**5. Conclusion**

transparency and conductivity lead to the best device performance among all the ITO‐free OSCs and verify the fact that the percolation threshold of Ag has been reduced to 9 nm by introducing a 2 nm MoO3 interlayer. As we know, the thickness of Ag film is strongly related to its transmittance and conductivity, and the percolation threshold thickness determines the smallest thickness for a metal film electrode, which is the most important parameter in the 3D growth of Ag film during the thermal evaporation process. Thus, the decrease in percolation threshold thickness not only could maintain the high conductivity, but also could enhance the

**Figure 12.** SEM images of (a) Ag (9 nm), (b) Ag (11 nm), (c) MoO3 (2 nm)/Ag (9 nm) and (d) MoO3 (10 nm)/Ag (9 nm) electrodes deposited on glass substrates. The white scale bar represents 100 nm. Reproduced with permission [11].

**Figure 13.** (a) Transmittance spectra with corresponding sheet resistances and (b) J‐V characteristics for ITO‐free OSCs fabricated on PET or glass substrates. (a) Adapted with permission [11]. Copyright 2014, Elsevier. (b)Reproduced with

optical transmission of Ag film and lower the fabrication cost as well.

Copyright 2014, Elsevier.

176 Nanostructured Solar Cells

permission [26]. Copyright 2015, IEEE.

In conclusion, the properties of electrode‐organic interfaces and transparent electrode mate‐ rials have significant impact on the efficiency of light absorption, charge transport and collection, which dominates the overall efficiency of OSCs. Nowadays, the interface engineer‐ ing and electrode engineering have attracted increased attentions all over the world. In this chapter, after a simple review of interfacial layers and transparent electrodes reported in OSCs, we have investigated two efficient modifying layers of ZnO and ultrathin Ca films, two potential ITO‐free electrodes of AZO and ultrathin Ag film, and their effect on the performance improvement of P3HT:PCBM based OSCs.

By utilizing an aqueous solution method processed ZnO interfacial layer at low temperatures, IOSCs have obtained an obvious improvement of device performance. The results show that the transition point of ZnO annealing temperature is approximately 80°C. When the ZnO annealing temperature is above 80°C, the corresponding IOSCs show senior photovoltaic performances with PCEs higher than 3.5%, and the flexible devices based on PET substrates also display a PCE of 3.26% as well as a good flexibility in bending tests. All devices show good repeatability and air stability. The improved device performance can be attributed to the well‐ aligned energy levels and improved charge transport between ITO and organic material. Thus, the low‐temperature ZnO deposition method based on aqueous solution is a promising technique in fabricating highly efficient IOSCs and flexible devices with long lifetime.

By utilizing an ultrathin Ca modifier and AZO transparent cathodes, ITO‐free IOSCs have achieved an obviously improved device performance and weakened light‐soaking issue. Although the AZO only IOSC show a very poor performance, IOSCs with a Ca modifier (5 nm or thicker) could obtain the remarkably increased *V*OC of 0.60 V and PCE of 2.69%, which is attributed to the well‐aligned energy levels at AZO/organic interface. When an ultrathin Ca modifier (~1 nm) is introduced, a further improved PCE above 3% is obtained and more importantly, the light soaking issue in the AZO only IOSC and AZO/Ca (5 nm) IOSC has been evidently weakened, which could be explained by the highly efficient electron transport at AZO/Ca/organic interface, while the more exact mechanism should be further investigated in the future work.

By utilizing an ultrathin MoO3 interlayer for Ag film growth, the MoO3 (2 nm)/Ag (9 nm) anode not only shows a low sheet resistance of 6.29 Ω/square but also presents a higher transparency with a maximum of 74% at 361 nm, and notably the percolation threshold of Ag film has been decreased from 11 to 9 nm according to the 3D island growth of Ag, confirmed by the SEM, sheet resistance, and transmittance study. The resulted ITO‐free OSCs with this MoO3/Ag anode show an improved PCE 2.71%, and the corresponding flexible device fabricated on PET substrates also achieves a comparable PCE of 2.50% to that of ITO‐based OSCs. Thus, the evaporated Ag film electrode with good transmittance and low resistivity is a potential candidate of ITO, and it would find more applications in flexible devices and roll‐to‐roll production. All investigations in this chapter enrich the understanding of interface and electrode engineering in OSCs, which may be instructive for further research on the improve‐ ment of device performance and the possible commercialization in the future.
