Harnessing solar energy is becoming more and more important in this era of space exploration and renewable energy. As a result, various material systems are being used, new designs implemented and systems customized for use in specific environments.
Whether the illuminating source be the artificial light or the sun, it is obviously desirable to maximize responsivity over a wide range of the broad source emission range. In fact, devices respond only to a restricted range of wavelengths, limited at short wavelengths by material absorption, and at long wavelengths by the material band gap. Device spectral response relies on a large number of parameters, including device design, electrical contact, material system, etc., and is limited for single p-n junction devices by the Schockley–Queisser limit. The combination of these factors may be evaluated in considering the device energy conversion efficiency.
Spectral Response/ EQE (IPCE)
A photovoltaic (PV) device’s spectral response (A W-1) offers information on the physics at play at the device, considering not only the convertor, but also the transmittance and reflectance of the device. The external quantum efficiency (EQE)/ internal photon conversion efficiency (IPCE), described as the number of electrons supplied to the external circuit per photon incident on the device, can be achieved directly from the spectral response.
This measurement is carried out by shining a monochromatic probe beam onto the sample and documenting the photocurrent produced as a function of wavelength. Users must take care to ensure that the probe beam is not shaded by electrical connections, or that shading is considered by correcting the resulting response. The probe’s beam power is characterized first by using a detector of known responsivity (A W-1). The subsequent measurement of the photocurrent produced by the device under test as a wavelength function allows for the determination of spectral responsivity.
Measurement Under Light Bias
A light bias at a level of one sun (an irradiance of 1000 WM-2) should be used to characterize the device under realistic carrier injection levels, and in the case of multi-junction devices, to make sure the junction under test be current limiting. The discriminating challenge between the photocurrent produced by the solar simulator from that produced by the monochromatic probe can be determined by modulating the monochromatic beam, provided that the carrier transport mechanism involved in the device under test enables a response at the modulating frequency.
Reflectance and Transmittance
In an ideal world, all photons that reach a PV device are transmitted only to the active area where the conversion process takes place. Due to the use of the refractive index of the materials, light shall be reflected from the device’s front surface (to mitigate which anti-reflection coatings are used). One must also consider, in the case of thin- film devices, that light might be transmitted through the sample. The device’s total reflectance (diffuse and specular) and the total transmittance (diffuse and normal) can be measured with the help of an integrating sphere.
Internal Quantum Efficiency
The EQE result can be altered to take into account transmittance and reflectance to consider only the part of the incident light reaching the active area, yielding the internal quantum efficiency. This allows a better understanding of the material properties of the device.
Calculation of Jsc
Under standard testing conditions of AM1.5, the measured spectral response may be used to predict the projected device short circuit current density, Jsc.
I-V measurements of PV devices, employed to determine device Jsc and Isc (among other things), must be carried out under AM1.5 illumination. Often, a reference cell is employed to determine the irradiance of the solar simulator used. A mismatch factor must be computed where the spectral response of the reference cell varies from the test cell. This needs knowledge of the device being tested and the spectral response of the reference device.
This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.
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