Types of efficiency
There are three types of efficiency: power conversion (PCE), external quantum (EQE), and internal quantum (IQE). PCE measures output to input power, EQE measures how many electrons are harnessed for how many photons hit the cell, and IQE is similar to EQE except that it takes optical losses into account.
Power conversion efficiency (PCE)
This is the number everyone cares about: how much energy comes out of a solar cell? PCE is the measure of how much electric power is generated when the Sun is shining on the solar cell. PCE is given by:
Where FF is the fill factor, J_sc is the short circuit current density, V_oc is the open circuit voltage, and P_in is the input power flux. The short circuit current density (or flux) is the measure of how much current there is per square unit of area. So a J of 1 A/m^2 translates to 1 coulomb of charge flowing per second in one square meter. For a circuit, the current (or current density) is a maximum when it is shorted. This essentially means that it is wired to itself.
V_oc is the open circuit voltage and is measured in volts (joule-meters). It is a maximum when the circuit is open, i.e., not connected to anything. This measures the potential gradient for carriers to move. A gradient drives the charges to move, so the higher the voltage, the more forcefully the carriers are pushed to move.
The P_in in the denominator should not be confused with power. It is actually power flux, as can be seen by unit analysis. The power flux is measured as watts per square meter. The power flux that goes into the cell is the irradiance of the Sun. When the Sun shines on the cell with an irradiance of 700 W/m^2, the power flux coming into the cell is 700 W/m^2. Spectral irradiance and power flux are seen here as the same thing.
J_sc and V_oc are very easy to measure. One would decide which spectral irradiance to emulate for P_in. FF has been calculated for various solar materials. By measuring the J_sc, V_oc, and applying a P_in, the PCE can be calculated. If desired, the PCE could be modeled as a function of irradiance.
External quantum efficiency (EQE)
This measures how many electrons are put out per photon hitting the solar cell. It does not take reflection and transmittance into account. It may not sound practical, but it is used to measure internal quantum efficiency. EQE is easy to measure: shine light on a solar cell and measure its current output. EQE is a function of either wavelength or energy. Different wavelengths will absorb, transmit, or reflect more easily than others, while different energies will either produce or not produce electrons. The wavelengths focus on the optical properties while the energies focus on the electronic properties of the semiconductor. In short, EQE tells us how many photons make it into the cell.
Internal quantum efficiency (IQE)
This takes into account optical losses. It takes the EQE and divides it by 1-R to take reflectance into account. The smaller the reflectance, the more photons are absorbed and thus the more carriers it can produce. As R decreases, IQE increases. In short, IQE tells us what the cell does with the photons that get absorbed.