Fundamentals of electrical material and device spectroscopies applied to thin-film polycrystalline chalcogenide solar cells

Abstract

This chapter examines the fundamentals of characterization methods using AC modulated electrical excitation and measurement of the complex-valued response as applied to chalcogenide polycrystalline thin-film solar cells. Capacitance voltage (CV), drive-level capacitance profiling (DLCP), admittance spectroscopy (AS), deep-level transient spectroscopy (DLTS) and its variants, so-called transient photocurrent (TPI) and transient photocapacitance (TPC), and deep-level optical spectroscopy (DLOS) are the members of this class sometimes referred to as capacitance-based techniques. The primary desirable trait of capacitance measurements compared to others is their ability, in ideal cases, to accurately quantify charge and thus densities of states as functions of space and energy. However, other material and device features may produce similar signals; and only in very restricted cases (such as an epitaxial pn homojunction with two Ohmic contacts) are traps or defects in the depletion width the only possible origin of a defect-like signature. The applicability of assumptions of capacitance spectroscopies and interpretations developed for single-crystalline and epitaxial materials and devices are examined in the context of today's chalcogenide thin-film photovoltaic (PV) technologies, which present both material and band-diagram nonidealities in this regard.