Intermediate‐band effect in hot‐carrier solar cells

Abstract

I have proposed that two-step photoexcitation of carriers is feasible in hot-carrier solar cells (HC-SCs), like that in intermediate-band solar cells (IB-SCs). The second excitation from the IB to the conduction band (CB) in an IB-SC is regarded to make the carriers in the unoccupied band hot. In an analogous way, in an HC-SC, a photoexcited carrier arising from the interband transition can be further excited by the next photon via the intraband transition. In other words, the lower-energy states in the CB function as the IB. Owing to this IB effect, the absorbed energy increases while the excited carrier number decreases under the optimal operating conditions; the latter leads to a decrease in the energy dissipation associated with the hot-carrier extraction. Consequently, the limiting conversion efficiency of the HC-SCs improves by up to 6% to 9% (absolute) compared with that with no IB effect, under the condition of the carrier thermalization time of 1 ns and 1000 times concentrated solar illumination. The detrimental impact of the increase in the emitted energy due to the intraband transition is well suppressed under the suitable operating conditions. Thus, the IB effect always provides positive results even if the effect is weak, contrasting to that the weak second excitation could lower the conversion efficiency of an IB-SC.

Electron Transfer at Quantum Dot–Metal Oxide Interfaces for Solar Energy Conversion

“…The constrain is linked to the definition of a univocal hot temperature for the photogenerated hot carriers in the QD absorber. Electrons populating different electronically isolated states in quantum confined systems are likely not in thermal equilibrium (i.e., each electronic state can be defined by a finite T ); as such, a low energy electron in the QD LUMO will require the assistance of a second IR photon to gain the required excess energy to reach a higher in energy selective contact. , This deviation from theory, imposed by quantum confinement in QD-based systems, makes any approach employing QD–MO interfaces closer to the intermediate band solar cell concept rather than to the HCSCs. , In the case a second IR photon is needed to extract an elecron from the absorber toward a selective contact, the upper efficiency limit should be redefined to ∼64%. − Independently of these technicalities and practical considerations, within the QD–MO field, many research groups have focused their attention on demonstrating viable extraction of hot electrons populating the QDs toward the MO electrode; highlights of the works on the topic from the perspective of kinetics are made in section .…”

“…Electrons populating different electronically isolated states in quantum confined systems are likely not in thermal equilibrium (i.e., each electronic state can be defined by a finite T); as such, a low energy electron in the QD LUMO will require the assistance of a second IR photon to gain the required excess energy to reach a higher in energy selective contact. 108,110 This deviation from theory, imposed by quantum confinement in QD-based systems, makes any approach employing QD−MO interfaces closer to the intermediate band solar cell concept rather than to the HCSCs. 110,111 In the case a second IR photon is needed to extract an elecron from the absorber toward a selective contact, the upper efficiency limit should be redefined to ∼64%.…”

Hot‐carrier solar cells and improved types using wide‐bandgap energy‐selective contacts

“…Two improved types of HC‐SCs using different mechanisms have been proposed for higher η PV . The HC‐SC with intraband transition (HC‐SC+intra) includes intraband transition that electrons excited from the VB to the CB are subsequently excited to the higher‐energy levels in the CB by sub‐bandgap photons, as illustrated in Figure 1B 27,28 . Holes in the VB are also excited in a similar manner.…”

“…These models have been improved for evaluating the impacts of finite values of these parameters, showing that the requisites for high conversion efficiency are a carrier thermalization time longer than 1 ns and an energy‐selection width of the QD‐ESC narrower than 0.1 eV 5,6,19–24 . Then, two improved types of HC‐SCs that utilize sub‐bandgap photons have been proposed for further higher efficiency: an HC‐SC with intraband transition (HC‐SC+intra) and an intermediate‐band‐assisted HC‐SC (IB‐HC‐SC), and their performance was analyzed using similar models 25–28 …”

“…5,6,[19][20][21][22][23][24] Then, two improved types of HC-SCs that utilize sub-bandgap photons have been proposed for further higher efficiency: an HC-SC with intraband transition (HC-SC+intra) and an intermediate-band-assisted HC-SC (IB-HC-SC), and their performance was analyzed using similar models. [25][26][27][28] In the present study, I construct a new model based on the previous ones to evaluate the impacts of three realistic points of great importance. The first is nonradiative recombination of photogenerated carriers.…”