Cross-Sectional Studies

The electronic working principle of a solar cell device can be illustrated and analyzed along the band diagram of the structure. However, experimental deter­mination of the band diagram of a device is very difficult if not impossible. With techniques such as photoemission spectroscopy (see Chapter 15), this problem is approached from the interface point of view. KPFM can be used to follow a different approach. The measurement of the work function along the cross section of a complete device provides valuable information about the electronic properties of the different layers [30], the presence of impurity phases [79] and built-in electric fields [80].

As one example of cross-sectional KPFM, we illustrate here the investigation of the Ga-distribution in a Cu(In!_x, Gax)S2 solar cell device [81]. A study by energy dispersive X-ray (EDX) diffraction in a scanning electron microscope (SEM) showed that the absorber exhibits two distinct layers, where the Cu(Inx_x, Gax)S2 shows a significantly higher Ga-content toward the Mo back contact and a significantly higher In-content toward the CdS buffer layer. KPFM imaging was performed on the very same position of the cross section, thus allowing to compare exactly the obtained electronic information to the compositional one. The KPFM image of the CPD and a line profile across the various layers are shown in Figure 11.7a and b, respectively. Clearly a higher work function for the back part of the absorber is observed. The fairly sharp transition between the two work function regions coincides with the transition from the In – to the Ga-rich part of the absorber layer. This increase of the electric

potential indicates the presence of a built-in electric field, which is oriented in such a way that it accelerates electrons toward the In-rich region of the absorber layer and therefore keeps them away from the back contact of the solar cell. This proves the existence of a back surface field in the device, which reduces recombination at the back contact. Comparison to the quantum efficiency (QE) of the device reveals an increased QE for photons with energies near the band gap, which are absorbed deep inside the absorber. For the generated electron-hole pairs, collection is improved by the presence of this back surface field [81].

The metallurgical junction in heterojunction thin-film solar cells, defined by a change in overall chemical composition, does not necessarily coincide with the electronic junction, defined by the change in electrical properties. In the CdTe/CdS cell, Te from CdTe and Cu from the cell’s back contact may, at sufficiently high concentrations, type-convert CdS [82]. S diffusion into the CdTe may type-convert some of the p-CdTe. Both effects would lead to a buried homo – rather than a heterojunction. Visoly-Fisher et al. have shown, by a combination of SCM and KPFM cross-sectional mapping, that the cell is a heterojunction, within the experimental uncertainty (50 nm), with no evidence for CdS-type conversion [26]. Figure 11.8 shows the layer sequence in the cell: the insulating glass substrate is coated with a low-resistance (LR) SnO2:F layer 300-500 nm thick. This layer shows unstable, noisy SCM signal probably related to unwanted current flow in the sensor circuit, due to high conductivity and lack of a surface dielectric layer, resulting in erroneous SCM results. The adjacent n-type layer (dark under any dc bias between —2 and + 2 V) consists ofboth a high-resistance (HR) SnO2 layer and the CdS layer, which are electronically indistinguishable. A structure lacking the CdS layer showed a layer sequence similar to that of a conventional cell, but with a 50-80 nm thinner n-type layer, adjacent to the CdTe. A layer of CdTe grains with very weak (almost zero) SCM signal is seen adjacent to the HR-SnO2/CdS layer, 200-350 nm

thick. These are interpreted to be recrystallized and regrown CdTe grains due to cell processing [83]. The SCM signal shows clear p-type behavior further into the CdTe layer.

A KPFM profile without external bias (not shown) indicates the change in CPD between the different layers, expected from their different work functions [26]. The junction’s built-in electric field is shown by a drop in the KPFM signal across certain layers, otherwise expected to show a constant signal related to their work function. Such voltage drop is noted across the HR-SnO2 layer, indicating that this layer supports the junction’s built-in electric field/open circuit voltage. This indicates that to support the high open-circuit voltage, the n-type layer must have some minimal thickness (around 300nm in the cells studied here). The role of the HR-SnO2 that replaces part ofthe CdS is to improve the cell’s blue response, due to its larger band gap, and make good electrical contact to CdS, due to alignment of the conduction band minima. A thin CdS layer is still needed to provide a photovoltaic junction with low defect concentration and high open – circuit voltage [84]. This work demonstrates how combined SCM and KPFM of CdTe/CdS cells show the location of the internal junctions and the roles of different layers in the structure.

294 I 11 Scanning Probe Microscopy on Inorganic Thin Films for Solar Cells

11.4

Summary

Thin-film solar cells are made of complex materials; hence the prediction of device properties cannot rely on the properties of model systems and simple junction physics. Physical characterization of the polycrystalline films in use requires the understanding of their spatially resolved properties on the nanoscale. Such characterization, provided by scanning probe microscopy in its numerous variations, in combination with macroscopic analysis, can link the material properties and device performance, and allow proper optimization of its energy conversion efficiency.