Bias dependent admittance spectroscopy of thin film solar cells: KF post deposition treatment, accelerated lifetime testing, and their effect on the CVf loss maps

Modern solar cell designs include an always larger variety of elements and additional layers. This approach is usually successful and leads to the development of increasingly efficient materials. However, scientifically, it makes the drawing of accurate conclusions always more challenging, due to the growing number of elements, possible defects, interfaces, and barriers. To try and remedy this problem, we developed a novel way to investigate solar cells by representing bias dependent admittance spectroscopy (CVf) measurement data in the form of a 2D loss map. In this contribution, we elaborate how this technique can be used experimentally, present some concrete results we obtained from measurements on ultra-thin CIGS solar cells and explain how they can be interpreted. We identify 3 major response domains on our typical CVf loss maps. One at high frequency that covers most of the bias range and can be related to series resistance. One at low frequency, mostly visible at strong positive and negative biases, which is related to shunt and dissipation. The last response domain, close to 100 kHz and impacting most of the bias range, can be identified as a defect response of the material. Using CVf measurements on samples with KF post deposition treatment, known for its grain boundary passivation properties, and samples that were previously submitted to accelerated lifetime testing in damp heat conditions, the impact of bulk defects, grain boundaries and conduction band offsets are investigated.