Dopants of Cu 2 ZnSnS 4 (CZTS) for solar cells

1. Dopants of Cu2ZnSnS4 (CZTS) for solar cells By Dr. Mohammad Junaebur Rashid Post Doctoral Researcher Supervisor: Professor Dr. Nowshad Amin Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia (UKM).

2. Introduction • CZTS → potential photovoltaic material for low cost thin film solar cells • Promising optical properties → Band gap: 1.4 - 1.6 eV • → High absorption coefficient: α ≃ 104cm−1 • → Abundant, low-cost, and nontoxic constituents. • Expected conversion efficiency is 32 % (theoretical) for thin film CZTS. • → Reported conversion efficiency is around 10%. [ JJAP 51 (2012) 10NC11 ] • → Conversion efficiency of 20.3% is reported for CIGS. • What about the cost? Minimum cost for raw materials for different PV technology 2

3. Introduction CZTS is a quaternary semiconductor: Cu2ZnSnS4 • Kesterite type structure (sphalerite-like crystal structure): primitive cell is based-centered-tetragonal with 8 atoms/cell. • Experimentally both CZTS (kesterite) and CZTSe (stannite) are observed to crystallize. • → Kesterite is the most common. 3

4. Electrical properties CZTS is self doped through the formation of intrinsic defects • Vacancies or point defects (VCu, VZn, VSn, and VS) These defects are formed during the growth of CZTS • Antisite defects (CuZn, Zncu, CuSn, SnCu, ZnSn, and SnZn) • Interstitial defects (Cui, Zni, and Sni) CZTS is generally p-type • Formation energy of acceptor defects is lower than the donor defects • → Makes CZTS p-type self doping • Arises mainly due to the CuZn antisite defects (relatively deeper acceptor level) • Inter-mixing of host atoms with different oxidation states such as Sn/Zn cation disorder • → Introduces deeper level in the band gap • Successfully fabricated CZTS solar cells are Cu-poor and Zn-rich • Resistivity:10-3 Ω∙cm to 10-1 Ω∙cm • Hole concentration: 1016 cm-3to 1018 cm-3 • Hall effect measurement: low hole mobility (1 to 10 cm2V-1s-1) 4

5. Intermediate band Insertion of intermediate states into the bandgap • Provides multiple absorption bands in a single junction structure • The absorption of photons is more efficient than in conventional single-gap cells • → Because the absorption of low energy photons causes transitions from the VB to the partially filled intermediate band (IB) and from there to the CB. → These transitions generate additional carriers to those generated for the usual process through photon absorption, promoting electrons from the VB to the CB. • Promising potential route to obtain high efficiency (> 60%) 5 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

6. Intermediate band • [Energetic position of the donor and acceptor are eD and eA, respectively] • An impurity as a donor: eD lies in the gap but eA does not • An impurity as a acceptor: eA lies in the gap but eD does not • An impurity with amphoteric behavior: both eD and eAare found in the gap • For low concentration of impurity the deep localized defect states act as effective nonradiative recombination centers. • However at high concentration the defect states lead to bands. → If the bands corresponding to the ionization energies overlap, there is the possibility of forming a partially filled IB. For a partially filled IB the donor and acceptor energies coincide with the Fermi energy, i.e. the IB has amphoteric behavior. → The donor and acceptor energies for the CrZn, IrSn, IrCu, and VZn substitutions are within the gap, i.e. are amphoteric. 6 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

7. Dopants for CZTS What are the dopants considered so far? • Self doping: Zn (p-type conduction) and Sn (n-type conduction) • Calculated the substitution energies of Cd atom for Cu, Zn or Sn atom for CZTS [ Density functional theory (DFT) with a generalized gradient approximation (GGA)] • → Cd-doped CZTS for the Cu atom (CdCu) exhibits n-type conduction • → The impurity level of CdCu is formed near the CB minimum [ JJAP 51 (2012) 10NC11 ] • → Cd-doped CZTS for Zn atom (CdZn) shows neutral charge • → The band structure of the CdCu+Vcu pair exhibits neutral charge • Dopants considered by C. Tablero: Iridium (Ir), Chromium (Cr) and Vanadium (V) • → DFT is used to calculate the electronic structure of the Cr-doped CZTS → To determine the optical properties, the complex dielectric function is calculated as a function of the photon energy → Other optical properties are obtained using the Kramers-Kronig relations 7 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

8. Dopants for CZTS Obtained results might be different, because • Different calculation methods are used → DFT is found currently tractable to explore isolated defects • Size of the supercell structure → 64-atom supercell, 216-atom supercell, etc. • Using different lattice parameters → The experimental lattice parameters of the natural CZTS (HLP) are aH = 5.427 Å and cH/2aH = 1.002. However, natural CZTS often contains Fe. → Experimental analyses of the synthetic (iron-free) CZTS reports different cell parameters (SLP) aS = 5.485 Å and cS/2aS = 0.997 • In CZTS the cation substructure turns out to be intrinsically disordered. → The exchange of two cations causes a significant symmetry lowering → Cu / Zn disorder is difficult to measure and their positions are not easy to differ → Some calculation does not consider cation disorder 8 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

9. Dopants for CZTS The formation or substitution energy • The substitution energy of a host A atom by M (MA substitution) in CZTS is estimated as where A = Cu, Sn or Zn, M= V, Cr and Ir, E(Cu2ZnSnS4:M) and E(Cu2ZnSnS4) are the total energies of the unit cell with and without the M impurity, and E(A) and E(M) are the energies of the elemental atomic reservoirs (isolated A and M atoms). • The incorporation will be favored if more M atoms are available (higher chemical potential µM), if more A places are available (lower chemical potential µA), and if the position of Fermi energy is lower. • E(Cu2ZnSnS4:Cr) and E(Cu2ZnSnS4) are lower by using SLP than the HLP, indicating that compounds with SLP are more stable energetically. → However the substitution energies are very similar with both lattice parameter. • In all substituted structures the substitution energy is negative, between -100 meV and -180 meV per atom, indicating that these substitution growth processes are favorable. → More stable 9 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

10. Dopants for CZTS The formation or substitution energy • Compounds are formed to minimize the total energy of the constituent particles. If the energy of formation is positive then you need to supply energy to cause reaction in contrast thenegative energy of formation means release of energy. • So those with negative energy of formation are more stable than those with positive energy of formation. • The stability of these impurity atoms at cation sites is also compared as regards the respective intrinsic cation vacancies, i.e. E[Cu2-xZnSnS4] + xE(M) - E[(MxCu2-x)ZnSnS4] for Cu. These energies are negative, between -110 meV and -230 meV per atom, indicating that these substitutions are favorable as regards the intrinsic cation vacancies. → Ability to reduce the point defects. 9 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

11. Dopants for CZTS Energy diagram of Cr-doped CZTS • One of the main effects of substituting Cu, Sn or Zn host atoms by Cr in the electronic structure of the CZTS host is to create an IB within the energy bandgap. → The Fermi energy is above, below, and within the IB for the CrCu, CrSn, and CrZn substitutions, respectively. Therefore, this IB is full, empty and partially full, respectively. → The main difference on the band structure between the HLP and SLP is a lower band gap (around 0.15 eV) when the SLP are used. However, the IB within the energy bandgap is very similar for all substitutions. 10 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

12. Conclusion The substitution energy for SLP (iron free CZTS) • For V and Ir the substitution energy is negative, between -100 meV and -180 meV per atom. • Cr-doped CZTS present several recombination paths depending on the IB occupation and on the cation substitution. • The IB is full, empty, and partially full for the CrCu, CrSn, and CrZn substitutions, respectively. → For the CrZn substitutions, the IB is partially full. The additional VB−IB and IB−CB transitions permit the absorption of lower energy photons than the host semiconductor. → The obtained absorption coefficients show an increase in light absorption below the host gap due to these additional absorption channels. • Cd doped CZTS can be used for n-type layer. 11 J. Phys. Chem. C 2012, 116, 23224−23230 C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and

13. Conclusion Implementation in MBE √ • Melting temperature: Cr → 19070C • Ir → 24660C • V → 19100C • Cd → 3210C • Si → 14140C • Silicon sublimation source: SUSI 13

14. Thank you for your attention Terima kasih atas perhatian anda 12