Investigation of valence plasmon excitations in GMZO thin film and their suitability for plasmon-enhanced buffer-less solar cells

Solar Energy 178 (2019) 114–124

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Investigation of valence plasmon excitations in GMZO thin film and their suitability for plasmon-enhanced buffer-less solar cells

T

⁎

Vivek Garga, , Brajendra S. Sengara, Amitesh Kumara, Gaurav Siddhartha, Shailendra Kumarb, Shaibal Mukherjeea a b

Hybrid Nanodevice Research Group (HNRG), Electrical Engineering, Indian Institute of Technology Indore, Madhya Pradesh 453552, India University Grants Commission, Department of Atomic Energy (UGC DAE), Consortium for Scientific Research, Indore, Madhya Pradesh 452017, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Plasmons UPS CIGSe Ultrathin solar cells

The approach of eliminating buffer layer in conjunction with plasmon-enhanced transparent conduction oxide (TCO) layer is an attractive methodology to realize low-cost ultrathin buffer-less solar cells (SCs) by introducing plasmon-enhanced absorption and reduced fabrication steps. Here, we report a novel method to generate wideband sputter-stimulated plasmonic feature in Ga-doped-MgZnO (GMZO) thin-films, which are observed due to the different metallic and metal-oxide nanoclusters formation. Through an extensive analysis of photoelectron spectroscopy, spectroscopic ellipsometry, and field-emission scanning electron microscope measurements the evaluation of plasmonic features and correlation of them with various nanoclusters inside GMZO thin-film is performed. Additionally, the suitability and expected performance of plasmon-enhanced GMZO thin-film based buffer-less SCs are probed through; 1) band-offset analysis at the plasmon enhanced-GMZO/CIGSe heterojunction; 2) simulation studies to analyze the effect of conduction band-offset (CBO) on the performance of the buffer-less SCs; 3) predicting the performance of the buffer-less SC using the parameters of GMZO thin-films with varying CBO, and 4) envisaging the concept of ultrathin buffer-less SC with calculated CBO and absorber layer thickness (300 nm) for ultrathin SCs. Moreover, at the experimentally calculated band-offset with ultrathin absorber layer thickness (300 nm), theoretically calculated buffer-less SC performance parameters estimated to be open-circuit voltage (Voc): 0.75 V, short-circuit current density (Jsc): 17.29 mA/cm2, fill-factor (FF): 80.5%, and efficiency (Eff): 10.46%.

1. Introduction Thin film solar cells (SCs) made up of a wide variety of semiconductors comprising CdTe, CIGS, CZTSSe, CTGSe, etc. offer the benefit of reduced material cost (Jackson et al., 2016; Kim et al., 2015; Londhe et al., 2018; Sengar et al., 2016; Wang et al., 2013). These SCs usually have buffer layers, such as CdS, ZnS, Zn(O, S) and In2S3, in between the absorber layer and transparent conducting oxides (TCOs) used as the electrode materials (Izaki et al., 2015; Kobayashi et al., 2015; Naghavi et al., 2010; Naghavi et al., 2015). The role of the buffer layer at the absorber/TCO interface is to prevent shunting (Sozzi et al., 2014). Buffer layers having the suitable properties for reduced recombination at the absorber/TCO interface certainly prevent the shunting. One of the critical parameters to reduce the recombination is controlling the conduction band-offset (CBO) parameter at absorber/ TCO heterojunction. A high-efficiency cell is obtained when the CBO at the buffer/absorber heterojunction is between −0.1 to 0.4 eV

⁎

(Minemoto and Julayhi, 2013; Sozzi et al., 2014). Furthermore, the high-efficiency device structures exploit CdS buffer layers to overcome the issue of the shunting. However, some critical issues with the CdS layer are: (1) it absorbs light with a wavelength shorter than ∼ 520 nm; (2) toxic Cd waste from chemical bath deposition (CBD); and (3) deposition using chemical route breaks the continuity of the deposition process makes it unbefitting for an in-line process for the mass production. In this study, we accordingly focus on SCs fabricated without a buffer layer by directly depositing TCO layer onto the absorber. The elimination of buffer layers provides the benefit of (1) the reduction of the SC fabrication steps, (2) elimination of Cd wastage, and (3) and using Cd-free materials, for environment-friendly SC realization. In addition, a simpler structure is aimed at helping to reduce process optimization steps to obtain high-efficiency devices. However, the removal of the buffer layer by directly depositing TCO layer onto the absorber results in the deterioration of the SC performance parameters.

Corresponding author. E-mail addresses: [email protected], [email protected] (V. Garg).

https://doi.org/10.1016/j.solener.2018.12.017 Received 6 September 2018; Received in revised form 2 December 2018; Accepted 6 December 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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and (4) envisaging the concept of ultrathin buffer-less SC with calculated CBO and absorber layer thickness (300 nm) for ultrathin SCs.

The reason behind the degradation in performance of SCs is the mismatching of the conduction band, which induces the recombination at the TCO/absorber junction. Thus, if the conduction band minima of the TCO can be precisely controlled and matched to that of the absorber, the interface recombination at the TCO/CIGSe interface can be suppressed efficiently. To increase the efficiency of the SCs while reducing manufacturing costs by reducing the thickness of SCs, several light management techniques have been proposed and investigated, such as metal nanoparticles, nanowires, and plasmonic metallic nanostructures for absorption improvement to counter the issue of the incomplete absorption originated due to the thinner absorbers (Adamovic and Schmid, 2011; Atwater and Polman, 2010; Beck et al., 2009; Ferry et al., 2010; Garg et al., 2016; Pillai and Green, 2010). Modelling studies of the plasmonic metallic nanostructures have shown great potential as a light management scheme in thin film SCs (Catchpole and Polman, 2008; Garg et al., 2016; Pillai and Green, 2010). Plasmonic is an area with very high potential for light harvesting in solar energy conversion because of its ability to sustain coherent electronic oscillations expedite electromagnetic field coupling and confinement (Adamovic and Schmid, 2011; Atwater and Polman, 2010; Ferry et al., 2010; Pillai and Green, 2010). There are reports available in the literature, which validate the concept of surface plasmon enhanced light trapping and absorption in thin film solar cells by incorporating metallic nanoparticles. (Shi et al., 2015; Zhu et al., 2013) Various fabrication related and technical issues are predominant such as scalable and economically viable techniques for controlled nanostructure patterning, parasitic optical absorption in the metallic nanostructures, and augmented carrier recombination near the metal–semiconductor interfaces (Garg et al., 2018a). Another design challenge for the realization of plasmon-enhanced photovoltaic devices is the need for ultrathin TCOs having low enough resistivity and high transmittance to be used for top contact electrodes. With ultrathin TCOs, transmittance increases, but at the same time resistivity increases. The ultrathin-films showed a tendency to be either form continuous film with amorphous behavior or form isolated islands. This outcome in the poor electrical behavior of ultrathin TCOs. To overcome this challenge, a novel approach of producing ultra-thin, high quality films grown by dual ion beam sputtering (DIBS) is proposed. The latter films allow tunable plasmon enhancement in a wide range and can also mitigate the loss associated with the usage of metal nanoparticles. Additionally, this approach gives an advantage of wide-band plasmonic excitation without the need of the incorporation of external metallic nanoclusters in the material. Here, we report a novel approach to excite plasmons in Ga-doped MgZnO (GMZO) thin-films by utilizing DIBS system. By using the DIBS system, plasmonic generation in TCOs in the broad spectral range of 1.87–10.48 eV, are reported in our previous studies (Awasthi et al., 2017; Awasthi et al., 2016; Awasthi et al., 2015; Garg et al., 2018a; Garg et al., 2018b). In continuation of our earlier studies on the sputterinstigated plasmon generation in GZO thin-films, plasmon generation in GMZO is studied using: (1) electron energy loss analysis using ultraviolet photoelectron spectroscopy; (2) quantification of plasmon energies within broad plasmonic peaks; (3) finding the possibility of a particular type of plasmon, i.e., particle plasmon (PP) (in the air, or within the material), surface plasmons (SP), and bulk plasmon (BP) generation at the air/thin film interface, and within a thin film; (4) verification of metallic clusters formation in thin GMZO films; (5) plasmonic verification using spectroscopic ellipsometry. Additionally, to check the suitability and expected performance of plasmon-enhanced GMZO thin film based buffer-less SCs (device structure depicted in Fig. 1(a)), additional studies have been performed as follows: (1) bandoffset analysis at the GMZO/CIGSe heterojunction; (2) simulation studies to analyze the effect of CBO on the performance of the buffer-less SCs; (3) predicting the performance of the buffer-less SC device using the parameters of the DIBS grown GMZO thin-films with varying CBO

2. Experimental details GMZO thin-films with a thickness of 150 nm is deposited at 300 °C on Si substrates using DIBS system by utilizing 1 atomic% Ga-doped Mg0.20Zn0.80O (99.99% pure) target, employing a radio frequency (RF) primary deposition source. Furthermore, a DC coupled assist-ion source is also deployed to ensure the improved growth uniformity and thin film adhesion as a result of a reduction in the columnar growth. A detailed description of the deposition system is reported elsewhere (Garg et al., 2018a; Kumar et al., 2017). Further, an optimized DIBS process conditions reported in our earlier publications are used to grow GMZO thin-films (Awasthi et al., 2015). Additionally, CIGSe thin film of 150 nm is deposited on the Si substrate at 300 °C. Consequently, a 5 nm layer of GMZO is grown on CIGSe thin-films on Si substrates to envision the GMZO/CIGSe heterojunction. The crystalline quality of GMZO and CIGSe thin-films are analyzed using Rigaku SmartLab X-ray diffraction (XRD) system with Cu Kα radiation (1.54 Å). For studying the morphological properties of samples, field-emission scanning electron microscope (FESEM) based on Supra55 Zeiss lens is used. Elemental properties, band-offset and plasmonic properties of the samples are examined by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) utilizing PHOIBOS 100 analyser. The angle integrated photoelectron spectroscopy (AIPES) beamline of INDUS-1 synchrotron source is used for ultraviolet photoelectron spectroscopy (UPS) measurements. The UPS setup is equipped with an AIPES experimental chamber, maintained at a background pressure of ∼2 × 10−9 mbar. Before mounting the samples in the chamber, Ar+ion sputtering is carried out at 2.66 × 10−5 mbar partial pressure (at room temperature) separated from the experimental chamber by a gate valve. After 5 min of mild Ar +ion sputtering, the samples are transferred from the preparation chamber to the experimental chamber and, within 10 min after the sputtering process; the UPS spectra are recorded. Further, the photoelectron counts are collected in the steps of 20 meV and with the pass energy and incident photon energy of 20 and 100 eV, respectively. Moreover, a detailed description of the UPS measurement system is reported elsewhere.(Awasthi et al., 2017; Garg et al., 2018a) Optical properties, such as dielectric functions, dispersion curves, and absorption, of the samples, are analyzed by using M-2000D J.A. Woollam spectroscopic ellipsometry (SE). 3. Results and discussions 3.1. Thin film analysis 3.1.1. Structural characterization XRD measurements suggest a high crystalline quality of GMZO and CIGSe films with a comparatively strong preferred orientation in (0 0 2) crystal plane at 2θ = 34.41°, and in (1 1 2) crystal plane at 2θ = 26.7°, respectively. The 2θ value of 34.41° in the XRD spectra of GMZO is associated with hexagonal wurtzite structure of ZnO and confirms the high c-axis oriented growth of thin-films. For GMZO (0 0 2) and CIGSe (1 1 2) peaks, the values of full-width at half-maximum (FWHM) are calculated to be 0.22° and 0.34°, respectively, and the corresponding crystallite size values are 37.28 and 24.30 nm, respectively, as shown in Fig. 1 (b) and (c). 3.1.2. Electron energy loss analysis UPS spectra of GMZO film depicted in Fig. 2(a), demonstrate the broad plasmon peaks for GMZO samples. Prominent broad plasmon peaks, as designated by P1-P4 in Fig. 2(a), depicted in the UPS spectra are formed due to the loss of kinetic energy of Ga-3d core photoelectrons after scattering with valence plasmons because of inelastic 115

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40

45

50

55

(224)

MgO (111)

MgO (200)

log (Intensity) (a.u.)

(002) Mg

log (Intensity) (a.u.)

35

(c) CIGSe

(112)

(b) GMZO

20

30

Angle 2θ, (degree)

40

50

60

70

80

Angle 2θ, (degree)

Fig. 1. (a) device structure of buffer-less solar cells, XRD spectra of (b) GMZO and (c) CIGSe thin-films.

scattering mechanism (Awasthi et al., 2017; Garg et al., 2018a; Kövér et al., 1995; Kumar and Mukherjee, 2012). The source of origin of the broad plasmon peaks can be the valence bulk plasmon (VBP) or valence surface plasmon (VSP) or particle plasmon resonance (PPR) of Zn, Mg, MgO, ZnO, Ga, and GaO nanoclusters embedded within GMZO matrix. Theoretically, the valence bulk plasmon energy (ℏω b) , in a material is evaluated using Eq. (1) (Awasthi et al., 2016; Egerton, 2009; Garg et al., 2018a; Yadav et al., 2013).

(ℏω b)2 = (4π ℏ2e2Nv /m∗)

2 ωpp =

∑ x i ni ∑ x i Ai

(1)

(2)

where ρ is the density of the material, Na is the Avogadro number, x i , ni , and Ai number of atom present in the molecule, the number of valence electron, and atomic weight of ith element, respectively. In order to determine the energy of VSP (VSPE), Eq. (3) is employed (Oates et al., 2011; Vishnu et al., 2015):

VSPE = VBPE/ 2

(4)

where, εa (ZnO ∼ 8.59 and air ∼1), is the dielectric constant of surrounding medium of nanoclusters, ε∞ is the high-frequency dielectric constant. The values of different constants necessary for plasmon energy calculations following equations (1–4) are populated in Table 1. Due to unavailability of data in the literature, the value of ε∞ is considered to be ∼3 for Zn, Mg, and Ga, nanoclusters. It should be noted here, VBPE, VSPE, and PPRE of nanoclusters embedded in GMZO are calculated and, the data are populated in Table 2. The plasmonic peak positions, the distance of the plasmonic peak from Ga 3d core-level peak position, and their corresponding energy values for VBPs, VSPs, and PPRs of Ga, Zn, Mg, GaO, MgO, and ZnO nanoclusters in sputtered GMZO thin-films are tabulated in Table 3 and plotted in Fig. 2(b). From the GMZO sample shown in Fig. 2(a), broad plasmon peaks P1P4 are observed; and their respective peak positions and the plasmon peak energy difference with the core-level peak Ga−3d are tabulated in Table 3. The tabulated energy difference is the plasmon energy can be ascribed to the collective contribution of surface plasmon resonance energy, and particle plasmons energy of Ga, GaO, Zn and ZnO nanoclusters in ZnO and air media. Additionally, broad plasmon peaks are deconvoluted to further quantify the contribution from the individual nanoclusters. The broad plasmon peaks, P1-P4, are de-convoluted and plotted in Fig. 2(c). The de-convolution of plasmon peaks gives insights into the contribution of individual metallic nanoclusters and their combinations to the broad plasmonic peak generation. The generation of plasmon is primarily governed by the contribution from VSP and PPR

where m* is the effective mass of electrons, and Nv is the density of valence band electrons, which can be calculated by Eq. (2):(Garg et al., 2018a)

Nv = Na × ρ ×

ωb2 ε∞ + 2εa

(3)

where VBPE is the energy of VBP. In addition, the particle plasmon resonance energy (PPRE, ℏωpp ) of different nanoclusters can be evaluated by using Eq. (4) (Garg et al., 2018a; Oates et al., 2011; Vishnu et al., 2015): 116

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Fig. 2. UPS Spectra of (a) GMZO, (b) calculated energy of different types of plasmons, (c) expanded deconvoluted plasmon peaks and (d) FESEM micrograph of GMZO thin-films (Inset shows magnified FESEM image).

comparison to VBP. The odds of PPR in air medium is also as high as VSP at the air/GMZO interface. In GMZO bulk, there is a high probability of PPR within few nm of GMZO film, of metal and metal oxide nanoclusters. The contributions of different nanoclusters possible in broad plasmon peaks are properly indexed by considering the abovementioned hypothesis and depicted in Table 4 and Fig. 2(c).

Table 1 Parameters utilized for plasmon energy calculation. Clusters

Density (gm.cm−3)

No. of valence electrons

Atomic weight (amu)

High-frequency dielectric constant

Ga Zn GaO ZnO Mg MgO

5.91 7.14 6.44 5.61 1.71 3.58

3 2 – – 2 –

69.72 65.38 – – 24.31 –

3 3 3.57 3.76 3 4.52

3.1.3. Morphological characterization FESEM analysis is performed to verify the formation of metal and metal oxide nanoclusters in GMZO thin films and shown in Fig. 2(d). FESEM micrograph confirms the formation of nanoclusters with variable cluster sizes in the GMZO thin films. Further, variable clusters sizes observed in GMZO thin-films are assessed by processing of FESEM micrographs using the ImageJ software. Estimated average size obtained from image processing of different nanoclusters present in GMZO is in the range of 50–80 nm. The formation of variable cluster sizes in GMZO thin films and, contribution of the different metal and metal oxide nanoclusters in different clusters leads to the formation of wide band plasmonic generation in GMZO thin-films.

Table 2 Different plasmon energies for metal and metal oxide nanoclusters in GMZO film. Thin film

GMZO

Nanocluster

Ga Mg Zn ZnO MgO GaO

VBPE (eV)

14.53 10.89 13.46 21.38 24.28 23.69

VSPE (eV)

10.27 7.70 9.52 15.32 17.17 16.75

PPRE, ℏωpp (eV) (in ZnO medium)

(in air medium)

3.23 2.86 2.99 4.67 5.25 5.19

6.5 4.87 6.02 8.91 9.75 10.37

3.1.4. Optical characteristics Modelling of the SE measured data analysis has been performed with a three-layer optical model consisting of a Si substrate, a GMZO layer, and a top surface roughness layer using completeEase software supplied by J.A. Woolam Co. Inc. (J.A. Woollam Co, 2011; Pandey et al., 2015b; Sengar et al., 2016). General Oscillator (GenOsc) model consisting of Tauc-Lorentz (T-L) oscillator is used to obtain theoretical spectra by fitting GenOsc parameters. Obtained theoretical spectra are in good agreement with the experimental data, with low values of mean square error (MSE) of 8.75, and confirm the appropriateness and reliability of model fitting parameters. Tauc-Lorentz (T-L) oscillator is utilized in GenOsc model to take account of valence electron

in air medium at air/GMZO interface, and PPR in ZnO medium, due to the generation of different metallic and metal oxide nanoclusters. Different possibilities of the plasmon generation in GMZO thin-films are at (a) GMZO bulk and (b) air/GMZO interface. At the air/GMZO interface, the possibility of the existence of VSP is high due to the low dielectric constant (air)/high dielectric constant (GMZO) interface, in 117

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contributions, as the T-L model is suitable for modelling of the interband transition at the energy band critical points and the real and imaginary parts of dielectric function which are related to each other by Kramers-Kronig dispersion relations (Woollam Co and I, 2011; Sengar et al., 2018a,b. Moreover, wide-band absorption in TCOs are efficiently illustrated with the T-L model. Therefore, the T-L model is suitable for the modelling of the GMZO thin films in the wide spectral of 1.24–6.5 eV. (Fujiwara and Kondo, 2005; Sharma et al., 2017) The spectral variation of real (ε1) and imaginary (ε1) part of the complex dielectric function, ε (E ) = ε1 (E ) + iε2 (E ) in the energy range of 1.91–6.5 eV, obtained from SE measurement is depicted in Fig. 3(a). ε1 and ε2 are strictly associated to the electronic polarizability of ions and the localized fields inside the material, and, so, the dielectric function have a substantial role in designing optoelectronic and optical devices (Neumann and Reccius, 1979; Pandey et al., 2015a). The origin of ε1 peak in GMZO is because of the exciton-phonon complex transitions. From Fig. 3(a), the onset ofε2 , which corresponds to the absorption at the fundamental band gap (Eg), observed at 3.85 eV for GMZO. There is a peak in ε2 spectra of GMZO at photon energy larger than Eg is observed, which can be ascribed to additional absorption instigated from the plasmonic feature generation in the GMZO thin-films. The observed supplementary absorption might be caused by particle plasmons produced by metal or metal oxide nanoclusters in GMZO thinfilms (Awasthi et al., 2016; Kumar et al., 2008). The refractive index (n) and extinction coefficient (k) plots of GMZO are depicted in Fig. 3(b). Fig. 3(c and d) depicts the absorption coefficient (α) vs. incident photon energy for GMZO and CIGSe films, with a band gap of 3.85 ( ± 0.03) and 1.28 ( ± 0.03) eV, respectively. Error bars for the bandgap are determined from selecting different measurement and fitting regions. The onset of absorption spectra above Eg ∼ 3.85 eV, is observed in GMZO thin-films, which is caused by the interband transition. Additionally, peak present at 4.38 eV in the extinction coefficient (k) spectra of GMZO thin film as shown in Fig. 3 (b), corresponds to the plasmonic resonance peaks, which are observed because of the formation of nanoclusters in GMZO thin film. (Awasthi et al., 2016; Garg et al., 2018b; Chen et al., 1998; Kumar et al., 2008). The observed plasmonic energy contribution can improve the absorption cross-section by scattering mechanism, where the optical path length in the active medium increases, and hence ultimately augments the conversion efficiency of the SCs.

– (ZnO + Zn) 44.8 P4

24.5

MgO, GaO, (Mg + Ga), (Mg + Zn)

ZnO (GaO + ZnO), (GaO + MgO) (Ga + Zn + Mg + GaO + MgO)

Mg (Ga + Mg), (Mg + Zn), GaO, ZnO, MgO (ZnO + MgO), (GaO + ZnO), (Mg + Zn + GaO), (Mg + Ga + GaO) (ZnO + Mg + GaO), (Mg + ZnO + MgO) – Ga, Zn (Ga + Zn) – Mg ZnO 24.4 30.3 40.69 P1 P2 P3

4.1 10.0 20.39

Nanoclusters with corresponding PPRE in air medium Nanoclusters with corresponding VSPE Nanoclusters with corresponding VBPE Distance of plasmon peak from Ga 3d (20.3 eV) peak position (eV) Plasmon peak position (eV) Peak

Table 3 Calculation of plasmon energies of different peaks and their indexing with different plasmonic nanoclusters.

Nanoclusters with corresponding PPRE (in ZnO medium)

V. Garg et al.

4. Heterojunction analysis 4.1. Band alignment study The band structure of the GMZO/CIGSe heterojunction is probed using UPS and XPS measurements with an incident photon energy of 100 and 1486 eV, respectively. A short and mild Ar+ ion sputtering on GMZO film surfaces for 5 min is performed to clean the surfaces. This short duration Ar+ ion sputtering on the surface of GMZO thin-films efficiently removes the surface contaminants. Further, a polycrystalline Au foil is also loaded along with GMZO thin-films in order to get the reference spectra to evaluate the Fermi energy level (EF) of the metallic sample holder and to characterize the monochromatic synchrotron emission. Additionally, UPS measurements have been performed on individual thin-films of GMZO and CIGSe to obtain the valence band maxima (VBM) or valence band onset (VBOn). Wide scan UPS plots of GMZO and CIGSe are depicted in Fig. 4(a) and (b), respectively. The measured values of VBOn for CIGSe and GMZO are 0.58 ± 0.02 and 3.29 ± 0.02 eV, respectively, which correspond to the locations of VBM below the corresponding EF. The valence band-offset (VBO, ΔEV ) at GMZO/CIGSe heterojunction interface is estimated using Kraut’s Eq. expressed in Eq. (5), based on the core-level photoemission (Ajimsha et al., 2015; Kraut et al., 1980). The values of VBOn are used in Eq. (5) to evaluate VBO (ΔEV ). First, in order to obtain VBO, peaks present in 118

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Table 4 Possible contribution of nanoclusters in the generation of broad plasmon peaks indexed by α, β, and γ with subscripts of corresponding plasmonic peak numbers. At Air/GMZO interface

P1 P2 P3 P4

Within GMZO

PPR (Air medium)

VSP

PPR (GMZO medium)

[Mg] β1 [ZnO, MgO] α2, [(Mg + Zn), GaO] β2 , [(Ga + Mg)] γ2 [(ZnO + MgO), (GaO + ZnO)] α3 , [(Mg + Zn + GaO), (Mg + Ga + GaO)] β3 [(ZnO + Mg + GaO), (Mg + ZnO + MgO)] α4

– [Ga] β2, [Zn] α2 (Ga + Zn) α3 [(ZnO + Zn)] β4

[ZnO] α1 [(GaO + ZnO), (GaO + MgO)] α2 (Ga + Zn + Mg + GaO + MgO) α3 –

4

ε1

4

ε2 2

2

1 1

0 2

3

4

5

0.8

1.8 0.6

1.6 0.4

1.4

0.2

1.2 1.0

6

0.0

2

3

4

5

6

Energy (eV) 4 -1 2

(c) GMZO

(αhυ)2 × 10 (eV-cm )

4

14

(αhυ)2 × 1014(eV-cm-1)2

1.0

2.0

Ener gy (eV) 6

(b) GMZO

2.2

3

3

0

1.2

2.4

(a) GMZO

5

Extinction Coefficient (k)

1 2 3 4

Peak

Refractive Index (n)

S. No.

2

Eg = 3.85 eV

(d) CIGSe

3 2 1

Eg = 1.28 eV

0

0 2.0

2.5

3.0

3.5

4.0

1.0

Energy (eV)

1.5

2.0

2.5

3.0

3.5

Energy (eV)

Fig. 3. Spectra of (a) ε1 and ε2 , (b) n and k. Tauc’s plots of (c) GMZO and (d) CIGSe thinfilms.

core-levels at the GMZO/CIGSe heterojunction interface (as depicted in Fig. 4 (e)). By substituting all required values in Eq. (5), the value VBO at GMZO/CIGSe heterojunction using five different sets of core-level peaks is estimated to be 2.376 eV. In the VBO calculation, a difference of 0.01 eV is observed, which corresponds to the measurement error. Considering similar error bar in the evaluation of core-level and VBM, one can conclude that the VBO at GMZO/CIGSe heterostructure is 2.366 ± 0.03 eV (including error bar associated with VBOn and core levels). Additionally, to investigate the influence of strain related effects on the calculation of band-offset parameters, critical thickness is evaluated for the GMZO/CIGSe heterojunction. For GMZO and CIGSe, the thin film thickness (∼150 nm) is well above the critical thickness (tc), and thus the strain between the substrate and thin film is considered to be completely relaxed. In the case of GMZO/CIGSe heterojunction, tc is calculated using an empirical Eq. (6): (Ajimsha et al., 2015)

the UPS spectra of GMZO, as shown in Fig. 4(c), are at 10.76, 20.38 eV, and 50.25 eV, which is identified as Zn 3d, Ga 3d and Mg 2p, core-level photoelectron peak, and peaks present in UPS spectra of CIGSe shown in Fig. 4(d) are at 18.72 and 55.01 eV, which are identified as Ga 3d and Se 3d peak, respectively, are considered as reference peaks. Additionally, other core-level peaks such as Ga 3 s, Zn 2p, Mg 2 s, Se 3d and, In 3d are also considered as reference peaks while evaluating band-offset parameters. The estimated core-level energy position is defined to be the centre of the peak width at the half of the peak height to obtain high precision core-level peak positions (Kraut et al., 1980). Moreover, UPS spectra of the GMZO/CIGSe heterojunction is depicted in the Fig. 4(e). Altogether five different core-level peak combinations used for band-offset calculation to verify the results and the data are tabulated in Table 5. VBO (ΔEV ) value can be evaluated from Eq. (5): (Kraut et al., 1980) GMZO CIGSe CIGSe ΔEV = ΔECL + (EGMZO Ga3d − E VBOn ) − (ESe3d − E VBOn )

(ECIGSe Se3d

(5)

tc =

ECIGSe VBOn )

− is the energy difference of Se 3d coreIn Eq. (5), GMZO level and VBOn in CIGSe film, (EGMZO Ga3d − E VBOn ) is the energy difference of Ga3d core-level and VBOn in GMZO film, and CIGSe ΔECL = (EGMZO Ga3d − ESe3d ) is the energy difference of Ga 3d and Se 3d

az2 2|aZ − ac |

(6)

whereaZ ∼ 0.32 nm, andac ∼ 0.54 nm are the lattice constants of GMZO and CIGSe, respectively. Using these values, tc is evaluated to be 119

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Fig. 4. Valence band edge of (a) GMZO, and (b) CIGSe, UPS spectra of (c) GMZO, and (d) CIGSe, (e) GMZO/CIGSe heterojunction, and (f) band diagram of GMZO/ CIGSe heterojunction at thermodynamic equilibrium. Table 5 Parameters used in band-offset calculation with the corresponding calculated VBO, CBO, and band bending of the GMZO/CIGSe heterojunction. S. No.

Core-level photoelectron for individual film

Core-level photoelectron for heterojunction

Band-offset

GMZO (eV)

CIGSe (eV)

CIGSe/GMZO (eV)

VBO (eV)

CBO (eV)

Se3d:55.01 In3d:445.78 In3d:445.78 Ga3s:164.99 In3d:445.78

Ga3d:19.66 Zn3d:10.79 Ga3s:164.78 Zn2p:1022.69 Mg2s: 89.81

2.38 2.37 2.37 2.38 2.38 2.366 ± 0.03

0.19 0.20 0.20 0.19 0.19 0.184 ± 0.06

1 Ga3d: 20.32 2 Zn3d:10.76 3 Ga3s: 164.75 4 Zn2p:1022.57 5 Mg2s: 89.77 Average value in eV

Se3d:54.62 In3d:446.15 In3d: 446.15 Ga3s:164.78 In3d: 446.15

120

Band bending (eV)

0.33 0.34 0.34 0.33 0.33 0.334 ± 0.01

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In addition, to consider the majority carrier recombination in CIGSe (holes) and TCO (electrons) at the heterojunction interface, a layer is inserted in between CIGSe and TCO, termed as interface defect layer (IDL). The list of parameters utilized in the device simulation is tabulated in Table 6. Here, the CBO is defined as (a) positive: indicates the conduction band minima of TCO is higher than that of CIGSe, and (b) negative: indicates the conduction band of TCO is lower than that of CIGSe. In the simulation, the electron affinity values of TCO and IDL are altered to achieve the required CBO at the junction. To consider the majority carrier recombination at the interface, few assumptions are made in the model: i) the value of electron affinity of IDL is the same as that for CIGSe when the CBO value is positive, as shown in Fig. 5(a) and (ii) the value of the electron affinity of IDL is the same as that for TCO when CBO is negative, as shown in the Fig. 5(b). In both cases of positive and negative CBO, the band gap of IDL is modified to fix the position of the valence band maxima to that of CIGSe.

0.233 nm, which is well below the thickness of the GMZO layer. Calculated tc approves that the GMZO layer with a thickness of 5 nm is thick enough to be considered as completely strain relaxed, which, on the other hand, confirms the reliability of VBO calculations, as described above. In addition, the conduction band-offset (CBO, ΔEc ) can be derived using Eq. (7).(Kraut et al., 1980; Pandey et al., 2015a; Sengar et al., 2018a,b)

ΔEC = EgCIGSe − EgGMZO − ΔEV

(7)

where, EgCIGSe and EgGZO are the values of energy band gap of CIGSe and GMZO, respectively. Moreover, the band gap (Eg) of CIGSe and GMZO obtained from SE analysis using Tauc’s plot are 1.28 and 3.85 eV, respectively. By substitution of the values of Eg and VBO in Eq. (7), the average value CBO at the GMZO/CIGSe heterojunction using five different sets of core level peaks is found to be 0.184 ± 0.06 (including error bars associated with the bandgap and VBO) eV. The CBO calculations following Eqs. (5) and (7) at GMZO/CIGSe heterojunction interface considering Ga 3d, Zn 3d, Zn 3s, Se 3d, In 3d, and Zn 2p reference core-level peaks are populated in Table 5. In general, the binding energy of any particular core energy level shifts from its original value in discrete thin-films to that for film at the heterojunction. Shifting of core level outcomes in substantial band bending at the heterojunction interface such as GMZO/CIGSe in this case. Band bending (Ebb ) at the heterojunction can be calculated using Eq. (8). (Garg et al., 2018b)

Ebb = (EclCIGSe − EclCIGSe (i)) + (EclGMZO (i) − EclGMZO )

Ecla

5.2. Device simulation results The conduction band diagram of the solar cell is governed by the values of conduction band discontinuities at absorber/TCO interface for buffer-less SCs. The barrier at the absorber/TCO junction is defined by the contribution of the of the conduction band discontinuities for the positive CBO, as indicated in Fig. 4(e). Conduction band discontinuity affects band bending in absorber and TCO, and therefore, the built-in voltage is affected. The Figures of merit of the SC as a function of CBO at heterointerface of TCO/absorber (GMZO/CIGSe) are analyzed as depicted in Fig. 6(a). In the case of positive CBO at the GMZO/CIGSe interface, the cell performance is observed to be marginally dependent on CBO. On the other hand, the higher the value of CBO, the more significant becomes the built-in potential, mainly due to two reasons: (1) improvement in the collection of photo-generated carriers in the absorber, while slight reduction in the carrier recombination because of the formation of spike at the GMZO/CIGSe junction which acts as barrier for photogenerated electron flow towards the top electrode while activation energy (Ea) equals to Eg,abs, as depicted in Fig. 5(a), results in the improvement in the short circuit current density (Jsc) and open circuit voltage (Voc); (2) reduction of the dark saturation current, and also contributing to a slight improvement of the photo-current, for increased CBO from 0.0 to 0.6 eV, as depicted in Fig. 6. Moreover, when the builtin potential at the GMZO/CIGSe junction becomes small when compared to the blocking effect of the energy barrier from the CIGSe to the GMZO, the cell performance starts to degrade (Sozzi et al., 2014). Here, with increase in the CBO from the 0.6 to 0.7 eV, above mentioned condition is achieved, and it results in the excess of electrons in the absorber which augments the electron component of diffusion current which is in the reverse direction to the photo-generated current, while hole component of the photo-generated current drops, and as a consequence, the fill factor (FF) and thereby the efficiency (Eff) of SC deteriorates (Sozzi et al., 2014). As a result, one may observe a nonmonotonic dependence of the cell performance parameters with varying CBO towards positive values at the GMZO/CIGSe for the GMZO/CIGSe based buffer-less solar cell. Moreover, when the value of the band reduces from 0 to −0.7 eV, the values of Voc and Jsc decrease monotonically. However, a slight decline in the value of FF is recorded with such reduction in CBO. At negative CBO values cliff formed at the absorber/TCO junction does not obstruct the photogenerated electron flow towards front electrode, results in only a slight change of Jsc, when compared to other performance parameters of the device. However, the interface recombination mechanism becomes dominant when the Ea for the carrier recombination [Eg, abs - |CBO|] becomes lower than Eg of the absorber as depicted in Fig. 5(b) for the condition of negative CBO values (Minemoto and Murata, 2015). Therefore, as the negative CBO values are increased towards higher negative values, the Ea becomes smaller compared to Eg,abs values, results in reduced Voc values due to

(8)

Eclb

and are the values of core-level energy levels of two Here, selected elements in the bulk region of (a) CIGS and (b) GMZO, Ecla (i) and Eclb (i) represent the corresponding core-level energies measured at the heterojunction interface. Calculated values of band bending at the interface for different core level combinations are populated in the Table 5. The energy band bending for p- and n-type materials at the p-n heterojunction depicts downward and upward band bending, respectively. Under the thermodynamic equilibrium condition at GMZO/ CIGSe heterostructure interface, the schematic of energy band diagram based on the calculated and measured parameters is demonstrated in Fig. 4(f). The estimated average values of ΔEV and ΔEC at room temperature are 2.366 ± 0.03 and 0.184 ± 0.06 eV, respectively. A marginal positive value of ΔEC indicates that CBM of CIGSe is little below than that of GMZO at the heterojunction interface. A positive value of ΔEV and ΔEC indicate type-I band alignment (straddling type) of the heterojunction. It can be observed that EF is close to CBM of GMZO and VBM of CIGSe thin film. In addition, type-I band alignment in these conditions has to form a spike at the heterojunction, a required criterion to be satisfied for the use of GMZO layer as an n-type layer for CIGSe based thin film SCs. 5. Simulation studies 5.1. Simulation model To assess the effects of CBO at absorber/TCO interface, different combinations of affinity are simulated using one dimension SCAPS simulation software (Burgelman et al., 2000). This software solves the Poisson’s equation and continuity equations at each mesh point. A simple model is used in the SCAPS simulations for a better understanding of the effect of plasmon-enhanced thinner TCO layers in CIGSe thin film SC structure for buffer-less SCs. GMZO thin film parameters obtained from thin film analysis, i.e., carrier concentration, absorption profile, band gap, etc. are utilized in the simulation model. Moreover, the baseline model is obtained from the highest efficiency reported simulation model presented by Friedlmeier et al. of ZSW Research Center (Friedlmeier et al., 2015). The model used in the simulation study is a simple three-layer structure, as shown in Fig. 5. 121

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Fig. 5. Device structure of buffer-less CIGSe solar cell simulation for (a) positive CBO and (b) negative CBO at TCO/CIGSe interface. IDL is inserted between TCO and CIGSe layers.

the optical absorption decreases, while other parameters such as Voc and FF remain primarily unaffected. Appreciable performance parameters of Voc: 0.75 V, Jsc: 17.29 mA/cm2, FF: 80.5%, and Eff: 10.46% are recorded for buffer-less SC realized by utilizing plasmon-enhanced and CBO-optimized GMZO layer on top of ultrathin CIGSe (thickness of 300 nm) (cell area ∼ 1 cm2). Moreover, considering the error bar estimated for the CBO (∼0.184 ± 0.06 eV), there is very small variation in the performance parameters (< 0.1%) of the simulated buffer-less device. Hence, it can be concluded that the theoretical and experimental studies reported above give insights and essential guidelines for the realization of plasmon-enhanced TCO/absorber based ultrathin buffer-less SCs.

Table 6 Material parameters used in the simulation studies performed in the SCAPS tool. Parameter

CIGSe

IDL

Ga:MgZnO

Thickness d [nm] Eg [eV] χe [eV] ɛr NCB NVB Vt, c Vt, b µe µh ND NA A

2500

10

180

1.28 4.5 13.6 2.2E18 1.8E19 1E7 1E7 100 25 1E16 8E16 4.3E4

Variable Variable 13.6 2.2E18 1.8E19 — — 100 25 — 8E16 —

3.85 4.5 9 2.2E18 1.9E19 1E8 1E8 100 31 1E20 0 Experimentally measured

Double-donor Single 1E13 1.1E−15/1E−12 ECB−0.24, ECB−0.34

Acceptor Single 1E20 1.0 × 1012/1.0 × 1012

Defect 1 Distribution Nt σe/h Et EA WG Defect 2 distribution Nt σe/h Et

6. Conclusions We investigated the generation of plasmons in the GMZO thin films under the influence of secondary ion source, and its effect on the junction properties of GMZO/absorber heterojunction. The verification of plasmon in the GMZO thin film is investigated using UPS, FESEM, and spectroscopic ellipsometry measurements. Afterward, investigation of plasmon generation at the GMZO/CIGSe heterojunction is performed. Finally, after verification of plasmon generation at the heterojunction, the band-offset properties at the junction of plasmon-enhanced-GMZO/CIGSe heterojunction is performed. Moreover, for further understanding of the effect of band offset on the performance of the buffer-less SC simulation studies are performed using the material parameters of GMZO and CIGSe obtained from the material studies of the individual thin films. Simulation studies suggest the optimum position of the conduction band is derived from being 0.0–0.4 eV higher than the conduction band of the absorber. These simulation results should be the essential guidelines for designing new material combination of TCO and absorber for buffer-less solar cells. The experimental and the simulation studies performed confirms the GMZO/CIGSe heterojunction is a good TCO/absorber combination for the buffer-less SC realization.

–

0.45 0.20 Double-acceptor single 1E13 1.1E−12/1E−15 EVB + 0.29, EVB + 0.58

–

Acceptor Single 1E16 1.1E−15/1E−13 Et

increased interface recombination at the junction. Hence, at the negative CBO values, Voc is directly proportional to the activation energy (Ea). Using experimentally calculated material parameters and band offset parameters at the heterojunction, theoretically calculated performance parameters of the plasmonic-enhanced-GMZO/CIGSe based buffer-less SCs are Voc: 0.78 V, Jsc: 24.79 mA/cm2, FF: 84.1%, and Eff: 18.16% (cell area ∼ 1 cm2). Additionally, SC performance parameters for varying absorber layer thickness with a fixed value of CBO (0.184 eV) are investigated and plotted in Fig. 6(b). With the decline in the absorber layer thickness, only Jsc is mostly observed to reduce since

Acknowledgments This work is partially supported by the Clean Energy Research Initiative (CERI), Department of Science and Technology (DST), Government of India (DST/TM/CERI/C51(G)), DST-RFBR Project under India-Russia Program of Cooperation in Science and Technology (No. 122

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26.66

(a) 2

27.49 mA/cm

25.80 24.94 0.84

0.42

V oc (V)

0.63

V oc (V)

23.6

2

J sc ( mA/cm )

27.52

2

J sc ( mA/cm )

V. Garg et al.

0.78 V

0.21

5.9 0.779 0.760

0.75 V

0.741 0.722 83.2

FF ( % )

FF ( % )

2

17.29 mA/cm

11.8

84 70

84.1 %

56 42

81.9

80.5 %

80.6 79.3

E ff. ( % )

16.8

E ff. ( % )

(b)

17.7

11.2

18.16 %

5.6 0.0 -0.50

-0.25

0.00

0.25

15.6 11.7

3.9

0.50

10.46 %

7.8 500

1000

1500

2000

2500

Absorber Thickness (nm)

Band Offset (eV)

Fig. 6. Simulated device performance parameters: Voc, Jsc, FF, and Eff variation (error bar with (a) conduction band-offset and (b) absorber thickness. [Text in the inset shows the device performance parameters for (a) absorber thickness of 2500 nm and CBO of 0.184 eV, and (b) absorber thickness of 300 nm and CBO of 0.184 eV].

DST/INT/RFBR/IDIR/P-17/2016). We are thankful to DIBS, FESEM, EDX, and XRD facilities equipped at Sophisticated Instrument Centre (SIC) at IIT Indore. The authors Vivek Garg acknowledge UGC and, Brajendra S. Sengar and Amitesh Kumar acknowledge CSIR India for their fellowships. Prof. Shaibal Mukherjee is thankful to Ministry of Electronics and Information Technology (MeitY) Young Faculty Research Fellowship (YFRF) award under Visvesvaraya Ph.D. Scheme for Electronics and Information Technology and DST and IUSSTF for BASE Fellowship award. This publication is an outcome of the R&D work undertaken in the project under the Visvesvaraya PhD scheme of Ministry of Electronics & Information Technology, Government of India, being implemented by Digital India Corporation (formerly Media Lab Asia). We are thankful to Dr. D. M. Phase, Dr. R. J. Chaudhary and Mr. A. Wadikar for using AIPES beam line Indus facility at RRCAT.

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