Molecular approaches to next-generation photovoltaic-energy conversion

Solar photovoltaic energy presents a tremendous opportunity for society to make the transition to sustainability. However, economic barriers remain to widespread uptake of solar electricity, especially in countries such as Australia, where cheap coal abounds. The imperative is, therefore, to drive down the cost of solar electricity by getting ‘more bang for your buck.’ Two distinct strategies to achieve this are to make solar cells more efficient or to collect sunlight over a large area and concentrate it onto a much smaller slab covered with solar cells. Both must be achieved at low incremental cost.

Organic, molecular materials have highly reproducible and controllable optical properties. We are investigating ways in which these attributes may be exploited to increase the efficiency of solar-light harvesting. Here we give an overview of projects underway that are aimed at slashing the cost of solar electricity.

By design, single-energy-threshold solar cells are limited to a maximum energy-conversion efficiency of 32.9% under the standard AM1.5G spectrum (representing average solar irradiation at the Earth's surface). While this depends on the material's absorption threshold, many first-generation technologies, such as crystalline silicon, are approaching this performance limit. By sacrificing efficiency, cost savings can be made by employing cheaper processing methods and a smaller amount of photoactive material. These second-generation solar cells include technologies such as thin-film silicon and organic (plastic) photovoltaics. The field of third-generation photovoltaics seeks to build on the cost savings of second-generation cells while circumventing the single-energy-threshold limit. One strategy employed to achieve this goal is to transform the solar spectrum by upconversion, where long-wavelength light used poorly by a single-threshold solar cell is converted to a shorter wavelength that is of more use to the photovoltaic device.

Figure 1. Red light is upconverted to blue in a molecular solution. The upconversion margin is 0.96eV.

We have been investigating triplet-triplet annihilation upconversion (TTA-UC), which uses organic molecules to harvest low-energy light and conjoin the energy of two photons to generate higher-energy radiation. This technique is extremely flexible with regard to wavelength. Using a range of porphyrin molecules synthesized by Max Crossley's research group at the University of Sydney (Australia), we have succeeded in upconverting red to blue (see Figure 1), green, and yellow, as well as green to blue light. Our experiments have shown that TTA-UC can proceed with high efficiency. In the tens of microseconds following laser excitation, we find that as many as 33% of absorbed quanta take part in the TTA-UC process, with instantaneous efficiencies exceeding 40%.^1,2 Moreover, kinetic analysis reveals that the process would reach its maximum at a value exceeding 60% (where 100% represents the maximum quantum efficiency of 50%). The kinetic parameters derived from these experiments allow us to evaluate the expected efficiency under solar irradiation. Although this remains in the few-percent range under sunlight, we anticipate steady improvement as we continue to optimize kinetic parameters. Nonlinear, detailed balance modeling shows that the efficiency of a 2eV-bandgap solar cell that uses annihilation-based upconversion can be improved by more than 50%. However, we are also interested in improving crystalline silicon cells, which have bandgaps of approximately 1.1eV. We have, therefore, synthesized a dye capable of absorbing light below the silicon threshold.

By engineering the upconversion mechanism into the solar cell directly, one may harvest electrons from the excited dye rather than upconverted photons. A dye-sensitized solar cell operating under these conditions has an energy-conversion-efficiency limit exceeding 40% at a bandgap of 2eV.³ In collaboration with Ned Ekins-Daukes at Imperial College (London, UK), we are working on the underlying molecular photophysics to make molecular intermediate-band solar cells a reality.

Luminescent solar concentrators (LSCs) were proposed in the 1970s as a way to collect a large area of sunlight and direct this onto a small slab of solar cells. The principle is simple: light falls onto a planar waveguide doped with a highly fluorescent material such as an organic dye. The ensuing fluorescence is isotropic. Approximately 75% of the re-emitted light is trapped inside the waveguide by total internal reflection and guided onto the edges of the slab, which are dressed with photovoltaic cells. A key advantage of LSCs is their ability to concentrate diffuse light. Therefore, they do not need to track the sun like parabolic reflectors and can be integrated into architecture as functional windows.

However, reabsorption is a potential problem. Even if the dye is 100% fluorescent, each reabsorption event may result in photon escape from the waveguide. To address this problem, we have been investigating luminophore alignment, where the fluorescent dye's electronic-transmission moment is held perpendicularly to the waveguide.⁴ This results in demonstrated decreases in emission from the top and enhanced emission from the edges of the waveguide as the percentage of trapped radiation increases to 94%. The expected efficiency enhancements should see commercial deployment of next-generation LSCs in the next few years.

Part of this work was done by PhD students Yuen-Yap Cheng, Rowan MacQueen, and Murad Tayebjee, undergraduate student Derrick Roberts, and postdoctoral researchers Raphaël Clady and Burkhard Fückel at the University of Sydney.