Preface

Photovoltaics covers an extremely wide range of different fields of science and technology that are in a state of continuous development and improvement for decades. Solar cells and models that have been developed to the level of industrial production or prototype samples, as well as the devices of exploratory types are divided into the so-called generations of photovoltaics. Chapters, which concern the problems of the first, second and third generations of solar cells are included in the relevant three books of this edition. Chapters that are general in nature or not related specifically to these generations, some novel scientific ideas and technical solutions, which has not properly approved, new methods of research and testing of solar cells and modules have been collected in the fourth book of the four-volume edition of "Solar cells". General issues of the efficiency of a direct conversion of solar radiation into electrical energy in solar cell and through hydrogen production in photoelectrochemical solar cell are discussed in several chapters of the book. Considerable attention is paid to the quantum-size effects in solar cells both in general and on specific examples of AlGaAs superlattices, CdSe quantum dots, etc. New materials, such as cuprous oxide as an active material for solar cells, AlSb for use as an absorber layer in p-i-n junction solar cells, InGaAsN as a promising material for high efficiency multi-junction tandem solar cells, InP in solar cells with semiconductorinsulator-semiconductor structures are discussed in several chapters. Other chapters are devoted to the analysis of both status and perspective of organic photovoltaics as well as specific issues, such as polymer/fullerene solar cells, poly(p-phenylenevinylene) derivatives, photovoltaic textiles, photovoltaic fibers, etc.

It appears that the fourth book of the edition of "Solar Cells" will find many interested readers.

The editor addresses special thanks to the contributors for their initiative and high quality work, and to the technical editors that conveyed the text into a qualitative and pleasant presentation.

> **Professor, Doctor of Sciences, Leonid A. Kosyachenko**  National University of Chernivtsi Ukraine

**1** 

*1China 2Singapore*

**Effects of Optical Interference and Annealing on** 

Polymer solar cells are of tremendous interests due to their attractive properties such as flexibility, ease of fabrication, low materials and energy budget. However, organic materials have short exciton diffusion length and poor charge mobility, which can greatly decrease the performance of polymer solar cells. These challenges can be effectively overcome through the use of the bulk heterojunction (HJ) structure because it can guarantee the effective exciton dissociation and carrier transport simultaneously if a proper bicontinuous interpenetrating network is formed in the active layer. Based on this structure, the

The performance of a polymer solar cell is mainly determined by the short-circuit current density (*JSC*), the open circuit voltage (*VOC*), and the fill factor (*FF*), given that *η=JSCVOCFF/Pin* (where *η* is power conversion efficiency, *PCE*, and *Pin* is the incident optical power density). *VOC* has a direct relationship with the offset energies between the highest occupied molecular orbital of Donor (*D*) material and the lowest unoccupied molecular orbital of Acceptor (*A*) material (Cheyns et al., 2008). Since the *D* and *A* materials are intimately mixed together in the bulk HJ structure and their interfaces distribute everywhere in the active layer, it is difficult to increase *VOC* by changing *D*/*A* interface property for a given material system (such as poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester,

performance of polymer solar cells has been improved steadily in the past decade.

P3HT:PCBM). Thus the usually used optimization method is to improve *JSC* and *FF*.

needs to be well optimized according to the optical interference.

*JSC* greatly depends on the optical interference effect in polymer solar cells. Because of the very high optical absorption ability of organic materials, the active layer is very thin and typically from several ten to several hundred nanometers. This thickness is so thin compared to the incident light wavelength that the optical interference effect has to be carefully considered. Depending on the thicknesses and optical constants of the materials, the optical interference causes distinct distributions of the electric field and energy absorption density. Due to this effect, *JSC* shows an obvious oscillatory behavior with the variation of active layer thickness. In order to gain a high *PCE*, the active layer thickness

**1. Introduction** 

**the Performance of Polymer/Fullerene Bulk** 

**Heterojunction Solar Cells** 

Chunfu Zhang1, Hailong You1, Yue Hao1,

Zhenhua Lin2 and Chunxiang Zhu2 *1School Of Microelectronics, Xidian University,* 

*2ECE, National University of Singapore,* 
