High voltage Cu(In,Ga)S2 solar modules

Sulfurcell (SC) has been running a pilot production for thin-film solar modules using CuInS₂-chalcopyrite (CIS) as absorber material since 2004. Since then production technology has been constantly improved with module power values exceeding 64 W, corresponding to an aperture area efficiency level of about 9%. Small area (0.5 cm2) cells cut out of such CIS modules reach maximum efficiencies close to 11%. Strong efforts have been made to develop a new sequential Cu(In,Ga)S₂ (CIGS) process suitable for production of large-scale CIGS solar modules thereby enabling module efficiencies above 10%. CIGS-based solar cells are—quite similar to CIS-based modules—prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor. Such Cu(In,Ga)S₂ solar cells reach material record efficiencies about 13%. The cells are characterized by high open-circuit voltages up to 890 mV. Based on the results of the “Helmholtz Zentrum Berlin” (HZB), Sulfurcell has successfully scaled this process to our typical module size of 125 cm × 65 cm and is currently piloting the process for mass production. This paper will give an overview of electrical and structural parameters of world's first large-scale CIGS modules. CIGS module and cell parameters will be compared with standard CIS module and cell parameters and measured CIGS efficiency temperature coefficients will be compared with typical temperature coefficients of modules based on established PV technologies.     

Manufacturing and application of CIS solar modules

The pilot production of Cu(In,Ga)Se₂ (CIS) modules at Würth Solar has progressed steadily, and the pilot line could be transferred successfully into a continuous operation reaching maximum capacity in 2005 of 1.5 MW_p. Best modules on the standard size of 60 cm × 120 cm reached 85 W_p, which corresponds to 13% aperture area efficiency. The average module efficiency has been steadily improved reaching values between 11% and 12% in the year 2005. The overall process yield of the pilot line could be increased and stabilised at high values well above 80%.In April 2005 the Würth Group has decided to invest in a new production line with a starting capacity of 15 MW_p/a. This capacity will be available at the end of 2006. The new building at the new location in Schwäbisch Hall/Germany will be ready in mid 2006.The long-time reliability of Würth Solar CIS modules could be proven by passing successfully the certified test according to EN61646 and by stable operation in the field for several years. Additionally, outdoor results with CIS modules in various applications show high energy ratings which are at least as good as the best c-Si systems. Furthermore, various CIS module types have been developed for building integration and other applications.     

Comparative study of sputtered and electrodeposited CI(S,Se) and CIGSe thin films

Copper indium disulphide CuInS₂ (CIS) and diselenide CuInSe₂ (CISe) and their alloys with gallium CuIn_1 − xGa_xSe₂ (CIGSe) thin films have been prepared using both high- and non-vacuum processes. The well known two-stage process consisting in a sequential sputtering of Cu and In thin layers and a subsequent sulfurisation has led to the formation of good quality CuInS₂ ternary compound. The films exhibit the well known chalcopyrite structure with a preferential orientation in the (112) plane suitable for the production of the efficient solar cells. The absorption coefficient of the films is higher than 10⁴ cm^− 1 and the band gap value is about 1.43 eV. A non-vacuum technique was also used. It consists on a one step electrodeposition of Cu, In and Se and in a second time Cu, In, Se and Ga. From the morphological and structural point of view, the films obtained are similar to those prepared by the first technique. The band gap value increases up from 1 eV for the CIS films to 1.26 eV for the CuIn_1 − xGa_xSe₂ with 0 < x < 0.23. The resistivity at room temperature of the films was adjusted to 10 Ωcm after annealing. The films exhibit an absorption coefficient more than 10⁵ cm^− 1. The most important conclusion of this study is the interesting potential of electrodeposition as a promising option in low-cost CISe and CIGSe thin film based solar cells processing.     

Transfer of CuInS2 thin film by lift-off process and application to superstrate-type thin-film solar cells

Transfer of a CuInS₂ thin film grown on a Mo/soda-lime glass substrate was investigated using a lift-off process. The CuInS₂ thin film was flatly exfoliated, with preferential peeling occurring in the CuInS₂/MoS₂ interface vicinity. This suggests that the interfacial MoS₂ layer behaves as a sacrificial layer. The lift-off process was also applied to solar cell fabrication. A superstrate-type CuInS₂ thin-film solar cell was fabricated and exhibited no significant degradation of conversion efficiency compared with a substrate-type CuInS₂ thin-film solar cell. The lift-off process could therefore also be applied to fabricate the upper part of a tandem solar cell structure.     

Characterization of superstrate type CuInS2 solar cells deposited by spray pyrolysis method

CuInS₂ films were deposited on glass/FTO/TiO₂/In₂S₃ air ambient air at 300 °C by spray pyrolysis, resulting in superstrate-structured solar cells. The crystallinity of the spray-deposited CuInS₂ films was generally good. The CuInS₂ films with a thickness of below 2 μm showed only one layer and good adhesion. On the other hand, the CuInS₂ films with a thickness of more than 3 μm were formed with several layers, and were easily peeled off during deposition. The band gap value of CuInS₂ samples was around 1.3 eV. The performance of the best cell obtained was V_oc = 0.37 V, J_sc = 11.2 mA/cm², FF = 0.35, and had an efficiency = 1.7%. For large size solar cells (2 × 2 cm²), the effect of In₂S₃ film thickness on the cell performance was significant. In order to characterize the spray-deposited CuInS₂ films, the results of EPMA, XRD, XPS, and UV-vis absorption spectra have been discussed.     

Nanostructured solar cell by spray pyrolysis: Effect of titania barrier layer on the cell performance

ZnO nanostructured solar cells with CuInS₂ absorber layer were prepared by chemical spray method. In order to increase chemical stability of ZnO nanorods against dissolution in the next steps of the cell preparation, and reduce the electrical shorts between the front and back contacts, an amorphous TiO₂ layer was deposited on ZnO nanorods by ALD or sol-gel spray technique. The thicknesses of the layer (≤ 5 nm by spray and ≤ 1 nm by ALD), which did not impede the collection of carriers, were determined. TiO₂ thicknesses above these optimal values led to s-shaped I-V curves, causing the decrease in solar cell efficiency from 2.2 to 0.7% due to the formation of an additional junction blocking charge carrier transport in the device under forward bias. Nanostructured cells suffered from somewhat higher interface recombination but showed still two times higher current densities (~ 10 mA/cm²) than the planar devices did.     

The influence of base doping density on the performance of evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The phosphorous base doping dependence of evaporated poly-Si thin-film solar cell by aluminium-induced crystallization solid-phase epitaxy (ALICE) has been investigated. It is found that the open-circuit voltage (V_oc) of the the poly-Si thin-film solar cell increases with the decrease of base doping density due to the defect-rich nature of poly-Si thin-film material and effectiveness of the back surface field. Meanwhile, the short-circuit current (J_sc) also increases with the decrease of the base doping density as a result of the reduced doping-induced defects. Therefore, the maximum V_oc and J_sc are simultaneously achieved when the lowest phosphorous base doping density (~ 5.5 × 10¹⁵ cm^− 3) is applied.     

Thin tantalum-silicon-oxygen/tantalum-silicon-nitrogen films as high-efficiency humidity diffusion barriers for solar cell encapsulation

Flexible thin-film solar cells require flexible encapsulation to protect the copper-indium-2 selenide (CIS) absorber layer from humidity and aggressive environmental influences. Tantalum-silicon-based diffusion barriers are currently a favorite material to prevent future semiconductor devices from copper diffusion. In this work tantalum-silicon-nitrogen (Ta-Si-N) and tantalum-silicon-oxygen (Ta-Si-O) films were investigated and optimized for thin-film solar cell encapsulation of next-generation flexible solar modules.CIS solar modules were coated with tantalum-based barrier layers. The performance of the thin-film barrier encapsulation was determined by measuring the remaining module efficiency after a 1000 h accelerated aging test. A significantly enhanced stability against humidity diffusion in comparison to non-encapsulated modules was reached with a reactively sputtered thin-film system consisting of 250 nm Ta-Si-O and 15 nm Ta-Si-N.     

Effect of temperature on structural, optical and photoluminescence properties of antimony (Sb) doped polycrystalline CuInS2 thin films prepared by spray pyrolysis

Copper indium disulphide (CuInS₂) is an absorber material for solar cell and photovoltaic applications. By suitably doping CuInS₂ thin films with dopants such as Zn, Cd, Na, Bi, Sn, N, P and As its structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified. In this work, Sb (0.01 mole (M)) doped CuInS₂ thin films are grown in the temperature range 300-400 °C on heated glass substrates. It is observed that the film growth temperature, the ion ratio (Cu/In = 1.25) and Sb-doping affects the structural, optical and photoluminescence properties of sprayed CuInS₂ films.The XRD patterns confirm that the Sb-doping suppresses the growth of CuInS₂ polycrystalline thin films along (1 1 2) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Sb. About 60% of light transmission occurs in the wavelength range 350-1100 nm. The absorption coefficient (α) is found to be in the order of 10⁵ cm⁻¹. The band gap energy increases as the temperature increases from 300-400 °C (1.35-1.40 eV). SEM photographs depict that large sized crystals of Sb-doped CuInS₂ (1 μm) are formed on the surface of the films. Well defined sharp blue and green band emissions are exhibited by Sb-doped CuInS₂ thin films. Defects-related photoluminescence emissions are discussed. These Sb-doped CuInS₂ thin films are prepared by the cost effective method of spray pyrolysis from the aqueous solutions of CuCl₂, InCl₃, SC(NH₂)₂ and SbCl₃ on heated glass substrates.     

DC reactive sputtering of aluminium doped zinc oxide films for solar modules controlled by target voltage

Reactive sputtering is an option to further reduce costs associated with the deposition of the transparent front contact in chalcopyrite-based solar modules. In view of the difficulties reported in scaling-up reactive ZnO sputtering we have chosen a simple and robust approach. It is comprised of a dual magnetron operated in DC/DC mode, a constant oxygen flow and the process is controlled by target voltage. After process optimisation, the optical and electrical properties of the reactively sputtered films are comparable to those of reference films (RF-sputtered from ceramic targets). Likewise, the efficiency of monolithically integrated CuInS₂-based module test structures is not affected by the modified ZnO process.     

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO₂ (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd_1 − xZn_xTe ternary compound films have been studied. In order to improve the back contact layer, Cd_0.4Zn_0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.     

Preparation and characterization of CuIn1 − xGaxSe2 − ySy thin film solar cells by rapid thermal processing

This paper describes the synthesis and characterization of CuIn_1 − xGa_xSe_2 − yS_y (CIGSeS) thin-film solar cells prepared by rapid thermal processing (RTP). An efficiency of 12.78% has been achieved on ~ 2 µm thick absorber. Materials characterization of these films was done by SEM, EDS, XRD, and AES. J-V curves were obtained at different temperatures. It was found that the open circuit voltage increases as temperature decreases while the short circuit current stays constant. Dependence of the open circuit voltage and fill factor on temperature has been estimated. Bandgap value calculated from the intercept of the linear extrapolation was 1.1-1.2 eV. Capacitance-voltage analysis gave a carrier density of 4.0 × 10¹⁵ cm^− 3.     

Temperature dependence of photocapacitance spectrum of CIGS thin-film solar cell

Deep levels in Cu(In_1 − x,Ga_x)Se₂ (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x, from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.     

Performance of CuIn1 − xGaxSe2/(PVD)In2S3 solar cells versus gallium content

The present work studies the influence of the Ga content (x = Ga / (Ga + In)) in the absorber on the solar cell performance for devices using (PVD)In₂S₃-based buffers. Input to the hypothesis of the relative conduction band positions can be found in the evolution of the device parameters with x. For experiments with x between 0 and 0.5 devices using (PVD)In₂S₃-based buffers are compared to reference devices using (CBD)CdS. Both buffers give similar cell characteristics for narrow band gap absorbers, typically E_gCIGSe < 1.1 eV. However, the parameters of the cells buffered with (PVD)In₂S₃ are degraded when the absorber gap is widened whereas (CBD)CdS reference devices are only slightly affected. Consequently, the solar cell efficiency is similar for both buffer layers at the lower x values and increases with x only in the case of (CBD)CdS. These evolutions are coherent with the existence of a conduction band cliff at the CIGSe/(PVD)In₂S₃ interface.     

Low-temperature processing of a solution-deposited CuInSSe thin-film solar cell

A low-temperature (~ 350 °C) solution-processed CuInSSe photovoltaic cell is reported. The CuInSSe film was solution-deposited via spin-coating from a precursor solution consisting of metal chalcogenides (Cu₂S and In₂Se₃) dissolved in hydrazine (N₂H₄). X-ray diffraction data indicated a full conversion from the hydrazine precursor to CuInS_xSe_2−x structure at 350 °C with an average crystallite size of approximately 45 nm. Bandgap tuning of the CuInS_xSe_2−x was achieved by varying the excess amount of sulfur in the precursor solution. Based on the (220) reflection of the XRD pattern, the bandgap of CuInS_xSe_2−x ranged from 1.00 to 1.14 eV. Standard testing conditions at 1-sun intensity resulted in a power conversion efficiency of 7.43%.     

Influence of contaminations on the performance of thin-film silicon solar cells prepared after in situ reactor plasma cleaning

This paper addresses the influence of the chemical memory effect (CME) of in situ plasma cleaning by using the fluorinated gases on the properties of subsequently deposited thin-film silicon solar cells and discusses methods to avoid or reduce this effect. Secondary ion mass spectrometry (SIMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) profiles analysis showed a high impurity concentration in the intrinsic (i)-layer of p-i-n solar cells prepared directly after in situ cleaning. With increasing number of cell depositions these contaminations decrease and the solar cell performance recovers to the standard value. Restoring solar cell performance is accompanied by a decrease of contaminants concentration in the i-layer. The intentional variation of the F-content in the i-layer obtained by adding SiF₄ to the process gas mixture during a-Si:H i-layer preparation reveals that for solar cells a fluorine content above 1.5 × 10¹⁹ cm^− 3 is critical. We applied NF₃ or SF₆ + O₂ as cleaning gases and optimized the cleaning procedure. In case of using NF₃ as the cleaning gas, the CME was less pronounced as compared to the SF₆ + O₂ case and by additional procedures, like increasing the total gas flow rate during deposition, hydrogen plasma treatment of reaction chamber, the high solar cell quality could be achieved directly after in situ reactor cleaning. Low concentration of impurities in such cells was observed. Also the long-term illumination test (light-soaking for 1000 h, AM1.5 radiation) shows the same stabilized efficiency as compared to reference cells.     

Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu₁₁In₉ precursors to be employed as light absorbing CuInS₂ (CIS) layers in CIS-CdS heterojunction thin-film solar cells has been investigated. The Cu₁₁In₉ precursor films were produced by DC-sputtering from a single-phase Cu₁₁In₉ target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In₂S compound, the formation of which is favoured by the phase transition of Cu₁₁In₉ to Cu₁₆In₉ at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS_x surface phase.     

Manufacturing of CdTe thin film photovoltaic modules

The technology to fabricate CdTe/CdS thin film solar cells can be considered mature for a large-scale production of CdTe-based modules. Several reasons contribute to demonstrate this assertion: a stable efficiency of 16.5% has been demonstrated for 1 cm² laboratory cell and it is expected that an efficiency of 12% can be obtained for 0.6 × 1.2 m² modules; low cost soda lime float glass can be used as a substrate; the amount of source material is at least 100 times less than that used for single crystal modules and is a negligible part of the overall cost. The fabrication process can be completely automated and a production yield of one module every 2 min can be obtained, which implies a production cost substantially less than 1€/W_P. A further cost reduction will render this kind of energy production competitive with the energy obtained from fossil fuels by approaching the so-called grid-parity. Some new companies have recently announced the start of production or plan to do so in the near future. Many of these plants are located in Germany, some in the USA. In Italy, a new company has been constituted in 2008, with the aim of building a factory with a capacity of 18 MW/year. In this article, we will describe and compare the basic principles of CdTe solar cells and modules. We will include an overview of the potentials of these technologies and of the R&D issues under investigation. This paper describes how the large-area mass production of CdTe solar modules is realized in the Italian factory and presents a worldwide overview of the current production activities.     

Locally resolved surface photo voltage spectroscopy on Zn-doped CuInS2 polycrystalline thin films

Incorporation of small amounts of Zinc (< 1 at.%) in polycrystalline CuInS₂ thin films for solar cells leads to an increased open circuit voltage. Here we investigate the optoelectronic effect of Zn doping by local surface photovoltage spectroscopy (SPS). SPS is measured using Kelvin probe force microscopy (KPFM) to obtain the surface photovoltage (SPV) and SPS with high lateral resolution, and thereby study the homogeneity of the doping. In our KPFM experimental setup, illumination is realized by a Xe arc lamp and monochromator in the visible spectrum range by means of an optical fiber into the UHV system of the KPFM.We compare CuInS₂ thin film samples with and without Zn doping. The pure CuInS₂ samples show a sharp onset of SPV at the band gap of 1.48 eV, whereas for Zn-doped CuInS₂ we observe a two step onset, with a steep increase of SPV at 1.48 eV. However, already below this band gap, we observe a slight SPV response, even down to about 1.40 eV. This indicates the presence of states in the band gap, likely resulting from disorder induced by the Zn-doping. The absence of lateral differences in the observed SPV spectra favors an explanation by Urbach-tails over the possible existence of a Zn foreign phase. These results are in agreement with transmission/absorption measurements.     

Improvement of a-Si:H solar cell performance by SiH4 purging treatment

Amorphous silicon (a-Si:H) thin film solar cells were prepared in a single chamber large area plasma enhanced chemical vapor deposition (PECVD) system. A purging process using silane (SiH₄) gas was developed to remove the residual contaminations in the reactor after a nitrogen trifluoride (NF₃) plasma dry cleaning process. Such a purging treatment leads to a clear improvement in initial fill factor (FF) and in efficiency of as-prepared a-Si:H solar cells. Secondary ion mass spectroscopy (SIMS) results demonstrate that fluorine impurity concentration [F] at the p-layer as well as p/i interface of solar cells reduces by more than one order of magnitude after this purging process. Additionally, high [F] is accompanied with high oxygen impurity concentration [O] which plays a great role in the solar cell performance. Low degradation rate of open circuit voltage (V_oc) and fill factor (FF) of solar cells after a purging process after a 1000 h light soaking further illustrates an improvement in the material properties. Implanting such a purging process in the practical production line, about 2 W in power for a-Si:H solar modules (1.1 m × 1.3 m) are gained and meanwhile the champion solar module (1.1 m × 1.3 m) of stabilized power of 113 W with 160 nm thick intrinsic layer has been achieved.