(536c) Combinatorial CdZnS Thin Films Deposited with a Continuous Flow Microreactor

Chemical bath deposited CdS thin films are commonly used as buffer layers in CdTe and CIGS photovoltaics because they form a high quality p-n junction with the absorber.  However, light absorption by the CdS reduces solar cell efficiency.  Cd_1-xZn_xS has a wider band gap than CdS, offering the potential to reduce deleterious absorption of light in the 300-550 nm range and improve current densities by over 2 mA/cm².  To maximize energy conversion efficiency, the Cd_1-xZn_xS should be a single-phase ternary alloy and the stoichiometry should be optimized to ideally position the conduction band edge for increased transmissivity while retaining good junction properties. 

   We report on chemical bath deposition of combinatorial Cd_1-xZn_xS thin films using a continuous flow microreactor.  The microreactor uses a sub-millimeter reaction channel, with the substrate acting as one reactor wall.  The microreactor behaves like a plug flow reactor whereby the bath composition changes as material is deposited on the substrate.  While the bath composition changes with respect to position down the reaction channel, the composition at any position is time-invariant.  The graded bath composition results in deposition of graded thin films whose stoichiometry and optoelecronic properties change significantly over length scales of millimeters to centimeters.  Spatially resolved characterization of the substrate enables rapid and direct correlation of material properties to growth conditions, which is not possible with a batch reactor where bath composition changes with time.

    Graded Cd_1-xZn_xS films grown with 1:200 Cd:Zn bath composition at the inlet had composition that varied from x=0.0 to x=0.17 over a distance of 12 mm on a single substrate.  Film stoichiometry was determined by x-ray photoelectron spectroscopy (XPS) mapping.    Stoichiometry changes because of differences in speciation and reaction kinetics of the cation species as they flow down the channel.  XPS and XRD confirm that CdZnS is a single phase material.  Deep level emission in photoluminescence and XPS indicate that O and OH is also incorporated into the film and is bound to Zn, with amount increasing further down the reaction channel.  The graded stoichiometry causes the absorption edge to blue-shift by over 80 nm, as determined by UV-vis transmission and reflectance measurements.  Blue-shifting band edge and changes in defect density are also seen by photoluminescence.  The continuous flow microreactor offers new potential for deposition of graded thin films that act as combinatorial libraries for high throughput screening and accelerated materials discovery.