CdS

Basic Information:

Molecular Weight: 144.48 gm (77.81% Cd, 22.19% S)

CdS is a naturally occurring mineral that shows up with two different crystal structures as greenockite and hawleyite.

Greenockite [1] [2]:

  • Named after Lord Greenock (1783-1859), the owner of the land where it was discovered.
  • Discovered in 1840 in Greenock, Scotland.
  • Found in Traprock cavities and ore veins.
  • An uncommon mineral of sulfide deposits

Density (calculated): 4.824 g/cm3

Member of: Wurtzite Group

Lustre: Adamantine, Resinous

Color: Yellow to red

Streak: Orange-yellow to brick red

Hardness (Mohs): 3-3 ½

Tenacity: Brittle

Cleavage: Distinct/Good

  • Distinct on {1122}, imperfect on {0001}

Fracture: Conchoidal

Hawleyite [4]:

  • Named after Professor James Edwin Hawley (1897-1965), a Canadian mineralogist at Queen's University.
  • Discovered in the Hector-Calumet mine in Canada.
  • Found as a coating on fine-grained sphalerite and siderite.

Density (calculated): 4.87 g/cm3

Member of: Sphalerite Group

Lustre: Metallic

Color: Bright Yellow

Streak: Light Yellow

Hardness (Mohs): 2½ - 3

Diaphaneity: Opaque

Cleavage: None

Common Uses [3]:

  • Color for soaps, textiles, paper and rubber.
  • Used in printing inks, ceramic glazes, fireworks, x-ray fluorescent screens and in body temperature detectors.
  • Provides stability against oxidation and UV radiation in some industrial products.
  • Used to create pigments with colors ranging from yellow to deep red that have high tolerance to heat and light. These pigments are mostly used in coloring plastics, ceramics and paints.
  • Used in small amounts in electric batteries and other electric components.

Crystal Structure

Greenockite [2]: Dihexagonal Pyramidal

Crystal System: Hexagonal

Cell Dimensions: a = 4.136Å, c = 6.713Å, Z = 2; Den(Calc)= 6.06

Axial Ratios: a:c = 1 : 1.623

Unit Cell Volume: 99.45 Å3

Space Group: P63mc

Morphology: Crystals hemi hemimorphic pyramidal. Earthy coatings (sphalerite).

Twinning: Twin plane {1122} rare forming trillings.

X-ray Diffraction [6]:

                  X-RAY WAVELENGTH:         1.541838

                  MAX. ABS. INTENSITY / VOLUME**2:          122.0494781

 

2-THETA INTENSITY D-SPACING H L K
24.82 59.21 3.5824 1 0 0
26.52 42.68 3.3574 0 0 2
28.20 100.00 3.1607 1 0 1
36.64 25.83 2.4497 1 0 2
43.72 45.25 2.0683 1 1 0
47.87 45.29 1.8982 1 0 3
50.93 6.79 1.7912 2 0 0
51.87 33.85 1.7610 1 1 2
52.85 15.27 1.7307 2 0 1
54.62 2.33 1.6787 0 0 4
58.33 5.62 1.5803 2 0 2
60.88 2.38 1.5201 1 0 4
66.83 14.14 1.3985 2 0 3
69.33 4.60 1.3540 2 1 0
70.94 10.80 1.3273 2 1 1
72.44 5.12 1.3034 1 1 4
75.54 9.29 1.2575 1 0 5
75.66 4.52 1.2557 2 1 2
80.33 5.66 1.1941 3 0 0
83.34 13.69 1.1585 2 1 3
86.40 6.04 1.1251 3 0 2

Hawleyite [5]: Hextetrahedral

Crystal System: Isometric

Cell Dimensions: a = 5.818 Å, Z = 4

Unit Cell Volume: 196.93Å3

Space Group: F4 3m

Morphology: Fine grained powdery coatings

X-ray Diffraction [7]:

X-RAY WAVELENGTH:       1.541838

MAX. ABS. INTENSITY / VOLUME**2:       213.1595047

2-THETA INTENSITY D-SPACING H L K Multiplicity
26.54 100.00 3.3590 1 1 1 4
30.74 23.16 2.9090 2 0 0 6
44.02 51.93 2.0570 2 2 0 12
52.14 39.02 1.7542 3 1 1 12
54.65 5.46 1.6795 2 2 2 4
64.01 7.15 1.4545 4 0 0 6
70.56 13.46 1.3347 3 3 1 12
72.68 5.95 1.3009 4 2 0 24
80.95 12.96 1.1876 4 2 2 12
87.03 6.95 1.1197 5 1 1 12
87.03 2.32 1.1197 3 3 3 4

PV Applications [10]:

 
Polycrystalline CdS thin films have properties that are suitable for solar cell fabrication. They have good optical transmittance, a wide band-gap and good electrical properties. CdS thin films also have a high absorption coefficient, electron affinity, low restivity, and easy ohmic contact, also making them suitable for solar cell applications. Because of these qualities, a lot of research is being done on direct band-gap thin films, especially because of its intermediate band-gap. CdS thin films are usually grown by chemical bath deposition because it is a more simple technique that produces quality films. Other techniques are sometimes used such as electrodeposition, screen printing, sputtering, and spray pyrolysis. The thin films with a and b phase depending on the deposition conditions. a-CdS grows with columnar structure along the c-axis perpendicular to the substrate so there are no grain boundaries parallel to the junction. CdS is used as a window electrode because of its stability, reasonable conversion efficiency, and the deposition technique is low-cost.
 
A diagram of CdS acts as a window layer can be found from reference [9].
 
For Cu(In,Ga)Se2 (CIGS) solar cells which contains chemical-bath-deposited CdS have attained a record efficiency of 20.3%. In this case, the CdS is the buffer layer. This instance had the following parameters [11]:
 
Open-circuit voltage: 730 mV
 
Fill Factor: 77.7%
 
Shunt resistance: 880 Ω cm2
 
Short-circuit current density: 35.7 mA/cm2
 
Electron current density: 4.2E-11 A/cm2
 
Specific contact resistance: 0.23 Ω cm2
 
Electrode polarization: 880 Ω cm2
 
Photocurrent density: 35.6 mA/cm2
 
Diode ideality factor: 1.38
 
Cell Area: 0.50 cm2
 
Cell setup:
  • soda-lime glass (3 mm)
  • sputtered molybdenum (500–900 nm)
  • CIGS (2.5–3.0 µm)
  • chemical bath deposited CdS buffer layer (40–50 nm)
  • sputtered undoped ZnO (50–100 nm)
  • sputtered aluminium doped ZnO (150–200 nm)
  • nickel/aluminium-grid
 

Basic Parameters at 300 K [14]

 
Heat Capacity: 53.97 + 3.77.10-3T J mol-1 K-1
 
Melting Temperature: 1750 K, 1405(10) C
 
Volume Compressibility: 1.586.10-7 bar-1
 
 

Band Structure:

 
Band Gap[12]: 2.42 eV
 
CdS is a direct gap semiconductor with the smallest energy gap in the center of the Brillouin Zone. The topmost valence band is split due to crystal field and spin-orbit coupling into three spin-degenerate states. Exciton sates formed with holes in these valence band states are denoted A, B and C exciton, respectively.
 
Calculated energies of symmetry points of the band structure (relative to the top of the valence band E(Γ5V)):
 
A graph of the band structure from an empirical tight binding model [18K] compared to the pseudopotential band structure of [67B] can be found from reference [14] (Energy bands corresponding to the Cd 4d states and spin-orbit coupling are not considered in this calculation).
 
Carrier concentration of undoped CdS [13]: 1.19 × 1019 cm−3
 
Temperature Dependences:
 
A graph of the energy gap of the A gap vs. temperature, experimental points can be found from reference [14].
 
Effective Masses and Density of States [14]
 
   conduction band, effective masses
   From experiments the conduction band mass can beassumed to be nearly isotropic.
   mn                           0.25 m0                       T= 300 K                        thermoelectric power
                                    0.2...0.16 m0              T= 25...700 K                mobility analysis, OMS, PPS
   valence band, effective masses
                                    0.7 (1) m0                   T= 1.6 K                         exciton magneto-absorption
                                    5 m0
 
Basic Parameters of Electrical Properties [14]
 
   electromechanical coupling factors
   k31                           0.119                           T= 298 K                       ultrasound resonance
   k33                           0.262
   k15                           0.188
   kt                              0.154
 
Mobility and Hall Effect [14]
 
                                    ≥10000 cm2/Vs         T= 30...40 K                  peak mobilities in ultrapure crystals
                                    160 cm2/Vs                T= 300 K                       In-doped, n=5•1019 cm-3
 
Optical properties [14]
 
  refractive index and birefringence
 
  Δ(=n- n ﻠ c)
  ﻠ c                      nc                   Δn               λ [μm]                        T [K]
 2.573                 2.586                                     0.55                           293                prism
 2.479                 2.496                                     0.61
 2.417                 2.434                                     0.69
 2.358                 2.375                                     0.85
 2.296                       2.312                                                  1.50
 2.281                                                  1.678                          2                                       293                     interference
 2.258                                                  1.662                          6  
 2.187                                                  1.408                         14
 2.051                                                                                     18
 1.880                                                                                      24
 
  temperature dependence of the refractive index
 
                                                                          T [°C]         λ [μm]
(1/ﻠ)dﻠ/dT     26.8(3)٠10-6 K-1        35...80        10.3           interference
(1/n)dn/dT      27.8(2)٠10-6 K-1
d(Δn)/dT           3.07(12)٠10-6 K-1
 
 
 
Graphs of numerically calculated spectral dependence of the absorption coefficient α and normal-incidence reflectivity R for E T c (a) and E ‖ c (b) at 300 K can be found from refernece [14].
 
Elastic Constants [15]: sE11, sE12, sD33, sE55, sD33, sD55
 
Phonon Frequencies [14]
 
 
References
 
[1] WebMineral, “Greenockite Mineral Data”, http://webmineral.com/data/Greenockite.shtml.
 
[2] J. Ralph, I. Chau, mindat Available at: <http://mindat.org> (2012).
 
[3] United States Department of Labor, “Chemical Identification, Production, and use
 
of Cadmium,” Occupational Safety and Health Administration, April 23rd 1993 . http://
 
 
[4] WebMineral, “Hawleyite Mineral Data”, http://webmineral.com/data/Hawleyite.shtml.
 
[5] J. Ralph, I. Chau, mindat, (2012). http://www.mindat.org/min-1834.html
 
[6] Downs R T (2006) The RRUFF Project: an integrated study of the chemistry,
 
crystallography, Raman and infrared spectroscopy of minerals. Program and Abstracts of the
 
19th General Meeting of the International Mineralogical Association in Kobe, Japan. O03-13
 
 
[7] Downs R T (2006) The RRUFF Project: an integrated study of the chemistry,
 
crystallography, Raman and infrared spectroscopy of minerals. Program and Abstracts of the
 
19th General Meeting of the International Mineralogical Association in Kobe, Japan. O03-13
 
 
[8] S. Schorn, “Hawleyite,” Mineralienatlas, 2012. http://www.mineralatlas.eu/lexikon/
 
index.php/MineralData?mineral=Hawleyite
 
[9] Y. Wakchaure, “Solar Cells,” www.nd.edu/~gsnider/EE698A/Yogesh_Solar_cells.ppt
 
[10] G. Sasikala, P. Thilakan, C. Subramanian, “Modifcation in the chemical bath deposition
 
apparatus, growth and characterization of CdS semiconducting thin films for photovoltaic
 
applications,” Solar Energy Materials & Solar Cells, vol. 62, no. 2000, pp. 275-293, October
 
 
[11] P. Jackson, et al., “New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells
 
beyond 20%,” Progress in Photovoltaics: Research and Applications,vol. 19, no. 7, pp. 894-897,
 
November 2011.
 
 
[12] J. Britt, Thin‐film CdS/CdTe solar cell with 15.8% efficiency. Applied Physics 
 
Letters, 62, 2851-2852 (1993). http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?
 
arnumber=4880703&abstractAccess=no&userType=inst
 
[13] S. Kose, et al., ‘Optical characterization and determination of carrier density of
 
ultrasonically sprayed CdS:Cu films,” Applied Surface Science, vol. 256. No. 13, April 2010.
 
 
[14] Madelung, O. (2004). Semiconductors: Data handbook. (3rd ed.). Springer.
 
[15] I.B. Kobiakov, “Elastic, piezoelectric and dielectric properties of ZnO and CdS single
 
crystals in a wide range of temperatures,” Solid State Communications, vol. 53, no. 3, July 1980.
 

 

The development of these pages on photovoltaic materials’ properties was carried out at the University of Utah primarily by undergraduate students Jeff Provost and Carina Hahn working with Prof. Mike Scarpulla. Caitlin Arndt, Christian Robert, Katie Furse, Jash Sayani, and Liz Lund also contributed. The work was fully supported by the US National Science Foundation under the Materials World Network program award 1008302. These pages are a work in progress and we solicit input from knowledgeable parties around the world for more accurate or additional information. Contact earthabundantpv@eng.utah.edu with such suggestions. Neither the University of Utah nor the NSF guarantee the accuracy of these values.