This work investigates the optical performance of a textured back surface reflector (BSR) produced by a maskless solution-based wet chemical etch in Al $_\text{0.3}$ Ga $_\text{0.7}$ As for a 1.1- $\mu$ m-thick GaAs solar cell. Combining the maskless texture with the 94% reflective flat mirror resulted in a high haze in reflectance near 80% across the GaAs absorbing region. The increased diffuse reflectance from the maskless BSR provided a 19.7% increase in the integrated short-circuit current density from the base region of the 1.1- $\mu$ m GaAs solar cell with the maskless BSR compared to the simulated 1.1- $\mu$ m solar cell with no light trapping structures. Based on the principle concepts for the Fabry–Perot etalon, the lifetime enhancement factor (LEF), defined as the extended photon lifetime in an optical cavity due to light trapping, was derived for the BSR devices. The LEF for the maskless BSR device was measured to be 4.3 times greater than the single-pass photon lifetime, agreeing with the high haze in reflectance and improved photoabsorption near the GaAs band edge. This value agrees with the F factor in the analytical propagation model that fits the experimental external quantum efficiency curves and justifies using the LEF to quantitatively represent the optical performance of light trapping structures in sub- $\mu$ m thick solar cells.