Squaraine Donor Research Articles

Squaraine dyes containing diphenylamine group: Effects of different type structures on material properties and organic photovoltaic performances

Four donor-acceptor-donor (D-A-D’) unsymmetrical squaraines (USQs) with different molecular skeletons (XZ-type and YZ-type), containing diphenylamine group with/without methoxy substituent were synthesized as donor materials in bulk-heterojunction (BHJ) organic photovoltaics (OPVs). The introduction of methoxy group in USQs has little different effects on the overall photovoltaic performance. Conversely, the different molecular skeleton types of the USQs have significant influence on their material properties and photovoltaic performances. Compared to BIDPSQ and BIDPOMeSQ with XZ-type molecular skeleton, IDPSQ and IDPOMeSQ with YZ-type molecular skeleton display closer solid-state packing, much lower highest occupied molecular orbital (HOMO) energy level, higher hole mobility and smaller phase separation domain size. Consequently, YZ-type USQs exhibit the most excellent performance with power conversion efficiency (PCE) of ~4%, which is approximately 300% higher than those of XZ-type USQs. Surprisingly, even though IDPSQ and IDPOMeSQ show wide band gaps, the corresponding devices still achieve a highest PCE of ~4%, which is comparable to or even higher than the PCEs of some reported SQ-based devices with low band gaps. These results indicate the YZ-type molecular skeleton and the electron-donating diphenylamine group are very promising to construct highly efficient squaraine donor materials.

Performance, Morphology, and Charge Recombination Correlations in Ternary Squaraine Solar Cells

Ternary bulk-heterojunction organic solar cells (BHJ-OSCs) are demonstrated by combining two squaraine donors (USQ3OH and IDPSQ) having complementary optical absorption and PC71BM as the acceptor. ...

Coevaporated Bisquaraine Inverted Solar Cells: Enhancement Due to Energy Transfer and Open Circuit Voltage Control

There is currently enormous interest in the development of small molecule organic solar cells (SMSC), as in principle, these systems offer advantages over both conventional Si photovoltaics and organic polymer solar cells. Here, we report Förster Resonance Energy Transfer (FRET) enhanced inverted SMSC fabricated by coevaporating two different squaraine donors, a symmetrical squaraine (SQ, 2,4-bis-4-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine), and an asymmetrical squaraine (ASSQ, 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio) squaraine). ASSQ absorbs blue light (λmax 540 nm) and emits from 550 nm to the near-infrared region, which overlaps with SQ absorption (λmax 690 nm). Therefore, by utilizing a thin film containing the two squaraine donors as the active layer in a SMSC, we can both broaden the photovoltaic absorption spectrum, and reduce recombination loss as a result of FRET. This strategy has resulted in SMSC with power conversion efficiencies (PCE) which are up to 46% greater than those obtained by using a single squaraine donor. Ultrafast time-resolved photoluminescence and transient absorption spectroscopy provide clear evidence of FRET between the small molecules, with a rapid energy transfer time of ∼1 ps. At optimal blending, which correlates to the highest PCE measured, the efficiency of energy transfer is as high as 85%. Furthermore, in the devices containing two different squaraine molecules, the open circuit voltage (VOC) is proportional to the fraction of the two donors in the blend, allowing us to predict the VOC as the ratio of the two donors is changed. SMSC with inverted structures also demonstrate long-term stability in ambient conditions compared to devices employing a conventional architecture.

Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor-acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an "inverted" quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C60 cap layer. We call this apparently unprecedented growth process from a buried interface "inverse quasi-epitaxy" where the crystallites of these "soft" van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C60 acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of ηP = 5.4 ± 0.3% for single-junction and ηP = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donor-acceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells.

Polymer bulk heterojunction photovoltaics employing a squaraine donor additive

We investigate the effects of adding a functionalized squaraine donor 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ) into a conventional poly(3-hexylthiophene)(P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) polymer bulk heterojunction photovoltaic cell. The near infrared absorption of the blend was enhanced by the DPSQ additive, resulting in an increased power conversion efficiency of the P3HT:PCBM devices by >20%. A maximum power conversion efficiency of 3.4±0.3% and an external quantum efficiency as high as 55% was achieved for a P3HT:PCBM blend that included 5wt.% DPSQ.

Controlling J‐aggregate formation for increased short‐circuit current and power conversion efficiency with a squaraine donor

ABSTRACTA squaraine dye is tested for novel application in a near‐infrared‐active organic photovoltaic cell that is subsequently optimized to obtain a power conversion efficiency of 2.4 ± 0.3%. The optimization utilizes an Alq3 buffer layer and macroscopic structure control through the addition of co‐solvents in the spin‐casting process. Co‐solvent addition increases the amount of aggregates present as measured through linear absorption spectroscopy, and there is a concurrent increase in both efficiency and short‐circuit current. An interpretation of the greatly increased current density is presented that describes how increased J‐aggregation likely increases hole mobility and, as a result, charge separation of the photogenerated excited state. Copyright © 2012 John Wiley & Sons, Ltd.

Small‐Molecule Photovoltaics Based on Functionalized Squaraine Donor Blends

Two squaraine (SQ) donor molecules with different absorption bands are blended together for better coverage of the solar spectrum. The blend SQ device shows a significant improvement compared with single SQ donor devices. By applying a solvent annealing process and a compound buffer layer, a power-conversion efficiency of 5.9 ± 0.3% is achieved under 1 sun illumination.

Measurement of exciton diffusion lengths in optically thin organic films

Spectrally resolved photoluminescence quenching (SR-PLQ) is a convenient and accurate method for measuring the exciton diffusion length of organic materials; however, the requirement of optically thick films demanded by this technique poses practical limitations to its implementation. Through simulations of the optical field and exciton dynamics, we extend SR-PLQ to the case of optically thin organic films; i.e., films whose thickness is comparable to or less than that of the optical absorption length across the entire optical absorption spectrum. This allows for the characterization of films whose thickness is comparable to that used in practical organic optoelectronic devices. Using this method, we measure the diffusion lengths of several squaraine donors, the acceptor 3,4,9,10 perylenetetracarboxylic dianhydride, and the relationship between the donor, boron subphthalocyanine thickness and diffusion length.

Arylamine-Based Squaraine Donors for Use in Organic Solar Cells

Squaraine (SQ) dyes are notable for their exceptionally high absorption coefficients extending from the green to the near-infrared. In this work, we utilize the functionalized SQ donor: 2,4-bis [4-(N-phenyl-1-naphthylamino)-2,6-dihydroxyphenyl] squaraine (1-NPSQ) by substitution of isobutylamines in the common "parent SQ" with arylamines to improve stacking and hence exciton and charge transport. The strong electron-withdrawing arylamine group results in a highest occupied molecular orbital energy of 5.3 eV, compared to 5.1 eV for the parent SQ, making 1-NPSQ a suitable donor when used with a C(60) acceptor in an organic photovoltaic cell. Optimized and thermally annealed, nanocrystalline heterojunction 1-NPSQ/C(60)/bathocuproine solar cells with an open circuit voltage of 0.90 ± 0.01 V, fill factor of 0.64 ± 0.01, and short circuit current of 10.0 ± 1.1 mA/cm(2) at 1 sun, AM1.5G illumination (solar spectrally corrected) result in a power conversion efficiency of 5.7 ± 0.6%. Crystallograpnic data suggest that the intermolecular stacking of 1-NPSQ molecules is closer than that of the parent SQ, thereby reducing the device series resistance and increasing its fill factor.

High efficiency organic photovoltaic cells based on a vapor deposited squaraine donor

2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) is used as a donor material in vapor deposited organic heterojunction photovoltaic cells. Devices with the structure indium tin oxide/SQ (x)∕C60 (400Å)/bathocuproine (100Å)∕Al (1000Å), where x=65, 110, 150, and 200Å were compared. Devices with x=65Å exhibited a power conversion efficiency of 3.1% under 1sun, AM1.5G simulated solar irradiation, giving an open circuit voltage of 0.76±0.01V, a short circuit current of 7.01±0.05mA∕cm2, and a fill factor of 0.56±0.05. Thicker SQ films lead to lower short circuit currents and fill factors, giving conversion efficiencies in the range of 2.6% to 3.2%. The demonstration of sublimable SQ as a donor material opens up a family of compounds for use in small molecule based heterojunction photovoltaics.