Structural and Optical Properties of Nonstoichiometric Titanium Hydride, Vanadium Hydride and Zirconium Hydride as Hot Carrier Solar Cell Absorbers

Photovoltaic devices directly convert incident solar energy to electricity. Above bandgap incident photons excite carriers (electron and holes) deep into their respective bands. These excited carriers are called hot carriers and they undergo several physical loss processes to lose their excess energy to the lattice and thermalize to the band edge. In conventional photovoltaics, these carriers are collected from the band-edge before they recombine. The Hot Carrier Solar Cell (HCSC) is a promising third generation photovoltaics concept for energy loss reduction in solar cells by inhibiting the thermalization losses in the absorber. It has a limiting efficiency of 85%, well beyond the Shockley–Queisser limit of 33% which is the upper threshold for conventional photovoltaic cells e.g., Si solar cells. The primary mission of the hot carrier absorber engineering is to achieve a cooling rate of the carriers from 100s of picoseconds to nanoseconds, comparable to radiative recombination. One way to achieve a slower hot carrier cooling rate in the absorber is to acquire a phonon bottleneck effect. This would impede phonon assisted hot carrier relaxation. We have investigated the use of transition metal hydrides as HCSC absorber materials because of several potential advantages. The large atomic mass difference between the transition metal and hydrogen atoms leads to a large phononic band gap and a separation of the optical and acoustic phonon branches. This large phononic gap can inhibit the Klemens mechanism whereby longitudinal optical phonon decays into two acoustic phonons – the main route by which optical phonons decay - hence slowing down the hot carrier thermalization. In this work, we report on the fabrication and preliminary results of non-stoichiometric titanium hydride, vanadium hydride and zirconium hydride thin films for their quality and useability as HCSC absorbers. The thin films were prepared using electron beam evaporation and characterized using XRD, UV-VIS/IR, Ellipsometry and Raman spectroscopy. The results show that the amount of hydrogen in TiH
x
is such that x=1, for VH
x
x= 0.81 and for ZrH
x
x=1. From UV/VIS/IR, the reflection from all samples is dominated by a Pd layer used to prevent oxidation. The suitability of these materials as hot carrier absorbers is yet to be fully assessed, but the data presented here are important initial parameters in making that determination.