Dopants of Cu2ZnSnS4 (CZTS) for solar cells

By

Dr. Mohammad Junaebur Rashid

Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia (UKM).

Post Doctoral Researcher

Supervisor: Professor Dr. Nowshad Amin

Introduction

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Promising optical properties → Band gap: 1.4 - 1.6 eV → High absorption coefficient: α 10≃ 4 cm−1

→ Abundant, low-cost, and nontoxic constituents.

CZTS → potential photovoltaic material for low cost thin film solar cells

Expected conversion efficiency is 32 % (theoretical) for thin film CZTS.

→ Reported conversion efficiency is around 10%. [ JJAP 51 (2012) 10NC11 ]

What about the cost?

→ Conversion efficiency of 20.3% is reported for CIGS.

Minimum cost for raw materials for different PV technology

Introduction

CZTS is a quaternary semiconductor: Cu2ZnSnS4

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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.

Electrical properties

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CZTS is self doped through the formation of intrinsic defects

Vacancies or point defects (VCu, VZn, VSn, and VS)

Antisite defects (CuZn, Zncu, CuSn, SnCu, ZnSn, and SnZn)

Interstitial defects (Cui, Zni, and Sni)

These defects are formed during the growth of CZTS

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)

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)

Inter-mixing of host atoms with different oxidation states such as Sn/Zn cation disorder

Successfully fabricated CZTS solar cells are Cu-poor and Zn-rich

→ Introduces deeper level in the band gap

Intermediate band

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Insertion of intermediate states into the bandgap

Provides multiple absorption bands in a single junction structure

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

→ These transitions generate additional carriers to those generated for the usual process through photon absorption, promoting electrons from the VB to the CB.

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.

Intermediate band

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An impurity as a donor: eD lies in the gap but eA does not

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

→ The donor and acceptor energies for the CrZn, IrSn, IrCu, and VZn substitutions are within the gap, i.e. are amphoteric.

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.

→ 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.

However at high concentration the defect states lead to bands.

[Energetic position of the donor and acceptor are eD and eA, respectively]

Dopants for CZTS

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What are the dopants considered so far?

Self doping: Zn (p-type conduction) and Sn (n-type conduction)

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

Dopants considered by C. Tablero: Iridium (Ir), Chromium (Cr) and Vanadium (V)

Calculated the substitution energies of Cd atom for Cu, Zn or Sn atom for CZTS

→ Cd-doped CZTS for the Cu atom (CdCu) exhibits n-type conduction

→ The impurity level of CdCu is formed near the CB minimum

→ Cd-doped CZTS for Zn atom (CdZn) shows neutral charge

→ The band structure of the CdCu+Vcu pair exhibits neutral charge

[ JJAP 51 (2012) 10NC11 ]

[ Density functional theory (DFT) with a generalized gradient approximation (GGA)]

→ 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

Dopants for CZTS

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Obtained results might be different, because

Different calculation methods are used

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

Size of the supercell structure

→ DFT is found currently tractable to explore isolated defects

→ 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

→ The exchange of two cations causes a significant symmetry lowering

In CZTS the cation substructure turns out to be intrinsically disordered.

→ Cu / Zn disorder is difficult to measure and their positions are not easy to differ

→ Some calculation does not consider cation disorder

Dopants for CZTS

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The formation or substitution energy

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

The substitution energy of a host A atom by M (MA substitution) in CZTS is estimated as

)]()([)():()( 4242 MEAEZnSnSCuEMZnSnSCuEAE

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).

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

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.

Dopants for CZTS

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The formation or substitution energy

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

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.

→ Ability to reduce the point defects.

Dopants for CZTS

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

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.

Conclusion

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The substitution energy for SLP (iron free CZTS)

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

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.

For V and Ir the substitution energy is negative, between -100 meV and -180 meV per atom.

Cd doped CZTS can be used for n-type layer.

Conclusion

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Implementation in MBE

Melting temperature: Cr → 19070C Ir → 24660C V → 19100CCd → 3210C Si → 14140C

Silicon sublimation source: SUSI

√

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Thank you for your attention

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