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Solar & Alternative Energy

Improving the efficiency of plastic solar cells

Modeling plastic solar cells creates opportunities for targeted improvement of their performance.
3 July 2006, SPIE Newsroom. DOI: 10.1117/2.1200606.0289

As the need for renewable energy sources becomes increasingly apparent, solar cells are attracting more attention. Those made from thin plastic films are particularly attractive because they are relatively easy to produce, structurally flexible, and can be applied to large areas at low cost. Additionally, their material properties can be tailored by modifying their chemical makeup, resulting in greater customization than traditional solar cells allow. Although significant progress has been made, the efficiency of plastic solar cells—the proportion of sunlight energy that they successfully convert into electric energy—is currently limited to 4–5%,1 whereas traditional devices typically achieve around 20%. To improve the efficiency of plastic solar cells, it is crucial to understand what limits their performance.

In contrast to semiconductors, light absorption in plastics does not directly result in free charge carriers. Instead, it produces neutral excitons, each one comprised of an electron and a hole, or complimentary positive charge. Therefore, all plastic solar cells must consist of two materials: an electron donor and an electron acceptor. By carefully matching these materials, the breaking up of the exciton by transferring an electron to the acceptor is energetically favored. After breakup, the electrons and holes exist in isolation from each other in the two materials, reducing the probability that they will recombine. Instead of simply stacking these two materials in layers, far more efficient solar cells are created by intimately mixing these materials to provide a larger area over which electron transfer can occur

Clearly, plastic solar cells are a far cry from their traditional semiconductor counterparts in terms of materials, structure, and characteristics. Our efforts are focused on understanding and modeling the electrical and optical properties of plastic solar cells.

In the metal-insulator-metal (MIM) model, we have demonstrated that the blend of the donor and the acceptor can be treated as one imaginary intrinsic semiconductor, or effective medium.2 The properties of the donor and acceptor materials determine the transport of holes and electrons, respectively, through this effective medium.

The MIM model consistently describes a variety of material combinations and properties. One important issue is the dependence of the short-circuit current Jsc (the maximum current a solar cell can supply) and the open-circuit voltage Voc (the cell's maximum voltage output) on incident light intensity I. Our model has shed new light on the nature of these dependencies.

A phenomenon specific to an effective medium described by the MIM model is the existence of a space charge limit to the photocurrent. This phenomenon has also been observed experimentally,3 but is not present in the pn junctions used in traditional solar cells. Such a space charge limit is characterized by a square-root dependence on effective voltage Vo – V and a I3/4 dependence on incident light intensity (see Figure 1).3 We have shown that the power law dependence of Jsc on intensity  is not related to bimolecular recombination as expected, but rather is caused by the space charge that results from a large difference in electron and hole mobility. 4 The MIM model consistently describes these effects (see Figure 2).


Figure 1. At various light intensities, photocurrent Jph varies as a function of effective voltage Vo–V.
 

Figure 2. The short-circuit current density, Jsc, of a plastic solar cell increases with light intensity.
 

A pn-junction model also cannot explain the incident light intensity dependence of the open-circuit voltage.5 We have been able to derive a formula for Voc within the MIM model,5,6 that explains this observed behavior (see Figure 3).


Figure 3. The open-circuit voltage Voc of a plastic solar cell increases with light intensity.
 

Plastic solar cells are a promising alternative to traditional semiconductor devices, but a better understanding of them is crucial for more directed efforts to improve their performance. Based on our model, we have proposed enhancements that may improve their efficiency to more than 10%.7


Authors
Lambert Jan Anton Koster
Materials Science Centre Plus and DPI, University of Groningen
Groningen, The Netherlands
Jan Anton Koster is a PhD student in the Molecular Electronics group at the University of Groningen. In addition to modeling organic solar cells, he is also involved in fabricating and characterizing hybrid organic/inorganic solar cells.
Valentin D. Mihailetchi, Paul W. M. Blom
Materials Science Centre Plus, University of Groningen
Groningen, The Netherlands
Valentin Mihailetchi received his PhD in physics from the University of Groningen, working on modeling and the characterization of organic solar cells. Recently, he has moved to the Energy Research Centre of the Netherlands where he is working on inorganic solar cells.
Paul Blom was appointed in May 2000 as a Professor at the University of Groningen, where he heads a group studying the electrical and optical properties of organic semiconducting devices. At present, the group's main focus is on the device physics of polymeric light-emitting diodes, transistors, and solar cells.