Every day, we face the possibly greatest energy machine ever. It is the sun, and with the help of solar cells, its energy can be used to generate electrical current.

Most available solar cells are made of inorganic materials (this means that they do not contain any carbohydrates), mainly silicon, which is quite common in nature. Nevertheless, it has been tried to improve these solar cells and the idea of organic solar cells came up. They are said to be low-weight, flexible and transparent (which is important for panoramic roofs). Research about such cells was also started, because the production of organic solar cells was thought to be cheaper, but this has changed in the meantime
. Unfortunately, organic solar cells have a big disadvantage: Their energy conversion efficiencies are far lower (4-5%) than the energy conversion efficiencies of conventional solar cells (up to 39%) [1]. This means that they can transform less energy from the sun than inorganic ones.That is the main project in laboratories at the moment: Trying to enhance the energy conversion efficiency of organic solar cells [2] [3] [4].

Here I describe a series of experiments at the Institute of Chemistry and Technology of materials at the Technical University Graz, in which I investigated the influence of different production parameters on the energy conversion efficiency of polymer fullerene solar cells (a special type of organic solar cells). What is the energy conversion efficiency? It is the ratio between the generated electrical current and the energy from the sun. It tells you, how much of the sun‘s energy can be transformed into electrical energy. This article should give you a short overview of this topic.

HOW DO ORGANIC SOLAR CELLS WORK?

Organic solar cells are made of organic molecules (containing carbohydrates). The basic design can be compared with a sandwich. An electron donor and an electron acceptor are placed between two electrodes [5]. If solar cells are lighted, the electrons of the donor material reach a higher level of energy. The electrons are transferred from the donor to the acceptor material, which is able to absorb electrons. There is a flow of electrons and since electrical current is defined as a flow of charge, electrical current is being generated (see figure 1) [6].

THE EXPERIMENT

In the laboratory, I have produced bulk heterojunction solar cells, a special type of organic solar cells. Simplified, the design consists of a glass plate with anode material on it. Then the donor and acceptor layer are mixed and coated onto the plate. Finally, a metal electrode is added to the plate (see figure 2) [7].

The donor materials in my experiments were three different polymers, abbreviated: P3HT, PCDTBT and PTB7. Different polymers were investigated and finally compared, because the chemical structure has a major influence on the energy conversion efficiency of solar cells. The fullerene phenyl-C61-butyric acid methyl ester (PCBM) was the acceptor in this series of experiments. Fullerene is a big organic molecule, similar to graphite that looks like a football (see figure 3) [8].

PRODUCTION OF THE ORGANIC SOLAR CELLS

After cleaning the glass plate and applying the different layers onto the substrate, aluminium contacts were vapour deposited as electrodes. Then the cells were tempered for 30 minutes (see figure 4).

To find out how to improve my solar cells, I tried out four different temperatures. In another series of experiments I added process additives to the active layer to investigate their impact on the energy conversion efficiency. Process additives are chemicals which are said to have an impact on the properties of the finished layer. They can either improve or worsen the energy conversion efficiency of the solar cells.This has to be found out. Among other parameters, the energy conversion efficiency of the obtained solar cells (see figure 5) was measured with the help of a measuring box (see figure 6). The solar cells were exposed to light and the generated electrical current was measured. The electrical current was then compared with the incoming energy from the light and the energy conversion efficiency was calculated.

RESULTS

I know, this was a lot of (probably) complicated information and as you might already be thinking, I got a lot of data. That is why I will take two examples which I want to explain to you: Have a look at figure 7: After applying the polymer fullerene solution onto the plate, the latter was heated. The diagram shows the achieved efficiencies of solar cells produced with the polymer P3HT. The solar cells had been exposed to four different temperatures. The first measuring point shows the efficiency of solar cells which only experienced room temperature. The highest total energy conversion efficiency was achieved when the plate was previously heated to 140°C for 30 min. The energy conversion efficiency at 140°C is higher than at other temperatures, because thermal annealing (heating the plates) changes the structure of the polymer fullerene layer. In detail, thermal annealing leads to a larger interfacial area between donor and acceptor and improves the pathways for the electrons.

This leads to increased absorption of light and improved charge transport to the electrodes. This is important since electrical current is a flow of charge. But the energy conversion efficiency decreases when annealing temperatures are too high, because the structure of the molecules is changed. This makes the production of free electrons more difficult [9]

. The second parameter investigated was the process additives.

When solar cells are produced, process additives are added to the polymer fullerene solution before it is coated onto the substrate. Figure 8 shows the influence of process additives on P3HT solar cells. The efficiency is plotted against the amount of process additive. Each of the used process additives: 1,3 diiodooctane (DIO) and benzene-1,3-dithiol (BED) has an impact on the energy conversion efficiency. In general, process additives are said to improve the mobility of holes, which can be described as positive charge carriers.

Diiodooctane is said to increase this mobility by adapting the morphology of the polymer fullerene film [10]. So, solar cells are still a big field for investigations. There are always new ideas and although they are rarely available so far, organic solar cells might be the future of solar cell technology, since they have advantages like flexibility and transparency. This may lead to new possibilities in architecture, such as panoramic roofs. Every single change in the production can have a huge impact and there is a lot of  work behind a single solar cell. Perhaps you realize now what happens inside some of your daily gadgets. And when you see a football next time, you might react just the same as I do: „Oh my god, the football looks like a huge fullerene!“

Figures: Author

Vocabulary:

electrical current: Strom

to enhance: erhöhen

silicon: Silizium

energy conversion efficiency: Energieumwandlungseffizienz

carbohydrates: Kohlenwasserstoffe

to vapour-deposit: aufdampfen

to temper: tempern, härten, wärmebehandeln