Development and studies of thin film nanocrystal based photovoltaics. By Mauricio Andrade ChE 389 Department of Chemical Engineering Summer 2007

Overview Objective Introduction  Photovoltaic  Photovoltaic semi conductive materials Design of thin film PV  Manufacturing design  Synthesis  Capping Ligands  Application of thin film PV Thin Film Challenges  Capping Ligands  Monodispersity Solutions- Absorbing Layer Treatment technique  Chemical treatment  Thermal treatment Future Work Design Questions

Objective colloidal nanocrystals by providing a commercial route to low-cost manufacturable photovoltaic devices. To develop a high-efficiency photovoltaic solar cell made of colloidal nanocrystals by providing a commercial route to low-cost manufacturable photovoltaic devices.

Introduction A photovoltaic cell is composed of semiconducting materials that release charge carriers upon excitation with light. Electron hole pairs are then separated by an induced potential difference present in the device.

PV Materials

Manufacturing Designs Top down Develop devices from externally controlled assemblies.  Products are efficient with high degree of purity.  Expensive to manufacture Bottom up Self assembly of smaller components due to their chemical or physical properties.  Less expensive to manufacture  Purity and efficiency is reduced.

Synthesis Objective:  Develop monodisperse photovoltaic nanomaterials  Preferably Copper Indium Gallium Diselenide  Capable of forming several nanometric size layers

Capping Ligands By Doh, Lee, unpublished data  Photovoltaic nanocrystals are stabilized by ionic/covalent bonding between the surface of the nanocrystal and hydrocarbon molecules.  Self-assembly is caused by the equilibrium between: Van Der Waals forces Steric hindrance

Application of Thin Film PV By Dr Bryan Korgel, unpublished data

Thin Film Challenges Capping Ligands  Inhibit contact between photovoltaic nanoparticles  Poor attachment to the substrate Monodispersity  Hard to achieve as the surface area to volume ratio is increased Elimination of elements as components are synthesized (Indium)

Solution- Absorbing layer treatment techniques 1 Chemical treatment Goal: Remove the capping ligands. Reduction of film volume leads to crack formation. Current method involves exposure to a hydrazine solution. Three different outcomes have been observed:

1. (a),(d) Conductive films where the cracks does not isolate the fragments of the film, making a continuously conductive film. 2. (b),(e) Limited conductive films due to deep cracks on certain regions 3. (c),(f) Nonconductive films that arise from deep cracks in the film and further flecking of conductive materials from the surface.

Solution- Absorbing layer treatment technique Thermal treatment High temperature exposure of material yield larger grain sizes that increase mobility of charges  At temperatures below boiling temperature of the capping ligands weight loss is observed.  At high temperatures (>500 C) metallic undesired reactions may yield Cu/In alloy or In 2 O 3 depending on the annealing environment.

Future Work Design

Questions?