Interface Study of ITO / ZnO and ITO / SnO 2 Complex Transparent Conductive Layers and Their Effect on CdTe Solar Cells

Transparent ITO/ZnO and ITO/SnO2 complex conductive layers were prepared by DCand RF-magnetron sputtering. eir structure and optical and electronic performances were studied by XRD, UV/Vis Spectroscopy, and four-probe technology. e interface characteristic and band offset of the ITO/ZnO, ITO/SnO2, and ITO/CdS were investigated by Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS), and the energy band diagrams have also been determined. e results show that ITO/ZnO and ITO/SnO2 �lms have good optical and electrical properties.e energy barrier those at the interface of ITO/ZnO and ITO/SnO2 layers are almost 0.4 and 0.44 eV, which are lower than in ITO/CdS heterojunctions (0.9 eV), which is bene�cial for the transfer and collection of electrons in CdTe solar cells and reduces the minority carrier recombination at the interface, compared to CdS/ITO. e effects of their use in CdTe solar cells were studied by AMPS-1D soware simulation using experiment values obtained from ZnO, ITO, and SnO2. From the simulation, we con�rmed the increase of EEff, FF, VVoc, and IIsc by the introduction of ITO/ZnO and ITO/SnO2 layers in CdTe solar cells.


Introduction
Transparent conducting oxide (TCO) layers have been extensively studied because of their use as transparent electrodes in displays and in photovoltaic devices [1].By incorporating a high resistance layer, the thickness of a conducting cadmium sul�de (CdS) layer can be reduced, which signi�cantly improves the blue response of CdTe devices [2] and makes CdTe thin-�lm solar cells more competitive [3].Wu has reported the efficiency of 16.5% with <100 nm CdS thickness [4].However, as the thickness of CdS is decreased, the �lms would become discontinuous leading to the formation of localized CdTe/TCO junction, which leads to excessive shunting and therefore lowers the solar cell efficiency [5].Using complex transparent conductive layers is known as a feasible method to improve the characteristics of CdTe thin �lms solar cells.Indium-tin oxide (ITO) systems, SnO 2 , and ZnO have been used as the high resistance layer because of their excellent electrical and optical properties [6,7] and the improvement of device performance [8].
e complex transparent conductive layers are always heterojunction structure, which are rather complicated systems for their different electron affinities, band gaps.e band offset and interface properties of a heterostructure are some of the most important properties.Sheng et al. have studied the n-layer/transparent conducting oxide (n/TCO) interfaces in amorphous silicon (a-Si:H) and microcrystalline silicon (mc-Si:H) materials by XPS [9].Liu et al. studied the interface properties and band alignment of Cu 2 S/CdS heterojunction, and the band offsets are obtained [10].Horn studied electronic structure at the interface, relating to band bending and the evolution of transport barriers such as the Schottky barrier and the heterojunction band offset [11].Bernède and Marsillac.have measured the band offsets of SnO 2 /-In 2 Se 3 heterojunction by XPS and estimated the conduction band discontinuity Δ  to be −0.3 ± 0.3 eV [12].Mönch discussed the electronic properties and chemical interactions at GaAs (110) and InP (110) surfaces [13].
In this present work, ITO, ZnO, and SnO 2 �lms have been successfully prepared on ITO coated glass substrate by DC-and RF-magnetron sputtering and characterized by XRD, UV/Vis spectra, and four-probe apparatus.UPS and XPS were used to characterize the band offset of ITO/CdS, e TCO/CdS structure was analyzed by X-ray diffraction (DX-2500, Dandong Fangyuan Instrument LLC) using Cu Κa radiation (  00 nm).e sheet resistance was measured with a Digital Four-Probe Tester (SZT-2, Suzhou Tongchuan Electronics).e thickness of each �lm was measured with a stylus pro�ler (XP-2, Ambios Technology Inc.).e optical transmission was measured by UV/Vis spectrometer (Perkin Elmer Inc., Lambda-950).e XPS and UPS were measured by the multifunctional X-ray Photoelectron Spectroscopy (AXIS Ultra DLD , Kratos Analytical Inc.) with the base pressure ∼5.0 ×0 −9 Torr, the X-ray of Al K  , and the X-ray tube power of 130 W. e samples were etched by 21.2 eV He + beam and were biased with 7.36 volts to obtain reproducible cut-off results.e work functions were determined from the low-kinetic energy cut-off in the UPS spectra; that is, the intersection of the linear extrapolation with the baseline.In this experiment, samples were cleaned and thinned by sputtering with He + ions in HUV.
AMPS-1D has been employed to model and analyze the CdTe solar cells, and the different cells con�guration is shown in Figure 1.e parameters used in the simulation are shown in Table 1.e electron affinity energies and mobility were obtained from [15][16][17].e thickness of CdTe �lm was set as 6 m.

Results and Discussion
3.1.XRD. Figure 2 shows XRD patterns of ZnO and SnO 2 �lms deposited on ITO �lms.e main diffraction peaks (400), (440), and (222) and so on come from the ITO �lms.Only one weak peak of ZnO was observed in the spectra labeled as (101), and one weak peak of SnO 2 was observed labeled as (200) at 360 ∘ diffraction angle.is indicates that the ZnO and SnO 2 �lms have been deposited successfully onto the ITO �lms.

3.2.
Transmittance. e optical and electrical properties of ITO, ITO/ZnO, and ITO/SnO 2 �lms were measured.e thickness was 400 nm for ITO �lms and 150 nm for ZnO and SnO 2 �lms.Figure 3 shows the optical transmission of the asdeposited ITO/ZnO and ITO/SnO 2 �lms.At the wavelength from 500 nm to 850 nm, the average transmittance is 82% for ITO/ZnO and 81.64% for ITO/SnO 2 �lms, respectively, which are not lower than ITO �lms (85%) too much.On the other hand, in the blue region, the red shis of the effective absorption edge of ITO/ZnO and ITO/SnO 2 �lms are clearly observable.e sheet resistance of the ZnO was obtained as 108 Ω/□ while 104 Ω/□ for the SnO 2 �lms, which are higher than ITO (13.2 Ω/□).e deposition of ZnO or SnO 2 �lms as high resistance transparent (HRT) on ITO  �lms can passivate the CdS surface than ITO, which could eliminate the leakage current caused by the pinhole effects of CdS [15] and thus improves the short circuit current remarkably.

XPS/UPS.
Before analysis with XPS/UPS, all of the samples were cleaned by sputtering with He + ions for 1 minute in HUV to eliminate surface effects.e layers were pro�led using XPS and UPS by taking spectra aer every pro�ling time intervals until it revealed to the ITO �lm by He + sputtering.F 4: UPS pro�ling spectra of ITO/CdS �lms.

ITO/CdS
System. Figure 4 shows the UPS of ITO/CdS structure at different times (9-12 min).e "valence band offset" corresponds to be (  −  VBM ) and provides a direct measure of the Fermi level at the sample surface.e results show that the valence band maximum (VBM) in the interface of ITO/CdS �lms increases from 2.14 eV to 3.10 eV.e  cutoff (secondary electron onset) is the abscissa value on the le side when the intensity is 0 in the later UPS, so the work function can be obtained by e ℎ is the excitation energy of Helium (21.2 eV), and the  Fermi is set to be 0, so the Φ would be concluded to be 4.7 eV for CdS and 4.2 eV for ITO.And the interface dipole  of ITO and CdS �lms can be obtained as 0.41 eV and 0.2� eV by     − VBM. ( e XPS at various times in Figure 5 shows the band energy variation at the interface of the ITO/CdS �lms.e pro�ling of samples starts CdS �lms and ends at 46 min, when the intensity peak of Cd3d is zero.e O1s emission gradually increases in intensity and the Cd3d decreases during the pro�ling process.At 1-9 min the intensity of O1s is weak and the B.E. is of no change because the content of oxygen atoms is very lower when the CdS �lms have been etched for 1 min.At 9-11 min, the intensity of O1s increases while the Cd3d decreases and the B.E. of Cd3d increases about 0.14 eV.at is to say, that the surface pro�ling occurs at 9-11 min.e thickness of CdS �lms is 150 nm, so the pro�ling speed is estimated as 15 nm/min.e intensity of Cd3d decreases gradually and passes off at 46 min.e results show a shi in all the core level lines to larger binding energies, which indicates the formation of a space charge layer (band bending) in the substrate.
In order to construct the band energy diagram, the position of   within the bulk must be known.e difference between the vacuum level Δ vac can be obtained by subtracting the overall band bending from the difference of the work functions by ( is electron affinity) Δ vac   2 +  2  −  1 +  1   0.56 eV.
(3) en the valence band offset Δ  can be determined by using the band energy difference between the O1s and Cd3d at intermediate coverage and the binding energies of the core levels with respect to the valence band.One has Using the Δ  and the band gaps given before, the conduction band offset Δ  was calculated to be Δ  = Δ vac +  1 −  2  = 0.9 eV. ( Having determined band bending, band offset, and interface dipole, the �nal band alignment at the interface ITO/CdS heterojunction is presented in Figure 6.e conduction band bends upward in the ITO layer at the surface while the CdS layer bends downward.e electrons need to overcome a huge energy barrier (about 0.9 eV) when transferring from the CdS to ITO �lms.e �PS at various pro�ling times in Figure 8 shows the variations in the band energy of the ITO/ZnO �lms at the interface (20�23 min).e pro�ling of samples starts ZnO �lms and ends at 61 min.e In3d (ITO) emission gradually increases in intensity and the Zn2p (ZnO) intensity decreases along the pro�ling progress.e thic�ness of ZnO �lms is 150 nm, so the pro�ling speed is estimated as 7 nm/min, which is obviously lower than CdS.And Δ vac = 0.05 eV was obtained and the valence band offset Δ  and conduction band offset Δ  were calculated to be 0.4 eV and 0.05 eV, respectively.

ITO/ZnO System. e characterization of the layer at different pro�ling times is illustrated in
e �nal band alignment at the interface ITO/ZnO heterojunction is presented in Figure 9. e conduction band bends downward in the ITO layer while ZnO layer bends upward at the interface.e barrier energy is about 0.4 eV, which is lower than ITO/CdS heterojunction potential barrier.at is to say the introducing ZnO �lm is bene�cial for the transfer and collection of electrons.

ITO/Sn𝑂𝑂
2 System.e interface pro�ling from 9 to 20 min is presented in Figure 10.At the interface of ITO/SnO 2 , the VBM value decreases from 3.38 eV to 3.28 eV.e Φ of SnO 2 �lms would be calculated to be 4.14 eV and 4.06 eV for ITO �lms.And the interface dipole  of ITO and SnO 2 �lms can be obtained as 0.44 eV and 0.73 eV.
e �PS at various pro�ling times is showed in Figure 11.e pro�ling of samples starts SnO 2 �lms and ends at 42 min, when the intensity of Sn3d passes off.It con�rms that the In3d (ITO) emission increases in intensity and the Sn3d   (SnO 2 ) decreases during the pro�ling process.At 20�23 min, the pro�ling came to the interface, and the pro�ling speed is estimated as 7 nm/min and the thickness of SnO 2 �lm is 150 nm.e adding of SnO 2 �lms between ITO and CdS �lms also changes the energy structure.e Δ vac = 0.13 eV, Δ  = 0.44 eV, and Δ  = −0.06eV were also obtained.e energy band diagram is presented in Figure 12. e conduction band bends downward in the ITO layer at the surface while SnO 2 layer bends upward.e results shows that electrons must overcome an energy barrier (0.44 eV) when transferring from the SnO 2 to ITO �lms, which is also less than that in the ITO/CdS heterojunction.us adding SnO 2 layer is also bene�cial for the transfer and collection of electrons.

Device Simulation.
Based on the previous analysis, we have simulated the effect of ITO/ZnO and ITO/SnO 2 �lms in CdTe cells by AMPS-1D. Figure 13 shows the electric �elds, dark I-V curves, and simulated output performance of different cells.e results show that inserting of ZnO or SnO 2 �lms changes the electric �eld distribution, with the electric �eld strength decreasing at the ITO/CdS interface and a new electric �eld appearing at the ZnO (or SnO 2 )/CdS interface.ese electric �eld distributions are bene�cial for the transfer and collection of electrons in CdTe cells.e introduction of ITO/ZnO or ITO/SnO 2 �lms in CdTe solar cells improves the efficiency ( ff ), open voltage ( oc ), and short circuit current ( sc ) signi�cantly.
Also, we fabricated CdTe solar cells with or without HRT �lms.e cells with ZnO �lms have the efficiency of 12.17% ( oc = 742 mV,  sc = 26 mA/cm 2 , FF = 62.6%, and area = 0.5 cm 2 ) and the sample with SnO 2 �lms has the efficiency of 11.4% ( oc = 724 mV,  sc = 25.8 mA/cm 2 , FF = 61.2%,and area = 0.5 cm 2 ), while the sample without HRT layers has a much lower efficiency of 8.7% ( oc = 689 mV,  sc = 23.55 mA/cm 2 , FF = 53.6%,and area = 0.5cm 2 ).e results also show that the introduction of HRT layers decreases the series resistance, which was partly attributed to good interface properties between HRT and CdS layers.

Conclusion
e ITO/ZnO and ITO/SnO 2 �lms were successfully deposited on a glass substrate by DC-and RF-magnetron sputtering.e optical transmittance of the ITO/ZnO and ITO/SnO 2 as complex TCO layers was 82% and 81.64% from 500 nm-850 nm, respectively.e measured sheet resistances of ITO/ZnO and ITO/SnO 2 layers were 10 5 Ω/□ and 37.5 Ω/□, respectively.e interface compositions of the TCO layers were characterized by UPS and XPS, and the energy band diagrams were determined.e energy barriers at the interface of ITO/ZnO and ITO/SnO 2 layers are almost 0.4 and 0.44 eV, which are lower than those at ITO/CdS heterojunctions (0.9 eV).e ITO/ZnO and ITO/SnO 2 as complex transparent conductive bene�t the transfer and collection of electrons in CdTe solar cells and reduce the minority carriers recombination at the interface, compared to CdS/ITO.Furthermore, we have also simulated and analyzed the effects of the ITO/ZnO and ITO/SnO 2 �lms on CdTe cells by AMPS-1D.e results show that the electric �eld distribution changes a lot by the introduction of ZnO and SnO 2 �lms between ITO and CdS.e  ff , FF,  oc , and  sc are improved signi�cantly, that is to say, the ITO/ZnO and ITO/SnO 2 complex transparent conductive layers are bene�cial for the performance of CdTe solar cells.

F 5 : 2 F 6 :
XPS spectrum of the O1s and Cd3d reigns for n-ITO/n-CdS isotype heterojunction at various pro�ling times.Energy band diagram of the ITO/CdS isotype heterojunction.

F 8 :F 9 :
�PS spectrum in the In3d and Zn2p reigns for n-ITO/n-ZnO isotype heterojunction under various pro�ling time.Energy band diagram of the ITO/ZnO isotype heterojunction.
Figure 7. e VBM increases from 3.11 eV to 3.21 eV at the interface of the ITO/ZnO �lms.And the Φ would be calculated to be 3.8 eV for ZnO and 4.1 eV for ITO.And the interface dipole  of ITO and ZnO �ms can be obtained as 0.51 eV and 0.16 eV.

F 10 :F 11 :
e UPS spectra of ITO/SnO 2 complex layers.XPS spectrum in the In3d and Sn3d regions of an n-ITO/n-SnO 2 isotype heterojunction at �arious pro�ling times.

F 13 :
e simulated output performance for different structure: the electric �eld distribution (a), the dark IV curve (b),  sc and  ff , (c) and FF &  oc (d).
T 1: e parameters used in the AMPS-1D simulation.
Energy band diagram of the ITO/SnO 2 isotype heterojunction.