Hall effect is among the most fundamental effects in physics and plays important role in semiconductor research. It tells us the most fundamental information of the majority charge carriers in semiconductor: their type (P or N), density (n) and mobility (μ). Consider a semiconductor under light illumination such as a solar cell absorber that has both majority and minority carriers. Until recently, we did not know how to extract both majority and minority carrier information such as mobility and density simultaneously. This information have been hidden in the photo-Hall measurement and unknown since the discovery of the Hall effect in 1879. The insight comes from a new equation expressing the hole and electron mobility difference which can be expressed as: ΔµH = d(σ2H)/dσ where ΔµH is the hole and electron Hall mobility difference, σ is the conductivity and H is the Hall coefficient. This equation allows us to extract both majority and minority carrier information simultaneously and unlocks an astonishing array of charge carrier parameters such as their mobility, photo-carrier density, recombination lifetime, diffusion lengths and recombination coefficients. The experiment is enabled by new advances in high sensitivity ac Hall measurement using a parallel dipole line system. We applied this technique in high performance perovskite and kesterite solar cell films, demonstrating a new exciting capability of characterizing semiconductor materials with unprecedented details. I will also share some behind-the-screen story of this discovery at IBM Research.