Au heterostructure solar cells

The electrical properties such as resistivity, carrier mobility and carrier concentration of the CdTe and the Cu-doped CdTe ﬁlms deposited on glass substrates at different substrate temp[r]

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Original Article

Photovoltaic device performance of electron beam evaporated Glass/TCO/CdS/CdTe/Au heterostructure solar cells

K Punithaa, R Sivakumara,*, C Sanjeevirajab aDepartment of Physics, Alagappa University, Karaikudi, 630 003, India

bDepartment of Physics, Alagappa Chettiar College of Engineering and Technology, Karaikudi, 630 003, India

a r t i c l e i n f o

Article history: Received August 2017 Received in revised form 21 November 2017 Accepted December 2017 Available online 14 December 2017 Keywords:

CdTe

Grain boundary effect Optical tailoring Recombination losses Ideality factor Parasitic resistances Conversion efﬁciency

a b s t r a c t

We report on substrate temperature and Cu addition induced changes in the photovoltaic device perfor-mance of Glass/TCO/CdS/CdTe/Au heterostructures prepared by the electron beam evaporation technique Prior to the photovoltaic study, the structural and optical properties of CdTe, CdTe:Cu and CdS/CdTe, CdS/ CdTe:Cu layers were studied X-ray diffraction (XRD) analysis showed that the depositedfilms belong to a zinc blende structure The existence of the Te peak in the XRD pattern revealed the presence of excess Te in the depositedfilm structures, which confirmed the p-type conductive nature of the films This was further substantiated by the electrical study The low resistivity of 1 103Ucm was obtained for wt.% of the Cu-doped CdTefilm, which may be due to the substitutional incorporation of more efcient Cu2ỵ(Cd2ỵ) into the CdTe lattice The decrease in band gap with increasing Cu content may be attributed to the existence of shallow acceptor level formed by the incorporation of Cu into the CdTe lattice The efficiency of the cell was increased with increasing Cu concentration and the cell prepared at room temperature with wt.% of Cu addition possessed the maximum conversion efficiency of 1.68% Further, a good photoresponse of the device is achieved as the Vocand Iscare increased with increase in the input power

© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Nowadays, energy conversion is a critical challenge to get and utilise the energy in an efﬁcient, cost-effective and sustainable way, due to limited nature of fossil fuel reserves within the earth's crust

[1,2] In the effort tofight this serious threat for the survival of mankind the solar energy is found as a perennial bountiful, safe, clean, diversely convertible and sustainable source that offers an inexhaustible supply Solar energy can be harvested in many ways Among them, photovoltaic conversion of solar energy has paved a promising way to meet the increasing energy demands As the solar cell converts light energy directly into electrical energy, its performance and efficiency depends on the properties of the ma-terial used Solar cells started to emerge with Si as an absorber material, which produces photogenerated carriers in the incident of light To outsmart the high material and processing cost of Si based solar cells, chalcogenide based thin film solar cells have been developed, which includes, Cu(In,Ga)Se2, CuInSe2, CdSe, HgTe and

CdTe Among these materials, a significant focus is being given to the CdTe-based solar cells in a renewed attention as an attractive potential light absorbing layer with a high absorption coefficient and a direct energy band gap of 1.45 eV which is an optimal match with the solar spectrum, and thus facilitates its efficient utilization (as it can absorb 90% of solar radiation with 1e2mm thickness of CdTefilms, whereas, Si requires 20mm thickness offilm to absorb the similar range of solar radiation) of solar light

Normally, CdTe crystallizes in the zinc blende structure (as the stable form) The zinc blende lattice has two types of surface po-larities, namely (111)A and (111)B, and hence, there will be an electrostatic attraction between these different planes[3] Such an attractive force makes it difficult to separate between these planes Also, if viewed perpendicular to the direction of the (111) plane, it appears to consist of stacked planes of hexagonally packed alter-nating Cd and Te layers[4] In addition, its strong ionicity of 72% and its chemical bond>5 eV makes it extremely stable (both chemically and thermally) Besides, CdTe can be prepared with both n-type conductivity and p-type conductivity, which makes it useful for both homojunction and heterojunction based solar cell figura-tions The extended and point defects in CdTe are electrically active states Therefore, they have a strong effect on the optical and photoelectric properties of CdTe films, which considerably

* Corresponding author Fax: ỵ91 4565-225202

E-mail address:krsivakumar1979@yahoo.com(R Sivakumar) Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2017.12.001

2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

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determine its solar cell efﬁciency In addition, the optical and electrical properties of CdTe can be tuned by suitably incorporating an appropriate dopant into the CdTe matrix Cu is an amphoteric type of dopant (i.e., Cu acts as a donor in the interstitial sites (Cui) and as an acceptor when substituting Cd (CuCd))[5]suitable for the CdTe matrix In order to realize the p-type conductivity with optimal transport properties, the compensation of the CuCd ac-ceptors by the Cuidonors should be avoided The inherently nested valence bands in Te enable the approximate hole packets of 4, leading to a p-type semiconductor Hence in this work, a controlled level of Cu is doped into the CdTe matrix to maintain the p-type conductivity and also to obtain the Te-rich CdTeﬁlms

The photovoltaic performance of a solar cell is basically depends on the structural properties of the absorber layer, which in turn are greatly controlled by the technique that is adopted for the ﬁlm deposition Till now, various techniques have been employed to de-posit CdTe thinﬁlms, viz thermal evaporation [6], closed space sublimation[7], liquid-phase deposition[8], pulsed laser deposition

[9], pulsed laser evaporation [10], molecular beam epitaxy [11], chemical vapor deposition [12], and electrodeposition [13] It is worthwhile to mention here that the electron beam evaporation (EBE), one of the physical vapor deposition methods, has been considered largely for the preparation of device quality thinfilms owing to the maximum possibility of direct transfer of energy to the source Though the production cost offilms by the EBE technique is high (compared to the chemical methods), it imparts some feasible devised-based qualities to the films, which are the key factors determining the suitable performance of thefilms for developing the specialised devices To date, very few reports are available on CdTe thinfilms prepared by the EBE technique[14] For instance, Murali et al.[14]studied the effect of substrate temperature on the electrical properties of CdTefilms deposited by EBE technique However, no attempt was made to understand the photovoltaic device perfor-mance of electron beam evaporated CdTe thin films Hence, the present work focuses on the photovoltaic device performance of the FTO/CdS/CdTe:Cu/Au structure prepared by the EBE technique Prior to the photovoltaic study, the structural and optical properties of deposited layers were investigated The effect of the substrate tem-perature on the properties mentioned above was also studied Experimental

Thinfilms of CdTe, and Cu doped CdTe (CdTe:Cu; Cu ¼ 2, and wt.%) were deposited onto thefluorine doped tin oxide (FTO) coated glass substrates using the EBE technique (HINDHIVAC vac-uum coating unit model 12A4D with the electron beam power supply model EBG-PS-3K) under a chamber vacuum of better than 5 105mbar CdTe powders (SigmaeAldrich; 99.99% purity) were casted into pellets of 10 mm diameter with mm thickness The pelletized CdTe ingots were placed in a graphite crucible (12 mm outer diameter 10 mm inner diameter mm depth) and kept on water-cooled copper hearth of the electron gun, inside the vacuum chamber The distance between the substrate and the target material wasfixed as 12 cm The chamber was evacuated to a high vacuum of better than 2 105mbar using rotary and diffu-sion pumps and the chamber pressure was measured by pirani and penning gauges In the electron gun, the electrons extracted from a dc-heated cathode of tungstenfilament, by the application of an electric field, pass through an anode, and deflected through an angle of about 180by the magneticfield to reach the target ma-terial The surface of the CdTe pellet on the graphite crucible was scanned by the resultant and deflected electron beam with an accelerating voltage of kV and a power density of about 1.5 kW cm2 The ablated material was evaporated and the vapor phase condensed and deposited as thin film on the precleaned

substrate The homogeneous distribution of the evaporated CdTe particles on the substrate was attained by continuously rotating the substrate during deposition The deposition time was 10 and the deposition rate was 0.1mm/min The thickness of deposited film was in the range of ~1.00 (±0.03)mm, measured by surface profilometer (Mitutoyo, SJ-301) The films were deposited at different substrate temperatures (Tsub) like room temperature (RT), 100C, 150C and 200C Similarly, CdS/CdTe and CdS/CdTe:Cufilm structures were deposited (without breaking the chamber pres-sure) onto the FTO substrate with 100 nm thickness of CdS, which would ever serve as the window layer In order to improve the crystallinity, the depositedfilms were annealed in air (post depo-sition heat treatment) (Tannea) at 400C for 10

Prior to the photovoltaic study, the structural and optical properties were investigated for the CdTe, CdTe:Cu, CdS/CdTe and CdS/CdTe:Cu thinfilms The structural property of the films was analyzed by X-ray diffraction (XRD; X'pert Pro PANalytical) using Cu-Karadiation (l¼ 0.154 nm) over a 2qscan range of 10e80 The surface morphology of CdS/CdTe:Cu thinfilm was studied using scanning electron microscopy (SEM; TESCAN VEGA 3) The optical properties offilms were studied with a UV-Vis-NIR spectropho-tometer (JASCO) The photoluminescence (PL) property of thefilms was studied using a photoluminescence spectrometer (Cary eclipse VARIAN), whereas a xenonflash lamp (15 W) and a photomultiplier tube were used as the source of excitation and the detector, respectively In addition, the electrical properties of the CdTe and CdTe:Cu films deposited on the glass substrate and annealed at 400C were studied by the Van der Pauw configuration Finally, solar cell characteristics of the FTO/CdS/CdTe:Cu/Au structure was studied using a solar simulator (4200 Keithley Semiconductor Characterization System)

3 Results and discussion

3.1 Structural and surface morphological properties

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Fig XRD patterns of CdTe and CdTe: Cu (2, and wt%) thinﬁlms deposited on FTO substrate at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and annealed at 400C

Fig XRD patterns of CdS/CdTe and CdS/CdTe: Cu (2, and wt%) structures deposited on FTO substrate at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and annealed at 400C

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The degree of crystallinity is high for the RT deposited annealed film and the deterioration in the crystallinity is observed for the film deposited at the substrate temperature of 200C and annealed at 400C The observed higher degree of crystallinity for RT deposited annealedfilm may be due to the primary nucleus formed on the substrate surface at room temperature which can easily be driven towards the location with lower surface free energy This leads to the effective growth of the material On the other hand, the dete-rioration of the crystallinity for thefilm deposited at the substrate temperature of 200C and annealed at 400C may be due to the re-evaporation of adatoms from the substrate surface Also, at higher substrate temperatures, there is a possibility for the dissociation and desorption of adatoms that makes the films thermodynamically unstable and deteriorates the crystallinity[19] Besides, from the XRD patterns of the CdS/CdTe and CdS/CdTe:Cu structures (Fig 2), the signature of CdS corresponding to the hexagonal phase (JCPDS card No.: 02-0563) was identified from the small peaks of the (100), (103) and (106) planes, which raised from the window layer Further, the signature of the Te peak revealed the presence of excess Te in the depositedfilm for the solar cell structures

The crystallite size of the CdTe and Cu doped CdTe ﬁlms deposited on the FTO substrate was calculated using the Scherrer's formula and the result is given inTable The crystallite size is found to increase with the increase of the Cu content However, one can observe that the crystallite size in the wt.% Cu doped CdTe ﬁlm is lower than in the undoped one This may be due to the lattice distortion induced by the incorporation of Cu into the CdTe matrix Since the ionic radius of Cu2ỵ(0.72 ) is lower than that of Cd2ỵ (0.97 ), the decrease in the crystallite size may be attributed to the substitutional incorporation of the Cu2ỵions instead of the Cd2ỵ ions[9] It is worthwhile to mention here that copper is the fast migrating impurity in the CdTe compound Cu migration in single crystalline CdTe and in other II-VI compounds is characterized by the two component diffusion The fast diffusion component has been assigned to the interstitial copper (Cuiỵ), while the slower one has been assigned to the substitutional copper (CuCd) and the Cu complexes, such as (CuiỵỵVCd2) and (CuỵeCuCd)[20] Upon the in-crease in the concentration of Cu (3 and wt.%) together with the annealing treatment, the Cuiỵ may diffuse fast and occupy the substitutional Cd vacancy or the Cu complexes, which in turn in-crease the crystallite size

The surface morphology of the prepared FTO/CdS/CdTe:Cu wt.% structure at the substrate temperatures of RT, 100, 150 and

200C and then annealed at 400C was studied using the scanning electron microscopy and the obtained images are shown in

Fig 3(aed) The influence of the substrate temperature on the morphology is clearly seen from the micrographs The film deposited at RT and annealed at 400C (Fig 3(a)) shows an uniform morphology with netted surface When the film deposited at RT was subjected to the annealing treatment, the adatoms may gain kinetic energy from the thermal energy and start to form clusters from the nucleation sites These nuclei grow large enough to touch each other, coalescence takes place at the interface between them which will minimize the surface free energy[16] This results in the growth of grains with netted-surface-like morphology On the other hand,Fig 3(bed) show the SEM images of the films deposited at the substrate temperatures of 100, 150 and 200C and subse-quently annealed at 400C The morphology of thefilm deposited at 100C shows an uniform distribution of very small crystal grains, whereas, thefilm deposited at 150C shows a different morphology with large sized grains grown outwards to form a netted feature The SEM image of thefilm deposited at 200C shows the deteri-oration in the grain growth, which is consistent with the result of the X-ray diffraction study

3.2 Optical property

When the light of sufficient energy is incident onto a material, the electromagnetic radiation interacts with the discrete atomic energy levels and induces the transition of electrons from occupied states below the Fermi energy to unoccupied states above the Fermi energy A quantitative study of these transitions provides the un-derstanding of the initial andfinal states involved in the transition and hence knowledge of the band structure Also, it is known that the efficiency of any photovoltaic device depends on the amount of photons absorbed by the material, which in turn is related with the energy of the photon and the band gap of the material In order to evaluate the energy band gap of the CdTe, CdTe:Cu, CdS/CdTe and CdS/CdTe:Cu thinfilms deposited on the FTO substrate, the optical transmittance of thefilms were measured in a UV-Vis-NIR spectro-photometer During the optical transmission measurements, the respective contributions from FTO and FTO/CdS have been nullified by introducing them as the references and hence the information corresponding to CdTe and CdTe:Cu only was observed The ab-sorption coefficient was calculated from the optical transmittance using the formula,

aẳ lnTị

t (1)

where T is the transmittance and t is the thickness of theﬁlm The energy band gap is related to the absorption coefﬁcient (a) through the Tauc relation[21],

ahy¼ B hy Eg

n (2)

where B is a constant which arises from the Fermi's Golden rule of fundamental electronic transition within the framework of the parabolic approximation for the dispersion relation, Egis the energy band gap and n takes the values depending upon the type of the transition CdTe is a direct band gap material and hence the Tauc plot drawn between (ahy)2and photon energy (hy) is expected to show a linear behavior in the higher energy region and the extrapolation to the linear region ata¼ gives the Egof theﬁlms (graph not shown here) It is observed that the value of Egchanged from 1.48 eV to 1.38 eV for the CdTe and CdTe:Cuﬁlms deposited on the FTO substrate and from 1.57 eV to 1.38 eV for the CdS/CdTe and CdS/CdTe:Cu structures deposited also on the FTO substrate, as

Table

Crystallite size of CdTe and CdTe:Cu thin ﬁlms deposited on FTO substrate

Sample conditions Crystallite size (nm)

Tsub¼ RT; Tannea¼ 400C

FTO/CdTe 61

FTO/CdTe:Cu wt.% 41

FTO/CdTe:Cu wt.% 49

FTO/CdTe:Cu wt.% 80

Tsub¼ 100C;Tannea¼ 400C

FTO/CdTe 41

FTO/CdTe:Cu wt.% 40

FTO/CdTe:Cu wt.% 43

FTO/CdTe:Cu wt.% 50

FTO/CdTe 54

FTO/CdTe:Cu wt.% 39

FTO/CdTe:Cu wt.% 47

FTO/CdTe:Cu wt.% 53

FTO/CdTe 46

FTO/CdTe:Cu wt.% 45

FTO/CdTe:Cu wt.% 46

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summarized inTables and This decreasing band gap may be attributed to the existence of the shallow acceptor level formed by the incorporation of the Cu dopant into the CdTe lattice Since Cu is an amphoteric nature of dopant, it acts as a donor when occupies the interstitial sites and as shallow acceptors when substituting Cd (CuCd), and also in forming the structure complexes with Cd va-cancies such as (CuiỵVCd2) and (CuiỵeCuCd) It was reported that the activation energy of the CuCdacceptor center is about 0.15 eV above

the valence band[22] Further, thermal annealing creates Cd va-cancies (VCd) to facilitate the substitution of the Cu atoms in the Cd sublattices[5,20] The observed Egvalues are in agreement with those reported by Dharmadasa et al.[23]and Hu et al.[22]for CdTe layers deposited on the FTO substrate for the fabrication of FTO/ CdS/CdTe heterostructure solar cells The reduction in Egvalue with the increasing Cu concentration revealed the dopant acting as a substitutional impurity in the Cd vacancy, i.e CuCd This result is

Fig SEM images of CdS/CdTe: Cu wt% structures deposited on FTO substrate at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and annealed at 400C

Table

Optical energy band gap values of pure and Cu doped CdTe thinﬁlms deposited on FTO substrate

Sample conditions Eg(eV)

FTO/CdTe 1.44

FTO/CdTe:Cu wt.% 1.42

FTO/CdTe 1.47

FTO/CdTe 1.45

FTO/CdTe 1.48

Table

Optical energy band gap values of FTO/CdS/CdTe structures

Sample conditions Eg(eV)

CdTe 1.44

CdTe:Cu wt.% 1.42

CdTe:Cu wt.% 1.41

CdTe 1.45

CdTe:Cu wt.% 1.42

CdTe:Cu wt.% 1.40

CdTe:Cu wt.% 1.38

CdTe 1.46

CdTe:Cu wt.% 1.44

CdTe:Cu wt.% 1.43

CdTe:Cu wt.% 1.42

CdTe 1.44

CdTe:Cu wt.% 1.57

CdTe:Cu wt.% 1.56

CdTe:Cu wt.% 1.48

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consistent with the report of Ding et al.[24], where they have observed the band gap narrowing for CdTe thinﬁlms deposited with different substrate temperatures

3.3 Photoluminescence study

The optical quality of the CdTe, CdTe:Cu, CdS/CdTe, and CdS/ CdTe:Cufilms deposited on FTO substrates was further studied by photoluminescence spectroscopy with an excitation wavelength of 600 nm and the recorded spectra are shownFigs and The broad single emission peak localized around 822 nm corresponds to the band to band radiative recombination of CdTe The energy value corresponding to the emission peak is found as 1.51 eV, which approximately matches with the energy band gap obtained from the optical measurement Also, it is observed that the intensity of emis-sion peaks are increased with the Cu content up to wt.% The in-crease in the peak intensity may be due to the existence of the defect traps which may lead to the emission of the large number of excitons releasing large amount of emitted energy However, a decrease in the PL peak intensity is observed for the wt.% of Cu doped CdTefilm This may be due to the complete saturation of defects and some of these defects may trap two electrons or holes (doubly excited), which increase the activation energy[25] In addition, the dopant concen-tration and substrate temperature induced variation in the intensity of the PL emission peaks may be attributed to the change in surface state density of thefilms Further, the broadeningof the emission peak may be due to the photo-assisted transition

3.4 Electrical properties

The electrical properties such as resistivity, carrier mobility and carrier concentration of the CdTe and the Cu-doped CdTe films deposited on glass substrates at different substrate temperatures and annealed at 400C were measured using the Van der Pauw configuration The conductive nature of the films was found using the hot probe technique, where, the current was observed toflow from hot to cold junction in all thefilms, which revealed the p-type nature of conductivity This observation is consistent with the result of de Moure-Flores et al.[9], where the authors have observed the p-type nature of conductivity for the Cu-doped CdTefilms up to wt.%, which was changed to n-type conductivity when the dopant con-centration was increased to 10 wt.% The substrate temperature and Cu concentration induced changes in the electrical parameters of thefilms are given inTable The resistivity of thefilms is found to decrease with the increasing Cu content It may be mentioned that de Moure-Flores et al.[9]have observed the lowest resistivity of 68.8 103 Ucm for the wt.% Cu doped CdTe sample and 21 104Ucm for the wt.% Cu doped CdTe samples prepared at the substrate temperature of 300 C However, our result shows the lowest resistivity of 1 103Ucm for the wt.% Cu doped CdTefilm. This may be due to the substitutional incorporation of more efcient Cu2ỵ(Cd2ỵ) in the CdTe lattice, which in turn leads to the increase in the mobility and the free carrier concentration of thefilms 3.5 Photovoltaic study

3.5.1 Construction of p-n heterostructure solar cells

The schematic sketch for the construction of a p-n hetero-structure solar cell and the IeV graph of an ideal solar cell is shown in

Fig 6(a) and (b) The transparent ordinary window glass (about -3 mm thick) was used to protect the active layers from the envi-ronment The transparent conducting oxide of FTO acts as a front contact of the device because of its high work function and larger mechanical stability A thin layer of n-CdS (about 100 nm) was employed as the window layer of the device owing to its wide band

gap and transparent nature down to the wavelength of about 500 nm The p-type CdTe (1mm thick) was used as an active absorber layer It is the effective region of the device, where the generation and collection of carriers occur The back contact provides a low resis-tance electrical connection to the CdTe A thin gold (Au) layer (few tens of nm thick) was used as back contact on CdTe layer The cur-rentevoltage (IeV) characteristics of this cell structures were measured using the solar simulator (4200 Keithley Semiconductor Characterization System) The photocurrent was measured by illu-minating the cell with the white light using a halogen lamp The conversion efﬁciency of the cell was measured with a power density of 100 mW/cm2 The photoresponse of the solar cell was measured by varying the power density (60, 80, and 100 mW/cm2)

3.5.2 IeV characteristics

The currentevoltage (IeV) characteristics of the cell (Glass/TCO/ CdS/CdTe:Cu/Au) structure, prepared at the substrate temperatures of RT, 100, 150 and 200C and post heat treated at 400C are shown inFig The span of the IeV curve ranges from the short circuit current (Isc) at zero volts, to zero current at the open circuit voltage (Voc) (Fig 6(b)) The‘knee’ of the IeV curve is the maximum power point (Imax, Vmax), i.e the point at which the solar cell generates the maximum electrical power At voltages well below Vmax, theflow of the photogenerated electrical charge to the external circuit is relatively independent of the output voltage Near the‘knee’ of the curve, this behavior starts to change As the voltage further in-creases, an increasing percentage of the charges recombines within the solar cells rather thanflowing out through the external circuit At Voc, all of the charges recombine internally The maximum power point, located at the knee of the curve, is the (I, V) point at which the product of the current and the voltage reaches its maximum value The various solar cell parameters, such as the open circuit voltage (Voc), the short circuit current (Isc), thefill factor (FF), the efficiency (h), the series resistance (Rs) and shunt resistance (Rsh) were evaluated from the IeV curve and the results are presented in

Table It is observed that the Vocvaries between 290 and 643 mV and the Iscchanges from 2.87 to 4.75 mA/cm2 It is also seen from

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However, the lower Iscvalues are responsible for the low con-version efficiency values Bhandari et al.[27]fabricated CdTe solar cells with CdCl2surface treatment and produces a solar conversion efficiency of 8.7% with an Au back contact The efficiency was

further improved to 11.4% when the Cu/Au back contact was used It was said that Cu in the Cu/Au back contact reduces the width of the space charge region Moreover, in the case of CdTe, the height of the Schottky barrier (qFB), which is measured between the top of the

Fig Photoluminescence spectra (excited at 600 nm) of CdTe and CdTe: Cu (2, and wt%) thinﬁlms deposited on FTO substrate at Tsub¼ RT, 100C, 150C and 200C and further annealed at 400C

Fig Photoluminescence spectra (excited at 600 nm) of CdS/CdTe and CdS/CdTe: Cu (2, and wt%) structures deposited on FTO substrate at Tsub¼ RT, 100C, 150C and 200C and further annealed at 400C

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semiconductor valence band and the Fermi level at the metal-semiconductor interface, is given by,

qfBẳ qfM 4:3eV ỵ 1:5eVị ẳ qfM 5:8eV (3)

This equation implies that the barrier can only be reduced to zero if a metal with a work function of at least 5.8 eV has to be applied However, due to its covalent nature, CdTe does not follow the Schottky theory rigorously[28] To balance the surface change, the bands of CdTe bend towards its surface giving rise to a space charge region Also, it is reported that Paudel et al have obtained the efﬁciency of 0.5% for undoped CdTe solar cells[29] However, in the present work, Au was used as a the back contact which has a high work function of 5.1 eV and this acts as a better contact and produced measurable conversion efﬁciency without the step of surface treatment for the CdTe surface

The observed low value of the short circuit current may be attributed to the surface recombination This may be explained as follows: the absorption coefficient (a) of CdTe steeply increases in a narrow range at hyz Egand becomes higher than 104cm1at hy> Eg As a result, the penetration depth of photons (a1) is less than ~ 1mm When the electricfield in the space charge region is not strong enough, these photogenerated electrons may recombine before running through the external circuit which leads to the insufficient collection of charge carriers and hence lowers the short circuit current[26] In addition, it was stated that the short circuit current density will be lowered if a significant portion of radiation is absorbed outside the space-charge region[26]

Further, the diffusion component of the short circuit current depends on the thickness of the CdTe layer The losses of the diffusion component of the short-circuit current are 5, and 19% for 10, and 2mm of the CdTeﬁlm layer, respectively[26] The higher the thickness, the lower the losses of the diffusion component In our case, the thickness of the CdTe layer is 1mm from which the losses of the diffusion component of the short circuit current may be expected more because of the higher absorption coefﬁcient (>104cm1) and so the lower penetration depth (<1mm)[26] Thus, the carriers arisen outside the space-charge region diffuse into the neutral part of the CdTe layer shall penetrate deeper into the ma-terial Carriers reaching the back surface of the layer will recombine and not contribute to the photocurrent If the layer thickness is low, recombination may take place even at the back surface which annihilates the photogenerated electrons Thus, thinning the CdTe

layer reduces the short circuit current density due to the incom-plete charge collection in the neutral part of the CdTefilm Besides, if the space charge region is too wide, the electricfield becomes weak and cannot keep the mobile charge carriers separated until they run through the external circuit and hence the short circuit current is reduced Further, the secondary cell parameters like the saturated current density (Jo) and the ideality factor (n) were also calculated for the solar cell fabricated at Tsub¼ RT These parame-ters can be evaluated from the graph plotted between ln JDvs the anode voltage (V) which is shown inFig The intercept of the linear portion gives the saturated current density (Jo) and the slope gives the ideality factor (n) For an ideal pn-junction, the current is carried by the thermionic emission of carriers over the junction barrier[29] For the purely thermionic emission, the ideality factor is always Any deviation in the value of n from is attributed to other current transport mechanisms like tunneling through the barriers and/or to the presence of a the generation/recombination current in the junction region[30] In such cases, the IeV curve will be less than square and the corresponding values of Imaxand Vmax will be proportionally smaller Thus, the process of the recombi-nation of carriers in the depletion region is an important cause for high values of the ideality factor In the CdS/CdTe solar cell, the lattice constant of CdTe is 6.48 Å and of CdS is 5.82 Å This lattice mismatch gives rise to large interfacial defect states which act as recombination sites at the interface and deteriorate the device performance Besides, the disorders due to their amorphous nature also cause the defect states as interstitials and impurities These defects are distributed in energy in the band gap and act as recombination centers Deep defects, sometimes called mid-gap defects, are located near the center of the band gap and usually act as recombination centers The calculated saturate current density and ideality factor are 7.59 104 mA/cm2 and 3.20, respectively This clearly infers that there are recombination losses which may occur either in the interface region and/or through the deep defect states Paudel et al.[29]have reported that the ideality factor (n) and the reverse saturation current density (Jo) of CdTe films vary from 2.7 to 1.7 and from 1.01 104to 1.38 107mA/ cm2, respectively It was stated that the back contact interface recombination influences the parameters n and Jo

However, observation of this measurable currentﬂow is due to the grain growth attained by the post-deposition heat treatment of the fabricated CdS/CdTe heterostructure The heat treatment

Table

Electrical parameters of CdTe and CdTe:Cu thinﬁlms

Samples Resistivity (r) (103)Ucm Mobility (m) cm2/Vs Carrier concentration

(N) (1011)/cm3 Tsub¼ RT; Tannea¼ 400C

Pure CdTe 55 39.5 29.7

CdTe:Cu wt.% 49 48.9 25.6

CdTe:Cu wt.% 24 53.8 48.6

CdTe:Cu wt.% 01 89.0 403.2

Pure CdTe 59 41.1 25.4

CdTe:Cu wt.% 45 56.1 24.9

CdTe:Cu wt.% 44 68.2 20.8

CdTe:Cu wt.% 58 81.2 131.7

Pure CdTe 48 48.1 26.6

CdTe:Cu wt.% 26 63.4 37.6

CdTe:Cu wt.% 06 71.4 130.9

CdTe:Cu wt.% 05 75.8 154.9

Pure CdTe 53 46.9 25.0

CdTe:Cu wt.% 49 49.7 25.1

CdTe:Cu wt.% 47 60.8 21.4

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reduces the defect density, grain boundaries (which act as recom-bination centers in CdTe) and promotes the interdiffusion between the CdTe and CdS layers that reduces the recombination rate to some extent[31] During operation, the efﬁciency of the solar cells is reduced by the dissipation of power across the internal

resistances These parasitic resistances can be termed as series resistance (Rs) and parallel shunt resistance (Rsh) Series resistance (Rs) is caused by the ohmic losses in the surface of the solar cell The parallel shunt resistance (Rsh) is caused by the losses due to the leakage current which arises because of the non-idealities and

Fig (a) Schematic diagram of thinﬁlm solar cell with various active layers and metal contacts and (b) IeV graph of an ideal solar cell under dark and light conditions K Punitha et al / Journal of Science: Advanced Materials and Devices (2018) 86e98

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impurities near the junction and causes the partial shorting of the junction near the solar cell edges[32] For an ideal cell, the series resistance (Rs) should be zero, resulting in no further voltage drop before the load, and the shunt resistance (Rsh) should be infinite and would not provide an alternate path for the current toflow From our results, it is observed that both the ohmic losses through the higher series resistance and the leakage current loss through the lower shunt resistance may be the reason for the obtained solar cell parameters and hence, the conversion efficiency This can also be observed from the IeV graph (Fig 7) The effects of these para-sitic resistances on the IeV characteristic of the solar cell fabricated

with the wt.% Cu doped CdTe absorber layer are graphically shown inFig For an ideal solar cell, the IeV graph near Iscwill be flat The slope of the IeV curve between Impand Iscis affected by the amount of shunt resistances Reduced shunt resistance results in a steeper slope in the IeV curve near Iscand a reducedfill factor This decrease in the shunt resistance may be due to changes within the device Similarly, the slope of the IeV curve between Vmpand Vocis affected by the amount of series resistances Increased series resistance reduces the steepness of the slope and also reduced the fill factor However, the decreasing trend of the series resistance and the increasing trend of the shunt resistance with respect to the

Fig Effects of substrate temperature and Cu concentration on the change in I-V characteristics of the Glass/TCO/CdS/CdTe/Au heterostructure solar cell

Table

Solar cell parameters of Glass/TCO/CdS/CdTe/Au structures

Samples and conditions VocmV IscmA/cm2 VmaxmV ImaxmA/cm2 FF h(%) RsUcm RshUcm

Pure CdTe 576 3.52 376 2.83 0.53 1.07 60.7 825

CdTe:Cu wt.% 615 4.33 398 3.40 0.51 1.35 50.3 909

CdTe:Cu wt.% 620 4.52 406 3.66 0.53 1.48 44.9 946

CdTe:Cu wt.% 643 4.60 476 3.55 0.57 1.68 31.4 953

Pure CdTe 457 2.91 314 2.21 0.52 0.69 46.5 878

CdTe:Cu wt.% 465 3.26 344 2.27 0.52 0.78 34.2 978

CdTe:Cu wt.% 473 3.84 342 2.83 0.53 0.96 26.5 1120

CdTe:Cu wt.% 482 4.14 378 3.02 0.57 1.21 13.4 1650

Pure CdTe 550 4.09 390 3.05 0.53 1.19 32.5 496

CdTe:Cu wt.% 576 4.40 387 3.37 0.52 1.32 29.2 496

CdTe:Cu wt.% 594 4.61 431 3.49 0.55 1.51 26.7 503

CdTe:Cu wt.% 609 4.75 469 3.32 0.53 1.53 34.8 620

Pure CdTe 290 2.87 229 1.60 0.44 0.36 23.5 240

CdTe:Cu wt.% 304 3.23 214 2.20 0.48 0.47 20.6 308

CdTe:Cu wt.% 309 3.59 220 2.50 0.49 0.54 17.0 326

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increasing Cu concentration lead to the increase in Iscand in the efficiency (Table 5) Paudel et al.[33]reported the similar range of shunt resistance andfill factor values for the CdS/CdTe solar cells Madhu et al.[34]have reported the Vocof 209 mV, Iscof 2.3 mA/cm2, FF of 0.3, andhof 1.88% for the electrochemically deposited CdS/ CdTe solar cells Literature survey shows that the efficiency of CdTe (1 mm) based solar cells have been enhanced by improving the crystallinity and the richness of Te through the CdCl2treatment along with surface etching Han et al.[35]observed the conversion efficiency of 2.62%, with the open circuit voltage of 465 mV and the fill factor of 37.6% The improved collection of the short circuit current may be due to the passivation of the grain boundaries through the surface treatment From the trend of our results, it is

well understood that the incorporation of the Cu dopant and the temperature treatment facilitate the maintenance of the Te rich-ness or the p-type conductivity The increasing trend of the short circuit current density infers that the grain boundary effect is nulliﬁed to some extent with the improvement of the crystallinity facilitated by the temperature treatment Hence, it may be mentioned that the obtained solar cell parameters may further be improved by the above mentioned CdCl2heat treatment and sur-face etching However, the photoresponse of the device is good as the Vocand Iscis increased with the increase in the input power as observed fromFig 10

4 Conclusion

The structural and optical properties of undoped CdTe, Cu doped (2, and wt.%) CdTe, CdS/CdTe, and CdS/CdTe:Cu layers deposited on FTO substrate were studied Thefilms exhibit a polycrystalline state in the cubic zinc blende structure for the CdTe and in the hexagonal structural phase for the CdS compounds The crystallite size was found higher for the samples deposited at RT and annealed at 400C The absence of the oxide and other elemental peaks except Te in the XRD patterns inferred that the prepared materials are single-phase of cell structure nature The surface morphological study showed the formation of netted surface features The pho-toluminescence study further confirmed the optical quality of the prepared cell structures The Cu-concentration-induced decrease in band gap values of the samples was observed The p-type conductive nature of the preparedfilms was revealed by the hot probe method The low resistivity (1 103Ucm) of the CdTe:Cu (4 wt.%)film is due to the substitutional incorporation of the more efcient Cu2ỵ(Cd2ỵ) in the CdTe lattice as compared to otherfilms, which increased the free carrier concentration The study of the IeV characteristics of the heterojunction solar cell structures has shown

Fig Plot of ln (JD) vs anode voltage (V) of the Glass/TCO/CdS/CdTe: Cu (4 wt%)/Au heterostructure solar cell prepared at RT and annealed at 400C

Fig Change in current density and power with anode voltage of Glass/TCO/CdS/CdTe: Cu (4 wt%)/Au heterostructure solar cells prepared at different substrate temperatures (Tsub¼ RT, 100, 150 and 200C) and annealed at 400C

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a low efficiency of the solar cells, which might be due to the nature of the CdTe and CdS layers, the junction formation, and the grain boundary effects In addition, the low values of Iscwere observed, which can be ascribed to the internal electricfield being not strong enough to keep the liberated electrons and holes separated to pass through the external circuit and also due to the low minority carrier lifetime that leads to the recombination losses Moreover, the ef-ficiency is increased with increasing the Cu concentration The study reveals that the solar cells prepared at RT with wt.% Cu addition possess the maximum conversion efficiency of 1.68% Further, the device shows a good photoresponse as the Vocand Isc are increased with increase in the input power

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