Device performances

We revealed the dependence of device performances on the Ar pressure during sputter deposition of NiOx, and the results are summarized in Table 3.1 and Fig. 3.11. It is

enhanced and beyond that pressure, it declined (Fig. 3.11 (a)). This phenomena can be explained with the optical and electrical properties of the sputtered NiOx thin films. As shown in Fig. 3.10, at lower pressure, the NiOx thin films absorbed some part of the incident sunlight. At higher pressure, although the transmittance is better, higher resistance of the films reduces the device performance, as seen from the increased series resistance (Rs) for 5.0 and 6.5 Pa. Therefore, we selected the devices with NiOx HTL prepared at 3.5 Pa Ar pressure for further study of thickness-dependent device performance, and the results are summarized in Table 3.2.

Table 3.1. Performance of the devices with different Ar pressure of the deposition chamber during sputter deposition of NiOx. Data collected from at least 12 cells for each condition. (NiOx film thickness: 60–70 nm for 0.5–3.5 Pa and ~40 nm for 5.0–6.5 Pa).

Ar Pressure

(Pa)

 (%)

Jsc

(mA/cm²)

Voc

(V)

FF

Rs

(Ω･cm²)

Rsh

(Ω･cm²) x10³ 0.5 11.020.46 17.670.71 0.970.01 0.630.03 5.010.58 2.070.19 2.0 12.210.58 18.700.83 0.980.01 0.630.02 5.160.47 2.350.72 3.5 14.760.39 19.860.85 1.010.02 0.680.02 5.410.62 3.250.33 5.0 13.610.61 19.790.69 0.980.01 0.660.03 8.850.83 2.300.46 6.5 12.090.57 18.880.48 0.980.01 0.650.04 8.740.92 1.230.57

0 1 2 3 4 5 6 7 6

7 8 9 10 11 12 13 14 15 16

Efficiency (%)

Pressure (Pa)

(a)

400 600 800

0 20 40 60 80 1000 20 40 60 80 100

EQE (%)

Wavelength (nm) 20 nm 50 nm 70 nm 100 nm 150 nm 250 nm

(b)

EQE (%)

0.5 Pa 2.0 Pa 3.5 Pa 5.0 Pa 6.5 Pa

Fig. 3.11. Device performances. (a) Ar pressure dependent PCE, (b) EQE of the devices with NiO HTL prepared at different Ar pressure and thickness.

Table 3.2. Performance of the devices with different thickness of NiOx prepared at 3.5 Pa. Data collected from at least 12 cells for each thickness.

NiOx

Thickness (nm)

 (%)

Jsc

(mA/cm²)

Voc

(V)

FF

Rs

(Ω･cm²)

Rsh

(Ω･cm²) x10³ 202 8.250.41 18.470.23 0.980.01 0.460.11 21.481.31 0.860.15 502 13.430.56 19.890.65 1.000.01 0.620.02 5.070.45 1.470.38 703 14.760.39 19.860.85 1.010.02 0.680.02 5.410.62 3.250.33 1005 13.640.67 19.010.41 0.950.01 0.690.01 5.820.33 3.050.46 1505 14.120.35 18.740.64 0.930.02 0.730.01 5.350.52 2.830.63 2507 11.850.58 16.110.72 0.900.02 0.740.01 5.800.61 2.760.29

It was found that when the NiOx layer was too thin (e.g., 20 nm), the devices showed lower PCE. The very thin film may be not sufficient to block the photogenerated electrons because of the insufficient coverage of the ITO film with the NiOx layer, which in turn decreases the fill factor (FF) [23], [30]. In fact, the improved FF values with increasing thickness were observed possibly due to the elimination of pinholes.

However, with a very thick NiOx HTL, the FF was satisfactory, but the PCE was again low. With higher thickness, the transmittance decreased, with the small deviation at 150 nm thickness. From Fig. 3.11 (b), we can see that with higher thickness, the EQE of the devices decreased significantly at lower wavelength, which reduces the short circuit current density (Jsc). Devices with a NiOx HTL thickness of 703 nm showed best performance (Fig. 3.12). Although the hysteresis behaviors of the perovskite solar cells are an important issue, which is frequently observed and reported in the literature [43],

our NiOx devices showed almost no hysteresis behaviors (Fig. 3.13). The devices showed a good reproducibility, with a limited deviation of PCE, as shown in Fig. 3.14.

Histograms of solar cell efficiencies were collected from 32 cells with NiOx HTL of 70 nm3 thickness prepared at 3.5 Pa Ar pressure.

0 50 100 150 200 250

8 12 16 12 16 20 0.6 0.8 1.0 1.2 0.40 0.60 0.80

Effic ie ncy (%)

Thickness (nm) Js c ( mA/cm

) Vo c ( V) FF

Fig. 3.12. Device performances. NiOx HTL thicknesses dependent PCE, Jsc, Voc and FF.

Fig. 3.13. J-V curve of the best device with NiOx HTL (3.5 Pa, 70 nm) under one sun condition measured at forward scan (-0.05 V  1.2 V, step 0.02 V, delay time 200 ms) and reverse scan (1.2 V  -0.05 V, step 0.02 V, delay time 200 ms).

Fig. 3.14. The PCE distribution histogram of devices with NiOx HTL prepared at 3.5 Pa.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-10 -5 0 5 10 15 20 25

Forward Reverse

Jsc= 20.33 (mA/cm²) Voc= 1.08 (V) FF = 0.69

Rs= 5.99 (Ω･cm²) Rsh= 1.34 X 10³(Ω･cm²) η = 15.15 %

Pint= 100 (mA/cm²) J (mA/cm2)

Voltage (V)

Counts (%)

PCE (%)

14.00 14.25 14.50 14.75 15.00 15.25 15.50 0

10 20 30 40 50 60

No significant performance degradation was observed for encapsulated devices with NiOx HTL stored at ambient temperature under dark condition for five months. The significant improvement of the stability over previous NiOx based devices [7], [14], [16]

[18]-[37] was achieved possibly due to the synergy of the NiOx HTL, with the high optoelectronic quality of the MACl treated perovskite layer [39]. We also observed that the PCE and open circuit voltage (Voc) gradually increase with time at ambient temperature under dark condition. The improvement can be explained by the ion migration and chemical doping of the PCBM layer by iodide [8], [44]. On the other hand, under continuous 1 SUN illumination (no UV-light filtering) and MPPT condition at 30℃, the performance first degraded gradually and then the degradation rate decreased; and it eventually reached 87% of initial efficiency after 670 h of operation, as shown in Fig. 3.15. The PEDOT:PSS-based devices degrade rapidly, and within 400 h, they retain only 20% of the initial PCE, possibly due to the chemical nature of the PEDOT:PSS layer [13]-[16]. The lifetime of solar cells may be defined as the operation time until the output of the device has fallen below a certain level, that is, 70% of nominal efficiency for more than 40 years was expected from some commercial Si solar cells. In fact, the NiOx based devices showed surprisingly high stability and it would require significant testing time to see further degradation at 30℃ (Fig. 3.16).

0 100 200 300 400 500 600 700 0.2

0.4 0.6 0.8 1.0 1.20.2 0.4 0.6 0.8 1.0 1.20.2 0.4 0.6 0.8 1.0 1.20.2 0.4 0.6 0.8 1.0 1.2

Normalized PCE

Time (h)

Continuous illumination-NiOx Dark-NiOx

Continuous illumination-PEDOT:PSS

Normalized J sc Normalized V oc Normalized FF

Fig. 3.15. Stability of the encapsulated device at 30℃ (~50%RH) under ambient in dark condition and under MPPT condition (1 SUN) (The device was kept at MPPT condition between the periodical J-V measurements)

We studied the modulation of the work function of NiOx thin films to realize high Voc

and subsequently high performance of PVSC. It is noticed that the work function of sputter deposited NiOx films is dependent on the applied radio frequency power supply during sputtering process. Fig. 3.16 represents the XRD patterns of NiOx films prepared on the commercially available ITO coated glass substrates. To better understand the peaks of NiOx films, we also added the XRD patterns of commercially available ITO coated glass substrates. All sputtered deposited NiOx films are polycrystalline in nature and preferred orientation along (200) plane, other weak peaks are (220) and (222).

Fig. 3.16. XRD pattern of NiOx films prepared at different applied rf power supply.

Fig. 3.17 shows the SEM photographs of sputtered deposited NiOx films prepared on the commercially available ITO coated glass substrates and the grain size became smaller with increasing applied rf power supply.

Fig. 3.17. SEM photographs of NiOx films prepared at different applied rf power supply.

The effect of applied rf power on work function and Voc is presented in Fig. 3.18. From Fig. 3.18, we can observe that the work function of sputter deposited NiOx films changes with the applied rf power and at lower power (50 W), the value of work function is 5.68 eV; with increasing applied rf power (up to 200 W), the value of work function decreases to 5.40 eV. Subsequently, the open circuit voltage (Voc) of the devices decreases with increased applied rf power. So, overall cell performance can be enhanced by modulating the work function of sputter deposited NiOx films with changing applied rf power. The best performance cells were found with the NiOx HTL sputter deposited at applied rf power of 50 W. However, the reason behind the changing of work function with applied rf power supply is still unknown. Further investigation is necessary to find out the reason. Fig. 3.19 reveals the determination of work function of NiOx films sputter deposited at different applied rf power supply.

40 60 80 100 120 140 160 180 200 220

5.40 5.45 5.50 5.55 5.60 5.65 5.70

Workfunction Voc

Power (W)

Workfunction (eV)

0.90 0.92 0.94 0.96 0.98 1.00 1.02

Voc (V)

Fig. 3.18. Variation of work function of NiOx films and corresponding Voc of the devices with different applied rf power supply.

Fig. 3.19. Work function of NiOx films sputtered at (a) 50 W, (b) 100 W and (c) 200 W applied rf power supply.

Energy (eV)

Yield (cps)

Some reports show that the Voc is mostly independent of the ionization potential of organic hole transport layer [45], while, other report shows that the low Voc of PEDOT:PSS HTL based PVSC might be due to the reduction of PEDOT:PSS by methylammonium iodide (MAI) solution [46] and the reduction would be resulted in the decrease of WF [47] and the Voc could be enhanced by using intrinsically nonreduction-active HTL instead of PEDOT:PSS HTL [46]. So, it is very interesting to observe that the Voc of perovskite solar cells is strongly dependent on the work function of inorganic NiOx HTL and this observation is important to improve the performance of inorganic hole transport layer for perovskite solar cells.

The solar cell performances of the devices are presented in Table 3.3. Solar cell with the NiOx HTL sputter deposited at applied rf power of 50 W shows an Rsh value of 3.25 x 10³, which is larger than the Rsh value of solar cells with the NiOx HTL sputter deposited at applied rf power of 100 W and 200 W (2.36 x 10³ and 1.85 x 10³ respectively).This result also represents the low recombination loss and high Voc of solar cells with the NiOx HTL sputter deposited at applied rf power of 50 W.

Table 3.3. Solar cell performances. Data presented as the average of 8 cells.

Power (W)

 (%)

Jsc

(mA/cm²) Voc

(V)

FF Rs

(Ω･cm²)

Rsh (Ω･cm²)

x10³

50 15.02 19.97 1.09 0.69 5.99 3.25

100 12.19 18.20 0.93 0.72 5.65 2.36

200 11.01 17.29 0.91 0.70 6.03 1.85

Fig. 3.20. shows the J-V curve of the devices with NiOx HTL prepared at different applied rf power supply and from the J-V curves, it is revealed that the device with NiOx HTL prepared at lower rf power (50 W) shows high Voc and with increasing rf power (e.g. 100 W and 200 W), the values of Voc decrease. This is the key reason behind the high performance of the devices with NiOx HTL prepared at lower rf power.

Fig. 3.20. J-V curve of the devices with NiOx HTL prepared at different applied rf power supply.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-10 -5 0 5 10 15 20 25

50W-Forward 50W-Reverse 100W-Forward 100W-Reverse 200W-Forwad 200W-Reverse

J (mA/cm2)

Voltage (V)