Impedance spectroscopy as a diagnostic tool for charge transport and interface limitations in advanced silicon solar cells

Abstract

Advanced silicon solar cell technologies employ multilayer architectures and offer significant potential to improve power conversion efficiency. However, fabrication-induced nonidealities often lead to charge-transport issues and reduced photovoltaic performance. Therefore, a systematic assessment of the device is essential to pinpoint the specific regions of performance loss and to enable targeted optimization. This study applies detailed impedance spectroscopy (IS) with a broadband AC signal in the range of 1 Hz to 1 MHz for silicon heterojunction (SHJ) solar cells; one is defect-dominated charge transfer in a-Si:H layer, another one is hindered charge transport at the p-a-Si:H/ITO hole-selective contact, and an optimized SHJ cell. These effects manifest within a distinct frequency range of the IS response, thereby enabling the identification of the dominant charge-carrier resistive and recombination-loss mechanisms. A deeper analysis of the Nyquist plot, together with frequency-dispersed phase shifts and real and imaginary impedance responses (Z′, Z″), provides clear signatures of the specific location of the performance loss. It is observed that the optimized device has a well-established depletion region and minority-carrier diffusion, with negligible resistive drop across the device. However, in unoptimized devices, additional charge-delay and impedance features appear within a specific frequency range, revealing distinct origins of the performance loss: one associated with the i-a-Si:H layer and the other with the ITO contact. Therefore, IS provides a powerful basis for diagnosing distortions in photocurrent–voltage graphs, degradation pathways, and transport bottlenecks. The frequency-resolved IS can be a critical tool for guiding interface engineering and process optimization of any optoelectronic device.