Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact

Accepted Manuscript Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact Eun Seok Cha, Young Min Ko, Seon Cheol Kim, Byung Tae Ahn PII:

S1567-1739(16)30289-9

DOI:

10.1016/j.cap.2016.10.014

Reference:

CAP 4349

To appear in:

Current Applied Physics

Received Date: 18 August 2016 Revised Date:

18 October 2016

Accepted Date: 20 October 2016

Please cite this article as: E.S. Cha, Y.M. Ko, S.C. Kim, B.T. Ahn, Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact, Current Applied Physics (2016), doi: 10.1016/j.cap.2016.10.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact

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Eun Seok Cha, Young Min Ko, Seon Cheol Kim, Byung Tae Ahn* Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea *[email protected] Key words: CdTe solar cells, ZnO buffer layer, Cu solution contact, short circuit current, quantum efficiency.

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ABSTRACT

Conventional CdTe solar cells have a CdS window layer, in which an absorption loss of

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photons with more than 2.4 eV occurs through the CdS layer. A thinner CdS layer was applied to enhance light transmission and a ZnO buffer layer with a band gap of 3.3 eV was introduced to suppress shunting through the thinner CdS window layer. A 100-nm thick ZnO layer sputter-deposited at 300°C had uniform coverage on a transparent conductive oxide

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(TCO) after a subsequent high-temperature process. The ZnO layer was effective in preventing shunting through the CdS window layer so that the open-circuit voltage and fill factor of the CdTe solar cells were recovered and the short-circuit current was enhanced over

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that of the conventional CdTe solar cell. In the ZnO/CdS/CdTe configuration, the short-circuit current was further improved throughout the visible wavelength region by replacing the Cu-

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metal contact with a Cu solution contact. As a result the short-circuit current from 21.7 to 26.1 mA/cm2 and the conversion efficiency of the CdTe solar cell increased from 12 to 15% without antireflective coating. Our result indicates that the Cu solution back contact is a critical factor for achieving a higher cell efficiency in addition to ZnO buffer layer.

ACCEPTED MANUSCRIPT 1. Introduction Conventional CdTe cells consist of a CdS/CdTe heterojunction in which CdS is a window material and CdTe is a light absorbing material. Since the band gap of CdS is 2.4 eV

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and the thickness of CdS is around 200 nm, incident light with photon energies of more than 2.4 eV is absorbed and lost in the CdS window layer. As a result, the blue light is lost and the light-generated current is limited. In order to reduce absorption loss in the blue wavelength

increase the band gap of the window layer.

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region by the window material, it is necessary to reduce the thickness of the CdS layer or

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The increase in the band gap of the window layer was achieved by changing the composition of the window layer to CdS:O [1,2]. However, with a thinner window layer, it was necessary to introduce a buffer layer such as SnO2 and ZnSnO4 between the transparent conducting oxide (TCO) and CdS to avoid junction shunting [1-3]. Previously, we used In2S3

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as a buffer layer and reported that the cell performance was better in the TCO/CdS/In2S3/CdTe structure than in the TCO/In2S3/CdS/CdTe structure [4]. Our result indicated that the band alignment at the TCO/CdTe interface dominated the good lattice

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match of the CdS/CdTe interface. Even though recent progress by GE and First Solar reported above 20% cell efficiency, little information on the structure of the cell was known

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[5]. As a result, scientific and technological progress from the academic side has so far been very limited. To achieve theoretical efficiency of above 25%, it is still necessary to correctly understand the physics behind the CdTe solar cell that has a much lower efficiency. With a thinner CdS window layer, shunting can occur between TCO and CdTe by tunneling through the thin CdS layer, resulting in a lower open-circuit voltage (Voc) [6,7]. Therefore, to avoid shunting of the thin CdS layer, a buffer layer such as SnO2 and Zn2SnO4 has been considered [1-3]. In our experiment, we selected ZnO as the buffer layer for CdTe cells to increase the short circuit current (Jsc) of CdTe cells. The ZnO buffer was first applied

ACCEPTED MANUSCRIPT to CdTe by Perrenoud et al. [6,8] and the ZnO buffer demonstrated an increase in Jsc only in the blue wavelength region, not in the red wavelength region. The performance degradation by Cu contamination is well explained in literature [9,10].

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We first reported that Jsc value was strongly suppressed by the screening of light by a CdS layer with an 803 nm absorption level due to Cu contamination [11]. In other words, the portion of Jsc in the red wave region is limited by Cu contamination in CdS. Therefore, it was

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necessary to reduce Cu contamination of CdS during the formation of the Cu back contact. For that purpose, we needed to supply less Cu on the CdTe surface to an amount as small as

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possible. In this study, we applied a Cu solution coating process to the CdTe surface to replace the conventional Cu metal deposition.

Here we report a systematic study of CdTe cell fabrication with the application of a ZnO buffer layer in combination with a thin CdS layer to improve the blue wave response and the

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application of a Cu solution back contact to improve the red wave response. We found that the combination of a ZnO buffer and CdS window enhanced the short-circuit current of CdTe

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cells and that a Cu solution back contact replacing a Cu metal back further increased the Jsc.

2. Experimental details

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CdTe solar cells were fabricated in a TCO/ZnO/CdS/CdTe superstrate configuration. A textured fluorine-doped tin oxide (FTO) coated borosilicate glass with a size of 2x5 cm2 was used as a substrate. Substrates were cleaned ultrasonically in acetone and ethanol for 15 minutes each and dried in a forced convection oven at 70°C for 15 minutes. A ZnO buffer layer was deposited on the FTO substrate by radio frequency (RF) sputtering of a 4-inch ZnO target at a sputtering power of 135 W. The base pressure and working pressure were 2x10-6 and 2x10-3 Torr, respectively. The substrate temperature for the ZnO sputtering process was varied. Then, a CdS layer was deposited on the ZnO-coated sample by immersing the

ACCEPTED MANUSCRIPT substrate into a reaction beaker filled with distilled water, Cd(CH3COO)2, NH4(CH3COO), and CH4N2S and by increasing the reaction beaker to 85°C. The thickness of the CdS layer was varied by controlling the reaction time.

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The CdS/ZnO-coated sample was inserted into a close-space sublimation (CSS) chamber, in which the distance between the CdTe source plate and the sample plate was 2 mm. The CSS chamber was pumped down to 10 mTorr and filled with oxygen to 5 Torr. The source

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temperature and sample temperature were 620 and 575°C, respectively. A 4-µm-thick CdTe layer was deposited with a growth rate of about 1 µm/min. After CdTe deposition, the sample

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was dipped into a CdCl2 solution and dried immediately with an N2 blow gun. Then the sample was annealed at 380°C in a chamber filled with 80% He+20% O2 for 10 minutes. Following the CdCl2 heat treatment, residual CdCl2 on the CdTe surface was rinsed off. A p+ back contact was formed by depositing 1-nm thick Cu metal on the CdTe surface, which was etched by a nitric phosphoric (NP) acid and by annealing at 220°C for 15 minutes.

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After the p+ contact process, a mechanical scribing was performed to make cell outlines and to expose the FTO front contact. Then a 100-nm thick Au layer was deposited by thermal

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evaporation with a deposition rate of 2Å/s. Finally, second scribing was performed within the previously defined cell outline. Four CdTe solar cells with a size of 1x0.5 cm2 were

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fabricated for one glass substrate. The formation of the p+ back contact with a Cu solution is described in the paragraph that describes Fig. 11. The morphology and layer thickness of ZnO and CdS films were measured using a scanning electron microscope (SEM). The surface morphology was examined with an atomic force microscope (AFM) and the crystal quality of the ZnO film was investigated using x-ray diffraction (XRD). The photovoltaic properties of the CdTe solar cells were measured using a solar simulator at an AM 1.5 one sun condition that was calibrated with a silicon reference cell. The external quantum efficiency (EQE) was measured using a QEX10 solar-cell

ACCEPTED MANUSCRIPT quantum efficiency (QE) system, produced by PV measurement Inc.

3. Results and discussion

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3-1. Effect of ZnO buffer layer Figure 1 shows a typical cross-sectional SEM morphology of CdTe solar cells in a ZnO/CdS/CdTe configuration. The CdTe and FTO layers are clearly seen while the ZnO and

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CdS layers are not clearly seen in Fig. 1 because those layers are too thin.

Figure 2 shows the changes in photovoltaic properties: the open-circuit voltage (Voc),

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short-current (Jsc), fill factor (FF), and cell efficiency (η) of the conventional CdS/CdTe solar cells with various CdS thicknesses. The reference CdTe cell with a 200-nm-thick CdS has 11.93% cell efficiency without antireflective coating (ARC). The Voc, Jsc, and FF of the cell were 0.81 V, 21.68 mA/cm2, and 67.69%, respectively. The shunt resistance (Rsh) and series

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resistance (Rs) of the reference cell are 2,110 and 1.6 Ω·cm2, respectively. Compared to the reference cell, the Voc value of the cell dropped little as the CdS thickness decreased to 90 nm, and then it dropped fast to 0.47 V when the CdS thickness was near 40 nm. Our result

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showed that a tunneling recombination through CdS layer began when the CdS thickness was around 70 nm. The result is in good agreement with the literature [7].

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The Jsc increased little as the thickness of CdS decreased to 90 nm, and it increased fast below 90 nm CdS. The optical transmittance, T(λ), of a photon with an energy larger than 2.4 eV is expressed as

T(λ)=Io(λ) [1-R(λ)] exp(-α(λ)·d)

(1),

where Io(λ) is the incident beam intensity, R(λ) the reflectivity, α(λ) the absorption constant, and d the thickness of CdS layer. It is seen that this relation is effective when the thickness of the CdS layer is below 90 nm. The FF degraded more sensitively on the thickness of CdS layer than Voc or Jsc. The FF

ACCEPTED MANUSCRIPT degraded significantly even with a 90-nm-thick CdS layer. The FF value is affected by Rsh and Rs with the following relation: FF/FFo=1-JscRs/Voc-Voc/JscRsh,

(2),

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where FFo is the value with Rs=0 and Rsh=∞ Ω·cm2. The terms JscRs/Voc and Voc/JscRsh are the FF loss ratio by the Rs and Rsh, respectively, and the term JscRs/Voc+Voc/JscRsh is the total FF loss ratio by Rs and Rs with respect to FFo. The Rsh values of the CdTe cell with 40, 70, and 90-nm-thick CdS layers were 185, 247, and 943 Ω·cm2, respectively, and the Rs values were

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2.2, 1.6, and 2 Ω·cm2, respectively. With the Rsh and Rs values, the total FF loss ratio of the

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CdTe cells with 200, 90, 70 and 40-nm thick CdS layers were 6, 10, 18, and 21%, respectively. In our calculation, the lower Rsh value with a thinner CdS layer critically affected the degradation of FF instead of the Rs value. Therefore, we concluded that the Rsh value is one of the key electrical parameters that can confirm the sturdiness of a junction. The

by absorbing blue waves.

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Rs value is nearly the same regardless of CdS thickness because the CdS layer is conductive

The small Rsh of the cell with a thinner CdS may have originated from the tunneling

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recombination of the electrons at the conduction band of FTO and the holes at the valence band of CdTe through the very thin CdS layer. Another possibility is the direct contact

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between FTO and CdTe due to poor step coverage of CdS on FTO. Therefore, it was necessary to add a ZnO buffer to prevent the tunneling through the CdS window layer. With this understanding we moved to investigating the ZnO buffer. Figure 3 shows the reproducibility of the photovoltaic properties of ZnO/CdS/CdTe solar cells with various sputter-deposition temperatures of the ZnO buffer layer. The thicknesses of ZnO and CdS were 100 and 70 nm, respectively. The deposition temperature of the ZnO layer was varied to room temperature, 100, 200, and 300°C. Four sets of the same experiment were conducted to confirm the reproducibility of our experiment. This kind of reproducibility

ACCEPTED MANUSCRIPT result has not been reported even though the application of ZnO as a buffer layer was reported in the literature [6,8]. With the CdTe cell with a ZnO layer deposited at room temperature, 100, and 200°C, the photovoltaic parameters including Voc, Jsc, and FF showed

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an extreme scattering from 0 to 13.5%. In particular, the Voc and FF values experienced a very large scattering, even at 200°C. At a 300°C deposition temperature, no significant scattering of Voc and FF were observed, which resulted in a reproducible efficiency, ranging

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from 12 to 13%. We then investigated the origin of such scattering using a cross-sectional image of the FTO/CdTe interface.

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To understand the interface morphological difference at the CdTe/FTO interface, the surface morphology of the ZnO film with two different deposition temperatures was analyzed using SEM and AFM. The AFM surface morphologies of the ZnO deposited at RT and at 300°C are very similar each other. No significant difference in surface morphology of the

not shown here.

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ZnO layer with different deposition temperatures was noticed from AFM images which were

Figure 4 shows the SEM images of the FTO/ZnO/CdS/CdTe interface with a ZnO buffer

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deposited at room temperature (a) and 300°C (b) after the CdTe deposition process. The thicknesses of ZnO and CdS were 100 and 70 nm, respectively. In Fig. 4a, no clear interlayer

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was observed between CdTe and FTO and also many voids along the CdTe/FTO interface were observed. This suggests that ZnO/CdS are intermixed and agglomerated. Wu at al. reported an intermixing with a similar ZnSnO4/CdS bilayer [12]. The void might have been generated by the volume shrinkage of the ZnO film which was in an amorphous state at room temperature. In Fig. 4a, direct contact between FTO and CdTe is possible at some point, which results in an extremely low Voc in Fig. 2. On the other hand, a darker interlayer with a continuous and uniform thickness is clearly seen between CdTe and FTO in Fig. 4b. A few voids were still observed along the boundary but the interlayer is continuous even near the

ACCEPTED MANUSCRIPT region with voids. With the continuous interlayer, the cell efficiency was reproducible with more than 13%. In our experiment, we confirmed that there was a uniform and continuous layer with ZnO deposition at 300°C. The quantum efficiency curve shown later in this paper

optical absorption starting point by CdS was the same.

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indicated that intermixing of ZnO and CdS was negligible because the wavelength of the

Figure 5 shows the XRD patterns of the ZnO films deposited on a FTO substrate at room

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temperature, 200, and 300°C. The peaks at 2θ=33.6 and near 34.3° are the FTO (101) peak and the ZnO (002) peak, respectively. The 2θ values of the ZnO film deposited at room

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temperature, 200, and 300°C are 34.2, 34.3, and 34.4°, respectively. The shift of 2θ toward a lower angle and smaller diffraction intensity at room temperature and 200°C indicate that the ZnO structure deposited at lower temperature is more open and disordered than that deposited at 300°C. In particular, the strong peak intensity at 300°C indicates that the crystallinity of

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ZnO film deposited at 300°C is much better than those of ZnO films deposited at room temperature and 200°C. In other words, the crystallinity of ZnO deposited up to 200°C is poor and is more vulnerable to the chemical etching in a basic solution with the PH=10.5

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during a CBD process. Furthermore, the film is vulnerable to intermixing of ZnO/CdS by the subsequent CdTe deposition process at 600°C and the CdCl2 annealing process at 400°C.

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These results indicate that a good crystallinity of the ZnO layer is a key factor in maintaining a uniform and stable interlayer at the FTO/CdTe interface during the subsequent hightemperature process. We have seen that a reliable and reproducible result can be obtained with a ZnO deposition temperature of 300°C. At a ZnO deposition temperature of 300°C, the thickness of ZnO was varied in the FTO/ZnO/CdS/CdTe configuration to find an optimized ZnO thickness for CdTe solar cells. Figure 6 shows the photovoltaic properties of the CdTe solar cells in a ZnO/CdS/CdTe configuration. The ZnO was deposited at 300°C with various thicknesses and the CdS

ACCEPTED MANUSCRIPT thickness was fixed at 90 nm. The Voc value of the CdTe cell with a 100-nm thick ZnO increased to 0.82 V, which is even larger than 0.81 that can be obtained from the CdTe solar cell with a conventional 200-nm thick CdS window layer. The FF value also recovered to the

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conventional value of 0.68 when the ZnO thickness was 100 nm. The Rsh of the CdTe cell with 0, 35-, 60-, and 100-nm thick ZnO buffer were 943, 1183, 1702, and 1738 Ω·cm2, respectively. The Rsh value increased as the ZnO thickness increased. These results showed

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that the ZnO buffer prevented the tunneling recombination between FTO and CdTe and a sturdier junction formed with a thicker ZnO layer. The Jsc value of the CdTe cell with a 100-

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nm thick ZnO buffer layer was also the same value as that of the conventional CdTe cell with 200-nm thick CdS only. Our results demonstrated that employing a 100-nm thick ZnO layer deposited at 300°C completely recovered the CdTe properties with a 90-nm thick CdS layer. In other words, a 100-nm thick ZnO is necessary to maintain the Voc and FF in the

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ZnO/CdS/CdTe configuration. With a 100-nm thick ZnO buffer, we changed the thickness of the CdS window layer to find the optimum CdS thickness. Figure 7 shows the photovoltaic properties of the CdTe solar cells in ZnO/CdS/CdTe

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configuration with 100-nm thick ZnO and various CdS thicknesses. The reference cell is a CdS/CdTe cell with 200 nm CdS thickness. The highest cell efficiency of 13.5% with

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Voc=0.79V, Jsc=25.1mA/cm2, and FF=0.68 was achieved with a 70-nm thick CdS layer. We had a 1.6% point efficiency gain compared to that of the conventional cell. The main contribution of the cell efficiency is the increase of Jsc from 21.7 to 25.1 mA/cm2. Figure 8 shows the QE curve of the CdTe cells with a 100-nmk thick ZnO buffer and a 70-nm thick CdS window. The origin of the Jsc increase can be analyzed from quantum efficiency (QE) curves. It is seen that the QE values in the blue wave and UV region (marked as ①) became much larger due to the increased optical transmittance through the thinner CdS layer. A slight increase in the QE value in the range of 550 to 750 nm (marked as ②)

ACCEPTED MANUSCRIPT was also observed due to less light scattering through the thinner CdS layer. The light scattering event by the grain boundaries in CdS layer will be reduced as the thickness of CdS

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layer decreases.

3-2. Effect of Cu solution contact

However, little enhancement in the QE value was seen near the wavelength of 800 nm or

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the IR region, marked as ③ in Fig. 8. The biggest issue in the CdTe solar cell was development of a stable p+ back contact. The most suitable method is Cu doping on CdTe, but

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the CdTe cell is easily degraded by Cu contamination at the CdS/CdTe junction [10, 13-15]. Degradation of the cell has several reasons: shunting at the CdS/CdTe interface [13] and light shielding through an impurity level in the CdS layer for a photon energy of 1.55 eV (λ=803 nm) [11]. With the Cu contamination, the QE value near the 800 nm wavelength, marked as

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③, was drastically reduced [11,15,16], which supported the light shielding by Cu contamination in CdS. Therefore, it is imperative to reduce the Cu contamination to further increase the QE value near the 800 nm wavelength region. For that purpose, the total amount

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of Cu in the back contact source should be as low as possible, while the Cu concentration at the back surface should be as high as possible for the p+ contact.

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The standard contact method is as follows. The CdTe surface was etched by a NP acid, a thin Cu metal layer was deposited on it, and the contact was formed by annealing. To avoid the Cu accumulation in CdS, it was necessary to minimize the Cu supply from the back contact. One of our previous techniques for this purpose was the application of Cu2-xTe on CdTe, in which less Cu was supplied from the back contact source [17]. However, the reproducibility of the back contact was poor. An alternative was to apply a Cu solution on the CdTe back contact [10]. Here, we selected Cu-solution back contact to minimize Cu contamination and applied to the optimized ZnO/CdS/CdTe configuration.

ACCEPTED MANUSCRIPT Figure 9 shows the difference in the SEM surface morphologies of CdTe film with and without NP acid etching. With NP acid etching, the pinhole and grain boundaries were well delineated so the possibility of Cu contamination through the pinholes and grain boundaries

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was high. On the other hand, no NP acid etching was necessary for the Cu solution deposition method so the possibility of Cu contamination through pinholes of grain boundaries was low. Figure 10 shows the photovoltaic parameters of CdTe solar cells at various annealing

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temperatures with a Cu-solution coating on the as-deposited CdTe. The thickness of ZnO and CdS were 100 and 70 nm, respectively. The annealing time was 10 minutes. It is seen that the

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optimum annealing temperature and time for Cu solution contact were 250°C and 10 min. At the optimum condition, Voc, Jsc, and FF were highest. With an increasing annealing time at 250°C, the cell performance degraded. The main parameter that affected the degradation was Jsc. This indicates that the Jsc is very sensitive to the Cu diffusion from the back contact to the

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front junction. In a severe environment, A CdTe PV module might be degraded after long time. For CdTe production it might be necessary to develop a Cu diffusion barrier layer. Figure 11 shows the illuminated J-V curves of the CdTe cells with a 100-nm thick ZnO

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buffer and 70-nm thick CdS window. In comparison, the cell with a Cu metal back contact and the cell with a conventional CdS buffer are also demonstrated. The photovoltaic

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parameters of the cells are summarized in Table 1. The first notable result is the increase of Jsc from 25.1 to 26.1 mA/cm2 by replacing the Cu metal back contact with a Cu solution back contact. The second noticeable result is the increase of Voc from 0.70 to 0.82 V and the increase of FF from 0.67 to 0.70. As a consequence, the cell efficiency increased from 13.5 to 15.1% without ARC.

Table 1. PV parameters of the CdTe cell with ZnO (100 nm)/CdS (70 nm) buffer. ZnO enhanced the Jsc and the Cu solution contact further increased all parameters.

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Jsc

FF

η

Rsh

Rs

(V)

(mA/cm2)

(%)

(%)

(Ω·cm2)

(Ω·cm2)

Cu metal

0.81

21.7

67.7

11.9

1018

1.56

ZnO/CdS

Cu metal

0.79

25.1

66.7

13.5

614

1.79

ZnO/CdS

Cu solution

0.82

26.11

70.39

15.1

950

1.25

Back contact

CdS

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Buffer

Figure 12 shows the QE curve of the CdTe cells with a 100-nmk thick ZnO buffer and 70-nm thick CdS window. In comparison, the cell with the Cu metal back contact and the cell with a conventional CdS buffer are also demonstrated. The QE values significantly increased

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in a wide range of spectrum from 350 to 810 nm by changing the metal contact technology. In

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particular, the QE value near the 800 nm wavelength region, marked as ③, increased greatly with the Cu solution back contact. It is clear that the Jsc is very sensitive to the Cu contact technology, suggesting that the Cu contamination in CdTe and CdS should be minimized to achieve a higher Jsc value.

The increase of overall QE values in the spectrum range of 350 to 810 nm can be

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explained as two ways: less shunting and less light shielding. First, we assume Cu atoms can diffuse through the interstitial site in CdTe and they stay at the interstitial sites as a donor. In

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that case, the CdTe will be less p-type. In other words, CdTe becomes more p type by avoiding Cu diffusion, resulting in the less shunting [13]. Second, Cu diffusion generates an

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impurity level in the CdS layer for a photon energy of 1.55 eV (λ=803 nm) and reduces transmittance through the CdS layer [11]. With less Cu contamination, the impurity level in CdS might be suppressed.

The QE curve suggests that a Jsc value of 26 mA/cm2 is the limit of a cell with a ZnO/CdS/CdTe configuration. To achieve a higher Jsc, the absorption loss by the thin CdS should be eliminated by replacing the CdS layer with a wide ban gap window material [18]. Our cell is vulnerable to Cu contamination. In the future, we may need to introduce a Cu diffusion barrier [19] or passivate the grain boundary [20] or develop s new contact with no

ACCEPTED MANUSCRIPT Cu element [21].

4. Conclusions

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In order to increase the light transmission through the CdS window layer in CdTe solar cells, we reduced the thickness of CdS. However, a thinner CdS caused large shunting between FTO and CdTe, resulting in a Voc drop, even though the increase of Jsc was possible

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with a CdS layer below 90-nm thick. We introduced a resistive ZnO buffer layer with a band gap of 3.3 eV between FTO and CdS to suppress the junction tunneling. The ZnO buffer layer

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was deposited by a sputtering process, and we found that the crystallinity of ZnO is very important to obtaining a reliable cell efficiency. The ZnO film deposited at 300°C showed a continuous and uniform interlayer after a high temperature CdTe deposition process. As a result the cell performance was reproducible.

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The optimum thickness of the ZnO buffer and CdS window were 100 and 70 nm, respectively. With the optimum thickness the cell efficiency increased from 11.9 to 13.5%. The main contribution of the increase was the increase of short circuit current from 21.7 to

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25.1 mA/cm2 by the increased light absorption in the blue wave and UV regions. However, little improvement was observed in the red wavelength region.

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By replacing the Cu metal contact on CdTe with a Cu solution contact, the cell efficiency was further increased to 15.06% with no ARC. With the Cu solution contact, Jsc, Voc, and FF improved. In particular, the QE values across the entire spectrum increased. As a result, the Jsc value increased from 21.7 to 26.1 mA/cm2 compared to the CdTe cell with only a CdS layer. Our approach demonstrated that the introduction of a ZnO buffer and a Cu solution back contact was quite successful in increasing the Jsc. However, our result also indicated that Cu contamination is very critical to enhancement of the cell performance. To further increase the efficiency of the cell, a Cu diffusion barrier or other contact with no Cu element should be

ACCEPTED MANUSCRIPT developed.

Acknowledgement

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This work was financially supported by the Korean Ministry of Science and Technology through the Climate Change Research Hub of KAIST (N01150136).

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References

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1. D. M. Meysing, C. A. Wolden, M. M. Griffith, H. Mahabaduge, J. Pankew, M. O. Reese, J. M. Burst, W. L. Rance, T. M. Barnes, Properties of reactively sputtered oxygenated cadmium sulfide (CdS:O) and their impact on CdTe solar cell performance, J. Vacuum Sci. Technol. A, 33, 021203 (2015).

2. X. Wu, J. Zhou, A. Duda, J. C. Keane, T. A. Gessert, Y. Yan, R. Noufi, 13.9%-efficient

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CdTe polycrystalline thin-film solar cells with an infrared transmission of similar to 50%, Prog. Photovol., 14, 471 (2006).

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3. K. Jeyadheepan, M. Thamilselvan, K. Kim, J. Yi, C. Sanjeeviraja, Optoelectronic properties of R-F magnetron sputtered Cadmium Tin Oxide (Cd2SnO4) thin films for

AC C

CdS/CdTe thin film solar cell applications, J. Alloys Compounds, 620, 185 (2015). 4. M. S. Kim, L. Larina, J. H. Yun, B. T. Ahn, Fabrication of CdTe solar cell using an Inx(OOH,S)y/CdS double layer as a heterojunction counterpart, Curr. Appl. Phys. 9, 455 (2009).

5. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Solar cell efficiency tables (Ver. 45), Prog. Photovolt. Res. Appl., 23, 1 (2015). 6. J. Perrenoud, L. Kranz, S. Buecheler, F. Pianezzi, A. N. Tiwari, Thin Solid Films, The use of Al-doped ZnO as transparent conductive oxide for CdS/CdTe solar cells, 519, 7444

ACCEPTED MANUSCRIPT (2011). 7. B. A. Korevaar, A. Halverson, J. Cao, J. Choi, C. Collazo-Davila, W. Huber, High efficiency CdTe cells using manufacturable window layers and CdTe thickness, Thin Solid

8.

T. Liu, X. He, J. Zhang, L. Feng, L. Wu, W. Li, G. Zeng, B. Li, Effect of ZnO films on CdTe solar cells, J. Semiconductors, 33, 093003 (2012).

I. Visoly-Fisher, K.D. Dobson, J. Nair, E. Bezalel, G. Hodes, D. Cahen, Factors affecting

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9.

RI PT

Films, 535, 229 (2013).

the stability of CdTe/CdS solar cells deduced from stress tests at elevated temperature,

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Adv. Funct. Mater. 13 (2003) 289 (2003).

10. B. A. Korevaar, R. Shuba, A. Yakimov, H. Cao, J. C. Rojo, T. R. Tolliver, Initial and degraded performance of thin film CdTe solar cell devices as a function of copper at the back contact, Thin Solid Films, 519, 7160 (2011).

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11. B. T. Ahn, J. H. Yun, E. S. Cha, K. C. Park, Understanding the junction degradation mechanism in CdS/CdTe solar cells using a Cd-deficient CdTe layer, Curr. Appl. Phys., 12, 174 (2012).

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12. X. Wu, S. Asher, D.H. Levi, D. E. King, Y. Yan, P. Gessert, P. Sheldon, Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline thin film solar

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cells. J. Appl. Phys. 89, 4564 (2001). 13. H. C. Chou, A. Rohatgi, E. W. Thomas, S. Kamra, A. K. Bhat, Effects of Cu on CdTe/CdS Heterojunction Solar Cells with Au/Cu Contacts, J. Electrochem. Soc., 142, 254 (1995).

14. D. Grecu, A. D. Compaan, D. Young, U. Jayamaha and D. H. Rose, Photoluminescence of Cu-doped CdTe and related stability issues in CdS/CdTe solar cells, J. Appl. Phys. 88, 2490 (2000). 15. V. Evani, M. Khan; S. Collins, V. Palekis, P. Bane, D. Morel, C. Ferekides, Effect of Cu

ACCEPTED MANUSCRIPT and Cl on EVT-CdTe solar cells, Proc. 42nd Photovolt. Special. Conf. (PVSC), 1-5, New Orleans (2015). 16. S. Demtsu and J. Sites, D. Albin, Role of Copper in the Performance of CdS/CdTe Solar

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Cells, Proc. 4th World Conf. Photovolt. Energy Conv. (WCPEC-4), 523-529, Waikoloa (2006).

17. J. H. Yun, K. H. Kim, D.Y. Lee, B.T. Ahn, Back contact formation using Cu2Te as a Cu-

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doping source and as an electrode in CdTe solar cells, Sol. Energy Mater. Sol. Cells, 75, 203 (2003).

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18. M. Gloeckler, I. Sankin, Z. Zhao, IEEE J. Photovolt., CdTe solar cells at the threshold to 20% efficiency, J. Photovolt., 3, 1389 (2013).

19. J. H. Yun, E. S. Cha, B. T. Ahn, H. S. Kwon, E. A. Al-Ammar, Performance improvement in CdTe solar cells by modifying the CdS/CdTe interface with a Cd

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treatment, Curr. Appl. Phys., 14, 630 (2014).

20. B. A. Korevaar, J. R. Cournoyer, O. Sulima, A. Yakimov, J. N. Johnson, Role of oxygen during CdTe growth for CdTe photovoltaic devices, Prog. Photovolt. Res. Appl., 22,

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1040 (2014).

21. K. C. Park, E. S. Cha, B. T. Ahn, Sodium-doping of ZnTe film by close-spaced

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sublimation for back contact of CdTe cell, Curr. Appl. Phys., 11, S109 (2011).

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thicknesses. Fig. 3. Variation of photovoltaic properties of ZnO/CdS/CdTe solar cells with various ZnO deposition temperatures. Thickness: ZnO 100 nm and CdS 70 nm.

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Fig 4. Cross-section SEM images of CdTe solar cell with ZnO deposited at room temperature (a) and 300°C (b). Thickness: ZnO 100 nm and CdS 70 nm.

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Fig. 5. XRD patterns of the ZnO films deposited on an FTO substrate at RT, 200°C, and 300°C.

Fig. 6. Photovoltaic properties of the CdTe solar cells in a ZnO/CdS/CdTe configuration. The ZnO was deposited at 300°C with various thicknesses and the CdS thickness was

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fixed at 90 nm.

Fig. 7. Photovoltaic properties of CdTe solar cells in a ZnO/CdS/CdTe configuration. The ZnO thickness is 100 nm and the CdS thickness is varied.

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Fig. 8. QE curves of the CdTe solar cells in a ZnO (100 nm)/CdS(70 nm)/CdTe configuration and the CdTe cell with 200-nm thick CdS as a reference cell.

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Fig. 9. SEM surface morphologies of CdTe film before and after NP etching. Fig. 10. Photovoltaic properties of CdTe solar cells in a ZnO/CdS/CdTe configuration after annealing the Cu solution back contact at various temperatures.

Fig. 11. Comparison of J-V of the CdTe solar cells in a ZnO (100 nm)/CdS(70 nm)/CdTe configuration with a Cu metal back contact and a Cu solution back contact. The data from a reference CdTe cell with 200-nm thick CdS is also included. Fig 12. Comparison of QE curves of the CdTe solar cells in a ZnO (100 nm)/CdS(70 nm)/CdTe configuration with a Cu metal back contact and a Cu solution back contact.

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CdTe solar cell has low efficiency due to the blue-wave light absorption by CdS and Cu contamination by Cu back contact. This paper systematically investigated by apply ZnO buffer in combination of thinner CdS layer and applying Cu solution back contact method. Frist, the short-circuit current was improved from 21.7 to 25.1 mA/cm2 by the thinner CdS while the open-circuit voltage was maintained as before by the ZnO buffer layer as seen in the blue QE curve in the figure below. The short-circuit current was further improved from 25.1 to 26.1 mA/cm2 by applying Cu solution contact in replacement of the Cu metal contact as shown in the red QE curve in the figure below. As result, the cell efficiency was improved from 12 to 15% without antireflective coating. The physics behind the improvement of the short-circuit current was discussed in the paper.

Prof. Byung Tae Ahn Deptartment of Maetrials and Engineering Korea Advanced Institute of Science and Technology (KAIST) [email protected] Tel: 82-42-350-4220, 82-10-9687-4220