Polymers' Self-Organization: Air Bubbles and Efficient PV Cells, Slides of Nanotechnology

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Self-Organization of Polymers

and Applications in Solar Cells

Jung Hwan Woo

Outline

 Introduction

 Self-assembly and distinctive features  Ordered air bubble in polymer film example  Polymer PV cells

 Application

 Highly efficient polymer PV cells using self- organized polymer  Bulk heterojunction solar cells

 Future Improvements

Introduction

 Self-assembly

 Defined as spontaneous and reversible organization of molecular units into ordered structures

 Distinctive features

 Order  High order  Interactions  Weak bonds play a role  Building blocks  Nano and mesoscopic structures http://nimet.ufl.edu/

Introduction

 Types

 Self-ordered/assembled…  Nanocomposites  Semiconductor islands  Pore structures  Carbon nanotubes  Quantum wires and dots

www3.interscience.wiley.com

Introduction

 Different methods to template

 Ordered array of colloidal particles  Templating using an emulsion  Honeycomb structures by polymer with rod-coil architecture  Self-organized surfactants, i.e. mesoporous silica  Microphase-separated block copolymers  bacteria

Templating Example

 Ordered array of colloidal particles

 Procedure  Colloidal crystals infiltrate with a fluid which fills and solidifies in the space between the crystals  Spheres removed by thermal decomposition or solvent extraction  Solidified fluid forms 3D array of pores  Main drawback  Length of pores cannot be controlled

Self-Ordered Array of Air

Bubbles in a Polymer Film

 Procedures include forced air flow with moist atmosphere over a volatile solvent (polystyrene)  High vapor pressure and velocity drives the temperature to 0°C  Condensed water droplets form a structured array and sinks into the solution  When new water droplets condense previous array provide a template for the next layer  Size range from 0.2 to 20 μm

 Final product

Self-Ordered Array of Air

Bubbles in a Polymer Film

Srinivasarao, Science, (2001)

 Hole depth profile

 Discontinuity of holes seen at around 5 μm in depth

Self-Ordered Array of Air

Bubbles in a Polymer Film

Srinivasarao, Science, (2001)

 Parameters

 Solvent  2D porous films obtained when CS 2 is used whereas 3D films obtains for polystyrene  Air velocity  30 m/min => 6-μm pores  300 m/min => 0.5 μm pores

Self-Ordered Array of Air

Bubbles in a Polymer Film

 Why do water droplets form close

packed bubbles?

 Liquid droplets fail to coalesce with the same liquid in some situations  This phenomenon studied by Rayleigh in 1879  This behavior driven by thermocapillary convection  The presence of lubricating medium (air) between two liquid droplets keeps them from coalescence.

Self-Ordered Array of Air

Bubbles in a Polymer Film

Outline

 Introduction

 Self-assembly and distinctive features  Ordered air bubble in polymer film example  Polymer PV cells

 Application

 Highly efficient polymer PV cells using self- organized polymer  Bulk heterojunction solar cells

 Future Improvements

Polymer PV Cells

 Requirements for higher efficiency

 High fill factor  Ordered structure  Efficient absorption of solar radiation  Increased thickness. However, this results in higher series resistance  Lower series resistance  Ordered structure can reduce R (^) s

Outline

 Introduction

 Self-assembly and distinctive features  Ordered air bubble in polymer film example  Polymer PV cells

 Application

 Highly efficient polymer PV cells using self-organized polymer  Bulk heterojunction solar cells

 Future Improvements