**1. Introduction**

For a long time, solar cells based on CuIn1-xGaxSe2 (Cu(In,Ga)Se2, CIGS), similar to CdTe-based devices, keep a stable position in thin-film photovoltaics as an alternative to solar modules based on mono- and poly-silicon wafers. The I-III-VI CIGS alloy is a solid solution of copper indium diselenide (CIS) and copper gallium diselenide (CGS). This alloy is tolerant in terms of material stability and miscibility in a wide range of *x*, when the semiconductor bandgap varies continuously from about 1.0 eV for CIS to about 1.65 eV for CGS. Irrespective of the material composition, CIGS is a direct-bandgap semiconductor and strongly absorbs sunlight. Because of its high optical absorption coefficient, a much thinner film is required compared to many other semiconductor materials (1–2 μm is enough to absorb most of the sunlight).

To date, mass production of cost-effective thin-film polycrystalline and monolithically interconnected CIGS-based modules has been achieved by many companies worldwide. Since 2007, one Japanese company Solar Frontier, alone for example, delivered over 1 GW CIGS modules. The CIGS devices demonstrate excellent long-term stability; can be fabricated on lightweight and flexible substrates that are desirable for portable, building-integrated photo‐ voltaics and many other applications when solar panels are used to replace conventional building materials in parts of the roof or facades. It should also be noted that the CIGS modules, in addition to their long-term stability, have shown higher resistance to ionizing radiation compared to crystalline Si and III-V solar cells, i.e., such devices are promising for space application [1].

The energy conversion efficiency of CIGS modules is in the range of 12-15%, but for small area laboratory cells, the efficiency milestone of > 20% was achieved in 2010 [2]. In 2014, Solar Frontier has achieved 20.8% energy conversion efficiency for small area CIGS cells and Zentrum für Sonnenenergie-und Wasserstoff-Forschung Baden-Württemberg (ZSW) shortly

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improved the cell efficiency to 20.9% [3,4]. Since the efficiency of CIGS modules lie in the 12-15% range (except for a 16.6 % world record [5]), which is about half of the theoretical limit 28-30%, there are plenty of opportunities to contribute towards the scientific and technological advancement of CIGS PV technology.

The device architecture of CIGS and CdTe solar cells are alike, similar to other p-n heterojuc‐ tions. In both cases, the CdS/absorber heterostructure is the key element in determining the electrical and photoelecnric characteristics of the device. In CIGS solar cells, the heterojunction is formed between the p-CIGS and n-CdS. The conductivity of CIGS is determined to a large degree by intrinsic defects, while the n-CdS is doped to a much larger extent by donors. This asymmetric doping causes the space charge region (SCR) to extend much further into the CIGS. As in CdTe solar cell, a thin layer of CdS serves as "window", through which radiation penetrates into the absorber.

The difference between these devices lays in their popular superstrate (CdTe) and substrate (CIGS) configurations. In superstrate configuration devices, the sunlight enters the absorber through the glass substrate and transparent conductive oxide layer (TCO, usually SnO2:F) while only through the TCO layer (usually ZnO:Al) in substrate configuration. These design features are not of fundamental importance from the point of view of the physical processes taking place, but demand different device fabrication technologies. The physical models used for the interpretation of the CdTe solar cell characteristics has been successfully applied with some modifications to the CIGS devices [6,7].

Based on the above reasoning, in this chapter a detailed analysis of the optical and recombi‐ nation losses in CIGS devices are presented, which are important causes of poor quantum efficiency (QE), leading to low solar-to-electric energy conversion efficiency in solar cells. A quantitative determination of the losses is presented and some possible pathways to reduce them are identified. Calculations of the optical losses are carried out using the optical constants, refractive indices and extinction coefficients, of the materials used. Equally important are the recombination losses, which are determined using the continuity equation considering the drift and diffusion components of the photocurrent and all possible recombination losses. In order to discuss the influence of the electrical parameters of the heterojunction on the photo‐ electric conversion efficiency of the device, an analysis of the current-voltage characteristics recorded in dark and under illumination is also included in this chapter. It seems that the analysis of the physical process involved in the photoelectric conversion is useful from a practical point of view since undoubted successes in the development of efficient CIGS solar cells have been achieved mainly empirically [8].
