Understanding the structural and electrical behaviors of organic-inorganic (hybrid) halide perovskites under practical environments is critical for building an efficient and stable optoelectronic device. In this thesis, we report a light-activated interlayer contraction in 2D hybrid halide perovskite. This effect is reversible and is strongly dependent on the structural phase and interlayer distance. X-ray photoelectron spectroscopy and density function theory simulation suggest that photogenerated hole carriers are accumulating at the inter-slab iodide atoms which results in the enhancement of interlayer I---I interactions across the organic barrier. In-situ structural (device) and transport (space charge limited current-SCLC) measurements directly correlate the light-induced interlayer contraction to the onset of a three-fold increase in carrier mobility and conductivity. Furthermore, light intensity dependent SCLC measurement reveals a percolation based mechanism for the enhancement of the charge transport. The increase in transport properties boost the photovoltaic efficiency of Dion-Jacobson 2D n=4 perovskite solar cells from 15.6% to 18.3%.