Polycrystalline thin film solar cells based on copper indium diselenide (CuInSe 2) and its alloys and cadmium telluride (CdTe) appear to be the most promising candidates for largescale application of photovoltaic energy conversion because they have shown laboratoryefficiencies in excess of 15%. Heterejunction devices with n-type cadmium sulfide (CdS) films show very low minority carrier recombination at the absorber grain boundaries and at the metallurgical interface which results in high quantum efficiencies. Open circuit voltages of these devices are relatively low owing to the recombination in the space charge region in the absorber. Further improvement in efficiency can be achieved by reducing this recombination current, especially in devices based on CuInSe 2 and its alloys. Low-cost manufacturing of modules requires better resolution of a number of other technical issues. For modules based on CuInSe 2 and its alloys, the role of Na and higher deposition rates on device performance need to be better understood. In addition, replacing the chemical bath deposition method for CdS film deposition with an equally effective, but more environmentally acceptable process are needed. For modules based on CdTe, more fundamental understanding of the effect of chloride/oxygen treatment and the development of more reproducible and manufacturable CdTe contacting schemes are necessary.

We have determined the activation energies of sodium diffusion from the soda-lime glass substrate through the Mo back-contact layer, as well as through copper indium gallium diselenide (CIGS) deposited on the Mo back-contact layer of CIGS thin-film solar cells. The activation energies were determined by X-ray photoelectron spectroscopy (XPS) to measure surface sodium concentrations before and after thermally induced diffusion. The activation energies were found to be similar for the diffusion of Na through the Mo/glass and CIGS/Mo/glass thin films, approximately 8Á6 and 9Á6 kcal/mol, respectively. Furthermore, the sodium diffusion was found to occur by annealing in an environment of 1Á0 Â 10 À5 Torr of air, oxygen, or water vapor, but not in vacuum of less than 1 Â 10 À8 Torr. In addition, the diffusion of Na was found to occur faster in the presence of oxygen than in water under identical annealing conditions.

Molybdenum (Mo) coated soda‐lime glass is a commonly used substrate for Cu(InGa)Se2 solar cells as it also acts as the sodium (Na) source, which improves the efficiency of these devices. In this work, we investigate how oxygen controls the segregation and accumulation of Na on the Mo surface. A direct relationship between the concentration of surface oxygen and the amount of Na accumulation is showed. Values for the surface segregation ratio and grain boundary diffusion coefficient for Na in Mo are obtained by fitting diffusion data at several temperatures to a model for grain boundary diffusion. The results of this model reveal that surface oxygen controls the Na saturation level through its effect on the surface segregation of Na. An activation energy for grain boundary diffusion of Na is estimated and is similar to that of MoO bond dissociation in MoO3 suggesting the involvement of this bond during Na transport. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2365–2372, 2014

The Na content of (Ag,Cu)(In,Ga)Se2 films was cyclically adjusted using a novel method involving cycles of water rinsing at 60 °C followed by heating in air at 200 °C to remove Na and evaporation of NaF to re-introduce Na back into the film. The low temperatures and short heating times ensure that Na is removed only from grain boundaries while leaving grain interiors unaffected. Cross-grain conductivity and Seebeck coefficient were measured during this removal procedure and both measurements decreased when Na was removed and both recovered upon the re-addition of Na, consistent with an increase in compensating donor defects in the absence of Na. These results demonstrate that Na reversibly affects the electrical properties of grain boundaries. We propose that Na reversibly passivates donor-like defects such as InCu double donors at grain boundaries.

CuInGaSe 2 films on 6-inch wide Mo coated polyimide web substrate in a roll-to-roll vapor deposition system from elemental sources is described. Material transport from the sources to the moving web substrate has been modeled by combining an evaporative effusion model and the gas flow kinetics and by experimentally determining the flux intensity profile. The model gives a reasonably good approximation of the Ga profile in the films. Poor adhesion of the selenide film to molybdenum has been resolved by depositing a thin layer of b-(Ga 0Á8 In 0Á2 ) 2 Se 3 precursor layer. Two-dimensional compositional mapping by energy dispersive spectroscopy of 5feet-long web gave 0Á88 AE 1Á9% and 0Á28 AE 2Á5% respectively for Cu/(Ga þ In) and Ga/(Ga þ In) ratios, indicating highly uniform film composition. This compositional uniformity translated to the uniformity in the devices fabricated on the web. The open-circuit voltages of the devices from the centerline of the 5-feet web were measured to be 0Á529 AE 0Á86%. A two-dimensional device efficiency survey gave 9Á2 AE 1%. The process was able to produce high-quality material, as defined by the best device efficiency, for a wide range of Cu/(Ga þ In) ratios. The best efficiency achieved so far was 12Á1% with MgF 2 anti-reflection coating.

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The incorporation of sodium from sodium fluoride in single-crystal CuInSe2 (CIS) is investigated to provide insight into the intra-granular aspects of sodium incorporation in CIS-based thin films. Sodium was incorporated by evaporating NaF onto two CIS crystals of varying compositions and defect structures followed by heating under vacuum. Diffusion profiles show a near-surface reaction before a deeper diffusion zone which follows a complementary error function, confirming Na diffusion into the crystals. Transmission electron microscopy analysis indicates that dislocations do not control the diffusion process. The activation energy of diffusion is ∼0.7 eV for both crystals. This low activation energy suggests that Na diffusion occurs rapidly through the bulk at temperatures as low as 300 °C and helps explain the uniform Na concentration often observed in grain interiors of polycrystalline Cu(InGa)Se2 thin films.