SnS-based thin film solar cells: perspectives over the last 25 years

Jacob A. Andrade-Arvizu1 • Maykel Courel-Piedrahita1 • Osvaldo Vigil-Galan1

Received: 30 January 2015 / Accepted: 4 April 2015

� Springer Science+Business Media New York 2015

Abstract New types of thin film solar cells made from

earth-abundant, non-toxic materials and with adequate

physical properties such as band-gap energy, large ab-

sorption coefficient and p-type conductivity are needed in

order to replace the current technology based on

CuInGaSe2 and CdTe absorber materials, which contain

scarce and toxic elements. One promising candidate ab-

sorber material is tin monosulfide (SnS). The constituent

elements of the SnS film are abundant in the earth’s crust,

and non-toxic. If this compound is used as the absorber

layer in solar cells, high efficient devices should be fabri-

cated with relative low cost technologies. Despite these

properties, low efficiency SnS-based solar cells have been

reported up to now. In this work, we present a review about

the state of the art of SnS films and devices. Finally, an

analysis about different factors that are limiting high effi-

ciency solar cells is presented.

1 Introduction

The production of solar cells with low cost, high efficiency

and by means of environmental friendly processes is the

current challenge for terrestrial applications. Thin film

photovoltaics (TFP) technology, known as second gen-

eration of solar cells, emerged to meet some of these ex-

pectations. Themassive use of solar cells requires increasing

the conversion efficiency of the devices and effective

lowering of the manufacturing costs [1]. It goes without

saying that thin films have the potential to revolutionize the

present cost of photovoltaic devices as alternative to silicon

technology [2]. Additionally, in terms of cost and efficiency,

the second generation of solar cells could be seen as a future

resource for sustainable energy.

Many have been the attempts made over the past century

until today for searching non-toxic and cost-effective new

materials with adequate properties for photovoltaic applica-

tions. Certainly, the developments of binary and ternary ma-

terials, such asGalliumArsenide (GaAs), CadmiumTelluride

(CdTe), Copper-Indium-Gallium-Selenide (CIGS), etc., have

created pathways compared to the first generation solar cells

technology, due to the favorable properties and simplicity in

their synthesis. Lately, the most advanced materials for

making thin film solar cells are based on the use of CdTe or

CIGS as absorber layer materials [3]. However, problems

related to toxicity [4–7] and scarcity of someof the constituent

elements of these compounds have been reported as issues to

overcome, in a mass production [8].

To achieve cost-effective thin film solar cells for large-

scale production, the absorbing semiconductor material

used in the device needs to satisfy many requirements.

First, the constituent elements should be inexpensive, non-

toxic and abundant in the earth’s crust. Second, to obtain

high conversion efficiencies, the material should have ap-

propriate optical and electrical properties such as a suitable

optical band gap, a high optical absorption coefficient, a

high quantum yield for excited carriers, a long minority

carrier diffusion length, and a high minority carrier lifetime

(a low recombination velocity) [9]. Compounds such as tin

sulfide (SnS) have been considered as materials that can

meet these requirements; however, far lower efficiency

values have been reported in solar cells based on SnS as

absorber.

& Maykel Courel-Piedrahita

maykelcourel@gmail.com

1 Escuela Superior de Fısica y Matematicas-Instituto

Politecnico Nacional (IPN), C.P. 07738 Mexico, D.F.,

Mexico

123

J Mater Sci: Mater Electron

DOI 10.1007/s10854-015-3050-z

In material science, efforts persist on finding new

sustainable materials. New approaches have considered

kesterite compounds for solar cell applications. However,

several problems have been identified as principal hurdles

to boost solar cell performance, reaching efficiencies

lower than 13 % [1, 10–14]. On the other hand, among

many semiconducting metal Chalcogenides, tin sulfides

have attracted extensive interest due to its physical and

chemical properties for solar energy conversion, espe-

cially by its photoconductive character [15, 16]. Despite

this material has been widely studied; as far as we know,

there is no report that embraces results obtained for both

SnS physical properties, and its application in solar cells.

Besides, the main critical aspects to improve both mate-

rial properties and solar cell performance have not yet

been identified.

In this work, a review about SnS properties and its ap-

plication results in thin film solar cells is presented. The

most used techniques and results about SnS thin film pro-

cessing are presented. In order to improve SnS-based thin

film solar cell performance, several hurdles are identified.

Finally, a critical discussion about a further solar cell ef-

ficiency improvement is presented.

2 SnS: a potential absorber for thin film solar cellapplications

2.1 Brief history

The tin sulfide (SnS) compound was first reported by a

German mineralogist Robert Herzenberg in the year 1932

[17], being this the main reason for the mineral to be

recognized as herzenbergite, also known as: Kolbeckine.

Since then, many are the available studies on the struc-

tural, optical and electronic properties as well as its pe-

culiarities with synthesis parameters and fabrication

techniques. In this regard, the p-T-x diagram for the Sn-S

system was then determined in the early 60 s by Albers

et al. [18] especially in the region of the SnS compound.

Also, this investigation group showed that the existence

region of solid SnS lies entirely with a high degree of

probability at the excess sulfur side. This diagram also

results to be quite helpful for the understanding of the

feasibility of Tin binary sulfides formation such as SnS,

and SnS2 even at low temperatures and with a suitable

Sn/S ratio. Some years later, in 1979, an exhaustive report

on various SnS minerals was stated by Kissin and Owens

[19]. After almost 25 years later, Dittrich et al. [20]

studied the sulfo-salts emphasizing their potential impor-

tance in the field of photovoltaic devices.

2.2 IV–VI semiconductors: the tin sulfides family

In nature, it is found that tin sulfide compounds exist in a

variety of phases such as: SnS (Orthorhombic/Zinc Blen-

de), SnS2 (Hexagonal/Trigonal), Sn2S3 (Orthorhombic),

and Sn3S4 (Tetragonal), as well as in numerous polysulfide

anions due to bonding characteristics of tin and sulfur.

Among them, only SnS, SnS2 and Sn2S3 are discrete

phases. The oxidation state may take ?2, ?4 for tin and

-2,-1, 0 for sulfur. The tin sulfide chemistry is then further

enriched by the catenation ability of sulfur. In addition, it

has been observed by Jiang and Ozin [21] that other metallic

and non-metallic elements can be incorporated into the tin

sulfide structures to yield ternary and quaternary materials or

the successful doping of the compound.

Besides, all these binary compounds behave as semi-

conductor materials, and exhibit n or p type conductivity

depending on the tin concentration. Among the tin

chalcogenide thin films, SnS and SnS2 are the most im-

portant ones because of its suitable opto-electronic prop-

erties for photovoltaic applications. Due to optimum band

gap, high absorption coefficient and p-type conductivity,

SnS results to be a suitable candidate for absorber layer in

thin film solar cells whereas SnS2 for window layer be-

cause of the wider band gap and n-type conductivity. In

fact, SnS2 has been considered as an alternative non-toxic

buffer layer to CdS.

Therefore, three of these binary compounds based on the

Sn–S system (SnS, SnS2, and Sn2S3) result to be interesting

from a technological point of view. The Sn2S3 is classified as

a type I mixed valence compound (II and IV valence) with a

semiconductor behavior, whose optoelectronic properties

are dependent on the crystalline structure and stoichiometry.

These compounds could be used to build photovoltaic p–n or

p–i–n structures with a conversion efficiency of about 25 %

according to the Loferski’s limit [22]. These structures

would be low-cost devices, because the materials involved

are cheap, nonstrategic, and abundant in nature. In addition,

Sn2S3 is a semiconductor material that appears to be suitable

for preparing near-lattice-matched heterojunctions in sys-

tems such as Sn2S3/CdTe, Sn2S3/GaSb, Sn2S3/AlSb, etc.

These systems have applications in the detection and gen-

eration of infrared radiation [23]. Table 1 displays structure,

lattice constants, band gap value and conductivity type for

the tin sulfide family. Among the different phases, SnS with

direct band gap shows adequate properties to be used as an

absorber due to its suitable band gap *1.4 eV and p-type

conductivity. On the other hand, SnS2 compound with hex-

agonal structure is characterized by a high band gap value

(2.2 eV) with direct transitions and n-type conductivity

making it attractive for buffer layer applications.

J Mater Sci: Mater Electron

123

2.3 Tin sulfide (SnS) as a proper candidate

for second generation solar cells

As remarked in the previous section, one promising ma-

terial for absorber layer in thin film solar cells is the metal

monochalcogenide tin monosulfide (SnS). This material

has a near optimum energy band gap and exhibits an am-

photeric character promising the possibility of grain

boundary passivation. Not only does SnS consist of

relatively abundant elements but large scale production

processes already exist for producing thin films of tin and

for converting metals into the corresponding sulfur com-

pounds using a range of chemical, physical and sulfuriza-

tion processes. Some of the main properties of SnS

compound are presented in Table 2.

In addition, this compound is chemically stable in acidic

as well as alkaline media due of its amphoteric character

[53]. According to the Shockley–Queisser criteria [54], a

maximum efficiency up to 33 % could be achieved for this

material (for a direct band gap energy of approximately

1.3 eV). Nevertheless, this result seems to be a very first

rough approximation. On the other hand, another possible

estimation for conversion efficiency limit could be eluci-

dated for different materials as a function of their optical

band gaps from the theory contained in the Loferski dia-

grams [22]. Under this approach, an efficiency value higher

than 24 % was obtained. This result along with the prop-

erties reported in Tables 1 and 2 emphasize the acceptance

of SnS as a good candidate for an absorber material. SnS

crystals present natively p-type conductivity due to the

small formation enthalpy of tin vacancies, which generate

shallow acceptor-like vacancy defects [55]. Amorphous

and crystalline SnS films show a direct band gap of:

1.0–1.65 eV and an indirect of 1.0–1.45 eV [56] with an

absorption coefficient of around 104 cm-1 which means an

absorption length about 1 lm. Besides, SnS compound has

shown an inert surface with few surface states [40, 56].

This might reduce the carrier recombination losses due to

defects at p-n junctions and at grain boundaries [31].

Furthermore, Hofmann [57] have considered the SnS-

orthorhombic structure as pseudo-tetragonal one. Every Sn

atom stays surrounded by three S atoms forming bond

angles around 90�. SnS has a unit cell with lattice pa-

rameters as presented on Tables 1 and 2. On the other

hand, a comparative study on binary metallic chalcogenide

semiconductor materials demonstrated that the ionicity and

transverse charges increased in the order of GeS ? Ge-

Se ? SnS ? SnSe [58, 59]. Ettema et al. [60] reported the

existence of SnS in two different forms such as a-SnS and

b-SnS with similar charge distribution. They suggested that

the a-SnS is a low temperature phase compound with lower

symmetry than the high temperature b-SnS phase. In ad-

dition, a-SnS phase shows a higher band gap (indirect

1.6 eV, direct 1.8 eV) respect to the b-phase. On the other

hand, some properties of the films related with lattice

mismatch, defects, and an improvement in the grain size

could significantly improve the efficiency of the device

[40].

3 SnS thin film fabrication

3.1 Routes of synthesis

In the last three decades, thin films of SnS have been

synthesized by means of diverse physical and chemical

routes; among them, Table 3 shows the most representative

synthesis pathways. Each method has its own advantages

and disadvantages in the production of homogeneous and

defect-free thin film materials. New ways of synthesis

routes are being evolved in order to produce controlled size

and shape of desired morphology. In general, the measured

properties of SnS samples depend on the synthesis route. A

brief description of each method is described below.

3.1.1 Physical routes

3.1.1.1 Two-step process The basic principle of this

synthesis route consists of the sulfurization of thin metallic

tin precursor layers. Reddy et al. [61] reported a detailed

study of the structural properties of the SnS thin films

deposited using the TSP. Here, the sulfurization process

Table 1 Structure, lattice constants, band gap value and conductivity type for the tin sulfide family

Material Structure a (A) b (A) c (A) EG (eV) Type of conductivity References

SnS (tin monosulfide) Orthorhombic 3.64 11.21 3.99 1.35 (direct)/

1.1 (indirect)

P/N [24–26]

SnS2 (tin disulfide) Hexagonal 3.64 3.64 5.90 2.22 (direct) N/P [27]

2H-SnS2 Orthorhombic 3.64 3.64 5.89 – P [28]

Sn2S3 (tin sesquisulfide) Orthorhombic 8.84 14.02 3.74 2.05 (direct)

0.53 (indirect)

N [29]

J Mater Sci: Mater Electron

123

was carried out at different temperatures (100–400 �C) in a

sulfur rich environment. Some years later, Gordillo et al.

[64] synthesized SnS:Bi thin films out of this technique.

The characterization studies revealed that the SnS:Bi films

tend to grow with a mixture of the SnS and Bi2S3 phases. It

was also found that the SnS:Bi films exhibit an absorption

coefficient [104 cm-1 and a direct energy band gap

ranging from 1.37 to 1.47 eV, indicating that this com-

pound shows the required optimal properties for an ab-

sorber layer in thin film solar cells.

3.1.1.2 Atomic layer deposition Homogeneous and ultra-

thin films of various materials can be coated on a variety of

substrates by means of this route. SnS was deposited by

Table 2 Main properties of SnS compound

Type of property References

Crystal structure Orthorhombic Herzenbergite polycrystalline

(2/m 2/m 2/m)

Also: hexoganol (Wurtzite) and cubic (Sphalerite) crystal

forms which are less stable than herzenbergite at room temperature

[30, 31]

Space group Pnma, Cmcm, Pbnm [31–33]

Atoms/cm3 4.15 9 1026 [30]

Unit cell volume (A3) 192.67 [33]

Atoms per unit cell 4

Molecular weight (g) 150.78

Density (g/cm3) 5.08 [34]

Lattice constants (A) a b c [31, 35]

3.64 11.21 3.99

Effective hole masses m (||a) m (||b) m (||c) [34]

0.2 m0 0.2 m0 1.0 m0

Conductivity type p-type: Stoichiometric [36–38]

n-type: S vacancies–Sb doped

Intrinsic resistivity (X cm) 13–20 [31, 35]

Effective electron density (cm-3) 6.3 9 1014–1.2 9 1015 [35, 39]

Effective hole density (cm-3) 1018 [35]

Hole mobility (Hall Mobility) (cm2/Vs) T = 300 K T = 2 9 103 K [31, 35]

50–90 77

Work function (eV) 4.2 [35, 40]

Binding energy (eV) S2p3/2 S3p5/2 [31, 35, 41]

161.4 485.3

Dielectric constant e(0) e(0) [34, 42]

E||a : 32 E||a : 14

E||b : 48 E||b : 16

E||c : 32 E||c : 16

a (cm-1) (absorption edge) 104 [24]

Refraction index 3.52 [24]

Band gap energy (eV) Egdirect Egindirect [24, 26, 40, 42–51]

1.35

Reported in the range

of values: 1.0–1.65

1.1

Reported in the range

of values: 1.0–1.45

Debye temperature (K) 270 [52]

Linear thermal expansion coefficient (K-1) 2.8 9 10-7 [34]

Heat capacity (J/mol K) Cp @300K Cp @80K [34]

45 29.3

Melting point (K) 1154 [31]

J Mater Sci: Mater Electron

123

this technique using sequential exposures of Sn(II) 2,4-

pentanedionate (Sn(acac)2)and H2S. It has been shown that

using the ALD method, the growth rate of SnS thin films

stayed independent of substrate temperature in the range of

125–225 �C. X-ray fluorescence studies showed that Sn/S

atomic ratio was *1.0 for the SnS films. Also, XPS

measurements revealed that the films contained oxygen

impurities at 15–20 % atomic after air exposure. The films

had a band gap of *1.87 eV which was higher than the

value of the bulk (*1.3 eV) [65].

3.1.1.3 Co-evaporation Polycrystalline SnS thin films

have been grown on glass substrates using this procedure

involving chemical and physical reactions between the si-

multaneously evaporated precursor species. Thin films of

SnS were prepared by this method ranging from 200 to

400 �C. The chemical composition of the SnS films results

to be regulated by controlling the evaporation temperature

and/or the amount of the S and Sn evaporated masses. It

was found by Cifuentes et al. [66] that using an adequate

set of deposition parameters; it is possible to get tin sulfide

films in the SnS phase with orthorhombic structure (111) or

in the SnS2 phase with tetragonal structure (001). In this

paper, it is also reported that the refractive index of the SnS

films is significantly greater than that of the SnS2 films.

On the other hand, Koteeswara et al. [67] deposited thin

films at a temperature of 300 �C. A high crystalline quality

along with a dominant SnS phase (040) films were ob-

tained. They also reported that the growth rate of the layers

was directly proportional to the temperature. Nearly

stoichiometric, low-resistive and single-phase SnS films

with an optical band gap of 1.37 eV were synthesized and

reported which could be used as an absorber in the fabri-

cation of thin film heterojunction photovoltaic devices.

3.1.1.4 Close-spaced sublimation/close-spaced vapor

transport As far as we know, Yanuar et al. [68] reported

for the first time the synthesis of SnS thin films by the

CSVT method. The as-grown films showed natively p-type

conductivity with a hole concentration of 1017cm-3, an

energy band gap of 1.32 eV, an absorption coefficient of

104 cm-1 and a low Hall mobility of 3.73 cm2/Vs. The

films also showed a single phase and an excellent crys-

talline quality with prismatic shaped crystallites of about

1 lm. Years later, Xiao et al. [70] synthesized SnS by the

CSS technique at higher temperatures (650 and 720 �C)showing that with the increase of the source temperature

the grain sizes increased around 650 %. They also reported

the values for the direct band gap energies of 1.21 and

1.15 eV for each temperature, respectively.

3.1.1.5 Hot wall vacuum deposition Thin films and

nanorods of Sn1-xPbxS (0.00 B x B 0.45) with

orthorhombic (001) crystal structure and c-axis oriented

perpendicular to the substrate surface presenting a strong

(004) oriented peak were synthesized by the HWVD

method. The as-deposited films were pin-hole free and

strongly adherent to the surface of the substrate. Sn1-xPbxS

layers appeared grey in color and the evaluated thickness

varied from 2.0 to 4.0 lm [71, 72].

3.1.1.6 Molecular beam epitaxy A systematic investiga-

tion of optoelectronic properties of SnS grown by MBE

technique is reported by Wang et al. [73]. Energy band

Table 3 Physical and chemical routes used in the deposition of SnS thin films

Physical routes Chemical routes

Synthesis route References Synthesis route References

Two-step process [61–64] Brush plating [82]

Atomic layer deposition [16, 65] Chemical bath deposition [44–49, 78, 83–95]

Co-evaporation [63, 66, 67] Chemical vapor deposition [96–99]

Close-spaced vapor transport/

close-spaced sublimation

[68–70] Pulsed CVD [100]

Hot wall vacuum deposition [71, 72] Plasma-enhanced CVD [101]

Molecular beam epitaxy [73] Dip deposition [102]

Thermal evaporation [42, 43, 74–78] Electron beam evaporation [103]

Vacuum evaporation [24, 25] Electrodeposition [26, 50, 51, 95, 104–120]

Rf—sputtering [79, 80] Multilayer-based solid-state reaction [121]

Physical vapor deposition [81] Successive ionic layer adsorption and reaction [122–124]

Spray pyrolisis [23, 35–38, 63, 125–139]

J Mater Sci: Mater Electron

123

simulation indicates that SnS had an indirect bandgap of

0.982 eV. Furthermore, the report indicates that elemental

Cu could be used as a p-type dopant for the absorber ma-

terial. Besides, the authors have noticed a significant re-

duction in the rocking curve FWHM over the existing

published values. Also, a crystallite size in the range of

2–3 lm was observed.

3.1.1.7 Radio frequency sputtering In the IEEE confer-

ence of the year 1994, Guang-Pu et al. [140] reported the

preparation of SnS thin films by RFS method for the first

time. The chemical composition, crystal structure, optical

and electrical properties were also reported in this work.

Besides, a critical discussion about the relationship be-

tween the SnS film properties and the sputtering parameters

was also added. Using Sb as a dopant and combining it

with thermal annealing at elevated temperatures (about

400 �C); n-type SnS thin films were obtained. Years later,

Hartman et al. [139] sputtered films with a SnS target by

means of using argon plasma. They have found that the

resistivity, stoichiometry, phase, grain size, shape, band

gap, and the optical absorption coefficient can be varied by

modifying argon pressure for a fixed deposition time in

these films. They have also reported values for the indirect

band gap of 1.08–1.18 eV.

By the year of 2014, Xu et al. [79] utilized polyamide

substrates to improve flexibility of SnS thin film hetero-

junctions. ZnO/SnS heterojunctions were deposited by

magnetron sputtering and their properties were studied.

Post annealing of the films enhanced the crystalline quality

of the compound as observed by other techniques. With a

preferential orientation along the (040) plane and grain

sizes of 18 nm, the compositions of as-deposited and an-

nealed flexible SnS thin films were close to the stoichio-

metry of SnS. The fabricated ZnO/SnS flexible

heterojunctions showed rectifying properties with a recti-

fying ratio of 6.85 and a diode ideal factor of 1.23.

3.1.1.8 Thermal evaporation Nahass et al. [43] have

synthesized SnS thin films by this physical deposition

route. In this work, the authors investigated the structural

transformation after an annealing process at the tem-

perature range of (160–300) �C. Additionally, an exhaus-

tive report on the various optical properties of SnS films

was also included in this paper. The substrate often plays a

critical role in determining the properties of the deposited

film, especially when the deposition technique is thermal

evaporation. Devika et al. [41] have investigated the effect

of annealing on the composition, crystal structure, surface

features and opto-electronic properties of thermally

evaporated SnS thin films and found that with annealing

temperature increase, the composition of the film changed

due to re-evaporation of Sulfur. The SnS structure in the as-

deposited and annealed films remained orthorhombic. With

an increase in substrate temperature, the grain size and the

surface roughness were reduced. Also, the electrical re-

sistivity decreased. As a peculiar observation, the films

annealed at 100 �C showed some unusual features com-

pared to those annealed at other temperatures. Miles et al.

also deposited SnS thin films on glass and onto SnO2:

coated glass substrates, by using thermal evaporation with

the aim of optimizing the properties of the film for using in

solar cells. In particular, they investigated the effects of

source temperature, substrate temperature, deposition rate

and film thickness on the chemical and physical properties

of the films [76].

3.1.1.9 Vacuum evaporation There are few reports of

SnS thin film synthesis by the direct VE of tin sulfide

compound powder. Noguchi et al. [25] prepared tin sulfide

(SnS) films by the vacuum evaporation technique. As-

grown SnS films showed p-type conduction with a resis-

tivity of the order of 13–20 X cm, a carrier density of

6.3 9 1014–1.2 9 1015 cm-3, and Hall mobility values in

the range of 400–500 cm2/Vs. Devika et al. [24] discussed

the influence of annealing on physical properties of

evaporated SnS films and observed that desulfurization

took place owing to the re-evaporation of S from SnS films.

They also showed that at higher annealing temperatures,

defragmentation of grain size could be observed. Devika

and collaborators also studied the influence of substrate

temperature on surface structure and observed that films

grown at higher substrate temperatures showed a large

grain size distribution compared to that at room tem-

perature. Besides, they observed an indirect band gap for

all the prepared SnS films lying somewhere between 1.37

and 1.42 eV.

3.1.1.10 Physical vapor deposition The PVD method

describes a variety of vacuum deposition methods used to

deposit thin films by the condensation of a vaporized form

of the desired material. Revathi et al. [81] deposited SnS

films onto different substrates such as glass, ITO, and Mo-

coated glass. The role of the substrate was then analyzed

depending upon the compositional, microstructural and

photoelectrochemical properties of the SnS films.

Orthorhombic structures with nearly stoichiometric ele-

mental compositions (Sn/S ratios of *1.01) were observed

for all the substrates. Particularly, SnS films deposited on

ITO and Mo-coated glass substrates, exhibited (040) plane

as preferred orientation whereas the films deposited on

glass showed (111) plane as predominant. Also, the Raman

spectra showed SnS single phase contribution. The p-type

conductivity and high photoresponse for films deposited on

the ITO substrate suggested them as a good candidate for a

solar cell absorber.

J Mater Sci: Mater Electron

123

3.1.2 Chemical routes

3.1.2.1 Brush plating Thin films of polycrystalline

orthorhombic (040) p-SnS were brush plated onto tin oxide

coated glass substrates from aqueous solution containing

SnCl2 and Na2S2O3 [82]. The variation of space charge

capacitance, with applied potential (Vapp), was recorded for

the PEC cell with a p-SnS/Fe3?, Fe2?/Pt structural system.

Also, the spectral response of the PEC cell formed with the

SnS photoelectrode was studied and reported. The crys-

tallite size was reported to be in the range of 0.3–0.7 lm.

3.1.2.2 Chemical bath deposition (CBD) Numerous re-

ports are available about the fabrication of SnS thin films

using different precursor solutions, which render Sn2? and

S2- ions for the synthesis of the thin films. CBD is a simple

and very popular technique for thin film deposition, espe-

cially for the deposition of metal chalcogenide thin films.

Pramanik et al. [83] developed this technique for the syn-

thesis of SnS thin films on glass substrate at room tem-

perature. The deposited films were amorphous and

presented n-type conductivity with an optical band gap of

1.51 eV. Two years later, in 1989, Ristov et al. [84] syn-

thesized p-type tin sulfide thin films from SnCl2 and Na2S

precursor salts. An interesting observation was revealed

when annealing this film several hours. Above 280 �C the

conductivity changed to n-type without any detectable

change in the composition, but annealing for a longer time

(over 24 h) changed the composition to SnS2. For higher

temperatures (300–400 �C) in open air, oxidation of the

films was observed, changing its composition to SnO2. In

the next year, Engelken et al. [85] dissolved sulfur in

propionic acid, after which SnCl2�2H2O, potassium glu-

conate or tartaric acids were added. The solution was

heated from room temperature to 90 �C and the bath turned

in a yellowish color, after which the color changed to slate

grayish within a 60 s period. After 30 min, SnS films of

1 lm thickness were obtained. One year later, Lokhande

et al. [86] reported the procedure for depositing various

metal chalcogenide thin films including SnS by employing

CBD technique. By the same year, a much simplified

technique for preparing good quality SnS films was re-

ported by Nair et al. [87]. They were able to deposit uni-

form p-type films with thicknesses up to 1.2 lm. These

films exhibited slight photosensitivity as well. In the same

year and by the same group, the application of these SnS

films as tubular solar collectors owing to its high ab-

sorbance was accentuated [78]. Nair et al. [88] reported the

deposition of good-quality thin films of SnS, CdS, CdSe,

etc., establishing the possible phase transformation from

SnS to SnO2. Thin tin sulfide films deposited by Tanu-

sevski [89] were polycrystalline with an orthorhombic

structure. These films were thermally treated in argon

atmosphere at 300 �C showing an increment in the crys-

tallinity degree. The value of 1.38 eV for the optical

bandgap was determined for the direct transitions and

presented no change after the thermal treatment. From the

red edge of the photoconductivity spectrum, a band gap of

Eg(ph) = 1.24 eV was determined. An impurity level with

an activation energy of 0.39 eV and a thermal band gap of

Eg(T) = 1.19 eV were determined by the temperature de-

pendence of the dark resistance of the films. SnS thin films

with porous structure were synthesized by Lei et al. [90] on

glass using Triethonalamine (TEA), Thioacetamide,

SnCl2�H2O and aqueous ammonia as reactants and aqueous

NH4Cl solution as buffer. All the samples were polycrys-

talline with an orthorhombic structure. The properties of

these porous structures are in principle more suitable for

third generation photovoltaic devices. Two years later,

Akkari et al. [92] investigated the influence of TEA con-

centration on the properties of SnS films for a post-opti-

mization of the deposition parameters. These films

exhibited an orthorhombic structure and a direct band gap

about 1.65 eV. Hankare et al. [44] deposited SnS films on

non-conducting glass substrate at room temperature. The

thin films resulted to be uniform, well adherent and

brownish colored. The optical band gap of the samples was

1.0 eV. The films exhibited p-type conductivity with an

activation energy of 0.62 eV.

By multi-deposition runs, SnS thin films with a Zinc

Blende structure were synthesized by Akkari et al. [46].

The precursors were aqueous solution containing 30 ml

TEA, 10 ml Thioacetamide, 8 ml Ammonia solution and

10 ml of Sn2? (0.1 M). The crystallinity of the films was

improved with film thickness and the band gap energy was

about 1.76 eV for a film prepared after six deposition runs.

By the same year (2010), Guneri et al. [47] deposited SnS

on glass substrates by keeping the bath for 24 h. The films

were nearly stoichiometric (Sn/S = 1.18) and presented

p-type conductivity. Its resistivity and mobility values were

found to be 2.53 9 105 X cm and 8.99 9 105 cm2/Vs,

respectively. Besides, an activation energy of 0.527 eV

was calculated. Optical band gap values of direct and

indirect transitions were estimated to be 1.37 eV and

1.05 eV, respectively. In a following work, they reported

the effect of deposition time on the structural, electrical and

optical properties of SnS thin films at different deposition

times (2–10 h) at 60 �C [48]. All the deposited films were

polycrystalline in nature and presented an orthorhombic

structure with small crystal grains. Despite this, the mi-

crostructures presented substantial changes with the depo-

sition time and were nearly stoichiometric. Hall

measurements showed that the obtained films had p-type

conduction and resistivity values of SnS films also changed

with deposition time. Band gap measurement indicated that

for allowed direct, allowed indirect, forbidden direct and

J Mater Sci: Mater Electron

123

forbidden indirect transitions, band gap values varied in the

range 1.30–1.97, 0.83–1.36, 0.93–1.49 and 0.62–1.23 eV,

respectively for these set of samples. The influence of the

pH value (pH 1.5–3.5) in this technique on the growth and

properties of SnS thin films were investigated by Kassim

et al. [49]. The authors obtained relatively uniform grain

size, good coverage and thicker films for lower pH value

such as pH 1.5. Band gap values were found to be

1.2–1.6 eV for the films deposited under various pH

values.

Over 3 years later, Safonova et al. [93] showed that SnS

films of 100–500 nm can be deposited onto ZnS and CdS

substrates using tin chloride and thiosulfate as the precur-

sors. The deposition was carried out at 25 �C. In a single

deposition way, film thicknesses in the range of

110–170 nm were achieved. On the other hand, by using

two more successive depositions, film thicknesses in the

range of 450–500 nm were obtained.

3.1.2.3 Chemical vapor deposition Price et al. [97] de-

posited SnS films employing atmospheric pressure CVD

technique using Tri-n-Butyltin Trifluoroacetate and H2S as

precursors at 350–600 �C under a nitrogen atmosphere. An

exhaustive Raman analysis was performed on the samples

and the results were discussed. Barone et al. employed

Aerosol assisted CVD to deposit SnS thin films. In this

work, they used (PhS)4Sn as the precursor solution and H2S

as the co-reactant. They found that at 500 �C, uniform

films of SnS were deposited [98]. Juarez et al. [99] de-

posited SnS thin films with the assistance of ‘‘Plasma En-

hanced Chemical Vapor Deposition’’ (PECVD) method.

The electrical and optical characterizations of thin films

based on Sn-S bonds (SnS2, Sn2S3) as a function of the

relative concentration of the precursor vapors, SnCl4 and

H2S were carried out. In all the studied cases, the deposited

films were formed by polycrystalline materials. The au-

thors have reported that SnS2 compound produced under

certain deposition conditions has a forbidden band gap

around 2.2 eV. This compound showed n-type electrical

conductivity with a dark value of 2 9 10-2 (X cm)-1 at

room temperature. Also, it showed the typical semicon-

ductor dependence of its electrical conductivity on the

temperature, with an activation energy of about 0.15 eV.

However, thin films of a mixture of SnS2 and Sn2S3compounds were deposited with higher values of the

relative concentration of vapor sources than those used to

obtain the SnS2 compound. They have also observed that

the optical band gap shows a decreasing trend as the

relative concentration increases. A similar trend was ob-

served for dark electrical conductivity.

3.1.2.4 Dip deposition process Sekhar et al. [141] have

reported the fabrication of SnS and SnS2 thin films. By

means of this technique, a substrate was dipped into an

alcoholic solution of the corresponding tin chloride and

thiourea, then withdrawn vertically at a controlled speed,

and finally baked in a high temperature furnace at atmo-

spheric conditions. Good quality films were obtained at a

baking temperature of 300 and 360 �C. Values of band gap

for SnS and SnS2 obtained from spectral response of

photoconductivity were 1.4 and 2.4 eV, respectively.

Open-air annealing of both SnS and SnS2 films at 400 �Cconverted them into transparent conducting SnO2.

3.1.2.5 Electrodeposition This synthesis route is an

economical method to deposit large area thin films and can

lead to the real mass-production of solar cells. Tin sulfide

thin films were fabricated using conventional three elec-

trode method via constant voltage model. Ichimura et al.

[106, 107] described a three-electrode cell that was used

for ECD with a saturated calomel electrode (SCE) as the

reference electrode. An In2O3 coated glass sheet was used

as the substrate and a stainless sheet as the counter elec-

trode. An aqueous bath used for the deposition contained

SnSO4 and 100 mM of Na2S2O3. SnSO4 concentration [Sn

(II)] was varied between 1 and 7.5 mM. The pH of the

solution was changed in the range between 3.0 and 4.0 by

adding H2SO4 and thin films of SnS were grown at cath-

ode’s surface. The deposition period was 1 h, and the bath

was set to room temperature. Tin sulfide can also be fab-

ricated by constant current Electrodeposition method using

a platinum electrode [112, 142]. SnS films for PV appli-

cation were successfully deposited employing electro-

chemical deposition technique and a detailed report on

electrical characterization results was given by Sato et al.

[26]. On the other hand, SnS thin films deposited over

flexible metallic substrates were studied by Khel et al. [95].

They deposited SnS thin films on Al sheet through this

technique from aqueous solutions containing SnSO4 and

Na2S2O3. The films were polycrystalline with an

orthorhombic structure. Composition was found to be

S-rich in acidic pH and Sn-rich at higher pH values. Re-

lation between film properties and deposition parameters

were also studied in this work to optimize the deposition

conditions. SnS thin films were synthesized by Zainal et al.,

in aqueous media in the presence of Triethanolamine using

this method [110]. Effect of deposition potential, Tri-

ethanolamine concentration, and deposition time on the

properties of SnS films were studied. The presence of

Triethanolamine showed improvement in reproducibility,

adherence and crystallinity of the films. XRD studies

indicated formation of polycrystalline compounds. The

highest photo response was obtained for the film deposited

at -0.80 V in the presence of 0.06 M Triethanolamine.

Films deposited at longer deposition time showed higher

photo response. Absorbance studies revealed that the band

J Mater Sci: Mater Electron

123

gap energy was about 1.20 eV with indirect transitions.

Jain et al. cathodically electrodeposited SnS thin films on

SnO2-coated conducting glass substrates, from an aqueous

solution containing SnCl4 and Na2S2O3 [111]. The films

had Herzbergite orthorhombic crystal structure. Flake/

needle-like crystal structures present in as-deposited sam-

ples were due to the anisotropic growth of various crystal

planes. Interestingly they could observe that annealing of

these needle-like structures resulted in the growth of pla-

telet-like structures. These films were polycrystalline with

an optical band gap of 1.4 eV. Pulse-form electro-deposi-

tion technique was used by Cheng et al. to deposit SnS

films on ITO coated glass substrates [112].

3.1.2.6 Multilayer-based solid-state reaction Tin mono-

sulfide films were synthesized by means of this route by

varying the layer thickness ratios of tin to sulfur; Sn-S films

with different chemical compositions were prepared. The

SnS film with approximate stoichiometric ratio was ob-

tained when the layer thickness ratio of tin to sulfur was set

to 1. The film resulted to be well crystallized with strong

(040) preferential orientation and having a direct band gap

of 1.45 eV. The resistivity and activation energy were

found to be 500 X cm and 0.06 eV, respectively, at room

temperature. In addition, the carrier concentration value

was 2.8 3 1016 cm-3 [119].

3.1.2.7 Spray pyrolysis The spray pyrolysis is a versatile

and economic technique that has been extensively utilized

to synthesize tin monosulfide thin films. In addition, this

route allows an easy way of doping compounds either by

deficiencies in stoichiometry, or by adding an external agent

to the solution with the precursor ion salts. In the year of

1994, Lopez et al. [36] published some results of the SnxSysynthesis by the SP technique. In that work, they reported

that at substrate temperatures of 370–390 �C, the SnS phase

was the predominant phase. Besides, they also determined

its direct band gap energy (1.27 eV) and the activation

energy (0.54 eV). Later, the group of N. K. Reddy et al.

[123] synthesized this compound in the temperature range

of 300–350 �C and calculated an indirect band gap energy

of 1.0 eV. The same group, a year later [124] grown thin

layers of tin monosulfide from equimolar solutions of tin

chloride and N,N-dimethyl thiourea into Corning 7059

glasses. The nozzle presented 2 automatized degrees of

freedom during the deposition (the x–y plane). Single-phase

polycrystalline thin films with a strong preferential orien-

tation along the (111) direction were obtained with a crys-

tallite size of 0.35 lm. Later, Thangaraju et al. [125]

synthesized n-type SnS films at a temperature of 350 �C.The films were highly resistive and amorphous. An indirect

band gap energy of 1 eV was also found for these films. The

films exhibited a photoconductivity character (10 times the

dark conductivity) as well. But, since then, no further re-

ports are available on the application level of these photo-

sensitive films deposited by spray technique. On the other

hand, Ramakrishna et al. [126] deposited SnS single-phase

polycrystalline thin films. All the grown films presented

good adherence to the substrate surface and were pin-hole

free. The layers showed a (111) preferential orientation with

a grain size of 0.35 lm. The films also showed a resistivity

of 30 X cm, a net carrier concentration of 1.2 9 1015 cm-3

and a mobility[180 cm2/Vs with a calculated bandgap of

1.32 eV. Later, these values were corroborated by other

reports. Reddy et al. [127] reported spray pyrolytic depo-

sition of SnxSy films on Sb doped SnO2 glass substrates in

2005. They deposited films at different substrate tem-

peratures and investigated their physical properties. It was

found that films formed in the temperature range of

300–375 �C were nearly stoichiometric with a SnS single

phase and an average grain size of 0.36 lm. The resistivity

of the films was 30 X cm, while the band gap and net carrier

concentration were 1.32 eV and 2 9 1015 cm-3, respec-

tively [127–130].

Moreover, Ramakrishna et al. [63] synthesized and

characterized SnS thin films obtaining similar results to the

previously reported. They also fabricated a heterojunction

solar cell using sprayed SnS as the absorber layer and

indium doped cadmium sulfide as the window layer. Three

years later, Devika’s investigation group [131] explored the

structural behavior of nanocrystalline tin monosulfide

structures with respect to temperature changes. Their

studies also emphasized the dependence of structural

properties of nanocrystalline SnS structures on the sur-

rounding temperature. One year later, Sajeesh et al. [37,

132] reported that the growth temperature sets phase for-

mation: the SnS phase is formed for the growth tem-

perature of 350\Ts\ 400 �C while SnS2 for

Ts[ 400 �C, and Sn2S3 for Ts\ 300 �C. Jeyaprakash

et al. [133] prepared and reported SnS thin films by a home

built microcontroller based spray pyrolysis unit. The X-ray

diffraction analysis confirmed the presence of nanocrys-

talline SnS phase formation with preferential orientation

along (111) plane. By these means, they noticed that a

better crystallinity takes place at higher temperatures. A

direct allowed band gap lying in the range of 1.30–1.40 eV

was noticed with temperature increment. On the other

hand, a study of the photoconductivity and thermoelectric

properties of SnxSy was carried out by Fadavieslam et al.

[134]. Thin films of tin sulfide (SnxSy) with atomic ratios of

y/x = 0.25–1.50 have been prepared on glass substrates at

a temperature of 420 �C. The initial materials for the

preparation of thin films were an alcoholic solution con-

sisting of tin chloride (SnCl4�5H2O) and thiourea

(CS(NH3)2). The SnxSy thin films showed a polycrystalline

structure with a nearly uniform surface and cluster-type

J Mater Sci: Mater Electron

123

growth. By increasing the atomic ratio of (y/x) in films, the

optical band gap, photosensitivity, thermal activation en-

ergy and the Seebeck coefficient changed from 2.72 to

2.37 eV, from 0.05 to 0.78, from 0.07 to 0.48 eV (in the

high temperature range) and from ?0.17 to -0.22 mVK-1

(@ T = 350 K), respectively. In addition, the SnS thin

films structure tends to a nearly single-crystal state in (001)

preferred orientation corresponding to SnS2 phase with

increasing (y/x) ratio. These structure conditions consid-

erably influence the photosensitivity and thermoelectric

properties of thin films.

A broader literature on spray pyrolysis deposition

technique of SnS thin films can be found in the references

[23, 35–38, 63, 125–139].

4 SnS-based thin film solar cell results

Despite the efforts made recent last years in improving

SnS-based solar cells, low efficiency values have been

obtained, with the best value of 4.36 % reached in 2014

[143].

This section is dedicated to a condensed historical re-

view of the works where SnS thin films have been used in

solar cells. Back on 1988, Sharon and Basavaswaran re-

ported a photo conversion efficiency of 0.6 % for a

Photoelectrochemical cell (PEC) with the structure:

n-SnS/Ce4?/Ce3?/Pt. SnS thin films were synthesized by

passing H2S through an solution of SnCl2 [53]. By the

year of 1994, Noguchi et al. [25] successfully deposited

SnS thin films by vacuum evaporation technique and

fabricated an ITO/n-CdS/p-SnS/Ag structure. It exhibited

a short circuit current density (Jsc) of 7 mA/cm2, an open

circuit voltage (VOC) of 0.12 V, a fill factor (FF) of 0.35

and a conversion efficiency (g) of 0.29 %. On the other

hand, a SnS/CdS photovoltaic study was done by Ra-

makrishna et al. [63] in the year 2006 where the SnS

films were synthesized by SP technique and exhibited an

efficiency and quantum efficiency (QE) of 1.3 and 70 %,

respectively. Ghosh et al. [80] fabricated a SnS/CdS

heterojunction by evaporating CdS and SnS films.

A CdCl2-post treatment applied to the window material

presented an increment of grain size. Efficiency values for

solar cells based on window layers with and without

treatment were 0.08 and 0.05 %, respectively, under 1

Sun of intensity. The fabricated cells showed rectification

characteristics. The investigation group of Avellaneda

et al. [144] chemically deposited thin films of SnS in two

different crystalline structures: Orthorhombic SnS (OR)

and Zinc-Blende SnS (ZB). These films showed p-type

conductivity and band gap energies of 1.2 and 1.7 eV,

respectively. A SnO2:F/CdS/SnS(ZB)/SnS(OR) photo-

voltaic structure with an evaporated Ag-electrode was

reported in this paper, exhibiting the following values:

VOC = 370 mV, JSC = 1.23 mA/cm2, FF = 0.44 and

g = 0.2 % under 1 kW/m2 (1 Sun) of illumination. The

methodology on the preparation as well as chemical,

structural and physical characterization results of the Mo/

p-SnS/n-CdS/ZnO heterojunctions were reported by

Bashkirov et al. [71] in 2012. The SnS thin films were

grown by HWVD method on the Mo-coated glass sub-

strates at 270–350 �C. The CdS buffer layers were de-

posited onto the SnS films by chemical bath deposition.

The ZnO window layers were deposited by a two-step rf-

magnetron sputtering method, resulting in a ZnO bilayer

structure: the first layer consists of undoped i-ZnO and the

second of Al-doped n-ZnO. The best junction exhibited

the following parameters: VOC = 132 mV, JSC = 3.6 mA/

cm2, FF = 0.29 and g = 0.5 %. One year later, an opti-

mization of the SnS conduction band offset (CBO) as the

light absorbing layer and Zn1-xMgxO as the buffer layer

in SnS thin film solar cells was carried out to improve the

solar cell conversion efficiency by Ikuno et al. [145]. The

CBO was experimentally controlled by varying the Mg

content of the buffer layer. The optimum CBO value

range for improved solar cell performance was deter-

mined to be from -0.1 to 0 eV. A SnS thin film solar cell

sample with the optimum CBO value exhibited a con-

version efficiency of approximately 2.1 %. Hegde et al.

[74] synthesized SnS polycrystalline thin film by thermal

evaporation (TE) technique in 2013. Electrical resistivity

of the deposited films was about 32.5 X cm with a direct

optical band gap of 1.33 eV. Carrier concentration and

mobility of charge carriers estimated from the Hall

measurements were found to be 6.24 9 1015 cm-3 and

30.7 cm2/Vs, respectively. This research group reported

the fabrication of heterojunction solar cells in superstrate

configuration using TE-SnS as an absorber layer and

CdS:In as window layer. The resistivity of pure CdS thin

film of 320 nm thickness was about 1–2 X cm and was

reduced to 40 9 10-3 X cm upon indium doping. The

solar cells with indium doped CdS window layer showed

improved performance as compared to pure CdS window

layer. The best device had a conversion efficiency of

0.4 % and a fill factor of 33.5 %. A record efficiency was

achieved for SnS-based thin film solar cells by varying

the oxygen-to-sulfur ratio in Zn(O,S) by Prasert et al.

[96]. Studies showed that increasing the sulfur content in

Zn(O,S) raises the conduction band offset between

Zn(O,S) and SnS to an optimum slightly positive value. A

record SnS/Zn(O,S) solar cell with a S/Zn ratio of 0.37

exhibits short circuit current density (Jsc), open circuit

voltage (Voc), and fill factor (FF) of 19.4 mA/cm2,

0.244 V, and 42.97 %, respectively, with an active-area

efficiency of 2.46 %. Recently, this efficiency value was

further improved by adding a SnO2 layer of few

J Mater Sci: Mater Electron

123

nanometers at SnS/Zn(O,S) interface, reaching a new

record efficiency of 4.36 % as reported [143]. A bifacial

SnS solar cell consisting of glass/FTO/SnS/CdS/ZnO/ITO

demonstrated front- and back-side power conversion ef-

ficiencies of 1.2 and 0.2 %, respectively [146]. Low val-

ues for the Jsc during back-side illumination were

exhibited; the diffusion length of minority carrier elec-

trons in the p-type SnS appears to be less than the

thickness of the device because the photogenerated mi-

nority carrier electrons at the back side do not get col-

lected adequately at the SnS/CdS interface to contribute

significantly to Jsc. Both the low back-side Jsc and the

virtual shunt indicate that the Jsc can be increased by

improving charge carrier collection. Two options for im-

proving charge carrier collection include: (1) improving

the minority carrier diffusion length in the SnS bulk by

increasing minority carrier mobility, minority carrier

lifetime or both and (2) using a thinner SnS layer to

reduce charge carrier collection distance necessary. For

the latter, light trapping techniques may be necessary to

maintain adequate light absorption. Nanostructuring the

p-n junction can also reduce the charge carrier collection

distance required. [133].

Some of the solar cell performance parameters of SnS-

based heterojunctions reported by various groups have

been presented in Table 4.

5 Important points to be considered in orderto improve SnS solar cell efficiency

Despite SnS absorber is a potential material for photo-

voltaic applications, solar cells efficiencies based on this

chalcogenide semiconductor are far below those reported

for other absorbers such as CIGS, CdTe, and kesterites. In

order to study if a further solar cell efficiency improvement

is possible, several difficulties that degrade solar cell per-

formance should be highlighted. In general, the following

limiting factors are considered important to improve solar

cell performance:

– Band alignment and density of defect states at SnS/

buffer interface

– SnS bulk defects and secondary phases formation

– Short minority carrier lifetime and diffusion length

– SnS-back contact and solar cell configuration

Table 4 Photovoltaic properties of tin monosulfide thin film solar cells

Cell VOC

(mV)

JSC(mA/cm2)

FF

(%)

g(%)

RS

(kX cm2)

RSH

(kX cm2)

n J0 9 10-7

(mA/cm2)

Year References

p-SnS/Fe3? 320 0.65 65 0.54 – 2.5 – – 2001 [147]

SnS/CdS 260 9.6 53 1.3 0.023 – – – 2006 [63]

SnS/CdS 274 0.31 40 0.08 0.505 2.14 1.83 – 2008 [80]

ZnO/SnS 120 0.04 33 0.003 5.57 1.64 1.28 1.0 2009 [148]

SnS(OR)/SnS(ZB)/CdS 370 1.23 44 0.2 0.5 18 – 5 2009 [149]

SnS/TiO2 471 0.3 71 0.1 0.27 0.0349 – – 2010 [150]

SnS/CdS 183 2.7 34 0.17 – – – – 2011 [151]

p-SnS/n-SnS 280 9.1 29.9 0.74 – – – – 2012 [152]

Mo/SnS(CVD)/CdS/ZnO 132 3.63 29 0.5 0.04 0.35 4 0.2 2012 [71]

Zn0.83Mg0.17O/SnS 270 12.1 64 2.1 – – – – 2013 [145]

CdS/SnS 208 17.9 38 1.6 0.005 0.0312 – – 2013 [153]

SnS/CdS:In 302 3.98 33.5 0.4 0.169 0.503 – – 2013 [74]

SnS/Zn(O,S) 220 16.8 47.7 1.8 – – – – 2013 [96]

244 19.42 42.97 2.04 – – – –

TCO/CdS(0.3 llm)/

SnS(1.8 llm)/C/Ag

270 6 44 0.7 – – – – 2014 [144]

Mo/SnS/Zn(O,S)/ZnO/ITO 261 24.9 44.4 2.9 – – – – 2014 [154]

Glass/FTO/SnS/CdS/ZnO/ITO

Front side 200 15 40 1.2 – – – – 2014 [146]

Back side 140 1.3 34 0.2 – – – –

Mo/SnS/SnO2/Zn(O,S)/ZnO/ITO 372 20 58 4.36 – – – – 2014 [143]

J Mater Sci: Mater Electron

123

5.1 Band alignment and density of defect states

at SnS/buffer interface

One of themost important issues in SnS solar cells is low open

circuit voltage. In fact, for the best solar cell efficiency re-

ported up to date (4.36 %), an open circuit voltage value of

372 mVusing Zn(O,S) as buffer layer is reported [143]. Band

alignment at SnS/buffer is commonly assumed to be the main

hurdle that produces low open circuit voltage values.

To find one adequate buffer material could be one of the

greatest challenges for SnS technology. In order to overcome

that problem, someworks have been focused onfinding a better

buffer candidate. For that purpose, band alignment studies have

been reported for CdS [155, 156], SnO2 [155], ZnMgO [157],

and Zn(O,S) [158]. Among them, Zn(O,S) showed a better

alignment with SnS. In particular, for S/Zn ratios lower than

0.5, small positive conduction band offsets (spike-like) as

shown in Fig. 1a were reached. From a theoretical point of

view, a small CBO in the range of 0 eV to 0.4 eV is the optimal

band alignment [159, 160]. Outside this range, a negative CBO

(cliff-like), as shown in Fig. 1b, suffers from increasing inter-

face recombination, while a large positive CBO forms a barrier

that reduces photocurrent collection. Despite Zn(O,S) align-

ment was better, no significant improvements in solar cell ef-

ficiency have been reached. Therefore, lowopen circuit voltage

could be related to other aspects such as defects at SnS/buffer

interface, bulk defects, etc. If there is a significant lattice mis-

match between the two component materials, a high interfacial

defect density at SnS/buffer interface could characterize SnS/

buffer junction reducing open circuit voltage value. So far,

there is no report about defect states study at SnS/buffer junc-

tion and its effect on solar cell performance. Such study could

help to clarify Voc losses.

5.2 SnS bulk defects and secondary phases

formation

The low performance could also be blamed on poor film

quality, i.e., polycrystalline films with the presence of

multiple secondary phases. Diode quality factors for SnS

devices are reported in the range of 1.83–1.92, [63],

indicating substantial recombination in the space charge

region (SCR). Short-circuit current collection in SnS de-

vices is on par with that achieved in kesterite devices [1],

which along with the high diode quality factor (*2) is

consistent with losses primarily due to recombination.

These losses are mainly related to interface defects and/or

bulk states which highly depend on deposition technique.

Therefore, an improvement in crystalline quality is re-

quired to improve solar cell output. Furthermore, further

studies are also needed in order to elucidate the influence

of both interface and bulk defects and which of them has

a major impact on the Voc value. On the other hand, at

least two secondary phases could be formed along with

SnS compound (Sn2S3, and SnS2). Despite secondary

phases have shown an important role in kesterite com-

pounds [1], the recent application of atomic layer and

chemical vapor deposition (ALD/CVD) techniques has

succeeded in producing high-quality phase pure films [65,

161] without a significant increase in efficiency. There-

fore, secondary phases could have a low impact on solar

cell performance. As a result, future progress in SnS solar

cells should be rely on understanding the defects that

lower Voc and the nature of defects near the SnS/buffer

heterojunction.

5.3 Short minority carrier lifetime and diffusion

length

SnS compounds are characterized by low minority carrier

diffusion length values (0.18–0.23 lm) [162]. This implies

low lifetime and mobility values. Low minority carrier

diffusion length values determine low values of Voc. Im-

proving the morphological properties and the crystalline

quality of the films involved in the solar cells will guar-

antee an improvement in the charge carrier transport

parameters.

Fig. 1 Band alignment for SnS/

buffer heterojunction: a spike-

like configuration and b cliff-

like configuration

J Mater Sci: Mater Electron

123

5.4 SnS back contact and solar cell configuration

The majority of SnS solar cells produced to date have been

based on adopting the architecture developed for CIGS,

where the absorber (SnS) is deposited on Mo-coated glass

as displayed in Fig. 2a. However, this metal could de-

compose SnS compound to form secondary phases in the

back contact such as MoS2 which can behaves as a

blocking back contact. A similar problem is presented in

kesterite compound which solar cell configuration is

mostly based on Mo as back contact [1]. This could explain

the complex temperature dependence observed for SnS

Ohmic contacts [163]. Therefore, the option of using other

types of metals is an open problem.

On the other hand, most solar cells reported have been

fabricated with the structure of substrate or CdS/CIGS

(Fig. 2a). The option of fabricating devices in a superstrate

structure, i.e. the deposition of buffer onto TCO, followed

by the deposition of the absorber film as shown in Fig. 2b,

could favor the properties of the solar cells, especially

when deposition methods of absorber imply high roughness

of the films such as spray pyrolysis. In this way, the

roughness should have less impact in the properties of the

solar cells.

It is also important to mention that a Metal–Insulator-

Semiconductor (MIS) behavior has been observed in CdTe

and kesterite Cu2ZnSnSe4 compounds and results have

been published [164–166]. This performance is due to the

buffer layer can be considered as an insulator between the

TCO that behaves like a metal and the p-type semicon-

ductor. As a result of this character, an improvement in Voc

values has been observed when CdS is doped with Cu,

which implies an increase in buffer layer resistivity value.

In other words, better solar cell efficiencies have been

obtained for thinner and resistive buffer layers. If SnS solar

cells behave as a MIS structure, a Voc enhancement should

be reached by increasing buffer resistivity, but further

studies are needed to demonstrate that performance. This

could help to improve one of the most important limiting

parameters in SnS-based technology.

In summary, in addition to high-quality material, SnS

cells must have optimized window and back-contact in-

terfaces to minimize recombination, resistive losses, and

parasitic absorption. While the present Zn(O,S) buffer may

be adequate to address some of the most urgent absorber

development needs, moving to higher band-gaps and im-

proved light transmission will require buffer development

similar to that being carried out for CIGS devices. In ad-

dition to determining and optimizing the interfacial band

offset, understanding band bending, interfacial defect

concentration, inter-diffusion and stability among other

properties is necessary to minimize recombination.

6 Conclusions

In this work, we present a review about the state of the art of

SnS films and devices along with the analysis about different

factors that are limiting solar cell performance. In summary,

there are several issues that must be understood in order to

overcome present limits in Voc values. In order to reach a

further improvement in SnS solar cell efficiency, it is re-

quired to obtain high-quality SnS material. Besides, some

other important aspects such as recombination at SnS/buffer

interface and bulk materials must be improved. On the other

hand, to find an adequate back-contact for replacing Mo,

could improve solar cell performance. Furthermore, in ad-

dition to determining and optimizing the interfacial band

offset, understanding band bending, interfacial defect con-

centration, inter-diffusion and stability among other prop-

erties is necessary to minimize recombination.

Acknowledgments This work was partially supported by CeMIE-

Sol-207450/P26. J.A. Andrade-Arvizu thanks Raul Andrade & Car-

men Arvizu for everything. J.A. Andrade-Arvizu and M. Courel thank

Conacyt and BEIFI fellowship supports. O. Vigil-Galan acknowl-

edges support from COFAA and EDI of IPN.

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