There are two important interfaces in HSCs, i. e., the polymer/inorganic nanocrystal interface where the charge separation takes place and the active layer/electrode interface where the free charges are collected. Therefore, their properties are crucial to the device performance. Efficient interface modification could not only facilitate the charge transport but also retard the backward recombination and remarkably improve the conversion efficiencies of the solar cells [61, 62, 65, 104–119].
One of the most important work on interface modification was reported by Goh et al. in 2007 , who systematically studied the effect of interface modification in TiO2/P3HT-based HSCs using different types of modifiers as shown in Fig. 9.15, and the corresponding device physics was also provided by the authors. They concluded that there were two kinds of dipoles existed at the TiO2/P3HT interface when the interface modifiers were introduced; namely, the molecular dipole and the interfacial dipole that generated by the interaction between the carboxylic group and TiO2. They both could lead to the TiO2 band edge shift, and thus affect the Voc of the device. A nearly 2-fold improvement of the PCE was achieved by using Ruthenium dye which could mediate the interface charge transfer and slow down the recombination kinetics.
Fig. 9.15 a Schematic of the interface modification of the bilayer TiO2/polymer solar cell. The table lists the calculated dipole of the benzoic acid with different R group. b molecular structures of benzene carboxylic molecules. c molecular structure of Ruthenium dyes. Reproduced with permission from Ref. 
Interface modification was also reported to affect the crystallinity of the interfacial P3HT layer in a bilayer ZnO/P3HT device. The crystallinity of P3HT was decreased when it was casted on ZnO surface as identified by the blue shift (50 nm) of its absorption peaks. Upon modification by alkanethiol, the crystallinity of P3HT was recovered and enhanced Jsc was observed since the more ordered P3HT has broader absorption spectrum and higher hole mobility . Similar effects were also reported for the Ruthenium dye .
Ruthenium dyes are most commonly used interface modifiers. However, the surface of inorganic acceptors cannot be fully covered by single dye molecules due to their large molecular size. Tai et al.  were able to solve this problem by using a small molecule 3-phenylpropionic acid (PPA) as a co-modifier which could cover the voids that were inaccessible for Ruthenium dye (see Fig. 9.16) and improved performance was achieved for the co-modified device due to the better passivation of the backward recombination kinetics.
It is now a standard procedure to introduce the electron blocking layer (dense TiO2, ZnO, etc.) and/or hole blocking layer (typically PEDOT) in HSCs for more efficient charge collection, which both fall into the category of the interface modification (the active layer/electrode interface). Besides, Takanezawa et al.  used VOx as a buffer layer between active layer and Ag electrode to prevent the recombination at the organic/Ag interface and the photovoltaic performance of the device was greatly enhanced accordingly. Qian et al.  demonstrated the use of solution-processed ZnO nanoparticle buffer layer in HSCs based on the blend of CdSe quantum dots and P3HT. Upon ZnO modification, the PCEs of the devices were increased by 30-80 %. More interestingly, the stability of the device was also dramatically improved.
Polymer/inorganic HSC has grown to be an important member of the new generation of photovoltaics. An increasing attention has been paid to HSCs in the past years for their potential high efficiencies and low fabrication costs. Many efforts have been made toward the improvement of device performance, including the optimization of the device architecture and the active layer morphologies, interface engineering and design of new materials, etc. Although the current PCEs of HSCs are still lower than conventional silicon solar cells, DSSCs, and BHJ OSCs, if looking back upon the
history of the development of the HSCs, we could find that the PCEs of HSCs have increased all the way. Despite the slow increasing rate of PCE, the trend is obvious and much higher efficiencies are expected upon the further understanding of the device physics and the development of nanofabrication techniques.
 Ri-1,iR i—1 ,i—2