Xiaowei Zhan

Rylene and Related Diimides for Organic Electronics

Organic electron-transporting materials are essential for the fabrication of organic p-n junctions, photovoltaic cells, n-channel field-effect transistors, and complementary logic circuits. Rylene diimides are a robust, versatile class of polycyclic aromatic electron-transport materials with excellent thermal and oxidative stability, high electron affinities, and, in many cases, high electron mobilities; they are, therefore, promising candidates for a variety of organic electronics applications. In this review, recent developments in the area of high-electron-mobility diimides based on rylenes and related aromatic cores, particularly perylene- and naphthalene-diimide-based small molecules and polymers, for application in high-performance organic field-effect transistors and photovoltaic cells are summarized and analyzed.

An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells

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 efficiencies of up to 6.8%, a record for fullerene-free PSCs.

n‐Type Organic Semiconductors in Organic Electronics

Organic semiconductors have been the subject of intensive academic and commercial interest over the past two decades, and successful commercial devices incorporating them are slowly beginning to enter the market. Much of the focus has been on the development of hole transporting, or p-type, semiconductors that have seen a dramatic rise in performance over the last decade. Much less attention has been devoted to electron transporting, or so called n-type, materials, and in this paper we focus upon recent developments in several classes of n-type materials and the design guidelines used to develop them.

Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole

Aggregation greatly boosts emission efficiency of the silole, turning it from a weak luminophor into a strong emitter.

Non-fullerene acceptors for organic photovoltaics: an emerging horizon

Although fullerenes and their derivatives, such as PCBM, have been the dominant electron-acceptor materials in organic photovoltaic cells (OPVs), they suffer from some disadvantages, such as weak absorption in the visible spectral region, limited spectral breadth and difficulty in variably tuning the band gap. It is necessary to explore non-fullerene electron acceptors that will not only retain the favorable electron-accepting and transporting properties of fullerenes but also overcome their insufficiencies. After a decade of mediocrity, non-fullerene acceptors are undergoing rapid development and are emerging as a hot area of focus in the field of organic semiconductors. Solution-processed bulk heterojunction (BHJ) OPVs based on non-fullerene acceptors have shown encouraging power conversion efficiencies of over 4%. This article reviews recent developments in several classes of solution-processable non-fullerene acceptors for BHJ OPVs. The remaining problems and challenges along with the key research directions in the near future are discussed.

High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics

We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = -5.66 eV, LUMO = -3.93 eV; ITIC: HOMO = -5.48 eV, LUMO = -3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-band-gap polymer donors and wide-band-gap polymer donors. ITIC-Th has higher electron mobility (6.1 × 10(-4) cm(2) V(-1) s(-1)) than ITIC (2.6 × 10(-4) cm(2) V(-1) s(-1)) due to enhanced intermolecular interaction induced by sulfur-sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-band-gap and wide-band-gap polymer donors. In one case, a power conversion efficiency of 9.6% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.

A Facile Planar Fused-Ring Electron Acceptor for As-Cast Polymer Solar Cells with 8.71% Efficiency

A planar fused-ring electron acceptor (IC-C6IDT-IC) based on indacenodithiophene is designed and synthesized. IC-C6IDT-IC shows strong absorption in 500-800 nm with extinction coefficient of up to 2.4 × 10(5) M(-1) cm(-1) and high electron mobility of 1.1 × 10(-3) cm(2) V(-1) s(-1). The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibit power conversion efficiencies of up to 8.71%.

High-performance fullerene-free polymer solar cells with 6.31% efficiency

A nonfullerene electron acceptor (IEIC) based on indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile was designed and synthesized. IEIC exhibited good thermal stability, strong absorption in the 500–750 nm region with an extinction coefficient of 1.1 × 105 M−1 cm−1 at 672 nm, deep LUMO energy level (−3.82 eV) close to those of fullerenes, and a relatively high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. Fullerene-free polymer solar cells (PSCs) based on the blends of the IEIC acceptor and a low-bandgap polymer donor PTB7-TH, using a perylene diimide derivative as a cathode interlayer, showed power conversion efficiencies (PCEs) of up to 6.31%, which is among the best PCEs reported for fullerene-free PSCs.

A Solution-Processable Star-Shaped Molecule for High-Performance Organic Solar Cells

Organic solar cells (OSCs) exhibit tremendous potential in the world’s energy strategy due to their predominant advantages such as low cost, light weight, and large-area fabrication on fl exible substrates. [ 1–4 ] Small molecule semiconductors for bulk heterojunction (BHJ) OSCs are attractive because of their advantages over their polymer counterparts, which include welldefi ned molecular structure, defi nite molecular weight, and high purity without batch to batch variations. [ 5–7 ] In recent years, a great amount of effort has been dedicated to develop this class of materials, including dendritic oligothiophenes, [ 8–10 ] staror X-shaped molecules, [ 11–15 ] linear analogs with donor–acceptor– donor (D–A–D) structures, [ 16–21 ] fused polycyclic arene, [ 22 ] and other organic dyes. [ 23–25 ] However, the highest power conversion effi ciencies (PCEs) reported for small molecule/fullerene derivative BHJ OSCs were only ≈ 3–4.4%, [ 5–7 ] lower than that ( ≈ 6–8%) of polymer/fullerene derivative BHJ solar cells. [ 1–4 ] The lower effi ciency of small molecul BHJ OSCs became the biggest hindrance to their application. Triphenylamine (TPA) has been regarded as a promising unit for effi cient photovoltaic materials due to its good electrondonating and high hole-transporting capabilities. [ 26 ] Benefi ting from its special propeller starburst molecular structure, amorphous materials with isotropic optical and charge-transporting properties could be expected when combining TPA with linear π -conjugated systems. Solution-processable TPA-based small molecules have been widely investigated for application in OSCs with PCEs below 3%. [ 12–15 , 27–29 ] On the other hand, solutionprocessed BHJ OSCs based on oligothiophenes exhibited PCEs as high as 3.7%. [ 8–11 , 30 ] Here, we report the synthesis and characterization of a new 3D, star-shaped, D–A–D organic small molecule with TPA as the core and donor unit, benzothiadiazole as the bridge and acceptor unit, and oligothiophene as the arm

Single‐Junction Binary‐Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push-pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC ). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71 BM (PCE = 5.22%).