Effect of electron acceptor structure on stability and efficiency in polymer solar cells: a combinatorial approach Michael Tro, Alexis Sarabia, David Oparko, Emma Lewis, Maxwell J. Giammona, Justin Isaac, Thorsteinn Adalsteinsson, Brian McNelis and Richard P. Barber, Jr. Department of Physics and Department of Chemistry & Biochemistry, Santa Clara University, Santa Clara CA 95053 Introduction

How the Devises Are Made and Measured

Organic solar cells offer a cheap alternative to silicon based  solar cells. They are easy to manufacture, and do not  compete for raw materials.  They also can be used in a larger  range of applications.  However, they suffer from  low power  conversion efficiency, which tends to degrade over time.   While low efficiencies could be considered a trade‐off for the  low costs, the poor device lifetime remains a problem.  We  seek to understand the pathways for this degradation and  how they can be improved in order to make these device  commercially viable.  

Motivation for Library Approach

Side view: PCBOD: long life time but poor  efficiency Optimum anneal temp: same  as PCBM PCBO: medium life time and  efficiency Optimum anneal temp: lower  than PCBM and PCBOD

R= octyl

Fabrication

Background

Acid etch: ‐Indium‐Tin‐Oxide (ITO)  *

Top view:

Spin coat (and anneal):

octadecyl

‐PEDOT:PSS ‐Active layer

Library of R-groups

Deposited via thermal evaporation: ‐LiF

The generic structure of an organic solar cell, a bulk  heterojunction has two distinct and continuous layers.   One consists of an electron donor, this layer is usually  fluorescent, while the other layer is an electron acceptor.   When the electron donor is exposed to light it its put  into an excited state.  The excited electron is then  transferred to the electron acceptor molecule and  transported to the cathode.  The corresponding hole  travels through the donor layer to the anode, creating a  potential difference which can be used as electric power. 

Electron Acceptor / Donor

‐Al Measurement Shine light on sample Vary voltage, and measure current Take measurements every 15 minutes for 2 days

Typical Data Curves Main figure: degradation of  PCBO:P3HT devise in ambient  condition.  The arrow indicates  the progression of time. Voltage at I = 0 is the open  circuit voltage Current at V= 0 is the short  circuit current Inset: a semilog plot of the  power conversion efficiency as a  function of time.  The solid line fit  shows the slope used to extract  characteristic time, τ, where

P3HT PCBM

Figures of Merit for Five PCB-esters

efficiency ~              .

The left three compounds in this  figure are of comparable molecular  weight and they are  unfunctionalized, yet our data  shows they perform dramatically  different. It is interesting to note that there  is still a inverse relationship  between efficiency and life time in  these samples.  

Acknowledgements Supported by an SCU Sustainability  Grant, an SCU IBM Faculty Research  Grant and a grant from IntelliVision  Technologies.

*http://www.greengroup.engin.umich. edu/electronics.html

Effect of electron acceptor structure on stability and ...

The generic structure of an organic solar cell, a bulk heterojunction has two distinct and continuous layers. One consists of an electron donor, this layer is usually.

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