1. Dopants of Cu2ZnSnS4 (CZTS) for solar cellsBy
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: eV
→ High absorption coefficient: α ≃ 104 cm−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. These defects are formed during the growth of CZTS
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-3 to 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%)
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 5

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 eA are 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.
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 6

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
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 7

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 = Å and cH/2aH = However, natural CZTS often contains Fe.
→ Experimental analyses of the synthetic (iron-free) CZTS reports different cell parameters (SLP) aS = 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
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 8

9. Dopants for CZTS The formation or substitution energy
The formation energy needed to incorporate one positively (negatively) charged M atom into place B (MB substitution) in the host H semiconductor, and thus form a donor (acceptor), is
where denotes the total incorporation energy, EH is the total energy of the host, EF is the Fermi energy and µB and µM denote the chemical potentials of the B and M. These chemical potentials represent the energy of the reservoirs with which atoms are being exchanged.
The incorporation will be favored if more M atoms are available (higher µM ), if more B places are available (lower µB), and if the position of Fermi energy is lower.
The ionization energies correspond to the value of EF at which the impurity or defect changes from to MB, i.e., and
For the host semiconductor, when B = M, the acceptor and donor energies correspond with the CB and VB edge energies, i.e., eC and eV, and the gap is
→ To obtain the donor (eD) and acceptor (eA) ionization energies we used total-energy differences.
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 9

10. 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.
Compunds form so as to minimise the total energy of the constituent particles. If the energy of formation is positive ...you have to supply energy to cause reaction while those with negative energy of formation instead release energy.  So those with negative energy of formation are more stable than those with positive energy of formation.
→ 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
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 9

11. 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 the negative 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.
Compunds form so as to minimise the total energy of the constituent particles. If the energy of formation is positive ...you have to supply energy to cause reaction while those with negative energy of formation instead release energy.  So those with negative energy of formation are more stable than those with positive energy of formation.
→ Ability to reduce the point defects.
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 9

12. 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.
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 10

13. 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.
We use –ve sign to say that, energy is released. The low formation energy is favorable.
→ 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.
C. Tablero, Journal of Alloys and Compounds 586 (2014) 22–27 and
J. Phys. Chem. C 2012, 116, 23224−23230 11

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

15. Thank you for your attention
Terima kasih atas perhatian anda
PC design less sensitive to residual disorder. 12