Quantum efficiency

The quantum efficiency of a solar cell is a very important measure for solar cells as it gives information on the current that a given cell will produce when illuminated by a particular wavelength. If the quantum efficiency is integrated (summed) over the whole solar electromagnetic spectrum, one can evaluate the current that a cell will produce when exposed to the solar spectrum. The ratio between this current and the highest possible current (if the QE was 100% over the whole spectrum) gives the electrical efficiency of the solar cell. With solar cells, one often measures the external quantum efficiency (EQE, sometimes also simply referred to as QE), which is the current obtained outside the device per incoming photon.

:$\backslash text\{Efficiency\}\; =\; \backslash frac\{\backslash text\{output\}\}\{\backslash text\{input\}\}$ :$\backslash text\{EQE\}\; =\; \backslash frac\{\backslash text\{electrons/sec\}\}\{\backslash text\{photons/sec\}\}=\; \backslash frac\{\backslash text\{current\}/\backslash text\{(charge\; of\; 1\; electron)\}\}\{(\backslash text\{total\; power\; of\; photons\})/(\backslash text\{energy\; of\; one\; photon\})\}$

The external quantum efficiency therefore depends on both the absorption of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A "good" material avoids charge recombination and therefore a drop in the external quantum efficiency. EQE should not be confused with internal quantum efficiency which is the ratio of current to absorbed photons.

Spectral responsivity

To convert from responsivity (R_λ, in A/W) to QE_λ (on a scale 0 to 1):

:$QE\_\backslash lambda=\backslash frac\{R\_\backslash lambda\}\{\backslash lambda\}\backslash times\backslash frac\{h\; c\}\{e\}\backslash approx\backslash frac\{R\_\backslash lambda\}\{\backslash lambda\}\; \{\backslash times\}\; (1240\backslash ;\{\backslash rm\; W\}\backslash cdot\; \{\backslash rm\; nm/A\})$

where λ is in nm, h is the Planck constant, c is the speed of light in a vacuum, and e is the elementary charge.

Determination

where $N\_e$ = number of electrons produced, $N\_\backslash nu$ = number of photons absorbed.

:$N\_\backslash nu/t\; =\; \backslash Phi\_o\; \backslash frac\{\backslash lambda\}\{hc\}$

Assuming each photon that is absorbed in the depletion layer produces a viable electron-hole pair, and all other photons do not,

:$N\_e/t\; =\; \backslash Phi\_\{\backslash xi\}\backslash frac\{\backslash lambda\}\{hc\}$

where t is the measurement time (in seconds) $\backslash Phi\_o$ = incident optical power in watts, $\backslash Phi\_\{\backslash xi\}$ = optical power absorbed in depletion layer, also in watts.

See also

References

Category:Quantum electronics Category:Electronics terms Category:Spectroscopy Category:Engineering ratios Category:Physical quantities