Effect of Buffer Layer Formed by Intentionally Induced Heterogeneous Reaction on CIGS Solar Cells

A modified chemical surface deposition (mCSD) method was introduced to confirm the advantages of buffer layers deposited heterogeneously using a solution process mechanism. In chemical bath deposition (CBD), an absorber is immersed in a mixed aqueous solution containing all cation and anion precursors; in chemical surface deposition (CSD), only the absorber surface participates in the reaction using mixed precursor solutions; and in mCSD, each cation and anion precursor reacts separately on the absorber surface, resulting in a heterogeneous reaction. Optimum conditions to form a buffer layer via a heterogeneous reaction in the mCSD process are determined by changing the deposition order of the precursor solution and solution combination. The CdS or Zn(S,O,OH) buffer layers formed under optimal mCSD conditions indicated higher photovoltaic performance in solar cells compared to that of the conventional CdS buffer layer formed by the CBD method. Temperature-dependent photovoltaic characteristics, capacitance-voltage measurements, and drive-level capacitance profiling were performed to investigate carrier transport behaviors, confirming that the solar cell with mCSD-CdS had less interface recombination. Further, the admittance spectroscopy for defect analysis indicated that a solar cell with the mCSD-processed buffer layer did not form deep defects compared to that with the CBD-processed buffer layer.