Hot carrier solar cell with semi-infinite energy filtering aim at 70% efficiency limit

A global paradigm shift from economies fueled by carbon and other non-renewable energy sources is being coerced by the inescapable global climate extremes being witnessed today. In fact, the effects of global warming are a reality that the human race will have to contend with regardless of any shift. In this view, much attention has been directed towards renewable energy sources such as wind, solar, hydrothermal, hydroelectric- among others. Of concern here is the solar energy which represents the most abundant energy source available on earth. The challenge to utilizing this ‘ocean’ of energy has been anchored on how to harness it. Presently, advancement of solar cells has yielded promising results; regardless, little in terms of efficiency has been reported. To this end, several approaches to increase the solar cells’ efficiency have been reported: the most successful so far being the multijunction solar cell and the hot carrier solar cell concept. Unfortunately, the former requires infinite number of layers made of infinite number of different semiconductors, in order to reach their efficiency limit. Based on published literature, the latter on the other hand, can achieve the same efficiency limit with only three layers: absorber and two energy selective contacts.

The auspicious hot carrier solar cell can further be improved by adopting energy barriers as energy selective contacts. This approach however raises serious functional queries that ought to be resolved before full adoption of the concept. To this end, scientists from the University of Applied Science Jena, Carl Zeiss Promenade: Professors Igor Konovalov and Bernd Ploss, proposed to address the shortfalls presented by the adoption of energy barriers as energy selective contacts by extending the thermionic emission theory. They particularly focused on presenting a specific model case of a hot carrier solar cell. Their work is currently published in the research journal, Solar Energy.

Generally, the electrical parameters of a symmetrical hot carrier solar cell double heterostructure with metallic absorber layer and high-pass energy filters were calculated within thermionic emission theory. During this endeavor, the authors limited their consideration to a symmetrical with respect to electrons and holes structure utilizing a thin metallic film as absorber, with the goal being to limit the number of free parameters.

The authors reported that the theoretical limit of efficiency for the double heterojunction design was slightly below the black body limit and exceeded 70% for full concentration. Realistic parameters describing carrier cooling in a typical bulk absorber were seen to deteriorate the efficiency limit down to 0.3% at full concentration. In addition, besides the protraction of the carrier cooling time, two other parameters were identified as influencers of the heat flow from the e-gas to the lattice.

In summary, the study used the thermionic emission theory to explain both linear and curved I-V characteristics of hot carrier solar cells with double heterojunction design and semi-infinite energy filtering. Generally, an efficiency limit of 73% was predicted for infinite characteristic cooling time and full solar concentration. Moreover, carrier cooling was treated within linear thermal conductivity model. Overall, Professors Igor Konovalov and Bernd Ploss established that the optimal absorption of light together with small thermal loss to the lattice required high carrier mobility in the absorber layer.