**2.2. Results and discussion**

The fabricated OSCs have a structure shown in **Figure 1**, wherein P3HT:PCBM acts as the photoactive layer, the ZnO film plays the roles of electron transport layer and hole blocking layer. This solution processed ZnO interfacial layer is used to lower ITO work function, modify the ITO polarity, and align the energy levels at ITO/P3HT:PCBM interface. In details, ZnO has the conduction band energy of −4.2 eV and the valence band energy of −7.5 eV, which suggests that electrons from PCBM can be transported into ZnO, while holes from P3HT can be blocked. Meanwhile, the MoO3 acts as the hole transport layer and electron‐blocking layer, and the ITO and Ag play the roles of cathode and anode, respectively.

**Figure 2a** shows the typical J‐V curves under simulated AM 1.5G illumination for IOSCs at different ZnO annealing temperatures. It is well known that the J‐V characteristics of solar cells can be described by the single‐diode model under illumination, and the relation of J and V is given as follows:

$$J = J\_o(\exp(\frac{q(V - R\_sJ)}{nk\_BT}) - 1) + \frac{V - R\_sJ}{R\_{sh}} - J\_{ph} \tag{1}$$

where *J*0 is the saturation current, *q* the electron charge, *n* the ideality factor, *kB* the Boltzmann constant, *T* the temperature, *Rs* the series resistance, *Rsh* the shunt resistance, and *Jph* the photocurrent. Based on Eq. (1) and our previous reported method [16], these photovoltaic parameters extracted from J‐V characteristics are shown in **Table 1**. As shown in **Figure 2a**, the experimental data are represented as symbols and the curves calculated from our method are indicated as solid lines. It is clear that the measured data of all devices are well reproduced by the fitting curves.

thickness around 100 nm. Finally, the MoO3 (8 nm)/Ag (100 nm) anode was thermally evapo‐

**Figure 1.** Schematic illustration and ideal energy diagram of materials for the inverted OSCs [6]. Copyright (2015) The

The current density‐voltage (J‐V) curves were measured under simulated AM 1.5G solar simulator (Sanei Electric XEC‐300M2) using a source‐measure unit (Keithley 2400). The illumination intensity is kept at 100 mW/cm2 using a calibrated Si solar cell. The transmission spectra, film surface morphology, and ZnO film crystal quality were characterized by the ellipsometer (J. A. Woollam WVASE 32), atomic force microscopy (AFM; Agilent 5500), and

The fabricated OSCs have a structure shown in **Figure 1**, wherein P3HT:PCBM acts as the photoactive layer, the ZnO film plays the roles of electron transport layer and hole blocking layer. This solution processed ZnO interfacial layer is used to lower ITO work function, modify the ITO polarity, and align the energy levels at ITO/P3HT:PCBM interface. In details, ZnO has the conduction band energy of −4.2 eV and the valence band energy of −7.5 eV, which suggests that electrons from PCBM can be transported into ZnO, while holes from P3HT can be blocked. Meanwhile, the MoO3 acts as the hole transport layer and electron‐blocking layer, and the ITO

**Figure 2a** shows the typical J‐V curves under simulated AM 1.5G illumination for IOSCs at different ZnO annealing temperatures. It is well known that the J‐V characteristics of solar cells can be described by the single‐diode model under illumination, and the relation of J and V is

*B sh*

where *J*0 is the saturation current, *q* the electron charge, *n* the ideality factor, *kB* the Boltzmann constant, *T* the temperature, *Rs* the series resistance, *Rsh* the shunt resistance, and *Jph* the

*ph*


( ) (exp( ) 1) *s s*

*qV RJ V RJ J J <sup>J</sup> nk T R*

Japan Society of Applied Physics.

164 Nanostructured Solar Cells

**2.2. Results and discussion**

given as follows:

photoluminescence (PL) spectra (325 nm, He‐Cd laser).

and Ag play the roles of cathode and anode, respectively.

0

.

rated through a shadow mask and the resulted devices have an active area of 10 mm2

**Figure 2.** (a) Measured and calculated J‐V characteristics of IOSCs with the ZnO interlayer annealed at various temper‐ atures from 50 to 150 °C. (b) Photovoltaic parameters as a function of ZnO annealing temperatures [6]. Copyright (2015) The Japan Society of Applied Physics.


**Table 1.** Photovoltaic parameters of inverted OSCs with the ZnO interlayer annealed at different temperatures [6]. Copyright (2015) The Japan Society of Applied Physics.

**Figure 2a** and **Table 1** show the J‐V curves and photovoltaic parameters of IOSCs with ZnO annealed at various temperatures. The device with 150°C annealed ZnO obtains an overall PCE of 3.62% with *V*OC = 0.63 V, *J*SC = −9.30 mA/cm2 , and FF = 62.07%. If the annealing temper‐ ature of ZnO decreases from 150 to 80°C, the IOSCs perform well and achieve the almost unchanged PCE of 3.57%, 3.55%, and 3.54 %. However, when the temperature is further decreased (50 or 70°C), the device performance rapidly deteriorates. Although the degradation of the device performance is mainly due to the decrease in photocurrent, it can still be observed that both *V*OC and FF also begin to decrease. The effect of the ZnO annealing temperature on the device PCE is also shown in **Figure 2b**. The statistical results in **Figure 3a** show that the temperature approximately 80°C is the transition point, which corresponds to the above discussion. The data are obtained from at least seven devices for each annealing temperature of ZnO interlayer. Meanwhile, the fabricated devices in **Figure 3b** also show good air stability and their PCE could maintain about 90% of the original PCE values after 20 days stored in air.

**Figure 3.** (a) Performance statistical results of the IOSCs with the ZnO interlayer annealed at different temperatures [6]. Copyright (2015) The Japan Society of Applied Physics. (b) PCE degradation of IOSCs in air during 20 days.

**Figure 4.** (a) Measured J‐V characteristics of and PCE statistical result of flexible IOSCs. (b) Photovoltaic parameters as a function of the number of bending times up to 1000 cycles with a bending radius of about 50 mm [6]. Copyright (2015) The Japan Society of Applied Physics.

Furthermore, IOSCs based on flexible PET substrates were also fabricated with the structure of PET/ITO/ZnO/P3HT:PCBM/MoO3/Ag. As shown in **Table 1** and **Figure 4a**, the flexible device with ZnO annealed at 80°C shows a PCE of 3.26% with *V*OC = 0.64 V, *J*SC = −9.10 mA/ cm2 , FF = 56.0%, and the PCE statistical result also shows good device repeatability, which is comparable to that of the reference devices based on glass substrates. Then, the relatively good

ZnO quality achieved at 80°C annealing was again confirmed in the flexible devices. To quantitatively evaluate the device flexibility, a bending test with a number of bending times up to 1000 cycles was carried out. From **Figure 4b**, it could be seen that all the photovoltaic parameters nearly remain unchanged during the first 400 bending cycles, whereas *J*SC decreases to −9.0 mA/cm2 , FF degrades to 55.4%, and the overall PCE only decreases by 2% from 3.26% to 3.19% after continuous 1000 cycles of bending test. As a result, the device fabricated on flexible PET substrates shows superior flexibility, which proves the potential of low‐tempera‐ ture ZnO deposition method in fabricating flexible IOSCs and other electron devices.

that both *V*OC and FF also begin to decrease. The effect of the ZnO annealing temperature on the device PCE is also shown in **Figure 2b**. The statistical results in **Figure 3a** show that the temperature approximately 80°C is the transition point, which corresponds to the above discussion. The data are obtained from at least seven devices for each annealing temperature of ZnO interlayer. Meanwhile, the fabricated devices in **Figure 3b** also show good air stability and their PCE could maintain about 90% of the original PCE values after 20 days stored in air.

**Figure 3.** (a) Performance statistical results of the IOSCs with the ZnO interlayer annealed at different temperatures [6].

**Figure 4.** (a) Measured J‐V characteristics of and PCE statistical result of flexible IOSCs. (b) Photovoltaic parameters as a function of the number of bending times up to 1000 cycles with a bending radius of about 50 mm [6]. Copyright

Furthermore, IOSCs based on flexible PET substrates were also fabricated with the structure of PET/ITO/ZnO/P3HT:PCBM/MoO3/Ag. As shown in **Table 1** and **Figure 4a**, the flexible device with ZnO annealed at 80°C shows a PCE of 3.26% with *V*OC = 0.64 V, *J*SC = −9.10 mA/

, FF = 56.0%, and the PCE statistical result also shows good device repeatability, which is comparable to that of the reference devices based on glass substrates. Then, the relatively good

(2015) The Japan Society of Applied Physics.

cm2

166 Nanostructured Solar Cells

Copyright (2015) The Japan Society of Applied Physics. (b) PCE degradation of IOSCs in air during 20 days.

**Figure 5.** (a) Transmission and (b) PL spectra of ZnO on glass annealed at different temperatures [6]. Copyright (2015) The Japan Society of Applied Physics.

**Figure 5a** shows the transmission spectra of ZnO/glass samples annealed at 150, 80, 70, and 50°C. All the spectra are very similar and the samples show a transmittance about 88% in the wavelength region from 400 to 1000 nm. And their difference only comes from the shift of transmission edge for ZnO material in the short wavelength region from 300 to 400 nm. It can be seen in the illustration that the transmission edge locates at the lowest wavelength when the ZnO annealing temperature is 50°C and gradually shifts to longer wavelength with the increase in temperatures. Generally, if the material transmittance edge is located at a short wavelength, it has a wide bandgap; otherwise, it has a narrow band gap. A narrow band gap usually means a higher crystalline degree for ZnO [17], so ZnO annealed at higher tempera‐ tures has better crystalline quality. From the inset of **Figure 5a**, the relatively smaller shift of the transmission edge for ZnO annealed at 80–150°C means that the ZnO bandgap almost remains unchanged when the annealing temperature is above 80°C, which corresponds to the similar device performance for ZnO annealed at 80–150°C. Meanwhile, from the PL spectra of ZnO in **Figure 5b**, the intrinsic peak in the 350–400 nm range is related to the near‐band edge emission of ZnO. With an increase in ZnO annealing temperatures, the intrinsic peak position of ZnO shifts to the longer wavelength and the peak intensity also increases. What is more, the wide peaks in the range from 500 to 800 nm are usually related to the defects in ZnO film, such as interstitial zinc or oxygen atoms, zinc atom vacancy, and oxygen atom or ion vacancy [18]. With an increased temperature, the defect‐related peaks are weakened, and thus, the number of defects is decreased and better ZnO film quality is obtained. Simultaneously, for different ZnO film annealing temperatures of 50, 70, 80, and 150°C, the corresponding root‐mean‐square surface roughness are 0.673, 0.867, 1.108, and 1.145 nm, respectively. The increased surface roughness corresponds to the more sufficient ZnO crystallization at a high annealing temper‐ ature. It is thought that, when the annealing temperature is below 80°C, the ZnO morphology changes markedly, and when the annealing temperature is above 80°C, the ZnO morphology almost remains unchanged. From the morphology results, it is concluded that an annealing temperature of 80°C is sufficient for ZnO crystallization. And the high temperature only slightly improves the film quality, a result agreed to the similar photovoltaic performance of IOSC with ZnO annealed from 80 to 150°C.

According to above discussion, one can draw the conclusion that the 80°C is sufficient for ZnO annealing to obtain a relatively high film quality and act as an interfacial layer, and the resulted devices based glass or PET substrates show the similar photovoltaic performance when the ZnO annealing temperature is higher than 80°C. In short, the senior device performance and good stability show that the aqueous solution method is a more promising low‐temperature technique for depositing ZnO in IOSCs and it may be widely applied in flexible and printing devices in the future.
