On the use of complex band structure to study valley photovoltaics: toward efficient hot carrier extraction

Hot carrier solar cells were first proposed many decades ago. Over the intervening years, there has been a continuing quest to create these cells, which hold promise to shatter the Shockley-Queisser efficiency limit on single-junction solar cells. One approach considered is to use satellite valleys of the conduction band as metastable states for storing hot electrons until they can be extracted. Experimental efforts, however, have shown the presence of a barrier between the two materials, likely at the heterostructure interface between the absorber and extraction layer. Transfer across the interface is a real-space event rather than a momentum-space process. If the two bands from, and to, which the electron moves are not perfectly aligned, then tunneling must occur. The determination of the evanescent wave numbers that appear in tunneling coefficients are not the simple ones in textbooks but must be found from the full complex band structure of the two materials. Here, the nature of these evanescent states and their role in the tunneling of carriers across typical interfaces is examined using empirical pseudopotential methods.