**3.3. Absorptive capacity of the CIGS absorber layer**

In the previous sections while discussing the photocurrent, we assumed 100% light-to-electric conversion in the CIGS layer. However, there are losses related to the incomplete absorption of light in this layer. Considering this fact, while estimating the exact values of *integrated* absorptive capacity of the CIGS layer the spectral distribution of solar radiation and absorption coefficient of the material must be taken into account rather than determining the absorptive capacity for one wavelength in the range *hν* > *E*g.

It should be noted that in the photon energy range from *E*<sup>g</sup> to 4.1 eV electron-hole pairs arise independently of the energy absorbed. In other words, the energy needed for the band to band transition is less than the absorbed energy. So while considering the dependence of the absorptivity of solar radiation power *A*<sup>Φ</sup> (rather than photon flux *A*hν) on the thickness of the semiconductor this fact must be considered. For this reason, the number of electron-hole pairs, and hence the photocurrent generated in the solar cell, is not proportional to the power of solar radiation. Hence, the dependences of *A*hν and *A*Φ on the thickness of the semiconductor are different and for calculation of the photocurrent, the absorptivity of photon flux *A*hν should be used.

The *integrated* absorptive capacity of the semiconductor layer depends on two factors: (i) the spectral distribution of solar radiation, and (ii) the optical transmission of the ZnO and CdS layers that affect the spectral distribution of solar radiation reaching the absorber. With this consideration the integrated absorptivity can be taken as the ratio of the number of photons absorbed in the CIGS layer to the number of photons penetrated through the ZnO and CdS layers [18]:

$$A\_{\rm hv}(d) = \frac{\sum\_{i} T(\lambda) \frac{\Phi\_{i}}{h\nu\_{i}} [1 - \exp(-\alpha\_{i} d)] \,\Delta\lambda\_{i}}{\sum\_{i} T(\lambda) \frac{\Phi\_{i}}{h\nu\_{i}} \Delta\lambda\_{i}}.\tag{18}$$

Here summation is for the spectral range starting from *λ*=300 nm and the upper limit depends on the *E*g of the material, which corresponds to *λ*=1200, 1080 or 900 nm respectively for CuInSe2, CuIn0.69Ga0.31Se2 and CuIn0.34Ga0.66Se2 solar cells.

According to Eq. (18) for CuInSe2, CuIn0.69Ga0.31Se2 and CuIn0.34Ga0.66Se2 solar cells, 99.4, 99.6 and 99.8% photon absorption happens for a layer thickness of 2 μm. But with a layer thickness of 1 μm the photon absorptivity values for these solar cells are equal to 97.0, 98.3 and 97.7% respectively, i.e., optical losses become noticeable. On the other hand, in another direct-gap semiconductor CdTe the photon absorptivity of 99.4–99.8% takes place at a thickness of about 30 μm [18].

It should also be borne in mind that CIGS solar cell is a substrate configuration device having metallic substrate as one of the electrodes. This implies that long wavelength radiation with low absorption coefficient can reflect back from the rear surface. In the event of 100% reflec‐ tance from the back surface, the absorptivity is the same as for the double thickness of the absorber layer, i.e., the *A*hν(*d*) value in such a solar cell can be found by doubling the thickness *d* in Eq. (18). This means that 99.4–99.8% photon absorption in CIGS takes place at a thickness of 1 μm rather than 2 μm.
