Improving Solar Efficiency Through Tandem Design

Michael A. Greenwood

Organic solar cells have several potential benefits. Compared with their nonorganic brethren, they are lightweight, inexpensive and easy to produce. Unfortunately, they remain relatively ineffective, converting much less sunlight into usable electricity than other approaches.

A research team at National University of Singapore seeking to bolster this critical factor reports that it has devised a tandem photovoltaic cell that covers more of the spectrum and that exhibits promising performance.

As shown in this figure, CuPc and PCBM form a bilayer heterojunction subcell. Excitons produced in CuPc will diffuse to the interface, and because of the energy offset between CuPc and PCBM, these excitons can be dissociated at the donor-acceptor (CuPc/PCBM) interface and collected by the electrodes. Meanwhile, the PCBM and P3HT blend forms the bulk heterojunction subcell. Excitons are dissociated at the interfaces throughout the bulk of the blend layer, and free electrons are collected. PCBM is used to form simultaneously the bilayer and the bulk heterojunction subcells. Courtesy of Chunxiang Zhu.

Although tandem solar cells have been devised before, team member Chunxiang Zhu said that his group’s design does not require the intermediate metal layer that results in the loss of valuable light from absorption and reflection. Omitting the layer not only reduces light loss but also simplifies the fabrication process.

Investigators designed the tandem cell on top of indium-tin-oxide-coated glass substrates, coating this with a 50-nm layer of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate). A layer of copper phthalocyanine (CuPc) was added next, followed by a blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). An aluminum electrode was deposited on the top.

This design resulted in two subcells, expanding the spectral absorption to almost the entire visible range. Additionally, the CuPc layer enhanced the cell by not blocking hole transport to the bottom electrode.

During experiments, the tandem cell was subjected to a simulated light intensity of 100 mW/cm². The absorption spectra were measured with a Shimadzu UV-VIS-NIR spectrophotometer.

The investigators found that, with the combination of materials, the tandem solar cell achieved a power conversion efficiency of 2.79 percent, nearly equal to the sum that its component parts achieved on their own. The highest-performing organic cells, meanwhile, currently exhibit efficiencies in excess of 6 percent.

Layer thickness in the tandem cells was a key parameter investigated by the scientists. They found that if the CuPc layer was too thick, excitons were lost and performance suffered. A thickness of 8 nm was determined to be optimal.

Zhu said that the team is working to further improve the efficiency of the tandem cells, and he is optimistic that the devices will produce more power.

Applied Physics Letters, Feb. 28, 2008, Vol. 92, 083310.