IMPROVED CuGaSez-BASED SOLAR CELL PERFORMANCE BY In-S SURFACE TREATMENTS

David Fuertes Marrdn, Alexander Meede;, Sebastian Lehmann, Marin Rusu, Thomas SchedeCNiedrig, and Martha Ch. Lux-Steiner

Hahn-Meitner lnstitut Berlin, Glienicker Strasse 100, D14109 Germany *Present address: SULFURCELL Solartechnik GmbH, Barbara Mc-Clintock Strasse 11, D-I 2489 Berlin, Germany

ABSTRACT

CuGaSe;! (CGSe) thin films for photovoltaic applications have been subjected to surface treatments based on In-S by means of chemical vapor deposition. Structurat and electronic characterization of as-grown films and processed devices show the effective incorporation of In and S in the near-surface region of CGSe thin films and a positive impact on the solar cell performance.

INTRODUCTION

Thin-film solar cells based on wide gap Cwntaining chalcopyrite absorbers show poor performances compared to their low gap counterparts. To a large extent, limitations in their performance have their origin in absorberhuffer interface related issues, in the form of a non-ideal band alignment between components andlor due to high densities of interface states [I]. In contrast, the absorberbuffer interface does not seem to represent a fundamental limitation for the performance of low gap, high efficiency chalcopyrite solar cells using the same partner as buffer layer, namely CdS [2]. It can be argued that a single compound may not be the optimal choice for building the interface as buffer layer with the entire family, of Cu-containing chalcopyrite absorbers, ranging in energy gaps between -1 eV for CulnSw and -2.4 eV for CuGaS2. However, the question arises, in how far wide gap chalcopyrites can be forced to work with CdS as partner with minor electronic losses at the interface.

In this contribution we explore the possibility of modifying the nearsurface region of the wide gap CGSe compound by means of postgrowth dry chemical treatments based on In and S. The goal of this approach is to achieve a situation resembling that of low gap chalcopyrites in terms of interfacial issues, while still maintaining the potential of wide gap absorbers, particularly regarding opewcircuit voltage figures, in a single device. A complete characterization program, both structural and electronic, has been carried out to determine the impact of such treatments on the film and device quality, as a function both of processing time and starting film composition.

EXPERIMENTAL

CGSe thin films were grown onto Mecoated soda- lime glass by chemical vapor transport in a close-spaced system (CCSVr) [3]. For these experiments, series of samples were prepared showing different compositions, hereafter referred to in terms of [Ga]/[Cu] ratios of the films. Two identical samples from each run were used thereof, one directly processed as a solar cell following the standard procedure, and one treated in an open-tube chemical vapor deposition (CVD, see for example refs. [4,5]) system under In- and S-containing atmosphere. Binary selenide source materials used in the previous references have been replaced by ln2S3 (99.99% purity) for these experiments. The chemical transport works with the aid of HCI as transport agent and N2 as carrier gas at source temperatures of -600°C. The stability of In& under reactive gases has been monitored by means of X- ray diffraction (XRD). No changes in source material have been observed from the dihctograms before and after processing in the CVD reactor even after 20 hours at operating temperatures, pointing to a stoichiometric volatilization of the compound. The gaseous species are injected into the substrate side. where the CGSe film is kept at -530'C onto a rotating holder. Different treatment times have been tested, ranging from 2 up to 30 min. Solar cell processing was carried out thereof in the standard way, together with the reference samples, first with provision of CdS buffer by chemical bath, and subsequent sputtering of a double ZnO layer. Ni-AI contacts were evaporated as front grids. No etching step prior buffer deposition was included in the process.

Complete cells were characterized by means of I(V,T) measurements, both under AMI .5 in a solar simulator and in a home-made system provided with a cryostat unit. Quantum efficiency (QE) measurements were carried out on selected samples. The two best performing samples, one reference and one treated absorber from the same CCSW run, were prepared for TEM analysis by mechanical polishing and ion milling. A PhiIips Cl2 microscope equipped with an energy-dispersive X-ray measurement unit was used for that purpose. Additionally, selected samples were grown and treated under identical conditions as those used in the general analysis, but were not processed as cells. These samples were used for structural and compositional characterization by means of X-ray diffraction (XRD) in a Brucker system operated at 40 kV and 40 mA. and X-ray fluorescence (XRF) by means of

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a Philips MagicPRO system. Scanning electron microscopy (Aigemini) was used to image surface and cross sections of selected samples.

RESULTS

Film properties

XRD characterization of treated films revealed the formation of secondary phases and some degree of inter- phase alloying as a function of processing time. Fig. 1 shows a comparison of two XRD measurements, theta- Ztheta vs. grazing incidence (GI) at 0.7" incoming beam, of a 15 min. treated CGSe film with original lGa]flCu]-l.l. Scans have been normalized to the maximum intensity of (1 12) peaks. GI measurements probe the upper part of the film, roughly estimated in -500 nm for the conditions of choice, whereas th-2th probe the entire layer thickness, as evidenced by the prominent (110) peak from the Mo- coated substrate around 40.5". Characteristic peaks of CGSe are readily visible in both scans. Additional features show up at higher angles as shoulders at all CGSe peaks. which are attributed to the formation of CulnS2 (CIS) and CuGaSz (CGS). Peak identification was carried out with JCPDF cards 35-110 (CGSe), 47-1372 (CIS), and 25- 0279 (CGS), represented in the figure as bars with square, diamond, and circle heads, respectively. From this th- 2thlGI comparison it is concluded that,ths formation of secondary phases takes place in the uppermost part of the film, as evidenced by the relative increase in intensity of the corresponding features for the case of GI scan. This fact has been proved by electron microscopy studies, which helped identifying two characteristic features of treated samples, compared with bare absorbers. First, a different morphology characterized by a sub-micrometer texturing, showing a remarkable anisotropy. Second, the formation of a thin surface phase in the last few tens of nanometers of the absorber film, as shown in Fig. 2. Both features agree well with the observed XRD patterns, as explained above, and account also for changes in the relative peak intensities of distinct reflections observed when comparing bare and treated CGSefilms.

The formation of alloys and secondary phases has been studied as a function of processing time. The Iowa panel of Fig. 1 shows a comparison of GI scans (0.7') of samples treated during 5 and 15 minutes under In-S- containing atmosphere. Peak identification according tu the standards considered point to a sequential process in the formation of secondary phases in the samples under study. After 5 minutes treatment, changes in the relative magnitude of CGSe peaks and appearance of small shoulders on their high-angle side reveal the formation of CIS. With longer treatment times CIS peaks develop and those of CGS become clearly visible. In addition, shoulders become broader and point to formation of alloys of the type Cu(ln,Ga)(S,Se)Z after I 5 minutes treatment. No evidences have been found from XRD analysis of CulnSez formation.

25 30 A0 -9

2-Theta

Kl

2-Theta 10

Fig, 1. XRD pattems of treated CGSe films. Upper panel: comparison of theta-2theta (grey) and grazing incidence (0.7'. black) scans of a 15 min. treated CGSe, normalized to (1 12) peak intensities. Lower panel: grazing incidence (0.7") scans of 5 (black) and 15 min. (grey) treated samples in log scale. See t e ~ for phase identification.

Fig. 2. SEM cross section of a complete devicewith 5 min. treated CGSe absorber. White arrows indicate the formation of a secondary phase in the uppermost part of the absorber forming the hetero-junction with CdS buffer.

Device Properties

In the following, results will be presented of samples which undetwent 5 min. treatments, focusing on the composition dependence observed. Fig. 3 shows the recorded best PV parameters under standard illumination conditions of the sample series of untreated and treated samples, as a function of the [Ga]/[Cu] ratio of as-grown films. Efficiency figures of treated samples (Fig. 3a) exceed those of untreated counterparts over the whole

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composition range studied. The effect mainly stems from higher short-circuit current densities (3b), reaching values between 15 and 22 W c m 2 for [Ga]/[Cu]-1.0 and [Ga]r[Cu]-4 -4, respectively. The short-circuit current shows a nearly linear dependence with film composition, increasing with higher Ga-contents. in contrast with a nearly composition-independent value around 15 W c m Z for the case of untreated samples in the Ga-rich range. In bare CGSe devices, all parameters show independently a certain transition from lower values, associated with Cu- rich and stoichiometric compositions, to higher vafues achieved at sufficient Ga contents. For the samples under study, the composition values at which the transition takes place differ between PV parameters. From Figs. 3b. c and d, it can be seen that higher currents and fill factors are achieved at Ga contents slightly above ideal stoichiometry, whereas in the case of the open-circuit voltage, the transition appears shifted toward higher Ga-contents. Devices are generally tolerant of high-Ga off- stoichiometry, and high efficiency devices can still be obtained from absorbers showing [Ga]l[Cu]-l.4. For the case of treated samples, the transition is readily visible in the fill factor plot, whereas the behavior of the open-circuit voltage appears hard to predict, with a pronounced minimum right at transitiorrrelated compositions in untreated absorbers, and clearly higher figures on the Ctk rich side when compared to references. This last aspect is also extensive to all PV parameters. Whereas on the Cu- rich range the treatment may consume detrimental Cup- S e and still grow some few nanometers outwards, on the Ga-rich side the process kinetics may be different and limited by diffusive processes.

Fig. 4 shows I-V curves of cells based on untreated (left) and treated CGSe absohrs from the same CCSVT run, with [Gaj/[Cu]-l.Z, measured under standard conditions. These are so far the best results obtained from treated absorbers. Comparing the PV parameters, it is readily observed that the major impact on the efficiency improvement arises from a higher short-circuit current density (by nearly 25%), whereas only minor losses are recorded in the open-circuit voltage (-30 mV). The fill factor of the device slightly improved with the treatment, and the overall effect leads to an efficiency over 9%, to be compared to 7.2% from the reference sample.

QE measurements of the cells shown in Fig. 4, and the corresponding derivative with respect to the wavelength, are shown in Fig. 5 (lower and upper panels, respectively). A wider reckresponse is visible for the treated sample compared to the reference cell, resulting from a broad tail in the long-wavelength range. Carriers generated by 1000 nm-photons are still partly collected at the junction, implying an effective optical gap narrowing from CGSe values takes place in treated samples. This is better seen in the derivative plot. Dotted lines indicate band gap energies of different compounds building up the devices. Positive peaks are related with window layers (CdS and ZnO), whereas negative ones correspond to absorber compounds.

mj I ,

m 300 0.9 1.0 1.1 12 13 1A 1.5 0.9 1.0 1.1 19 1.3 1.4 1.5

Composition [Gay[Cu]

Fig. 3. PV parameters of solar cells based on 5 min. treated absorbers (filfed symbol) and untreated references (open), as function of the [Gay[Cu] ratio of reference samples, (a) efficiency, (b) short-circuit current. (c) fill factor, and (d) open-circuit voltage.

0.1 CGSe-ref In,S, treated

J, - 19.07 mAlon’

VI C

U

4.0 4.5 0.0 0.5 1.0 -1.0 65 0.0 o s 3.0

Voltage (V) Voltage (V)

Fig. 4. Current-voltage characteristics of untreated CGSe reference [GaJ(Cu]-l.2 (left) and treated CGSe for 5 minutes under In-S at -530’C (right).

For the untreated sample the two features at 1.63 and 1.78 are attributed to the first two gaps of CGSe, stemming from crystal-field and spin-orbit splitting of the triply degenerated uppermost valence band. These values are lower than those normally attributed to crystalline CGSe, but in agreement with Ga-rich off-stoichiometries in thin films. For the case of the treated sample, a prominent feature shows up at 1.52 eV, which corresponds to the lowest CIS gap. Two additional features of this plot

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deserve some attention: (a) the broad tail on the long wavelength range, which implies that collection occurs in the device even for lower energies than that required for minority carrier generation in CIS; the origin of this broad tail remains unclear. It can be speculated that, though no evidences of CulnSez formation have been found in treated samples by XRD, the possibility of some degree of alloying in the form of Culn(S,Se)z resulting from the treatment (no more than a few monolayers), cannot be excluded. (b) The features at 1.63 and 1.78 are still observed in the treated sample, implying that a significant part of the light is absorbed in CGSe. This result is in agreement with microscopy studies, showing that the modified absorber region after the treatment extends only some tens of nanometers from the surface into the bulk,

1.0, : 1 . , . ' , . _ I ,

0.0 , . , , , . , , 400 600 800 1000

Wavelength (nm)

Fig. 5. QE (lower panel) and derivative of QE with respect wavelength of 5 minutes treated (solid line) and untreated reference (dashed, [Ga~[Cu]=l.Z) samples. Band gaps of different phases are labeled with thin dotted lines.

CONCLUSIONS

The results of this contribution show that there is still some room for improving the performance of wide-gap CGSe-based solar cells. While maintaining the bulk properties of CGSe, it is possible to modify the near- surface region of the absorber film and improve its interface properties witb the buffer. In and S have been chosen for this goal as natural candidates, showing an ideal solubility in the CGSe chalcopyrite matrix, and being present in high efficiency, low gap absorbers with well behaved interfaces. It can nonetheless be argued that the samples we report on in this contribution can be considered as 'poor-CIGSS" performing devices (certainly with a peculiar concentration and gap gradient), rather than 'good CGSe-based" solar cells. In this respect, the following aspects should be considered:

- The modified region of samples treated during 5 minutes is contained within the depletion region, so the bulk, including part of the space-charge region and the region where transport is primarily diffusive, still remains CGSe. This can be already concluded from QE measurements (see Fig.5), where CGSe-gap-related features are visible together with those attributed to CIS. and supported by SEMREM studies. - Variations in the band gap (and composition gradients) are therefore expected (and supported experimentally by I(V,T) measurements) within the depletion region. The presence of CIS phase, with lower gap than CGSe, assists camer generation, accounting for higher current densities of cells based on treated absorbers. The built-in electric field associated with the junction assists drift transport over/through eventual potential barriers at interfaces in the absorber. - Numerical simulations assuming "reasonable" parameters, not included in this contribution due to limited space, have shown that the combination of CGSe-based devices with some 50 nm of CIS as uppermost part of the absorber layer can lead to improved JSC. FF, and efficiency figures compared to CGSe devices, while maintaining VOC values above those expected from CIS solely. In this respect, the low VOC values recorded experimentally from CGSe references hinders a conclusive answer to the question "good CGSehad CIGSSe". I

ACKNOWLEDGMENTS

C. Ketch, M. Kirsch, S. Wiesner, U. Bloeck, P. Schubert-Bischoff, N. Allsop and R. Klenk are gratefully acknowledged.

REFERENCES

[l] M. Gloeckler, J.R. Sites, "Efficiency limitations for wide- band-gap chalcopyrite solar cells", Thin Solid Films (in press).

[Z] U. Rau, H.W.. Schock, "Electronic properties of Cu(fn,Ga)SeZ heterojunction solar cells", Appl. Phys. A 69, 1999, pp. 131-147.

[3] M. Rusu, et al. 'CuGaSe2 thin films prepared by a novel CCSVT technique for photovoltaic application", Thin Solid Films 451452,2004, pp. 556-561.

[4] N. Meyer, et at. 'Solar cells based on HCVD grown CuGaSez absorbers with high operrcircuit voltages", Sixteenth EPVSEC, Glasgow, 2000, pp. 429433.

[5] D. Fuertes Marrbn et al. 'Improvements on CVT-based CuGaSe;! thin-film solar cells". Proceedings I d . Conf PV in Europe, from Technology to Energy Solutions, Rome, 2002, pp. 421 424.

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