Solar cells based on CuInS2—an overview

Solar cells based on CuInS2—an overview

Thin Solid Films 480–481 (2005) 509 – 514 Solar cells based on CuInS2—an overview R. Klenka,*, J. Klaera, R. Scheera, M.C...

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Thin Solid Films 480–481 (2005) 509 – 514

Solar cells based on CuInS2—an overview R. Klenka,*, J. Klaera, R. Scheera, M.Ch. Lux-Steinera, I. Luckb, N. Meyerb, U. Rqhleb a

Hahn-Meitner-Institut, Glienickerstr. 100, D-14109 Berlin, Germany SULFURCELL Solartechnik GmbH, Barbara-McClintock-Str. 11, D-12489 Berlin, Germany


Available online 22 January 2005

Abstract Over the recent years, CuInS2 has emerged as a promising absorber material for thin film photovoltaic modules. A pilot production line for full size (12060 cm2) modules is currently being established. CuInS2 preparation, material properties, and cell structure are in many aspects comparable to those of the more widely researched selenium-containing chalcopyrite absorbers but there are also unique features of each of the material systems. In this contribution, we give an overview of the historical development, current status, and near to mid-term future of CuInS2-based devices. D 2004 Elsevier B.V. All rights reserved. Keywords: Solar cells; CuInS2; Photovoltaic modules

1. Historical notes A working solar cell based on a thin-film CuInS2 homojunction was described in 1977 [1] (Table 1). The claimed efficiency of an electrochemical cell using an ntype crystal was already close to 10% in 1986 [2]. Following these proofs of concept, research had been focused mainly on thin-film heterojunctions. Except for a unique approach using a copper-tape substrate [3], the basic cell structure, the processes and materials used for back contact, buffer layer, and window layer are usually the same as those developed for the standard low-gap Cu(In,Ga)Se2-based cells. The most successful absorber preparation methods are multi-source evaporation and two-step (sulfurization of metal precursor films) processes. While an efficiency of 7.3% was reported in 1986 using presumably the latter method [4], most of the pioneering work was based on the former. Scheer et al. [5] were the first do report an efficiency exceeding 10%.

* Corresponding author. Tel.: +49 30 80622625; fax: +49 30 80623199. E-mail address: [email protected] (R. Klenk). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.042

The group at the University of Stuttgart later reported an xefficiency of 12.2% [6]. The two-step processes are pursued due to their superior potential for industrial production. Here, the 10% landmark efficiency was reached in 1996 [7,8]. Rapid thermal processing (RTP) was introduced shortly after that, reducing typical annealing times from 1 h to 3 min. Cells based on RTP absorbers have reached a confirmed total area efficiency of 11.4 % [9].

2. The role of Cu-excess in chalcopyrite absorber preparation The beneficial effects of Cu excess for the growth of chalcopyrite thin films were recognized already during early stages of development. Since excess copper cannot be accommodated in the chalcopyrite lattice it leads to the formation of secondary Cu–S,Se phases which appear to play an active role in the growth mechanism [10]. However, the highly conducting secondary phases must be removed before forming the heterojunction, which, in case of the selenide films, has lead to a number of recipes (bilayer [11], three-stage process [12]) where the secondary phase is converted to chalcopyrite in later


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Table 1 Efficiency of selected CuInS2-based solar cells Ref.


Efficiency (%)







[4] [5] [6] [7] [8]

1988 1993 1996 1996 1996

7.3 10.2 12.2 10.4 10.5






evaporation of compound+S, homo junction crystal, electrochemical cell 2 step ? co-evaporation co-evaporation 2 step, sulfur 2 step, H2S, Pt substrate 2 step, RTP, sulfur

Total area, confirmed

stages of film growth by increasing the In/Cu flux ratio. For CuInS 2, the Cu-excess has to be maintained throughout preparation because the secondary phase is required to overcome the kinetic limitation of sulfur incorporation [13]. The CuS segregates to the surface when the film is cooled down to room temperature and is removed easily by selective etching [14]. Films prepared without Cu-excess are generally semi-insulating which we believe is due to a high concentration of sulfur vacancies acting as compensating donors. Some success has been achieved by very carefully incorporating gallium and sodium to promote p-type conductivity [15].

for two different Cu/In ratios in the precursor films indicating that 50% variation in precursor stoichiometry leads to typically only 40 mV variation in open circuit voltage. The current baseline process sequence is depicted in Fig. 2. Sputtering is used for the majority of deposition steps, having the advantage of being a process well established in industry for large-area deposition. Neither an extra process for depositing sulfur, nor a separately controlled effusion source, nor the handling of H2S gas are necessary as sulfur powder is simply heated together with the metal-coated substrate. Fig. 3 shows open circuit voltages and fill factor of cells that have been prepared within a 6-month period and demonstrates that even under the conditions of a research laboratory this baseline process can be quite stable. Analysis of samples obtained by interrupting the annealing process at different stages and model experiments [17] had lead to a clarification of the reaction sequence in the two-step process. The metal precursor undergoes several phase transitions during the initial temperature increase. The chalcopyrite compound forms directly from these Cu–In alloys reacting with sulfur, in

3. The baseline process sequence Development efforts at the Hahn-Meitner Institute (HMI) in recent years have been consistent with requirements for industrial production, i.e., simple device structure, fast and reproducible processes, a good understanding of reaction kinetics, and methods for process control and quality assurance. CuInS2 as an absorber material, Cu-excess throughout preparation and the RTP two-step process are believed to be of advantage in implementing these goals. CuInS2 has an ideal band gap, hence, it does not necessarily require alloying with other materials for band gap adjustment. Growth assistance by Cu–S eliminates kinetic limitation of compound formation and makes the very fast RTP process feasible. Furthermore, it stabilizes the p-type conductivity thereby again eliminating the need for controlling additional elements such as sodium or gallium. In combination with the selective chemical etching, the Cu-rich preparation leads to a self-adjusting process with respect to the stoichiometry of the absorber film and hence a very wide process window. This has been studied especially in the older two-step process using a sulfur effusion source and a conventional substrate heater [16]. Fig. 1 shows the statistical distribution of open circuit voltages

Fig. 1. Distribution of open circuit voltages for precursor compositions Cu/ In=1.2 (top) and Cu/In= 1.8 (bottom).

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Fig. 2. Baseline process sequence for manufacturing of CuInS2-based modules.

contrast to what has been reported for annealing of metal/ selenium stacks where the reaction proceeds via binary Cu–Se and In–Se compounds [18]. This may be advantageous in view of the low thermodynamic driving force for the transition from binary chalcogenides to the ternary chalcopyrite. The growth surface is mainly located at the sample surface, i.e., metals diffuse through the already formed chalcopyrite to react with sulfur. The film recrystallizes under the influence of Cu–S phases in the final stages of the process. Many aspects of this model have been recently refined by in situ observations using Xray diffraction, laser light scattering [19] and Raman spectroscopy. Through these investigation, it is now also feasible to use laser light scattering or Raman spectroscopy for on-line process control. The latter method also appears to be useful for quality assurance after absorber preparation because certain features of the Raman spectrum of the absorber are found to correlate with the performance of the solar cell [20].

Transition to larger areas was relatively straightforward owing to the self-adjusting process which greatly reduces the requirements for homogeneity. In addition to developing a reliable process for cell preparation, a further task for scaling-up has been to demonstrate the feasibility of monolithical interconnects. Walter et al. [21] were the first to do so using evaporated absorber films. They noted, however, a systematic decrease of fill factor with the number of cells connected in series. Our RTP processed films do not suffer from this effect. As shown in Fig. 4, an efficiency of 9.7% has been achieved and verified independently on a substrate measuring 55 cm2 [22]. Lock-in thermography is used routinely to locate and trace defects in module test structures. The next step towards full industrial production is the implementation of a pilot-line with a projected capacity

Fig. 3. Open circuit voltage and fill factor of baseline cells prepared over a period of 6 months.

Fig. 4. jV characteristics of a CuInS2 module test structure under simulated AM 1.5 illumination.


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5. Further development

Fig. 5. Partial view of the SULFURCELL facilities. The in-line sputtering system for back contact and precursor deposition is visible in the background.

of 1 MWp/a by SULFURCELL (Fig. 5). The deposition systems have been designed for a standard module size of 12060 cm2. All systems have been installed and first test depositions are currently carried out. Modules will be manufactured starting in early summer of 2004.

4. Stability In general, the stability of CuInS2 cells is comparable to that of the selenide-based cells. Illumination-induced degradation is not observed, on the contrary, light-soaking tends to increase rather than decrease the efficiency. The inherent physical stability is also evident from experiments where cells and modules were found to be stable with accelerated aging at 350 K in dry nitrogen [22]. Furthermore, radiation hardness [23] has been reported. On the other hand, elevated temperatures together with high humidity cause significant degradation of non-encapsulated modules, and, to a lesser degree, cells. This shortcoming in chemical stability is not surprising in view of similar observations made with selenide-based modules. There the transparent front contact and interconnects, which are more or less identical in both module types, play a major role in degradation under damp heat. A transmission-line test structure has been developed to independently assess different contributions to damp heat-induced changes in module performance. Detailed results will be presented elsewhere [24]. Prober encapsulation ensures chemical stability and further development of a cost-effective, industrially feasible laminate is a significant goal at SULFURCELL. In addition, improvement of the inherent chemical stability remains the subject of fundamental investigations at the HMI.

While the CuInS2 cell has reached a development status that makes mass production attractive, research efforts are continuing to further improve preparation technologies and module properties. Typical examples include investigations on the feasibility of reactive sputtering [25] for absorber preparation, electrochemical etching of CuS [26], Cd-free buffer layers [27], and modifying the absorber by incorporating additional elements [6,28]. Incorporation of gallium is routinely used with selenide absorbers to increase the band gap which is too low in pure CuInSe2. With CuInS2, where the band gap is already at 1.5 eV without any gallium, the motivation is less straightforward [29]. Nevertheless, in terms of improving the efficiency potential of the solar cell, the incorporation of gallium into the absorbers has produced promising results [30,31] (Table 2). In two-step processes, most of the gallium is found close to the back contact after sulfurisation [32], therefore the band gap in the active region of the cell increases only by some 10 meV. The gain in open circuit voltage can, however, be significantly higher than that [33,34]. Similar observations have been made using a sequential multi-source evaporation process [35]. The latter process has achieved the highest efficiency of a selenium-free chalcopyrite-based cell. We have also shown that it is a good starting point for the development of wide-gap absorbers as required for tandem configurations by demonstrating an efficiency of more than 10% (Table 2). A screening of different transparent conductive oxide films is currently under way to find a material that could serve as a transparent back contact to this cell type [36].

6. Recombination Efficiencies under standard reporting (STR) condition as high as the maximum values of selenide-based cells have not been achieved with CuInS2. Partly, this may be due to concentrating on scaling-up rather than cultivating lab-scale processes. The high band gap of CuInS2 should result in a better annual yield in terms of kWh/kWp which reduces the significance of the difference in STR efficiency. Nevertheless, the open-circuit voltage is surely

Table 2 Efficiency of selected Cu(In,Ga)S2-based solar cells Reference

Eg (eV)

Voc (mV)

Eff. (%)


[35] [31] [34] [30] [35]

1.53 1.5

776 723 772 763 831

12.3 c12 11.6 10.4 10.1

total area, confirmed area 0.01 cm2 total area confirmed, on flexible steel foil total area, confirmed

1.5 1.65

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reducing the excessive tunneling, or, starting from cells with negligible tunneling and establishing a full barrier under illumination. The current experimental work focuses on the latter approach, i.e., preventing interface recombination by deliberate band gap grading.

7. Conclusions

Fig. 6. Corrected saturation current densities of a baseline CuInS2-based solar cell as a function on inverse temperature. Without illumination the activation energy E a corresponds to the band gap of the absorber.

not as high as expected from the band gap and the good bulk properties which can be deduced from the excellent current collection. Transport analysis has been carried out for different CuIn(Ga)S2-based cells. Comparison of results to those obtained with low- and wide-gap selenide cells [37] reveals unique features concerning the dominant recombination which determines the open-circuit voltage of the CuIn(Ga)S2 cells. Typically, the dominant process is found to change with illumination (Fig. 6). The ideal border cases are recombination over a reduced barrier at the interface without major assistance by tunneling (under illumination) and recombination in the space charge region with significant tunneling assistance (dark). Reduced barrier height causes losses in open circuit voltage. It appears to be a general problem in wide-gap chalcopyrite-based solar cells, especially when the absorber is grown Cu-rich and has to be etched before buffer layer deposition [38]. Chalcopyrite thin films with an overall slightly Cu-poor composition show a very Cudeficient surface with a band gap wider than that of the bulk. Such an inherent band gap grading can significantly reduce interface recombination. It has been argued that preparation under Cu-excess prevents formation of the surface layer and that, consequently, interface recombination becomes the dominant process [38]. On the other hand, depending on conduction band line-up and interface recombination velocity, inversion at the interface should in principle be sufficient to suppress interface recombination [39]. In some of our samples, particularly those containing Ga [35], illumination has almost no influence on saturation current and diode ideality factor, the latter being strongly temperature dependent. In this case, even when avoiding errors in the evaluation described in Ref. [40], the theory of merely tunneling enhanced recombination cannot be applied. Here, the location of the dominant recombination remains uncertain. In principle, there are two approaches to higher open circuit voltage: starting from cells with high barrier and

Despite the limited research efforts on CuInS2, a stable baseline process sequence could be developed which produces module test structures with efficiencies close to 10%. Its wide process window and high throughput, the availability of process monitoring and quality assessment methods and the use of deposition processes already proven in an industrial environment makes it a feasible candidate for scaling-up and mass production of thin film photovoltaic modules. A 1-MWp/a pilot line currently being established at SULFURCELL is the next step towards this goal.

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