Light- and bias-induced metastabilities in Cu(In,Ga)Se2 based solar cells caused by the (VSe-VCu) vacancy complex

We investigate theoretically light- and bias-induced metastabilities in Cu(In,Ga)Se2 (CIGS) based solar cells, suggesting the Se–Cu divacancy complex (VSe-VCu) as the source of this hitherto puzzling phenomena. Due to its amphoteric nature, the (VSe-VCu) complex is able to convert by persistent carrier capture or emission from a shallow donor into a shallow acceptor configuration, and vice versa, thereby changing in a metastable fashion the local net acceptor density inside the CIGS absorber of the solar cell, e.g., a CdS/CIGS heterojunction. In order to establish a comprehensive picture of metastability caused by the (VSe-VCu) complex, we determine defect formation energies from first-principles calculations, employ numerical simulations of equilibrium defect thermodynamics, and develop a model for the transition dynamics after creation of a metastable nonequilibrium state. We find that the (VSe-VCu) complex can account for the light-induced metastabilities, i.e., the “red” and “blue” illumination effects, as well as for the reverse-bias effect. Thus, our (VSe-VCu) model implies that the different metastabilities observed in CIGS share a common origin. A defect state in the band gap caused by (VSe-VCu) in the acceptor configuration creates a potentially detrimental recombination center and may contribute to the saturation of the open circuit voltage in larger-gap Cu(In,Ga)Se2 alloys with higher Ga content. Therefore, the presence of metastable defects should be regarded as a concern for solar cell performance.We investigate theoretically light- and bias-induced metastabilities in Cu(In,Ga)Se2 (CIGS) based solar cells, suggesting the Se–Cu divacancy complex (VSe-VCu) as the source of this hitherto puzzling phenomena. Due to its amphoteric nature, the (VSe-VCu) complex is able to convert by persistent carrier capture or emission from a shallow donor into a shallow acceptor configuration, and vice versa, thereby changing in a metastable fashion the local net acceptor density inside the CIGS absorber of the solar cell, e.g., a CdS/CIGS heterojunction. In order to establish a comprehensive picture of metastability caused by the (VSe-VCu) complex, we determine defect formation energies from first-principles calculations, employ numerical simulations of equilibrium defect thermodynamics, and develop a model for the transition dynamics after creation of a metastable nonequilibrium state. We find that the (VSe-VCu) complex can account for the light-induced metastabilities, i.e., the “red” and “blue” illumination effect...

[1]  Jinwoo Lee,et al.  The determination of carrier mobilities in CIGS photovoltaic devices using high-frequency admittance measurements , 2005 .

[2]  H. Weinert,et al.  Infrared Faraday Effect in n-Type CuInSe2 , 1977 .

[3]  U. Rau,et al.  A model for the open circuit voltage relaxation in Cu(In, Ga)Se2 heterojunction solar cells , 1999 .

[4]  S. Wasim,et al.  Sound Velocities and Elastic Moduli in CuInTe2 and CuInSe2 , 1990 .

[5]  W. Shafarman,et al.  Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling , 2004 .

[6]  B. Alder,et al.  THE GROUND STATE OF THE ELECTRON GAS BY A STOCHASTIC METHOD , 2010 .

[7]  A. Rockett,et al.  Effect of Ga content on defect states in CuIn 1¿x Ga x Se 2 photovoltaic devices , 2002 .

[8]  Payne,et al.  Periodic boundary conditions in ab initio calculations. , 1995, Physical review. B, Condensed matter.

[9]  K Schulten,et al.  VMD: visual molecular dynamics. , 1996, Journal of molecular graphics.

[10]  Su-Huai Wei,et al.  Effects of Ga addition to CuInSe2 on its electronic, structural, and defect properties , 1998 .

[11]  W. Shafarman,et al.  Distinguishing metastable changes in bulk CIGS defect densities from interface effects , 2003 .

[12]  M. Bodegård,et al.  The ‘defected layer’ and the mechanism of the interface-related metastable behavior in the ZnO/CdS/Cu(In,Ga)Se2 devices , 2003 .

[13]  P. Fons,et al.  Anion vacancies in CuInSe2 , 2001 .

[14]  G. Kresse,et al.  Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set , 1996 .

[15]  A. Zunger,et al.  Metal-dimer atomic reconstruction leading to deep donor states of the anion vacancy in II-VI and chalcopyrite semiconductors. , 2004, Physical review letters.

[16]  A. Zunger,et al.  Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot? , 2004 .

[17]  Brian E. McCandless,et al.  Device and material characterization of Cu(InGa)Se2 solar cells with increasing band gap , 1996 .

[18]  A. Yamada,et al.  Molecular beam epitaxial growth and characterization of CuInSe2 and CuGaSe2 for device applications , 2002 .

[19]  U. Gerstmann,et al.  Charge corrections for supercell calculations of defects in semiconductors , 2003 .

[20]  A. Zunger,et al.  Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors , 2005, cond-mat/0503018.

[21]  U. Rau,et al.  Spectral dependence and Hall effect of persistent photoconductivity in polycrystalline Cu(In,Ga)Se2 thin films , 2002 .

[22]  M. Edoff,et al.  Compensating donors in Cu(In,Ga)Se2 absorbers of solar cells , 2005 .

[23]  A. Rothwarf,et al.  Time-dependent open-circuit voltage in CuInSe2/CdS solar cells: theory and experiment , 1987 .

[24]  H. Schock,et al.  The metastable changes of the trap spectra of CuInSe2‐based photovoltaic devices , 1996 .

[25]  H. Schock,et al.  Phase segregation, Cu migration and junction formation in Cu(In, Ga)Se2 , 1999 .

[26]  H. Schock,et al.  Device Analysis of Cu(In,Ga)Se2 Heterojunction Solar Cells - Some Open Questions , 2001 .

[27]  T. Ohshima,et al.  Piezoelectric photothermal investigation of proton irradiation induced defects in CuInSe2 epitaxial films , 2005 .

[28]  A. Zunger,et al.  n -type doping of CuIn Se 2 and CuGa Se 2 , 2005 .

[29]  M. Usuda,et al.  All-electron GW calculation based on the LAPW method: Application to wurtzite ZnO , 2002, cond-mat/0202308.

[30]  R. Crandall,et al.  Strongly temperature-dependent free-energy barriers measured in a polycrystalline semiconductor , 2005 .

[31]  D. Cahen,et al.  Direct evidence for diffusion and electromigration of Cu in CuInSe2 , 1997 .

[32]  Jean-François Guillemoles,et al.  CU(IN, GA)SE2 SOLAR CELLS : DEVICE STABILITY BASED ON CHEMICAL FLEXIBILITY , 1999 .

[33]  G. Sawatzky,et al.  Density-functional theory and NiO photoemission spectra. , 1993, Physical review. B, Condensed matter.

[34]  J. Zaanen,et al.  Density-functional theory and strong interactions: Orbital ordering in Mott-Hubbard insulators. , 1995, Physical review. B, Condensed matter.

[35]  A. Zunger,et al.  Self-interaction correction to density-functional approximations for many-electron systems , 1981 .

[36]  A. Zunger,et al.  Defect physics of the CuInSe 2 chalcopyrite semiconductor , 1998 .

[37]  A. Zunger,et al.  Halogen n-type doping of chalcopyrite semiconductors , 2005 .

[38]  U. Rau,et al.  Classification of metastabilities in the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se2 solar cells , 2001 .

[39]  U. Rau,et al.  Persistent photoconductivity in Cu(In,Ga)Se2 heterojunctions and thin films prepared by sequential deposition , 1998 .

[40]  A. Zunger,et al.  CORRIGENDUM: Momentum-space formalism for the total energy of solids , 1979 .

[41]  Jinwoo Lee,et al.  Detailed study of metastable effects in the Cu(InGa)Se2 alloys: Test of defect creation models , 2005 .

[42]  T. Irie,et al.  Electrical Properties of p- and n-Type CuInSe2 Single Crystals , 1979 .

[43]  G. Kresse,et al.  From ultrasoft pseudopotentials to the projector augmented-wave method , 1999 .

[44]  Blöchl,et al.  Projector augmented-wave method. , 1994, Physical review. B, Condensed matter.

[45]  M. Bodegård,et al.  Reversible changes of the fill factor in the ZnO/CdS/Cu(In,Ga)Se2 solar cells , 2003 .