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Cadmium telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal band gap of 1.45 eV, its high optical absorption coefficient and availability of various device fabrication methods. Superstrate CdTe solar cells fabricated on glass have to-date exhibited efficiencies of 16.5%. Work on substrate devices has been limited due to difficulties associated with the formation of an ohmic contact with CdTe. Foil substrate curvature, flaking, delamination and adhesion as a result of compressive strain due to the coefficient of thermal expansion (CTE) mismatch between the flexible foil substrate and the solar cell films has been studied. Thin films have been characterized by AFM, XRD, SEM, ASTM D3359-08 tape test, and solar cells have been characterized using J-V and spectral measurements. Adhesion improves by minimizing the mismatch of the CTE . A CdCl2 treatment is important for high efficiency solar cells. The treatment influences the microstructure and interface properties of the layers. The effect of the current CdCl2 chemical treatment increases flaking and has to be optimized for the CdTe substrate cell on foil. We have also fabricated solar cells on tantalum, molybdenum and tungsten foils, all with lower CTEs than SS430. We are currently producing solar cells with a VOC of 630mV, a 50% fill factor and over 6.0% efficiency.
In this study, 1 μm thick polycrystalline CdTe films were deposited by magnetron sputtering using a variable argon pressure, 2.5 ≤ pAr ≤ 50 mTorr, and a fixed substrate temperature, Ts = 230°C. Real time spectroscopic ellipsometry (RTSE) was performed during deposition in order to analyze the nucleation and coalescence, as well as the evolution of the surface roughness thickness ds with bulk layer thickness db and the depth profile in the void volume fraction fv. A linear correlation was found between the final ds value measured by RTSE at the end of deposition and the root-mean-square (rms) surface roughness measured by atomic force microscopy (AFM) ex situ after deposition. A monotonic decrease in RTSE-determined roughness thickness is observed with decreasing Ar pressure from 18 to 2.5 mTorr. The lowest pressure also leads to the greatest bulk layer structural uniformity; in this case, fv increases to 0.04 with increasing CdTe thickness to 1 μm. The photovoltaic performance of CdTe films prepared with the lowest pressure of pAr = 2.5 mTorr is compared with that of previously optimized CdTe solar cells with pAr = 10 mTorr.
This work reports the use of strategies based on Raman scattering for process monitoring of electrodeposited based S rich CuIn(S,Se)2 solar cells. Main vibrational modes in the Raman spectra are sensitive to features related to the crystalline quality, chemical composition and presence of secondary phases in the chalcopyrite layers, being all these features relevant for the optoelectronic properties of the final devices. Ex-situ and in-situ measurements during the electrochemical step allow the direct assessment on the formation of Se rich secondary phases which are related to the stoichiometry of the grown precursors. The analysis of the relative intensity of the spectral contribution from these phases allows early detection of deviations of precursor stoichiometry in relation to the optimum composition range in terms of solar cell efficiency. The applicability of the technique for the in-situ monitoring of the electrodeposition process is also discussed
Chalcopyrite-based devices show highest conversion efficiencies among present thin film architectures with values of 20% at laboratory scale. This outstanding performance has been achieved for quaternary Cu(Inx,Ga1-x)Se2 (x˜0.7) compound material. However, a strong correlation between the performance and the gallium content or, in other words, low versus high bandgap materials has been recognized. One critical issue in this discussion is the formation of a copper-depleted near-surface phase with 1:3:5 and 1:5:8 stoichiometries. In earlier reports, surface phases with corresponding compositions have been found on CuInSe2, CuGaSe2 and Cu(Inx,Ga1-x)Se2 thin films. These near-surface phases show a positive influence on the performance of cells based on low bandgap Cu(Inx,Ga1-x)Se2 material due to n-type inversion and band gap widening compared to bulk properties. A tendency towards a neutral or even a negative impact of the near-surface phase on wide band gap material (high gallium content) has recently been reported . Nevertheless, the structural models of copper-poor chalcopyrite-related compounds have been controversially discussed in literature but a stannite-type structural model is most suitable as will be presented. In any case, the relation of the structural properties between chalcopyrite and 1:3:5 phases is crucial for the performance of related devices.
In this contribution we will report about the structural analysis of the Cu(Inx,Ga1-x)3Se5 solid solution series by means of anomalous x-ray scattering using synchrotron radiation, powder and single crystal neutron diffraction. Contributions of the isoelectronic species Cu+ and Ga3+ could be separated by these experiments. Bulk samples synthesized from the elements and heat treated at 650°C after the main reaction step - the latter in order to allow equilibrium structure formation - were investigated. Structural data like lattice parameters, tetragonal distortion and cation distribution were obtained for the complete Cu(Inx,Ga1-x)3Se5 solid solution series. The stannite-type structural model was assigned to all members of the investigated 1:3:5s which will be strengthened by simulations. We observed that the tetragonal distortion vanishes for compositions close to a gallium content as used for highest efficiency Cu(Inx,Ga1-x)Se2 devices. However, the tetragonal distortion depends critically on the cation distribution which is in turn controlled by the thermal history of the sample, as we have recently reported for pure CuGaSe2 . This means that we can plot a direct correlation for the misfit between chalcopyrite and 1:3:5 phases depending on the gallium content and the thermal treatment of the considered thin films. These results will widen the understanding of the chalcopyrite-based thin film photovoltaic devices.
 S. Lehmann et al., Phys. Stat. Sol. A (in press)
In this work we compare ZnS-based buffer layers prepared by atomic layer deposition, ALD, and chemical bath deposition, CBD. Both material and device properties are compared. CBD buffer layers are amorphous with a Zn(OH,S) composition while ALD buffer layers used in devices are crystalline with a Zn(O,OH,S) composition. Devices with ALD buffer layers are stable while for CBD, large lightsoaking effects are seen. Stable devices with CBD buffer layers are obtained by including an ALD-(Zn,Mg)O layer on top of the CBD layer.
We report on the chemical deposition and electronic properties of CuInS2/Zn(S,O) interfaces. The Zn(S,O) buffer was grown by a new chemical bath deposition (CBD) process that allows the tailoring of the S/O ratio in the films. Resulting Zn(S,O) films exhibit transparencies above 80% (for λ>390 nm) and an optical energy band gap of 3.9 eV which decreases to 3.6 eV after annealing in air at 200°C. Production line CuInS2 (CIS) absorbers provided by Sulfurcell Solartechnik GmbH are used as substrates for the investigation of the CIS/Zn(S,O) interface and the chemical composition of Zn(S,O). A ZnS/(ZnS+ZnO) ratio of 0.5 is found by X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy (XPS and XAES). The valence band offset between the heterojunction partners (ΔEV = 1.8 ± 0.2 eV) has been determined by means of XPS and ultraviolet photoelectron spectroscopy (UPS). Considering the energy band gap of the CIS absorber and the measured band gap of Zn(S,O), the conduction band offset (ΔEC) is calculated as: resulting in a spike of 0.5±0.3 eV in the conduction band at the heterojunction before annealing. After the heat treatment, the valence band offset is reduced to 1.5±0.2 eV and the calculated conduction band offset remains at 0.5±0.3 eV.
Thin films of tin sulfide (SnS) were deposited on TCO-coated glass substrates by pulse electrodeposition. Cyclic voltammetry showed that SnS deposition occurs in the -0.8 V to −1 V range. The films deposited using the potential pulses of -0.95V (Von) and +0.1V (Voff) are of orthorhombic crystal structure with lattice parameters and grain size similar to those of the thin films of orthorhombic structure obtained by chemical deposition. The optical band gap of the films was 1.3 eV. In CdS/SnS heterojunctions an open circuit voltage110 mV, short circuit current density 0.72 mA/cm2 and fill factor of 0.32 are reported here.
A rapid screening method is reported in which material processing parameters are investigated as a function of the CdTe absorber thickness in CdTe/CdS solar cells. It has been used to investigate i) the optimum absorber thickness for CdCl2 processing at 380°C for 10 mins, and ii) the effect on device performance of post-growth annealing of CdS layer with H2, N2, and O2. It was found that the optimum thickness of CdTe compatible with the processing was ∼3μm. The device results were independent of the post-growth treatment of the CdS for the conditions investigated here. The bevel method allowed for ∼30 data points to be obtained from each sample, this giving a significant advantage over conventional experimental methods.
Hydrogen diffusion in zinc oxide thin films was studied by secondary ion mass spectrometry (SIMS) measurements, investigating the spreading of implanted deuterium profiles by annealing. By effusion measurements of implanted rare gases He and Ne the microstructure of the material was characterized. While for material prepared by low pressure chemical vapour deposition an interconnected void structure and a predominant diffusion of molecular hydrogen was found, sputter-deposited ZnO films showed a more compact structure and long range diffusion of atomic hydrogen. Hydrogen diffusion energies of 1.8 – 2 eV, i.e. higher than reported in literature were found. The results are discussed in terms of a H diffusion model analogous to the model applied for hydrogen diffusion in hydrogenated amorphous and microcrystalline silicon.
We investigate the origin of fill factor changes induced by reverse bias treatment. Evolution of current-voltage characteristics have been measured during application of reverse voltage bias. Two different cell behaviors have been identified. At elevated temperatures one kind of the devices strongly deteriorates and exhibit so called double diode behavior. On the other hand, in the same conditions another cells keep their fill factor almost constant. We correlate the fill factor changes with the kinetics of capacitance and show that although increased number of shallow acceptors itself cannot induce this severe FF deterioration, it may strongly influence position of the Fermi level at the heterointerface that in a presence of an electron barrier is crucial for the device behavior.
The crystalline, optical and electrical properties of In2S3 containing copper thin films are investigated. Increasing the amount of copper within the In2S3 crystalline matrix yields reduced bandgap value and hindered conductivity. The films investigated being synthesized at low temperature (200 °C), it is likely they have similar properties as the materials formed at the CuIn1-xGaxSe2/In2S3 interface.
In the present work we studied the influence of selenisation temperature, Se vapour pressure and duration of the process on the properties of MoSe2 layer formed on Mo-foil and on sputtered Mo layers on soda lime and Mo-on-ITO glasses. We found that MoSe2 layer thickness (dL) on Mo-foil depended linearly on selenisation time. The thickness of MoSe2 layer on Mo-foil (dL) depended on Se vapour pressure as a function , where n ≈ 0.5. The same dependence was also found for sputtered Mo layers in low Se vapour pressure region of 13 – 133 Pa. MoSe2 layer thickness depended on the origin of Mo layer which is related with the density of Mo layer: MoSe2 on Mo foil was thicker than on sputtered Mo. All the MoSe2 layers were full of cracks if Se vapour pressure was higher than 1333 Pa. All tested MoSe2 layers showed p-type conductivity.
We report on recent advances in the development of a luminescence spectroscopy based on scanning tunneling microscopy (STM) and its application to fundamental aspects of Cu(In,Ga)Se2 (CIGS) thin films. Relevant to our discussion is the specifics of the surface electronics. The CIGS shows pronounced stoichiometric deviations at the surface and, consequently, distinct surface electronics that has been shown to be critical in achieving high efficiency. Cathodoluminescence (CL), a luminescence spectrum imaging mode in scanning electron microscopy (SEM), provides a direct correlation between the microstructure of the CIGS and its electronic properties. As such, cathodoluminescence can resolve the emission spectrum between grain boundaries and grain interiors or be used to investigate the influence of local orientation and stoichiometry on the electronic properties of the CIGS at the microscale. Cathodoluminescence is not a surface microscopy, however, and resolving the electronic structure of the CIGS surface remains elusive to all luminescence microscopies. With this motivation, we have developed a luminescence microscopy based on STM, in which tunneling electrons are responsible for the excitation of luminescence (scanning tunneling luminescence or STL). The hot-tunneling-electron excitation is confined to the surface and, consequently, the tunneling luminescence spectrum reveals the electronic states near the surface. The STM is integrated inside the SEM and, therefore, both CL and STL can be measured over the same location and compared. Using this setup, the transition from the grain interior to the surface can be investigated. We have improved the collection of our optics to a level in which tunneling luminescence spectrum imaging can be performed. Here we present a detailed account on our investigation of the surface electronics in CIGS deposited in the regime of selenium deficiency as defined by <Se>/(<Cu> + <In> + < Ga >) = 1.
Cyclic voltammogram studies were performed on H2SeO3, CuSO4, In2(SO4)3, GaCl3, H2SeO3 + CuSO4 + In2(SO4)3 and H2SeO3 + CuSO4 + In2(SO4)3 + GaCl3 to understand the electrodeposition mechanism. The reduction potential from the cyclic voltammogram studies indicates that the first deposited layer is Cu from the Cu-In-Se and Cu-In-Ga-Se solution mixture. The subsequent deposition of the In and Ga layer is more favorable on the first-deposited Cu layer.
A theory of non-crystalline recombination junctions is developed and compared to the experimental data. Junction transport is represented as hopping in both real and energy spaces, dominated by rare yet exponentially effective optimum channels having favorable configurations of localized states. Our work correlates the current-voltage characteristics of non-crystalline devices with material parameters and predicts large non-ideality factors increasing under light, and possible variations between nominally identical devices.
The role of thermal annealing and of CdCl2 as a main source of electrically active but vaporizable chlorine doping in chemical bath deposited CdS thin films is studied. The films were deposited on glass substrates from aqueous solution of either CdCl2, NH4Cl, NH4OH, and thiourea, or CdSO4, (NH4)2SO4, NH4OH, and thiourea. Films deposited in the presence of CdCl2 and annealed in H2 atmosphere at 310 and 420 °C show a resistivity lower than 10 Ω·cm, one order of magnitude less than for identically annealed films deposited in absence of CdCl2. Annealing at 420 °C in closed ampoules, where a counter pressure of CdCl2 builds up, leads to a lower resistivity on the order of 10−1 Ω·cm, confirming the key role of chlorine on the electronic properties. However, further characterization via photoluminescence raises new questions about chlorine-related defects and their role in the mechanisms that govern film resistivity.
Magnetron sputtering (MS) of CdTe and related II-VI materials facilitates low energy ion and electron bombardment that promotes good film growth at substrate temperatures well below those needed for other physical vapor deposition methods. MS also provides good control of deposition rates while allowing scale-up to large areas. In this paper we review the use of MS for deposition of polycrystalline thin films of CdS, CdTe and related materials for solar cells with a focus on reducing the thickness. We relate the deposition conditions and plasma properties determined by Langmuir probe measurements to some of the materials properties of the films through spectroscopic ellipsometry and high resolution electron microscopy. For cells with CdTe layers from 0.35 to 2.5 μm, we have done a first-order optimization of chloride treatment conditions and back contact structure. We discuss the influence of CdTe thickness and post-deposition processing on the efficiency, open-circuit voltage, short-circuit current, and fill factor and show that 10% efficient cells can be fabricated with 0.5 μm of CdTe.
Preliminary studies on the properties of undoped and Ti-doped In2S3 thin films grown on soda lime glass by chemical solution deposition under different conditions are presented. Different physical, chemical and morphological properties of the films have been analysed. At the beginning of the deposition of In2S3 films, In2O3 and In(OH)3 deposited by electroless-chemical reaction are dominant. The optical properties observed for Ti-In2S3 films show the partial contribution of the electronic transitions. The study is completed with SEM analysis which shows the influence of the deposition time and the precursor used, in the morphology for incorporation of titanium at the beginning of deposition and X-ray diffraction when is observed the amorphous nature of the films.
We prepared fine Cu(In,Ga)Se2 (CIGS) powder suitable for screen printing using a mechanochemical synthesis and wet bead milling. Particulate precursors were deposited in a layer by a screen-printing technique, and the porous precursor layer was sintered into a dense polycrystalline film by atmospheric-pressure firing in an N2 gas atmosphere. The microstructure of CIGS powder and fired CIGS film were observed in an SEM. The wet bead milling was effective for the reduction and homogenization of the average grain size of CIGS powder. The CIGS grains in the film were well sintered and the size of CIGS grains was as large as about 2 μm. The CIGS solar cell showed an efficiency of 3.1%, with Voc of 0.279 V, Jsc of 28.8 mA/cm2 and FF of 0.386.
We report a systematic study of the influence of the target-substrate distance and rf power on the structural and optical properties of ZnO thin films grown by rf magnetron sputtering in Ar atmosphere from ZnO sputtering target. Sharp (002) peak showed by XRD indicates a c-axis crystalline growth of ZnO films. Growth rate remained almost constant for short target-substrate distances. However, the grain size increases with the rf power decreasing the compressive stress in ZnO films. As-grown ZnO films have average transmittance more than 80% in the visible region. Optical bandgap (Eg) increases from 3.18 to 3.27 eV as increase the target-substrate distance probably due to low stress compression in ZnO films. In addition, when rf power is above 100 W, the optical band gap increases as increase of the stress compression.