Requisites for Highly Efficient Hot-Carrier Solar Cells

Abstract

We have constructed new models based on detailed balance of particle and energy fluxes to clarify the operating principle of hot-carrier solar cells (HC-SCs) and find the requisites for high conversion efficiency. Energy dissipation due to thermalization of photogenerated carriers can be significantly reduced, even though the thermalization time is not sufficiently long. Instead, the energy dissipation related to entropy generation associated with hot-carrier extraction is remarkable. The thermalization time must be several nanoseconds to exceed the Shockley-Queisser limit under the 1 sun solar irradiation and over 10 ns to compete against triple-junction solar cells at 1,000 sun. The other requisites unique to hot-carrier extraction are a short carrier equilibration time being around one-thousandth of the thermalization time, and an energy-selection width of energy-selective contacts (ESCs) for mono-energetic carrier extraction, which is needed to match the quasi-Fermi levels in the hot absorber and in the cold electrodes, being narrower than 0.1 eV. It seems extremely challenging to fulfill all the requisites, although investigations for material development as well as new concepts for post conventional HC-SCs are underway. IntroductionIn a conventional solar cell, a photon whose energy is higher than the bandgap of the light-absorbing material used in the cell is absorbed to generate a carrier with an energy equal to the photon energy. However, carrier energies in excess of the bandgap of the absorber immediately dissipate by emitting phonons whose temperature equals the room temperature, namely, thermalization of carriers occurs within several picoseconds in most cases. Therefore, the excess carrier energies cannot be

The effects of intraband and interband carrier‐carrier scattering on hot‐carrier solar cells: A theoretical study of spectral hole burning, electron‐hole energy transfer, Auger recombination, and impact ionization generation

“…Recent studies show very promising results from perovskite materials which have rather slow carrier energy relaxation and thus can provide better opportunities to achieve higher carrier temperatures, though the exact physical mechanism responsible for these phenomena is not fully understood yet. In short, the possibilities and potentials of hot‐carrier solar cells are still waiting to be explored and exploited …”

“…It should be particularly noted that the effects of carrier intraband energy relaxation processes due to electron‐phonon interactions on hot‐carrier solar cells are not investigated in this work. Previous studies show that the performance of hot‐carrier solar cells can be severely limited by these intraband energy relaxation processes . However, such a limitation could possibly be overcome, and the performance of hot‐carrier solar cells can be significantly improved, if the lifetimes of longitudinal optical (LO) phonons can be prolonged, as explored by Conibeer et al and Levard et al However, the study of electron‐phonon interactions in hot‐carrier solar cells is certainly beyond the capacity and scope of this work.…”

“…In short, the possibilities and potentials of hot-carrier solar cells are still waiting to be explored and exploited. 20…”

“…Previous studies show that the performance of hot-carrier solar cells can be severely limited by these intraband energy relaxation processes. 17,20,21,45 However, such a limitation could possibly be overcome, and the performance of hot-carrier solar cells can be significantly improved, if the lifetimes of longitudinal optical (LO) phonons can be prolonged, as explored by Conibeer et al 11,[46][47][48][49][50][51][52][53][54] and Levard et al 55 However, the study of electron-phonon interactions in hot-carrier solar cells is certainly beyond the capacity and scope of this work.…”

“…However, a recent theoretical study has suggested that carrier temperatures can be significantly higher than the lattice temperatures in conventional solar cells, especially when photovoltaic devices operate near a short-circuit condition. 14 On the contrary, for hot-carrier solar cells [15][16][17][18][19][20][21][22][23][24] in which the kinetic energy of carriers is extracted to become output electrical power, the carrier temperature becomes the main focus [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] ; hence, intraband carrier-carrier scattering is crucial and cannot be ignored.…”