Understanding Solvent Manipulation of Morphology in Bulk-Heterojunction Organic Solar Cells

Constructing Multiscale Fibrous Morphology to Achieve 20% Efficiency Organic Solar Cells by Mixing High and Low Molecular Weight D18

This study underscores the significance of precisely manipulating the morphology of the active layer in organic solar cells (OSCs). By blending polymer donors of D18 with varying molecular weights, a multiscale interpenetrating fiber network structure within the active layer is successfully created. The introduction of 10% low molecular weight D18 (LW-D18) into high molecular weight D18 (HW-D18) produces MIX-D18, which exhibits an extended exciton diffusion distance and orderly molecular stacking. Devices utilizing MIX-D18 demonstrate superior electron and hole transport, improves exciton dissociation, enhances charge collection efficiency, and reduces trap-assisted recombination compared to the other two materials. Through the use of the nonfullerene acceptor L8-BO, a remarkable power conversion efficiency (PCE) of 20.0% is achieved. This methodology, which integrates the favorable attributes of high and low molecular weight polymers, opens a new avenue for enhancing the performance of OSCs.

Universal and versatile morphology engineering via hot fluorous solvent soaking for organic bulk heterojunction

After explosive growth of efficiency in organic solar cells (OSCs), achieving ideal morphology of bulk heterojunction remains crucial and challenging for advancing OSCs into consumer market. Herein, by utilizing the amphiphobic nature and temperature-dependent miscibility of fluorous solvent, hot fluorous solvent soaking method is developed to optimize the morphology with various donor/acceptor combinations including polymer/small-molecule, all-polymer and all-small-molecule systems. By immersing blend film into hot fluorous solvent which is utilized as liquid medium with better thermal conductivity, the molecular reorganization is accelerated. Furthermore, fluorous solvent can be miscible with the residue of chloroform and chloronaphthalene above upper critical solution temperature. This mixed solvent diffuses around inside the active layer and selectively promotes molecular reorganization, leading to optimized morphology. Compared to widely-used thermal annealing, this approach processed under mild conditions achieves superior photovoltaic performance, indicating the practicality and universality for morphological optimization in OSCs as well as other optoelectronic devices.

Bis(thieno[3,2-b]thieno)cyclopentafluorene-Based Acceptor with Efficient and Comparable Photovoltaic Performance under Various Processing Conditions

The morphology of a bulk heterojunction (BHJ) blend within a polymer solar cell (PSC) device plays a crucial role in its performance. The ideal morphology is generally achieved through molecular engineering and optimization under film processing conditions. Under different processing conditions, the deviation of the resulted morphology characteristics from the ideal one leads to the dispersion of device performance. For a specific donor/acceptor BHJ blend, it is of great challenge to maintain an efficient and comparable photovoltaic performance under various processing conditions. The solution to this challenge would be of great value in offering more choices for a suitable processing technology in practical applications. Based on the acceptor BTTFIC with the core of bis(thieno[3,2-b]thieno)cyclopentafluorene (BTTF) in our previous work, we chemically modified BTTFIC by fluorination of the end groups of 1,1-dicyanomethylene-3-indanones (IC) and the switching part of octyls in BTTF with 4-hexylphenyls to offer a novel acceptor (BTTFIC4F-Ar). The inverted PBDB-T-2Cl:BTTFIC4F-Ar blend device provided an average power conversion efficiency (PCE) of 10.61, 11.08, and 11.55% when processed under solvent annealing (SA), thermal annealing (TA), and additive treatment with 1,8-diodooctane (DIO), respectively. Different from the reported discrete performance under various processing conditions for a specific donor/acceptor BHJ blend, a low mean absolute performance deviation of 3% was attained. This slight enhancement trend was unexceptionally reflected on charge generation, transportation, and recombination within the blend films from SA, TA, and DIO conditions. A slightly improved ordering of BTTFIC4F-Ar within the DIO blend was observed. Meanwhile, very similar molecular packings as well as a close amorphous domain size of the mixture of PBDB-T-2Cl and BTTFIC4F-Ar within the three blends were observed. These morphological characteristics are in good agreement with the photoelectrical conversion performance of the blends under the three processing conditions. Furthermore, similar attenuation behaviors in performance were also observed. This investigation may provide new guidance on the molecular engineering of nonfullerene acceptors to achieve an efficient BHJ blend with more options for a suitable and cost-effective processing method in practical applications.

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.

Note: Setup for chemical atmospheric control during in situ grazing incidence X-ray scattering of printed thin films

In order to tailor the assembling of polymers and organic molecules, a deeper understanding of the kinetics involved in thin film production is necessary. While post-production characterization only provides insight on the final film structure, more sophisticated experimental setups are needed to probe the structure formation processes in situ during deposition. The drying kinetics of a deposited organic thin film strongly influences the assembling process on the nanometer scale. This work presents an experimental setup that enables fine control of the atmosphere composition surrounding the sample during slot die coating, while simultaneously probing the film formation kinetics using in situ grazing incidence X-ray scattering and spectroscopy.

Synergetic Solvent Engineering of Film Nanomorphology to Enhance Planar Perylene Diimide-Based Organic Photovoltaics

Solvent additive has proven as a useful protocol for improving the film nanomorphology of polymer donor (D): fullerene acceptor (A) blends in bulk heterojunction (BHJ) photovoltaic cells. By contrast, the effect of such solvent additive on nonfullerene BHJ cells based on perylene diimide acceptor, for instance, is less effective because of their highly planar structure and strong π-aggregation in solid state. Here we choose N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) and thieno[3,4-b]thiophene-alt-benzodithiophene (PTB7) as a model D:A blend system to investigate how solvent engineering strategy synergistically impacts the blend film nanomorphology. Based on the differences of solvent volatility and solubility, various host solvents-chloroform (CF) and chlorobenzene (CB) and solvent additives-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) are selected for comparative studies. It is found that the π-aggregation of PDIs can be largely suppressed by using low-boiling point (Tb) CF solvent, yet enlarged by using high-Tb CB. Moreover, CN additive provides good solubility of PDI molecules and hence reduces large PDI aggregates in CB system, while DIO exhibiting poor solubility works oppositely. By contrast, DIO that presents larger Tb difference with CF prolongs the film-forming, which assists in optimizing the PDI aggregation and increases the intermixed PTB7:PDI phases more significantly than CN in CF system, yielding the finest phase-separation morphology and balanced charge mobility. Consequently, the inverted BHJ cells based on CF-processed PTB7:PDI blend film with 0.4 vol % DIO exhibit the highest PCE of 3.55% with a fill factor of 56%, both of which are among the best performance for such a paradigm PTB7:PDI blend-based BHJ cells.

Semiconducting end-perfluorinated P3HT–fullerenic hybrids as potential additives for P3HT/IC70BA blends

Hybrid materials based on polythiophene–fullerene species covalently attached through aziridine bridges are presented, as potential stabilizers of P3HT:IC₇₀BA active layers for BHJ devices.