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Keywords:

  • Cu(In,Ga)Se2;
  • CIGS;
  • thin-film solar cells;
  • molybdenum (Mo);
  • chalcopyrites

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

In this work, we investigate the effect of ageing Mo-coated substrates in a dry and N2 flooded cabinet. The influence was studied by preparing Cu(In,Ga)Se2 solar cells and by comparing the electrical performance with devices where the Mo layer was not aged. The measurements used for this study were current–voltage (J-V), external quantum efficiency (EQE), secondary ion mass spectroscopy (SIMS) and capacitance–voltage (C-V). It was concluded that devices prepared with the aged Mo layer have, in average, an increase of 0.8% in efficiency compared with devices that had a fresh Mo layer. Devices with aged Mo exhibited a nominal increase of 12.5 mV of open circuit voltage, a decrease of 1.1 mA/cm−2 of short circuit current and a fill factor increase of 2.4%. Heat treatment of fresh Mo layers in oxygen atmosphere was also studied as an alternative to ageing and was shown to provide a similar effect to the aged device's performance. Copyright © 2013 John Wiley & Sons, Ltd.

INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

In recent years, Cu(In,Ga)Se2 (CIGS) thin-film solar cells have started to make a successful transition from research to industry. Besides the announcement of production increase for several manufacturers, a world record module from regular production of 14.4% and a world record sub-module of 17.4% efficiency were presented by Q-Cells/Solibro and Solar Frontier, respectively [1, 2]. In the research field, the world record for solar cells was recently improved to 20.3% [3]. The gap between cell record efficiency and module efficiencies is closing, but the fundamental understanding is still insufficient regarding the process steps in the module fabrication. This paper deals with the Mo back contact layer and the influence of waiting time between Mo deposition and CIGS processing.

In CIGS thin-film solar cells, it is common to use a Mo bilayer as back contact [4-6], deposited by sputtering. Sputtering is a well-known technique for uniform thin-film deposition on large area substrates and, as such, is used in many industrial applications. One sputtering drawback is that it is difficult to deposit a Mo film simultaneously characterised by high conductivity and strong adhesion because different process settings are used to optimise for each attribute [7]. These properties are related to the stress state of the film and can be changed for example by varying the sputter power and pressure [4, 7]. The use of a Mo bilayer, where the first layer is designed to have good adhesive properties and the second to be low ohmic and conduct the current, is one way to mitigate this constraint. Apart from conductivity and adhesion, other properties are important as well. Because the presence of Na is a prerequisite for obtaining high CIGS solar cell efficiencies, the Mo/Na interaction is a matter of importance. If a soda lime glass (SLG) is used for the Na supply, the Na diffusion through the Mo layer must be uniform and sufficient. Another asset with the use of Mo is its ability to form an ohmic contact with the CIGS layer, presumably by forming an interfacial layer of MoSe2 [8]. The properties and quality of the back contact may be dependent on the surface condition of the Mo layer, for example, the oxidation state of the surface [9]. The properties of the Mo layer itself also influence the properties of CIGS [10-14], as does the use of different substrates [15-17] and the cleaning of the substrate [18]. Alternatives to Mo have been studied [19], but Mo seems to be the best choice.

A common argument [20, 21] that is widely accepted is that substrates coated with the Mo layer need to be transferred from its deposition chamber into the CIGS chamber as fast as possible in order to avoid oxidation of the Mo surface. Some state-of-the-art deposition systems even use clustering tools, where vacuum is never breached between the deposition of Mo and CIGS. In industry, pre-coated substrates or buffering of substrates between the Mo and CIGS deposition steps is widely used, and it is important to understand the influence of the waiting time between the steps on module efficiency. To evaluate whether the transfer time of Mo to the CIGS deposition systems is a critical parameter, we have prepared a set of solar cells on Mo-coated SLG where Mo aged in dry nitrogen atmosphere at room temperature is compared with freshly deposited Mo. The results indicate that by storing the Mo films in a dry nitrogen flooded cabinet, no adverse effect on cell efficiency is observed and may provide a small improvement. As an alternative to ageing at room temperature, accelerated ageing by exposing freshly deposited Mo films to an O2 atmosphere at 200 °C during 60 min was performed.

EXPERIMENTAL

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

Growth

The deposition of the Mo bilayer was made by direct current sputtering in Ar atmosphere using an MRCII in-line sputter system. A sputter power of 1500 W was used for both the bottom layer and the top layer, whereas Ar pressures of 15 mTorr for the first layer and 6 mTorr for the top layer were used [5]. This recipe provides bilayers with a total thickness of around 350 nm and a sheet resistance lower than 0.5 Ω. The aged Mo layers were stored in a N2 flooded cabinet where the temperature is kept constant at 22 °C. The ageing was carried out by leaving the samples in the cabinet during a certain number of weeks. The cabinet doors were typically only opened for seconds a few times per day. This created a limited exposure of the samples to oxygen. All samples of the ageing test were deposited simultaneously and subsequently aged, whereas the fresh Mo layers used as reference samples were deposited within 18 h prior to every CIGS run. The fresh samples were also stored in the cabinet between deposition and further processing into solar cells.

Additionally, freshly deposited Mo bilayer films on SLG substrates were deliberately oxidised in a tubular furnace in O2 atmosphere at 200 °C during 60 min, after which they were immediately loaded into the CIGS deposition system without further delay.

The growth of the CIGS layers was carried out in a batch system using a simulation of an in-line evaporation approach described elsewhere [22]. The process was carried out in a vacuum chamber pumped to a base pressure below 2 × 10−6 mbar, holding six 2.5 × 5 cm2 substrates per run. The maximum substrate temperature during the CIGS deposition was 540 °C. The cells were prepared using the group baseline: SLG/Mo/CIGS/CdS/i-ZnO/ZnO : Al/Ni–Al–Ni grid [23]. No anti-reflective (AR) coating was used in this work.

Characterisation details

The composition of the CIGS films was determined by X-ray fluorescence performed in a Spectro X-lab 2000. The symbols x:[Ga]/([In] + [Ga]) and y:[Cu]/([In] + [Ga]) are used in the following to denote the atomic ratios of gallium versus both group III elements and copper versus the group III elements, respectively. The elemental depth profiles, Cu, In, Ga, Se, Mo and Na, were measured using secondary ion mass spectrometry (SIMS).

Cells were characterised in a in-house current versus voltage (J–V) set-up with illumination from an ELH lamp. Fill factor (FF), short circuit current (Jsc) and open circuit voltage (Voc) were estimated from the J–V curves. The external quantum efficiency (EQE) was determined under ambient light, using chopped monochromatic light that was scanned through the wavelength interval of 360–1200 nm in 2-nm steps. Cells with atypical characteristics were excluded from the J–V statistics, and when mentioned, the presented values of cell performances are medians of 12 cells. C–V characterisation was performed with an in-house system, utilising four-point C–V measurements with an Agilent 4284A Precision LCR Meter. The frequency of the probing AC signal used in the measurements was 500 kHz, and the level of the signal was 30 mV.

Comparison details

Our baseline produces cells with efficiencies varying between 15.5% and 17%. Within each run, there are small variations of less than 0.5%. A fresh Mo served as reference for each aged layer. The positions of test and reference samples were randomised to minimise systematic errors. The testing period lasted 7 weeks with an experimental run being performed every week.

RESULTS AND DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

Time evolution of the Mo ageing

The typical CIGS film was 2 µm thick with x close to 0.40 ± 0.02 and y close to 0.92 ± 0.02. There are small variations of the composition between each run, but they are within the X-ray fluorescence measurement error. In Figure 1, the median solar cell parameters are presented. For reference, in Table 1, the relative differences in the parameters between the solar cells from reference and the aged Mo layers are shown.

image

Figure 1. Median electrical parameters of the reference and aged Mo cell as function of Mo storage time.

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Table 1. Difference of the electrical parameters between the aged Mo devices and their references.
ΔAge (days)ΔVoc (mV)ΔJsc (mA/cm2)ΔFF (% absolute)Δη (% absolute)
717−610.3
1420−0.61.90.8
2100.20.50.2
28100.77.92.3
3522−0.31.80.8
426−0.71.10.2
Average12.5−1.12.40.8
Average without 28 days outlier13–1.51.30.5

For the fresh Mo, the efficiencies of the solar cells vary between 14.2% and 16.2%. This variation is slightly higher than we normally observe and was caused by the reference cell used in the 28-day test, which was worse than expected, mostly because of a low FF. This was an unidentified and isolated problem. It may be connected with a more resistive ZnO : Al layer or more resistive grid, but because it was an outlier, it was not further studied. For comparison purposes, this value will not be considered. For the aged Mo, the efficiencies vary between 15.5% and 16.9%. Although the values mentioned before are medians, the variation of the most efficient cells of each sample follow the same trend, between 15.5% and 16.9% for the reference cell and between 15.7% and 17.8% for the cells with aged Mo. Graphs showing J–V and EQE curves of typical cells are presented in Figures 2 and 3, respectively. In both cases, the reference cell has an efficiency of 16.7%, and the aged Mo one presents an efficiency of 17.2%. The EQE plot shows no significant difference in the cells.

image

Figure 2. Plot of the reference cell and an experimental cell where the Mo was aged for 35 days.

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image

Figure 3. External quantum efficiency for the reference cell (grey) and the aged Mo one (black) in Figure 2.

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Although the efficiency difference is small, it is still significant. The cells with the aged Mo have better median efficiencies than their reference fresh counterparts. Most of the films follow the similar tendencies in the other parameters, that is, an increase of the Voc and FF and a small decrease of the Jsc for the cells with the aged Mo. By considering these variations and by having in mind that the position of the samples was randomised, it is possible to state that the aged Mo provides a small but beneficial effect to the cell performance. It is also interesting to observe that there is no apparent correlation between the length of the ageing period and the evolution of the parameters. As it is shown in Table 1, the ageing of the Mo results, in average, in an increase of the Voc by 12.5 mV, a decrease of the Jsc by 1.1 mA/cm2, an increase of the FF by 2.4% and an increase of the efficiency by 0.8%, absolute percentages. These differences are also visible in the J–V plot shown in Figure 2. To have a better validity of these results, this experiment was reproduced using two other set of samples, and the observed results were similar, that is, a small improvement of the efficiency but with no correlation with the length of the Mo ageing after the initial 18 h. The best cell prepared in this set had an efficiency of 17.8% without AR coating, and it was achieved with a 35-day-aged Mo. Removing the 28 days outlier changes the variation of the FF from 2.4% to 1.3%, but it only lowers the difference of the average efficiency from 0.8% to 0.5%, where for the Jsc and the Voc, there is almost no difference, so basically the conclusions are valid with or without this point. Both variations are presented in Table 1, and for consistency purposes, the values without the outlier are going to be considered.

The differences in the Voc and Jsc seen in Table 1 may be explained by a higher carrier concentration in the CIGS layer for the cells with the aged Mo. A higher carrier concentration in the CIGS layer may lead to a higher built-in field between the n-type and p-type sides of the junction and thereby a higher Voc. On the other hand, higher doping in the CIGS part of the junction reduces the depletion layer width; therefore, the current collection may drop. It is well known that carrier concentration in CIGS increases with increasing concentration of Na [24-26]. A correlation between oxygen and Na in Mo layers has also been observed [27]. It is therefore reasonable to believe that the ageing of the Mo layer prior to CIGS deposition would change the diffusivity of the Mo layer for Na from the SLG substrate and thereby change the Na concentration in the CIGS layer. Regarding the increase of the FF with the ageing of the Mo, this can only partly be explained with the increase in Voc. This difference can be explored using the following expression, which correlates FF with Voc [28]:

  • display math

where voc is the normalised voltage defined as Voc/(AkT/q), A is the ideality factor of the cell, k the Boltzmann's constant, T the temperature and q the electron charge. By using the two different average Voc values of Table 1, one reaches a difference in FF of 0.2–0.3% by using A values between 1 and 2. This is much less than 1.3% averaged difference seen in Table 1; therefore, the increase in the Voc does not explain why the FF increases so much.

Secondary ion mass spectrometry results

In Figure 4, SIMS analysis and a comparison of Na concentration of the CIGS layers shown in Figures 2 and 3 are presented. The left parts of the plots correspond to the CIGS/CdS interface, and the right part of the plots is the Mo/CIGS interface. For both films, the Se and Cu signals are flat all the way through the film, whereas In and Ga show concentration profiles with opposite slopes. There is more Ga at the back part of the film, that is, close to the Mo, and more In towards the CdS interface as intended [29]. The interesting point to note in these profiles is the similarity of the Na concentration. Both samples show the same Na profile and the same Na concentration, from around 1.5 × 1019 atoms/cm3 to 2 × 1019 atoms/cm3. Both samples also show a Na bump near the CdS interface. Closer to the Mo interface, the Na signal is much higher for two reasons. One is an interface SIMS problem, which may be explained by an increased oxygen concentration at the CIGS/Mo interface, and the other is that the Na content in the Mo itself is higher than in the CIGS and that the sputtering induced roughness leads to a smearing at the interface. From the SIMS data, it is hard to find any significant difference between the samples that can explain the differences in cell behaviour. The minor differences in the shape of the profiles lie mostly within the measurement error of the method, but possibly, there is an indication of a higher Na concentration near the surface of the CIGS in the aged Mo sample.

image

Figure 4. Secondary ion mass spectrometry profile measurements of the (a) reference and (b) aged Mo cell with Mo aged for 35 days. The Na values are calibrated concentrations in atoms per cubic centimetre where the rest of elements are shown in atomic percentage. Note that the scale of the depth axis starts at 0.5 µm, where the Cu(In,Ga)Se2 starts, because the top of the sample consisting of the ZnO and CdS layers has been shifted outside of the graph. The right most part of the graph corresponds to the Mo back contact.

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C–V results

C–V measurements were performed to obtain an estimation of the carrier concentration. The doping profiles for the same cells as the ones used for the SIMS analysis are shown in Figure 5. Both cells are good representatives of the average behaviour of the six cells measured for each sample. Typical non-uniform profiles are observed, as a result of a different response from interface and bulk states [30]. The net acceptor concentration for both cells is between 1 × 1016 and 2 × 1016 cm−3. In comparison with the reference cells, slightly higher concentration is observed for intermediate distances from the junction for the aged Mo cells while the whole profile is shifted by around 20 nm towards the junction. There is also a shift regarding the net acceptor concentration: the curves for the two samples are shifted around 0.5 × 1016 cm−3. If one considers the following equation, Vbi = (q/KBT)ln(inline image/(NA*ND)) [31], where Vbi is the built-in voltage, q the electron charge, KB the Boltzman's constant, T the temperature, inline image the concentration of the intrinsic carriers, NA the concentration of acceptors and ND the concentration of donors, and assume that between the samples, only the concentration of acceptors changes, then a ratio between the Vbi of the two samples is possible to estimate. For the first approximation, one can use the estimated net acceptor concentration as the concentration of acceptors and estimate the Vbi, which is closely related to the Voc. For the estimated values between 2 × 1016 and 2.5 × 1016 cm−3, inline image = 0.994. If this difference of 0.6% is applied to the average reference Voc, one would have an absolute difference around 8 mV. Considering that for these calculations many approximations are carried out, one can say that the net acceptor concentration alone is not explaining the increase of the Voc for the aged devices, but it may explain part of it.

image

Figure 5. Doping profile of a reference cell and of an aged Mo cell.

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According to our hypothesis of increased carrier concentration as an explanation of increased Voc and decreased Jsc, one would expect CIGS deposited on aged Mo to have a higher carrier concentration. Keeping in mind the measurement errors associated with C–V measurements, we can tentatively say that the shape of the curves is in accordance with the SIMS profiles with a small increase of Na concentration near the surface of the CIGS for both samples. The aged sample also shows a slightly higher net carrier concentration than the reference sample. This effect would change the Voc in accordance with what we saw before, an increase for the aged samples, but it does not explain the effect totally.

Oxidisation test

Solar cells were prepared on top of the Mo layers that were oxidised in an oxygen furnace, and the average parameters for 12 cells are presented in Table 2. The J–V plots for a representative from these cells are shown in Figure 6. The oxidised Mo showed a higher Voc, a lower Jsc, a similar FF and an overall 0.2% improvement in the efficiency. Because in this case the FFs are similar for both devices, it is not clear that the benefits given by both studies are coming from the same mechanism. It has been suggested [32] that water vapour also influences the performance of CIGS devices, and although no water is expected to be absorbed by the Mo, one would need more detailed analysis, such as X-ray photoelectron spectroscopy [33], to investigate the oxidation states and composition of the surface of the Mo films. Such detailed study was performed by Yoon et al. [34], but the correlation of this effect with electrical performance was not made.

Table 2. Electrical parameters of a reference cell and of a cell where Mo was oxidised.
 Voc (mV)Jsc (mA/cm2)FF (%)η (%)
Oxidised Mo68132.8176.5717.2
Reference67032.9177.2517.0
image

Figure 6. J–V behaviour of a reference cell and an oxygenated Mo one.

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CONCLUSIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

For the in-line direct current magnetron sputtered Mo samples in this experimental series, we have shown that there is no major negative impact on device parameters due to storing Mo-coated samples for many weeks in a dry atmosphere containing N2 prior to the CIGS deposition, but rather, a minor beneficial effect has been observed. This effect is, however, independent on the number of weeks that the samples have been stored after the initial 18 h of waiting time between the Mo deposition and the CIGS evaporation.

By deliberately oxidising the Mo films in a furnace containing oxygen, the results were similar, that being a small efficiency improvement following the oxidation.

We hypothesised that the small beneficial effect was due to increased doping of the CIGS layer linked to differences in Na concentration. This could not be supported by SIMS profiling of the Na concentration. However, the CV carrier density profiles showed that there are differences between the samples, even though they are relatively small. With more sensitive methods or with more statistical data, a better picture of the effects that are giving the electrical changes may be revealed.

We note that the efficiency values without AR coating are in the 15.5–16.5% range for the median values and with best cells clearly above 17% for an industrially feasible CIGS process with a deposition time of less than 20 min. This indicates that the aged Mo layers are still suitable for high efficiency solar cells and also that they probably are sufficiently or close to sufficiently oxidised in their as-deposited state.

ACKNOWLEDGEMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES

The other members of Ångström Solar Center are greatly acknowledged for their valuable discussions. This work was supported by the Swedish Energy Agency through project Ångström Thin Film Solar Center and STandUP.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
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