About this Research Topic
Photovoltaic (PV) devices directly convert sunlight into electricity, which offer a practical and sustainable solution to address the challenge of ever-increasing energy demand in a clean way. Intensive research is being conducted to search for high efficiency solar cells with low-cost fabrication. Currently, PV devices based on various inorganic materials including silicon (Si), III-V group semiconductors, CdTe and CIGS dominate the entire market. However, partially due to the high production cost and related environmental issues, conventional PV technology raises obvious constraints on the further manufacturing capacity of scale-up and system-cost and their wide adoption. In recent years, there has been growing interest in emerging polymer-based PV technologies owing to their synthetic variability, low-temperature processing, and the possibility of producing lightweight, flexible, easily manufactured, and inexpensive solar cells.
Since the first efficient bulk heterojunction (BHJ) polymer solar cells were independently realized by the groups of Heeger and Friend in 1995 in polymer:fullerene and polymer:polymer blends, polymer solar cells have dominated the field of high-efficiency organic solar cells. However, fullerene-based acceptors have intrinsic limitations including limited tunability of energy levels, weak absorption ability, high synthetic cost as well as morphological instability, which limit future applications in a living environment. The aforesaid drawbacks have motivated the design of novel n-type molecules or polymers as acceptors. For efficient solar cells, the absorption spectrum of the photoactive polymer layer should match the solar emission spectrum and the layer should be sufficiently thick to absorb all incident light. Meanwhile, the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor (polymer) and highest occupied molecular orbital (HOMO) of the acceptor should be well-matched for optimizing driving force for the dissociation of Frenkel excitons. In addition, the charge transport through polymer phase to the charge extraction layer should be fast and efficient. Therefore, breakthroughs in molecular design for novel materials are highly desirable for improving light utility, charge dissociation, and charge transport for polymer solar cells.
The bulk heterojunction microstructure (the donor:acceptor phase separation) also plays a critical role in achieving proper charge transport channels for collecting the electrons and holes. However, it is difficult to well-control the crystallization and phase separation behavior in a polymer blend system owing to the significantly reduced entropic contributions, rigidity characteristics of polymers and the entanglement among polymer chains. As a result, it is urgent to control the microstructure precisely and establish the correlation between morphological structure and photophysical process of solar cells.
This Research Topic will highlight the important aspects in the research of emerging polymer solar cells from a fundamental research point of view, ongoing challenges, and opportunities in the field. The Research Topic is intended to cover the molecular design of novel materials and microstructure control of the BHJ layer for high-performance polymer solar cells. Themes to be explored here include, but are not limited to:
- Molecular design and synthesis for the novel materials for polymer solar cells.
- Fundamental aspects of condensed state physics and photophysical process in conjugated polymer blend system.
- Establishing the relationship between molecular structure, morphology and device performance.
- Developing thick film polymer solar cells compatibility to the roll-to-roll printing process.
Keywords: solar cells, efficiency, synthesis, microstructure
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