The effectiveness of solar panels is hampered by the problem of "Gold Bears": light must have the right amount of energy to be converted to voltage. Too little energy and photons (packets of light energy) pass right through the panel. Too much and excess energy disappears as heat. Several tricks have been tried to collect high-energy photons. Scientists at the University of Groningen and Nanyang University of Technology have shown that by combining two materials, excess energy is used rather than wasted as heat. This can potentially increase the energy efficiency of solar panels.
Semiconductors convert photon (light) energy to 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 are looking for ways to collect this excess energy. Scientists at the University of Groningen and Nanyang Technological University (Singapore) have now shown that this may be easier than expected by combining perovskite with acceptor material for "hot electrons". Their proofs of principle were published in Advances in Science on November 15, 2109.
In photovoltaic cells, semiconductors will absorb photonic energy, but only from photons that have the required amount of energy: too little and photons pass right through the material, too much and excess energy is lost as heat. The exact amount is determined by the range: the difference in energy levels between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO).
"The excess energy of hot electrons produced by high-energy photons is very quickly absorbed by the material as heat," explains Maxim Pshenichnikov, professor of ultrafast spectroscopy at the University of Groningen. Materials with a greater range should be used to fully capture the energy of hot electrons. However, this means that hot electrons must be transported to this material before losing their energy. The current common approach to collecting these electrons is to slow down energy loss, for example by using nanoparticles instead of bulk materials. "In these nanoparticles, electrons are less likely to release excess energy as heat," explains Pshennikov.
Together with colleagues at Nanyang Technological University, where he has been a visiting professor for the past three years, Wheatmannikov is studying a system in which an organic-inorganic hybrid perovskite semiconductor is combined with the organic compound batophenanthrolin (bphen), a large-band material. Scientists used laser light to excite electrons in the perovskite and study the behavior of the hot electrons that were generated.
"We used a method called pump and push drilling to excite electrons in two stages and investigate them at femtosecond time ranges," explains Pshennikov. This allowed scientists to produce electrons in perovskites with energy levels just above the range of bphen, without exciting electrons in bphen. Therefore, all the hot electrons in this material would come from the Perovskite.
The results show that the hot electrons from the perovskite semiconductor are readily absorbed by bphen. "It happened without the need to delay these electrons and, moreover, in bulk materials. So, without any tricks, the hot electrons were collected. "However, scientists have noticed that the energy required is slightly higher than that of bphen." This was unexpected. Obviously, a little extra energy is needed to overcome the barrier between the two materials. "
However, the study provides evidence for the principle of collecting hot electrons in bulk semiconductor materials from perovskite. Pshennikov: "The experiments were carried out with a realistic amount of energy comparable to visible light. The next challenge is to build a true device using this combination of materials. "
Reference:" Extraction of hot carrier in CH 3 NH 3 PbI 3 revealed by pump-piston 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 Tim Chien Sum, November 15, 2019, Scientific Advances .
doi: 10.1126 / sciadv.aax3620