Highly reflective back contact

transparency of the cap annealed substrate. The lower sub-band gap absorption of the cap annealed ZnO layer is likely to be due to reduced defect absorption as discussed in the previous chapter for vacuum annealed ZnO. To check this, the PL spectrum of both ZnO layers were recorded and displayed in Fig. 10.12b. The cap annealed ZnO has a pronounced band to band PL peak with minor defect induced PL intensity in the sub-band gap region. The PL spectrum of the standard ZnO shows a strong contribution of the defect PL peaks in the sub-band gap region with only little band to band radiative recombination. Thus it is likely that the lower sub-band gap absorptivity of the cap annealed film originate from a reduced defect concentration. As discussed in the previous chapter, this is believed to originate from structural disorder possibly induced by the dopants.

By cleaving the sample with the technique described in Sec. 2.5.2 it was possible to measure the change in transmission and PL spectra due to the deposition of a CIGSe layer at 525‰. The results are shown in Fig. 10.12a and b. The cap annealed ZnO shows an increased band to band PL signal, which indicates further improved structural order due to the high temperature CIGSe deposition. The infrared absorption is slightly reduced, indicating, that the charge carrier density become reduced during the CIGSe deposition, similar as for the vacuum annealed sample in the previous section. The sub-band gap absorption and sub-sub-band gap PL intensity is slightly increased after the CIGSe deposition, which shows that the CIGSe deposition has a negative influence on the ZnO layer. However, the average light absorption between 400 nm and 1100 nm is still reduced from 6.5 % to 2.1 % which yields a photo-current increase of 1.6 mA/cm² compared to the sodium free ZnO annealed during the CIGSe deposition.

Summary

ZnO annealing in vacuum was shown to increase the electron mobility and decrease the defect absorption in ZnO layers. The increase in mobility was shown not to originate from structural changes or reduced grain boundary scattering, but from the reduced defect con-centrations in the ZnO bulk. The same effect is on observed due to the CIGSe deposition onto as-deposited ZnO layers. However, the best ZnO properties were obtained from the cap annealed ZnO, even though the CIGSe deposition slightly deteriorates these. The vacuum annealing has the advantage, that it does not require any additional processing step, as the ZnO gets annealed during the CIGSe deposition. If sodium diffused from the glass into the ZnO layer, the annealing effect was not beneficial, but the defect absorption actually increased.

(a) (b)

Figure 10.13: a) Calculated reflection at the interface between CIGSe and MoSe_x/Mo, MoO_3-x/Ag and Au. b)Simulated EQE spectra for a 500 nm thick CIGSe device with different back contacts and without recombination in the CIGSe bulk. A gain in short circuit current of 1.5 mA/cm⁻²can be achieved due to the highly reflective MoO_3-x/Ag back contact.

better charge carrier collection if high doping levels limit the SCR width within the CIGSe.

CIGSe devices in the substrate configuration utilize molybdenum as the back contact material, since it has the tendency to form a thin layer of MoSe_x during the CIGSe deposition, which is beneficial for the back contact quality [150]. The disadvantage of the Mo/MoSe stack is the very low reflectivity.

The calculated light reflectivity at the interface between CIGSe and the MoSe_x/Mo stack is shown in Fig. 10.13. The n and k values for CIGSe were taken from [181] and for the metals from [182]. The Reflectivity is between 20 and 40 %, which leads to a reduced EQE in the infrared region for thin absorber layers, as shown in Fig. 10.13b. The reduced EQE translates into a short circuit current loss of 1.5 mA/cm⁻² for a 500 nm thin CIGSe layer, compared to a device with a highly reflective contact like Au. The high material costs of Au makes it no option for industrial application though. As shown in Fig. 10.14 alternative metals to Au are only Pt and Ni, with Pt being as expensive as Au and Ni being less reflective. Other highly reflective metals, like Al, Ag and Cu are not well suitable due to their high low work function of 4.2, 4.5 and 4.7, respectively.

Copper has the further disadvantage of high diffusivity, which leads to shunted devices.

Aluminium oxidises very quickly, leading to a highly resistive contact. Leaving silver as the best option. Devices fabricated with Ag back contacts were tested, but they suffered from a strong Schottky contact which suppressed any photovoltaic activity.

Hole transport layers, like PEDOT:PSS or MoO3-x, typically used in organic solar cells were also tested and surprisingly, MoO_3-x in combination with Au lead to the best devices. In organic solar cells MoO_3-x is a standard material for hole extraction [183].

It is highly resistive, but 5-10 nm layer thickness were shown to be sufficient to achieve ohmic contacts with ITO (WF 4.7 eV) [184] [183] and also Ag (WF 4.5 eV) [185]. The

Figure 10.14: J−V curves of ZnO/CIGSe stacks with different metals as the back contact, without sodium doping. The dashed line is for a ZnO/CIGSe/Au device doped with sodium.

calculated reflectivity for the MoO_3-x/Ag stack is also shown in Fig. 10.13a.

So far, only the MoO_3-x/ITO stack was tested for semi-transparent CIGSe devices [184].

But the MoO_3-x reacted with CIGSe to GaO_x during the deposition of CIGSe on top of it. The MoO_3-x/Ag stack has not yet been used for CIGSe solar cells. Here it is tested for application as a highly reflective back contact in CIGSe superstrate devices. The MoO_3-x was thermally evaporated from a MoO₃ powder source onto two CIGSe coated ZnO substrates at room temperature. One CIGSe layer was NaF post treated directly after the CIGSe deposition. Reference cells were fabricated with Au back contacts, directly after the CIGSe deposition, whereas, prior to the MoO_3-x/Ag deposition, both CIGSe layers were stored for 3 months in vacuum with several short vacuum breaks in between.

No further treatments were performed to remove potential oxidation of the surface on the CIGSe layer.

The J−V curves of devices with Au and MoO3-x/Ag as the back contact are shown in Fig. 10.15a. The devices are slightly Cu-poor at the back contact interface, which leads to a high ohmic contact above 4 Wcm² for the devices fabricates without sodium. The series resistance is slightly higher for the device with MoO3-x/Ag, probably induced by the high resistive MoO_3-x layer [186]. To increase the back contact quality it was shown in Sec. 6.1, that it is necessary to dope the CIGSe surface with sodium. However, it was not possible to perform the NaF PDT after the MoO3-x deposition, as it was done with the 10 nm Mo layer described in Sec. 5.2.4, since the sodium did not diffuse through the MoO_3-x layer, even at 300‰. Fig. 10.15b shows the J −V curve of the sample with the NaF PDT performed prior to the MoO3-x deposition. Again, the MoO3-x layer increases the series resistance by 0.5 Wcm² compared to the Au reference device. The MoO_3-x/Ag contact does not reduce the shunt resistance or the short circuit current. The variations in the FF and the VOC are relatively small and likely to originate from typical sample quality fluctuations and not from the MoO_3-x/Ag contact.

(a) (b)

Figure 10.15: Experimental J −V curves of superstrate CIGSe solar cells with Au and MoO_3-x/Ag as the back contact,a) without any sodium presentb)with NaF PDT at 300‰ for both samples.