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CdTe solar cells
Solar Energy Materials & Solar Cells 128 (2014) 411–420
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Te/Cu bi-layer: A low-resistance back contact buffer for thin film CdS/CdTe solar cells Wei Xia a,1, Hao Lin a, Hsiang Ning Wu a, Ching W. Tang a,b,c,n, Irfan Irfan b,c, Chenggong Wang b, Yongli Gao b a
Department of Chemical Engineering, University of Rochester, Rochester, NY 14627, United States Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, United States c Department of Chemistry, University of Rochester, Rochester, NY 14627, United States b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 March 2014 Received in revised form 19 May 2014 Accepted 3 June 2014 Available online 27 June 2014
A buffer layer based on a Te/Cu bi-layer useful for forming ohmic contact to p-CdTe has been developed for application in CdS/CdTe solar cells. The bi-layer buffer was prepared by vapor deposition and a thermal annealing ( 200 1C) was required for activation. Enhanced efficiency and stability were obtained by optimizing the Cu/Te compositions and the thermal activation conditions. Characterization by XRD, XPS, and PL indicates that under the thermal activation conditions Cu diffuses rapidly in the Te without forming CuxTe compounds. The enhanced stability can be attributed to the mediation of Cu diffusion into CdTe by the Te layer. The Te/Cu buffers are particularly useful for the fabrication of ultrathin CdS/CdTe solar cells. & 2014 Elsevier B.V. All rights reserved.
Keywords: CdTe Solar cell Back contact Stability Tellurium Copper
1. Introduction Among the hurdles to producing high-efficiency and long-life n-CdS/p-CdTe thin-film solar cells, the formation of an ohmic and stable back contact to p-CdTe remains the most challenging [1]. Due to the generally high resistivity and deep valence band edge associated with p-CdTe, it is practically impossible to achieve a good ohmic contact to p-CdTe with a metal including those with a high work function such as Au or Ni. A common approach to lowering the contact resistance is to create a high work function Te or Te-rich buffer layer on the CdTe surface prior to the deposition of the metallic back electrode. The buffer layer serves to align the work function of the back electrode closer to the valence band edge of p-CdTe and thus reduces the energy barrier for hole transport between the back electrode and p-CdTe. Chemically etching CdTe with bromine-methanol or nitric-phosphoric (NP) acid solution [2,3] has been found to be highly effective in producing such a Te-rich buffer layer, and good ohmic contacts have been achieved with common metals such as Ni or Au for the
n Corresponding author at: Department of Chemical Engineering, University of Rochester, Rochester, NY 14627, United States. Tel.: þ1 585 275 3552; fax: þ1 585 273 1348. E-mail address: [email protected] (C.W. Tang). 1 Defect Elimination, Samsung Austin Semiconductor, Austin, TX 78754, United States.
http://dx.doi.org/10.1016/j.solmat.2014.06.010 0927-0248/& 2014 Elsevier B.V. All rights reserved.
back electrode. However, there remain major drawbacks with the chemical method in producing the buffer layer including the need for a thick CdTe film to avoid the creation of electrical shunts from over etching in solution [4] and the incompatibility with the overall physical vapor deposition processes for the CdS/CdTe solar cell production. Alternative methods other than chemical etching have been used to produce ohmic back contacts for CdS/CdTe solar cells. Typically these methods involve the use of a semiconductor (MX) as the buffer layer between CdTe and the back electrode. CuxTe [5], ZnTe:Cu [6], NiTe2 [7], Sb2Te3 [8] and As2Te3 [9] have been found to be useful. Presumably they are capable of producing a lowresistance CdTe/MX tunnel junction by creating a heavily p-doped CdTe surface with Cu or Sb as the shallow acceptor [8]. In particular buffers based on CuxTe have been used to produce highly efficient CdS/CdTe solar cells. However, the detailed formulation of CuxTe buffer is critical in achieving an ohmic back contact that is also stable. Whereas Cu deficiency in CuxTe can lead to non-ohmic behaviors, an excess amount of Cu can cause fast device degradation [5,10]. To be effective, these semiconductor MX buffer layers are often introduced in addition to the Te or Te-rich buffer produced by chemical etching. Vapor-deposited Te layers have also been used as the back contact buffer in CdS/CdTe solar cells [11,12]. With HgTe-doped graphite as the electrode, Niles et al. [11] showed that vapordeposited Te films can be used to replace the chemically produced
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Te as the back contact buffer layer, resulting in solar cells with only slightly lower efficiencies. Uda et al. [12] used Cu as the back electrode along with evaporated Te as the buffer with the suggestion Cu could lower the resistivity of the CdTe layer. However, no device stability data was presented in these previous studies concerning the use of evaporated Te as the buffer layer. In particular, the possible interaction between Cu and Te and how such interaction can be used to control Cu diffusion into CdTe and thereby the device stability have not yet been sufficiently explored. In this work we present a more effective method for producing an ohmic and stable back contact to p-CdTe without chemical etching. The method utilizes a vapor-deposited Te/Cu bi-layer as the buffer, where the Cu to Te ratio is precisely controlled by the thicknesses of the Te and Cu layers. In addition, a thermal annealing step is used to activate the back contact after the cell is completed with a Ni back electrode. Based on this method, CdS/ CdTe solar cells with a power conversion efficiency up to 14.8% have been produced. Furthermore, efficiencies as high as 13.5% have been demonstrated in cells with an ultra-thin CdTe layer of only about 1 μm.
2. Experimental details The fabrication of thin-film CdS/CdTe solar cells has been described in detail in a previous paper [13]. Briefly, CdS films ( 200 nm) on fluorine-doped tin oxide (FTO) coated glass substrates were provided by Sunflux Inc. [14]. Prior to CdTe deposition, the CdS films were subjected to a vapor cadmium chloride (VCC) treatment [15] under the following conditions: the CdS coated substrates and the CdCl2 powder source were maintained in a close-space configuration at a temperature of 390 410 1C in an O2:N2 (1:4) ambient of 6.7 kPa for 6 min. CdTe films ranging from 1.0 to 5.0 mm were deposited by a close-spaced sublimation (CSS) method. Similarly, the CdTe films were also subjected to the VCC treatment for 4.5 7.5 min, depending on the CdTe thickness. The back contact comprising a Te/Cu bi-layer as the buffer and Ni as the electrode was prepared by sequentially depositing Te and Cu on the CdTe surface followed by Ni, all by sputtering in Ar. For the reference cells, the CdTe films were etched in a NP solution (2.2 ml HNO3:175 ml H3PO4:70 ml H2O) for 10 30 s to form a Te-rich layer and rinsed in de-ionized water. For the cells with a vapor-deposited Te/Cu buffer, a thermal activation step was
applied after the Ni back electrode (200 nm) was deposited. The activation conditions were varied over a range of temperatures (150–275 1C) and durations (0.2–20 min). For thermal stress tests, the CdS/CdTe solar cells were annealed in a tube furnace under N2. The annealing temperature was maintained at 200 1C with a ramp-up and ramp-down rate being 40 1C/min and 10 1C/min, respectively. Scanning electron microscopy (SEM) (Hitachi, TM 3000) and X-ray diffraction (XRD) (Philip, X'pert MRD) were used to examine the morphological and crystalline properties of the CdTe films with and without a buffer layer. X-ray photoemission spectroscopy (XPS) measurements were carried out using a modified VG ESCA Lab system. Low-temperature photoluminescence (PL) was recorded using an Ocean Optics spectrometer (USB 4000) with a 12 mW He-Ne laser (Research Electro-Optical) as the excitation source. Current–voltage (J–V) data were obtained using a Keithley sourcemeter (Model 2400) and a tungsten-lamp-based solar simulator (Solux 3SS4736 50W), which was calibrated with a silicon photodiode (Hamamatsu S1787-12). The incident light intensity was 80 mW/cm2. All current densities for the solar cells reported in this work have been normalized to an illumination of 100 mW/cm2 AM 1.5 solar spectrum. Spectral response measurements were obtained using a calibrated ¼ m monochromator (ARC SpectroPro 275).
3. Results and discussions 3.1. Effects and optimization of thermal activation In Fig. 1 we compare the J–V characteristics of CdS/CdTe cells with four different buffer configurations: (1) without a buffer, (2) Te-rich buffer by NP treatment, (3) 100 nm Te buffer by sputtering deposition, and (4) Te (100 nm)/Cu (1.1 nm) buffer by sequential sputtering deposition. The back contact electrode is Ni. After Ni electrode deposition, the cells were subjected to thermal activation for a short duration ( 0.2 min) at 270 1C. The photovoltaic parameters extracted from the J–V plots are summarized in Table 1. It can be seen that all four cells exhibit relatively good ohmic behavior without any apparent roll-over in the forward bias. Despite its ohmic behavior, the cell without a buffer layer produces a very low Voc of only 478 mV compared to a Voc of 800 mV for the cells with a buffer layer. This large reduction in Voc can be attributed to Ni diffusion into CdTe and the consequent lowering of the CdS/CdTe junction potential [16]. The effect of the buffer layer on Jsc is relatively modest, indicating that photogeneration in CdTe is independent of the nature of back contact, at least for CdTe thickness of 4–5 μm. The cell with the Te/Cu buffer exhibits the highest efficiency of 14.1%, which is largely due to the significantly improved fill factor of 77.6%. The cell with the NP buffer is similar to the cell with the sputtered Te buffer in overall performance, in agreement with previous studies [17,18]. Aiming to further optimize the CdS/CdTe cell performance, we have examined in detail the Te/Cu buffer compositions and the Table 1 Photovoltaic parameters for CdS/CdTe solar cells with various back contact buffer layers. Thermal activation for the cells was 270 1C for 0.2 min.
Fig. 1. J–V characteristics of CdS/CdTe solar cells with various back contact buffer layers: (1) no buffer layer, (2) Te-rich layer created by NP, (3) 100 nm Te, and (4) 100 nm Te plus 1.1 nm Cu. The metal electrode is 200 nm Ni. The Te and Cu layers of (3) and (4) are vapor-deposited.
Back contact buffer/ process
Back contact electrode
Jsc Voc (mA cm 2) (mV)
FF (%)
η (%)
None NP Te Te/Cu
Ni Ni Ni Ni
20.9 21.5 21.4 22.2
60.5 71.5 73.9 77.6
6.0 12.0 12.7 14.1
478 780 800 820
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Fig. 2. Effect of thermal annealing on the photo J–V characteristics of CdS/CdTe solar cells with a Te/Cu back contact: (a) accumulative thermal annealing for various durations at a fixed temperature and (b) accumulative annealing with incremental temperatures for a fixed duration of 20 min at each temperature. Only the power quadrant is shown in Fig. 2b.
Table 2 Photovoltaic parameters for CdS/CdTe solar cells with Te/Cu back contact before and after accumulative thermal activation. The Te and Cu thickness is 100 nm and 4.5 nm, respectively. Annealing order
0th 1st 2nd 3rd 4th
Thermal activation Temperature (1C)
Duration (min)
– 150 150 150 225
– 20 20 20 5
Jsc (mA cm2)
21.2 21.3 21.0 20.8 22.7
Vov (mV)
744 740 740 735 810
FF (%)
62.7 64.6 64.5 65.3 71.7
η (%)
9.0 10.2 10.0 9.9 13.2
thermal activation process. We have found that the thermal activation step is crucial in forming the back contact. In Fig. 2a we show the effect of cumulative thermal annealing at 150 1C on a CdS/CdTe cell with a Te (100 nm)/Cu (4.5 nm) buffer. It can be seen that annealing even at such a modest temperature for only 20 min can significantly improve the J–V by reducing the roll-over. Further annealing for a cumulative duration of 60 min only slightly improves the J–V. However, as shown in Fig. 2a and Table 2, additional annealing at a higher temperature of 225 1C for only 5 min increases the cell efficiency from 10% to 13.2%, largely by improving the Voc (from 735 to 810 mV) and fill factor (from 65.3% to 71.1%). In Fig. 2b we show the J–V dependence on the annealing temperature for a fixed duration of 20 min. It is apparent that the threshold temperature for thermal activation of the Te/Cu buffer is around 180 1C at which significant improvement in cell performance is realized. It is also apparent that annealing at 225 1C for 20 min can cause deterioration. We present in Table 3 the photovoltaic parameters for another set of cells which have been subjected to thermal activation at temperatures from 200 to 275 1C for durations from 0.2 to 20 min. As expected, optimal cell performance can be obtained by adjusting the activation temperature and duration. With increasing temperatures the duration must be shortened and vice versa. With activation temperature at or above 200 1C, cell efficiency higher than 13.5% can be readily obtained. The highest efficiency (14.8%) has been obtained with an activation temperature of 275 1C for only 0.2 min. To better understand the effects of thermal activation, we have used SEM, XRD, XPS and PL to characterize the CdTe films and buffers and to track the changes in structural and interfacial properties with thermal activation. In Fig. 3 we show the SEM
Table 3 Effect of thermal activation on CdS/CdTe solar cells with Te/Cu back contact buffer. Thermal activation
Voc (mV) Jsc (mA cm 2) FF (%) η (%)
Temperature (1C) Duration (min) 200 225 235 245 260 275
20.0 4.0 2.0 1.5 0.5 0.2
810 810 810 810 812 830
22.2 22.1 22.3 22.5 22.7 22.9
76.2 76.3 75.1 74.8 77.0 77.8
13.7 13.7 13.5 13.6 14.2 14.8
images of (a) CdTe film without a buffer, (b) a 100 nm Te buffer on a CdTe film before and (c) after thermal activation at 225 1C for 4 min. It can be seen that the Te buffer on CdTe before thermal activation (Fig. 3b) is smooth and clearly conforms to the hexagonal facets of the underlying CdTe film (Fig. 3a). After thermal activation, tiny nodules (marked by arrows in Fig. 3c) appear to decorate the CdTe facet surfaces. These nodules appear to be crystallized elemental tellurium from XRD analysis. The crystallinity of these films has been analyzed using XRD. As shown in Fig. 4a, where the XRD spectra of the films before and after thermal activation are overlaid on top of each other, the (111), (220) and (311) peaks of CdTe overlap closely with the (100), (102) and (200) of the hexagonal Te, respectively [18,19]. The pronounced (101), (111), and (201) peaks indicate that the asdeposited Te layer is already highly crystallized. After thermal activation, the intensity of (101), (102), (200) and (201) peaks of Te is slightly increased, indicating the crystallinity of the Te film is further enhanced. Fig. 4b shows an enlarged view of the XRD patterns between 401 and 55o and the enhancements in the Te (111) and (201) peaks with thermal activation. The changes in XRD patterns are consistent with the SEM images of Fig. 3, which reveal the growth of Te nodules after the thermal activation. It is also noteworthy that no XRD peaks assignable to TeOx can be detected in these films. XPS has been used to analyze the Te/Cu buffers on CdTe and to track the surface composition change with thermal activation. Specifically, the sample was a Te (50 nm)/Cu (4 nm) buffer on CdTe (4 μm) and the thermal activation was carried out entirely in the ultra-high vacuum XPS system at 260 1C for up to 1 h. XPS data were collected between consecutive annealing durations and after the sample was cooled down to room temperature. Fig. 5 shows the progression of the XPS spectra of Te 3d5/2 and Cu 2p peaks
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Fig. 3. SEM images of (a) as-made CdTe, (b) Te (100 nm) buffer on CdTe before thermal activation, and (c) same as (b) after thermal activation at 225 1C for 4 min. The imaged films are the top surface of CdS/CdTe solar cells.
Fig. 4. (a) XRD spectra of (i) as-made CdTe, (ii) Te(100 nm) buffer on CdTe before thermal activation, and (iii) same as (ii) after thermal activation at 225 1C for 4 min. (b) Enlarged view of the XRD spectra between 401 and 551.
with thermal annealing. For visual clarity the XPS spectra have been normalized to equal peak intensity. With an initial annealing for only 5 min, there is a large shift of 2 eV towards lower binding energy (BE) for both Te 3d5/2 and Cu 2p peaks, which can be attributed to the removal of surface impurities. Subsequent annealing up to 20 min does not cause any further shift in these peaks which remain at 572.5 eV and 932.6 eV for Te 3d5/2 and Cu 2p, respectively, suggesting that both Te and Cu are still in the elemental form [20]. It is only after annealing for 60 min that shifts in both peaks are observed. The Te 3d5/2 peak is shifted 0.3 eV towards lower BE while the Cu 2p peak is shifted 0.1 eV towards higher BE. These peak shifts indicate that chemical reactions between Te and Cu have occurred. The Cu to Te atomic ratio on the surface of the Te/Cu buffer can be calculated from the XPS data by measuring the area under Cu 2p and Te 3d5/2 peaks after correcting for individual atomic
sensitivity factors. The peak area was measured by fitting the XPS data with one or more Gaussian peaks. As Fig. 6 shows, with increasing annealing duration, the Cu/Te ratio is decreased, suggesting the diffusion of Cu into the Te bulk layer. Apparently Cu diffusion is substantial within the first 5 min at 260 1C. Te/Cu buffers have been investigated by Wu et al. [18] and Zhou et al. [21]. In their studies, Te/Cu stack was prepared by depositing a layer of Cu on CdTe that had been previously subjected to chemical etch (NP etching) to produce a Te or Te-rich surface. The Te/Cu stack was subjected to thermal annealing to induce the formation of CuxTe of various phases depending on the annealing conditions. From XRD analysis, they concluded that there was no evidence of CuxTe formation at annealing temperatures below 260 1C for Te/Cu stacks in which the Cu to Te molar ratio was less than 0.55. In agreement with these results, our XPS studies also indicate no chemical reaction occurred below 260 1C. Furthermore,
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Fig. 5. XPS analysis of Te/Cu buffer layer. The sample structure is: glass substrate/FTO/200 nm CdS/4 μm CdTe/50 nm Te/4 nm Cu. The time markers indicate different annealing durations.
Fig. 6. Surface Cu/Te molar ratio of a Te (50 nm)/Cu (4 nm) buffer from XPS analysis. The annealing temperature was 260 1C.
our studies show that the formation of CuxTe is not necessary for the formation of ohmic back contact since the optimal temperature we found for the thermal activation of Te/Cu buffer is around 225 1C. These results suggest that thermal activation under our optimized conditions (225 1C for 4 min) can only promote the diffusion of Cu in Te to form a more homogeneous mixture or solution without causing chemical reaction. Although we cannot assess the diffusion of Cu into the CdTe film from XPS or XRD studies, we can infer that such diffusion is reduced due to the dilution of Cu in Te. In order to trace Cu diffusion in CdTe, we conducted PL measurements directly on CdS/CdTe solar cells with the following structure: glass substrate/200 nm–CdS/4.0 μm–CdTe/100 nm–Te/ 1.1 nm–Cu/200 nm–Ni, where the buffer is Te/Cu. PL data were collected at 40 K with the laser beam directed at the CdS/CdTe interface through the glass substrate. Fig. 7 shows the PL spectra
Fig. 7. Normalized PL spectra of a Glass/CdS/CdTe/(Te/Cu)/Ni solar cell with excitation through the glass substrate. The CdTe layer was 4.0 mm and the Te/Cu buffer was 100 nm Te and 1.1 nm Cu. Thermal activation (TA) was 225 1C for 4 min; thermal stress test (TST) was 200 1C for 9 h.
before and after thermal activation (225 1C, 4 min), and also after an additional annealing at 200 1C for 9 h. The prominent PL features are two broad bands with peaks at 1.380 eV and 1.542 eV. According to Halliday et al. [22] and Caraman et al. [23], the 1.542 eV band can be attributed to the donor–acceptor (D–A) pair associated with Cu in CdTe. Before thermal activation, this D–A pair band is not observable, presumably due to the absence of Cu in CdTe. The growth of this band with thermal activation indicates that Cu diffusion into CdTe layer does occur even under a relatively mild annealing condition and that Cu can easily diffuse through a 4-mm thick CdTe film into the vicinity of CdS/CdTe junction. The band centering at 1.38 eV is the most intense in the PL spectra. It has a shoulder at 1.42 eV, which is noticeably reduced in intensity with thermal annealing as shown in Fig. 7. The
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Fig. 8. PL spectra and the corresponding deconvoluted peaks (a) without and (b) with thermal activation; (c) with thermal activation and 9 h thermal stress at 200 1C; (d) the evolution of the Cu þ –VCd band. PL spectra were obtained with interface excitation at 40 K. TA stands for thermal activation.
Table 4 Position and relative intensity ratio of the deconvoluted peaks in Fig. 8. Mechanism Cd1 xSxTe
As-made After TA TA þ TST
Peak (eV)
I/I0
Peak (eV)
1.381 1.378 1.379
0.675 1.438 0.532 – 0.563 –
Cu þ –VCd
VCd–ClTe
Surface defects I/I0
Peak (eV)
0.263 – – 1.448 – 1.452
I/I0
Peak (eV)
– 1.421 0.118 1.425 0.102 1.428
I/I0
0.038 0.108 0.157
I0 is the intensity of the primary peak located at 1.380 eV; TA stands for thermal activation.
assignment of the 1.38 eV band is complicated, but it has been attributed to various species, including Cd1 xSxTe (1.381 eV) [24], VCd–ClTe (1.450 eV) [22] and Cu þ –VCd (1.425 eV) [25]. In Fig. 8, the 1.38 eV band has been deconvoluted [22,26–27] into four Gaussians (using MicroLab Origin 8.0) with peak locations at 1.381 eV, 1.350 eV, 1.425 eV and 1.450 eV to account for the relative contributions from these assigned species. The exact peak locations are varied slightly among the PL spectra in order to obtain the best fit with the original PL spectra (the R square values for all the curve fittings are higher than 0.9999). In Table 4, the Gaussian peak positions and their correlated species are listed along with the relative PL intensity, I/I0, which represents the fractional area
of each Gaussian component. It can be seen that the 1.42 eV band (assigned to Cu þ –VCd) shows the largest increase in the relative PL intensity from 0.038 to 0.108 upon thermal activation and to 0.157 with additional thermal stress. In contrast, the relative PL intensity for the other Gaussian bands remains more or less unchanged. Based on SEM, XRD, XPS and PL studies described above, the major effect of thermal activation on the back contact with a Te/Cu buffer is to cause Cu to diffuse in the Te layer without forming any CuxTe compounds. The threshold temperature required for activation is relatively low, at 200 1C, indicating that Cu diffusion in Te is fast. Furthermore, Cu from the Te/Cu buffer can diffuse through the CdTe layer into CdS/CdTe junction even at this temperature.
3.2. Effects of Te/Cu buffers on CdS/CdTe stability The fact that Cu can diffuse readily in Te and CdTe films even at a modest temperature can be a major issue affecting the long-term device stability of CdS/CdTe solar cells with Te/Cu as the back contact buffer. To address this issue we have examined the dependence of device stability on the Cu content of the Te/Cu buffers. With the vapor deposition method, it is straightforward to produce Te/Cu buffers with various Te and Cu thicknesses and estimate the molar ratio of Cu to Te, assuming that Cu diffuses freely in the Te layer at the thermal activation temperature. For stability evaluation we prepared two sets of Te/Cu buffers having a
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Fig. 9. (a) Photo and (b) dark J–V characteristics for cells with Te/Cu buffers of various Te thicknesses. The Cu thickness is 1.1 nm. Thermal activation was 225 1C for 4 min.
Table 5 Device performance of CdTe solar cells using Te/Cu back contact buffer with different Te thicknesses. Te/Cu thickness (nm)
Te/Cu molar ratio
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
0/1.1 50/1.1 100/1.1 200/1.1 300/1.1
0 16.3 32.7 65.4 98.1
500 810 820 825 820
20.1 21.6 21.4 21.3 21.3
63.8 74.0 77.1 74.0 74.8
6.4 12.9 13.5 13.0 13.1
fixed Cu thickness in one set and a fixed Te thickness in the other. Thermal activation was carried out at 225 1C for 4 min. The photo and dark J–V shown in Fig. 9 are for the first set of Te/Cu buffers having a fixed Cu thickness (1.1 nm) and Te thickness varied from 0 to 300 nm. The photovoltaic parameters are summarized in Table 5. It can be seen that as long as Te is present in the buffer layer the solar cell performance is normal with efficiency around 13%, and Voc and FF above 810 mV and 74%, respectively. In contrast, the cell without Te (Cu only in the buffer) exhibits rather low efficiency (6.4%), mainly due to a poor Voc (500 mV). This correlates with the low onset voltage for the dark forward-biased current (Fig. 9b), suggesting a reduction in the CdS/CdTe junction potential or a substantial contribution of the recombination current at the junction. Since Cu is in direct contact with CdTe in this cell, enhanced Cu diffusion into CdTe (without mediation by a Te layer) is the likely cause. For the Te/Cu to be effective as the back contact buffer, the minimum Te thickness appears to be about 50 nm. With or without Te, the J–V characteristics indicate that the back contact appears to be ohmic as long as Cu is present. We evaluated the stability of this set of CdS/CdTe cells under thermal stress at 200 1C in N2 for up to 9 h. In Fig. 10 the relative efficiency loss as a percentage of the initial efficiency is plotted against the thermal stress duration. It can be seen that the cell stability can be correlated with the thickness of the Te layer in Te/ Cu buffer. With a thicker Te layer the cell is more stable. The relative efficiency loss is only about 10% for the cells with 100– 300 nm Te in the buffer layer as compared to 35% for the cell with 50 nm Te. Clearly, the efficiency of the cell with 0 nm Te degraded much faster with a 43% relative efficiency loss after 9 h of thermal stress. This affirms that direct contact of Cu with CdTe is detrimental to the solar cell performance and suggests that the Te layer
Fig. 10. Cell efficiency degradation under thermal stress test at 200 1C. The thickness of the Cu layer is 1.1 nm in the Te/Cu buffers. Thermal activations were carried out at 225 1C for 4 min.
is needed to mediate the diffusion of Cu into CdTe by diluting the Cu concentration at the CdTe surface. The second set of CdS/CdTe solar cells was prepared with a Te/ Cu buffer having a fixed Te layer (100 nm) and a Cu layer varied from 0 to 2.2 nm. As shown in Fig. 11a and b, the initial photo and dark J–V characteristics are similar for all the cells. The photovoltaic parameters are summarized in Table 6. The J–V characteristics also indicate that the back contact is ohmic for all the cells regardless of the Cu content in the buffer. However, as shown in Fig. 12, the cell stability with respect to thermal stress is dramatically dependent on the Cu content. After thermal stress for 9 h at 200 1C, the relative efficiency loss is about 20% for the cell without Cu in the buffer layer as compared to 3–9% for cells with Cu. It is noteworthy that the Te/Cu buffer with only 0.3 nm Cu appears to be the most stable. Assuming that Cu is uniformly distributed in the Te layer with thermal activation, this minute amount of Cu is equivalent to a Cu to Te molar ratio of 8.34 10 3, which is far less than what have been used in previous studies [12,18,21]. According to Zhang et al. [29], Cl can accumulate at the CdTe grain boundaries and improve p-doping by creating a shallow acceptor complex VCd–ClTe. There was also evidence that the diffusion and accumulation of Cu and Cl in bulk CdTe may be correlated, in the
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Fig. 11. (a) Photo and (b) dark J–V for CdS/CdTe solar cells with Te/Cu buffers. The Cu layer is varied from 0 to 2.2 nm and the Te layer is fixed at 100 nm. The cells were subjected to a thermal activation at 225 1C for 4 min.
Table 6 Device performance of CdTe solar cells using Te/Cu back contact buffer with different Cu thicknesses. Te/Cu thickness (nm)
Cu/Te molar ratio
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
100/0.0 100/0.3 100/0.6 100/1.1 100/2.2
0 8.3 10 3 16.6 10 3 30.5 10 3 61.1 10 3
790 815 810 800 810
21.0 21.2 21.1 21.3 21.1
71.0 75.0 73.7 74.4 74.7
11.8 13.0 12.6 12.7 12.8
form of a CuCd–ClTe complex. Energetics studies [29] showed that Cu favors Cd substitution to form interstitial CuCd and Cl favors replacing Te (ClTe). The defect states having an energy level below the conduction band minimum can be removed by co-passivation of Cl and Cu. Neither Cl nor Cu itself can completely passivate the grain boundaries. The co-passivation of Cl and Cu on the defects states of grain boundaries may be beneficial for device stability [30]. In agreement with previous back contact studies [28–30], we show that Cu plays a critical role in forming ohmic contact to p-CdTe back contact and enhanced stability can be obtained by carefully controlling the Cu content in the buffer layer. 3.3. Application of Te/Cu bi-layer on ultra-thin CdTe solar cells The effects of NP etching on the morphology of an ultra-thin CdTe film (1.5 μm) are illustrated in Fig. 13. Compared with the asmade CdTe film in Fig. 13a, the CdTe film after etching (Fig. 13b) has smoother grains with fewer facets and broader grain boundaries. As shown in the cross-section view in Fig. 13c and d, boundaries of the CdTe column grains are more prominent after etching. These morphology changes suggest that the grain boundaries of CdTe are vulnerable to NP etching. The broad grain boundaries can greatly reduce the shunting resistance of CdTe films and also provide rapid-diffusion channels for metal diffusion from the back contact. Both effects can cause deterioration of cell performance. Ultra-thin CdTe solar cells with CdTe thicknesses of 0.9 μm and 1.5 μm were prepared for comparison of NP treatment and Te/Cu buffer. With NP treatment, the cell with a 0.9 μm CdTe layer was shorted. In contrast, with the vapor-deposited Te/Cu buffer the cell
Fig. 12. Device efficiency degradation with a correlation of stability test duration: (a) thermal stress test. The Te thickness is 100 nm. Thermal activations were carried out at 225 1C for 4 min.
was functional with an efficiency of 11.1%. The efficiency of 11.1% is comparable to the best efficiency of ultra-thin CdTe solar cells achieved by Compaan's group using magnetron sputtering fabrication procedure [31,32] to our best knowledge. As shown in Fig. 14 and Table 7, for cells with 1.5 μm CdTe, the Te/Cu buffer improves the cell efficiency over the NP treatment to 13.5% from 10.1%. This can be attributed to improvements in both contact resistance and reduced leakage current.
4. Summary We developed a useful buffer based on a vapor-deposited Te/Cu bi-layer for forming ohmic contact to p-CdTe in CdS/CdTe solar cells, including cells using ultra-thin CdTe. Thermal annealing was required to activate the buffer. Enhanced efficiency and stability were obtained in CdS/CdTe cells by optimizing the Cu/Te composition and the conditions for thermal activation. Characterization by XRD, XPS, and PL indicates that Cu diffuses rapidly in the Te at 200 1C without forming CuxTe compounds and Cu diffusion into CdTe can be mediated by the Te/Cu buffer.
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Fig. 13. SEM images of ultra-thin CdTe film before (a: top view, c: cross-section view) and after NP etching (25 s) (b: top view, d: cross-section view). CdTe film thickness is 1.5 mm.
Fig. 14. (a) Photo and (b) dark J–V characteristics of ultra-thin CdTe solar cells using NP treatment and Te/Cu bi-layer in the ohmic contact formation.
Table 7 Device performance of ultra-thin CdTe solar cells using NP treatment and Te/Cu back contact buffer layer in the ohmic contact formation. CdTe thickness (μm)
Te/Cu thickness (nm)
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
0.9 1.5
100/1.1 100/1.1
790 820
20.4 21.5
69.0 76.4
11.1 13.5
CdTe thickness (μm)
NP etch duration (s)
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
1.5
25 s
750
21.4
63.0
10.1
420
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Acknowledgments W. Xia thanks Sunflux Inc. for providing VTD-CdS substrates and Dr. Qing Du for taking the SEM images. C. Wang and Y. Gao acknowledge the support of the National Science Foundation, United States. Grant no. DMR-1303742. References [1] N. Romeo, A. Bosio, R. Tedeschi, V. Canevari, Back contacts to CSS CdS/CdTe solar cells and stability of performances, Thin Solid Films 361–362 (2000) 327–329. [2] C.S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P. Selvaraj, D.L. Morel, High efficiency CSS CdTe solar cells, Thin Solid Films 361–362 (2000) 520–526. [3[ Y.S. Tyan, Semiconductor devices having improved low-resistance contacts to p-type CdTe, and method of preparation. US. Patent 1982, 4,319,069. [4[ H. Lin, W. Xia, H.N. Wu, C.W. Tang, CdS/CdTe solar cells with MoOx as back contact buffers, Appl. Phys. Lett. 97 (2010) (123504/1–123504/3). [5] B. Späth, K. Lakus-Wollny, J. Fritsche, C.S. Ferekides, A. Klein, W. Jaegermann, Surface science studies of Cu containing back contacts for CdTe solar cells, Thin Solid Films 515 (2007) 6172–6174. [6[ U.S. Avachat, N.G. Dhere, Preparation and characterization of transparent conducting ZnTe:Cu back contact interface layer for CdS/CdTe solar cell for photoelectrochemical application, J. Vac. Sci. Technol. A 24 (2006) 1664–1667. [7] O. Rotlevi, K.D. Dobson, D. Rose, G. Hodes, Electroless Ni and NiTe2 ohmic contacts for CdTe/CdS PV cells, Thin Solid Films 387 (2001) 155–157. [8] N. Romeo, A. Bosio, R. Tedeschi, A. Romeo, V. Canevari, A highly efficient and stable CdTe/CdS thin film solar cells, Sol. Energy Mater. Sol. Cells 58 (1999) 209–218. [9] N. Romeo, Method for the formation of a non-rectifying back-contact a CdTe/ CdS thin film solar cell, US Patent Application Publication 2010, 2010/ 0186810A1. [10] X. Wu, High-efficiency polycrystalline CdTe thin-film solar cells, Sol. Energy 77 (2004) 803–814. [11] D.W. Niles, X. Li, D. Albin, D. Rose, T. Gessert, P. Sheldon, Evaporated Te on CdTe: a vacuum-compatible approach to making back contact to CdTe solar cell devices, Prog. Photovoltaics: Res. Appl, 4 (1996) 225–229. [12] H. Uda, S. Ikegami, H. Sonomura, Te-metal contact on evaporated CdTe film for a CdS/CdTe solar cell, Sol. Energy Mater. Sol. Cells, 35 (1994) 293–298. [13] W. Xia, H. Lin, H.N. Wu, C.W. Tang, Effects of high-temperature annealing on ultra-thin CdTe solar cells, Thin Solid Films 520 (2011) 563–568. [14] G.M. Hanket, B.E. McCandless, W.A. Buchanan, S. Fields, R.W. Birkmire, Design of a vapor transport deposition process for thin film materials, J. Vac. Sci. Technol. A 24 (2006) 1695–1701. [15] W. Xia, J.A. Welt, H. Lin, H.N. Wu, M.H. Ho, C.W. Tang, Fabrication of Cd1 xZnxS films with controllable zinc doping using a vapor zinc chloride treatment, Sol. Energy Mater. Sol. Cells 94 (2010) 2113–2118.
[16] I. Irfan, H. Lin, W. Xia, H.N. Wu, C.W. Tang, Y. Gao, The effect of MoOx interlayer on thin film CdTe/CdS solar cells, Sol. Energy Mater. Sol. Cells 105 (2012) 86–89. [17] X. Li, D.W. Niles, F.S. Hasoon, R.J. Matson, P. Sheldon, Effect of nitric-phosphoric acid etches on material properties and back-contact formation of CdTe-based solar cells, J. Vac. Sci. Technol. A 17 (1999) 805–809. [18] X. Wu, J. Zhou, A. Duda, Y. Yan, G. Teeter, S. Asher, W.K. Metzger, S. Demtsu, S.H. Wei, R. Noufi, Phase control of CuxTe film and its effects on CdS/CdTe solar cell, Thin Solid Films 515 (2007) 5798–5803. [19] S.K. Batabyal, C. Basu, A.R. Das, G.S. Sanyal, Self-assembly of tellurium nanorods, in: Proceedings of the 5th IEEE Conference on Nanotechnology, Nagoya, Japan, 2005, pp. 627–630. [20] B.V. Crist, Handbook of The Elements and Native Oxides, XPS International, Inc., Mountain View, CA (1999) 273–274. [21] J. Zhou, X. Wu, A. Duda, G. Teeter, S.H. Demtsu, The formation of different phases of CuxTe and their effects on CdTe/CdS solar cells, Thin Solid Films 515 (2007) 7364–7369. [22] D.P. Halliday, J.M. Eggleston, K. Durose, A photoluminescence study of polycrystalline thin-film CdTe/CdS solar cells, J. Cryst. Growth 186 (1998) 543–549. [23] I. Caraman, G.I. Rusu, L. Leonite, The photoluminescence of CdTe thin films in SnO2–CdS/CdTe–Ni solar cells thermally treated in CdCl2 solution, J. Optoelectron. Adv. Mater. 9 (2007) 1546–1550. [24] J. Aguilar-Hernandez, G. Contreras-Puente, A. Morales-Acevedo, O. Vigil-Galan, F. Cruz-Gandarilla, J. Vidal-Larramendi, A. Escamilla-Esquivel, H. HernandezContreras, M. Hesiquio-Garduno, A. Arias-Carbajal, M. Chavarria-Castaneda, G. Arriaga-Mejia, Photoluminescence and structural properties of cadmium sulphide thin films grown by different techniques, Semicond. Sci. Technol. 18 (2003) 111–114. [25] T. Okamoto, Y. Matsuzaki, N. Amin, A. Yamada, M. Konagai, Characterization of highly efficient CdTe thin film solar cells by low-temperature photoluminescence, Jpn. J. Appl. Phys. 37 (1998) 3894–3899. [26] R. Ahmad-Bitar, H. Moutinho, F. Abulfotuh, L. Kazmerski, Analyses of photoluminescence spectra of CdTe thin films at different temperatures, Renew. Energy 6 (1995) 553–558. [27] C. Kraft, H. Metzner, M. Hädrich, U. Reislöhner, P. Schley, G. Gobsch, R. Goldhahn, Comprehensive photoluminescence study of chlorine activated polycrystalline cadmium telluride layers, J. Appl. Phys. 108 (2010) (124503/1– 124503/8). [28] K.D. Dobson, P.D. Paulson, B.E. McCandless, R.W. Birkmire, The dynamic of cadmium telluride etching, Mater. Res. Soc. Symp. Proc. 763 (2003) 107–118. [29] L. Zhang, J.L.F. Da Silva, J. Li, Y. Yan, T.A. Gessert, S.H. Wei, Effect of copassivation of Cl and Cu on CdTe grain boundaries, Phys. Rev. Lett. 101 (2008) (155501/1–155501/4). [30] N.R. Paudel, D. Kwon, M. Young, K.A. Wieland, S. Asher, A.D. Compaan, Effects of Cu and CdCl2 treatment on the stability of sputtered CdS/CdTe solar cells, in: Proceedings of the 35th IEEE Photovoltaic Specialists Conference, Hawaii, United States, 2010, pp. 1009–1013. [31] V. Plotnikov, X. Liu, N. Paudel, D. Kwon, K.A. Wieland, A.D. Compaan, Thin-film CdTe cells: reducing the CdTe, Thin Solid Films 519 (2011) 7134–7137. [32] A. Gupta, V. Parikh, A.D. Compaan, High efficiency ultra-thin sputtered CdTe solar cells, Sol. Energy Mater. Sol. Cells 90 (2006) 2263–2271.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Te/Cu bi-layer: A low-resistance back contact buffer for thin film CdS/CdTe solar cells Wei Xia a,1, Hao Lin a, Hsiang Ning Wu a, Ching W. Tang a,b,c,n, Irfan Irfan b,c, Chenggong Wang b, Yongli Gao b a
Department of Chemical Engineering, University of Rochester, Rochester, NY 14627, United States Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, United States c Department of Chemistry, University of Rochester, Rochester, NY 14627, United States b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 March 2014 Received in revised form 19 May 2014 Accepted 3 June 2014 Available online 27 June 2014
A buffer layer based on a Te/Cu bi-layer useful for forming ohmic contact to p-CdTe has been developed for application in CdS/CdTe solar cells. The bi-layer buffer was prepared by vapor deposition and a thermal annealing ( 200 1C) was required for activation. Enhanced efficiency and stability were obtained by optimizing the Cu/Te compositions and the thermal activation conditions. Characterization by XRD, XPS, and PL indicates that under the thermal activation conditions Cu diffuses rapidly in the Te without forming CuxTe compounds. The enhanced stability can be attributed to the mediation of Cu diffusion into CdTe by the Te layer. The Te/Cu buffers are particularly useful for the fabrication of ultrathin CdS/CdTe solar cells. & 2014 Elsevier B.V. All rights reserved.
Keywords: CdTe Solar cell Back contact Stability Tellurium Copper
1. Introduction Among the hurdles to producing high-efficiency and long-life n-CdS/p-CdTe thin-film solar cells, the formation of an ohmic and stable back contact to p-CdTe remains the most challenging [1]. Due to the generally high resistivity and deep valence band edge associated with p-CdTe, it is practically impossible to achieve a good ohmic contact to p-CdTe with a metal including those with a high work function such as Au or Ni. A common approach to lowering the contact resistance is to create a high work function Te or Te-rich buffer layer on the CdTe surface prior to the deposition of the metallic back electrode. The buffer layer serves to align the work function of the back electrode closer to the valence band edge of p-CdTe and thus reduces the energy barrier for hole transport between the back electrode and p-CdTe. Chemically etching CdTe with bromine-methanol or nitric-phosphoric (NP) acid solution [2,3] has been found to be highly effective in producing such a Te-rich buffer layer, and good ohmic contacts have been achieved with common metals such as Ni or Au for the
n Corresponding author at: Department of Chemical Engineering, University of Rochester, Rochester, NY 14627, United States. Tel.: þ1 585 275 3552; fax: þ1 585 273 1348. E-mail address: [email protected] (C.W. Tang). 1 Defect Elimination, Samsung Austin Semiconductor, Austin, TX 78754, United States.
http://dx.doi.org/10.1016/j.solmat.2014.06.010 0927-0248/& 2014 Elsevier B.V. All rights reserved.
back electrode. However, there remain major drawbacks with the chemical method in producing the buffer layer including the need for a thick CdTe film to avoid the creation of electrical shunts from over etching in solution [4] and the incompatibility with the overall physical vapor deposition processes for the CdS/CdTe solar cell production. Alternative methods other than chemical etching have been used to produce ohmic back contacts for CdS/CdTe solar cells. Typically these methods involve the use of a semiconductor (MX) as the buffer layer between CdTe and the back electrode. CuxTe [5], ZnTe:Cu [6], NiTe2 [7], Sb2Te3 [8] and As2Te3 [9] have been found to be useful. Presumably they are capable of producing a lowresistance CdTe/MX tunnel junction by creating a heavily p-doped CdTe surface with Cu or Sb as the shallow acceptor [8]. In particular buffers based on CuxTe have been used to produce highly efficient CdS/CdTe solar cells. However, the detailed formulation of CuxTe buffer is critical in achieving an ohmic back contact that is also stable. Whereas Cu deficiency in CuxTe can lead to non-ohmic behaviors, an excess amount of Cu can cause fast device degradation [5,10]. To be effective, these semiconductor MX buffer layers are often introduced in addition to the Te or Te-rich buffer produced by chemical etching. Vapor-deposited Te layers have also been used as the back contact buffer in CdS/CdTe solar cells [11,12]. With HgTe-doped graphite as the electrode, Niles et al. [11] showed that vapordeposited Te films can be used to replace the chemically produced
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Te as the back contact buffer layer, resulting in solar cells with only slightly lower efficiencies. Uda et al. [12] used Cu as the back electrode along with evaporated Te as the buffer with the suggestion Cu could lower the resistivity of the CdTe layer. However, no device stability data was presented in these previous studies concerning the use of evaporated Te as the buffer layer. In particular, the possible interaction between Cu and Te and how such interaction can be used to control Cu diffusion into CdTe and thereby the device stability have not yet been sufficiently explored. In this work we present a more effective method for producing an ohmic and stable back contact to p-CdTe without chemical etching. The method utilizes a vapor-deposited Te/Cu bi-layer as the buffer, where the Cu to Te ratio is precisely controlled by the thicknesses of the Te and Cu layers. In addition, a thermal annealing step is used to activate the back contact after the cell is completed with a Ni back electrode. Based on this method, CdS/ CdTe solar cells with a power conversion efficiency up to 14.8% have been produced. Furthermore, efficiencies as high as 13.5% have been demonstrated in cells with an ultra-thin CdTe layer of only about 1 μm.
2. Experimental details The fabrication of thin-film CdS/CdTe solar cells has been described in detail in a previous paper [13]. Briefly, CdS films ( 200 nm) on fluorine-doped tin oxide (FTO) coated glass substrates were provided by Sunflux Inc. [14]. Prior to CdTe deposition, the CdS films were subjected to a vapor cadmium chloride (VCC) treatment [15] under the following conditions: the CdS coated substrates and the CdCl2 powder source were maintained in a close-space configuration at a temperature of 390 410 1C in an O2:N2 (1:4) ambient of 6.7 kPa for 6 min. CdTe films ranging from 1.0 to 5.0 mm were deposited by a close-spaced sublimation (CSS) method. Similarly, the CdTe films were also subjected to the VCC treatment for 4.5 7.5 min, depending on the CdTe thickness. The back contact comprising a Te/Cu bi-layer as the buffer and Ni as the electrode was prepared by sequentially depositing Te and Cu on the CdTe surface followed by Ni, all by sputtering in Ar. For the reference cells, the CdTe films were etched in a NP solution (2.2 ml HNO3:175 ml H3PO4:70 ml H2O) for 10 30 s to form a Te-rich layer and rinsed in de-ionized water. For the cells with a vapor-deposited Te/Cu buffer, a thermal activation step was
applied after the Ni back electrode (200 nm) was deposited. The activation conditions were varied over a range of temperatures (150–275 1C) and durations (0.2–20 min). For thermal stress tests, the CdS/CdTe solar cells were annealed in a tube furnace under N2. The annealing temperature was maintained at 200 1C with a ramp-up and ramp-down rate being 40 1C/min and 10 1C/min, respectively. Scanning electron microscopy (SEM) (Hitachi, TM 3000) and X-ray diffraction (XRD) (Philip, X'pert MRD) were used to examine the morphological and crystalline properties of the CdTe films with and without a buffer layer. X-ray photoemission spectroscopy (XPS) measurements were carried out using a modified VG ESCA Lab system. Low-temperature photoluminescence (PL) was recorded using an Ocean Optics spectrometer (USB 4000) with a 12 mW He-Ne laser (Research Electro-Optical) as the excitation source. Current–voltage (J–V) data were obtained using a Keithley sourcemeter (Model 2400) and a tungsten-lamp-based solar simulator (Solux 3SS4736 50W), which was calibrated with a silicon photodiode (Hamamatsu S1787-12). The incident light intensity was 80 mW/cm2. All current densities for the solar cells reported in this work have been normalized to an illumination of 100 mW/cm2 AM 1.5 solar spectrum. Spectral response measurements were obtained using a calibrated ¼ m monochromator (ARC SpectroPro 275).
3. Results and discussions 3.1. Effects and optimization of thermal activation In Fig. 1 we compare the J–V characteristics of CdS/CdTe cells with four different buffer configurations: (1) without a buffer, (2) Te-rich buffer by NP treatment, (3) 100 nm Te buffer by sputtering deposition, and (4) Te (100 nm)/Cu (1.1 nm) buffer by sequential sputtering deposition. The back contact electrode is Ni. After Ni electrode deposition, the cells were subjected to thermal activation for a short duration ( 0.2 min) at 270 1C. The photovoltaic parameters extracted from the J–V plots are summarized in Table 1. It can be seen that all four cells exhibit relatively good ohmic behavior without any apparent roll-over in the forward bias. Despite its ohmic behavior, the cell without a buffer layer produces a very low Voc of only 478 mV compared to a Voc of 800 mV for the cells with a buffer layer. This large reduction in Voc can be attributed to Ni diffusion into CdTe and the consequent lowering of the CdS/CdTe junction potential [16]. The effect of the buffer layer on Jsc is relatively modest, indicating that photogeneration in CdTe is independent of the nature of back contact, at least for CdTe thickness of 4–5 μm. The cell with the Te/Cu buffer exhibits the highest efficiency of 14.1%, which is largely due to the significantly improved fill factor of 77.6%. The cell with the NP buffer is similar to the cell with the sputtered Te buffer in overall performance, in agreement with previous studies [17,18]. Aiming to further optimize the CdS/CdTe cell performance, we have examined in detail the Te/Cu buffer compositions and the Table 1 Photovoltaic parameters for CdS/CdTe solar cells with various back contact buffer layers. Thermal activation for the cells was 270 1C for 0.2 min.
Fig. 1. J–V characteristics of CdS/CdTe solar cells with various back contact buffer layers: (1) no buffer layer, (2) Te-rich layer created by NP, (3) 100 nm Te, and (4) 100 nm Te plus 1.1 nm Cu. The metal electrode is 200 nm Ni. The Te and Cu layers of (3) and (4) are vapor-deposited.
Back contact buffer/ process
Back contact electrode
Jsc Voc (mA cm 2) (mV)
FF (%)
η (%)
None NP Te Te/Cu
Ni Ni Ni Ni
20.9 21.5 21.4 22.2
60.5 71.5 73.9 77.6
6.0 12.0 12.7 14.1
478 780 800 820
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Fig. 2. Effect of thermal annealing on the photo J–V characteristics of CdS/CdTe solar cells with a Te/Cu back contact: (a) accumulative thermal annealing for various durations at a fixed temperature and (b) accumulative annealing with incremental temperatures for a fixed duration of 20 min at each temperature. Only the power quadrant is shown in Fig. 2b.
Table 2 Photovoltaic parameters for CdS/CdTe solar cells with Te/Cu back contact before and after accumulative thermal activation. The Te and Cu thickness is 100 nm and 4.5 nm, respectively. Annealing order
0th 1st 2nd 3rd 4th
Thermal activation Temperature (1C)
Duration (min)
– 150 150 150 225
– 20 20 20 5
Jsc (mA cm2)
21.2 21.3 21.0 20.8 22.7
Vov (mV)
744 740 740 735 810
FF (%)
62.7 64.6 64.5 65.3 71.7
η (%)
9.0 10.2 10.0 9.9 13.2
thermal activation process. We have found that the thermal activation step is crucial in forming the back contact. In Fig. 2a we show the effect of cumulative thermal annealing at 150 1C on a CdS/CdTe cell with a Te (100 nm)/Cu (4.5 nm) buffer. It can be seen that annealing even at such a modest temperature for only 20 min can significantly improve the J–V by reducing the roll-over. Further annealing for a cumulative duration of 60 min only slightly improves the J–V. However, as shown in Fig. 2a and Table 2, additional annealing at a higher temperature of 225 1C for only 5 min increases the cell efficiency from 10% to 13.2%, largely by improving the Voc (from 735 to 810 mV) and fill factor (from 65.3% to 71.1%). In Fig. 2b we show the J–V dependence on the annealing temperature for a fixed duration of 20 min. It is apparent that the threshold temperature for thermal activation of the Te/Cu buffer is around 180 1C at which significant improvement in cell performance is realized. It is also apparent that annealing at 225 1C for 20 min can cause deterioration. We present in Table 3 the photovoltaic parameters for another set of cells which have been subjected to thermal activation at temperatures from 200 to 275 1C for durations from 0.2 to 20 min. As expected, optimal cell performance can be obtained by adjusting the activation temperature and duration. With increasing temperatures the duration must be shortened and vice versa. With activation temperature at or above 200 1C, cell efficiency higher than 13.5% can be readily obtained. The highest efficiency (14.8%) has been obtained with an activation temperature of 275 1C for only 0.2 min. To better understand the effects of thermal activation, we have used SEM, XRD, XPS and PL to characterize the CdTe films and buffers and to track the changes in structural and interfacial properties with thermal activation. In Fig. 3 we show the SEM
Table 3 Effect of thermal activation on CdS/CdTe solar cells with Te/Cu back contact buffer. Thermal activation
Voc (mV) Jsc (mA cm 2) FF (%) η (%)
Temperature (1C) Duration (min) 200 225 235 245 260 275
20.0 4.0 2.0 1.5 0.5 0.2
810 810 810 810 812 830
22.2 22.1 22.3 22.5 22.7 22.9
76.2 76.3 75.1 74.8 77.0 77.8
13.7 13.7 13.5 13.6 14.2 14.8
images of (a) CdTe film without a buffer, (b) a 100 nm Te buffer on a CdTe film before and (c) after thermal activation at 225 1C for 4 min. It can be seen that the Te buffer on CdTe before thermal activation (Fig. 3b) is smooth and clearly conforms to the hexagonal facets of the underlying CdTe film (Fig. 3a). After thermal activation, tiny nodules (marked by arrows in Fig. 3c) appear to decorate the CdTe facet surfaces. These nodules appear to be crystallized elemental tellurium from XRD analysis. The crystallinity of these films has been analyzed using XRD. As shown in Fig. 4a, where the XRD spectra of the films before and after thermal activation are overlaid on top of each other, the (111), (220) and (311) peaks of CdTe overlap closely with the (100), (102) and (200) of the hexagonal Te, respectively [18,19]. The pronounced (101), (111), and (201) peaks indicate that the asdeposited Te layer is already highly crystallized. After thermal activation, the intensity of (101), (102), (200) and (201) peaks of Te is slightly increased, indicating the crystallinity of the Te film is further enhanced. Fig. 4b shows an enlarged view of the XRD patterns between 401 and 55o and the enhancements in the Te (111) and (201) peaks with thermal activation. The changes in XRD patterns are consistent with the SEM images of Fig. 3, which reveal the growth of Te nodules after the thermal activation. It is also noteworthy that no XRD peaks assignable to TeOx can be detected in these films. XPS has been used to analyze the Te/Cu buffers on CdTe and to track the surface composition change with thermal activation. Specifically, the sample was a Te (50 nm)/Cu (4 nm) buffer on CdTe (4 μm) and the thermal activation was carried out entirely in the ultra-high vacuum XPS system at 260 1C for up to 1 h. XPS data were collected between consecutive annealing durations and after the sample was cooled down to room temperature. Fig. 5 shows the progression of the XPS spectra of Te 3d5/2 and Cu 2p peaks
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Fig. 3. SEM images of (a) as-made CdTe, (b) Te (100 nm) buffer on CdTe before thermal activation, and (c) same as (b) after thermal activation at 225 1C for 4 min. The imaged films are the top surface of CdS/CdTe solar cells.
Fig. 4. (a) XRD spectra of (i) as-made CdTe, (ii) Te(100 nm) buffer on CdTe before thermal activation, and (iii) same as (ii) after thermal activation at 225 1C for 4 min. (b) Enlarged view of the XRD spectra between 401 and 551.
with thermal annealing. For visual clarity the XPS spectra have been normalized to equal peak intensity. With an initial annealing for only 5 min, there is a large shift of 2 eV towards lower binding energy (BE) for both Te 3d5/2 and Cu 2p peaks, which can be attributed to the removal of surface impurities. Subsequent annealing up to 20 min does not cause any further shift in these peaks which remain at 572.5 eV and 932.6 eV for Te 3d5/2 and Cu 2p, respectively, suggesting that both Te and Cu are still in the elemental form [20]. It is only after annealing for 60 min that shifts in both peaks are observed. The Te 3d5/2 peak is shifted 0.3 eV towards lower BE while the Cu 2p peak is shifted 0.1 eV towards higher BE. These peak shifts indicate that chemical reactions between Te and Cu have occurred. The Cu to Te atomic ratio on the surface of the Te/Cu buffer can be calculated from the XPS data by measuring the area under Cu 2p and Te 3d5/2 peaks after correcting for individual atomic
sensitivity factors. The peak area was measured by fitting the XPS data with one or more Gaussian peaks. As Fig. 6 shows, with increasing annealing duration, the Cu/Te ratio is decreased, suggesting the diffusion of Cu into the Te bulk layer. Apparently Cu diffusion is substantial within the first 5 min at 260 1C. Te/Cu buffers have been investigated by Wu et al. [18] and Zhou et al. [21]. In their studies, Te/Cu stack was prepared by depositing a layer of Cu on CdTe that had been previously subjected to chemical etch (NP etching) to produce a Te or Te-rich surface. The Te/Cu stack was subjected to thermal annealing to induce the formation of CuxTe of various phases depending on the annealing conditions. From XRD analysis, they concluded that there was no evidence of CuxTe formation at annealing temperatures below 260 1C for Te/Cu stacks in which the Cu to Te molar ratio was less than 0.55. In agreement with these results, our XPS studies also indicate no chemical reaction occurred below 260 1C. Furthermore,
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Fig. 5. XPS analysis of Te/Cu buffer layer. The sample structure is: glass substrate/FTO/200 nm CdS/4 μm CdTe/50 nm Te/4 nm Cu. The time markers indicate different annealing durations.
Fig. 6. Surface Cu/Te molar ratio of a Te (50 nm)/Cu (4 nm) buffer from XPS analysis. The annealing temperature was 260 1C.
our studies show that the formation of CuxTe is not necessary for the formation of ohmic back contact since the optimal temperature we found for the thermal activation of Te/Cu buffer is around 225 1C. These results suggest that thermal activation under our optimized conditions (225 1C for 4 min) can only promote the diffusion of Cu in Te to form a more homogeneous mixture or solution without causing chemical reaction. Although we cannot assess the diffusion of Cu into the CdTe film from XPS or XRD studies, we can infer that such diffusion is reduced due to the dilution of Cu in Te. In order to trace Cu diffusion in CdTe, we conducted PL measurements directly on CdS/CdTe solar cells with the following structure: glass substrate/200 nm–CdS/4.0 μm–CdTe/100 nm–Te/ 1.1 nm–Cu/200 nm–Ni, where the buffer is Te/Cu. PL data were collected at 40 K with the laser beam directed at the CdS/CdTe interface through the glass substrate. Fig. 7 shows the PL spectra
Fig. 7. Normalized PL spectra of a Glass/CdS/CdTe/(Te/Cu)/Ni solar cell with excitation through the glass substrate. The CdTe layer was 4.0 mm and the Te/Cu buffer was 100 nm Te and 1.1 nm Cu. Thermal activation (TA) was 225 1C for 4 min; thermal stress test (TST) was 200 1C for 9 h.
before and after thermal activation (225 1C, 4 min), and also after an additional annealing at 200 1C for 9 h. The prominent PL features are two broad bands with peaks at 1.380 eV and 1.542 eV. According to Halliday et al. [22] and Caraman et al. [23], the 1.542 eV band can be attributed to the donor–acceptor (D–A) pair associated with Cu in CdTe. Before thermal activation, this D–A pair band is not observable, presumably due to the absence of Cu in CdTe. The growth of this band with thermal activation indicates that Cu diffusion into CdTe layer does occur even under a relatively mild annealing condition and that Cu can easily diffuse through a 4-mm thick CdTe film into the vicinity of CdS/CdTe junction. The band centering at 1.38 eV is the most intense in the PL spectra. It has a shoulder at 1.42 eV, which is noticeably reduced in intensity with thermal annealing as shown in Fig. 7. The
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Fig. 8. PL spectra and the corresponding deconvoluted peaks (a) without and (b) with thermal activation; (c) with thermal activation and 9 h thermal stress at 200 1C; (d) the evolution of the Cu þ –VCd band. PL spectra were obtained with interface excitation at 40 K. TA stands for thermal activation.
Table 4 Position and relative intensity ratio of the deconvoluted peaks in Fig. 8. Mechanism Cd1 xSxTe
As-made After TA TA þ TST
Peak (eV)
I/I0
Peak (eV)
1.381 1.378 1.379
0.675 1.438 0.532 – 0.563 –
Cu þ –VCd
VCd–ClTe
Surface defects I/I0
Peak (eV)
0.263 – – 1.448 – 1.452
I/I0
Peak (eV)
– 1.421 0.118 1.425 0.102 1.428
I/I0
0.038 0.108 0.157
I0 is the intensity of the primary peak located at 1.380 eV; TA stands for thermal activation.
assignment of the 1.38 eV band is complicated, but it has been attributed to various species, including Cd1 xSxTe (1.381 eV) [24], VCd–ClTe (1.450 eV) [22] and Cu þ –VCd (1.425 eV) [25]. In Fig. 8, the 1.38 eV band has been deconvoluted [22,26–27] into four Gaussians (using MicroLab Origin 8.0) with peak locations at 1.381 eV, 1.350 eV, 1.425 eV and 1.450 eV to account for the relative contributions from these assigned species. The exact peak locations are varied slightly among the PL spectra in order to obtain the best fit with the original PL spectra (the R square values for all the curve fittings are higher than 0.9999). In Table 4, the Gaussian peak positions and their correlated species are listed along with the relative PL intensity, I/I0, which represents the fractional area
of each Gaussian component. It can be seen that the 1.42 eV band (assigned to Cu þ –VCd) shows the largest increase in the relative PL intensity from 0.038 to 0.108 upon thermal activation and to 0.157 with additional thermal stress. In contrast, the relative PL intensity for the other Gaussian bands remains more or less unchanged. Based on SEM, XRD, XPS and PL studies described above, the major effect of thermal activation on the back contact with a Te/Cu buffer is to cause Cu to diffuse in the Te layer without forming any CuxTe compounds. The threshold temperature required for activation is relatively low, at 200 1C, indicating that Cu diffusion in Te is fast. Furthermore, Cu from the Te/Cu buffer can diffuse through the CdTe layer into CdS/CdTe junction even at this temperature.
3.2. Effects of Te/Cu buffers on CdS/CdTe stability The fact that Cu can diffuse readily in Te and CdTe films even at a modest temperature can be a major issue affecting the long-term device stability of CdS/CdTe solar cells with Te/Cu as the back contact buffer. To address this issue we have examined the dependence of device stability on the Cu content of the Te/Cu buffers. With the vapor deposition method, it is straightforward to produce Te/Cu buffers with various Te and Cu thicknesses and estimate the molar ratio of Cu to Te, assuming that Cu diffuses freely in the Te layer at the thermal activation temperature. For stability evaluation we prepared two sets of Te/Cu buffers having a
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417
Fig. 9. (a) Photo and (b) dark J–V characteristics for cells with Te/Cu buffers of various Te thicknesses. The Cu thickness is 1.1 nm. Thermal activation was 225 1C for 4 min.
Table 5 Device performance of CdTe solar cells using Te/Cu back contact buffer with different Te thicknesses. Te/Cu thickness (nm)
Te/Cu molar ratio
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
0/1.1 50/1.1 100/1.1 200/1.1 300/1.1
0 16.3 32.7 65.4 98.1
500 810 820 825 820
20.1 21.6 21.4 21.3 21.3
63.8 74.0 77.1 74.0 74.8
6.4 12.9 13.5 13.0 13.1
fixed Cu thickness in one set and a fixed Te thickness in the other. Thermal activation was carried out at 225 1C for 4 min. The photo and dark J–V shown in Fig. 9 are for the first set of Te/Cu buffers having a fixed Cu thickness (1.1 nm) and Te thickness varied from 0 to 300 nm. The photovoltaic parameters are summarized in Table 5. It can be seen that as long as Te is present in the buffer layer the solar cell performance is normal with efficiency around 13%, and Voc and FF above 810 mV and 74%, respectively. In contrast, the cell without Te (Cu only in the buffer) exhibits rather low efficiency (6.4%), mainly due to a poor Voc (500 mV). This correlates with the low onset voltage for the dark forward-biased current (Fig. 9b), suggesting a reduction in the CdS/CdTe junction potential or a substantial contribution of the recombination current at the junction. Since Cu is in direct contact with CdTe in this cell, enhanced Cu diffusion into CdTe (without mediation by a Te layer) is the likely cause. For the Te/Cu to be effective as the back contact buffer, the minimum Te thickness appears to be about 50 nm. With or without Te, the J–V characteristics indicate that the back contact appears to be ohmic as long as Cu is present. We evaluated the stability of this set of CdS/CdTe cells under thermal stress at 200 1C in N2 for up to 9 h. In Fig. 10 the relative efficiency loss as a percentage of the initial efficiency is plotted against the thermal stress duration. It can be seen that the cell stability can be correlated with the thickness of the Te layer in Te/ Cu buffer. With a thicker Te layer the cell is more stable. The relative efficiency loss is only about 10% for the cells with 100– 300 nm Te in the buffer layer as compared to 35% for the cell with 50 nm Te. Clearly, the efficiency of the cell with 0 nm Te degraded much faster with a 43% relative efficiency loss after 9 h of thermal stress. This affirms that direct contact of Cu with CdTe is detrimental to the solar cell performance and suggests that the Te layer
Fig. 10. Cell efficiency degradation under thermal stress test at 200 1C. The thickness of the Cu layer is 1.1 nm in the Te/Cu buffers. Thermal activations were carried out at 225 1C for 4 min.
is needed to mediate the diffusion of Cu into CdTe by diluting the Cu concentration at the CdTe surface. The second set of CdS/CdTe solar cells was prepared with a Te/ Cu buffer having a fixed Te layer (100 nm) and a Cu layer varied from 0 to 2.2 nm. As shown in Fig. 11a and b, the initial photo and dark J–V characteristics are similar for all the cells. The photovoltaic parameters are summarized in Table 6. The J–V characteristics also indicate that the back contact is ohmic for all the cells regardless of the Cu content in the buffer. However, as shown in Fig. 12, the cell stability with respect to thermal stress is dramatically dependent on the Cu content. After thermal stress for 9 h at 200 1C, the relative efficiency loss is about 20% for the cell without Cu in the buffer layer as compared to 3–9% for cells with Cu. It is noteworthy that the Te/Cu buffer with only 0.3 nm Cu appears to be the most stable. Assuming that Cu is uniformly distributed in the Te layer with thermal activation, this minute amount of Cu is equivalent to a Cu to Te molar ratio of 8.34 10 3, which is far less than what have been used in previous studies [12,18,21]. According to Zhang et al. [29], Cl can accumulate at the CdTe grain boundaries and improve p-doping by creating a shallow acceptor complex VCd–ClTe. There was also evidence that the diffusion and accumulation of Cu and Cl in bulk CdTe may be correlated, in the
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Fig. 11. (a) Photo and (b) dark J–V for CdS/CdTe solar cells with Te/Cu buffers. The Cu layer is varied from 0 to 2.2 nm and the Te layer is fixed at 100 nm. The cells were subjected to a thermal activation at 225 1C for 4 min.
Table 6 Device performance of CdTe solar cells using Te/Cu back contact buffer with different Cu thicknesses. Te/Cu thickness (nm)
Cu/Te molar ratio
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
100/0.0 100/0.3 100/0.6 100/1.1 100/2.2
0 8.3 10 3 16.6 10 3 30.5 10 3 61.1 10 3
790 815 810 800 810
21.0 21.2 21.1 21.3 21.1
71.0 75.0 73.7 74.4 74.7
11.8 13.0 12.6 12.7 12.8
form of a CuCd–ClTe complex. Energetics studies [29] showed that Cu favors Cd substitution to form interstitial CuCd and Cl favors replacing Te (ClTe). The defect states having an energy level below the conduction band minimum can be removed by co-passivation of Cl and Cu. Neither Cl nor Cu itself can completely passivate the grain boundaries. The co-passivation of Cl and Cu on the defects states of grain boundaries may be beneficial for device stability [30]. In agreement with previous back contact studies [28–30], we show that Cu plays a critical role in forming ohmic contact to p-CdTe back contact and enhanced stability can be obtained by carefully controlling the Cu content in the buffer layer. 3.3. Application of Te/Cu bi-layer on ultra-thin CdTe solar cells The effects of NP etching on the morphology of an ultra-thin CdTe film (1.5 μm) are illustrated in Fig. 13. Compared with the asmade CdTe film in Fig. 13a, the CdTe film after etching (Fig. 13b) has smoother grains with fewer facets and broader grain boundaries. As shown in the cross-section view in Fig. 13c and d, boundaries of the CdTe column grains are more prominent after etching. These morphology changes suggest that the grain boundaries of CdTe are vulnerable to NP etching. The broad grain boundaries can greatly reduce the shunting resistance of CdTe films and also provide rapid-diffusion channels for metal diffusion from the back contact. Both effects can cause deterioration of cell performance. Ultra-thin CdTe solar cells with CdTe thicknesses of 0.9 μm and 1.5 μm were prepared for comparison of NP treatment and Te/Cu buffer. With NP treatment, the cell with a 0.9 μm CdTe layer was shorted. In contrast, with the vapor-deposited Te/Cu buffer the cell
Fig. 12. Device efficiency degradation with a correlation of stability test duration: (a) thermal stress test. The Te thickness is 100 nm. Thermal activations were carried out at 225 1C for 4 min.
was functional with an efficiency of 11.1%. The efficiency of 11.1% is comparable to the best efficiency of ultra-thin CdTe solar cells achieved by Compaan's group using magnetron sputtering fabrication procedure [31,32] to our best knowledge. As shown in Fig. 14 and Table 7, for cells with 1.5 μm CdTe, the Te/Cu buffer improves the cell efficiency over the NP treatment to 13.5% from 10.1%. This can be attributed to improvements in both contact resistance and reduced leakage current.
4. Summary We developed a useful buffer based on a vapor-deposited Te/Cu bi-layer for forming ohmic contact to p-CdTe in CdS/CdTe solar cells, including cells using ultra-thin CdTe. Thermal annealing was required to activate the buffer. Enhanced efficiency and stability were obtained in CdS/CdTe cells by optimizing the Cu/Te composition and the conditions for thermal activation. Characterization by XRD, XPS, and PL indicates that Cu diffuses rapidly in the Te at 200 1C without forming CuxTe compounds and Cu diffusion into CdTe can be mediated by the Te/Cu buffer.
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Fig. 13. SEM images of ultra-thin CdTe film before (a: top view, c: cross-section view) and after NP etching (25 s) (b: top view, d: cross-section view). CdTe film thickness is 1.5 mm.
Fig. 14. (a) Photo and (b) dark J–V characteristics of ultra-thin CdTe solar cells using NP treatment and Te/Cu bi-layer in the ohmic contact formation.
Table 7 Device performance of ultra-thin CdTe solar cells using NP treatment and Te/Cu back contact buffer layer in the ohmic contact formation. CdTe thickness (μm)
Te/Cu thickness (nm)
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
0.9 1.5
100/1.1 100/1.1
790 820
20.4 21.5
69.0 76.4
11.1 13.5
CdTe thickness (μm)
NP etch duration (s)
Voc (mV)
Jsc (mA cm 2)
FF (%)
η (%)
1.5
25 s
750
21.4
63.0
10.1
420
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