**2. ITO-based OSCs with low-temperature ZnO interfacial layer**

In this section, IOSCs based on commonly used ITO electrodes and low‐temperature solution‐ processed ZnO interfacial layer are mainly investigated.

### **2.1. Device fabrication**

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

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

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

the future.

162 Nanostructured Solar Cells

PCE for IOSCs.

The aqueous precursor solution used for ZnO production is prepared as follows: ZnO powder (99.9%, particle size <5 μm, Sigma‐Aldrich) was dissolved in ammonia (25%, Tianjin Chemical Reagent) to form 0.1 M Zn(NH3)4 2+ solution; then, the solution was ultrasonically processed for 5 min and refrigerated for more than 12 h before use. P3HT and PCBM were purchased from Rieke Metals and Nano‐C, respectively. The commercial ITO‐coated glass substrate (Zhuhai Kaivo) has a sheet resistance below 10 Ω/square. While the ITO‐coated PET substrates show a relatively large resistance (60 Ω/square) and a thickness of 0.15 mm. All the materials are directly used in the device fabrication without any further purification.

**Figure 1(a)** shows the schematic structure of glass/ITO/ZnO/P3HT:PCBM/MoO3/Ag for fabricated IOSCs, and the device process was as following: ITO‐coated substrates (glass or PET) were ultrasonically cleaned with detergent (Decon 90), deionized water, acetone, and ethyl ethanol, and deionized water for 15–20 min, respectively. Then, the ZnO precursor solution was spin‐coated on a nitrogen‐dried ITO‐coated substrates at 3000 rpm for 40 s and then annealed in an oven at 50, 70, 80, 100, 130, and 150°C for 30 min or 1 h. The deposited ZnO interlayer has a thickness approximately 10 nm. Next, the P3HT:PCBM (1:0.8 wt% in 1,2‐ dichlorobenzene) solution was spin‐coated on ZnO at 1000 rpm for 60 s in a nitrogen‐filled glove box. After 150°C preannealing in nitrogen for 10 min, the obtained active layer has a thickness around 100 nm. Finally, the MoO3 (8 nm)/Ag (100 nm) anode was thermally evapo‐ rated through a shadow mask and the resulted devices have an active area of 10 mm2 .

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

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