Xiangjian Wan

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.

Small Molecules Based on Benzo(1,2-b:4,5-b')dithiophene Unit for High-Performance Solution-Processed Organic Solar Cells

Small molecules, namely, DCAO(3)TBDT and DR(3)TBDT, with 2-ethylhexoxy substituted BDT as the central building block and octyl cyanoacetate and 3-ethylrhodanine as different terminal units with the same linkage of dioctyltertthiophene, have been designed and synthesized. The photovoltaic properties of these two molecules as donors and fullerene derivatives as the acceptors in bulk heterojunction solar cells are studied. Among them, DR(3)TBDT shows excellent photovoltaic performance, and power conversion efficiency as high as 7.38% (certified 7.10%) under AM 1.5G irradiation (100 mW cm(-2)) has been achieved using the simple solution spin-coating fabrication process, which is the highest efficiency reported to date for any small-molecule-based solar cells. The results demonstrate that structure fine turning could cause significant performance difference and with that the performance of solution-processed small-molecule solar cells can indeed be comparable with or even surpass their polymer counterparts.

High‐Performance Solar Cells using a Solution‐Processed Small Molecule Containing Benzodithiophene Unit

A conjugated small molecule containing a benzodithiophene unit shows high performance in bulk heterojunction (BHJ) solar cells. Using the simple solution spinning process, a high PCE is achieved by employing this molecule as the donor in BHJ solar cells.

Graphene - a promising material for organic photovoltaic cells.

As a promising two-dimensional nanomaterial with outstanding electronic, optical, thermal, and mechanical properties, graphene has been proposed for many applications. In this Progress Report we summarize and discuss comprehensively the advances made so far for applications of graphene in organic photovoltaic (OPV) cells, including that for transparent electrodes, active layers and interfaces layer in OPV. It is concluded that graphene may very likely play a major role in new developments/improvements in OPVs. The future studies for this area are proposed to focus on the following: i) improving the conductivity without comprising the transparency as a transparent electrode material; ii) controlling the sheet sizes, band structure and surface morphology for use as a electron acceptor material, and iii) controlling and improving the functionalization and compatibility with other materials as interface layer material.

A perylene diimide (PDI)-based small molecule with tetrahedral configuration as a non-fullerene acceptor for organic solar cells

In this paper, a new perylene diimide (PDI)-based acceptor Me-PDI4 with tetrahedral configuration (or 3D) has been synthesized and characterized. Solution-processed organic solar cells (OSCs) based on Me-PDI4 have been investigated and our results show that the device performance can reach as high as 2.73%. Our new design with tetrahedral configuration (or 3D) could be an efficient approach to increase the PCE of OSCs with non-fullerene acceptors.

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.

Efficient solution processed bulk-heterojunction solar cells based a donor–acceptor oligothiophene

A solution processed small molecule with low band gap, dicyanovinyl-substituted oligothiophene (DCN7T), was used as the donor in bulk heterojunctions solar cells and a power conversion efficiency of 2.45% has been obtained for the device using simply spin-coating without special treatment.

Efficient small molecule bulk heterojunction solar cells with high fill factors via introduction of π-stacking moieties as end group

Three new oligothiophene derivatives with an acceptor–donor–acceptor structure incorporating 1,3-indanedione or the derivative of 1,3-indanedione units as the terminal acceptor groups—DIN7T, DINCN7T and DDIN7T—have been designed and synthesized for solution-processable small molecule BHJ solar cells. The impacts of these different end dye moieties on the optical absorption, solubility, electrochemical properties, morphology, mobility and solar cell performance were studied. All three compounds exhibit broad and highly efficient solar absorption with a low bandgap. The DIN7T-based BHJ solar cell device achieved a PCE of 4.93% and a high fill factor of 0.72, under illumination of AM 1.5, 100 mW cm−2.

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.

Photoconductivity of Bulk-Film-Based Graphene Sheets

Time-resolved photoconductivity measurements are carried out on graphene films prepared by using soluble graphene oxide. High photocurrent generation efficiency is observed for these graphene-based films, and the relationships between their photoconductivity and different preparation methods, incident light intensity, external electric field, and photon energies are investigated. Higher photoconductivity is observed with higher photon energy at same incident light intensity. By fitting the experimental data to the Onsager model, the primary quantum yields for charge separation to generate bound electron-hole pairs and the initial ion-pair thermalization separation distance are calculated.