We have developed a laser beam induced current imaging tool for photovoltaic devices and modules that utilizes diode pumped Q-switched lasers. Power densities on the order of one sun (100 mW/cm) can be produced in a ∼40 μm spot size by operating the lasers at low diode current and high repetition rate. Using galvanostatically controlled mirrors in an overhead configuration and high speed data acquisition, large areas can be scanned in short times. As the beam is rastered, focus is maintained on a flat plane with an electronically controlled lens that is positioned in a coordinated fashion with the movements of the mirrors. The system can also be used in a scribing mode by increasing the diode current and decreasing the repetition rate. In either mode, the instrument can accommodate samples ranging in size from laboratory scale (few cm) to full modules (1 m). Customized LabVIEW programs were developed to control the components and acquire, display, and manipulate the data in imaging mode.
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.
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