A Optical and Microstructural Characterization Study and Microstructural Model of Co-Evaporated Polycrystalline Thin Film Copper Indium Diselenide for Photovoltaic Applications
Abstract
The electrical, optical, microstructural, and morphological properties of polycrystalline thin film CuInSe _2 are investigated by X-Ray Diffraction, spectrophotometry, and Transmission Electron Microscopy as a function of compositional parameters and processing conditions. The film microstructure is redefined as a compositionally and temperature dependent polycrystalline aggregate mixture of CuInSe_2 and Cu_{rm x}Se/In _{rm y}Se minor phases, with a core crystallite exhibiting order-disorder phase separation and a CuInSe_2-CuIn_2 Se_{3.5} solid solution. The secondary phase phenomena includes Cu _{1.85}Se at grain boundaries and free surfaces, with sufficient amount for percolation in near-stoichiometric and Cu-rich compositions, and Cu _{rm x}Se inclusions for Cu-rich compositions, and for Cu-poor compositions at substrate temperatures of 500^circC. The inclusions are modeled by the Maxwell-Garnett theory as metallic inclusions with a filling fraction as little as eta = 0.01. The observed polymorphisms include phase separation of ordered chalcopyrite and disordered sphalerite regions, where the latter is the majority phase, and a solid solution of chalcopyrite CuInSe_2 and the ordered vacancy compound (OVC) CuIn_2Se _{3.5}, with an associated temperature dependent lattice shrinkage proportional to the deviation in molecularity. The polymorphisms reported here are the first in the CuInSe_2 material system. The stability of excessive Cu-vacancies observed in the OVC has significant ramifications on the electronic structure of CuInSe_2. The optical absorption coefficient, alpha , for thin film CuInSe_2 is reported as a function of composition and exhibits behavior indicative of a two-phase mixture and a solid solution for Cu-rich and Cu-poor compositions, respectively. The magnitude of alpha is significantly lower than previously reported values, and is substantiated by device modeling that accurately reproduces the measured internal quantum efficiency of CdS/CuInSe_2 photovoltaic devices. Compositional studies indicate a phase and defect chemistry on the Cu_2SeIn {_2}Se_3 pseudo-binary ti-line, with In_{rm Cu}/Cu_{rm In} and V_{rm Cu}/In _{rm Cu} majority defect pairs. The core crystallite is Cu-poor with no deviation from optimal valency, with the soluble Cu _{1.85}Se phase accounting for the Cu excess. The thin film growth process is modeled by intermediate Cu_{rm x} Se and In_{rm y} Se binary formation at the substrate surface. This study is the first of its kind in the literature and may be useful in the development of multinary thin film materials for photovoltaic applications.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1990
- Bibcode:
- 1990PhDT........59T
- Keywords:
-
- COPPER INDIUM ARSENIDE;
- Physics: Condensed Matter; Engineering: Materials Science; Engineering: Electronics and Electrical