A concentrator photovoltaic system can be built by coupling a lens array to a branched planar waveguide, which has one end with a solar cell attached and the other end divided into multiple branches. A right-angle prism is attached to each branch end, redirecting the focused sunlight into the waveguide via total internal reflection so that the light propagates inside the waveguide. With an appropriate design, the light leakage from the waveguide can be made negligible. Our ray-tracing simulations show that the ratio of the optical power exiting the waveguide to an optical power entering the lens-array is close to 87%, with the loss being mostly due to the Fresnel reflections at the lens (8%) and prism surfaces (3%). We can increase geometric concentration by cascading tapered waveguides. The number of cascades should be limited so that the light leakage from the tapered portions remains insignificant. A secondary optical element on each prism would collimate the light, easing this limitation, as well as making the system thinner. The absorption in the waveguide material imposes an inherent limit on the number of cascades and the concentration factor.