Enhancing solar cells with photochemical upconversion

Abstract

Photovoltaics have the potential to become a major contributor to future sustainable energy generation. To this end, device efficiency needs to be brought beyond present physical limits. Solar cells cannot absorb light with less energy than the bandgap (absorption threshold of the photovoltaic absorber material), and consequently this energy source is usually uncaptured. Upconversion (UC) devices harvest those unused subthreshold photons behind the solar cell, create one higher-energy photon out of (at least) two transmitted photons, and radiate upconverted light back toward the solar cell, thus expanding the usable solar spectrum (see Figure 1). Key requirements for UC units are broad absorption and high UC quantum yield under even the low-intensity incoherent illumination that is available to solar energy conversion devices. Upconversion was proposed as a means to increase solar cell efficiency about a decade ago,1 and research has since then been primarily focused on UC processes occurring in lanthanide-doped glasses or nanoparticles.2 Although transitions between the rare earth 4f orbitals lie in an energy range promising for combination with common crystalline silicon solar cells, UC systems based on erbium (Er) or ytterbium (Yb) suffer from weak absorption by Laporte-forbidden transitions, a narrow absorption bandwidth, and the resulting low efficiency. Recently, triplet-triplet annihilation (TTA) in organic molecules has been demonstrated to be an alternative UC mechanism highly promising for solar energy application.3 It proceeds efficiently even for low photon flux and displays highly tunable absorption characteristics. The key to high quantum yield from TTA-UC lies in the combination of two different classes of organic molecules dissolved together in organic solvents or in a polymeric host matrix.4 A socalled sensitizer species, which in many cases is a metalated porphyrin, absorbs a low-energy photon (process 1 in Figure 2) and undergoes fast intersystem crossing (ISC, process 2), whereby Figure 1. Principle of a solar cell assisted by upconversion (UC). The creation of usable electron-hole pairs in the photovoltaic absorber material (left) is augmented by light, usually transmitted unused, which is converted to higher photon energies in the UC unit (right).