The efficiency of solar collectors is hampered by a "Goldilocks" problem: the light must have just the right amount of energy to be transformed into a voltage. Too little energy and the photons (light energy packets) pass through the panel. Too much and the excess energy disappears as heat. Various tricks have been tried to extract the energetic photons. Scientists from the University of Groningen and the Nanyang University of Technology have now shown that the combination of two materials does not waste the excess energy as heat, but instead uses it. This may possibly increase the energy efficiency of solar modules.
Semiconductors convert energy from photons (light) into an electron current. However, some photons carry too much energy for the material to absorb. These photons produce "hot electrons" and the excess energy of these electrons is converted into heat. Materials scientists have been looking for ways to harvest this excess energy. Scientists at the University of Groningen and the Nanyang University of Technology in Singapore have now shown that this can be easier than expected by combining perovskite with a "hot electron" acceptor material. Their proof of principle was published in Science Advances on November 15, 2109.
In photovoltaic cells, semiconductors absorb photon energy, but only from photons with the correct amount of energy: too little and the photons penetrate the material too much and the excess energy is lost as heat. The correct amount is determined by the band gap: the difference in energy levels between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
& # 39; The excess energy of hot electrons generated by The energetic photons are rapidly absorbed by the material as heat, "explains Maxim Pshenichnikov, Professor of Ultrafast Spectroscopy at the University of Groningen. To completely capture the energy of hot electrons, materials with a larger band gap must be used. However, this means that the hot electrons should be transported to this material before they lose their energy. The current general approach to harvesting these electrons is to slow down the loss of energy by, for example, using nanoparticles instead of bulk materials. "In these nanoparticles, the electrons have fewer opportunities to release the excess energy as heat," explains Pshenichnikov.
Together with colleagues from the Nanyang Technological University, where he worked as a visiting professor for three years, Pshenichnikov studied a system in which an organic-inorganic hybrid perovskite semiconductor with the organic compound bathophenanthroline (bphen), a material with large band gap, was combined. The scientists used laser light to excite electrons in the perovskite and studied the behavior of the generated hot electrons.
& # 39; We used a method called pump-push probes to stimulate electrons in two steps and to study femtosecond timescale, "explains Pshenichnikov. This allowed the scientists to generate electrons in the perovskites with energy levels just above the band gap of bphen without exciting electrons in bphen. Hot electrons in this material would have come from the perovskite.
The results showed that hot electrons from the perovskite semiconductor could easily be absorbed by Bphen. This happened without these electrons had to be slowed down and beyond that in bulk solids. The hot electrons were thus harvested without tricks. "However, the scientists found that the energy requirement was slightly higher than the bphen band gap. "That was unexpected. Obviously, some extra energy is needed to overcome a barrier at the interface between the two materials. "
Nevertheless, the study provides evidence for the principle of harvesting hot electrons in perovskite semiconductor material. Pshenichnikov: "The experiments were carried out with a realistic amount of energy comparable to visible light. The next challenge is to construct a real device using this combination of materials.
Reference: "Hot Charge Extraction in CH 3 NH 3 PbI 3 revealed by pump-push probe spectroscopy "By Swee Sien Lim, David Giovanni, Qiannan Zhang, Ankur Solanki, Nur Fadila Jamaludin, Jia Wei Melvin Lim, Nripan Mathews, Subodh Mhaisalkar, Maxim S. Pshenichnikov and Tze Chien Sum, November 15, 2019, Advances in the Science .
DOI: 10.1126 / sciadv.aax3620