CuBIIICV^CVI = Se, S) compound semiconductors are part of the chalcopyrite (ch) family and are located in the middle of the ternary system Cu-BIII-CVI, on the pseudo-binary section Cu2CVI—B3IIC2V (see Fig. 5.5). The band gap (Eg) ranges from 1.0 to 1.5 eV for a single junction thin-film solar cell. The CuBIIICVI compounds crystallize in the chalcopyrite-type crystal structure, named after the mineral CuFeS2. This tetragonal crystal structure (space group /42d) consists of two specific cation-

sites. The monovalent cations are sited on the 4a (0 0 0) and the trivalent cations (In3 +, Ga3+) on 4b (0 0 lA) position. All cations are tetrahedrally coordinated by the anions (8d (x %%)) and vice versa.

A closer examination of the pseudo-binary tie-line reveals a stability of the ch — phase over a defined compositional-range. That means, the ch-phase accepts a deviation from ideal stoichiometry (CuBIIICVI) by maintaining the crystal structure, and without the formation of any secondary phase. The compound Cu1-yInySe05+y is single phase in the region of 0.513 < y < 0.543 and contains within this area only the chalcopyrite-type phase [15]. The common highly-efficient Cu(In, Ga)Se2 thin — film solar devices all exhibit an overall off-stoichiometric composition, due to the multi-stage process applied to grow these absorbers. Such deviations from stoi­chiometry always cause structural inhomogeneities and charge mismatches, which influence the material properties. One effect is the generation of point defects, which influences the electronic and optical properties of the compound semicon­ductor. In general 12 intrinsic point-defects can exist within the ch-type crystal structure.

• 3 vacancies: on the two cation and one anion sites (VCu, V™, V?)

• 6 anti-site defects: B&, CuB, , CuVI, B” VI, CVI III

• 3 interstitial defects: Cuj, BfI, CVI

These intrinsic point-defects cause different defect levels in the energy gap of the semiconductor (see Table 5.1) and therefore influence the electronic and optical properties, sensitively. Consequently, it is of great importance to know where the atoms are.

In addition to the generation of point defects, the anion position (x(CVI)) of chalcopyrite crystallites is also affected by off stoichiometry. A change of the anion position is proposed to be directly correlated with a change in Eg. The x-parameter controls the position of the valence-band maximum and conduction-band

minimum, and therefore Eg. Current studies have shown that Eg decreases for the Cu-poor composition in CuInSe2 caused by a change in x(Se), which is weakly dependent on the concentration of copper vacancies (VCu) [17]. The interplay between the crystal structure and the optical and electronic properties is a funda­mental problem, which has to be understood when tailoring high efficiency thin — film devices with a compound semiconductor as absorber layer. For instance, an uncontrolled change in Eg within an absorber layer is undesirable because it is less optimal for absorption of the incoming sunlight.

It is difficult to identify and quantify very small changes in the crystal structure, such as point defects or changes in atomic positions by imaging techniques. Therefore, it is preferred to study such effects by diffraction methods. The method of Rietveld refinement is applied to refine the crystal structure using an X-ray or neutron powder diffraction pattern of the off-stoichiometric compound in detail. This method provides information about the cation distribution and the position of the atoms within the structure with high accuracy. In the following section we discuss the reasons for the preferred use of neutrons in the description of structural changes in detail, and how point defects in compound semiconductors can be identified.