Density functional theory calculations of certain transition-metal  doped semiconductors show a  partially occupied relatively narrow band located between valence band and conduction band. These novel systems,  containing the metallic band, are called intermediate-band materials. They have enhanced optoelectronic properties  which allow an increase in solar energy conversion efficiency of conventional solar cells. We previously proposed   III-V, chalcopyrite and sulfide derived compounds showing desirable characteristics to produce the intermediate band  of interest inside the band-gap. In order to obtain further intermediate-band material proposals, this work focuses on  studing other compounds constituted mainly by tetravalent elements. The first proposal is vanadium substituting Sn  atoms in SnS2 , the second one is composed by type II silicon clathrate with two possibilities: vanadium substituting  Si and Ag occluded in the intra-crystalline cavities. UV-Vis-NIR spectra of some of these systems experimentally  synthesized show an agreement with previous theoretical prediction

Aguilera Bonet, Irene

Conesa, Jose Carlos

Lucena, Raquel

Palacios Clemente, Pablo

Seminóvski Pérez, Yohanna

Wahnón Benarroch, Perla

Archivo Digital UPM

New materials for intermediate band photovoltaic cells. A theoretical and experimental approach

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

NEW MATERIALS FOR INTERMEDIATE BAND PHOTOVOLTAIC CELLS. 
THEORETICAL AND EXPERIMENTAL APPROACHES. 
 
 
Perla Wahnón*a, Pablo Palacios a, Irene Aguilera a, Yohanna Seminóvski a, José C. Conesa b, Raquel Lucena b 
a Instituto de Energía Solar, ETSI de Telecomunicación, UPM. Ciudad Universitaria s/n, 28040 Madrid, Spain. 
b Instituto de Catálisis y Petroleoquímica, CSIC. Marie Curie 2, Cantoblanco, 28049 Madrid, Spain. 
* Phone: +34-913367274 Fax: +34-915446341 e-mail: perla@etsit.upm.es 
 
 
ABSTRACT: Density functional theory calculations of certain transition-metal doped semiconductors show a 
partially occupied relatively narrow band located between valence band and conduction band. These novel systems, 
containing the metallic band, are called intermediate-band materials. They have enhanced optoelectronic properties 
which allow an increase in solar energy conversion efficiency of conventional solar cells. We previously proposed  
III-V, chalcopyrite and sulfide derived compounds showing desirable characteristics to produce the intermediate band 
of interest inside the band-gap. In order to obtain further intermediate-band material proposals, this work focuses on 
studing other compounds constituted mainly by tetravalent elements. The first proposal is vanadium substituting Sn 
atoms in SnS2, the second one is composed by type II silicon clathrate with two possibilities: vanadium substituting 
Si and Ag occluded in the intra-crystalline cavities. UV-Vis-NIR spectra of some of these systems experimentally 
synthesized show an agreement with previous theoretical predictions. 
Keywords: intermediate band, quantum modelling, high efficiency, tin sulphide, silicon clathrate, transition element, 
thin film 
 
 
1  INTRODUCTION 
 
Nowadays, most used single gap solar cells (SC) have 
low efficiency, with maximum values ranging around 
20%. In the quest of solar cell efficiency improvement, 
the photovoltaic community has exposed some 
hypotheses that may be a key to the wide spread 
obtainment of an enhanced solar cell in the future. 
Between these hypotheses, a great attention is given to 
intermediate-band (IB) concept. 
The IB concept is based on having a narrow metallic 
band inside the semiconductor band-gap, which allows 
the absorption of two extra low energy photons that 
promotes a second electron-hole pair, increasing the 
system photocurrent without affecting the voltage. The IB 
efficiency had been theoretically predicted as high as 
63.2% and can be created by doping over the Mott limit a 
host semiconductor in which the band-gap is around 2 
eV[1].  
Recently we proposed different doped 
semiconductors as IB materials. The systems were 
constituted by partially substituted materials as: GaP [2]-
[4], CuGaS2 [5]-[10], and indium thiospinels [11]. The 
partial substitution in these materials was made by a 
transition-metal cation having a partially d shell. Some of 
these IB materials were synthesized and experimental 
evidences [12] are consistent with previous theoretical 
predictions [11]. 
Nevertheless, a feasible method to get IB material is 
not completely achieved yet.  Theoretical predictions, 
which can increment the IB materials proposals, are a 
fundamental support to get the solution to the present 
problem. In agreement with this, we present different 
tetravalent elements based materials as new IB-SC.  
 
 
2 THEORETICAL AND EXPERIMENTAL 
APPROACHES 
      
2.1  Theoretical approach: models and methods 
Theoretical systems modeling  
The first assembled system is formed by tin disulfide 
(SnS2) (Fig. 1), a semiconductor with 2.30-2.56 eV [13]-
[14] band-gap which is relatively near the optimum 
theoretical value found for an IB-SC. 
The partial substitution of tin by vanadium (25%) inside 
tetravalent SnS2 semiconductor forms a layered structure 
of V2Sn6S16 where vanadium and tin have octahedral 
coordination.  
 
 
 
Figure 1: Layered SnS2 semiconductor structure where 
Sn and S atoms are labeled. 
 
The second system is constituted by type II silicon 
clathrate (Si34), a network with 34 tetrahedrally bonded 
silicon atoms per primitive cell, similar bond as in a  
regular silicon lattice. This network has a bond topology 
such that the crystalline structure has internal cavities 
with 24 or 28 vertices, forming a metastable silicon 
allotropic phase, with a face-centered cubic Bravais 
lattice [15]-[17]. The band-gap experimentally found in 
this material is 1.9 eV [18]-[19], a value that is close to 
the optimum band-gap theoretically found for an IB-SC. 
Regular silicon used in photovoltaic applications has a 
band-gap of 1.1 eV, which is different from the Si34 
clathrate band-gap. 
The two systems proposed are vanadium partially 
substituted Si34 (2 of the 34 atoms in a unit cell) (Fig 2a) 
and silver occluded inside the largest cavities (Fig 2b). 
 
 
25th European Photovoltaic Solar Energy Conference and Exhibition /
5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
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a)  
 
 
 
b) 
 
Figure 2: Type II silicon clathrate with a) vanadium 
substitution b)  silver occluded in the larger polyhedral 
cavity. 
 
 
Theoretical calculation description 
Density functional theory (DFT) calculations, at the 
GGA level (with PBE functional) and including spin 
polarization were carried out, with program VASP [20]-
[21]. The electronic structure of periodic systems was 
expanded by the wave-functions in plane waves and 
represented the atomic core regions with the PAW 
method [22]-[23]. The expansion cutoff used was high 
enough to ensure the convergence of all studied 
materials, and the reciprocal space was sampled 
according to a dense Monkhorst-Pack grid. In all cases 
the structures were relaxed in both, atomic coordinates 
and lattice dimensions, to reach the minimum energy. 
The obtained relaxed structure was used as a starting 
point to obtain band structure, density of states (DOS) 
and projected density of states (PDOS) of the systems. 
 
2.2 Experimental approach: synthesis and 
characterization 
V-substituted SnS2 was synthesized hydrothermally 
from aqueous SnCl4 and VCl3 mixed with thiourea and 
adding Triton X-100 surfactant to stabilize crystal 
nanoparticles. The mixture was heated at 175ºC for 48 h 
in an autoclave and the resulting solid was separated by 
centrifugation, rinsed with deionized water and methanol 
and dried in air.   
Structural and optical properties of the resulting 
powders were characterized respectively by X-ray 
diffraction (DRX) and UV-Vis-NIR diffuse reflectance 
spectrometry (DRS) .  
 
 
3 RESULTS AND DISCUSSION 
 
3.1 Intermediate band formation in SnS2:V system.  
V2Sn6S16 DOS shows an IB which is not present in 
SnS2 structure (Fig 3a and Fig 3b). In this system, 
vanadium 3d electronic orbitals, that mainly form the IB, 
have t2g symmetry, and consistent with this, three signals 
of vanadium PDOS appear (Fig 3b). We can also observe 
that Fermi energy level is located inside the IB. 
 
a) 
 
 
b) 
 
 
 
Figure 3: a) DOS of the SnS2 semiconductor, b) spin up 
and down DOS (solid line) and vanadium PDOS (dashed 
line) of V2Sn6S16. In these figures the Fermi energy level 
in abscissa is set up at zero. 
 
0 10 20 30 40 50 60 70 80 90 100
0
1000
2000
3000
4000
5000
6000
7000
122022
012
114
121
210
023113004
201
112
200
013
111
110
003
011
002
100
D
iff
r. 
in
te
ns
ity
 (a
.u
.)
2θ (º)
Sn0,9V0,1S2 - ATATr3
001
Particle size = 33 nm 
 
 
Figure 4: Characterization by DRX demonstrating a 
mono-phase formation  (all peaks correspond to a SnS2 
lattice). 
25th European Photovoltaic Solar Energy Conference and Exhibition /
5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
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0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-ln
(R
)
E  /  eV
1,65 eV 2,19 eV
 
Figure 5: V0.1Sn0.8S2  optical properties characterization 
with  UV-Vis-NIR where we can see two absorption 
onsets (IB formation is deduced). 
 
DRX experimental characterization of SnS2:V 
compound formed by sorvothermal synthesis shows that 
only a SnS2 structure is formed (Fig 4). This is 
demonstrated by the multiple diffracted signals that are 
the same ones presented in parent semiconductor SnS2 
which means that no other phase is formed and vanadium 
perfectly substituted tin atoms. 
Subsequent UV-Vis-NIR optical spectrum shows two 
features, one of them corresponding to 2.19 eV which is 
related to transition of photon absorption by the host 
semiconductor band-gap (Fig 5) and the other signal 
corresponding to 1.65 eV. This signal is an experimental 
evidence of the IB formation. 
 
3.2 Type II silicon clathrate with two vanadium 
substitutions. 
Silicon clathrates are covalent cagelike crystals 
analogous to fullerene families. When pentavalent 
vanadium replaces some tetravalent silicon atoms in the 
Si34, then the whole system exhibits an electron excess.  
DOS of V2Si32 shows a spin up IB mainly constituted 
by 3d orbitals of the tetrahedrally coordinated vanadium 
(Fig 6).  Orbitals with e symmetry form the IB, 
meanwhile orbitals with t2 symmetry are located at higher 
energy values. Vanadium PDOS confirms these results. 
 
Figure 6: DOS and PDOS of vanadium substituted type 
II silicon clathrate forming V2Si32.  
 
Nevertheless, in this system vanadium have a V4+ 
oxidation state, and V4+ fundamentally exists in an 
octahedral environment, e.g. vanadium silicide VSi2 
where Si is 8-fold coordinated to Si [24]. In agreement 
with this, we expect vanadium substitution in Si34 
network will not be energetically favored. 
 
3.3 Type II silicon clathrate with an Ag atom occluded in 
the main polyhedral cavity.  
Clathrates, however, provide another alternative, the 
possibility of occluding guest atoms inside cavities. 
Group 11 elements have been tested and an IB results 
when Ag is inserted in the largest cavities.  
In the simplest case of Ag occluded in Si34 cavities, 
forming AgSi34 system, the lattice contains only one 
guest atom at zero oxidation state inside the main 
polyhedral cavity. 
No strong covalent interaction between network of 
clathrate and silver exists. Silver is a noble metal and we 
expect not to have a big electronic density transfer to or 
from the Si lattice and silver. 
 
Figure 7: DOS and PDOS of AgSi34 that exhibits an IB 
formation. 
 
In this case, where no bond interactions exist between 
silver and the clathrate, silver atoms indeed conserve a 
single electron in an orbital of s symmetry which is a 
partially filled electron orbital that may form the IB at an 
adequate energy position. The evidence of an IB 
formation by this structure is depicted in the AgSi34 DOS 
and silver PDOS (Fig 7). 
 
 
4 CONCLUSIONS 
 
Theoretical calculations and experimental 
characterization indicate the desired intermediate band 
materials formation in the proposed systems. 
SnS2 and Si34 based intermediate-band materials can 
be formed by vanadium partial substitution. Silver atoms 
occluded in the main polyhedral cavity of Si34 form a 
AgSi34 structure. This system can also form an 
intermediate band and is more likely to be realizable than 
V2Si32 formed by partial substitution of silicon by 
vanadium. 
The experimentally obtained V in SnS2 material has 
been synthesized in a polycrystalline form, and can be an 
optimum system for thin film devices. The resulting 
intermediate-band systems could be implemented as solar 
cells with high efficiency. 
 
5 ACKNOWLEDGEMENTS  
 
Thanks are given to projects GENESIS-FV (CSD2006-
0004), NUMANCIA2 (S2009/ENE1477) and 
FOTOMAT (MAT2009-14625-CO3-01). R.L. gives 
thanks for a PhD grant of CSIC (I3P programme) and 
Y.S would also like to acknowledge the MICINN for a 
FPI grant.  
 
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New materials for intermediate band photovoltaic cells. A theoretical and experimental approach

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