Efficient Ternary Organic Solar Cells with a New Electron Acceptor Based on 3,4-(2,2-Dihexylpropylenedioxy)thiophene

In this work, a ternary blend strategy based on PBDB-T and two small molecular acceptors (IDTT-OB and IDT-PDOT-C6) is demonstrated to simultaneously improve the photocurrent and reduce the voltage loss in organic solar cells (OSCs). The improved photocurrent is partially due to a broad absorption spectrum of the active layer. In addition, we find that the ternary system possesses a higher degree of crystallinity, smaller domain size, higher domain purity, and higher and more balanced charge-carrier mobilities in comparison with the two corresponding binary systems. The reduced voltage loss in the ternary device is mainly due to a lower energy loss (E_loss) of charge carriers. We achieve a E_loss of only 0.50 eV, which is one of the lowest values reported for the ternary nonfullerene OSCs. Our results have demonstrated that all photovoltaic parameters of ternary OSCs can be simultaneously improved by elaborately selecting the three active layer components.

Thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers for organic solar cells

Thieno[3,4-c]pyrrole-4,6-dione (TPD) based conjugated polymers as an electron donor, acceptor and single-component for application in organic solar cells in the past ten years have been intensively reviewed in this Feature Article.

Hole Transfer Originating from Weakly Bound Exciton Dissociation in Acceptor-Donor-Acceptor Nonfullerene Organic Solar Cells

The underlying hole-transfer mechanism in high-efficiency OSC bulk heterojunctions based on acceptor-donor-acceptor (A-D-A) nonfullerene acceptors (NFAs) remains unclear. Herein, we study the hole-transfer process between copolymer donor J91 and five A-D-A NFAs with different highest occupied molecular orbital energy offsets (ΔE_H) (0.05-0.42 eV) via ultrafast optical spectroscopies. Transient absorption spectra reveal a rapid hole-transfer rate with small ΔE_H, suggesting that a large energy offset is not required to overcome the exciton binding energy. Capacitance-frequency spectra and time-resolved photoluminescence spectra confirm the delocalization of an A-D-A-structured acceptor exciton with weak binding energy. Relative to the hole-transfer rate, hole-transfer efficiency is the key factor affecting device performance. We propose that holes primarily stem from weakly bound acceptor exciton dissociation, revealing a new insight into the hole-transfer process in A-D-A NFA-based OSCs.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.