Manufacturable Large Area CdS Thin Films for Solar Cell Applications Monitored with Optical Emission Spectroscopy
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ABSTRACT Manufacturable, sputtered, device-quality, CdS thin films are reported for high efficiency solar cell applications. The sputtering plasma is monitored during deposition using optical emission spectroscopy.
Optical emission spectroscopy (OES) is commonly used as an end point
detector in plasma etching processes, where the disappearance of the etch product wavelength signature provides an unambiguous indication of completion. OES is only now beginning to be examined for controlling deposition processes, primarily because the dependence between OES signal and film properties can frequently be a quite complex function of the electron and gas densities, the emitting species concentration, the electron impact excitation cross section, the electron energy distribution function, and the probability of inelastic collisions between plasma species. OES monitoring during CdS sputtering allows accurate determination of deposition rate. Both Cd and S emission peaks can be identified, allowing tracking of the results of preferential sputtering. The OES output has been tied directly into the chamber controls, resulting in automatic closed-loop control of deposition rate. The resulting CdS films are device-quality and well-suited to large-scale manufacturing. A photovoltaic efficiency of 12.1 % was obtained from sputtered CdS on CIGS absorber, compared to 12.9% for the traditional, but less manufacturable, chemical bath deposited CdS on the same batch of CIGS. The sputtering technique has many advantages over other deposition techniques, such as easy scaleablity to large areas, simple process control, compatibility with in-line manufacturing of layered devices and low cost. RF, or lower-cost pulsed DC, sputtering power supplies can be used with comparable deposition rates. The structure, optical, and electrical properties of the sputtered CdS thin films have been characterized. INTRODUCTION Cadmium sulfide (CdS) thin films continue to hold an important position in thin film solar cell applications. It can be made as a highly conductive n-type semiconductor with a wide band gap (> 2.4 eV). Currently most CdS is deposited on CuInSe 2 (CIS) or CuInGaSe 2 (CIGS) devices by dipping in a cadmium containing aqueous thiourea solution. A similar solution grown CdS is also used for high efficiency CdTe solar cells. Although the best efficiencies and open circuit voltages have been achieved using the solution growth technique, this process has serious limitations for high rate production including low deposition rates, difficulty in implementing inline manufacturing, and disposal of the spent solution. Sputtered CdS films received a great deal of interest during the early development of thin film solar cells, but sputtering was abandoned because solution grown CdS gave higher efficiencies. Now that laboratory efficiencies have reached 17.7% for CIGS [1] and 15.8% for CdTe [2], the development of a production worthy process for CdS growth is essential for the commercialization of CIGS and CdTe solar cells. An efficiency of 12.2%
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