**6. Conclusions**

The optical losses in Cu(In,Ga)Se2 solar cells caused by reflections from the interfaces and absorption in the ZnO and CdS layers have been calculated using the optical constants of the materials. When calculating the integral photoelectric characteristics of a solar cell, ignoring the multiple reflections and interference effects in the ZnO and CdS layers cannot cause remarkable errors. (i) The losses due to reflection (reducing the short-circuit current density) from the front surface of ZnO with an antireflection coating in CuInSe2, CuIn0.69Ga0.31Se2 and CuIn0.34Ga0.66Se2 solar cells are equal to 2.5, 1.9 and 1.4%, respectively (antireflection coating increases the photocurrent by 7.1%, 7.4% and 8.0%). The reflection losses at the ZnO/CdS interface are equal to 1.0, 0.9 and 0.9% for these cells, respectively, whereas for the CdS/CIGS interface the losses are 1.3, 1.2 and 1.1%, respectively. The total reflection losses for typical parameters of these solar cells are equal to 4.7, 4.0 and 3.5%, respectively (excluding shading by grid). (ii) The losses caused by absorption in the ZnO and CdS layers amount to 8.1, 7.8 and 9.6% for these solar cells, respectively. The losses due to insufficient absorptivity of the CIGS absorber are 0.6, 0.2 and 0.4%, respectively. The *total optical losses* with the antireflection coating amount to 17.4, 16.0 and 17.5% for solar cells with absorber bandgaps 1.04, 1.14 and 1.36 eV, respectively.

The recombination losses in the studied solar cells have been determined by comparing the measured quantum efficiency spectra with the calculation results. This approach allowed determining the real main parameters of the devices such as: lifetimes of charge carriers, concentration of uncompensated acceptors in the absorber, recombination velocity at the front and back surfaces, the thickness of the CdS film. (i) Recombination at the front surface of the absorber reduces the short-circuit current density by 2.2, 1.9 and 0.2% in solar cells with the CIGS bandgap 1.04, 1.14 and 1.36 eV, respectively. (ii) Recombination in the space-charge region causes a reduction of the short-circuit current density by 0.7, 0.1% and 1.0% for these devices, respectively. (iii) The losses in the neutral part of the absorber and the back surface amount to 2.5, 5.0 and 2.9%, respectively. (iv) Recombination only at the back surface of solar cells cause a decrease in the short-circuit current by no more than 0.2, 1.0 and 0.1%, respectively. *Total recombination losses* in the studied CIGS solar cells with bandgap of absorber 1.04, 1.14 and 1.36 eV are equal to 5.4, 7.0 and 4.1%, respectively. The recombination losses, which degrade the charge collection efficiency of solar cells, can be lowered by increasing the mobility and lifetime of electrons and holes within the literature data. Even greater improvement of the charge collection efficiency can be achieved by extending the SCR. However, positive impact of these measures is limited by the fact that the diffusion length of electrons and the SCR width can become larger than the absorber thickness, which for 100% charge collection efficiency should be increased.

Knowing the short-circuit current density and the dark *J–V* characteristic of CuIn1-xGaxSe2 solar cell, the *J–V* curve under illumination can be accurately calculated along with the fill factor, open-circuit voltage and energy conversion efficiency. A comparison of the calculat‐ ed and measured *J–V* curve under illumination gives the quantitative information on the electrical losses in solar cells indicated a significant impact of shunting, which increases with increasing the absorber bandgap. If to take into account the effect of shunts, the *J–V* curves of all samples are described by the theory of generation-recombination of charge carriers in the space-charge region. For a cell with the absorber bandgap 1.14 eV, shunt‐ ing leads to decreasing the fill factor from 0.73 to 0.70 and the efficiency from 13.3 to 12.7%, whereas for cell with the absorber bandgap 1.36 eV the efficiency loss amounts to 1.3%. In CuInSe2 solar cell there are no electric losses, the fill factor and efficiency are equal to 0.67 and 11.3%, respectively. Fabrication technology of CIGS solar cell at elevated temperature (600 – 650°C) allows eliminating the effect of shunting in solar cells with the absorber bandgaps up to 1.4–1.5 eV, but this is accompanied by shortening the carrier lifetime and results in lowering the open-circuit voltage.
