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Improved stability of CdTe solar cells by absorber surface etching
Solar Energy Materials & Solar Cells 162 (2017) 127–133
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Improved stability of CdTe solar cells by absorber surface etching a,1
a,2
a
a
MARK b
Ivan Rimmaudo , Andrei Salavei , Elisa Artegiani , Daniele Menossi , Marco Giarola , ⁎ Gino Mariottob, Andrea Gasparottoc, Alessandro Romeoa, a b c
Laboratory for Applied Physics, Department of Computer Science, University of Verona, Verona 37134, Italy Micro-Raman Laboratory, Department of Computer Science, University of Verona, Verona 37134, Italy Department of Physics and Astronomy “G. Galilei”, University of Padova, Via F. Marzolo 8, 35131 Padova, Italy
A R T I C L E I N F O
A BS T RAC T
Keywords: CdTe solar cells Back contact Etching Thin films
For CdTe solar cells copper seems to be necessary to achieve best energy conversion efficiencies, whilst it is known to be the main reason of cell performance degradation due to its tendency to diffuse through the bulk. Some studies have shown a direct connection between defect concentration and copper, but little has been discussed about its relation to the CdTe etching. Within this study many samples with Cu/Au back-contact have been prepared with different etching times and tested applying thermal, luminous and electrical stresses. We have analyzed the aging effects on the cell performance and on the nature and concentration of the defects by means of a variety of characterization techniques, like atomic force microscopy, Raman, current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopies. Results of the accelerated lifetime tests show that different performance degradation is observed for cells with differently etched absorber. Solar cells made with optimized etching have very low degradation while the strongest performance reduction is detected for unetched cells despite their initial efficiency is as high as for the case of etched absorbers.
1. Introduction CdTe solar cell is at the moment the thin film technology with the strongest market success, the main reason for this is that CdTe is a very simple, stoichiometric, robust material. CdS and CdTe deposition can be obtained by a wide variety of techniques achieving good efficiencies; the scalability of the fabrication process being, probably, the most interesting aspect of this device. However, some aspects are still not very clear, for example the role of Cu, which is concurrently considered either a dopant, or a necessary layer to avoid back contact rollover as well as the main degradation factor due to its fast diffusion. CdTe with a moderately wide band (Eg=1.5 eV) shows a high electron affinity (χ=4.4 eV) and there is no metal having a work function that is greater than the sum of the electron affinity plus the band gap of the absorber, to overcome this well-know limitations, different solutions have been proposed however insertion of Cu have been the most successful one [1]. High efficiency CdTe solar cells in superstrate configuration are typically fabricated treating the absorber surface by a chemical etch before the deposition of a thin Cu layer, coated by the metallic contact. The etching time is usually as short as possible to avoid damage of the
⁎ 1 2
stack. The etching process generates a tellurium rich layer (p+-type), thus increasing the back contact performance with a superior ohmic behavior [2]. Different etching procedures have been studied in the past; the most used ones are by nitric-phosphoric acid etching (N-P), Niles et al. have shown the formation of a Te rich layer [3] and by bromine-methanol etching (Br-MeOH) [4]. The effects of N-P etching on the electrical properties of the CdTe cells have been studied by Proskuryakov et at. [5]. However it has been shown that in some cases, high efficiency devices can be obtained even without CdTe surface etching [6] or by adding Te by evaporation [7]. On the other hand, as shown by Wu et al. (NREL) [2], it is possible to generate a CuxTe buffer, which stabilizes the Cu and reduces substantially the degradation of the device. In this work we propose the possibility to generate the CuxTe simply increasing the etching time and we analyze the effects of the CdTe surface etching on the stability of the back contact and of the device performance, following up the preliminary results presented at IEEE [8]. In that work we prepared CdTe solar cells with different etching conditions and with a standard Cu/Au stack. The devices were aged by accelerated stress test (AST) in a metal box with an illumination of one sun at a temperature of 85 °C [8]. Here instead the devices with different etching conditions are structurally studied by Raman spectro-
Corresponding author. Now at CINVESTAV, Merida, Mexico. Now at Calyxo Gmbh, Germany.
http://dx.doi.org/10.1016/j.solmat.2016.12.044 Received 9 March 2016; Received in revised form 19 December 2016; Accepted 25 December 2016 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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scopy and atomic force microscopy (AFM), their performance stability are tested for longer times to produce more definitive results, moreover, capacitance voltage (CV) and drive level capacitance profiling (DLCP) are presented at different frequencies and a detailed admittance spectroscopy (AS) is shown in order to address the different defects formed according to the provided etching. Moreover secondary ion mass spectrometry (SIMS) has been applied in order to verify the difference in copper diffusion for differently etched CdTe devices.
Table 1 Summary of different back contact configurations.
2. Methods
15%) have been previously obtained for 40 s-etching time [9]. If no etching is applied it is necessary to deposit 4 nm thick Cu to achieve high efficiencies, since with 2 nm thick Cu layer samples deliver efficiencies reduced to about 10%. In any case 2 nm thick Cu is a general value for back contact [10–12] where Cu acts not only as dopant but also as an ohmic contact, different is the case where Cu acts just as a dopant, mixed into a compound such as ZnTe or As2Te3 [13,14]. For the purposes of this work, we have concentrated on average results and not on the record values, for this reason the cell efficiencies where in the range of 10–12%. To assess the effect of our Br-MeOH etching, micro-Raman measurements have been carried out from unetched, 5 s-etched and 40 s-etched CdTe surfaces. The spectra shown in Fig. 1 reveal a tellurium peak only for etched samples that confirm the presence of tellurium rich layer. Moreover, they show similar peaks for the 5 setching and for the 40 s-etching. However taking into account grazing angle x-ray diffraction (GXRD) on the etched CdTe surfaces, measured previously [8], (see Fig. 2) we register a higher intensity of Te peak for the 40-s etched sample. Considering the higher penetration of x-rays this could suggest a thicker Te layer for the last case. Several GXRD peaks attributable to Cu2Te were observed only on not-etched samples and just few peaks for the 5 s-case, while only the CuTe phase occurs in the 40 s-etched samples, thus demonstrating that etching allows for the formation of different compounds, some of them (like CuTe) bringing to a better cell/device stability as mentioned by Wu et al. [2]. This because a Te surface is generated, which reacts with elemental Cu during annealing at 190 °C after deposition of Cu and Au. Moreover, etching changes the surface morphology and roughness of the CdTe surfaces compared to the as-grown ones: the height difference between the grooves at grain boundary positions and the peaks in the centre of grains is reduced by the action of the etching. As shown in Fig. 3 the morphology is very smooth for the 40 s-etched samples reducing Cu diffusion into the bulk, moreover the formation of CuTe as proved by XRD analysis [15], would further reduce Cu diffusion into the bulk. Less Cu results in more optimal doping with limited compensation.
CdTe solar cells are prepared, in our laboratory, by vacuum evaporation with a process that does not exceed the substrate temperature of 450 °C. Layers are deposited on a previously prepared transparent conductive oxide (TCO) coated soda lime glass with SiO2 sodium barrier, made by a sandwich of 400 nm thick indium tin oxide (ITO) and of 100 nm thick ZnO. The junction is made by the deposition of 300 nm CdS and 6 µm CdTe in the same vacuum chamber with substrate temperatures respectively of 100 °C and 340 °C at a pressure of 10−4 Pa. CdTe is recrystallized by rinsing CdTe with a CdCl2methanol saturated solution and annealing the stacks in air at 410 °C for 30 min. The previously mentioned etching is then applied by dipping the stack in a bromine-methanol bath (Br-MeOH), typically for a short time around 5 s [9]. The cell preparation is completed by depositing, at room temperature, a bi-layer of Cu and Au and subsequently annealing the cell at 190 °C for 20 min. The optimized thickness of the stack for a standard device consists of 2 nm for Cu and 50 nm for Au [9]. Current-voltage characteristics (JV) are obtained by using a Keithley SourceMeter 2420 at room temperature and 1000 W/m2. Drive level capacitance profiling (DLCP), capacitance-voltage (CV) and admittance spectroscopy (AS) measurements are carried out by a HP4284A LCR, the temperature is controlled by a Janis cryostat with Lakeshore 325 temperature controller in a vacuum of 10−4 Pa and in a temperature range between 100 K and 320 K. In order to study the junction between the back contact and the CdTe we have investigated the differently etched absorber surfaces by micro-Raman measurements. Raman spectra were recorded in backscattering geometry by exciting the samples with the 514.5 nm line of a mixed Ar–Kr ion gas laser. The scattered radiation was dispersed by a triple-monochromator (Horiba-Jobin Yvon, model T64000) mounting holographic gratings with 1880 lines/mm set in double subtractive/ single configuration and coupled to a nitrogen cooled CCD detector (1024×256 pixels). Roughness measurements by AFM were provided with a NT-MDT Solver Pro in semi-contact mode and NT-MDT nsg01 golden silicon tips. The ASTs were performed in a specific metal chamber, where a rack of halogen lamps and a temperature system control allows to keep the cells under one sun and 80 °C. SIMS depth profiles were obtained on a CAMECA IMS-4 f using an O2+ primary ion beam at a 12.5 kV accelerating voltage (corresponding to 8 keV impact energy) and detection of positive secondary ions. A mass resolution m/Δm ≈4000 was employed in the spectrometer to avoid mass interference between the 63Cu+ and the 126Te+ signals.
Type of Sample
Etching time (s)
Cu amount (nm)
1 2 3 4
0 0 5 40
2 4 2 2
3. Experimental Very similar solar cells but with a different preparation of the back contact were made following the above-described process. In particular for the CdTe etching, the bromine concentration was the same for all the samples (0.25 ml bromine in 40 ml of MeOH), but the etching time was varied from 0 (no etching) to 40 s and then combined with different amount of Cu (see Table 1). We have not observed significant differences in efficiency for different etching times. However our best devices (efficiency above
Fig. 1. Raman spectra of the unetched, 5 s-etched and 40 s-etched CdTe samples.
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Fig. 4. Relative efficiency (with standard deviation) during AST (accelerated stability test) over more than 1000 h, for devices with no etching (n.1-circles), 5 s (n.3-up triangles) and 40 s (n.4-diamonds) etching. An unetched sample with 4 nm of Cu (n.2squares) is compared. Upper graph: ageing in SCC, bottom graph ageing in OCC. Numbers identify the different type of samples according to Table 1. Fig. 2. GIXRD patterns of the unetched, 5 s-etched and 40 s-etched CdTe samples [8].
Fig. 3. Morphology profile by AFM of the unetched, 5 s-etched and 40 s-etched CdTe samples. Fig. 5. J–V curves of 4 samples (taken as examples, unetched with 2 and 4 nm, 5 setched and 40 s-etched CdTe samples) before (top graph) and after accelerated stability test (AST) (bottom graphs: left in OCC, right in SCC).
4. Results and discussion 4.1. Testing under accelerated lifetime stability conditions
reduced performance degradation, mainly due to open circuit voltage (Voc) reduction, while fill factor (FF) and short circuit current (Jsc) are only slightly reduced, back contact does not show rollover and series resistance is not affected by the ageing. Cells with double amount of Cu (4 nm) show very similar performance reduction, indicating that the degradation process is not determined by the Cu amount. The samples etched for 40 s show improved stability in both OCC and SCC with reduction of all the parameters. In this case rollover is always observed, but it is more pronounced in SCC than in OCC. Moreover it should be noticed that the cells were aged for more than 1200 h and the final efficiency was around 80% of the initial one. It has been reported that a temperature of 100 °C for 700 h could correspond to 10 years of service at 50 °C [23]. A more detailed analysis of the acceleration factor reported by Sites et Al. [20] and Mc Mahon et al. [21] brings to a rough estimation for 1200 h at 85 °C of an equivalent time of about 13 years of service. This shows that cells with a simple Cu based back contact (otherwise from having a ZnTe:Cu, As2Te3:Cu or Bi2Te3:Cu back contact [13,15]) would be able to provide a reasonable stability. Devices with 5 s Br-MeOH etched CdTe, in OCC are more stable than the unetched ones, but slightly less stable than the 40 s ones. After more than 1000 h, the efficiency reduces to 35%, 70% and 80% of the
The stability of the finished devices has been tested in the accelerated lifetime box, described in Section 2, and the device properties have been subsequently measured at different aging steps. The devices have been tested in both open circuit condition (OCC) and short current condition (SCC). In this last case the cells were aged with current flowing, as reported in [16–20]. This kind of analysis has been repeated on many solar cells (each set was composed between 20 and 30 solar cells) and the results are presented in Fig. 4. For better comparison all the efficiencies were normalized to 1. In general, the cells not subjected to etching show very different J-V curves before and after the AST (see Fig. 5). It is known that the back barrier height due to the Schottky junction at the back contact can be determined by analyzing the J-V curve and in particular the current at which the rollover curvature is maximum: this represents the reverse saturation current of the parasitic diode at the back contact. As a matter of fact the device is modeled as a connection in series of two diodes in opposite polarity: main junction and back contact [21,22] Under OCC the efficiency is dramatically affected and all the parameters are strongly reduced, also rollover appears at the back contact. On the other hand, under SCC the unetched samples show 129
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Fig. 7. DLCP (full dots) and CV (open dots) profiles at different ageing times under OCC at 100 kHz of 40 s-etched CdTe devices.
Fig. 6. DLCP (full dots) and CV (empty dots) analysis of devices with no etching (blue circles), with 5 s (green triangles) and with 40 s (red squares), at 10 kHz (top) and 1 MHz (bottom). Y-axe: difference between acceptors and donor concentration (the net acceptor concentration). X-axe: distance from the n/p junction. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article)
(blue full circles), that indicates different deep states connected with etching. Therefore, despite the low doping, the initial good performance of the not-etched samples could be attributed to the small deep defects concentration and hence, to reduced trap assisted recombination. Finally the 5 s-etched samples show profiles that are almost independent by the frequency. We refrain ourselves to give any exact explanation of the reason why DLCP response is smaller than CV one in case of 5 s-etching. Usually this effect can be avoided reverting the DC voltage sweep, which suggests a kind of “memory effect” of the defects. For reliable comparisons all the curves reported in Figs. 6 and 7 were measured sweeping the voltage from negative to positive values. Furthermore, we noted that high reverse bias could affect the samples (in particular notetched ones); hence, all the electrical measurements were performed keeping voltage higher than −2 V. This limitation avoids the depletion region extension towards the back contact, in particular for samples etches for 40 s due to the higher net charge density. On the other hand the maximum forward bias was set much below the voltage where the CV curve (not shown here) has a maximum, in order to avoid the influence of the parasitic back contact capacitance (more details in [27]). Taking into account all these aspects we exclude possible artifacts since our results are perfectly in line with values (1014 < Na-Nd < 1015) and curves recently reported in literature for not degraded CdTe devices [28–30].
initial value for the unetched, 5 s and 40 s respectively. On the other hand, in SCC the mean value of final efficiencies is very similar for the three cases (around 70% of the initial value). For cells without etching, the degradation strongly depends on the ageing conditions. In OCC the performance decreases fast and similar for both 2 and 4 nm Cu samples shown in Fig. 4 (which is evident by the high standard deviation) while in the SCC case these cells keep the performance similar to the etched ones and with small standard deviation. Limited degradation in SCC has been already observed [24] and explained by the fact that Cu diffuses as a positive ion hence it migrates towards CdS only if no external bias is applied (as in OCC conditions).
4.2. CV-DLCP analysis before aging In order to understand the effects of Br-MeOH etching we have performed CV and DLCP measurements on the above-mentioned samples before AST. In Fig. 6, we compare devices made with unetched, 5 s and 40 s-etched CdTe surface at frequencies of 1 MHz and 10 kHz. From these measurements it is possible to estimate the depletion region charge (Na-Nd) and profile it along the device. The shallow defects concentration is indicated by the DLCP curves (full dots) (since DLCP is only subjected to defects close to the valence band [25]), while the CV profiles (open dots) indicate the contribution of both deep and shallow defects to the charge concentration [26]. By comparing cells with same amount of Cu in the back contact but with different etching times, it is surprising that 40 s-etching deliver higher net charge concentration. At a first glance it is clear that the DLCP lines are at higher level for the 40 s-etched sample (full red squares): the etching affects the net shallow defects concentration, which can be identified as the doping density. At 10 kHz shallow defects concentration is in the order of 1014 cm−3 with an increase when getting close to the junction, where also deep defects are highly concentrated (open red squares). Similar considerations are suggested also by the red profiles at 1 MHz. For the 40 s samples the frequency increase from 10 kHz to 1 MHz determines a general reduction of the shallow defects response, in particular close to the junction. This makes more pronounced the contribution of the deep defects to the net charge. In general not-etched samples show smaller net charge concentration, moreover the frequency response is opposite to 40 s-etched case: increasing the frequency results in a smaller deep defects response (blue empty circles in Fig. 6) while the shallow defects stays constant
4.3. CV-DLCP analysis after aging The most stable case (40 s-etching under OCC) has been studied by CV-DLCP at 10 kHz and at different AST time up to more than 1500 h. As time flows, we observe a continuous reduction of the net charge NaNd both when measured by DLCP (Na depends only by shallow acceptor contribution) and CV (Na depends by the sum of shallow and deep acceptors). This effect could be explained by i) increase of the compensation in the absorber (increase of Nd) or ii) annihilation/transformation of the electronic state connected to an instable physical defect (decrease of Na). In the left side of Fig. 7 the DLCP profiles indicate a continuous reduction of the net shallow defects of almost one order of magnitude in 888 h, while in the same time the deep states concentration reduces only to one half (magenta empty stars in right side of Fig. 7). Since an increasing donors concentration (Nd) should affect similarly the DLCP and CV profiles (as for the unetched samples), we can conclude that for 40 s etching, both i) and ii) were registered, but the dominant effect in the doping reduction is mainly due to ii) and in minor part to compensation i). Other authors reported presence of shallow donors connected with 130
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but deep levels were registered (B1-B4). B2 and B3 are very similar to A2 and A3 respectively, while B1 and B1* can be attributed to the native CdTe acceptor, the isolated VCd-/2- [5,36]. Finally after 1600 h under AST similar deep levels are present: C2, C2*(same consideration as for A2) and C3 which could be the same defect of A3 and B3. However new AS peaks relative to shallow acceptors appear (C0, C1,C1*), such shallow defects are usually attributed to impurities that can from the glass (as Na) or from the back contact (as Ag) [5,37]. The latter could have been introduced by residues of silver paint used to contact the cell during electrical characterization. AS was also performed on the not-etched samples after 900 h under AST for comparison. In this case the deep and shallow acceptor levels are very similar to the ones registered in the 40 s samples at 0 h. However AS reveals a relative deep defects (D2) never registered in the 40 s sample for the 1600 h AST. This is commonly attributed to CuCd[30,36,37], theVCd–Cui complex is excluded from consideration due to its high instability as calculated by Krasikov et al. and supported by the measurements of Evani et al. [40], both identify CuCd- as the most probable form of copper related defect [41]. As mentioned from Krasikov et al. [41] cadmium vacancies are less likely to be found when copper is diffusing since they might be transformed in CuCd and this is observed in our AS results: VCd are present only for etched samples, VCd are however not completely excluded as also observed by Harvey et al. [42].
Table 2 Energy levels and cross sections of measured defects. Etching
AST
[s] 40
[h] 0
960
1600
unetched
900
Level
Ea
Error
□a
Identification
A0 A1* A1 A2 A3 B1 B1* B2 B3 C0 C1 C1* C2 C2* C3 D0 D1 D1* D2 D3 D4 D4*
[meV] 116 136 150 460 527 407 429 479 553 77 86 96 491 503 597 130 148 170 316 448 465 475
[eV] 7 4 1 14 11 13 11 16 13 2 2 3 1 12 20 4 6 3 15 13 15 5
[cm−2] 4,00E-16 3,00E-15 1,70E-14 1,20E-12 1,00E-11 2,73E-14 7,48E-14 4,17E-13 5,32E-12 4,72E-18 1,09E-17 2,63E-17 1,67E-12 4,12E-12 1,51E-10 1,37E-16 3,58E-16 3,88E-15 1,03E-16 1,30E-13 3,73E-15 7,02E-14
(VCd2-–ClTe+)complex of VCd2- or VTeId. Not identified Cation vacancy VCd-/2Id. Not identified Cation vacancy Impurity Id. Id. Not identified Id. Cation Vacancy (VCd2-–ClTe+)(2CuCd-–VTe+)-, Id. CuCd-, VCd-/2Not identified Id.
Cu [18,19,27,31], on the other hand Tiwari et al. [32] used Cu to dope CdTe but they also reported strong limitations in the possible amount of dopant to avoid compensation. In our case, considering the migration of Cu from the back contact as the source of the compensation, we can conclude that applying long etching to the CdTe surface reduces the Cu tendency to act as a compensator. To address the ii) component of the doping reduction, we have applied AS (see next section). Another interesting consideration is that after a very long time of AST the shallow states concentration slightly increases (purple diamonds in Fig. 7) while the deep defects contribution does not change and shifts toward the junction.
4.5. SIMS measurements In order to confirm the higher diffusion of copper for devices with not-etched CdTe, SIMS measurements have been carried out on different samples specially prepared for this analysis. In Fig. 8 the concentration of copper atoms in solar cells with and without Br-MeOH etching, after 650 h of ageing under AST condition, is presented together with their concentration. In absence of a Cu implanted standard sample, the Cu concentration was calculated by applying the Relative Sensitivity Factor (RSF) reported by Asher et al. in their paper [43], where they measured Cu in CdTe samples in the same experimental conditions used in the present paper. A possible calibration error due to this procedure applies in an identical way to all the measured samples so it does not influence the results of a direct comparison between the reported Cu profiles. It is evident how copper diffuses into the CdTe for the not-etched case, while for the etched case the Cu signal decreases very rapidly in the first nm beneath the surface layer. In particular we can observe at a depth of about 500 nm a difference in concentration of one order of
4.4. Admittance spectroscopy To highlight the effect of the 40 s-etching on the defects in the CdTe structure, AS measurements have been applied with a frequency from 300 Hz to 1 MHz and at a temperature from 90 K to 320 K. The characterization was repeated at different time of the AST in OCC (see Table 2). For not-etched samples AS is still possible even despite the high back contact barrier height as already demonstrated by [5], however after too long AST the AS peaks evolution with temperature as described in [33,34] seems to be no longer respected. The last reliable measurement on a not-etched sample was after 900 h AST and it is reported in Table 2 for comparison. Each measurement was performed by applying direct current (DC) bias of 0 V, −0.5 V and +0.5 V. Defects activation energy and capture cross section values were extracted and reported in Table 2; we have indicated as A, B, C defects respectively observed for the 40 s-etched sample at 0, 960,1600 aging hours and D for defects observed for the unetched case after 900 aging hours. We are aware that calculated cross sections have relative reliability since the numbers are very small, however they can contribute to the identification of the defects if confronted with energy values and compared with literature results. For the 40 s-etched samples, representing the most stable devices, four AS peaks before aging (0 h) were observed (A0–A3). They could be distinguished by applying different DC biases [35]. Three of them (A0, A1, A1*) are usually attributed to complexes involving Vcd- and an impurity atom, in general Cl and Cu [36,37]. A2 has been reported but never clearly assigned [38,39]. A3 could be attributed to a generic cation vacancy [36]. After 960 h under AST no shallow defects signature was detected,
Fig. 8. SIMS depth profiles of Cu after 650 h under AST conditions in devices with Br-MeOh etched CdTe (black circles) and with not-etched CdTe (red triangles). (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article)
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magnitude, while at a depth of about one μm the signal goes below the detection limit in these experimental conditions. We believe this is a clear demonstration of the fact that etching allows to appreciably reduce copper diffusion.
a dopant for their low activation energy, while compensation due to Cui+ is limited. In this case, the degradation is probably dominated by the slow decomposition of the above-mentioned shallow complexes. This generates deep states like VCd-/2- that have a negative impact since they increase the carriers’ recombination. Finally, after very long AST time, we observe formation of defects probably due to impurities coming either from the glass substrate or from the back contact, which could take part to the degradation. SIMS measurements (Fig. 8) confirm that copper diffusion is limited when etching is applied.
5. Discussion From the analysis of the JV, we can observe that, in accordance with other works [14] for our samples the etching is not necessary to achieve good efficiency. However for unetched samples the Cu layer thickness needs to be increased from 2 to 4 nm to achieve best efficiencies. This effect seems not to be connected to the back contact performance (none of these samples show rollover), but to a different Cu incorporation during the 190 °C annealing due to the absence of the etching. The CdTe surface has been analyzed by Raman (Fig. 1), GXRD (Fig. 2) and AFM (Fig. 3) to address the connection between morphology, Te rich layer, CuxTe compounds and the back contact performance. Subsequently to a long etching, CdTe surface appears smoother (as expected) and Raman spectra confirm the generation of a Te rich layer. However, 40 s-etching time does not result in a higher concentration of Te at the surface compared to the 5 s-etching time case. On the other hand the XRD reveals the presence of different compounds depending on the etching time. Cu2Te formation takes place when the etching time is short or even absent. On the contrary for samples etched for 40 s, Cu2Te is replaced by CuTe, which has been reported as a stable compound [2,19,20,31]. AST were performed in sets of more than 20 cells to study the changes in solar cells stability over a long time period (1200 h). The best stability was obtained for long time etching cases in SCC (20% loss in over 1200 h) (see Fig. 4). To our knowledge this is a remarkable result for a simple Cu/Au back contact since a similar performance stability is usually achieved introducing more complicated contacts [19,20,31]. Other studies [17,18,44] reported evidence of two different Cu diffusion components into the CdTe, characterized by different diffusion energies. The fast one has been connected to Cu+ diffusion via interstitial sites generating shallow donor states (Cui+, Ec−0.55 meV) [18,19,36]. The second component needs higher energy to generate and to dissociate (VCd2-– Cui+)-, (2CuCd-– Cui+)-, (2CuCd-–VTe+)- and CuCd-. The bias application under AST (SCC) to our unetched samples results in a degradation comparable to the etched ones, while in OCC it is dramatically faster, as also observed by other authors [19,20]. This indicates a strong drift component that confirms the Cu diffusion as positive ion [18,19]. On the other hand for the 40 s-etched samples the bias has small impact on the degradation excluding the massive presence of free Cu ions. Moreover CV-DLCP analysis on 40 s samples throughout the AST indicates the continuous reduction of the shallow acceptor concentration and the simultaneous deep states formation as the main degradation factor. The AS confirms: the presence of shallow complexes in the 40 setched samples before the AST, their absence after the AST and the presence of deep levels usually attributed to VCd–/2-. So we can state that, in the absence of etching, during the back contact annealing Cu reacts and generate Cu2Te at the surface. The following degradation is dominated by diffusion of Cu via interstitial sites that compensate the absorber generating shallow donors (Cui+, Ec-0.55 meV). As already mentioned Cu2Te is unstable and it results to be the main supplier of Cu+ to the solar cell [2,20]. In the absence of bias the cells without etching reduce their performance to 50% after only 300 h of AST. On the other hand, for long etching most of the Cu deposited in the back contact is used to form CuTe at the surface and complexes involving Cu like (VCd2-–Cui+)- in the bulk, which can be considered as
6. Conclusions Among the different reasons for degradation of CdTe solar cells (heat and moisture cause oxidation of the CdTe surface, as for example reported for Au contacted cells [45]), in presence of copper its diffusion into the CdTe bulk show high reduction of the photovoltaic performance [19]. The work presented in this paper shows that etching changes the way in which Cu reacts with CdTe surface: in fact by means of a 40 s Br-MeOh etching and 2 nm of Cu deposited on top of it, it is possible to obtain a CuTe and an extremely limited diffusion of copper. The aging shows that for the not-etched samples Cui+ ions are generated, as expected, while for the 40 s-etched samples no defects generated by copper are observed. Therefore, with a suitable optimization of the etching conditions it is possible to obtain a rather stable device even with a simple Cu based back contact. Acknowledgments We would like to acknowledge the financial support received within the Joint Project Program of the University of Verona grant number: B38I13000640003 with the specific NANOCAD project. References [1] A. Romeo, M. Terheggen, D. Abou-Ras, D.L. Bätzner, F.-J. Haug, M. Kälin, et al., Development of thin-film Cu(In,Ga)Se2 and CdTe solar cells, Prog. Photovolt. Res. Appl. 12 (2004) 93–111. [2] X. Wu, J. Zhou, a. Duda, Y. Yan, G. Teeter, S. Asher, et al., Phase control of CuxTe film and its effects on CdS/CdTe solar cell, Thin Solid Films. 515 (2007) 5798–5803. [3] D.W. Niles, X. Li, P. Sheldon, A photoemission interface determination of the band diagram of the Te / CdTe, J. Appl. Phys. 77 (1995) 4489–4493. [4] I.M. Dharmadasa, J.M. Thornton, R.H. Williams, Effects of surface treatments on Schottky barrier formation at metal/n‐type CdTe contacts, Appl. Phys. Lett. 54 (1989). [5] Y.Y. Proskuryakov, K. Durose, B.M. Taele, G.P. Welch, S. Oelting, Admittance spectroscopy of CdTeCdS solar cells subjected to varied nitric-phosphoric etching conditions, J. Appl. Phys. 101 (2007). [6] S. Mazzamuto, L. Vaillant, A. Bosio, N. Romeo, N. Armani, G. Salviati, A study of the CdTe treatment with a Freon gas such as CHF2Cl, Thin Solid Films 516 (2008) 7079–7083. [7] 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. Photovolt. Res. Appl. 4 (1996) 225–229. [8] I. Rimmaudo, A. Salavei, F. Rossi, C. Ferrari, A. Romeo, Etching effect of CdTe absorber on the stability of thin film solar cell devices, in: Proceedings of the 2013 IEEE 39th Photovolt. Spec. Conf., 2013, pp. 2029–2033. [9] Andrei Salavei, Ivan Rimmaudo, Fabio Piccinelli, Daniele Menossi, Nicola Romeo, Alessandro Romeo, Analysis of CdTe activation treatment with a novel approach, in: Proceedings of the 28th Eur. Photovolt. Sol. Energy Conf. Exhib., 2014, pp. 2140–2142. [10] C.R. Corwine, A.O. Pudov, M. Gloeckler, S.H. Demtsu, J.R. Sites, Copper inclusion and migration from the back contact in CdTe solar cells, Sol. Energy Mater. Sol. Cells 82 (2004) 481–489. [11] S. Demtsu, D. Albin, J. Sites, Role of Copper in the Performance of CdS /CdTe Solar Cells, in: Proceedings of the Conf. Rec. IEEE 4th World Conf. Photovolt. Energy Convers., 2006, pp. 523–526. [12] S.S. Hegedus, B.E. Mccandless, R.W. Birkmire, Analysis of stress-induced degradation in CdS/CdTe solar cells, in: Proceedings of the Conf. Rec. 28th IEEE Photovolt. Spec. Conf., 2000, pp. 535–538. [13] T.A. Gessert, S. Asher, S. Johnston, A. Duda, M.R. Young, T. Moriarty, Formation Of ZnTe: Cu/Ti Contacts at High Temperature for CdS/CdTe Devices, in: Proceedings of the Conf. Rec. 2006 IEEE 4th World Conf. Photovolt. Energy
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Improved stability of CdTe solar cells by absorber surface etching a,1
a,2
a
a
MARK b
Ivan Rimmaudo , Andrei Salavei , Elisa Artegiani , Daniele Menossi , Marco Giarola , ⁎ Gino Mariottob, Andrea Gasparottoc, Alessandro Romeoa, a b c
Laboratory for Applied Physics, Department of Computer Science, University of Verona, Verona 37134, Italy Micro-Raman Laboratory, Department of Computer Science, University of Verona, Verona 37134, Italy Department of Physics and Astronomy “G. Galilei”, University of Padova, Via F. Marzolo 8, 35131 Padova, Italy
A R T I C L E I N F O
A BS T RAC T
Keywords: CdTe solar cells Back contact Etching Thin films
For CdTe solar cells copper seems to be necessary to achieve best energy conversion efficiencies, whilst it is known to be the main reason of cell performance degradation due to its tendency to diffuse through the bulk. Some studies have shown a direct connection between defect concentration and copper, but little has been discussed about its relation to the CdTe etching. Within this study many samples with Cu/Au back-contact have been prepared with different etching times and tested applying thermal, luminous and electrical stresses. We have analyzed the aging effects on the cell performance and on the nature and concentration of the defects by means of a variety of characterization techniques, like atomic force microscopy, Raman, current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopies. Results of the accelerated lifetime tests show that different performance degradation is observed for cells with differently etched absorber. Solar cells made with optimized etching have very low degradation while the strongest performance reduction is detected for unetched cells despite their initial efficiency is as high as for the case of etched absorbers.
1. Introduction CdTe solar cell is at the moment the thin film technology with the strongest market success, the main reason for this is that CdTe is a very simple, stoichiometric, robust material. CdS and CdTe deposition can be obtained by a wide variety of techniques achieving good efficiencies; the scalability of the fabrication process being, probably, the most interesting aspect of this device. However, some aspects are still not very clear, for example the role of Cu, which is concurrently considered either a dopant, or a necessary layer to avoid back contact rollover as well as the main degradation factor due to its fast diffusion. CdTe with a moderately wide band (Eg=1.5 eV) shows a high electron affinity (χ=4.4 eV) and there is no metal having a work function that is greater than the sum of the electron affinity plus the band gap of the absorber, to overcome this well-know limitations, different solutions have been proposed however insertion of Cu have been the most successful one [1]. High efficiency CdTe solar cells in superstrate configuration are typically fabricated treating the absorber surface by a chemical etch before the deposition of a thin Cu layer, coated by the metallic contact. The etching time is usually as short as possible to avoid damage of the
⁎ 1 2
stack. The etching process generates a tellurium rich layer (p+-type), thus increasing the back contact performance with a superior ohmic behavior [2]. Different etching procedures have been studied in the past; the most used ones are by nitric-phosphoric acid etching (N-P), Niles et al. have shown the formation of a Te rich layer [3] and by bromine-methanol etching (Br-MeOH) [4]. The effects of N-P etching on the electrical properties of the CdTe cells have been studied by Proskuryakov et at. [5]. However it has been shown that in some cases, high efficiency devices can be obtained even without CdTe surface etching [6] or by adding Te by evaporation [7]. On the other hand, as shown by Wu et al. (NREL) [2], it is possible to generate a CuxTe buffer, which stabilizes the Cu and reduces substantially the degradation of the device. In this work we propose the possibility to generate the CuxTe simply increasing the etching time and we analyze the effects of the CdTe surface etching on the stability of the back contact and of the device performance, following up the preliminary results presented at IEEE [8]. In that work we prepared CdTe solar cells with different etching conditions and with a standard Cu/Au stack. The devices were aged by accelerated stress test (AST) in a metal box with an illumination of one sun at a temperature of 85 °C [8]. Here instead the devices with different etching conditions are structurally studied by Raman spectro-
Corresponding author. Now at CINVESTAV, Merida, Mexico. Now at Calyxo Gmbh, Germany.
http://dx.doi.org/10.1016/j.solmat.2016.12.044 Received 9 March 2016; Received in revised form 19 December 2016; Accepted 25 December 2016 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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scopy and atomic force microscopy (AFM), their performance stability are tested for longer times to produce more definitive results, moreover, capacitance voltage (CV) and drive level capacitance profiling (DLCP) are presented at different frequencies and a detailed admittance spectroscopy (AS) is shown in order to address the different defects formed according to the provided etching. Moreover secondary ion mass spectrometry (SIMS) has been applied in order to verify the difference in copper diffusion for differently etched CdTe devices.
Table 1 Summary of different back contact configurations.
2. Methods
15%) have been previously obtained for 40 s-etching time [9]. If no etching is applied it is necessary to deposit 4 nm thick Cu to achieve high efficiencies, since with 2 nm thick Cu layer samples deliver efficiencies reduced to about 10%. In any case 2 nm thick Cu is a general value for back contact [10–12] where Cu acts not only as dopant but also as an ohmic contact, different is the case where Cu acts just as a dopant, mixed into a compound such as ZnTe or As2Te3 [13,14]. For the purposes of this work, we have concentrated on average results and not on the record values, for this reason the cell efficiencies where in the range of 10–12%. To assess the effect of our Br-MeOH etching, micro-Raman measurements have been carried out from unetched, 5 s-etched and 40 s-etched CdTe surfaces. The spectra shown in Fig. 1 reveal a tellurium peak only for etched samples that confirm the presence of tellurium rich layer. Moreover, they show similar peaks for the 5 setching and for the 40 s-etching. However taking into account grazing angle x-ray diffraction (GXRD) on the etched CdTe surfaces, measured previously [8], (see Fig. 2) we register a higher intensity of Te peak for the 40-s etched sample. Considering the higher penetration of x-rays this could suggest a thicker Te layer for the last case. Several GXRD peaks attributable to Cu2Te were observed only on not-etched samples and just few peaks for the 5 s-case, while only the CuTe phase occurs in the 40 s-etched samples, thus demonstrating that etching allows for the formation of different compounds, some of them (like CuTe) bringing to a better cell/device stability as mentioned by Wu et al. [2]. This because a Te surface is generated, which reacts with elemental Cu during annealing at 190 °C after deposition of Cu and Au. Moreover, etching changes the surface morphology and roughness of the CdTe surfaces compared to the as-grown ones: the height difference between the grooves at grain boundary positions and the peaks in the centre of grains is reduced by the action of the etching. As shown in Fig. 3 the morphology is very smooth for the 40 s-etched samples reducing Cu diffusion into the bulk, moreover the formation of CuTe as proved by XRD analysis [15], would further reduce Cu diffusion into the bulk. Less Cu results in more optimal doping with limited compensation.
CdTe solar cells are prepared, in our laboratory, by vacuum evaporation with a process that does not exceed the substrate temperature of 450 °C. Layers are deposited on a previously prepared transparent conductive oxide (TCO) coated soda lime glass with SiO2 sodium barrier, made by a sandwich of 400 nm thick indium tin oxide (ITO) and of 100 nm thick ZnO. The junction is made by the deposition of 300 nm CdS and 6 µm CdTe in the same vacuum chamber with substrate temperatures respectively of 100 °C and 340 °C at a pressure of 10−4 Pa. CdTe is recrystallized by rinsing CdTe with a CdCl2methanol saturated solution and annealing the stacks in air at 410 °C for 30 min. The previously mentioned etching is then applied by dipping the stack in a bromine-methanol bath (Br-MeOH), typically for a short time around 5 s [9]. The cell preparation is completed by depositing, at room temperature, a bi-layer of Cu and Au and subsequently annealing the cell at 190 °C for 20 min. The optimized thickness of the stack for a standard device consists of 2 nm for Cu and 50 nm for Au [9]. Current-voltage characteristics (JV) are obtained by using a Keithley SourceMeter 2420 at room temperature and 1000 W/m2. Drive level capacitance profiling (DLCP), capacitance-voltage (CV) and admittance spectroscopy (AS) measurements are carried out by a HP4284A LCR, the temperature is controlled by a Janis cryostat with Lakeshore 325 temperature controller in a vacuum of 10−4 Pa and in a temperature range between 100 K and 320 K. In order to study the junction between the back contact and the CdTe we have investigated the differently etched absorber surfaces by micro-Raman measurements. Raman spectra were recorded in backscattering geometry by exciting the samples with the 514.5 nm line of a mixed Ar–Kr ion gas laser. The scattered radiation was dispersed by a triple-monochromator (Horiba-Jobin Yvon, model T64000) mounting holographic gratings with 1880 lines/mm set in double subtractive/ single configuration and coupled to a nitrogen cooled CCD detector (1024×256 pixels). Roughness measurements by AFM were provided with a NT-MDT Solver Pro in semi-contact mode and NT-MDT nsg01 golden silicon tips. The ASTs were performed in a specific metal chamber, where a rack of halogen lamps and a temperature system control allows to keep the cells under one sun and 80 °C. SIMS depth profiles were obtained on a CAMECA IMS-4 f using an O2+ primary ion beam at a 12.5 kV accelerating voltage (corresponding to 8 keV impact energy) and detection of positive secondary ions. A mass resolution m/Δm ≈4000 was employed in the spectrometer to avoid mass interference between the 63Cu+ and the 126Te+ signals.
Type of Sample
Etching time (s)
Cu amount (nm)
1 2 3 4
0 0 5 40
2 4 2 2
3. Experimental Very similar solar cells but with a different preparation of the back contact were made following the above-described process. In particular for the CdTe etching, the bromine concentration was the same for all the samples (0.25 ml bromine in 40 ml of MeOH), but the etching time was varied from 0 (no etching) to 40 s and then combined with different amount of Cu (see Table 1). We have not observed significant differences in efficiency for different etching times. However our best devices (efficiency above
Fig. 1. Raman spectra of the unetched, 5 s-etched and 40 s-etched CdTe samples.
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Fig. 4. Relative efficiency (with standard deviation) during AST (accelerated stability test) over more than 1000 h, for devices with no etching (n.1-circles), 5 s (n.3-up triangles) and 40 s (n.4-diamonds) etching. An unetched sample with 4 nm of Cu (n.2squares) is compared. Upper graph: ageing in SCC, bottom graph ageing in OCC. Numbers identify the different type of samples according to Table 1. Fig. 2. GIXRD patterns of the unetched, 5 s-etched and 40 s-etched CdTe samples [8].
Fig. 3. Morphology profile by AFM of the unetched, 5 s-etched and 40 s-etched CdTe samples. Fig. 5. J–V curves of 4 samples (taken as examples, unetched with 2 and 4 nm, 5 setched and 40 s-etched CdTe samples) before (top graph) and after accelerated stability test (AST) (bottom graphs: left in OCC, right in SCC).
4. Results and discussion 4.1. Testing under accelerated lifetime stability conditions
reduced performance degradation, mainly due to open circuit voltage (Voc) reduction, while fill factor (FF) and short circuit current (Jsc) are only slightly reduced, back contact does not show rollover and series resistance is not affected by the ageing. Cells with double amount of Cu (4 nm) show very similar performance reduction, indicating that the degradation process is not determined by the Cu amount. The samples etched for 40 s show improved stability in both OCC and SCC with reduction of all the parameters. In this case rollover is always observed, but it is more pronounced in SCC than in OCC. Moreover it should be noticed that the cells were aged for more than 1200 h and the final efficiency was around 80% of the initial one. It has been reported that a temperature of 100 °C for 700 h could correspond to 10 years of service at 50 °C [23]. A more detailed analysis of the acceleration factor reported by Sites et Al. [20] and Mc Mahon et al. [21] brings to a rough estimation for 1200 h at 85 °C of an equivalent time of about 13 years of service. This shows that cells with a simple Cu based back contact (otherwise from having a ZnTe:Cu, As2Te3:Cu or Bi2Te3:Cu back contact [13,15]) would be able to provide a reasonable stability. Devices with 5 s Br-MeOH etched CdTe, in OCC are more stable than the unetched ones, but slightly less stable than the 40 s ones. After more than 1000 h, the efficiency reduces to 35%, 70% and 80% of the
The stability of the finished devices has been tested in the accelerated lifetime box, described in Section 2, and the device properties have been subsequently measured at different aging steps. The devices have been tested in both open circuit condition (OCC) and short current condition (SCC). In this last case the cells were aged with current flowing, as reported in [16–20]. This kind of analysis has been repeated on many solar cells (each set was composed between 20 and 30 solar cells) and the results are presented in Fig. 4. For better comparison all the efficiencies were normalized to 1. In general, the cells not subjected to etching show very different J-V curves before and after the AST (see Fig. 5). It is known that the back barrier height due to the Schottky junction at the back contact can be determined by analyzing the J-V curve and in particular the current at which the rollover curvature is maximum: this represents the reverse saturation current of the parasitic diode at the back contact. As a matter of fact the device is modeled as a connection in series of two diodes in opposite polarity: main junction and back contact [21,22] Under OCC the efficiency is dramatically affected and all the parameters are strongly reduced, also rollover appears at the back contact. On the other hand, under SCC the unetched samples show 129
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Fig. 7. DLCP (full dots) and CV (open dots) profiles at different ageing times under OCC at 100 kHz of 40 s-etched CdTe devices.
Fig. 6. DLCP (full dots) and CV (empty dots) analysis of devices with no etching (blue circles), with 5 s (green triangles) and with 40 s (red squares), at 10 kHz (top) and 1 MHz (bottom). Y-axe: difference between acceptors and donor concentration (the net acceptor concentration). X-axe: distance from the n/p junction. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article)
(blue full circles), that indicates different deep states connected with etching. Therefore, despite the low doping, the initial good performance of the not-etched samples could be attributed to the small deep defects concentration and hence, to reduced trap assisted recombination. Finally the 5 s-etched samples show profiles that are almost independent by the frequency. We refrain ourselves to give any exact explanation of the reason why DLCP response is smaller than CV one in case of 5 s-etching. Usually this effect can be avoided reverting the DC voltage sweep, which suggests a kind of “memory effect” of the defects. For reliable comparisons all the curves reported in Figs. 6 and 7 were measured sweeping the voltage from negative to positive values. Furthermore, we noted that high reverse bias could affect the samples (in particular notetched ones); hence, all the electrical measurements were performed keeping voltage higher than −2 V. This limitation avoids the depletion region extension towards the back contact, in particular for samples etches for 40 s due to the higher net charge density. On the other hand the maximum forward bias was set much below the voltage where the CV curve (not shown here) has a maximum, in order to avoid the influence of the parasitic back contact capacitance (more details in [27]). Taking into account all these aspects we exclude possible artifacts since our results are perfectly in line with values (1014 < Na-Nd < 1015) and curves recently reported in literature for not degraded CdTe devices [28–30].
initial value for the unetched, 5 s and 40 s respectively. On the other hand, in SCC the mean value of final efficiencies is very similar for the three cases (around 70% of the initial value). For cells without etching, the degradation strongly depends on the ageing conditions. In OCC the performance decreases fast and similar for both 2 and 4 nm Cu samples shown in Fig. 4 (which is evident by the high standard deviation) while in the SCC case these cells keep the performance similar to the etched ones and with small standard deviation. Limited degradation in SCC has been already observed [24] and explained by the fact that Cu diffuses as a positive ion hence it migrates towards CdS only if no external bias is applied (as in OCC conditions).
4.2. CV-DLCP analysis before aging In order to understand the effects of Br-MeOH etching we have performed CV and DLCP measurements on the above-mentioned samples before AST. In Fig. 6, we compare devices made with unetched, 5 s and 40 s-etched CdTe surface at frequencies of 1 MHz and 10 kHz. From these measurements it is possible to estimate the depletion region charge (Na-Nd) and profile it along the device. The shallow defects concentration is indicated by the DLCP curves (full dots) (since DLCP is only subjected to defects close to the valence band [25]), while the CV profiles (open dots) indicate the contribution of both deep and shallow defects to the charge concentration [26]. By comparing cells with same amount of Cu in the back contact but with different etching times, it is surprising that 40 s-etching deliver higher net charge concentration. At a first glance it is clear that the DLCP lines are at higher level for the 40 s-etched sample (full red squares): the etching affects the net shallow defects concentration, which can be identified as the doping density. At 10 kHz shallow defects concentration is in the order of 1014 cm−3 with an increase when getting close to the junction, where also deep defects are highly concentrated (open red squares). Similar considerations are suggested also by the red profiles at 1 MHz. For the 40 s samples the frequency increase from 10 kHz to 1 MHz determines a general reduction of the shallow defects response, in particular close to the junction. This makes more pronounced the contribution of the deep defects to the net charge. In general not-etched samples show smaller net charge concentration, moreover the frequency response is opposite to 40 s-etched case: increasing the frequency results in a smaller deep defects response (blue empty circles in Fig. 6) while the shallow defects stays constant
4.3. CV-DLCP analysis after aging The most stable case (40 s-etching under OCC) has been studied by CV-DLCP at 10 kHz and at different AST time up to more than 1500 h. As time flows, we observe a continuous reduction of the net charge NaNd both when measured by DLCP (Na depends only by shallow acceptor contribution) and CV (Na depends by the sum of shallow and deep acceptors). This effect could be explained by i) increase of the compensation in the absorber (increase of Nd) or ii) annihilation/transformation of the electronic state connected to an instable physical defect (decrease of Na). In the left side of Fig. 7 the DLCP profiles indicate a continuous reduction of the net shallow defects of almost one order of magnitude in 888 h, while in the same time the deep states concentration reduces only to one half (magenta empty stars in right side of Fig. 7). Since an increasing donors concentration (Nd) should affect similarly the DLCP and CV profiles (as for the unetched samples), we can conclude that for 40 s etching, both i) and ii) were registered, but the dominant effect in the doping reduction is mainly due to ii) and in minor part to compensation i). Other authors reported presence of shallow donors connected with 130
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but deep levels were registered (B1-B4). B2 and B3 are very similar to A2 and A3 respectively, while B1 and B1* can be attributed to the native CdTe acceptor, the isolated VCd-/2- [5,36]. Finally after 1600 h under AST similar deep levels are present: C2, C2*(same consideration as for A2) and C3 which could be the same defect of A3 and B3. However new AS peaks relative to shallow acceptors appear (C0, C1,C1*), such shallow defects are usually attributed to impurities that can from the glass (as Na) or from the back contact (as Ag) [5,37]. The latter could have been introduced by residues of silver paint used to contact the cell during electrical characterization. AS was also performed on the not-etched samples after 900 h under AST for comparison. In this case the deep and shallow acceptor levels are very similar to the ones registered in the 40 s samples at 0 h. However AS reveals a relative deep defects (D2) never registered in the 40 s sample for the 1600 h AST. This is commonly attributed to CuCd[30,36,37], theVCd–Cui complex is excluded from consideration due to its high instability as calculated by Krasikov et al. and supported by the measurements of Evani et al. [40], both identify CuCd- as the most probable form of copper related defect [41]. As mentioned from Krasikov et al. [41] cadmium vacancies are less likely to be found when copper is diffusing since they might be transformed in CuCd and this is observed in our AS results: VCd are present only for etched samples, VCd are however not completely excluded as also observed by Harvey et al. [42].
Table 2 Energy levels and cross sections of measured defects. Etching
AST
[s] 40
[h] 0
960
1600
unetched
900
Level
Ea
Error
□a
Identification
A0 A1* A1 A2 A3 B1 B1* B2 B3 C0 C1 C1* C2 C2* C3 D0 D1 D1* D2 D3 D4 D4*
[meV] 116 136 150 460 527 407 429 479 553 77 86 96 491 503 597 130 148 170 316 448 465 475
[eV] 7 4 1 14 11 13 11 16 13 2 2 3 1 12 20 4 6 3 15 13 15 5
[cm−2] 4,00E-16 3,00E-15 1,70E-14 1,20E-12 1,00E-11 2,73E-14 7,48E-14 4,17E-13 5,32E-12 4,72E-18 1,09E-17 2,63E-17 1,67E-12 4,12E-12 1,51E-10 1,37E-16 3,58E-16 3,88E-15 1,03E-16 1,30E-13 3,73E-15 7,02E-14
(VCd2-–ClTe+)complex of VCd2- or VTeId. Not identified Cation vacancy VCd-/2Id. Not identified Cation vacancy Impurity Id. Id. Not identified Id. Cation Vacancy (VCd2-–ClTe+)(2CuCd-–VTe+)-, Id. CuCd-, VCd-/2Not identified Id.
Cu [18,19,27,31], on the other hand Tiwari et al. [32] used Cu to dope CdTe but they also reported strong limitations in the possible amount of dopant to avoid compensation. In our case, considering the migration of Cu from the back contact as the source of the compensation, we can conclude that applying long etching to the CdTe surface reduces the Cu tendency to act as a compensator. To address the ii) component of the doping reduction, we have applied AS (see next section). Another interesting consideration is that after a very long time of AST the shallow states concentration slightly increases (purple diamonds in Fig. 7) while the deep defects contribution does not change and shifts toward the junction.
4.5. SIMS measurements In order to confirm the higher diffusion of copper for devices with not-etched CdTe, SIMS measurements have been carried out on different samples specially prepared for this analysis. In Fig. 8 the concentration of copper atoms in solar cells with and without Br-MeOH etching, after 650 h of ageing under AST condition, is presented together with their concentration. In absence of a Cu implanted standard sample, the Cu concentration was calculated by applying the Relative Sensitivity Factor (RSF) reported by Asher et al. in their paper [43], where they measured Cu in CdTe samples in the same experimental conditions used in the present paper. A possible calibration error due to this procedure applies in an identical way to all the measured samples so it does not influence the results of a direct comparison between the reported Cu profiles. It is evident how copper diffuses into the CdTe for the not-etched case, while for the etched case the Cu signal decreases very rapidly in the first nm beneath the surface layer. In particular we can observe at a depth of about 500 nm a difference in concentration of one order of
4.4. Admittance spectroscopy To highlight the effect of the 40 s-etching on the defects in the CdTe structure, AS measurements have been applied with a frequency from 300 Hz to 1 MHz and at a temperature from 90 K to 320 K. The characterization was repeated at different time of the AST in OCC (see Table 2). For not-etched samples AS is still possible even despite the high back contact barrier height as already demonstrated by [5], however after too long AST the AS peaks evolution with temperature as described in [33,34] seems to be no longer respected. The last reliable measurement on a not-etched sample was after 900 h AST and it is reported in Table 2 for comparison. Each measurement was performed by applying direct current (DC) bias of 0 V, −0.5 V and +0.5 V. Defects activation energy and capture cross section values were extracted and reported in Table 2; we have indicated as A, B, C defects respectively observed for the 40 s-etched sample at 0, 960,1600 aging hours and D for defects observed for the unetched case after 900 aging hours. We are aware that calculated cross sections have relative reliability since the numbers are very small, however they can contribute to the identification of the defects if confronted with energy values and compared with literature results. For the 40 s-etched samples, representing the most stable devices, four AS peaks before aging (0 h) were observed (A0–A3). They could be distinguished by applying different DC biases [35]. Three of them (A0, A1, A1*) are usually attributed to complexes involving Vcd- and an impurity atom, in general Cl and Cu [36,37]. A2 has been reported but never clearly assigned [38,39]. A3 could be attributed to a generic cation vacancy [36]. After 960 h under AST no shallow defects signature was detected,
Fig. 8. SIMS depth profiles of Cu after 650 h under AST conditions in devices with Br-MeOh etched CdTe (black circles) and with not-etched CdTe (red triangles). (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article)
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magnitude, while at a depth of about one μm the signal goes below the detection limit in these experimental conditions. We believe this is a clear demonstration of the fact that etching allows to appreciably reduce copper diffusion.
a dopant for their low activation energy, while compensation due to Cui+ is limited. In this case, the degradation is probably dominated by the slow decomposition of the above-mentioned shallow complexes. This generates deep states like VCd-/2- that have a negative impact since they increase the carriers’ recombination. Finally, after very long AST time, we observe formation of defects probably due to impurities coming either from the glass substrate or from the back contact, which could take part to the degradation. SIMS measurements (Fig. 8) confirm that copper diffusion is limited when etching is applied.
5. Discussion From the analysis of the JV, we can observe that, in accordance with other works [14] for our samples the etching is not necessary to achieve good efficiency. However for unetched samples the Cu layer thickness needs to be increased from 2 to 4 nm to achieve best efficiencies. This effect seems not to be connected to the back contact performance (none of these samples show rollover), but to a different Cu incorporation during the 190 °C annealing due to the absence of the etching. The CdTe surface has been analyzed by Raman (Fig. 1), GXRD (Fig. 2) and AFM (Fig. 3) to address the connection between morphology, Te rich layer, CuxTe compounds and the back contact performance. Subsequently to a long etching, CdTe surface appears smoother (as expected) and Raman spectra confirm the generation of a Te rich layer. However, 40 s-etching time does not result in a higher concentration of Te at the surface compared to the 5 s-etching time case. On the other hand the XRD reveals the presence of different compounds depending on the etching time. Cu2Te formation takes place when the etching time is short or even absent. On the contrary for samples etched for 40 s, Cu2Te is replaced by CuTe, which has been reported as a stable compound [2,19,20,31]. AST were performed in sets of more than 20 cells to study the changes in solar cells stability over a long time period (1200 h). The best stability was obtained for long time etching cases in SCC (20% loss in over 1200 h) (see Fig. 4). To our knowledge this is a remarkable result for a simple Cu/Au back contact since a similar performance stability is usually achieved introducing more complicated contacts [19,20,31]. Other studies [17,18,44] reported evidence of two different Cu diffusion components into the CdTe, characterized by different diffusion energies. The fast one has been connected to Cu+ diffusion via interstitial sites generating shallow donor states (Cui+, Ec−0.55 meV) [18,19,36]. The second component needs higher energy to generate and to dissociate (VCd2-– Cui+)-, (2CuCd-– Cui+)-, (2CuCd-–VTe+)- and CuCd-. The bias application under AST (SCC) to our unetched samples results in a degradation comparable to the etched ones, while in OCC it is dramatically faster, as also observed by other authors [19,20]. This indicates a strong drift component that confirms the Cu diffusion as positive ion [18,19]. On the other hand for the 40 s-etched samples the bias has small impact on the degradation excluding the massive presence of free Cu ions. Moreover CV-DLCP analysis on 40 s samples throughout the AST indicates the continuous reduction of the shallow acceptor concentration and the simultaneous deep states formation as the main degradation factor. The AS confirms: the presence of shallow complexes in the 40 setched samples before the AST, their absence after the AST and the presence of deep levels usually attributed to VCd–/2-. So we can state that, in the absence of etching, during the back contact annealing Cu reacts and generate Cu2Te at the surface. The following degradation is dominated by diffusion of Cu via interstitial sites that compensate the absorber generating shallow donors (Cui+, Ec-0.55 meV). As already mentioned Cu2Te is unstable and it results to be the main supplier of Cu+ to the solar cell [2,20]. In the absence of bias the cells without etching reduce their performance to 50% after only 300 h of AST. On the other hand, for long etching most of the Cu deposited in the back contact is used to form CuTe at the surface and complexes involving Cu like (VCd2-–Cui+)- in the bulk, which can be considered as
6. Conclusions Among the different reasons for degradation of CdTe solar cells (heat and moisture cause oxidation of the CdTe surface, as for example reported for Au contacted cells [45]), in presence of copper its diffusion into the CdTe bulk show high reduction of the photovoltaic performance [19]. The work presented in this paper shows that etching changes the way in which Cu reacts with CdTe surface: in fact by means of a 40 s Br-MeOh etching and 2 nm of Cu deposited on top of it, it is possible to obtain a CuTe and an extremely limited diffusion of copper. The aging shows that for the not-etched samples Cui+ ions are generated, as expected, while for the 40 s-etched samples no defects generated by copper are observed. Therefore, with a suitable optimization of the etching conditions it is possible to obtain a rather stable device even with a simple Cu based back contact. Acknowledgments We would like to acknowledge the financial support received within the Joint Project Program of the University of Verona grant number: B38I13000640003 with the specific NANOCAD project. References [1] A. Romeo, M. Terheggen, D. Abou-Ras, D.L. Bätzner, F.-J. Haug, M. Kälin, et al., Development of thin-film Cu(In,Ga)Se2 and CdTe solar cells, Prog. Photovolt. Res. Appl. 12 (2004) 93–111. [2] X. Wu, J. Zhou, a. Duda, Y. Yan, G. Teeter, S. Asher, et al., Phase control of CuxTe film and its effects on CdS/CdTe solar cell, Thin Solid Films. 515 (2007) 5798–5803. [3] D.W. Niles, X. Li, P. Sheldon, A photoemission interface determination of the band diagram of the Te / CdTe, J. Appl. Phys. 77 (1995) 4489–4493. [4] I.M. Dharmadasa, J.M. Thornton, R.H. Williams, Effects of surface treatments on Schottky barrier formation at metal/n‐type CdTe contacts, Appl. Phys. Lett. 54 (1989). [5] Y.Y. Proskuryakov, K. Durose, B.M. Taele, G.P. Welch, S. Oelting, Admittance spectroscopy of CdTeCdS solar cells subjected to varied nitric-phosphoric etching conditions, J. Appl. Phys. 101 (2007). [6] S. Mazzamuto, L. Vaillant, A. Bosio, N. Romeo, N. Armani, G. Salviati, A study of the CdTe treatment with a Freon gas such as CHF2Cl, Thin Solid Films 516 (2008) 7079–7083. [7] 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. Photovolt. Res. Appl. 4 (1996) 225–229. [8] I. Rimmaudo, A. Salavei, F. Rossi, C. Ferrari, A. Romeo, Etching effect of CdTe absorber on the stability of thin film solar cell devices, in: Proceedings of the 2013 IEEE 39th Photovolt. Spec. Conf., 2013, pp. 2029–2033. [9] Andrei Salavei, Ivan Rimmaudo, Fabio Piccinelli, Daniele Menossi, Nicola Romeo, Alessandro Romeo, Analysis of CdTe activation treatment with a novel approach, in: Proceedings of the 28th Eur. Photovolt. Sol. Energy Conf. Exhib., 2014, pp. 2140–2142. [10] C.R. Corwine, A.O. Pudov, M. Gloeckler, S.H. Demtsu, J.R. Sites, Copper inclusion and migration from the back contact in CdTe solar cells, Sol. Energy Mater. Sol. Cells 82 (2004) 481–489. [11] S. Demtsu, D. Albin, J. Sites, Role of Copper in the Performance of CdS /CdTe Solar Cells, in: Proceedings of the Conf. Rec. IEEE 4th World Conf. Photovolt. Energy Convers., 2006, pp. 523–526. [12] S.S. Hegedus, B.E. Mccandless, R.W. Birkmire, Analysis of stress-induced degradation in CdS/CdTe solar cells, in: Proceedings of the Conf. Rec. 28th IEEE Photovolt. Spec. Conf., 2000, pp. 535–538. [13] T.A. Gessert, S. Asher, S. Johnston, A. Duda, M.R. Young, T. Moriarty, Formation Of ZnTe: Cu/Ti Contacts at High Temperature for CdS/CdTe Devices, in: Proceedings of the Conf. Rec. 2006 IEEE 4th World Conf. Photovolt. Energy
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