A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings of fullerene acceptors, for example, weak absorption in the visible spectral region and limited energy-level variability, is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion effi ciencies of up to 6.8%, a record for fullerene-free PSCs.

An electron acceptor challenging fullerenes for efficient polymer solar cells.

Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

https://science.sciencemag.org/content/sci/early/2018/08/08/science.aat2612.full.pdf

Organic and solution-processed tandem solar cells with 17.3% efficiency

18% Efficiency organic solar cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within t...

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

With the power conversion efficiency of polymer solar cells (PSCs) approaching the milestone value of 10%, their instability associated with a low work function metal cathode, particularly in the presence of oxygen and moisture, becomes a critical issue for real applications. To alleviate the air-sensitive problem, two easy-accessible solution-processed, environmentally friendly organic small-molecule cathode interlayers, with perylene diimides (PDI) as the core and amino (PDIN) or amino N-oxide (PDINO) as the terminal substituent, are explored. Benefitting from the extended planar structure of the PDI units, the two interlayer materials show high conductivities of ∼10−5 S cm−1, which make them capable of functioning efficiently in a wide thickness range of 6 to 25 nm. This is the first time that thickness-insensitive small-molecule-based cathode interlayers are reported. It is also found that the work function tuning effect of the two PDI-based interlayers allows high work function metals (such as Au and Ag) to act as the cathode. With the conventional device structure with PTB7 as a donor and PC70BM as an acceptor, the PDINO-based devices exhibit an efficiency of 8.24% with Al as the top electrode and 8.16% with Ag as the top electrode, much higher than that of the corresponding Ca/Al-based device (6.98%). The high efficiency of 8.35% is also achieved in the device with PTB7-Th as the donor. The success of the two PDI-interlayers indicates that π-delocalized planar structures with high electron affinities could be particularly useful in developing high-performance organic interlayer materials.

Perylene diimides: a thickness-insensitive cathode interlayer for high performance polymer solar cells

With the rapid evolution of photovoltaic polymer materials, power conversion efficiency of polymer solar cells has been markedly improved in recent years, and is now approaching a landmark value of 10 %. This review focuses on Donor–Acceptor (D–A) photovoltaic copolymers. Starting from briefly introducing the D–A concept, the fundamental donor and acceptor units for constructing polymer photovoltaic materials are introduced and classified. By summarizing the structure–property relationships of typical photovoltaic D–A copolymers, the important design rules for such materials are highlighted. Several crucial aspects, including proper combination of D–A units, high planarity of the backbone and proper incorporation of side chains are particularly emphasized. A new D–A architecture, namely main-chain donor and side-chain acceptor is introduced and reviewed. Moreover, the role of the electron-deficient group in fine-tuning energy levels of low-bandgap D–A photovoltaic polymers is discussed.

Structures and properties of conjugated Donor–Acceptor copolymers for solar cell applications

Thermostable single-junction organic solar cells with a power conversion efficiency of 14.62%

Open-circuit voltage (VOC) is the maximum voltage a solar cell can provide to an external circuit, which is derived from the splitting of hole and electron quasi-Fermi levels. In crystalline Si solar cells, the effective density of states at the bottom (top) of the conduction (valence) band is constant, and the quasi-Fermi level can be directly calculated via the Fermi–Dirac distribution. However, in organic materials, similar to amorphous Si, disorder induces gap tail states. Relaxation of carriers into these tail states brings the electron quasi-Fermi level down and the hole quasi-Fermi level up, and hence reduces VOC. Furthermore, carrier recombination of various kinds can cause additional loss of VOC. This article reviews the research progress in understanding the origin of VOC in organic solar cells. In particular, the dependence of VOC on four important factors, namely temperature, light intensity, work function of the electrode and material microstructure are discussed based on the model of density of states. Techniques to enhance VOC are also briefly introduced and their mechanisms are analysed.

Open-circuit voltage in organic solar cells

Graphdiyne (GD), a novel carbon allotrope with a 2D structure comprising benzene rings and carbon-carbon triple bonds, is successfully integrated with ZnO nanoparticles by a wet chemistry method. An ultraviolet photodetector based on these graphdiyne:ZnO nanocomposites exhibits significantly enhanced performance in comparison with a conventional ZnO device. GD may have diverse applications in future optoelectronics.

Jizheng Wang

Papers

Perylene diimides: a thickness-insensitive cathode interlayer for high performance polymer solar cells

Structures and properties of conjugated Donor–Acceptor copolymers for solar cell applications

Thermostable single-junction organic solar cells with a power conversion efficiency of 14.62%

Open-circuit voltage in organic solar cells

Graphdiyne:ZnO Nanocomposites for High-Performance UV Photodetectors.