Author: deibel

How Do Organic Solar Cells Function? – Part One

The first organic solar cells where based on an active layer made of a single material. By the absorption of light, strongly Coulomb-bound electron hole pairs where created, singlet excitons. As described in part zero, these have to be split in order to finally generate a photocurrent. In order to overcome the binding energy, one has to either hope on the thermal energy, or dissociate the exciton at the contacts. Unfortunately, both processes have a rather low efficiency: under normal conditions, the temperature is not high enough, and the sample thickness is much thicker than the exciton diffusion length. Early morning in north west Spain The consequence: excitons are mostly not dissociated, but recombine instead. This leads to luminescence, and light emitting solar cells do not belong to the most efficient… there is just not enough current output.

The introduction of a second layer was a quantum leap in terms of power conversion efficiency (though still on a low level): organic bilayer solar cells, presented in the mid eighties [Tang 1986]. The light is usually absorbed mainly in the so-called donor material, a hole conducting small molecule. The photogenerated singlet excitons now can diffuse within the donor towards the interface to the second material, the acceptor, which is usually strongly electronegative. A prominent example for an electron acceptor material is the buckminsterfullerene (C60).

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How Do Organic Solar Cells Function? – Part Zero

In a classical inorganic solar cell, pairs of charge carrier – an electron and a hole – are generated by the absorbed sunlight. These two oppositely charged carriers are only weakly Coulomb bound, due to the screening being rather efficient in this material class. The potential drop at the interface between a p- and an n-doped semiconductor layer (the pn junction), leads to their separation and subsequent transport to the respective contacts: a current flows. In organic semiconductors, things are somewhat different.

Here, the screening of opposite charges is much weaker as the dielectric constant is lower. This leads to a much stronger interaction of the photogenerated positive and negative charges. Spring in France Therefore, the primary optical excitation in organic materials is called (singlet) exciton, i.e., a strongly bound electron-hole pair. As this binding is more difficult to be overcome as compared to inorganic systems, the concept of organic solar cells has to be different… which we will come back to later.

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Why Disordered Materials?

So why disordered materials? Arguing from an application based (=engineer) point of view, disordered materials are usually easier and cheaper to be manufactured than (single or poly) crystalline ones. Looking at organic semiconductors, such as conjugated polymers or fullerene derivatives (“bucky balls”): they are soluble and can thus be deposited from the liquid phase – e.g., by printing (offset, inkjet, you name it). Device configuration of a Disordered Organic Solar Cell made of conjugated polymers (red chains) blended with fullerene molecules (bucky balls) The vision for the so called plastic electronics is to print circuits and devices on flexible substrates. This can be done at room temperature (low energy) and ideally with roll-to-roll processes (high throughput). Sounds good, eh?

Well, there are some drawbacks in terms of the application… though not for researchers;-) Printing semiconductors usually leads to rather disordered films, which have very low charge carrier mobilities as compared to Silicon and other inorganic semiconductors, and are thus not suitable for high-frequency applications. However, for photovoltaic devices, the low mobilities are not that much of a drawback. That said, organic solar cells are still at below 7% power conversion efficiencies… for very small areas. Single crystalline Silicon, on the other hand, sees already above 20% for modules.

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