Investigation of phonon scattering in superlattices for design of efficient multiple quantum-well hot carrier solar cells

热载流子太阳能电池是一种太阳能转换器，它利用半导体中光生电子和空穴的多余热能来产生电能。热载流子太阳能电池有望产生比传统限制更高的效率。一种很有希望实现高效热载流子太阳能电池的材料系统包括多个量子井，这些量子井由半导体材料组成，排列在交替的超晶格层中，以有效地减少热电子的能量损失。电子的能量损失是通过能量耗散到高能晶格振动，而高能晶格振动通过散射进一步消散到低能晶格振动。通过最小化散射将高能与低能晶格振动解耦，最终可以提高太阳能电池的效率。在拟议的研究中，项目组将解决这一挑战，通过设计超晶格组成和应变半导体晶体来设计高效热载流子太阳能电池。半导体中主要的声子散射机制是Klemens通道，它涉及将一个光学声子衰变为两个声学声子。通过改变超晶格的组成，可以改变声子色散中的能隙，从而为抑制声子散射中的类Klemens通道提供途径。这可以使声子寿命更长，导致非平衡声子布居，从而促进电子热化中的热声子瓶颈。应变同样可以改变声子的色散，再次允许减少声子散射的可能性。在两种超晶格系统--InAs/AlSb和AlAs/GaAs中，我们将研究超晶格组成和应变的作用。分析将通过第一原理方法进行，使用源自密度泛函理论的简谐和非简谐力相互作用以及声子玻尔兹曼输运方程的解。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The hot carrier solar cell is a type of solar energy converter that captures the excess thermal energy of photo-generated electrons and holes in a semiconductor to produce electric power. Hot carrier solar cells hold the promise of yielding significantly higher efficiency beyond traditional limits. A promising material system for achieving high efficiency hot carrier solar cells involves multiple quantum wells, comprised of semiconductor materials arranged in alternating layers known as superlattices, to effectively diminish energy loss from hot electrons. Energy loss from electrons occurs by dissipation of energy to high energy lattice vibrations, which further dissipate to low energy lattice vibrations by scattering. Decoupling the high energy from low energy lattice vibrations by minimizing scattering can ultimately enhance the solar cell efficiency. In the proposed research, the project team will tackle this challenge and design high-efficiency hot carrier solar cells through engineering the superlattice composition and by straining the semiconductor crystal. A dominant phonon scattering mechanism in semiconductors is the Klemens channel, which involves decay of an optical phonon into two acoustic phonons. By modifying superlattice composition, the energy gap in the phonon dispersion can be modified providing avenues to suppress Klemens like channels in phonon scattering. This can enable longer phonon lifetimes, resulting in non-equilibrium phonon populations, thus facilitating hot phonon bottleneck in the thermalization of electrons. Strain can similarly modify phonon dispersion, again allowing for the possibility to diminish phonon scattering. The role of superlattice composition and strain will be studied in two superlattice systems - InAs/AlSb and AlAs/GaAs. Analysis will be performed through a first-principles approach by using harmonic and anharmonic force interactions derived from density-functional theory along with a solution of the phonon Boltzmann transport equation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.