The surface chemical composition of Cu(In, Ga)Se2 (CIGS) alloys and CIGS/CdS interfaces are characterized by photoelectron emission spectroscopy. The results show that there is a severe but shallow (1-2 atomic layers) Cu depletion at {112} surfaces of CuInSe2 epitaxial films, consistent with earlier results. The depletion is not due to the existence of ordered defect compounds, but likely a result of the reconstruction of polar CIGS surfaces. Cd was found incorporated into the top 1-2 atomic layers of Cu-poor surfaces of CIGS, which may explain the n-type surface inversion in finished devices. These results and others formed the basis of a numerical model of CIGS solar cells. It is argued from the experimental and simulation results that the performance of devices based on high Ga content alloys may be reduced by failure to dope the surface of CuGaSe2 sufficiently n-type. The effect of observed defect states on the energy gap is considered, in particular with respect to pinning of the Fermi level near the heterojunction. By contrast, devices based on high In-content alloys show strong surface inversion in CIGS/CdS heterojunctions, which is suggestive of an effect of Cd incorporation in the CIGS surface.

P-type CdTe can be produced via acceptor doping with As. However, as with other II/VI materials, the dopant behaviour is not simple, as there is the potential for compensating species to be formed from intrinsic defects and dopant-defect complexes. A further complication is introduced by the presence of grain boundaries in polycrystalline material. This study demonstrates that dopant concentration is a function of VI/II ratio in the growth ambient, and that resistivity is minimised for a dopant concentration of < 2 × 1018 at.cm-3. Grain size is also affected by the VI/II ratio, increasing slightly as the growth ambient becomes more Te-rich.

ITN Energy Systems, Inc., (ITN) and Global Solar Energy, Inc., (GSE) have continued the advancement of copper indium gallium diselenide (CIGS) production technology through the development of a number of activities, including fault-tolerant control, in-situ sensors, and process improvements, with the goal of enabling uninterrupted continuous manufacturing of high quality CIGS materials that are very sensitive to processing conditions. In this paper real-time sensor fault detection and diagnosis strategies developed by ITN and GSE to enable fault-tolerant controllers for thin-film CIGS deposition process are presented. The results indicate that these strategies are very effective and they would play a major role in improving system reliability, thereby also improving yield, throughput, and efficiency. Sensing process reconfiguration methodologies that lead to fault-tolerant controllers also are briefly discussed.

This paper presents optimization studies on the formation of cadmium free buffer layers for high efficiency copper indium diselenide (CIGS) thin film solar cells using a vapor phase route. Indium sulfide layers have been deposited on CIGS substrates by Atomic Layer Deposition (ALD) at substrate temperatures between 140 and 260 °C using indium acetylacetonate and hydrogen sulfide precursors. The parametric study of the deposition temperature shows an optimal value at about 220°C, leading to an efficiency of 16.4 % which is a technological breakthrough. The analysis of the device shows that indium sulfide layers give an improvement of the blue response of the cells as compared a standard CdS processed cell, due to a high apparent band gap (2.7-2.8 eV), higher open circuit voltages (up to 665 mV) and fill factor (78 %). This denotes high interface quality of the system. Atomic diffusion processes of sodium and copper in the buffer layer are evidenced.

Photoconductive CdS (PC-CdS) in CdS/CdTe solar cells from five different sources is investigated using spectral sensitization of apparent quantum efficiency (AQE) and J-V analysis. Red bias light significantly enhances the blue AQE, commonly leading to AQE>1 below 550 nm, and blue bias light enhances the red AQE, but to a much smaller extent. These enhancements are more pronounced with increasing forward bias, after stress and in devices with intentionally Cu-doped CdS. This behavior is observed to some degree in all devices with CdS, but is absent in cells without CdS. These effects are consistent with blue light, either ac monochromatic or dc bias, increasing the CdS conductivity. This causes an increase in the field and depletion width in the CdTe to maintain balanced space charge, leading to increased collection of carriers from the CdTe. The CdS conductivity modulation can also change the AQE due to a change in equivalent circuit resistance. Analysis of J-V data measured with white, blue, red or no light indicates little dependence of series resistance or diode quality factor on the illumination spectrum. Thus, the PC-CdS resistance has little effect on the solar cell J-V performance, but does influence AQE.

Heavily In-doped CdTe and ZnTe crystals are investigated using perturbed γγ-angular correlation experiments. By means of characteristic electric field gradients, it is shown that in the Cd (Zn) poor regime, the compensation is governed by A-center formation. After annealing in Cd (Zn) rich atmosphere, the formation of DX-centers is observed and identified as the prevailing compensation mechanism in the Cd (Zn) rich regime.

Applying the (contactless) time-resolved microwave conductivity technique (TRMC) to CVD- and PVD-grown device grade CuGaSe2 thin films experimental data of hole mobility and lifetime are obtained. TRMC data of stoichiometric and Gallium-rich, front and back side illuminated thin films are presented. In the Gallium-rich samples an extended decay over several orders of magnitude is observed. In the stoichiometric samples the majority of excess carriers decays nearly instantaneously. Differences in charge carrier transport at the front and the back side are discussed in the framework of a two layer defect model, previously reported in such absorber layers.

We have investigated the compositional dependency on solar cell device quality for four different Cu(In, Ga)Se2 (CIGS) processes: Homogeneous and Ga-graded CIGS deposited at 60 min, Homogenous and Ga-graded CIGS deposited at 15 min. The Cu/(In+Ga) ratio was for varied between 0.65 and 1. An improved device performance was observed as the Cu/(In+Ga) ratio was increased from 0.65 up to 0.85 - 0.95 for all the different CIGS processes. This improvement was both due to a more efficient carrier collection at long wavelengths, increasing the Jsc, and an enhanced Voc. We explain theses improvements by an increased diffusion length in the CIGS material. The CIGS layers deposited at 15 min resulted in similar device performance as the CIGS layers deposited at 60 min, at low Cu/(In+Ga) ratios. However, for Cu/(In+Ga) ratios above 0.9 the efficiency difference increased to above 1 % unit, mainly due to a lower collection efficiency, especially at long wavelengths, resulting in a lower Jsc. By introducing a Ga-gradient in the CIGS layer the device performance improved for both deposition times, most clearly for the 15 min deposition time at Cu/(In+Ga) ratios below 0.9. At 15 minutes deposition time the reason for the improvement is a combination of small improvements in FF, Voc and Jsc, reflecting reduced recombination in general. For the single device, deposited at 60 minutes, that showed a significant improvement with Ga-grading, an improved carrier collection at long wavelengths was observed.

The highest efficiency CuIn1-xGaxSe2 (CIGS) solar cells use thin-film polycrystalline CIGS absorber layers. We have applied variable angle spectroscopic ellipsometry (VASE) to characterize the dielectric functions of polycrystalline thin films of CIGS with Ga: (In + Ga) ratios ranging from 0.18 to 1.0. The Cu: (In + Ga) ratios in these films are approximately 0.90, which is the ratio that yields the highest efficiency CIGS devices. Spectra were measured over the energy range 0.7 to 5.0 eV at room temperatures. Models used to analyze the ellipsometry data include the full multi-layer structure of the sample, which enables us to report the actual dielectric function rather than the pseudo-dielectric function. We present data on how the critical points change with composition, and compare and contrast our results with measurements of single-crystal and bulk polycrystalline samples reported in the literature. Auger electron spectroscopy, atomic force microscopy, and X-ray diffraction have been used to verify the homogeneity, surface roughness, and phase purity, respectively.

The development of a low cost roll-to-roll production process for CIGS remains an attractive goal. In the present approach, the absorber is prepared by electrodeposition techniques, while molybdenum, copper or stainless steel (SS) are used as flexible substrates. Two electrodeposition routes are evaluated: sequential plating of Cu, In and Ga followed by Se evaporation is compared to simultaneous (= ternary) electrodeposition of Cu, In and Se. Ternary electrodeposition yields 7.5 % efficiency on stainless steel. The sequential process leads to 9.0 % cell efficiency on copper and on stainless steel substrates.

The ternary alloy, InxGa1-xSb, is a compound semiconductor of tunable bandgap in the range of 0.18 - 0.72 eV, making it useful for infrared optoelectronic devices. Utilizing a unique system based upon vertical Bridgman technique, large diameter (50 millimeter) InxGa1-xSb polycrystals of composition ranging in x from 0.015 to 0.988 have been grown. Methods of mixing the melt during solidification, including the accelerated crucible rotation technique (ACRT), have been used in conjunction with optimization of the furnace temperature gradient profile to significantly reduce micro-cracking in the crystal boules while accelerating the growth rate from less than a millimeter per hour to three millimeters per hour. In this paper, the experimental system and crystal growth parameters for a set of ternary experiments will be detailed. Scanning electron microscopy (SEM) images of vertical cross-sections of In0.15Ga0.85Sb samples show a 95.8 percent reduction in micro-cracking resulting from specific melt mixing schemes. The mechanism for micro-crack removal during bulk alloy growth will be discussed briefly.

We investigate growth mechanisms of Cu(In, Ga)Se2 on Mo films on glass, as the typical back contact for Cu(In, Ga)Se2 solar cells. A thermal evaporation process deposits Cu(In, Ga)Se2of nominal 3 nm thickness at different rates R and substrate temperatures TSub. An ultrahigh resolution scanning electron microscope serves to investigate the nucleation behavior. The deposited Cu(In, Ga)Se2 forms three-dimensional isolated nuclei, known as Volmer-Weber growth mode. Deposition rate R and substrate temperature TSub control the areal density n of the Cu(In, Ga)Se2 nuclei. We observe a power law dependence between the island density n and the deposition rate R, and an exponential dependence of the island density n on substrate temperature TSub. The theory of homogeneous nucleation explains the Cu(In, Ga)Se2 cluster formation on polycrystalline Mo and the dependence of the island density on the growth conditions.

A study of the interfacial properties of wafer bonded InP/Si is presented. The electrical properties are measured by bonding InP to Si with different doping concentrations and by measuring the current voltage characteristics of the bonded pairs. Two different kinds of structures are studied: 1) n+InP doped to 3×1018 cm-3 bonded with n+Si doped to 1×1019 cm-3, and 2) n+InP doped to 3×1018 cm-3 bonded with p-Si doped to 6×1014 cm-3. After a 350°C anneal, structure 1 shows low resistive ohmic behavior while structure 2 is rectifying due to its higher sensitivity to interfacial chemistry. Indeed, both the reverse and forward current are reduced by annealing in structure 2, suggesting the formation of an interfacial barrier during annealing. This hypothesis is supported by High Resolution Transmission Electron Microscopy combined with Fourier Transform Infrared Spectroscopy, which indicate the formation of an amorphous interfacial oxide from the reaction of adsorbed water trapped at the InP/Si interface.

Voltage pulses of variable length were applied to CuIn1-xGaxSe2/CdS (0 < x < 1) junction solar cells. The resulting transient capacitance emission signal was recorded for several minutes. The amplitude of the capacitance emission signal increased linearly with the log of pulse time. These data do not follow the standard model for trap capture and emission of carriers. Instead they follow a simple electrostatic model based on electrostatic charging of traps.

We have used sputtered ZnTe:N and ZnO:Al as transparent electrodes for CdTe based solar cells. ZnTe:N is reactively sputtered with 3% N2 in the sputter gas. The ZnTe:N films typically have transmission near 85% above 750 nm and resistivity as low as 10 ohm-cm with fine grains of approximately 30 nm diameter. ZnO:Al is sputtered from a ZnO:Al2O3 (2%) target. The ZnO:Al films have resistivity as low as 4 × 10-4 ohm-cm with 80-95% transmission over the visible spectrum. To test the stability of the films, ZnTe:N films were annealed in argon, dry air and nitrogen at a range of temperatures from 200°C to 500°C. Annealing decreased the resistivity for temperatures up to 350°C and increased for temperatures greater than 350°C. ZnO:Al films were annealed in dry air at temperatures from 300°C to 550°C. Though the resistivity increased at higher annealing temperatures, there was no change in the transmission. Important to the function of a tandem solar cell is a transparent contact, like ZnTe:N/ZnO:Al, to replace the traditional metal contact to the CdS/CdTe solar cell. We investigate the ZnTe:N/ ZnO:Al bilayer as a possible recombination junction. The rf sputtered ZnTe:N and ZnO:Al films were characterized by AFM, STM, XRD, transmission, 4-point probe, and Hall measurements.

Although the deposition over large areas of polycrystalline thin-film semiconductors such as CdTe is possible by a variety of methods including close spaced sublimation, vapor transport deposition, physical vapor deposition, organometallic chemical vapor deposition, and electrodeposition, the use of a plasma-based method such as magnetron sputtering can have significant advantages. In this paper I review recent results from our group in the fabrication of CdS/CdTe cells using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film community address issues related to the challenges of fabricating high efficiency tandem cells with efficiencies over 25%. Recently we have achieved: improvements in sputtered cell performance with cells based on commercial SnO2:F as well as on substrates with our own sputtered ZnO:Al, progress in the use of reactive sputtering for the deposition of oxygen alloys of CdS and N-doped layers of ZnTe, and progress in the sputtering of wider and narrower bandgap alloys of CdTe with Zn, Mn, and Hg. Details of the sputtering process and some of the recent achievements are discussed below.

This article investigates chemical modifications of Cu(In, Ga)Se2 thin film by treatment in cadmium solutions with varying [NH3] concentration. Secondary ions mass spectroscopy and Xray photoelectron-spectroscopy analysis show that Cd-diffusion into the Cu(In, Ga)Se2 film is intimately tied to the reduction-oxidation of the Cu(In, Ga)Se2 surface, which is controlled by the [NH3] concentration. Reaction products, like Se and metal hydroxides of the redox process as wellas thediffusedCdinCu(In, Ga)Se2 film, result in degradation of solar cell performance.

Non-vacuum techniques have been used to deposit coatings of copper indium gallium selenide (CIGS) and metal oxide transparent conductors useful for fabricating thin-film photovoltaic modules. Coatings are formed from nanoparticulate precursor materials using spraying, printing and spin-coating methods. Sprayed layers exhibit non-planar morphologies and low particle packing, and CIGS films made from sprayed precursor layers exhibit related non-planar morphologies and residual void space. The surface roughness of spray-derived CIGS films reduces the sheet conductance of overlying coatings; thin coatings of transparent conductors deposited on rough CIGS films exhibit sheet resistances up to two orders of magnitude higher than equivalent coatings on planar surfaces. Slurry additives can improve layer morphology and sintered film properties, but organic additives can leave carbon contamination of the sintered CIGS films. The fabrication of multi-cell modules imposes additional constraints on transparent conductor sheet conductance.

Electroluminescence (EL) from polycrystalline CdTe/CdS solar cells was studied over the temperature range – 30 C to 25 C. We are able to observe above-background EL at forward current densities as low as 3 mA/cm2, allowing us to explore the EL behavior at current-voltage regimes within the normal operating parameters of the device. The EL spectrum is very similar to the photoluminescence (PL) spectrum, and is independent of applied voltage. We show that the EL most likely originates from injected electron-hole recombination at the CdTe/CdS junction. The total EL intensity is found to vary as a power-law function of current, EL ∼ Ib, where I is the forward current density and b is a constant. The value of b varies from sample to sample and decreases with increasing temperature. EL intensity typically is much more sensitive to device deterioration with light soak stress than is cell efficiency.

We report on the recent use of cathodoluminescence (CL) to probe the depth-dependent changes in radiative recombination that occur in CdTe devices during ZnTe:Cu contacting procedures. These types of CL measurements may be useful to assist in linking impurity diffusion (e.g., Cu) from the contact with depth-dependent variation in electrical activation within the CdTe layer. Variable-energy CL suggests that diffusion from the ZnTe:Cu contact interface may assist in reducing effects of shallow donors in the CdTe bulk, and yield higher acceptor levels in the region near the contact. CL analysis near abrupt metal discontinuities provides estimates of diffusion lengths for carriers associated with both excitonic and donor-to-acceptor pair recombination. Finally, CL measurements at increasing excitation levels (i.e., increasing electron-beam current) provides estimates of the defect state density, as well as providing evidence that discrete multiple defect bands may exist in CdTe prior to contacting.