Anti-solvent Engineering for Efficient Perovskite Solar Cell Using PVK as Hole-Transporting Layer

Perovskite solar cells were studied using PVK as hole-transporting layer and different anti-solvent process. We have fabricated CH3NH3PbI3 (MAPbI3) photoactive layer using chlorobenzene (CB) solution and PVK in CB solution as the anti-solvent solution, respectively. The results show that the power conversion efficiency is enhanced to 14.7% by using 2.5 mg/mL PVK in CB solution as the anti-solvent, which is significantly improved compared with 12.85% for the device treated using CB. PVK in CB as the anti-solvent solution facilitates the hole transport for MAPbI3/PVK heterojuction as well as good crystallinity and less PbI2 residues in MAPbI3 photoactive layer.

Conjugated polymer poly(9-vinylcarbazole) (PVK) is an efficient hole-transporting material in organic photoelectronics and can be easily deposited [14][15][16][17]. On the other hand, recently researchers employed anti-solvent approach for high efficient PSCs utilizing a few drops of nonpolar solvent during spin coating. This approach makes the precipitation process uniform across the whole surface, giving rise to a smooth crystalline film with large grains [18][19][20]. Based on these considerations, we have developed a simple way for high efficient PSCs using PVK as HTL and controlling anti-solvent engineering in this work. Under the optimal condition, the PSC device displays a PCE of 14.7%.

Material and Methods
FTO glasses (20×20mm 2 ) were sequentially cleaned with standard procedure, and dried in N2 IOP Conf. Series: Materials Science and Engineering 774 (2020) 012129 IOP Publishing doi:10.1088/1757-899X/774/1/012129 2 atmosphere. Then, a 500℃ spray pyrolysis method was employed to dposite TiO2 compact layer (c-TiO2). The mesoporous TiO2 (m-TiO2) layer was spin-coated on the top of the c-TiO2 layer. PbI2 (650 mg/mL) and MAI (220 mg/mL) were dissolved in dimethyl sulf-oxide (DMSO) and N,N-Dimethylformamide (DMF) (1:9), and the mixed solution was spin-coated at 4000rpm for 30s to deposite the perovskite absorber layer. As anti-solvent engineering, 70μl of CB solution or PVK in CB solution with different concentration were cast on the top of MAPbI3 layer at 4000 rpm for the last 24 s without stop during spin-coating, respectively. The substrates were dried at 65°C for 3 min and then dried at 100 °C for 5 min to form the MAPbI3 film. For the sample with CB as the anti-solvent, PVK was dissolved in CB to prepare 2.5mg/mL solution. The mixed solution was spin-coated at 4000 rpm for 24s and annealed at 100℃ for 10 min to fabricate the HTL film on the top of the MAPbI3 absorber layer. Finally, a 120 nm Au metal electrode was deposited by thermal evaporation method. Figure 1 presents the schematic of detailed fabrication steps of the perovskite device with a structure of FTO/c-TiO2/m-TiO2/MAPbI3/PVK/Au.

Result and Discussion
To explain the effect of the anti-solvent engineering on the characteristic of the perovskite absorber layer, we conducted UV-vis absorption spectroscopy, XRD patterns and PL spectra measurement of perovskite layer with a structure of FTO/c-TiO2/m-TiO2/MAPbI3 using CB solution and 2.5 mg/mL PVK in CB as the anti-solvents, respectively. Figure 2a shows the absorption curves with different anti-solvent. Both absorption curves exhibit broad band absorption characteristics from visible to NIR wavelength region. Compared to that of the perovskite layer treated with CB anti-solvent, the absorbance of the perovskite layer treated with PVK in CB anti-solvent with a concentration of 25 mg/mL increases at 300-470 nm, which is due to the absorption of PVK. This phenomenon indicates that PVK in CB anti-solvent has negligible effects on the absorption of MAPbI3 thin film in the range of visible light.
XRD measurement is conducted to elucidate the effect of various anti-solvents on the lattice structure of MAPbI3 thin films. As shown in figure 2b, XRD patterns of MAPbI3 thin film with CB anti-solvent reveal a nearly identical crystalline structure as reported in the previous literature [4]. It can be seen that the intensity of the (220) peak at 28.71 and the (310) peak at 32.01 decreased a little for the MAPbI3 thin film with PVK in CB as the anti-solvent. This shows that the crystal grains in perovskite film with PVK in CB anti-solvent have oriented grains and that the orientation intensities change a little which may be caused by the formation of grain boundaries or vacancies which origin from the partly extruded and segregated of the amorphous PVK during the MAPbI3 formation  [21]. It should be noted that there was no peak pertain to MAI and PbI2, that is to say, MAI and PbI2 were almost completely converted into MAPbI3. Figure 2c emerges the photoluminescence spectra measured for FTO/c-TiO2/m-TiO2/MAPbI3 with different anti-solvents. It was worth noting that the PL intensity of MAPbI3 with PVK in CB antisolvent was much weaker than that treated with CB at an excitation wavelength of 526 nm, although both films exhibited strong PL peak at about 750 nm. These results demonstrated that PL quenching was occurring in the MAPbI3/PVK heterojunction due to the efficient charge transfer from MAPbI3 to PVK [22].   We further investigated the function of anti-solvent engineering on the photovoltaic device performance and J-V characteristics of devices under simulated air mass at AM 1.5G solar irradiation are presented in figure 3a. For further investigate the different influences of various concentrations of PVK in CB solution, different concentrations of PVK were used (1 mg/mL, 2.5 mg/mL and 4 mg/mL). The photovoltaic performance values are summarized in Table 1. It is found that the application of PVK in CB anti-solvent significantly increases the short circuit current density (Jsc) compared to that with CB as the anti-solvent. Open-circuit voltages (Voc) of various photovoltaic devices have close values for different anti-solvents. The concentration of PVK dramatically influences the device performance and the optimized concentration of PVK is 2.5 mg/mL. The optimal device with a concentration of 2.5 mg/mL has an excellent Jsc reached 22.76 mA/cm 2 which is larger than that of the devices with other concentrations. The results show that the device fabricated with 2.5 mg/mL PVK in CB solution has a highest PCE of 14.7% which is enhanced compared with 12.85% of the device with CB anti-solvent, which may be due to enhanced photogenerated carriers transporting by the formation of heterojunction between the perovskite and PVK in perovskite bulk. Figure 3b shows the schematic charge transporting at heterojunction of the perovskite and PVK. With the anti-solvent of PVK in CB solution, the heterojunction of the perovskite and PVK is formed penetrating deeply in perovskite bulk and it will facilitate hole transporting in all device resulting a higher Jsc. With the anti-solvent of CB solution, the heterojunction of the perovskite and PVK is also formed when spin coating PVK after the perovskite absorb layer. However, the depth of penetration of heterojunction may be not enough and impede the hole transporting from the perovskite into PVK HTL layer. Figure 4 depicts the stability of the optimal perovskite solar cell with PVK in CB as anti-solvent without encapsulation. PCE of the device decreases to 13.3% from 14.7% after 168 h with a droop rate of 9.5%, which may be caused by the high hydrophobicity of PVK. From above evaluation, it is concluded that PVK in CB solution as anti-solvent effectively increases perfomance of the n-i-p perovskite photovoltaic devices using PVK as HTL.

Conclusions
In summary, we demonstrated the function of anti-solvent engineering in n-i-p PSCs with PVK as HTL. A PCE of 14.7% has been achieved using PVK in CB as anti-solvent with a concentration of 2.5 mg/mL. Finally we researched the stability of the optimal PSC and confirmed that the HTL solution can be ideal anti-solvent candidate to fabrication n-i-p perovskite photovoltaic devices.