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Searching Stable CuxS Structures for 

Photovoltaic Application

Qiang Xu

Bing Huang

Yufeng Zhao

Yanfa Yan

Rommel Noufi

Su-Huai Wei

Photovoltaic Module Reliability 

Workshop (PVMRW)

February 25-26, 2014

Golden, Colorado

NREL/PR-5900-61495
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Introduction

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CuxS is one of the promising solar cell absorber 

materials to replace CIGS. 

Solar cells based on CuxS have reached an efficiency 

as high as 10%. 

To further improve its efficiency and especially the 

stability, it is important to understand the stability and 

electronic structure of CuxS. 

However, due to the complexity of their crystal structures, 

no systematic studies have been carried out to 

understand the stability and electronic structure of the 

CuxS systems.
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CuxS Crystal Structure

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The experimentally identified stable 

compounds of CuxS (1<x≤2): 

• Chalcocite (Cu2S)

• Djurleite (Cu1.94S)

• Digenite (Cu1.8S)

• Anilite (Cu1.75S)

Three different structures of chalcocite 

Cu2S :

• monoclinic phase (low-chalcocite) 

(t<104 °C). 

• hexagonal phase (high-chalcocite) 

(104 °C<t<436 °C )

• cubic phase (cubic-chalcocite) 

(t>436 °C).
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Methods of Calculations

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 Using Vienna Ab-initio Simulation Package (VASP)

 All-electron-like projector augmented wave (PAW) potential

 The Perdew-Burke-Ernserhof (PBE) exchange correlation potential

 For more accurate calculation of band gaps, we also use hybrid 

functional (HSE) potentials

 Plane wave expansion up to 500eV

 The calculated cell are fully relaxed, which maximum force 

converges below 0.01eV/Å
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The Optimized Structure Parameters and Band Gaps

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The calculated lattice parameters, volume (V), band gap (Eg), and heat of 

formation per formula unit (ΔH) of CuxS.

 Monoclinic Cu2S is the most stable structure at x=2

 Anilite Cu1.75S has lowest formation energy among 

the calculated CuxS compound

The heat of formation ΔH(x) of CuxS
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The Band Structures of Chalcocite Cu2S

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All three chalcocites have direct 

band gaps at Γ with values around 

1.3–1.5 eV (HSE results).
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Calculated Heat of Formation of CuxS as a Function of x 

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The initial structures, i.e., x=2, are based on the three 

chalcocite Cu2S structures.

 The global minimum of the ΔH(x) 

occurs at about x=1.7 for the 

hexagonal and the monoclinic 

phases. 

 A phase transition from the 

monoclinic to the hexagonal 

phase can occur at xc≈1.70.

 However, the heat of formation of 

the low chalcocite and high 

chalcocite Cu1.75S is 20meV 

higher than that of the anilite

Cu1.75S. 

The anilite structure is indeed the most stable structure among 

all CuxS at the Cu-rich limit.
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The HSE Band Structure of Anilite Cu1.75S

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 It has a band gap of 1.39 eV

at the Γ point and is heavily 

hole-doped

 The hole carrier concentration 

can be controlled by doping 

anilite with donors such as Sn. 

 The large curvature at the 

band edge indicates that it 

should have good electron as 

well as hole conductivities. 

We suggest that anilite based CuxS could be promising 

materials for photovoltaic absorbers.
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Conclusion

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Cu2S is more stable in the low-chalcocite structure, in 

agreement with experimental observations. However, it is 

not stable against the formation of Cu vacancies. 

We identified that at the Cu-rich limit, the most stable crystal 

structure is Cu1.75S in the anilite structure, which has a band 

gap around 1.39 eV and could be a promising solar cell 

absorbers.
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