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Solar & Alternative Energy

The optimal band gap for plastic photovoltaics

Polymer solar cells operate differently from inorganic p-n junction-based solar cells, with consequences for their design and performance.
26 January 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0528

Solar cells are currently attracting much attention as potential energy sources. 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. Despite recent improvements and considerable effort, 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 To identify how to improve these cells, we have developed a numerical model that describes their electrical characteristics, as outlined in a previous SPIE Newsroom article.2 Here we argue that the optimal value of one of the key material parameters—the band gap of the light-absorbing plastic—is significantly different from that predicted for inorganic silicon-based solar cells.

Plastic (or polymer) solar cells consist of two materials, the polymer and an acceptor, to facilitate generation of free charge carriers. When a photon is absorbed, a bound state of an electron and a hole (or complementary positive charge) called an exciton is created (see Figure 1, process 1). Figure 1 shows the ionization potential (IP) and electron affinity (EA) of both the polymer and the acceptor phase. A small difference (∼0.4 eV) between the EA of the acceptor and the polymer is necessary to ensure efficient exciton dissociation (Figure 1, process 2).

Figure 1. The scheme illustrates the relation between the energy levels of polymer/acceptor solar cells and the processes of exciton creation (1) and electron transfer (2).

As Figure 1 shows, the maximum voltage that a plastic solar cell can supply, the open-circuit voltage Voc, is limited to the difference between the IP of the polymer and the EA of the acceptor. In fact, Voc is significantly smaller than this limit.3Voc can therefore be increased by reducing the difference between the EA of the acceptor and the polymer (typically 1 eV).

Another key parameter of solar cells is the current they can supply: the so-called short-circuit current, Jsc. The more photons are absorbed by the polymer, the higher Jsc can be. The solar spectrum is shown in Figure 2, together with the absorption spectrum of a polymer/acceptor film of typical thickness. It is clear that only a small fraction of the incident photons are absorbed. By decreasing the band gap of the polymer (currently usually 2.1 eV), more photons can be harvested, giving rise to higher Jsc.

Decreasing the band gap has an important side effect: the highest attainable Voc is lowered simultaneously, implying that there exists an optimal value for the band gap. It is well known that for inorganic p-n-junction-based solar cells, the best value of the band gap is 1.4eV. It is not clear, however, whether this also applies to plastic solar cells.

Our ability to accurately model polymer/acceptor solar cells enables us to calculate the effect of varying the band gap of the polymer while keeping the IPs of both materials such that not more than 0.5eV is lost in electron transfer.4 As a starting point, a much-studied materials combination with an efficiency of 3.5% is used.4 It turns out that the optimal value of the band gap lies around 1.9–2.0eV (see Figure 3), suggesting that the optimal value is equal to 1.4eV plus the offset between the EA of the polymer and the acceptor. Further calculations show that these devices can reach an efficiency of at least 10%.4

The high exciton binding energy typical for organic materials strongly influences the design and performance of solar cells made from them. As for polymer-based solar cells, the optimal value for the band gap thus differs from the value found for inorganic p-n-junction solar cells.

Figure 2. Comparison of the solar spectrum with that of a typical plastic solar cell shows how few incident photons are absorbed.

Figure 3. The the band gap of the absorbing polymer influences the efficiency of the solar device.

L. Jan Anton Koster, Valentin D. Mihailetchi, Paul W. M. Blom
University of Groningen
Molecular Electronics, Material Science Centre Plus and Dutch Polymer Institute, 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 and inorganic solar cells.

Valentin Mihailetchi received his PhD in physics from the University of Groningen, working on modeling and characterization of organic solar cells. Recently, he 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.