Thermodynamics of the mono-energetic energy selective Contacts of the hot carrier solar cell

TL;DR

This paper models the thermodynamic properties of mono-energetic energy selective contacts in hot carrier solar cells, revealing the extreme material requirements and challenges for practical implementation to achieve high efficiency.

Contribution

It provides a detailed thermodynamic analysis of ESCs using transport theory, highlighting the material properties needed for high-efficiency hot carrier solar cells.

Findings

ESCs require extremely high electron state densities (~3×10^{19} cm^{-3}) for >82% efficiency.

Optimal hot carrier temperatures exceed 2540 K as ESCs deviate from ideality.

The highest thermoelectric figure of merit (ZT≈8) is needed, which current materials do not exhibit.

Abstract

The hot carrier solar cell (HCSC) has the potential for converting solar energy into electrochemical energy with an efficiency of 85.4%. For this, in addition to an idealized light absorber, the HCSC has to be connected to the external load by means of the so-called \emph{mono-energetic energy selective contacts} (ESCs). However, the thermodynamic properties that these types of contact have to exhibit, such as their electric, thermal conductivity and Seebeck coefficient, have not been explored. This paper aims to fill this gap. In this respect, we model electron transport in non-ideal ESCs using the transport theory proposed by Datta and Landauer which has allowed us to calculate the value of these parameters as a function of the temperature and electrochemical potential of operation. Our findings also reveal that, to preserve the HCSC efficiency above 82%, the ESCs could require in the order of $3 \times 10^{19}$ cm$^{-3}$ electron states. As the ESCs depart from ideality, the temperature of the hot carriers at which optimum efficiency is obtained increases to above 2540 K. The mono-enenergetic selective contact characterized by the highest energy demands an electric, thermal conductivity and Seebeck coefficient that, when combined, are characterized by a high thermoelectric figure of merit $(ZT\approx 8)$. We are not aware of any material exhibiting this figure of merit which illustrates the difficulty in putting the HCSC concept into practice. Conversely, our work supports the idea that pursuing materials capable of transporting electrons ballistically through mono-energetic electron channels can provide the key for achieving materials characterized by high $ZT$