As the world slowly transitions to renewable energy sources, solar photovoltaic devices will play a key role. Maximizing the light energy-to-electrical energy conversion in photovoltaic devices is key to successful deployment of this technology. A perfect energy conversion efficiency of 100% would mean that all of the energy carried by the solar photons incident on the device is converted to electrical energy. Realistically, efficiency is limited by incomplete photon trapping, less-than-perfect conversion of every incident photon to an electron-hole pair (i.e. quantum efficiency less than 100%), and energy loss due to thermalization of electrons excited to energies much higher than the conduction band edge.

Recent work by Wang et al. from Kunming University of Science and Technology in Kunming, China tackles the last of the aforementioned inefficiencies. When a photon with energy larger than the semiconductor bandgap is incident on a photovoltaic, the energy difference between the large photon energy and the bandgap energy is lost as unusable heat in the material. Is there a way to reclaim this wasted energy? One approach is to use multijunction solar cells, where layered materials with tapered bandgaps can optimize energy conversion for photons across the solar spectrum. Indeed, multijunction solar cells hold the record for highest energy conversion efficiencies. The drawback is that they are expensive and complicated to fabricate.

Alternatively, instead of tailoring the material to absorb the incident solar spectrum, one can tailor the incident light spectrum to suit the photovoltaic. For example, if an incident photon has an energy twice the semiconductor bandgap energy, it could conceivably be energetically down-converted into two photons, each with half the initial photon energy, before entering the photovoltaic. The two low-energy photons can then be absorbed in the device with minimal thermalization loss. And because an initial high-energy SINGLE photon is down-converted into TWO low-energy photons which each give rise to en electron-hole pair, the quantum efficiency is doubled as well.

Of course, the photon down-conversion process comes with its own host of complications and inefficiencies. A leading method for down-conversion is doping crystalline host materials with rare-earth ions like praseodymium. However, dopants that absorb high energy photons efficiently do not necessarily down-convert them efficiently (and vice versa). To get around this, other approaches involve doping the crystalline host with two species, one of which is a good absorber of high-energy photons (like Tm³⁺) and the other is a good emitter of down converted photons (like Yb³⁺). On top of that, energy from the absorber ion must be efficiently transferred to the down-converter ion.

Wang et al. tackle this last issue in this Applied Optics article. Instead of transferring absorbed energy to the Yb³⁺ ion, Tm³⁺ can relax via several alternative mechanisms, one of which is simply to reemit the absorbed high-energy photon. To block this particular relaxation process, the authors use a three-dimensional photonic crystal with a photonic bandgap centered at the reemission wavelength. Because the photonic density of states at this wavelength is suppressed by the photonic crystal, the self-relaxation of Tm3+ is suppressed, which then favors the energy transfer process to the Yb³⁺ ions.

Wang et al. demonstrate this experimentally by fabricating 3D photonic crystals in the inverse opal structure made of YPO4 doped with Tm³⁺ and Yb³⁺. When illuminated with 480nm (high-energy) photons, reemission of Tm³⁺ at 650 nm is suppressed while emission via down-conversion in Yb³⁺ at 980nm is enhanced. The authors estimate the quantum efficiency of the process to be nearly 150% or an average of 1.5 electron-hole pairs generated for every 480nm incident photon.

Ultimately, the authors’ work provides insight into the energy transfer processes and it offers an effective way of controlling those processes. A possible next step is consideration of the radiation pattern of the down-converted photons. The key is ensuring that the photons go toward the absorbing region of a photovoltaic device.

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