Miaomiao Li

A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency

A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small‐Molecule Bulk‐Heterojunction Blends under Solvent Vapor Treatment

A series of solvents with different solubilities for DR3TBDTT and PC71 BM, and different boiling points, is used for solvent vapor annealing (SVA) treatment to systematically investigate the solvent-morphology-performance relationship. The presence of solvent molecules inside bulk-heterojunction (BHJ) thin films promotes the mobility of both donor and acceptor molecules, leading to crystallization and aggregation, which are important in modulating morphology.

Benzo[1,2-b:4,5-b′]dithiophene (BDT)-based small molecules for solution processed organic solar cells

Solution processed small molecule based solar cells have become a competitive alternative to their polymer counterparts due to the advantages of their defined structure and thus less batch to batch variation. With a large and rigid planar conjugated structure, the benzo[1,2-b:4,5-b′]dithiophene (BDT) unit has become one of the most widely used and studied building blocks for high performance small molecule based photovoltaic devices. In this review article, we review the progress made in the field of small molecules containing BDT units for solution-processed organic photovoltaic cells. Insights into several important aspects regarding the design and synthesis of BDT based small molecules are also included.

Graphene quantum dots as the hole transport layer material for high-performance organic solar cells

We present an investigation of organic photovoltaic (OPV) cells with solution-processable graphene quantum dots (GQDs) as hole transport layers (HTLs). GQDs, with uniform sizes and good conductivity, are demonstrated to be excellent HTLs in both polymer solar cells (PSCs) and small-molecule solar cells (SMSCs) with the blend of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) and small molecule DR3TBDT:[6,6]-phenyl-C71-butyric acid methyl ester (DR3TBDT:PC71M) as the active layer, respectively. The PSCs and SMSCs based on GQDs yield power conversion efficiencies of 3.51% and 6.82%, respectively, both comparable to those of solar cells with PEDOT:PSS as the HTLs. In addition, the cells with GQDs as HTLs exhibit much more reproducible performance and longer lifetime. In light of the high stability, low cost and easy processing, these results indicate that GQDs can be potentially used to replace PEDOT:PSS for producing high-performance and stable organic photovoltaic cells.

A simple small molecule as an acceptor for fullerene-free organic solar cells with efficiency near 8%

A simple small molecule acceptor named DICTF, with fluorene as the central block and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile as the end-capping groups, has been designed for fullerene-free organic solar cells. The new molecule was synthesized from widely available and inexpensive commercial materials in only three steps with a high overall yield of ∼60%. Fullerene-free organic solar cells with DICTF as the acceptor material provide a high PCE of 7.93%.

A high-performance photovoltaic small molecule developed by modifying the chemical structure and optimizing the morphology of the active layer

A small molecule (DR3TDOBDT) containing 4,8-dioctyl benzo[1,2-b:4,5-b′]dithiophene as the central block and 3-(2-ethylhexyl)-rhodanine as the end-capping groups has been designed and synthesized. A power conversion efficiency of 8.26% was achieved through the active layer morphology optimization process combining thermal annealing and solvent vapor annealing.

Effect of thermal annealing on active layer morphology and performance for small molecule bulk heterojunction organic solar cells

New molecule design and device optimization are two of the main strategies used to obtain high performance organic photovoltaics. In this paper, bulk heterojunction solar cell devices using a newly designed solution-processable small molecule (DRDTSBDTT) were systematically investigated for their J–V behavior and the morphology of the active layer under different annealing treatments to understand the impact of thermal annealing on open circuit voltage, short circuit current and fill factor. A strong relationship was found between thermal annealing and these factors. J–V behavior analysis indicates that this is because of the efficiency change of the fundamental exciton diffusion, charge separation and collection steps, which is supported by morphology studies for the active layer under different thermal treatments. The results show that for optimized performance of a given molecule, the morphology and phase control are the most important factors to achieve intrinsic best performance. With these, the power conversion efficiency was increased from 3.36% to 5.05% under optimized annealing treatment for DRDTSBDTT-based devices.

Evaluation of Small Molecules as Front Cell Donor Materials for High-Efficiency Tandem Solar Cells

Three small molecules as front cell donors for tandem cells are thoroughly evaluated and a high power conversion efficiency of 11.47% is achieved, which demonstrates that the oligomer-like small molecules offer a good choice for high-performance tandem solar cells.

Understanding the Halogenation Effects in Diketopyrrolopyrrole-Based Small Molecule Photovoltaics

Two molecules containing a central diketopyrrolopyrrole and two oligothiophene units have been designed and synthesized. Comparisons between the molecules containing terminal F (FDPP) and Cl (CDPP) atoms allowed us to evaluate the effects of halogenation on the photovoltaic properties of the small molecule organic solar cells (OSCs). The OSCs devices employing FDPP:PC71BM films showed power conversion efficiencies up to 4.32%, suggesting that fluorination is an efficient method for constructing small molecules for OSCs.

Novel donor–acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole for bulk heterojunction solar cells

Three new fluorinated D–A type conjugated polymers based on a novel building unit 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole have been synthesized through a Sitlle coupling reaction. The as-prepared polymers exhibit a narrow band gap (from 1.31 to 1.34 eV) and low lying energy levels with lowest occupied molecular orbital (LUMO) energy levels of −3.95, −3.97 and −4.15 eV, respectively. These polymers exhibit excellent solubility in common organic solvents due to the introduction of perfluorophenyl and long alkyl sidechains. The power conversion efficiency (PCE) of solar cells based on these as-prepared polymers and PC71BM could reach as high as 1.92%. Our results could provide a simple strategy for designing high performance D–A polymers based on this unit and a potential to further improve their performance.