ABSTRACT
There is little hope of finding a breakthrough in today’s photovoltaic (PV) conversion efficiency of inorganic materials if one stays with the well-understood macroscopic bulk approach. Such solar cell operation is based on the onestage fundamental process of optical electron-hole generation. In Si-based solar cells, which dominate PV applications, two energy-and volume-dependent maladjustments degrade this process: • the first concerns the incident sunlight (containing multi-energy photons) and
electron-hole extraction (at a fixed energy)—this effect appears in energy space; and
• the second consists of the spatial distribution of the photon absorption inside the conversion volume but a fixed extraction site (the metal-semiconductor contact)—this effect appears in geometrical space. Because of these two effects, there are losses in the carrier thermalization
and collection. Potential ways to enhance the conversion efficiency consist of • reducing the PV conversion volume (geometrical space) by requiring large
absorbance; and/or • adapting the conversion events, for example, by a multi-level multi-stage
process (energy space). In a conventional solar cell, there are two metastable light-generated carrier
populations that are collected at the average carrier energy. The quasi-Fermi levels
refer to the metastable carrier populations if their concentrations have a FermiDirac distribution.
