# Organic Photovoltaics

## Christopher Bruner October 24, 2010

### Submitted as coursework for Physics 240, Stanford University, Fall 2010

 Fig. 1: Schematic for operational principle of a solar cell. Light enters the active layer causing a splitting of positive and negative charge (represented by the blue and orange circles). This movement of charge towards the two electrodes produces and electrical current, i.e. electricity.

## Introduction

Within just the past 10 years, we have seen great strides towards development and understanding of renewable energy technologies; conjugated polymer-based organic solar cells represent just one of these many promising alternatives to burning fossil fuels. One estimate in 2007 estimated the world's electrical consumption is around 13 x 1012 Watts of energy to maintain the world and by 2050, this will increase by 10 x 1012 Watts. [1] To put this number into perspective, a common light bulb requires 60 Watts to operate, so 13 x 1012/60 is equivalent to about 2.2 x 1011 lightbulbs running simultaneously! Clearly, with fossil fuels being a finite resource, it becomes more and more attractive to harness the sun's energy for our needs.

## Inner Workings

Fig. 1 displays the most basic operational principle for a solar cell. [2] Light comes in from a source, in this case sunlight, and imparts energy upon the active layer. This imparted energy causes electrical charges, positive and negative, to separate to different electrodes. This is what is called the bound electron-hole pair creation, where the electron represents the negative charge and the holes represent the positive charge. This subsequent movement of charges to the different electrodes creates an electrical current or the movement of electricity. We can then harness this electrical current for work such as powering a light bulb, turning on a calculator, and operating many other common appliances.

What differentiates polymer-based organic solar cells from other solar cells is the choice of materials for the active layer. As we can see from Fig. 2, the active layer consists of essentially two different components: a polymer and a fullerene. [3] What makes using these materials so desirable compared to say, single crystal silicon active layers, is the relative low cost for producing these cells. How exactly is it low cost to produce? There are several reasons for this. First, the materials used in these cells can be dissolved in some solution and then simply laid down on an electrode. This means we can use mass production techniques such as inkjet printing (think of printers printing sheets of solar cells!) or spin coating. Plus, processing all takes place at relatively low temperature so you can "print" the solar cell on materials like plastic instead of just glass; this means the energy input to produce these cells is more efficient compared to silicon based solar cells. [4]

 Fig. 2: Active layer materials for polymer-based organic solar cells. PQT-12 and P3HT represent active layer polymers which give up a negative charge (electron) while PC60BM and PC17BM represent active layer fullerenes which take the negative charge."

## Challenges

So just how much energy can we extract from these solar cells? Well, we measure the amount of energy a solar cell produces based upon the power conversion efficiency, i.e. what percentage of the suns light is actually being converted to electrical energy. On a nice sunny day, the earth experiences about 1000 Watts per square meter. [5] So if a solar cell is rated for 15% power conversion efficiency, then 15% of the sunlight is converted to energy:

1000 Watts/m2 x 0.15 = 150 Watts/m2

This states that if you have a solar cell that is a square meter in area, then you gain 150 Watts, which is enough to power two and a half 60 Watt light bulbs. Fifteen percent is the best efficiency we obtain for single crystal silicon solar cells. [1] Unfortunately, polymer-based organic solar cells have yet to reach that limit, with the best up-to-date cell possessing a little below 8% power conversion efficiency. [4]

In addition to the lower overall power conversion efficiencies, polymer-organic solar cells are prone to chemical degradation and photo-bleaching. Coupled with the fact that these solar are very susceptible to defect propagation, the overall lifetime of the solar cell becomes an issue. [6] Silicon-based solar cells have predicted lifetime stabilities of over 25 years even in harsh climates. Should the active layer for organic-based polymer solar cells have water or oxygen present, the solar cell degrades very quickly. With proper encapsulation, however, device lifetimes in excess of 10,000 hours (417 days) have been found. [6] Unfortunately, these cells are thus not as robust as we could hope.

## Conclusion

In summation, the world will find itself in need of a renewable energy source in order to maintain the amount of energy consumption we have today. Organic solar cells present a novel method of taking sunlight and converting it into usable energy; however, there are still barriers that must be overcome before they can see widespread use, namely, increasing the overall efficiency and increasing the device lifetime. While organic solar cells may not be the answer in the long run, they will certainly help pave the way for advances in photovoltaic technology.

© 2010 Christopher Bruner. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

## References

[1] P. V. Kamat, "Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion," J. Phys. Chem. C 111, 2834 (2007).

[2] W. Cai, X. Gong, and Y. Cao, "Polymer Solar Cells: Recent Development and Possible Routes for Improvement in the Performance," Solar Energy Materials & Solar Cells 94, 114 (2010).

[3] S. Gunes, H. Neugebauer, and N. S. Sariciftci, "Conjugated Polymer-Based Organic Solar Cells," Chem. Rev. 107, 1324 (2007).

[4] A. C. Mayer , "Polymer-Based Solar Cells," Materials Today 10, 28 (2007).

[5] B. A. Gregg, "Excitonic Solar Cells," J. Phys. Chem. B 107, 4688 (2003).

[6] M. Jorgensen, K. Norrman and F. C. Krebs, "Stability/Degradation of Polymer Solar Cells," Solar Energy Materials & Solar Cells 92, 686 (2008).