3.1.

Precursor Characterization

SEM cross-sections and 3D microscope measurements of precursor layer surfaces are shown in Fig. 1. We observe a phase-segregated morphology consisting of a closed, compact underlayer and a rough, irregular top layer. For increasing deposition time, the closed underlayer increases linearly, while it decreases for increasing sputter gas pressure. As expected, the overall film thickness increases linearly with sputter time and decreases slightly with sputter pressure (determined via XRF, data not shown).

Fig. 1

SEM cross sectional images and 3D microscope measurements of the layer surfaces of (a) precursors sputtered at 100, 1000, and 2000 s and at a constant gas pressure of 2.5 μbar and (b) precursors sputtered at 1, 2.5, and 15 μbar and at a constant sputter time of 1000 s.

Purwins¹⁴ described the existence of a two-phase regime in the Cu-In-Ga phase diagram, consisting of elemental indium together with the copper-rich Cu₁₆(In_1-y,Ga_y)₉ and/or Cu₉(In,Ga)₄ phases. Indeed, our XRD measurements of a precursor indicate the existence of elemental indium as well as Cu₁₆(In_1-y,Ga_y)₉ and/or Cu₉(In,Ga)₄ within the layer (see Fig. 2). It was not possible to distinguish between the two phases within the XRD pattern. Furthermore, it is not clear if this phase combination evolves during the sputtering or if it is already present in the target.

Fig. 2

XRD data for a precursor layer and selenized layers of various selenization durations. The diffractogram of the precursor indicates the existence of elemental indium as well as Cu₁₆(In_1-y,Ga_y)₉ and/or Cu₉(In,Ga)₄ within the layer. For selenized layers, all undesired crystalline phases have vanished after 25 min and only CIGS can be found.

The atomic composition determined by EDX of the underlayer and the top layer is shown in Table 1. The atomic ratio CGI = Cu/(Ga+In) of ca. 1.8 for the underlayer fits well to the copper-rich Cu₁₆(In_1-y,Ga_y)₉ phase. The top layer contains much more In than the underlayer and we hence expect In to be present as a mixed alloy along with the Cu-In-Ga phase(s).

Table 1

The atomic composition of the underlayer and the top layer of a sputtered precursor, determined by EDX measurement, as well as the resulting atomic ratio CGI = Cu/(Ga+In).

The sputtered film generally contains less copper and less gallium than the original target. The graph in Fig. 3 shows the average atomic concentration of precursors which were sputtered at pressures from 1 to 15 μbar. We observe selective sputtering with a decreasing copper and gallium content as well as an increasing indium content for increasing sputter gas pressure. This phenomenon is probably related to differences in the sputter yield for elements with different atomic masses as is the case in an alloy target containing the light Cu and heavy In. It correlates with the decreasing thickness of the copper-rich underlayer with increasing pressure, as shown in the SEM cross-sections in Fig. 1.

Fig. 3

Atomic concentration of precursors sputtered at pressures from 1 to 15 μbar, determined by XRF. An increasing copper and gallium content as well as a decreasing indium content for decreasing sputter gas pressure is observed.

3.2.

Selenization and Solar Cells

XRD measurements of the CIGS layers are carried out for various selenization durations, as shown in Fig. 2. Already after 7 min selenization we find the CIGS phase beginning to evolve. For short selenization durations Cu(In,Ga)₃Se₅ and copper indium alloys can be seen.

Critical metallic phases are not detectable anymore in the diffraction pattern after 25 min of selenization, although shunting is still sometimes an issue. The shunting behavior could be related to incomplete CIGS transformation with residual (amorphous) metallic phases. Thus, although CIGS conversion is shown by XRD to be complete after about 25 min, a selenization time of 60 min is favorable in order to obtain maximum cell efficiencies.

Alberts et.al investigated the influence of elemental Se or H₂Se, respectively, and found that Se-vapor-treated samples had nonuniform surface morphologies correlated with a relatively large variation in the Cu/In atomic ratio and Se concentration through the depth of the samples.⁶ As shown in the SNMS depth profile [Fig. 4] our samples are more homogeneous. We find a relatively constant CGI [Cu/(Ga+In) ratio] as well as a constant Se/(Cu+In+Ga) ratio throughout the depth of the sample. We only see an increase of Se content at the molybdenum back contact, which is heightened with selenization time [Fig. 4]. We attribute this feature to the formation of molybdenum selenide. Furthermore, we observe a beneficial gallium gradient in the depth profile of the absorber layer.

Fig. 4

SNMS depth profile of a selenized CIGS layer (a) on molybdenum-coated glass. The intensities were multiplied with sensitivity factors obtained by XRF. The ratios SGI = Se/(Cu+Ga+In), CGI = Cu/(Ga+In), and GGI = G/(Ga+In) are multiplied by a factor of 1000 for clarity. (b) shows the selenium portion that increases at the back contact with selenization time.

A layer selenized for 60 min is shown in Fig. 5, indicating a polycrystalline structure. It has an atomic composition of Cu: 22.1 ± 0.5 at.%, In: 20.0 ± 0.9 at.%, Ga: 5.4±0.3 at.%, and Se: 52.6 ± 0.6 at.%, in agreement with the desired ratio for Cu(In,Ga)Se₂.

Fig. 5

(a) SEM image of a selenized layer completed to a solar cell. (b) Current-voltage characteristics and external quantum efficiency (inset) for the best cell. Eta = 13.7%; V_oc = 568 mV, FF = 72.4%; j_sc = 33.3 mA/cm², A = 0.24 cm².

With these devices we can attain very good current-voltage characteristics and quantum efficiencies. Table 2 shows current-voltage characteristics of different cells that have been selenized in different runs and finished without antireflective coating or metallic grids. The accordant curves for the best cell with a conversion efficiency of 13.7% are shown in Fig. 5.

Table 2

Current-voltage characteristics of different cells (A = 0.24 cm2), without antireflective coating or metallic grids.