**1. Introduction**

Organic solar cells (OSCs) have attracted much more attention due to their advantages of low cost, light weight, mechanical flexibility, simple process, and steady improved power conversion efficiency (PCE). Over the past 20 years, many researches about new materials, device structures,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

andfabricationprocesseshavebeenreportedforOSCs, andtheirPCEhas remarkablyincreased from 1% to 10% [1–5]. Nowadays, many efforts have been focused on the further improvement of PCE and long‐term stability. Besides the utilizing of novel photoactive materials and device structures, the interface engineering and electrode engineering play important roles in the improvement of device performance and the realization of cost‐effective mass manufacture in the future.

In general, the properties of electrode/organic interfaces and transparent electrode materials determine the efficiency of light absorption, charge transport, and collection, which is strongly associated with the open‐circuit voltage, short‐circuit current density, fill factor, and the overall PCE for IOSCs.

In inverted OSCs (IOSCs), by modifying the indium‐tin‐oxide (ITO) cathode with functional interface layers and by using high work function metals (Ag, Au) insensitive to air, the IOSCs can obtain improved air‐stability while maintaining a PCE comparable to that of conventional structure [6]. Over the past decade, many n‐type modifying materials (TiO2, ZnO) and ultrathin metal films (Ca, Al) have been used to modify the polarity of ITO, so that it can be more effective as an electron‐collecting electrode [7–9]. Among these materials, ZnO has a suitable work function, high electron mobility, good optical transmittance, and environmentally friendly nature. Further, it can be prepared by various methods [6, 10], such as the radiofrequency sputtering, atomic layer deposition, sol‐gel processing, and so on. All these methods are high cost or high temperature (over 200°C) process, which is not compatible with large area deposition and plastic substrates. For IOSCs, the solution method is time‐saving, inexpensive, simple, and compatible with printing techniques and flexible substrates, thus the solution method processed at low temperatures is more desirable. Simultaneously, the ultrathin metal modifier processed by the mature thermal evaporation is also a potential interfacial material, which has been successfully used to modify the ITO cathode and in efficient IOSCs [7, 9].

As we know, ITO is the most commonly used electrode in OSCs; however, the limited reserve of toxic indium element in earth and the increasing price of ITO force us to develop alternatives to ITO. So far, the reported replacements of ITO mainly include the metal films (such as Au, Ag, and oxide/metal/oxide), graphene, carbon nanotubes, and aluminum‐doped zinc oxide (AZO) electrodes [3, 11–14]. Among them, AZO is able to meet the requirements of electrode, what is more, Al and Zn are relatively rich in earth, nontoxic and the large area AZO film fabrication is relatively easy. Therefore, the commercial AZO may be more suitable to replace ITO electrode in OSCs. Meanwhile, a smooth and continuous metal thin film (e.g., Ag) can be easily deposited by simple thermal evaporation, suitable for application in the mass produc‐ tion. Moreover, due to their intrinsic flexibility and high conductivity [14], metal thin‐film electrodes are also suitable for application in roll‐to‐roll production of flexible OSCs. It is noted that making the Ag as thin as possible while maintaining its good optical and electrical properties is of vital importance to improve the performance of Ag thin‐film electrodes.

In this chapter, besides a simple review of interfacial layers and transparent electrodes, we would like to introduce two efficient modifiers of ZnO and ultrathin Ca films, and two potential ITO‐free electrodes of AZO and ultrathin Ag film in IOSCs or OSCs based on poly (3‐hexylth‐ iophene‐2,5‐diyl):[6,6]‐phenyl C61 butyric acid methyl ester (P3HT:PCBM) blend. Here, not only the optimization of device parameters, low‐temperature process, flexible device, and air stability; but also the energy levels alignment, interface charge transport, metal film growth, light‐soaking issue [15], and the underlying mechanism would be investigated. First, an aqueous solution method using low temperature is adopted to deposit a ZnO interlayer in IOSCs. The results show that the transition point of ZnO annealing temperature is approxi‐ mately 80°C. When the temperature is above 80°C, the corresponding IOSCs show senior photovoltaic performance with PCEs over 3.5%, and the flexible devices based on poly(ethyl‐ ene terephthalate) (PET) substrates also display a PCE of 3.26% as well as a good flexibility. Second, ITO‐free IOSCs based on AZO substrates and ultrathin Ca modifier are studied by optimizing the device parameters and discussing the unexpected light‐soaking issue in IOSCs. The results show that IOSCs with an ultrathin Ca modifier (~1 nm) could achieve a senior PCE above 3% and the highly efficient electron transport at AZO/Ca/organic interfaces, which obviously weakens the light soaking issue. Third, by introducing a MoO3 interlayer for Ag film electrode growth, ITO‐free OSCs with a MoO3 (2 nm)/Ag (9 nm) anode show an improved PCE of 2.71%, and the corresponding flexible device also achieves a comparable PCE of 2.50% to that of ITO‐based reference OSCs.
