The paper is co-authored by Matthew Beard, Yong Yan, Ryan Crisp, Jing Gu, Boris Chernomordik, Gregory Pach, Ashley Marshall, and John Turner. All are from NREL; Crisp also is affiliated with the Colorado School of Mines, and Pach and Marshall are affiliated with the University of Colorado, Boulder.

Beard and other NREL scientists in 2011 published a paper in Science that showed for the first time how MEG allowed a solar cell to exceed 100% quantum efficiency by producing more electrons in the electrical current than the amount of photons entering the solar cell.

The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current. We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds.

—Matthew Beard

The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption band edge lost to heat. The MEG process takes advantages of the additional photon energy to generate more electrons and thus additional chemical or electrical potential, rather than generating heat. QDs, which are spherical semiconductor nanocrystals (2-10 nm in diameter), enhance the MEG process.

Solar-to-chemical energy conversion for energy storage, fuels and feedstocks, particularly to address greenhouse gas emissions, is one of the primary goals for scientists in the field of solar energy generation. High solar conversion efficiency is key for a viable technology as that directly impacts the land area to be covered and ultimately impacts the cost of the system. The main loss limiting the conversion efficiency in solar conversion systems is that photons with energies greater than the semiconductor bandgap (E_g) produce hot carriers that relax via electron–phonon scattering and the subsequent phonon dissipation reduces the energy conversion efficiency.

Multiple exciton generation (MEG), within quantum-confined semiconductor nanocrystals (also referred to as quantum dots, QDs), describes a process where absorption of a high-energy photon produces hot carriers that cool via the generation of additional electron–hole pairs (excitons) rather than the generation of heat and thus, overcome part of the losses associated with hot-carrier cooling. The result is that the quantum efficiency (the number of excitons produced as a fraction of the number of photons absorbed) is greater than 100% for those wavelengths that can produce hot carriers with sufficient energy to drive the MEG process.

—Yan et al.

In the current report, the multiple electrons, or charge carriers, that are generated through the MEG process within the QDs are captured and stored within the chemical bonds of a H₂ molecule.

The system consists of two separated electrodes; one electrode consists of a conductive lead sulfide (PbS) QD layer deposited on top of a FTO/TiO₂ (fluorine-doped tin oxide) dielectric stack, while the second electrode consists of a platinum (Pt) mesh and is kept in the dark.

Light is absorbed at the photoanode within the QD layer producing free electrons and holes. Photogenerated holes oxidize sulfide to sulfur (nS²⁻ + 2(n − 1)h⁺ → S_n²⁻) at the QD/solution interface, while electrons make their way to the Pt electrode where they can reduce H⁺ to H₂(g) (2H⁺ + 2e⁻ → H₂).

The overall reaction is to H₂S to H₂ and S.

Photosplitting H₂S can be economical and has the added benefit of the production of H₂ gas, which can be used as either a chemical precursor or energy source. We note that in our experimental demonstration, developed here, the energy necessary to split H₂S is provided by a combination of the chemical bias (via a pH difference in each compartment) and the photovoltage of the QD PbS photoelectrode.

—Yan et al.

Schematic of the PEC apparatus. The QD layers make up the active region of the photoanode. Photogenerated holes oxidize sulfide in one compartment while electrons reduce hydrogen in the other compartment at the dark Pt cathode. A salt bridge connects the two compartments and transports H+ and Na+. Yan et al. Click to enlarge.

Funds for the research came from the Department of Energy’s Office of Science.

Resources

• Yong Yan, Ryan W. Crisp, Jing Gu, Boris D. Chernomordik, Gregory F. Pach, Ashley R. Marshall, John A. Turner & Matthew C. Beard (2017) “Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%” Nature Energy 2, Article number: 17052 doi: 10.1038/nenergy.2017.52

• Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozik, Matthew C. Beard (2011) “Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell” Science Vol. 334, Issue 6062, pp. 1530-1533 doi: 10.1126/science.1209845