External seminar archive:

The potential of type-II systems for third generation photovoltaics: Hot carrier absorbers and intermediate band solar cells

9 November 2017
Professor Ian R Sellers, University Of Oklahoma

Third generation photovoltaics aims to improve the efficiency of solar cell technology while keeping costs below levels that are prohibitive for practical implementation. The major loss processes in commercial solar cells include high-energy thermal losses and transmission of low energy photons that cannot be harnessed by conventional single-gap solar cells.

Quantum-engineered systems have long been considered an exciting route to enhance the performance of traditional solar cells via, for example, the absorption of low energy photons in intermediate band solar cells (IBSCs), or the extraction of high-energy photogenerated carriers extracted prior to thermalisation using a combination of hot carrier absorbers (HCAs) and energy selective contacts.

Several systems have been investigated for IBSCs and HCAs the majority of which utilise type-I quantum dots and quantum wells. Although there is some work on type-II structures these systems are much less developed.

Here, recent work at the University of Oklahoma on the potential of so-called ‘quasi-type-II systems’ will be presented. These systems display interesting band alignments in that there is strong and deep electron confinement in the conduction band; but the valence band is effectively degenerate at room temperature, facilitating the spatial diffusion and increased mobility of holes in the system.

These structures have several advantages over conventional type-I systems, which will be discussed, and prove particularly interesting for studies of HCAs and quantum dot (QD) IBSCs. Two specific systems that have been investigated in Oklahoma will be presented:

  1. InAs/GaAs1-xSbx QD IBSCs [1, 2].

  2. Type-II InAs/AlAsSb quantum wells for HCAs [3, 4].

Although in their infancy, Sb-capped QDs have demonstrated several interesting properties during development including non-conventional current-voltage properties and evidence of competing tunneling and thermal processes that dominate the carrier escape and transport as a function of temperature.

Experimental investigations of HCAs based on InAs/AlAs1-xSbx quantum wells have displayed evidence of robust hot carrier populations even at elevated temperatures where phonon relaxation of these carriers is expected to dominate.

This unusual behaviour is attributed to inhibited electron relaxation due to the spatial diffusion of holes resulting in an increased radiative recombination lifetime at higher temperature.


  1. Y. Cheng et al., Solar Energy Materials and Solar Cells, 147, 94 (2016).

  2. Y. Cheng et al., IEEE Journal of Photovoltaics, in press (2017).

  3. J. Tang et al., Applied Physics Letters, 106, 061902 (2015).

  4. H. Esmaielpour et al., Progress in Photovoltaics: Res. & Apps., 24, 591 (2016).