Influence of alkalis

Alkali treatments are usually performed on CIGSe absorbers to enhance thep-type doping density. Sodium and Potassium are the most common choices of the alkali metals. Often they are supplied via out-diffusion from alkali containing glass substrates through the Mo layer. Since, in contrary to the Mo back contacts, ZnO layers are good diffusion barriers for alkalis, the alkali dopants have to be supplied externally prior to the absorber

deposition by a precursor or after the CIGSe deposition by a post deposition treatment (PDT). In order to suppress the formation of NaOH and KOH, the fluoride compounds of the alkali metals, NaF and KF, are used. No negative influence of the fluorine atoms on the CIGSe absorber has been shown so far [133].

Sodium Fluoride Treatment (NaF) Fig. 5.21a shows theJ−V curves of superstrate devices with different NaF treatments. During the post deposition treatments, 10 nm of NaF were deposited on the CIGSe surface at a given temperature and annealed for 10 minutes at the same temperature. For the NaF precursor, 10 nm of NaF were deposited onto the ZnO prior to the CIGSe deposition. The absorber was deposited via a modified three-stage process starting with the co-evaporation of Ga-Se, as described in Sec. 1.3.1.

The NaF PDT at 100‰leads to aJ−V curve that is similar to the J−V curve of an untreated device (see Fig. 5.16) but with a decreased series resistance (details in Sec. 6.1).

The J_SC is 32 mA/cm², theV_OC 620 mV and the FF 28% leading to an overall η of 5.5%.

If the temperature during the NaF PDT is increased to 300‰, η increases to 10.1%, due to an increase in J_SC to 37 mA/cm², V_OC to 680 mV, and FF to 40%. The FF drops if the NaF PDT is performed at temperatures higher than 300‰. At 400‰the FF drops to 20%, while the V_OC remains 680 mV and the J_SC drops to 25 mA/cm². The device with NaF deposited as a precursor prior to the CIGSe deposition has a low J_SC of 5 mA/cm² in combination with a high V_OC of 740 mV and the injection current is pushed towards higher voltages. This indicates a strong electrical barrier in the device.

Fig. 5.21b shows the corresponding C−V curves of the post-treated devices. The NaF PDT at 100‰ leads to no change in the C−V curve compared to the untreated device shown in Fig. 5.16b. If the capacitance is interpreted as a space charge capacitance, then the high capacitance at positive bias translates with Eq. 2.7 into a 20 nm thick space charge region. The free charge carrier density can be calculated with Eq. 2.10 to be 6e+14 cm⁻³ in the CIGSe bulk. The NaF PDT at 300‰leads toC−V curves with high capacitances for the whole studied voltage range. Leading to a SCR width of 10 nm at 0.5 V and 30 nm at -0.5 V. The NaF PDT at 400‰ further increases the capacitance, lowering the SCR width even further.

Beside the deposition temperature, the diffusion velocity of sodium into the absorber may be important. In order to slow down the diffusion of sodium during the annealing time a 10 nm thick Mo layer was deposited prior to the NaF PDT on the absorber surface.

This leads to a different sodium depth profile compared to the same NaF PDT performed without Mo layer, shown in Fig. 5.22. The sodium concentration within the CIGSe bulk reaches a similar value for both samples, but close to the CIGSe/ZnO interface and especially at the interface, the concentration is considerably smaller for the sample with the Mo layer on the CIGSe surface.

The J−V andC−V curves of these two devices are compared to the untreated device in Fig. 5.23a and b. Compared to the untreated device both show high J_SC values of 37 mA/cm². However, theVOC of the Mo diffusion barrier device is lowered from 670 mV to 570 mV. Nevertheless η is increased to 10.8% due to an increase of the FF from 40%

(a) (b)

Figure 5.21: a)J−V curves of devices with CIGSe deposited at 525‰and with NaF PDT at performed at different temperaturesb)C−V curves of the same devices recorded at 293 K and 1 kHz.

Figure 5.22: GDOES depth profile of Na in CIGSe/ZnO stacks, with and without the depo-sition of a 10 nm thick Mo diffusion barrier prior to the NaF PDT.

(a) (b)

Figure 5.23: a)J−V curves of devices with CIGSe deposited at 525‰and with NaF PDT at performed at different temperaturesb)C−V curves of the same devices recorded at 293 K and 1 kHz.

to 51%. Employing Eq. 2.10 the charge carrier density can be calculated from the C−V curves in Fig. 5.23b. The Na PDT increased the charge carrier density in the CIGSe bulk of the device with the Mo layer from 5e+14 cm⁻³ to 9e+15 cm⁻³, and for the device without Mo layer to 5e+16 cm⁻³.

In forward voltage bias, the capacitance of the Mo layer device is unchanged compared to the sample without sodium. This indicates that the strong increase as observed for the NaF treated sample without the Mo layer is most likely due to the presence of sodium at the interface between CIGSe and ZnO.

Potassium Fluoride Post Deposition Treatment (KF PDT) The effect of potas-sium in superstrate devices is shown in Fig. 5.24. All samples were from the same deposi-tion run performed at 525‰. The post deposition treatment was performed at nominally 300‰. The deposition of potassium fluoride was performed at a high deposition rate of several nm/s and lead to a slight kink in the fourth quadrant of the J−V curve. Very similar to theJ−V curve of the sample with the NaF PDT in Fig. 5.21. Not shown here, but KF provided as a precursor leads to an even more pronounced kink within the J−V curve. This indicates, that potassium, similar to sodium, induces an electron barrier at the hetero-interface.

The pure NaF treatment performed in this sample variation was done with a very low NaF deposition rate of 1 nm/min for 10 minutes. This leads to a J −V curve similar to the one in Fig. 5.21 where the sodium diffusion rate was lowered by the additional Mo layer.

The third sample became the same NaF PDT for only 5 minutes, followed by a KF PDT with 1 nm/min also for 5 minutes. Compared to the pure NaF treatment, the combined NaF and KF treatment leads to an increase of the VOC from 525 mV to 580 mV. η of this device was 11.0 %, one of the highest measured efficiencies obtained

Figure 5.24: J−V curves of devices with absorbers from the same deposition run performed at 525‰, but with different PDTs performed at 300‰. The device with the NaF PDT followed by the KF PDT reached the highest efficiency of 11%. TheJ−V characteristics were obtained from a one-diode model fit of the illuminated and dark curve (l, J₀, R_s, R_p). The minimum band gap was 1.15 eV (GDOES) and the Cu/(In+Ga) = 0.89.

in this work, indicating further potential of improvements by fine tuning the alkali post deposition.

In summary, as shown before, low-rate deposition of sodium leads to an improved fill factor above 50 %. The combination of NaF and KF treatment was shown to improve the VOC similar as observed in substrate devices.