Manufacturable Large Area CdS Thin Films for Solar Cell Applications Monitored with Optical Emission Spectroscopy

ABSTRACTManufacturable, 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.