Band engineering of type-II ZnO/ZnSe heterostructures for solar cell applications
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Two kinds of type-II heterostructures (HSs) of ZnO (wurtzite)/ZnSe (wurtzite) [ZnO (WZ)/ZnSe (WZ)] and ZnO (wurtzite)/ZnSe (zinc blende) [ZnO (WZ)/ZnSe (ZB)] were designed for photovoltaic applications by first-principle calculations. The calculated effective bandgap of 1.51 eV for the ZnO (WZ)/ZnSe (WZ) HS is more favorable for solar cell applications compared to that of 1.69 eV for the ZnO (WZ)/ZnSe (ZB) HS. Furthermore, the electrons and holes are more effectively separated at the interface of ZnO (WZ)/ZnSe (WZ) HS due to the stronger misfit stress field. Finally, a strained ZB ZnSe layer was introduced to transport the separated holes from WZ ZnSe layer, and an optimal structure of ZnO (WZ)/ZnSe (WZ)/ZnSe (ZB) was proposed to realize a solar cell with near-infrared response.
I. INTRODUCTION
In recent years, photovoltaic (PV) devices have attracted considerable attentions due to the growing energy crisis.1–4 As for them, carrier separation is an essential procedure during the step of energy conversion, which is traditionally carried out by p–n junction.5 And recently, heterostructures (HSs) with type-II band alignment have been proposed as an alternative candidate to implement carrier separation,6 thus many kinds of type-II HSs such as ZnO/ZnS, ZnO/ZnTe, and ZnO/ZnSe have been suggested and developed.7,8 Effective bandgap of 2.37, 1.17, and 1.84 eV were theoretically predicted for the ZnO/ZnS, ZnO/ZnTe, and ZnO/ZnSe HSs, respectively, by Y. Zhang and J. Schrier group.7,8 However, these values are still different from the optimal bandgap (about 1.45 eV) based on the Shockley–Queisser detailed balance theory.9 Hence, it is quite necessary to modify the bandgap for the realization of highly efficient solar cell. Among the various HSs, the ZnO/ZnSe HS can offer larger valence band offset (VBO), which is more favorable for the high separation efficiency of the electrons and holes. Furthermore, the large lattice mismatch between the ZnO and ZnSe as well as the two types of phase structures [wurtzite (WZ) or zinc blende (ZB)] of ZnSe endows the ZnO/ZnSe HS with tunable electronic structures. In the light of these advantages, the ZnO/ZnSe HS is expected to reduce its effective bandgap to match the solar spectrum for solar cell applications by tuning the interfacial transition in spite of the large bandgap for either the ZnO or ZnSe Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2011.417 730
J. Mater. Res., Vol. 27, No. 4, Feb 28, 2012
http://journals.cambridge.org
Downloaded: 22 Jan 2015
semiconductors. Recently, well-aligned ZnO/ZnSe core/shell nanowire arrays have been fabricated, and the effective bandgap was reduced to as low as 1.60 eV by interfacial transition.8,10 However, the dependences of the effective bandgap or electronic structures on the heterojunction interface, strain, misfit stress field, etc. are still unclear, and little is studied on the microscopic mechanism of the carrier separation and the optimizations of solar cell materials. Th
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