## 6 Outlook and Perspective
**} \\
**Modification** & \multicolumn{2}{c}{} & & & & & & \\ \hline \multirow{11}{*}{Simplistic synthesis route} & ITO/SnO\({}_{2}\)-RT/PVSK/Spiro OMeTAD/Au & 21.8 & 1.084 & 0.767 & 18.1 & [119] \\  & ITO/SnO\({}_{2}\)-120 \({}^{\circ}\)C/PVSK/Spiro OMeTAD/Au & 22.0 & 1.098 & 0.785 & 19.0 & \\  & ITO/spin-coated SnO\({}_{2}\)/MAPb3/Spiro OMeTAD/Au & 21.8 & 0.901 & 0.549 & 10.78 & [128] \\  & ITO/spintered SnO\({}_{2}\)/MAPb3/Spiro OMeTAD/Au & 22.9 & 0.965 & 0.580 & 12.82 & \\  & ITO/SnO\({}_{2}\)-water/PVSK/Spiro-OMeTAD/Au & 21.6 & 1.119 & 0.740 & 17.87 & [129] \\  & ITO/SnO\({}_{2}\)-butyl acetate/PVSK/Spiro-OMeTAD/Au & 22.3 & 1.132 & 0.760 & 19.22 & \\  & ITO/SnO\({}_{2}\)-UVO (0 min)/PVSK/CIS/Au & 12.2 & 0.860 & 0.411 & 4.34 & [130] \\  & ITO/SnO\({}_{2}\)-UVO (30 min)/PVSK/CIS/Au & 20.39 & 0.920 & 0.583 & 10.96 & \\  & ITO/night-printed SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 21.1 & 1.100 & 0.750 & 17.37 & [133] \\  & ITO/SnO\({}_{2}\) NCs/CsFAMA/Spiro-OMeTAD/Au & 22.5 & 1.099 & 0.780 & 19.27 & [134] \\  & ITO/SnO\({}_{2}\) NCs-KOH/CsFAMA/Spiro-OMeTAD/Au & 22.6 & 1.148 & 0.790 & 20.50 & \\ \hline \multirow{11}{*}{Interface modifier} & ITO/SnO\({}_{2}\)/PVSK/CIS/Au & 19.9 & 1.010 & 0.600 & 12.95 & [135] \\  & ITO/SnO\({}_{2}\)-NH/LPI/PVSK/Spiro-OMeTAD/Au & 20.1 & 1.150 & 0.710 & 16.79 & \\  & ITO/SnO\({}_{2}\)-water/PVSK/Spiro-OMeTAD/Au & 20.8 & 1.080 & 0.680 & 15.27 & [136] \\  & ITO/SnO\({}_{2}\)-ethanol/PVSK/Spiro-OMeTAD/Au & 21.8 & 1.110 & 0.780 & 18.84 & \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 22.2 & 1.120 & 0.690 & 17.15 & [140] \\  & ITO/SnO\({}_{2}\)-TPAC/PVSK/Spiro-OMeTAD/Au & 23.11 & 1.160 & 0.715 & 19.17 & \\ \hline \multirow{11}{*}{Bilayer} & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 22.5 & 1.157 & 0.760 & 19.78 & [141] \\  & ITO/SnO\({}_{2}\)-NAT/F4C\({}_{2}\)TbNaT4/PVSK/ & 23.4 & 1.159 & 0.761 & 20.64 & \\  & ITO/SnO\({}_{2}\)/FAPbI\({}_{3}\)/Spiro-OMeTAD/Au & 24.8 & 1.060 & 0.750 & 20.10 & [142] \\  & ITO/SnO\({}_{2}\)-TBAF/FAPbI\({}_{3}\)/Spiro-OMeTAD/Au & 25.4 & 1.130 & 0.760 & 23.10 & \\ \hline \multirow{11}{*}{Functional groups} & ITO/Po/PVSK/Spiro-OMeTAD/Au & 24.1 & 1.080 & 0.706 & 18.45 & [143] \\  & ITO/NH2-2O68O/P/VSK/Spiro-OMeTAD/Au & 25.1 & 1.139 & 0.787 & 22.52 & \\  & ITO/Po/PVSK/Spiro-OMeTAD/Ag & 22.7 & 1.110 & 0.759 & 19.04 & [145] \\  & ITO/SnO\({}_{2}\)-NTC60-OH/PVSK/Spiro-OMeTAD/Ag & 23.4 & 1.130 & 0.807 & 21.39 & \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 22.0 & 1.108 & 0.781 & 19.01 & [149] \\  & ITO/SnO\({}_{2}\)-\(\rho\)/PVSK/Spiro-OMeTAD/Au & 22.8 & 1.120 & 0.784 & 20.05 & \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 23.2 & 1.097 & 0.746 & 18.99 & [150] \\  & ITO/SnO\({}_{2}\)-NDI graphene/PVSK/Spiro-OMeTAD/Au & 22.7 & 1.084 & 0.821 & 20.16 & \\  & ITO/SnO\({}_{2}\)/MAPb3/Spiro-OMeTAD/Au & 20.8 & 1.065 & 0.662 & 14.69 & [146] \\  & ITO/SnO\({}_{2}\)-SAM/MAPb1/Spiro-OMeTAD/Au & 21.2 & 1.160 & 0.692 & 17.03 & \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Ag & 21.8 & 1.150 & 0.732 & 18.41 & [152] \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Ag & 22.6 & 1.140 & 0.767 & 19.72 & \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 22.1 & 1.054 & 0.713 & 16.62 & [153] \\  & ITO/SnO\({}_{2}\)-MTMS/PVSK/Spiro-OMeTAD/Au & 22.8 & 1.105 & 0.746 & 18.75 & \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Ag & 23.7 & 1.113 & 0.778 & 20.1 & [154] \\  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Ag & 24.5 & 1.148 & 0.821 & 22.72 & \\  & ITO/SnO\({}_{2}\)/MPb3/Spiro-OMeTAD/Au & 21.2 & 1.030 & 0.587 & 13.30 & [155] \\  & ITO/SnO\({}_{2}\)-RBF/MAPb3/Spiro-OMeTAD/Au & 22.6 & 1.060 & 0.769 & 18.41 & \\ \hline \multirow{11}{*}{Vacancy defects} & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Au & 19.2 & 1.110 & 0.690 & 14.70 & [156] \\  & ITO/SnO\({}_{2}\)-NGO/PVSK/Spiro-OMeTAD/Au & 19.3 & 1.170 & 0.745 & 16.54 & \\ \cline{1-1}  & ITO/SnO\({}_{2}\)/PVSK/Spiro-OMeTAD/Ag & 23.9 & 1.060 & 0.741 & 18.85 & [157] \\ \cline{1-1}  & ITO/RNAi-SO/PVSK/Spiro-OMeTAD/Ag & 24.6 & 1.150 & 0.771 & 21.71 & \\ \cline{1-1}  & ITO/RNAi-SO/PVSK/Spiro-OMeTAD/Ag & 22.4 & 1.130 & 0.734 & 18.67 & [113] \\ \cline{1-1}  & ITO/SnO\({}_{2}\)-NH42S/PVSK/Spiro-OMeTAD/Ag &region binding, UV light resistance, strong charge extraction, and low photocatalytic activity compared with its many competitors.