| dc.description.abstract | In this research, solar energy conversion based on internal photoemission in metals are theoretically investigated. The pseudopotential method is applied to first obtain the actual band structures of aluminum (Al), copper (Cu), and silver (Ag). The energy distributions of the photoexcited electrons within metals are quantified under the assumption of direct or fully nondirect transition. With the incorporation of the spatial distributions of incident electromagnetic wave, the propagation loss, and finite emission probabilities of photoexcited electrons, the theoretical limits of the quantum yield (QY) and the power conversion efficiency (PCE) of n- and p-type planar Schottky devices as a function of the barrier height Phi_B and metallic film thickness t_m are evaluated. Moreover, the photocurrent and the corresponding net quantum yield generated within one period of a two-dimensional (2D) metallic grating in Al-TiO_2-Ag configuration are estimated.
The energy distributions of photoexcited electrons are found to be disparate markedly among Al, Cu, and Ag. Under the fully nondirect approximation, the continuous distributions of the electron density of states results in a relatively smooth and continuous energy distributions of photoexcited electrons in each metal considered here.The condition of direct transition (i.e. momentum conservation) only has little influence on the energy distributions of photoexcited electrons in Al. However, such transition criteria leads to the threshold energies in Cu and Ag for the incident photons at which vertical transitions can occur. Within the solar spectrum range, the assumption of constant optical matrix element is applicable for Al. However, such an assumption may underestimate the upward transition rates from the s-band and the top of d-bands in Cu.
For planar n-type Schottky devices under the assumption of direct transition approximation, the absence of the threshold incident photon energy gives rise to the maximum PCE up to 0.5530% in Al at Phi_B=0.95 eV and t_m=13.0 nm, the highest one among the 3 metallic materials. The QY of Al-based devices is much higher than the other two metals at small barrier heights but drops rapidly as the barrier height increases. For Cu-based devices, the slope of the QY versus Phi_B plot shows 2 distinct phases owing to the energy differences of the s-band and d-bands in Cu with respect to the Fermi level. Since electrons excited from the d-bands carry smaller excess energies and only part of them have sufficient energies to overcome or tunnel through the barrier, the maximum PCE in Cu is merely 0.0022% (at Phi_B=0.9 eV and t_m=17.0 nm). For Ag, excitations are restricted from the s-band within the solar spectrum range. The relatively high excess energies possessed by electrons form the s-band cause a weak dependence of the QY of Ag-based devices on the barrier height but a strong dependence on the metallic film thickness because of their shorter lifetimes. The maximum PCE in Ag is 0.0162% (at Phi_B=2.2 eV and t_m=5.0 nm). It is mainly limited by photon energies within a large part of the solar spectrum. When the momentum conservation condition is ignored, Al has the highest maximum PCE while Cu has the lowest value as a consequence of the low excess energies carried by electrons excited from the d-bands.
For Al-based p-type Schottky devices, the behaviors of the photocurrent as a function of the barrier height and metallic film thickness are similar to their n-type counterparts. The major difference originates form a shorter hole lifetime compared to that of electrons at the same excess energy, leading to a slightly smaller photocurrent. With nonconstant optical matrix elements considered, the maximum PCE of Al-based p-type devices is 0.2673% (at Phi_B=0.95 eV and t_m=11.0 nm), while that of its n-type counterpart is 0.2799% (at Phi_B=0.95 eV and t_m=13.0 nm) using the same calculation method. In contrast to n-type devices, holes generated by electrons excitations from the d-bands in Cu carry relatively high excess energies and thus shorter lifetimes. As a result, the QY of Cu-based p-type devices is a strong function of the metallic film thickness but is dependent weakly on the barrier height. $n$-type Schottky devices are therefore superior to p-type Schottky devices for Al and Cu in terms of their overall performances within the solar spectrum range.
Finally, the analysis on the 2D grating in Al-TiO_2-Ag configuration shows that when the momentum conservation condition is considered, the net QY within solar spectrum is found to be 7.9293%,%, while that of a planar Si_3N_4- Al-TiO_2-Ag device with identical dimension is 5.8515%. At a free-space wavelength of 612.61 nm, its net QY can reach up to 14.0073%. The major limiting factor of the present device is that, only around 15% of those electrons excited within the top metallic structure can reach and emit through the Al-TiO_2 interface. If the losses during the electron transport and finite emission angles at the Al-TiO_2 interface are ignored, theoretically the net QY of the present device with 2D gratings can exceed 38% under standard AM1.5G solar illumination. | en_US |