Why Efficiency for any energy converter is useful output power divided by input power, and for a photovoltaic cell the input is the optical power falling on it and the output is electrical. So efficiency is the fraction of the incident light energy that ends up as electrical current and voltage in the load. Typical silicon cells run roughly 10 to 20 percent, meaning most of the sunlight is lost as heat or passes through unabsorbed.
Watch out Open-circuit voltage divided by short-circuit current is just a resistance in ohms, not an efficiency, and it uses two conditions where the cell delivers no power at all. Lumens per watt describes a light source's luminous efficacy, the opposite direction of energy conversion.
Efficiency is always power out over power in. For a solar cell, in is light and out is electricity.