Solar cells

I-V characteristics

For the measurement of current-voltage characteristics, there are two measuring systems with constant light sources available at ISFH. Under standard testing conditions of 25 °C and an illumination intensity of 0.1 W/cm² (1 sun), the central solar cell parameters open-circuit voltage V_oc, short-circuit current density J_sc, fill factor FF, efficiency η as well as the current density J_MPP and voltage V_MMP measured at the point of maximum power output are determined. For a more comprehensive analysis of solar cells on the basis of the two-diode-model, in addition J_sc-V_oc-characteristics can be recorded.

Spectral response

During measurements of the quantum efficiency it is determined how many electrons per irradiated photon contribute to the short-circuit current of the solar cell. This is done by measurering the short-circuit current of the solar cell is measured under monochromatic illu-mination at a wavelength range of 300 nm to 1200 nm with simultaneous illumination with white light with an intensity between zero and one sun. In combination with a reflection measurement, the internal quantum efficiency (IQE) can be determined A detailed analysis of the IQE data provides information about emitter quality, diffusion length and surface recom-bination speed as well as information about the optical properties of the cell.

Electro- and photoluminescence (local series resistance)

Camera-based electroluminescence measurements open the possibility of characterizing a solar cell within a few seconds with a high local resolution. Electroluminescence is the transmission of light resulting from application of a forward bias voltage to a solar cell. The electrons injected into the solar cell recombine with the existing holes while the energy re-leased by this process is given off to a small extent in the form of a photon. The image taken with a CCD camera depicts the intensity distribution of luminiscence radiation. Generally all effects which lead to a local reduction of the charge carrier concentration are visible on the electroluminescence image. The causes of a reduced charge carrier concentration are mani-fold, but can be easily differentiated in most cases. Local resolution is limited by the number of pixels of the detector and comes to 150 µm with a solar cell with an edge length of 15 cm.

Shunthunting

In order to determine the local energy dissipation loss in solar cells with a high local resolu-tion, ISFH uses an infra red camera. In the process known as lock-in thermography, meas-urements in the dark (DLIT) and with illumination (ILIT) are differentiated. In the case of DLIT, charge carriers are injected into the base of the solar cell. Locations with high local recombination display a local rise in temperature. This rise in temperature can be measured by means of the lock-in technique with a resolution of up to 25 µK. In solar cell characteriza-tion, this process is used to identify shunts. In the case of ILIT, surplus charge carriers are optically created with the help of a light-emitting diode array. If a voltage is applied to the so-lar cell at the same time, a precise determination of the distribution of power losses under operational conditions is possible. With an appropriate lock-in frequency, local resolution is limited by the number of pixels of the detector and amounts to 625 µm with a solar cell with an edge length of 15 cm.

Spectrally resolved LBIC

Spectrally Resolved Light Beam Induced Current (SR-LBIC) permits the measurement of the local distribution of the short-circuit current of a solar cell and thus the determination of mate-rial and process related lateral inhomogenities. At ISFH a commercial system by Semilab with 4 lasers in the spectral range of 650 nm to 985 nm is available. A simultaneous measurement of the reflection permits the determination of the local internal quantum yield and thus the local effective diffusion length L_eff.

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