Eli Yablonovitch and coworkers have shown in an important paper that high external luminescence accompanies high solar cell efficiency, as it indicates low optical losses in the device.
E. Yablonovitch, O. Miller, and S. Kurtz, “Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit,” IEEE Journal of Photovoltaics, vol. 2, no. 3, pp. 303-311, July 2012
Here is prof. Yablonovitch explaining the issue at HOPV14 in Lausanne