Knowledge of the absorber bandgap is often needed for assessing the junction quality of a thin film solar cell, for example when computing the open-circuit voltage deficit. The bandgap is typically estimated from the routine measurement of the external quantum efficiency (EQE) of finished devices. However the extraction becomes ambiguous in the case of very thin absorbers, or in presence of bandgap gradients or collection issues of charge carriers. This work reviews several methods for the determination of the bandgap from EQE measurements and discusses their validity conditions. The numerical results are compared based on experimental EQE of several different thin film solar cells such as CuInGaSe 2 , CuInSe 2 , CdTe, CuZnSnSe and perovskite, and systematic trends are identified. Numerical simulations are also performed to illustrate the behavior of the different bandgap extraction methods in presence of a bandgap gradient and different magnitudes of the exponential tail states.
The performance improvement of conventional CdTe solar cells is mainly limited by doping concentration and minority carrier life time. Alloying CdTe with an isovalent element changes its properties, for example its band gap and behaviour of dopants, which has a significant impact on its performance as a solar cell absorber. In this work, the structural, optical, and electronic properties of CdTe1-xSex films are examined for different Se concentrations. The band gap of this compound changes with composition with a minimum of 1.40 eV for x = 0.3. We show that with increasing x, the lattice constant of CdTe1-xSex decreases, which can influence the solubility of dopants. We find that alloying CdTe with Se changes the effect of Cu doping on the p-type conductivity in CdTe1-xSex, reducing the achievable charge carrier concentration with increasing x. Using a front surface CdTe1-xSex layer, compositional, structural and electronic grading is introduced to solar cells. The efficiency is increased, mostly due to an increase in the short-circuit current density caused by a combination of lower band gap and a better interface between the absorber and window layer, despite a loss in the open-circuit voltage caused by the lower band gap and reduced charge carrier concentration.
Recent improvements in CdTe thin film solar cells have been achieved by using CdTe1−xSex as a part of the absorber layer. This review summarizes the published literature concerning the material properties of CdTe1−xSex and its application in current thin film CdTe photovoltaics. One of the important properties of CdTe1−xSex is its band gap bowing, which facilitates a lowering of the CdTe band gap towards the optimum band gap for highest theoretical efficiency. In practice, a CdTe1−xSex gradient is introduced to the front of CdTe, which induces a band gap gradient and allows for the fabrication of solar cells with enhanced short-circuit current while maintaining a high open-circuit voltage. In some device structures, the addition of CdTe1−xSex also allows for a reduction in CdS thickness or its complete elimination, reducing parasitic absorption of low wavelength photons.
Quantum efficiency measurements of state of the art Cu(In,Ga)Se (CIGS) thin film solar cells reveal current losses in the near infrared spectral region. These losses can be ascribed to inadequate optical absorption or poor collection of photogenerated charge carriers. Insight on the limiting mechanism is crucial for the development of more efficient devices. The electron beam induced current measurement technique applied on device cross-sections promises an experimental access to depth resolved information about the charge carrier collection probability. Here, this technique is used to show that charge carrier collection in CIGS deposited by multistage co-evaporation at low temperature is efficient over the optically active region and collection losses are minor as compared to the optical ones. Implications on the favorable absorber design are discussed. Furthermore, it is observed that the measurement is strongly affected by cross-section surface recombination and an accurate determination of the collection efficiency is not possible. Therefore it is proposed and shown that the use of an Al O layer deposited onto the cleaved cross-section significantly improves the accuracy of the measurement by reducing the surface recombination. A model for the passivation mechanism is presented and the passivation concept is extended to other solar cell technologies such as CdTe and Cu (Zn,Sn)(S,Se) .
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