Liangang Xiao

Deep Absorbing Porphyrin Small Molecule for High-Performance Organic Solar Cells with Very Low Energy Losses

We designed and synthesized the DPPEZnP-TEH molecule, with a porphyrin ring linked to two diketopyrrolopyrrole units by ethynylene bridges. The resulting material exhibits a very low energy band gap of 1.37 eV and a broad light absorption to 907 nm. An open-circuit voltage of 0.78 V was obtained in bulk heterojunction (BHJ) organic solar cells, showing a low energy loss of only 0.59 eV, which is the first report that small molecule solar cells show energy losses <0.6 eV. The optimized solar cells show remarkable external quantum efficiency, short circuit current, and power conversion efficiency up to 65%, 16.76 mA/cm(2), and 8.08%, respectively, which are the best values for BHJ solar cells with very low energy losses. Additionally, the morphology of DPPEZnP-TEH neat and blend films with PC61BM was studied thoroughly by grazing incidence X-ray diffraction, resonant soft X-ray scattering, and transmission electron microscopy under different fabrication conditions.

Multi-Length-Scale Morphologies Driven by Mixed Additives in Porphyrin-Based Organic Photovoltaics.

A new category of deep-absorbing small molecules is developed. Optimized devices driven by mixed additives show a remarkable short-circuit current of ≈20 mA cm(-2) and a highest power conversion efficiency of 9.06%. A multi-length-scale morphology is formed, which is fully characterized by resonant soft X-ray scattering, high-angle annular dark film image transmission electron microscopy, etc.

A complementary absorption small molecule for efficient ternary organic solar cells

Ternary solar cells are emerging as an attractive strategy to enhance the power conversion efficiencies (PCEs) of binary ones while maintaining the simple processing of single junction devices rather than that of the more complex tandem cells. However, only a few successful ternary blend solar cells have been reported with improved device performance. In this study, a low band gap small molecule DPPEZnP-O, which shows complementary absorption to PTB7, is introduced into PTB7 : PC71BM (PTB7: poly-thieno[3,4-b]thiophene/benzodithiophene; PC71BM: (6,6)-phenyl-C71-butyric acid methyl ester) to fabricate ternary solar cells. A PCE of 8.39% is achieved for the bulk heterojunction (BHJ) ternary solar cells with 20% DPPEZnP-O, which is a 12% enhancement in PCE compared with the binary OSCs based on PTB7 : PC71BM. DPPEZnP-O in PTB7 : PC71BM not only broadens light absorption to the near infrared region from 750 to 900 nm to harvest more photons but also plays a bridging role between PTB7 and PC71BM to provide more charge transfer routes at D/A interfaces. Unlike in many cases, 20% DPPEZnP-O in PTB7 : PC71BM does not form recombination centers to suppress the charge transport process, which also plays a role in the enhanced OSC performance.

High-Efficiency Small Molecule-Based Bulk-Heterojunction Solar Cells Enhanced by Additive Annealing

Solvent additive processing is important in optimizing an active layers morphology and thus improving the performance of organic solar cells (OSCs). In this study, we find that how 1,8-diiodooctane (DIO) additive is removed plays a critical role in determining the film morphology of the bulk heterojunction OSCs in inverted structure based on a porphyrin small molecule. Different from the cases reported for polymer-based OSCs in conventional structures, the inverted OSCs upon the quick removal of the additive either by quick vacuuming or methanol washing exhibit poorer performance. In contrast, the devices after keeping the active layers in ambient pressure with additive dwelling for about 1 h (namely, additive annealing) show an enhanced power conversion efficiency up to 7.78% with a large short circuit current of 19.25 mA/cm(2), which are among the best in small molecule-based solar cells. The detailed morphology analyses using UV-vis absorption spectroscopy, grazing incidence X-ray diffraction, resonant soft X-ray scattering, and atomic force microscopy demonstrate that the active layer shows smaller-sized phase separation but improved structure order upon additive annealing. On the contrary, the quick removal of the additive either by quick vacuuming or methanol washing keeps the active layers in an earlier stage of large scaled phase separation.

A‐D‐A Type Small Molecules Based on Boron Dipyrromethene for Solution‐Processed Organic Solar Cells

The unique properties of boron dipyrromethene (BODIPY) dyes including facile synthesis, high absorption coefficients, and delocalized molecular orbitals as well as excellent photochemical and thermal stability, make them promising as materials for organic solar cells. Accordingly, in this study three A-D-A structural small molecules of BDTT-BODIPY, FL-BODIPY, and TT-BODIPY have been synthesized, in which two BODIPY acceptor units are symmetrically conjugated to 4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene (BDTT), 9,9-dioctyl-9H-fluorene (FL), and thieno[3,2-b]thiophene (TT) donor cores, respectively. The manipulation of the structural parameters significantly improves the performances of the BHJ OSCs, which show power conversion efficiencies of 4.75%, 1.51%, and 1.67% based on [6,6]-phenyl C71 -butyric acid methyl ester (PC71 BM) as the acceptor material and BDTT-BODIPY, FL-BODIPY, and TT-BODIPY as the donor materials, respectively.

Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells

A porphyrin-based molecule DPPEZnP-BzTBO with bulky benzothiophene groups was designed and synthesized as an electron donor material for bulk heterojunction (BHJ) solar cells. The optimized devices under thermal annealing (TA) and then chloroform solvent vapor anneanling (SVA) for 80 s exhibited an outstanding power conversion efficiencie (PCE) of 9.08%. Contrasted with the smaller thienyl substituted analogues we reported previously, DPPEZnP-BzTBO-based BHJ solar cells exhibited a higher open circuit voltage due to the lower highest occupied molecular orbital energy level. The TA post-treatment of the active layers induced the formation of more crystallized components, and the subsequent SVA provided a driving force for the domain growth, resulting in more obvious phase segregation between the donor and the acceptor in nanoscale. Furthermore, the PCEs kept above 95% upon the further SVA treatment within the time range of 60 to 95 s probably because the bulky benzothiophene groups retard the too quick change of crystallinity, providing a wide processing window for the reproducible device fabrication.

Solution-processed bulk heterojunction solar cells based on porphyrin small molecules with very low energy losses comparable to perovskite solar cells and high quantum efficiencies

Two new A–D–A conjugated small molecules Por-Rod and Por-CNRod were developed using a porphyrin core as the donor unit and 3-ethylrhodanine and 2-(1,1-dicyanomethylene)rhodanine as the acceptor units. Por-Rod and Por-CNRod show broad absorptions up to ∼850 nm with optical energy bandgaps of 1.47 and 1.45 eV, respectively. Additionally, their blend films containing PC71BM show SCLC hole mobilities of 8.5 × 10−5 and 7.5 × 10−6 cm2 V−1 s−1, respectively. Although bulk heterojunction solar cells containing PC71BM that were processed either without additives, with thermal annealling alone or with only pyridine show very low power conversion efficiencies (PCEs), Por-Rod-based solar cells processed using pyridine and then thermal annealing, show a PCE of up to 4.97% with a remarkable VOC of up to 0.94 V and a very low energy loss of only 0.53 eV. This is the first report in which small molecule-based solar cells show such a low energy loss comparable to perovskite solar cells, while exhibiting a good PCE of about 5% and a maximum external quantum efficiency up to 61%. To further understand the effect of different processing conditions on the blend films, the morphology of the blend films was studied by grazing incidence X-ray diffraction and resonant soft X-ray scattering.

Doping ZnO with Water/Alcohol-Soluble Small Molecules as Electron Transport Layers for Inverted Polymer Solar Cells

By doping ZnO with porphyrin small molecules (FNEZnP-OE and FNEZnP-T) as cathode electron transport layers (ETLs), the inverted polymer solar cells (i-PSC) with PTB7:PC71BM (PTB7: polythieno[3,4-b]-thiophene-co-benzodithiophene, PC71BM:[6, 6]-phenyl-C71-butyric acid methyl ester) as the active materials exhibit enhanced device performance. While the power conversion efficiency (PCE) of the PSCs with pure ZnO ETL is 7.52%, that of the devices with FNEZnP-T-doped ZnO ETL shows a slightly improved PCE of 8.09%, and that of the PSCs with FNEZnP-OE-doped ZnO ETL is further enhanced up to 9.24% with an over 20% improvement compared to that with pure ZnO ETL. The better performance is contributed by the better interfacial contact and reduced work function induced by 9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorenes and 3,4-bis(2-(2-methoxy-ethoxy)-ethoxy)-phenyls in the porphyrin small molecules. More importantly, the PCE is still higher than 8% even when the thickness of FNEZnP-OE-doped ZnO ETL is up to 110 nm, which are important criteria for eventually making organic photovoltaic modules with roll-to-roll coat processing.

Highly efficient small molecule solar cells fabricated with non-halogenated solvents

Though much progress has been achieved for bulk-heterojunction (BHJ) organic solar cells (OSCs), the best performing devices are processed with halogenated solvents. However, the halogenated solvents should be replaced for the practical applications of OSCs due to the toxicity of halogenated solvents to human beings and the environment. In this study, OSCs based on a porphyrin small molecule are fabricated with toluene and o-xylene, which show high PCEs up to 5.46% and 5.85%, respectively. Both PCEs are the highest for small molecule-based OSCs processed with non-halogenated solvents, demonstrating that porphyrin small molecules are very promising for solar cell commercial applications.

A visible-near-infrared absorbing A–π2–D–π1–D–π2–A type dimeric-porphyrin donor for high-performance organic solar cells

Most of the currently available small molecule bulk heterojunction organic solar cells (BHJ OSCs) only utilize visible light and, to further increase the efficiency, the development of new organic materials that harvest near-infrared (NIR) light to produce an electric current is essential. Herein, a new A–π2–D–π1–D–π2–A type dimeric porphyrin-cored small molecule (CS-DP) is designed, synthesized and characterized. The use of CS-DP with a narrow bandgap (Eg) (1.22 eV) and the deep energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) affords the highest power conversion efficiency of 8.29% in BHJ OSCs with PC71BM as an acceptor, corresponding to a short circuit current of 15.19 mA cm−2, an open circuit voltage of 0.796 V and a fill factor of 70% under AM 1.5G solar irradiation. The high device performance is attributed to the visible-near-infrared light-harvesting capability of CS-DP, and the super low energy loss feature. The energy loss (Eloss) lies between 0.43 and 0.51 eV in the system, which is related to the very small energy offset of the LUMOs between the CS-DP donor and PC71BM (ΔELUMO = 0.06 eV). The value of ΔELUMO, which is considered as a driving force for the photoinduced charge separation, is much smaller than the empirical threshold of 0.3 eV, but would not be a limiting factor in the charge separation process. The results indicate that there may be room for further improving the PCE for low bandgap dimeric porphyrin systems.