Top Left: Extracted charge and electron density

of QDSSCs and QDFTO versus photovoltage,

both normalized to 1 cm3. Top Right: Number of

electron per nanoparticle in the nanocrystalline

TiO2 and the QD layer versus photovoltage. Left:

TPV measurements of QDSSCs and QDFTO

versus photovoltage.

Sensitization of wide band gap semiconductors using quantum dots (QDs) as light absorbers is currently of great interest in solar energy conversion systems. Studies concerning the fundamental

understanding of the QD-TiO2 system and other high surface area semiconductors have recently appeared. The QD-sensitization process is based on charge separation that happens at the wide

band gap semiconductor/QD/electrolyte interfaces. Fast electron injection from the QD to the semiconductor occurs upon illumination, followed by hole transfer to the electrolyte solution

(regeneration of the QD). Although quantum dot sensitized solar cells achieve efficiencies of 4-5% there is a lack in the physical understanding of the relative energies at the

semiconductor/QD/electrolyte interfaces. The QDs excited- state oxidation potential has to be more negative than the semiconductor conduction band potential in order to enable the electron

injection, and the oxidation potential of the QD must be more positive than the redox couple in the electrolyte solution in order to provide the driving force for the hole transfer. The fact that QDs

can build up a chemical potential under illumination and both the TiO2 and the QDs have Fermi levels, raises fundamental questions regarding the mechanism of the cell. Here we present a study

of the energy relationship at the interfaces of the TiO2/QD/Electrolyte. Advanced characterization utilizing charge extraction, open circuit voltage (Voc) decay and photovoltaic measurements were

employed to study the relative energetics of semiconductor/QD/electrolyte in a QDSSC. Further understanding of the system will be achieved with steady state absorbance, fluorescence and

ultrafast transient measurements, while combining with the above mentioned tools. Our goal is to acquire contactless information about the electronic state of QDSSCs under working conditions

while subjecting the cell to increasing light intensities.

Relative Energetics at the Semiconductor/Quantum Dot/Electrolyte Interfaces in Quantum Dot Sensitized

Solar cells (QDSSCs)

Ronen Gottesman, Menny Shalom, Yaakov Tischler and Arie Zaban

Chemistry Department, Bar-Ilan Institute of Nanotechnology and Advanced Materials

Left: QDSSCs : fast electron injection from the QD excited state directly to the TiO2-CB or through the

QD surface states (slower injection process) while holes are removed by the electrolyte. The main

recombination paths are (1) from the TiO2-CB to the electrolyte, (2) from the QD (CB or surface states) to

the electrolyte, and (3) internal recombination within the QD. Right: QDFTO: photo generated holes are

removed by the electrolyte while the excited electrons diffuse within the QDs layer. The major

recombination paths in QDFTO are (1) from the QD (CB or surface states) to the electrolyte and (2)

internal recombination within the QD.

Left: Quantum dot photo-electrochemical solar cells (QDSSCs). Right: solar cell consisting

solely of quantum dots, which are deposited directly of FTO glass (QDFTO).

Left: HRSEM image of a cross section that was done by a FIB (Focused Ion Beam). On the

bottom of the image we can see the relative thicker FTO layer, while on top of it a thinner

section of multilayers of CdSe QDs, Right: TEM image of CdSe QDs nanocrystals which

were deposited on the FTO layer using a CBD method.

General Scheme: Energy Band Diagram:

HRSEM and TEM:

Cell Structure:

Absorbance:

V characteristics:-I

IPCE:

Photoelectrochemical solar cell based on

QDs on FTO only.

and Transient Photovoltage: ExctractionCharge

A general illustration of the setup of the optical system for the steady state absorbance,

fluorescence and ultrafast transient measurements. The results will be correlated with

charge extraction and transient photovoltage (TPV) techniques and will acquire a

contactless information about electronic state of QDSSCs under working conditions while

subjecting the cell to increasing light intensities.

The Optical System:

Cell Performance: