Mechanism Analysis of Photoelectric Mismatch Loss in Curved CIGS Cells: An Indoor Experimental Study

Flexible photovoltaic (PV) technology not only has high power efficiency but also is thin and lightweight, enabling seamless adaption to the surface of curved buildings. However, the distinctive spatial geometry of curved surfaces leads to inhomogeneous irradiance, causing electrical mismatch losses. This study examines the geometric origin of photoelectric mismatch through controlled indoor experiments on bare flexible CIGS cells in six configurations: flat, length convex, length concave, width convex, width concave, and length wavy. The tests were conducted under standard test conditions at central angles from 0° to 180° and in two laboratory orientations. A two-dimensional microfacet optical model, accounting for direct-beam projection, self-shading, and inter-arm mutual reflection, was developed and calibrated against the measurements, giving an R2 = 0.85 and RMSE = 5.1%. The results show that width-bending configurations retain a performance ratio (PR) above 60% over the tested angular range because the irradiance gradient is distributed across the current-collection path. In contrast, length bending aligns the gradient with the current path and leads to much larger mismatch losses. For the concave cases, mutual reflection improves performance at intermediate central angles, but this benefit is lost when self-shading becomes dominant beyond approximately 135°. The most severe case is length concave in the horizontal orientation at 180°, which retains only 43.4% of the flat-reference power, corresponding to a 56.6% loss and 319.7 mW of mismatch loss. These findings clarify how bending direction, curvature, and orientation affect cell-level mismatch and provide design guidance for curved CIGS BIPV systems.