2S solution.)Under the applied potential charge recombination is minimized.
Tuning the Photoelectrochemical Responsethrough SizeQuantization.- incident photon to charge carrier efficiency(IPCE)Photocurrent action spectraA) OTE/TiO2(NP)/CdSe and(B) (Ti/TiO2(NT)/CdSe electrodes
nanotube TiO2 films generally absorb more light than nanoparticle TiO2 films, this difference accounts for a no more than a 5% increase in overall photons absorbed. Comparing this with a 10% improvement in IPCE of the nanotube film over the nanoparticle film demonstrates the measurable advantage of a nanotubearchitecture for facilitating electron transport in nanostructure-based semiconductor films.
Design of Rainbow Solar CellsArtistic Impression of a Rainbow Solar CellAssembled with Different-Sized CdSe Quantum Dots on a TiO2Nanotube Array
ConclusionConclusion Size dependent charge injection ( Tio2-CdSe)Size dependent charge injection ( Tio2-CdSe) Morphology dependenceMorphology dependence Overall power efficiency of about 1% with 3nmOverall power efficiency of about 1% with 3nmCdSe QDCdSe QD Maximum IPCE value (45%) obtained withCdSe/TiO2(NT) is greater than that ofCdSe/TiO2(NP) (35%).