A polymer light-emitting electrochemical cell (LEC) is a solid-state polymer device operating on in situ electrochemical doping and the formation of a light-emitting polymer p-n junction. Electrochemical doping of the luminescence polymer quenches the photoluminescence. The chemical potential difference between the p- and n-doped regions creates a built-in potential/field in the junction region, which can be probed by measuring the optical beam induced current (OBIC). In this study, the OBIC and photoluminescence profiles of the LEC have been simultaneously measured by scanning a focused light beam across a large planar LEC that has been turned on and cooled to freeze the doping profile. The photoluminescence intensity undergoes a sharp transition between the p- and n-doped regions. The OBIC photocurrent is only observed in the transition region that is narrower than the width of the excitation beam, which is about 35 μm. The results depict a static planar polymer p-n junction with a built-in field pointing from n to p. The electrode interface and the neutral regions do not produce a measurable photocurrent.