Dan Deng

Synergistic Effect of Polymer and Small Molecules for High‐Performance Ternary Organic Solar Cells

A ternary blend system with two donors and one acceptor provides an effective route to improve the performance of organic solar cells. A synergistic effect of polymer and small molecules is observed in ternary solar cells, and the power conversion efficiency (PCE) of the ternary system (8.40%) is higher than those of binary systems based on small molecules (7.48%) or polymers (6.85%).

Acceptor End‐Capped Oligomeric Conjugated Molecules with Broadened Absorption and Enhanced Extinction Coefficients for High‐Efficiency Organic Solar Cells

Acceptor end-capping of oligomeric conjugated molecules is found to be an effective strategy for simultaneous spectral broadening, extinction coefficient enhancement, and energy level optimization, resulting in profoundly enhanced power conversion efficiencies (of 9.25% and 8.91%) compared to the original oligomers. This strategy is effective in overcoming the absorption disadvantage of oligomers and small molecules due to conjugation limitation.

Understanding the Impact of Hierarchical Nanostructure in Ternary Organic Solar Cells

Ternary organic solar cells (OSCs), which blend two donors and fullerene derivatives with different absorption ranges, are a promising potential strategy for high‐power conversion efficiencies (PCEs). In this study, inverted ternary OSCs are fabricated by blending a highly crystalline small molecule BDT‐3T‐CNCOO in a low band gap polymer PBDTTT‐C‐T:PC71BM. As the small molecule is introduced, the overall PCEs increase from 7.60% to 8.58%. The morphologies of ternary blends are studied by combining transmission electron microscopy and X‐ray scattering techniques at different length scales. Hierarchical phase separation is revealed in the ternary blend, which is composed of domains with sizes of ≈88, ≈50, and ≈20 nm, respectively. The hierarchical phase separation balances the charge separation and transport in ternary OSCs. As a result, the fill factors of the devices significantly improve from 58.4% to 71.6%. Thus, ternary blends show higher hole mobility and higher fill factor than binary blends, which demonstrates a facile strategy to increase the performance of OSCs.

A conformational locking strategy in linked-acceptor type polymers for organic solar cells

In this paper, two novel linked-acceptor type D–A conjugated polymers POP and POM have been synthesized by Stille polymerization as donor materials for polymer solar cells (PSCs). The concept of introducing intramolecular noncovalent conformational locks into the main chain has been implemented to improve the coplanarity of the linked-acceptor polymers using alkyl substituted thiophenes as π bridges. In this paper, the alkyl side chain substituted in the thiophene bridge has been replaced with an alkoxy chain, in which the oxygen atoms on the side chain and the sulfur atoms on the neighbor thiophene unit could form a coulombic interaction and expand the conjugation degree of the polymers. As a result, the new polymer POP shows better planarity, absorbance ability and processability than that without conformational locks. Although the open-circuit voltage has a small decrease due to the stronger electron-donating nature of the alkoxy group, the fill factor and the current density values have been increased and the resulting best power conversion efficiency has been increased up to 8.18%. This work will become an impactful extending work of linked-acceptor type conjugated polymers and the result suggests that this conformational locking strategy might be a very promising method for the design and construction of novel highly efficient donor materials.

Small molecules incorporating regioregular oligothiophenes and fluorinated benzothiadiazole groups for solution-processed organic solar cells†

Small molecules incorporating regioregular oligothiophenes and fluorinated benzothiadiazole groups were synthesized for solution-processed organic solar cells. The length of oligothiophene units showed profound influences on light absorption, energy levels, crystalline behaviors and active layer morphologies. The photovoltaic performances were tested using mixed-solvent processing methods. When blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layer, the shorter molecule, 4TA4T, achieved a power conversion efficiency (PCE) of 3.81%, with an open circuit voltage (Voc) of 0.83 V, a short current density (Jsc) of 8.02 mA cm−2 and a Fill Factor (FF) of 0.57. The longer molecule, 6TA6T, showed a lower PCE of 3.21% due to a lower Voc of 0.74 V. This study provides detailed insights for the designing of small molecules by subtle changes in donor lengths.

Evolution of morphology and open-circuit voltage in alloy-energy transfer coexisting ternary organic solar cells

The oligomer-type small molecule PDT2FBT-ID is applied in a polymer/small molecule/fullerene ternary system. The PTB7-Th/PDT2FBT-ID/PC71BM ternary active layer shows complementary absorption spectra, enhanced face-on orientation and energy transfer properties. Compared to the binary system, the FF of the ternary system is improved from 66.72% to 76.14% and the Jsc is improved from 17.90 mA cm−2 to 18.92 mA cm−2. The maximum PCE of 11.1% is obtained in the ternary system with a common inverted device structure. Additionally, an alloy-like domain structure and energy transfer between the two donor materials are found to coexist in the ternary system. Based on the combined model, the variation in morphology and Voc is discussed in detail and compared with previous publications. An in-depth interpretation for the selection of a third compound in high performance ternary organic solar cells is provided. Finally, this facile design strategy could also be widely used in other systems.

High open-circuit voltage ternary organic solar cells based on ICBA as acceptor and absorption-complementary donors

Ternary organic solar cells (OSCs) are fabricated with indene-C60 bisadduct (ICBA) as an electron acceptor and the low-band-gap polymer PBDTTT-C-T and the highly crystalline small molecule n-BDT-3T-CNCOO as electron donors. A high open-circuit voltage of 0.98 V is achieved, which is 0.2 V higher than that of ternary OSCs based on phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. Incorporation of n-BDT-3T-CNCOO promotes the power conversion efficiency (PCE) from 5.01% for polymer binary devices to 5.51% for ternary devices. The improved PCE is attributed to the nanofibrous morphology with enhanced crystallinity of the donors and improved aggregation of the ICBA acceptor, which facilitate charge separation and charge transport. This work reveals that the ternary strategy of blending highly crystalline small molecules enhances PCEs of OSCs based on ICBA and other non-fullerene acceptors.

Dialkoxyphenyldithiophene-based small molecules with enhanced absorption for solution processed organic solar cells

Compared to the popular benzodithiophene (BDT) unit, dialkoxyphenyldithiophene (PDT) has the advantages of increased planarity along the molecular backbone (decreased dihedral angles between PDT and π bridges) and stronger donating ability, which may lead to better absorption, higher hole mobility and even higher power conversion efficiency (PCE). In this paper, two small molecules (CNO2TPDT and RD2TPDT) with PDT unit as a donor, bithiophene (2T) as π bridges, and oxoalkylated nitrile (CNO) or 3-ethylrhodanine (RD) as end-capped acceptors were synthesized and characterized. As expected, CNO2TPDT and RD2TPDT showed excellent absorption with edges of 720 and 745 nm, respectively, which are red-shifted by ca. 30 nm compared with their BDT analogues. A CNO2TPDT-based device exhibited relatively high Voc of 0.87 V but a low PCE of 4.16% due to a large phase separation. In contrast, a RD2TPDT-based device exhibited higher PCE of 6.64% with a Jsc of 12.74 mA cm−2 due to its red-shift absorption, better miscibility with [6,6]-phenyl-C71 butyric acid methylester (PC71BM) and higher hole mobility. This study demonstrated rational molecule design and effective morphology control can lead to good absorption ability and hole mobility, indicating the possibility for obtaining higher Jsc and PCE.

A novel small molecule based on naphtho[1,2-b:5,6-b′]dithiophene benefits both fullerene and non-fullerene solar cells

Small molecule solar cells have made great progress in recent years. Herein, we synthesized a novel small molecule donor NDTR with naphtho[1,2-b:5,6-b′]dithiophene (NDT) units as the building blocks. NDTR exhibited complementary absorption for both fullerene acceptor PC71BM and non-fullerene acceptor IDIC. Meanwhile, NDTR possessed a HOMO energy level of −5.23 eV and a LUMO energy level of −3.50 eV, which matched well with these two kinds of acceptors. When mixed with PC71BM, a high power conversion efficiency (PCE) of 7.75% was obtained, while the NDTR:IDIC system presented a PCE of 6.60%. The results indicated that NDTR was an all-round small molecule donor which can work well in both fullerene and non-fullerene systems.

Aromatic end-capped acceptor effects on molecular stacking and the photovoltaic performance of solution-processable small molecules

Aromatic end-capped acceptors are important in constructing donor materials and non-fullerene acceptors in organic solar cells. However, their features (such as electron-withdrawing ability and subtle change in planarity) and effects on molecular stacking and photovoltaic performance, lack a systematic study. This manuscript reports four molecules, namely, BT-RCN, BT-BA, BT-RA, and BT-ID, which are terminated by different acceptors in the same backbone. We quantify the molecular planarity through their dipole moment in the Z direction and investigate the effect of the degree of planarity on molecular properties and device performances. The four acceptors are classified into two groups based on acceptor strength: medium-strong and strong acceptors. Molecules based on medium-strong acceptors exhibit excellent efficiencies as follows: 10.1% for BT-ID, 9.6% for BT-RA, and 8.5% for BT-BA. Their decreased efficiency is quite consistent with their lowered hole mobility. Grazing incidence X-ray diffraction results demonstrate the positive relationship between the non-planarity of the acceptors and d-spacing distance in the π–π stacking direction, which are detrimental to mobility. BT-RCN, with a strong acceptor, obtains the lowest efficiency of 6.1%. These findings indicate the importance of matching the electron-donating ability of donor units and electron-withdrawing ability of acceptor units, and the subtle planarity change in molecular properties and aggregation.