July 20, 2012
Examining the film uniformity of my mPEDOT:PSS film samples.
The past few weeks have been filled with a series of trial and error experiments evaluating the effect of the interconnecting layer on a base layer of zinc oxide (ZnO). The interconnecting layer plays a crucial role in the structure of tandem solar cells as it provides stability between the multiple layers. Currently, organic compounds are being widely researched for their various applications in the fabrication of solar cells because of their potential to make them larger and more flexible and at a lower cost. Polymers with conjugated bonds work especially well for this purpose because of their easy electron movement. The polymer I am studying is poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), conveniently abbreviated PEDOT:PSS. Previous research has shown that PEDOT:PSS is able to form good quality films, has sufficient electrical conductivity, and high transparency, all of which are good qualities an interconnecting layer needs. However because PEDOT:PSS is acidic, modifications need to be made to alter its pH so that it does not dissolve the bottom ZnO layer. So, another aqueous polymer solution sodium polystyrene sulfonate (SPS) is mixed with PEDOT:PSS to create a modified version (mPEDOT:PSS). Because this mPEDOT:PSS is a novel addition to the organic solar cell field, the optimal ratio of PEDOT:PSS to SPS has yet to be found.
My job has been to create three different ratios of mPEDOT:PSS solutions and visualize their effect on a layer of ZnO. To do this, I first prepare the silicon wafers on which my films will be placed by washing the pieces in multiple baths of deionized water, acetone, and isopropanol and a final oxygen plasma cleaning. These processes are meant to insure a smooth surface so that the films may be as uniform as possible. The spin coating process is simple since the only step is to place the silicon wafer onto the platform, apply the film solution, and the machine will spin the sample at several thousand revolutions per minute to create a film that is nanometers thin. Thus far, my work has shown that PEDOT:PSS needs to be further diluted by SPS since ZnO films are mostly removed when contacted by any type of PEDOT:PSS solution.
A few of my samples with different concentrations of mPEDOT:PSS
In addition, I have tested the conductivity of mPEDOT:PSS itself by spin-coating a layer on a silicon wafer, evaporating a layer of gold that will serve as electrodes through which a current will be passed. By evaluating the IV curve (current vs. voltage), I will be able to assess the conductivity of the different mPEDOT:PSS solutions. Together with the polymer’s effect on ZnO, this conductivity data will help further the development of the use of PEDOT:PSS in the interconnecting layer of organic polymer solar cells.
June 24, 2012
Me, cutting small pieces of silicon to use as the base for my devices.
Upon my arrival in Hong Kong, I was naturally anxious. After exploring the campus at Chinese University of Hong Kong (CUHK) and parts of the city, I’ve spent my first weeks at the research lab at CUHK; I was introduced to basic solar cell technology and the developments being made in the field. Through reading published articles and papers and much Internet searching, I familiarized myself in the scientific “lingo” and principles on which my research will focus. The overall purpose of developing organic solar cells is to be able to produce cheaper photovoltaic systems with different materials. The challenge is to find materials that have the right physical and electronic properties to function effectively in a cell. To start, my objective will be to help a graduate student develop an intermediate layer within a cell whose efficiency is increased by having multiple layers of materials that absorbs a wider spectrum of light. The development of this intermediate layer is important to ensure stability of the solar cell itself while not inhibiting the absorption functions of the main layers. I look forward in the following weeks to creating my own solar cells, as well as spending more time exploring various Hong Kong neighborhoods.