The efficiency of solar panels is hampered by a "Goldilocks" problem: the light must have just the right amount of energy to convert to a voltage. Too little energy and the photons (light energy packets) pass right through the panel. Too much and the excess energy disappears as heat. Several tricks have tried to harvest high-energy photos. Researchers from the University of Groningen and Nanyang Technological University have now shown that by combining two materials, the excess energy is used rather than wasted as heat. This can potentially increase the energy efficiency of solar panels.
Semiconductors convert energy from photons (light) into an electron current. However, some photons carry too much energy for the material to be absorbed. These photons produce "hot electrons", and the excess energy from these electrons is converted to heat. Material scientists have been looking for ways to reap this surplus energy. Researchers from the University of Groningen and Nanyang Technological University (Singapore) have now shown that this can be simpler than expected by combining a perovskite with an acceptor material for "hot electrons." Their proof of principle was published Science Advances on November 15, 2109.
In photovoltaic cells, semiconductors will absorb photon energy, but only from photons that have the right amount of energy: too little and the photons pass right through the material, too much and the excess energy is lost as heat. The right amount is determined by the band gap: the difference in energy levels between the highest absorbed molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
"The excess energy from hot electrons produced by the high-energy photons is very rapidly absorbed by the material as heat," explains Maxim Pshenichnikov, professor of ultra-fast spectroscopy at the University of Groningen. To fully capture the energy from hot electrons However, this means that the hot electrons should be transported to this material before they lose their energy. The current general method of harvesting these electrons is to slow down the energy loss, for example, by using nanoparticles instead of bulk materials. " there are fewer alternatives for electrons to emit the excess energy as heat, "explains Pshenichnikov.
Together with colleagues from Nanyang Technological University, where he was a visiting professor for the past three years, Pshenichnikov studied a system in which an organic-inorganic hybrid perovski t semiconductor was combined with the organic compound bathophenanthroline (bphen), a material with a large band gap. The researchers used laser light to awaken electrons in the perovskite and studied the behavior of the hot electrons generated.
"We used a method called pump-push probing to wake electrons in two stages and study them in the femtosecond times," explains Pshenichnikov. This allowed the researchers to produce electrons in the perovskites with energy levels just above bphen's bandgap without exciting electrons in the bphen, therefore all the hot electrons in this material would come from the perovskite.
The results showed that hot electrons from the perovskite semiconductor were easily absorbed by the bphen. So, without any tricks, the hot electrons were harvested. "But the researchers noticed that the energy required was slightly higher than the bphen bandgap." This was unexpected. Obviously some extra energy is needed to overcome a barrier at the interface between the two materials. & # 39;
Nevertheless, the study provides evidence on the principle of hot electron harvesting in bulk perovskite semiconductor material. Pshenichnikov: 'The experiments were carried out with a realistic amount of energy, comparable to visible light. The next challenge is to design a real device using this combination of materials. "
Reference:" Extraction of Hot Carriers in CH 3 NH 3 PbI 3 was revealed with pump-push probe. spectroscopy ”by Swee Sien Lim, David Giovanni, Qiannan Zhang, Ankur Solanki, Nur Fadilah Jamaludin, Jia Wei Melvin Lim, Nripan Mathews, Subodh Mhaisalkar, Maxim S. Pshenichnikov, and Tze Chien Sum, November 15, 2019, Science Advances .
DOI: 10.1126 / sciadv.aax3620