This guide provides an overview of the spectral characterisation techniques used in the evaluation of their efficacy in the goal of harnessing a maximum of energy from the sun and artificial sources of light.
Along with an I-V measurement, the following quantities provide the most information of material and PV device function:
The spectral response (A W-1) of a PV device provides information on the physics at play in the global device, taking into account not only the material, but also the reflectance and transmittance of the device.
This measurement is performed by shining a monochromatic probe beam onto the sample and registering the photocurrent generated as a function of wavelength.
Care should be taken to ensure the probe beam is not shaded by electrical connections, or that the shading is taken into account by correcting the resulting response.
The probe is first characterised, using a detector of known responsivity (A.W-1) to determine the power in the beam. Subsequent measurement of the photocurrent generated by the device under test as a function of wavelength allows for the determination of spectral responsivity.
External Quantum Efficiency (IPCE)
The external quantum efficiency (EQE) is defined as the number of electrons provided to the external circuit per photon incident on the device, and is directly obtained from the spectral response measurement by the following argument.
The number, n, of electrons generated by the device, n=(It/e), where I is the generated current, t time and e the charge of the electron.
The number, m, of photons, incident on the sample, m= Pt/ Ev>, where P is the power in the beam, t time, and Ev the photon energy. The quantum efficiency, η, is defined as,
η = 100.n/m = 100.(It/e)/(Pt/Ev) = 100.(I/P).(Ev/e)
=> η= 100.S.(hc/e).(1/λ)≈1239.84.S/ λ (%)
Where S is the spectral responsivity in A.W-1 and λ the wavelength in nm. EQE can therefore be determined directly from a measurement of the spectral response.