Dithieno[3,2-b:2',3'-d]pyridin-5(4H)-one based D-A type copolymers with wide bandgaps of up to 2.05 eV to achieve solar cell efficiencies of up to 7.33

Asymmetrical Ladder-Type Donor-Induced Polar Small Molecule Acceptor to Promote Fill Factors Approaching 77% for High-Performance Nonfullerene Polymer Solar Cells

In this work, an effectual strategy of constructing polar small molecule acceptors (SMAs) to promote fill factor (FF) of nonfullerene polymer solar cells (PSCs) is first reported. Three asymmetrical SMAs of IDT6CN, IDT6CN-Th, and IDT6CN-M, which own large dipole moments, are designed and synthesized. The PSCs based on three polar SMAs exhibit apparently higher FFs compared with their symmetrical analogues. The asymmetrical design strategy accompanied with side chain and end group engineering makes IDT6CN-Th- and IDT6CN-M-based nonfullerene PSCs achieve high power conversion efficiency with FFs approaching 77%.

Fine-Tuning of Molecular Packing and Energy Level through Methyl Substitution Enabling Excellent Small Molecule Acceptors for Nonfullerene Polymer Solar Cells with Efficiency up to 12.54

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520-740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (J_SC ) of 18.41 mA cm^-2 , significantly higher than that of the device based on J71:ITCPTC (11.63% with a J_SC of 17.52 mA cm^-2 ). The higher J_SC of the PSC based on J71:MeIC can be attributed to more balanced μ_h /μ_e , higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

Thiophene-Based Organic Semiconductors

Thiophene-based π-conjugated organic small molecules and polymers are the research subject of significant current interest owing to their potential use as organic semiconductors in material chemistry. Despite simple and similar molecular structures, the hitherto reported properties of thiophene-based organic semiconductors are rather diverse. Design of high performance organic semiconducting materials requires a thorough understanding of inter- and intra-molecular interactions, solid-state packing, and the influence of both factors on the charge carrier transport. In this chapter, thiophene-based organic semiconductors, which are classified in terms of their chemical structures and their structure-property relationships, are addressed for the potential applications as organic photovoltaics (OPVs), organic field-effect transistors (OFETs) and organic light emitting diodes (OLEDs).

A carbon-oxygen-bridged ladder-type building block for efficient donor and acceptor materials used in organic solar cells

A carbon-oxygen-bridged ladder-type donor unit (CO5) was invented and prepared via an "intramolecular demethanolization cyclization" approach. Its single crystal structure indicates enhanced planarity compared with the carbon-bridged analogue indacenodithiophene (IDT). Owing to the stronger electron-donating capability of CO5 than IDT, CO5-based donor and acceptor materials show narrower bandgaps. A donor-acceptor (D-A) copolymer donor (PCO5TPD) and an A-D-A nonfullerene acceptor (CO5IC) demonstrated higher performance than IDT-based counterparts, PIDTTPD and IDTIC, respectively. The better performance of CO5-based materials results from their stronger light-harvesting capability and higher charge-carrier mobilities.

Side-Chain Effects on Energy-Level Modulation and Device Performance of Organic Semiconductor Acceptors in Organic Solar Cells

Two new non-fullerene acceptors, IDTC and IDTO, were designed and synthesized for the application in organic solar cells (OSCs). Compared with IDTC, the introduction of electron-donating alkoxy groups of IDTO leads to a higher LUMO level with a slightly blue-shifted absorption. Using the polymer PBDB-T as donor and the two small molecules as acceptors in the conventional device structure, the IDTC-based OSC exhibits a power conversion efficiency (PCE) of 9.35% with an open-circuit voltage (V_OC) of 0.917 V, a short-circuit current density (J_SC) of 16.56 mA cm^-2, and a fill factor (FF) of 61.61%. For the OSC based on IDTO, a higher PCE of 10.02% with a V_OC of 0.943 V, a J_SC of 16.25 mA cm^-2, and an FF of 65.41% are obtained. The more balanced μ_e/μ_h, evident aggregation, and phase separation contribute to the higher FF for the device based on IDTO. The increased J_SC for the device based on PBDB-T:IDTC can be attributed to the red-shifted and stronger absorption of the PBDB-T:IDTC blend film. These results indicate fine-tuning the electronic energy and absorption of non-fullerene acceptors is feasible to improve the performance of OSCs.

Synthesis and optical and electrochemical properties of a phenanthrodithiophene (fused-bibenzo[c]thiophene) derivative

We designed and developed a fused-bibenzo[c]thiophene, namely, 2,9-bis(tert-butyldimethylsilyl)phenanthro[9,8-bc:10,1-b'c']dithiophene (PHDT-Si), as a new π-building block in the emitters, photosensitizers and semiconductors for organic optoelectronic devices. Based on photophysical (photoabsorption, fluorescence and time-resolved fluorescence spectroscopy) and electrochemical measurements (cyclic voltammetry), and density functional theory (DFT) calculations, this work reveals that the fused-bibenzo[c]thiophene PHDT-Si, which is prepared by an efficient synthesis method, has a rigid, high planar and expanded π-conjugation structure, and possesses intense photoabsorption and fluorescence properties (λ = 598 nm (ε_max = 41 000 M^-1 cm^-1) and λ = 613 nm (Φ_f = 0.74) in toluene) in the long-wavelength region and undergoes an electrochemically reversible oxidation process, compared to non-fused 1,1'-bis(tert-butyldimethylsilyl)-4,4'-bibenzo[c]thiophene (BBT-Si).

Recent Advances in Wide-Bandgap Photovoltaic Polymers

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

A facilely synthesized lactam acceptor unit for high-performance polymer donors

A lactam acceptor unit BDTP_i was developed. PThBDTP_i:PC₇₁BM solar cells gave a PCE of 8.11%, indicating that BDTP_i is a promising building block for D–A copolymers.