Lead Selenide Thin Film

ECS Journal of Solid State Science and Technology, 2 (4) Q69-Q73 (2013) 2162-8769/2013/2(4)/Q69/5/$31.00 © The Electrochemical Society

Q69

Antimony Chalcogenide/Lead Selenide Thin Film Solar Cell with 2.5% Conversion Efficiency Prepared by Chemical Deposition M. Calixto-Rodriguez, Harumi Moreno Garc´ıa, M. T. S. Nair,∗,z and P. K. Nair Centro de Investigación en Energ´ıa, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, México Antimony sulfide-selenide solid solutions offer optical band gaps, Eg , in the 1–1.88 eV interval; and lead selenide offers Eg upward of its bulk value, 0.28 eV, depending on the extent of quantum confinement. In this work, thin film solar cells of SnO2 :F/CdS/Sb2 (Se/S)3 /PbSe/C-Ag are developed by chemical deposition of the thin films on transparent conductive oxide (TCO) glass. To prepare the solar cell, first a CdS thin film of 100 nm in thickness is deposited on the TCO from a solution containing Cd(II)-citrate complex. On this, a thin film is deposited from a solution containing potassium-antimony tartrate, thioacetamide and selenosulfate, which upon heating at 280◦ C in nitrogen ambient results in a Sb2 S1.2 Se1.8 film of 150 nm in thickness with an Eg of 1.67 eV. PbSe thin film 110 nm in thickness is deposited on it from a solution of Pb-citrate complex and selenosulfate, with crystalline grain diameter 10 nm, and Eg of 1.86 eV. The cell shows open circuit voltage (Voc ), 454 mV, short circuit current density (Jsc ) 12.5 mA/cm2 , fill factor (FF) 0.44 and conversion efficiency (η) 2.5%. The observed cell parameters are backed by a tentative energy level diagram and an estimate for the light generated current density. © 2013 The Electrochemical Society. [DOI: 10.1149/2.027304jss] All rights reserved. Manuscript submitted January 15, 2013; revised manuscript received February 5, 2013. Published February 21, 2013.

Theoretical analyzes relating optical bandgap (Eg ) to solar energy conversion efficiency shows that semiconductor materials with Eg in the 1.2–1.4 eV interval offers prospects as efficient photovoltaic absorbers.1,2 Antimony sulfide (Sb2 S3 ) and antimony selenide (Sb2 Se3 ) with direct-Eg for bulk materials at 1.88 and 1 eV, respectively3 produce solid solutions of Sb2 Sx Se3–x with Eg within this interval.4 Thin film solar cells of CdS/Sb2 S3 /PbS,5 and CdS/Sb2 Se3 /PbS,6 as well as CdS/Sb2 S3 /PbSe,7 with conversion efficiencies of up to 1% have been previously reported by us. In hybrid hetero-junction solar cells using antimony sulfide as the absorber, conversion efficiency of up to 3.9% are reported.8 Here we present thin film solar cells using antimony chalcogenide (Sb2 Sx Se3–x ) and PbSe obtained by sequential chemical deposition with conversion efficiency of up to 2.5% - the highest so far reported for this type of solar cell. Experimental Thin film deposition.— For optical and electrical characterization, thin films of CdS, Sb2 Sx Se3–x , and PbSe are deposited on Corning microscope glass slides of 7.5 cm x 2.5 cm, 1 mm in thickness. Solar cell structures of CdS/Sb2 Sx Se3–x and CdS/Sb2 Sx Se3–x /PbSe are developed on TEC-15 substrate (Pilkington, Toledo, USA) of a transparent conductive oxide (TCO) coating of SnO2 :F on glass, with nominal sheet resistance of 15 . The CdS thin films used as n-layer in the cell is 100 nm in thickness with Eg 2.6 eV, deposited for 1 h at 80◦ C from a chemical bath containing cadmium-citrate complex, reported first in 1994.9 For the deposition of Sb2 Sx Se3–x and PbSe thin films, a sodium selenosulfate solution was first prepared by refluxing 4 g of Se powder (ASARCO, 99.9%), 12 g of sodium sulfite (Na2 SO3 -Baker Analyzed), and 100 mL of distilled water at a temperature near 100◦ C for 5 h. The clear solution is of approximately 0.4 M in selenosulfate. Sb2 Sx Se3–x films.— 16.6 mL of 0.1 M potassium antimony tartrate (K2 Sb2 C8 H4 O12 · 3H2 O); 6.6 mL of 3.7 M triethanolamine (TEA), 2 mL of ∼ 15 M ammonia (aq.), 20 mL of 10−5 M silicotungstic acid (STA, H4 Si(W3 O10 )4 ), 6.6 mL of 1 M thioacetamide (TA), 2 mL of 0.4 M selenosulfate solution, and distilled water to take the mixture to 160 mL in volume in a 250 mL beaker. Substrates are vertically mounted in this bath mixture. The basic idea for this formulation comes from the work reported for Sb2 S3 thin films10,11 and later on used for solar cells.12 This formulation works well to deposit antimony chalcogenide thin films at 80◦ C in for 3 h on a CdS substrate layer; it is not found suitable for depositing a film directly on a bare glass substrate. The starting solution is clear with a pH 10.3. After ∗ z

Electrochemical Society Active Member. E-mail: [email protected]

the deposition, the filtered clear solution has a pH 9.6. When the film is heated at 280◦ C for 30 min, an amorphous-to-crystalline transformation produces an Sb2 Sx Se3–x film of 150 nm in thickness. A TCO/CdS/Sb2 Sx Se3–x solar cell is completed by applying colloidal graphite paint (SPI 05006-AB) over 5 mm x 5 mm area (0.25 cm2 in cell area) on the antimony chalcogenide film before heating. A TCO/Sb2 Sx Se3–x /PbSe solar cell has in addition a PbSe film deposited on the heated antimony chalcogenide thin film, as described below.

PbSe thin film.— For depositing this film, a chemical bath is prepared by mixing solutions of 1 mL of 1 M Pb(NO3 )2 , 7 mL of 0.4 M sodium citrate, 2 mL of 4 M NH4 OH, 1 mL of the selenosulfate, and distilled water to complete a volume of 80 mL. The film is deposited on Corning glass slides with a ZnS film (50 nm in thickness) as substrate layer13 or on TCO/CdS/Sb2 Sx Se3–x stack already heated at 280◦ C. A PbSe thin film of 110 nm in thickness is deposited in 90 min, when the bath is maintained at 40◦ C. To complete a TCO/CdS/Sb2 Sx Se3–x /PbSe cell structure, colloidal graphite paint of 0.25 cm2 in area is applied on the PbSe film and heated at 60◦ C for 30 min. On TCO/Sb2 Sx Se3–x /Cas well as TCO/CdS/Sb2 Sx Se3–x /PbSe/C- cells, colloidal silver paint (DuPont PV428) is printed on the dried graphite electrodes and allowed to dry at 60◦ C for 1 h. These cell structures are found to be stable during repeated measurement under sunlight for over eight months so far.

Characterization.— Film thickness is measured on an Ambios Technology XP-200 thickness measurement unit. Grazing Incidence X-ray Diffraction (GIXRD) patterns of a TCO/CdS/Sb2 Sx Se3–x /PbSe cell structure are recorded with the X-ray beam making an angle () of 0.2, 0.4, 0.6, and 1◦ with the plane of the sample. A Rigaku D-Max 2000 X-ray diffractometer using Cu-Kα radiation (λ = 1.5406 Å) is used for this measurement. The optical transmittance at normal incidence (T) and specular reflectance spectra (R) of the films are measured with a JASCO-670 spectrophotometer in the UV-VIS-NIR region, at 250–2500 nm wavelength range. To record the dark- and photoconductivity of the films, a bias of 10 V is applied across pairs of silver print electrodes of 5 mm in length at 5 mm separation. The current is recorded in the dark for 20 s and then under illumination (800 W/m2 of intensity from a tungsten-halogen lamp) for another 20 s followed by a 20 s dark decay. The current-voltage (I-V) characteristics of the cells are recorded in the laboratory using an intensity of illumination 1000 W/m2 from a tungsten-halogen lamp. The Voc and Jsc of the cells were then normalized using direct measurements on the cell outdoor under 1000 ± 50 W/m2 intensity of solar radiation of nearly1.5 air-mass during mid-day.

Downloaded on 2015-11-03 to IP 163.22.77.240 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Q70

ECS Journal of Solid State Science and Technology, 2 (4) Q69-Q73 (2013)

(a)

(200)

900

GIXRD: Ω = 0.2 − 1°

(220)

(111)

Intensity (cps)

(

600

PbSe 110 nm Sb2SxSe3-x150 nm CdS 100 nm SnO2:F 500 nm

0.4°

300

0.2° PbSe 06-0354

0 20

40

60

° CdS 41-1049 Sb2S3 42-1393 (201)

(200)

° *

* SnO2 46-1088

*

*

1000 1°

750 Sb2S3 (221)

Intensity (cps)

1250

d = 0.2868 nm d = 0.2765 nm

(b) 1500

(002) (110)

1750

d = 0.5250 nm

2θ (degree)

500

0.6°

(221)

(120)

250

Sb2Se3 06-0354

0 10

20

30

40

50

60

70

2θ (degree) Figure 1. (a) GIXRD patterns for the cell structure SnO2 :F/CdS/ Sb2 Sx Se3–x /PbSe shown in the inset recorded at = 0.2 and 0.4◦ ; (b) those recorded at = 0.6 and 1◦ .

Results and Discussion Structural characterization.— The GIXRD patterns on the cell structure (see the inset) SnO2 :F/CdS/Sb2 Sx Se3–x /PbSe are given in Fig. 1a to analyze the PbSe film and in Fig. 1b to analyze the solid solution, Sb2 Sx Se3–x . At = 0.2 and 0.4◦ , the patterns in Fig. 1a are dominated by the top-lying PbSe thin film (110 nm). They match the powder diffraction file (PDF) 06–0354 for the mineral clausthalite (PbSe from a mine in Clausthal, Germany). The sampling depths (SD) for the films for Cu-Kα radiation are estimated from the linear absorption coefficient for the X-ray for the constituent elements14 and the mass density of the substance,15 as described in previous reports.7,16 For PbSe the SD at = 0.2◦ is 23 nm and is 45 nm at = 0.4◦ . The peak positions of the sample maintain the same as that in the standard powder pattern, but a tendency for preferential growth with texture coefficient (TC >1) sets-in as the growth proceeds. This leads to crystalline planes in the grains arranged perpendicular to [111]: TC is 1.7 for the top 23 nm of the film, and 1.5 for the top 45 nm film and toward the Sb2 Sx Se3–x /PbSe interface (Fig. 1b). Average grain diameter of the crystallites estimated using the Scherrer formula for the top 23 nm is 10 nm and about 9 nm into the depth of the PbSe film. Peaks seen in the 2 θ region of 30–40◦ in the pattern recorded at = 0.4◦ correspond to underlying materials. The diffraction patterns recorded at = 0.6◦ (PbSe-SD: 70 nm) and = 1◦ (PbSe-SD: 115 nm) shown in Fig. 1b are also dominated by PbSe diffraction peaks. Since the PbSe film thickness (110) is approximately covered by the SD in both cases, the PbSe peak heights for (111), (200) and (220) planes are not altered in intensity. However,

increase in intensity is notable for the (110), (200) and (201) planes of SnO2 for the pattern recorded at = 1◦ compared with that recorded at = 0.6◦ . The prominent (002) peak of CdS-hex9 overlaps with the (110) peak of SnO2 , and hence cannot be distinguished. The position of the antimony chalcogenide (221) peak located in between that of Sb2 Se3 to the left and Sb2 S3 to the right, indicated in Fig. 1b, helps to estimate the chemical composition of this material. To estimate the value of x in the Sb2 Sx Se3–x solid solution, we assume a linear variation of inter-planar spacing d(221) of 0.2868 nm for Sb2 Se3 (PDF:06-0354) and 0.2765 nm for Sb2 S3 (PDF:42-1393) in the GIXRD patterns of the cell structure at = 0.6◦ and 1◦ . The GIXRD peak for the solid solution is located at 2θ = 31.86◦ , corresponding to d(221) of 0.2807 nm, which is seen almost independent of the sampling depth. This leads to the designation of the solid solution in the cell structure as Sb2 S1.2 Se1.8 . The presence of the PbSe and SnO2 peaks in the patters at the exact locations with reference to the respective PDF’s supports that there is no peak shift caused by any sample height mis-alignment in the measurement. Hence, the chemical composition estimated for the solid solution is reliable. We have observed that the content of selenosulfate in the deposition bath for Sb2 Sx Se3–x can shift the value of x in the solid solution. The present method of preparing the Sb2 Sx Se3–x solid solution is distinct from that reported by us earlier4 - in which a chemically deposited Sb2 S3 thin film was heated in contact with a chemically deposited selenium thin film. The crystallite grain diameter for the Sb2 S1.2 Se1.8 thin film in the cell structure is estimated from the non-overlapping peak of the solid solution located to the right of (120) peak of standard Sb2 Se3 indicated in Fig. 1b; it is about 16 nm. The GIXRD patterns in Fig. 1a and 1b also confirms that the cell structure is composed of stratified layers of CdS and Sb2 Sx Se3–x underneath the PbSe thin film. Optical and electrical properties of the Sb2 S1.2 Se1.8 and PbSe thin films.— Figure 2 shows the T, R, and T+R spectra of: (a) PbSe (110 nm) and (b) Sb2 S1.2 Se1.8 (150 nm) thin films. At wavelength (λ > 1000 nm), T+R remain fairly constant at approximately 95%, suggesting that the band-to-band optical absorption has not set in, and any loss due to diffuse reflectance on a powdery surface is minimal (up to 5%). At a wavelength below 750 nm, optical absorption across a bandgap occurs. The optical absorption coefficients (α) for photon energy (hν) of the materials are calculated by taking into account multiple reflections within the film of thickness d: 17 ⎧ 1/2 ⎫ 2 ⎨ ⎬ (1 − R)2 (1 − R)2 1 2 ln α= +R + [1] ⎩ 2T ⎭ d 2T These values are plotted in Fig. 2c, which indicate Eg for the materials (for α → 0): 1.84 eV for PbSe and 1.69 eV for Sb2 S1.2 Se1.8 . The inset (d) shows parts of (αhν)2/3 vs. hν plots at which optical absorption sets-in. This analysis is restricted to photon energy below 2.5 eV for Sb2 S1.2 Se1.8 to avoid the optical absorption in the CdS substrate layer and below 3.0 eV for PbSe film to avoid that in the ZnS substrate layer. The straight line plots confirm that the optical transitions are of the forbidden type across a direct bandgap18 of 1.86 eV for PbSe and 1.67 eV for Sb2 S1.2 Se1.8 thin film. These values of Eg will be used in further discussion. Discussion on the Eg .— Quantum confinement of excitons and consequent increase in Eg of PbSe19 and PbS20 nanocrystals (of a few nanometers in diameter) are well studied. With a large Bohr radius near 45 nm for the exciton ground state, drastic increase in Eg occurs in these materials when crystallite diameter descend toward 10 nm and less. In photovoltaic structures using PbSe quantum dots (QD’s), solar energy conversion efficiencies of up to 3.4% have been reported when the QD diameters are below 2 nm.21,22 Consideration of PbSe with a direct Eg of 0.278 eV at room temperature at the L-point of the fcc-Brillouin zone23 as a solar cell absorber owes to increase in Eg from 0.4 eV to 1.4 eV and consequent increase in Voc as the QD diameter drops from 10 nm toward 2 nm.19 At QD diameters above


ECS Journal of Solid State Science and Technology, 2 (4) Q69-Q73 (2013)

100

Q71

-5

10

(a) T+R

PbSe (110 nm)

PbSe (110 nm)

80 -6

-1

60

σ (Ω cm)

T, R (%)

10

T R

40

Sb2SxSe3-x (150 nm) -7

10

20 -8

10

0 500

1000

1500

2000

2500

λ (nm)

100

20

40

60

80

100

120

time (sec.)

(b) T+R

0

Sb2SxSe3-x (150 nm)

Figure 3. Photoconductivity response plots for PbSe (110 nm) and Sb2 Sx Se3–x (150 nm) thin films under an intensity of illumination, 800 W/m2 from a tungsten-halogen lamp.

T, R (%)

80 60

R

40

T

For Sb2 S1.2 Se1.8 thin film, a linear variation of Eg in the 1.0 eV (Sb2 Se3 ) – 1.88 eV (Sb2 S3 ) range3 would mean an Eg of 1.31 eV for the solid solution, in case quantum confinement does not exist at crystalline grain diameter 16 nm (result from GIXRD). The high static relative permittivity of > 100 for antimony chalcogenides,3 however, brings-in a large Bohr radius for the exciton ground state for the solid solution, just as in lead chalcogenides. This could explain why the Eg of 1.64 eV determined from Fig. 2c for Sb2 S1.2 Se1.8 is more than that is expected for Sb2 S1.2 Se1.8 based only on the chemical composition.

20 0 500

1000

1500

2000

2500

λ (nm) 6

10

(c)

Sb2SxSe3-x

5

PbSe

4

2/3

10

-1

(αhν) ,10 (cm eV)

3

3

10

1

1.84 eV

2

10

1.69 eV

2/3

α (cm)

-1

10

6 5 4

(d) Sb2SxSe3-x

3 2 1

1.67 eV 0 1.0 1.5

10

1.0

1.5

PbSe

2.0 2.5 hν (eV)

1.86 eV 2.0 2.5 hν (eV)

3.0

3.0

3.5

Figure 2. (a) Optical transmittance T and reflectance R versus wavelength plots for PbSe film and (b) Sb2 S1.2 Se1.8 thin film; (c) optical absorption coefficient versus photon energy plots for the films; inset (d): bandgap analysis for the films.

4 nm, the prospects for such application is reported to disappear. In the case of PbS, a compilation of data on the QD’s suggests that in order to bring Eg above 1.0 eV, the diameter has to be below 5 nm.20 Thus, it is intriguing that in the present case an Eg of 1.86 eV is estimated from the optical T and R data for a PbSe thin film, 110 nm in thickness (Fig. 2) for which the XRD data suggest an average crystalline diameter of 9-10 nm (Fig. 1). However, such a high Eg may be accepted as possible if one considers that unlike in the case of QD’s in which the experimental methodology for their preparation provides the QD diameter distribution strongly peaked near the stated value, no such control exists in the chemical bath deposition for PbSe reported here.

Electrical conductivity.— In the photoconductivity response plots given in Fig. 3 for both the absorber materials, the electrical conductivity under illumination is 1.9 × 10−7 ( cm)−1 for Sb2 S1.2 Se1.8 film and 2.4 × 10−6 ( cm)−1 for the PbSe film. Thermoelectric measurement on the PbSe film suggests that it is a p-type material. Due to the high electrical resistivity of Sb2 S1.2 Se1.8 , its conductivity type could not be ascertained. We assume that it is of n-type as reported for Sb2 S3 films deposited from chemical bath containing silicotungstic acid and used in solar cells.12 The majority carrier concentration (nn for n-type and pp for p-type) in the thin films under the illumination may be estimated by assuming carrier mobility of 10 cm2 V−1 s−1 – a usual value for polycrystalline semiconductor materials with crystallite diameters < 20 nm. Such estimates give nn , 1011 cm−3 for Sb2 S1.2 Se1.8 and pp , 1012 cm−3 for PbSe. The intrinsic carrier concentrations (ni ) are estimated from the optical band gaps from Fig. 2d using standard procedure,24 and are 104 cm−3 for Sb2 S1.2 Se1.8 and 3 × 102 cm−3 for PbSe. The approximate positions of the quasi-Fermi levels can thus be determined as 1.25 eV above the valence band for Sb2 S1.2 Se1.8 and 0.36 eV above the valence band for PbSe thin films. Overall, the characteristics of the PbSe thin film seen here are markedly different from that reported by us earlier for those used in TCO/CdS/Sb2 S3 /PbSe solar cells which showed Eg of 0.69–0.85 eV and electrical conductivity 0.5 ( cm)−1 . In that work the chemical formulations used for PbSe thin film deposition were distinct. Energy level diagram.— A tentative energy level diagram for the present case may be set up following the approach described previously7,25 from the electron affinity26 and ionization energy27 of the constituent elements of the semiconductor compounds. The electronegativity (EN) of the element is the arithmetic mean of its electron affinity and the ionization energy, and the EN of the compound is the geometric mean of the EN of the constituent elements, considered to coincide with the intrinsic Fermi level of the substance. Thus, an estimate for the electron affinity (χ) of the substance with respect to vacuum level is, EN – Eg /2, which is, −5.52 eV for Sb2 S1.2 Se1.8 and −5.00 eV for PbSe. A value of χ, − 4.5 eV for SnO2 is assumed,25 which also represents its work function, for being a TCO. Electron


Q72

ECS Journal of Solid State Science and Technology, 2 (4) Q69-Q73 (2013) Vacuum level

20

0 Sb2SxSe3-x/

-1

Energy (eV)

χ = 4.07 eV

JL (mA/cm2)

χ = 4.68 eV

-2

PbSe(110 nm)

15 χ = 4.5 eV

EFn= 5.1 eV

ϕ = 4.5 eV

EFp= 5.57 eV

EFn= 4.7 eV

-3

ϕ = 5 eV

14.8

11.5

10 7.2

5

-4

Sb2SxSe3-x

-5 SnO2:F -6

Eg= 1.86 eV

0

C

Eg= 1.67 eV

Eg= 2.48 eV

0

///////

200

300

400

Figure 5. Upper limit of the light generated current density (JL ) as a function of film thickness expected in solar cells using Sb2 Sx Se3–x and Sb2 Sx Se3–x /PbSe(110 nm) absorbers.

CdS

Figure 4. Tentative energy level diagram for the TCO/CdS/Sb2 Sx Se3–x / PbSe/C layers estimated from the results presented in Figs. (1)–(3).

affinity of −4.5 eV is a typical value considered for CdS.28 Its Eg is 2.48 eV because the thin film is subjected to heating (along with an overlying Sb2 Sx Se3–x thin film) at 280◦ C. The Eg of as-prepared CdS film (100 nm) by the citrate-complex method is 2.6 eV.9 The positions of the conduction band, valence band and the Fermi levels (dashed lines) with respect to the vacuum level estimated this way for the materials constituting the heterojunction are given in Fig. 4. The difference in the Fermi levels between the CdS window (−4.7 eV) and Sb2 S1.2 Se1.8 (−5.1 eV) suggests a built-in voltage (Vbi ) of nearly 400 mV for this junction. This value may be representative of the upper limit for the Voc for the TCO/CdS/Sb2 Sx Se3–x /C-Ag cell under the illumination level considered. With a PbSe absorber of p-type conductivity, a higher Voc could be expected from the energy level scheme due to additional Vbi of 470 mV across the Sb2 S1.2 Se1.8 /PbSe junction. A carbon electrode with Fermi level at −5 eV (work function, 5 eV), is not an appropriate contact for PbSe with its Fermi level at −5.57 eV. However, with the PbSe film placed behind Sb2 S1.2 Se1.8 , it is at a lower illumination level and hence the quasi−Fermi level will be closer to mid gap (−5.00 eV). The tentative energy level diagram proposed in Fig. 4 is a working model and requires refining, as further results come in on the conductivity type and chemical composition of the solid solution.

Sb2 S1.2 Se1.8 will absorb Nph (hν) of 1.60 × 1021 m−2 s−1 and produce JL of 25.7 mA/cm2 and PbSe will absorb Nph (hν) of 1.16 × 1021 m−2 s−1 and provide JL of 18.6 mA/cm2 . Values of JL for the ITO/CdS/Sb2 S1.2 Se1.8 and TCO/CdS/Sb2 S1.2 Se1.8 /PbSe(110 nm) cells as a function of Sb2 S1.2 Se1.8 film thickness are given in Fig. 5. We do not consider here reflection or recombination losses, multiple exciton generation, gain from any back reflection or absorption loss in glass, TCO or CdS. For an absorber thickness of 150 nm, ITO/CdS/Sb2 S1.2 Se1.8 will have a JL (or upper limit of Jsc ) of 11.5 mA/cm2 and that for the TCO/CdS/Sb2 S1.2 Se1.8 /PbSe(110 nm), it is 14.8 mA/cm2 . For a single absorber thickness of 110 nm, PbSe will give a JL of 7.2 mA/cm2 , but it adds only 3.3 mA/cm2 to the 11.5 mA/cm2 available from Sb2 S1.2 Se1.8 when placed behind the Sb2 S1.2 Se1.8 film. Solar cell structures.— Figure 6 shows the current vs. voltage (J-V) characteristics for the structures: (a) TCO/CdS/Sb2 Sx Se3–x /CAg showing an open-circuit voltage (Voc ) of 438 mV, short-circuit current density (Jsc ) of 8.68 mA/cm2 , fill factor (FF) of 0.38, and conversion efficiency (η) of 1.45%; and (b) TCO/CdS/Sb2 Sx Se3–x /PbSe/CAg, with Voc of 454 mV, Jsc of 12.52 mA/cm2 , FF of 0.45, and η of 2.56%. The values are normalized to represent AM 1.5 G (1000 W/m2 )

Voc

1.67eV

+ 0.1q

∞

η

FF )

454

4

12.52

% 0.45

2.56

2 0

-200

-100

0

100

-2

200

300

400

500

V (mV)

-4

(a)

-6 -8

-14

Ag

(b) C

Ag

PbSe

(a)

C Sb2S1.2Se1.8 CdS

CdS

(b)

SnO2:F

SnO2:F

-10 -12

N Ph (hν)e−α1 d1 [(1 − e−α2 d2 )]d E

1.86eV

Jsc 2

J (mA/cm2)

Light generated current density of solar cell absorbers.— The light generated current density (JL ) in optical absorber material represents the upper limit of Jsc when it forms the absorber element in a solar cell structure when illuminated with the same radiation.29 Value for JL may be estimated for AM 1.5 (1000 W/m2 ) photon flux density30 Nph (hν) for a single absorber of Eg1 or for two absorbers, of Eg1 followed at rear by another of Eg2 with film thickness d1 and d2 and absorption coefficients α1 and α2 , respectively as illustrated previously for Bi2 S3 /PbS31 and Bi2 S3 /Si32 heterojunction solar cells. In the TCO/CdS/Sb2 S1.2 Se1.8 /PbSe cell, absorber−1 is Sb2 S1.2 Se1.8 with Eg1 1.67 eV and absorber-2 is PbSe with Eg2 1.86 eV and its JL is: ∞ N Ph (hν)(1 − e−α1 d1 )d E JL (m A/cm 2 ) = 0.1q

500

Sb2SxSe3-x thickness (nm)

PbSe Sb2S1.2Se1.8

-7

100

Sunlight

Sb2S1.2Se1.8

Sunlight

[2] Here q is the electron charge, 1.602 × 10−19 C; Nph (hν) is of the order of 1021 m−2 s−1 (eV)−1 ; and dE is the differential photon energy hν in eV. For a single absorber with thickness d → ∞,

Figure 6. Current density versus applied bias normalized for AM 1.5 G (1000 W/m2 ) solar radiation for the cell type (a) and (b), illustrating the contribution of the PbSe thin film toward the cell performance (for layer thicknesses, refer to inset in Fig. 1a).


ECS Journal of Solid State Science and Technology, 2 (4) Q69-Q73 (2013) solar radiation. The increase in Voc when the PbSe absorber is added is very little (16 mV). However, Jsc is enhanced by nearly 50% establishing that PbSe indeed participate in the photo-carrier generation. The observed Jsc of 8.68 mA/cm2 is comparable with the estimate for JL , 11.5 mA/cm2 for the Sb2 Sx Se3–x (150 nm) absorber; and Jsc of 12.52 mA/cm2 is comparable with the estimate of 14.8 mA/cm2 for Sb2 Sx Se3–x (150 nm) + PbSe (110 nm) absorbers. Thus, Voc as well as Jsc are in reasonable proximity with the values expected from the tentative energy level diagrams in Fig. 4 and JL plots of Fig. 5. While the conversion efficiency of 2.56% reported for TCO/CdS/Sb2 S1.2 Se1.8 /PbSe/C cell is larger than what has been reported for TCO/CdS/Sb2 S3 /PbSe/C-Ag cells (≤1%), the Voc in the present case is significantly smaller than 690 mV reported for such cells.7 We may note from Fig. 4 that it is important to identify and apply a more adequate p-side contact with a work function of 5.57 eV or higher to avoid collection loss at the p-side. The series resistance of the TCO/CdS/Sb2 S1.2 Se1.8 /PbSe/C-Ag cell is high: > 10 cm2 and the parallel resistance is low, 400 cm2 , which keeps the fill factor, FF < 0.5 and hence the conversion efficiency below 3%. For higher efficiency cells, the series resistance is an order of magnitude lower and FF > 0.6. The composition of Sb2 Sx Se3–x may have to be reduced in x and appropriate heating/re-crystallization process should be established to increase the Voc . The thickness of CdS is not yet optimized for this type of cell. Further comments on Sb2 Sx Se3–x /PbSe solar cell absorbers.— We want to highlight here that other techniques exist for the preparation of Sb2 Sx Se3–x solid solutions. Pulsed vapor-liquid-solid growth of Sb2 S3 and Sb2 Se3 or solid solutions of the materials are reported using tris(dimethylantimony) – Sb(NMe2 )3 and diethyldiselenide (Et2 Se2 ) or H2 S in an adapted atomic layer deposition set-up.33 Synthesis of sulfurized Sb2 Se3 with composition Sb2 Se3-x Sx (0 ≤ x ≤ 2) in a microwave-stimulated solvothermal process uses thioglycolic acid (TGA)-ligated antimony salts and tri-n-octylphosphine (TOP)ligated selenium, with the sulfur content being varied by adjusting the TGA/TOP-Se2− molar ratio.34 Solid solutions of Sb2 Sx Se3–x may be produced as well by direct evaporation of Sb2 S3 -Se powder mixture.35 The resultant materials through these routes carry distinct crystalline, compositional and morphological features. In large area applications, safety and toxicity issues of antimony might become relevant. A study on antimony sulfide reported in 2002 describes the materials as relatively safe, but underlines various aspects to be considered among possible occupational hazards in handling such materials.36 Regarding safety issues of chemically deposited lead chalcogenide (PbS) films, a conclusion has been drawn that chemical deposition is a relatively safe method for the thin films and that the chalcogenides as products are relatively safer than volatile organic substances of lead.37 Conclusions For the first time, antimony chalcogenide/lead chalcogenide thin film solar cell has reached conversion efficiency above 2.5%, measured under solar radiation normalized to AM 1.5 G (1000 W/m2 ). This cell without any encapsulation had remained stable over a period of Feb. – Nov. 2012, in repeated measurements; thus the cell structure is accepted as stable. Reproducibility of the performance of the cell in repeated trials is good. Antimony chalcogenide absorber thickness presented is 150 nm, close to what is typically used in hybrid solar cells substituting the Ru-dye,8 for which conversion efficiency of up to 3.9% has been reported. Increase in the thickness of the antimony chalcogenide absorber beyond 150 nm through a successive chemical deposition did not yield better cell performance. This is due to the small crystalline grain diameters,