Kiarash Kiantaj EEC235/Spring 2008 Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture Kiarash Kiantaj EEC235/Spring 2008

Introduction Sensitization of mesoscopic Tio2 with dyes (11% efficiency) Short band gap semi-conductors to transfer electrons to large band gap semi-conductors Sensitizers: CdS, PbS, Bi2S3 CdSe, InP (short gap) TiO2 , SnO2 ( large gap)

Short band gap semi-conductors Harvesting visible light energy. Electron injection under visible light Fast charge recombination  low efficiency

Semiconductor Quantum dots Visible light harvesting assemblies Size quantization Tune visible response Vary band energies Open up ways utilize hot electrons and multiple carriers with single photon.

Quantized CdSe Particles and Their Deposition on TiO2 Particulate Films and Nanotubes Random versus Directed Electron Transport through Support Architectures, (a) TiO2 Particle and (b) TiO2 Nanotube Films Modified with CdSe Quantum Dots

- Absorption spectra of 3.7, 3.0, 2.6, and 2.3 nm diameter CdSe quantum dots in toluene. - Shift due to quantization

Deposition of QD on Tio2 films 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

40-50 nm particles ( diameter) Photograph of 2.3, 2.6, 3.0, and 3.7 nm diameter CdSe quantum dots in toluene, anchored on TiO2 particulate films (OTE/TiO2(P)/CdSe, (C) attached to TiO2 nanotube array (Ti/TiO2(NT)/CdSe). 40-50 nm particles ( diameter) Electro chemical etching of Ti in fluoride Tio2 nanotubes 80-90 nm ( diameter) , 8 um long

Growth details Constant absorption  monolayer CdSe Linear increase in absorption with TiO2 thickness CdSe quantum dots and TiO2 binding : bifunctional linker molecules (HOOC-CH2-CH2-SH) carboxylate and thiol functional groups

Absorption spectra Peaks due to the 1S exciton transitions Binding of CdSe to TiO2 Absorption spectra of (a) 3.7, (b) 3.0, (c) 2.6, and (d) 2.3 nm diameter CdSe quantum dots anchored on nanostructured TiO2 films (A) OTE/TiO2(NP)/CdSe (solid lines) and (B) (Ti/TiO2(NT)/CdSe (dashed lines).

Selectively harvest light CdSe maintains quantization properties after binding Absorbance = 0.7 more than 80% absorption of light below the onset. Uniform coverage of CdSe is similar to modified mesoscopic TiO2 with sensitizing dyes.

Photoelectrochemistry of TiO2 Films Modified with CdSe Quantum Dots Open circuit voltage Short current circuit Open circuit voltage is same for all. (650+-20 mV) Injected electrons relax to lowest conduction band conduction band level of TiO2+ redox = 600 mV

Photocurrent response depends on particle size Photocurrent response of (A) OTE/TiO2(NP)/CdSe and (B) (Ti/TiO2(NT)/CdSe electrodes. 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 TiO2 films (excitation >420 nm, 100 mW/cm2; electrolyte, 0.1 M Na2S solution).

Maximum photocurrent 3.0 nm CdSe Two opposing effects: 1- decreasing size shift of the conduction bad to more negative potential driving force for charge injection 2- decreasing size smaller response in visible region less 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 Response through Size Quantization. - incident photon to charge carrier efficiency (IPCE) Photocurrent action spectra A) 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 nanotube architecture for facilitating electron transport in nanostructure-based semiconductor films.

Design of Rainbow Solar Cells Artistic Impression of a Rainbow Solar Cell Assembled with Different-Sized CdSe Quantum Dots on a TiO2 Nanotube Array

Conclusion Size dependent charge injection ( Tio2-CdSe) Morphology dependence Overall power efficiency of about 1% with 3nm CdSe QD Maximum IPCE value (45%) obtained with CdSe/TiO2(NT) is greater than that of CdSe/TiO2(NP) (35%).