Solid State Theory of Photovoltaic Materials

Semiconductor materials such as CuInSe2, CdS, CdTe, and ZnO have been used widely in photovoltaic device applications. For example, CuInSe2 with a direct band gap of 1.04 eV is a prototype member of the family of I-III-VI2 chalcopyrite semiconductors. It has shown great potential as a low cost thin-film photovoltaic material, achieving 17% efficiency in polycrystalline form by alloying it with CuGaSe2. Unlike convensional binary semiconductors (e.g., GaAs or ZnSe), CuInSe2 appear to tolerate a large range of anion-to-cation off-stoichiometry (i.e., samples with a few percentage Cu-poor and/or In-poor stoichiometries are stable and useful). CuInSe2 can be doped p and n type to a low-resistivity level merely via introduction of native defects, without extrinsic impurities. However, the doping mechanism and other properties of CuInSe2 are not well understood at a fundamental level. It is crucial to understand these properties to further improve the cell efficiency and reduce the cost of solar energy.

The Solid State Theory Group at NREL has been a significant component of the fundamental research of NREL's PV program. It is the only systematic effort in applying modern condensed matter theory to photovoltaics. Researchers in this Group have pioneered theoretical research into the material properties and electronic structures of chalcopyrite semiconductors, providing early predictions for the beneficial effects of Ga-additions into CuInSe2, and guidelines for analysis of spectroscopic, transport and X-ray measurements on CuInSe2 including band offsets and alloying effects. These theoretical predictions and understanding have helped significantly experiments aimed at clarifying and assessing crucial PV functions.

Recently, using first-principles self-consistent electronic structure theory, the researchers at the Theory Group have calculated defect formation energies and defect transition energy levels in CuInSe2. Contrary to previously accepted assumptions in the analysis of defects in CuInSe2, they find that (i) it is much easier to form Cu vacancy in CuInSe2 than to form cation vacancies in II-VIs. (ii) Defect formation energies vary considerably both with the Fermi energy and the chemical potential of the atomic species, and (iii) Defect pairs such as (2VCu+InCu) have a remarkably low formation enthalpy. This explains the massive non-stoichiometry of CuInSe2 and the appearance of ordered defect compounds (ODCs) (e.g., CuIn3Se5 as ordered array of 2VCu+InCu defect pairs inside CuInSe2. The fact that CuInSe2 has good electrical properties despite this off-stoichiometry is explained in terms of the mutual passivation of donor InCu by acceptor VCu.

Other research includes studying defects in CdTe/CdS alloys and interfaces, and studying band offsets between photovoltaic semiconductors.

Selected References

  1. J. E. Jaffe and A. Zunger, Theory of the Band Gap Anomaly in ABC2 Chalcopyrite Semiconductors', Phys. Rev. B 29, 1882 (1984).

  2. S.-H. Wei, L. G. Ferreira, and A. Zunger, First-Principles Calculation of Order-Disorder Transition in Chalcopyrite Semiconductors, Phys. Rev. B Rapid Communication, 45 2533 (1992).

  3. S.-H. Wei, and A. Zunger, Band Offsets at the CdS/CuInSe2 Heterojunction, Appl. Phys. Lett. 63, 2549 (1993).

  4. S.-H. Wei, and A. Zunger, Band Offset and optical Bowing of Chalcopyrite and Zn-based II-VI Alloy, J. Appl. Phys. 78, 3846 (1995).

  5. S. B. Zhang, S.-H. Wei, and A. Zunger, Stabilization of Ternary Compounds via Ordered Array of Defect Pairs, Phys. Rev. Lett. 78, 4059 (1997).

  6. S. B. Zhang, S.-H. Wei, A. Zunger, and H. Katayama-Yoshida, Defects Physics of the CuInSe2 Chalcopyrite Semiconductor, Phys. Rev. B 57, 9642 (1998).

  7. S.B. Zhang, S.H. Wei and A. Zunger, "A Phenomenological Model for Systematization and Prediction of Doping Limits in II-VI and I-III-VI2 Compounds," J. Appl. Phys. 83, 3192 (1998).

  8. S.H. Wei, and A. Zunger, "Calculated natural band offsets of all II-VI and III-V semiconductors," Appl. Phys. Lett. 72, 2011 (1998).

  9. S.H. Wei, S.B. Zhang and A. Zunger, "The effects of Ga addition to CuInSe2 on its electronic, structural and defect properties," Appl. Phys. Lett. 72, 3199 (1998).

  10. S.H. Wei, S.B. Zhang and A. Zunger, "Effects of Na on the electrical and structural properties of CuInSe2," J. Appl. Phys. 85, pp. 7214-7218 (1999).

Other References

For a listing of all SST references on the topics "Ternary Chalcopyrites", "Semiconductor-Semiconductor Heterojunction Band Offsets", "Optical Properties and Bowing in Semiconductor Alloys", and "Isovalent Semiconductor Alloys", click on the "Get References" button below.