Interdiffusion and Doping Gradients at the Buffer/Absorber Interface in Thin-Film Solar Cells.

An accurate determination of the net dopant concentration in photovoltaic absorbers is critical for understanding and optimizing solar cell performance. The complex device structure of multilayered thin-film solar cells poses challenges to determine the dopant concentration. Capacitance-voltage ( C- V) measurements of Cu(In,Ga)Se2 thin-film solar cells typically yield depth-dependent apparent doping profiles and are not consistent with Hall measurements of bare absorbers. We show that deep defects cannot fully explain these discrepancies. We instead find that the space charge region capacitance follows the model of a linearly graded junction in devices containing a CdS or Zn(O,S) buffer layer, indicating that elemental intermixing at the absorber/buffer interface alters the dopant concentration within the absorber. For absorbers covered with MgF2, C- V measurements indeed agree well with Hall measurements. Photoluminescence measurements of Cu(In,Ga)Se2 absorbers before and after deposition of a CdS layer provide further evidence for a significant reduction of the near-surface net dopant concentration in the presence of CdS. We thus demonstrate that interdiffusion at the absorber/buffer interface is a critical factor to consider in the correct interpretation of doping profiles obtained from C- V analysis in any multilayered solar cell and that the true bulk dopant concentration in thin-film devices might be considerably different.

Extended ultrahigh-Q-cavity diode laser.

We report on a study of a 698 nm extended cavity semiconductor laser with intracavity narrowband optical feedback from a whispering gallery mode resonator. This laser comprises an ultrahigh-Q (>10(10)) resonator supporting stimulated Rayleigh scattering, a diffraction grating wavelength preselector, and a reflective semiconductor amplifier. Single longitudinal mode lasing is characterized with sub-kilohertz linewidth and a 9 nm coarse tuning range. The laser has a potential application for integration with the 1S0-3P0 strontium transition to create compact precision atomic clocks.