Effect of Anode Interfacial Modification by Self-Assembled Monolayers on the Organic Solar Cell Performance.

A series of self-assembled monolayer (SAM)-based benzoic acid derivatives such as 4-[5'-phenyl-2,2'-bitien-5-yl] benzoic acid (ZE-Ph), 4-[5'-(4-fluorophenyl)-2,2'-bitien-5-yl]benzoic acid (ZE-1F), and 4-[5'-(3,5-difluorophenyl)-2,2'-bitien-5-yl]benzoic acid (ZE-2F) were synthesized to use an interlayer between an ITO electrode and a MoO3 thin film layer in an organic solar cell (OSC) having poly-3 hexylthiophene (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) blend. The work function and surface wetting properties of the ITO were tuned by SAM molecules. The power conversion efficiency of fabricated OSC devices was improved compared to that of the control device from 1.93 to 2.20% and 2.22% with ZE-Ph and ZE-1F-modified ITO electrodes, respectively. The short-circuit current density (Jsc) was increased from 6.16 to 7.10 mA/cm2 and 6.94 mA/cm2 with control, ZE-Ph, and ZE-1F-modified solar cells, respectively. The increase in short-circuit current density (Jsc) shows that the hole-transporting properties between ITO and MoO3 were improved by the use of ZE-Ph and ZE-1F compared with that of the ITO/MoO3 electrode configuration. The open-circuit voltage (Voc) of the SAM-modified ITO-based devices was also improved compared with the Voc of unmodified ITO-based devices. These results show that using a monolayer as an interlayer in OSCs is an important strategy to improve the performance of OSCs. All the device parameters were characterized by Kelvin probe force microscopy, cyclic voltammetry, contact angle, and I-V measurements.

Enhanced Hole Mobility of p-Type Materials by Molecular Engineering for Efficient Perovskite Solar Cells.

Star-shaped triazatruxene derivative hole-transporting materials (HTMs), namely, 3,8,13-tris(4-(8a,9a-dihydro-9H-carbazol-9-yl)phenyl)-5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-TY1) and 3,8,13-tris(4-(8a,9a-dihydro-9H-carbazol-9-yl)phenyl)-5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-TY2), containing electron-rich triazatruxene cores and donor carbazole moieties, were synthesized and successfully used in triple-cation perovskite solar cells. All the HTMs were obtained from relatively inexpensive precursor materials using well-known synthesis procedures and uncomplicated purification steps. All the HTMs, including the 5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-H) main core, had suitable highest occupied molecular orbitals (HOMOs) for perovskite (TAT-H: -5.15 eV, TAT-TY1: -5.17 eV, and TAT-TY2: -5.2 eV). Steady-state and time-resolved photoluminescence results revealed that hole transport from the valence band of the perovskite into the HOMO of the new triazatruxene derivatives was more efficient than with TAT-H. Furthermore, the substitution of n-hexylcarbazole and 9-phenylcarbazole in triazatruxene altered the crystalline nature of the main core, resulting in a smooth and pinhole-free thin-film morphology. As a result, the hole mobilities of TAT-TY1 and TAT-TY2 were measured to be one order of magnitude higher than that of TAT-H. Finally, TAT-TY1 and TAT-TY2 achieved power conversion efficiencies of up to 17.5 and 16.3%, respectively, compared to the reference Spiro-OMeTAD. These results demonstrate that the new star-shaped triazatruxene derivative HTMs can be synthesized without using complicated synthesis strategies by controlling the intrinsic morphology of the TAT-H main core.

Simultaneous Optimization of Charge Transport Properties in a Triple-Cation Perovskite Layer and Triple-Cation Perovskite/Spiro-OMeTAD Interface by Dual Passivation.

Molecular engineering of additives is a highly effective method to increase the efficiency of perovskite solar cells by reducing trap states and charge carrier barriers in bulk and on the thin film surface. In particular, the elimination of undercoordinated lead species that act as the nonradiative charge recombination center or contain defects that may limit interfacial charge transfer is critical for producing a highly efficient triple-cation perovskite solar cell. Here, 2-iodoacetamide (2I-Ac), 2-bromoacetamide (2Br-Ac), and 2-chloroacetamide (2Cl-Ac) molecules, which can be coordinated with lead, have been used by adding them into a chlorobenzene antisolvent to eliminate the defects encountered in the triple-cation perovskite thin film. The passivation process has been carried out with the coordination between the oxygen anion (-) and the lead (+2) cation on the enolate molecule, which is in the resonance structure of the molecules. The Spiro-OMeTAD/triple-cation perovskite interface has been improved by surface passivation by releasing HX (X = I, Br) as a byproduct because of the separation of alpha hydrogen on the molecule. As a result, a solar cell with a negligible hysteresis operating at 19.5% efficiency has been produced by using the 2Br-Ac molecule, compared to the 17.6% efficiency of the reference cell.

Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics.

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV-vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.