Multiprobe Characterization of Inversion Charge for Self-Consistent Parameterization of HIT Cells

The performance of modern a-Si/c-Si heterojunction (HIT) solar cells is dictated by a complex interplay of multiple device parameters. A single characterization experiment [e.g., light current-voltage (I-V)] can be fitted with a set of parameters, but this set may not be unique and is, therefore, questionable as the basis for future design/optimization. In this paper, we use multiple (quasi-orthogonal) measurement techniques to uniquely identify the key parameters that dictate the performance of HIT cells. First, we study the frequency, voltage, and temperature response of inversion charge (Q_Inv) to create the theoretical basis for characterization of key device parameters, namely, the thickness of the i-layer at the front interface (t_ia-Si), a-Si/c-Si heterojunction valence band discontinuity (ΔE_V), built-in potentials in a-Si (φ_a-Si) and c-Si (φ_c-Si) regions, etc. Next, we simulate various characterization measurements, such as capacitance-voltage (C-V) and impedance spectroscopy, which probe Q_Inv and explain the parameter extraction procedure from these measurements. Subsequently, we use the algorithm/procedure just developed to extract the aforementioned parameters for an industrial-grade HIT sample. Finally, we extend this quasi-orthogonal characterization framework by correlating the C-V characteristics with the ubiquitous light and dark I-V characteristics to demonstrate the consistency of the developed theory and uniqueness of the parameter extracted. The unique parameter set thus obtained can simultaneously provide a basis for the interpretation of the experimental measurements and can also be used for the design/optimization of these solar cells.