IPCE Response Research Articles

Aniline and Indoline Donors Based Far‐Red Active Unsymmetrical Squaraine Dyes for Dye Sensitized Solar Cells

AbstractIn dye‐sensitized solar cells (DSSC), controlling the dye‐aggregation on the metal‐oxide surface by appending the alkyl groups around the donor or π‐spacer unit of the dye is a potential approach to enhance DSSC efficiency. Further, rigidification of the dye structures by cyclization modulates the photophysical properties of the sensitizer. Here a series of donor‐acceptor‐donor (D‐A‐D) type far‐red active unsymmetrical squaraine dyes (SQA) have been designed and synthesized, where N,N‐dimethylaniline, methylated‐ and branched‐indoline have been used as donor units. These dyes showed absorption between 629–654 nm (λmax) with the molar extinction coefficient of 1.49–1.94×105 M−1 cm−1. Systematic enhancements in DSSC device efficiency have been observed due to the cyclization and alkyl‐groups incorporation in this set of dyes which were further enhanced with the addition of chenodeoxycholic acid (CDCA). The highest DSSC device efficiency of 4.78 % (Jsc of 8.77 mA/cm2 and Voc of 692 mV) has been achieved for SQA3. The IPCE profile of SQA dyes indicates the contribution of aggregated structures for the photocurrent generation. Further, co‐sensitization of SQA3 dye with a complementary visible light active dye AK4 showed the enhanced device efficiency of 6.27 % with panchromatic IPCE response.

Harnessing of low-energy IR photons via oxygen-vacancies in SrTiO3 nanocrystals for photocatalytic hydrogen production

The surface-oxygen vacancies were generated to enhance the near-IR light response of the SrTiO3 photocatalyst by heating SrTiO3 in a highly reducing environment. The UV–visible diffuse reflectance spectra of SrTiO3-Ov revealed strong absorption between 400 and 800 nm, while IPCE response was extended beyond 800 nm, confirming the near-IR light response of SrTiO3-Ov. The hydrogen production rates for SrTiO3-Ov at pH 3.0 under full solar spectrum, visible and IR radiation were 100, 25 and 10 μlg−1h−1, respectively. The respective normalized hydrogen with respect to light intensity were found to be 100, 50 and 33 μlg−1h−1 while hydrogen production of pristine SrTiO3 was less than 2 μlg−1h−1 under full solar spectrum. The mid-gap Ov and Ti3+ states in SrTiO3-Ov are key factors in the origin of the near-IR responses of the catalyst and the feasibility of using low-photonic-energy IR light to produce hydrogen by water splitting was demonstrated.

Indoline-Based Donor-π-Acceptor Visible-Light Responsive Organic Dyes for Dye-Sensitized Solar Cells: Co-sensitization with Squaraine Dye for Panchromatic IPCE Response

Indoline-Based Donor-π-Acceptor Visible-Light Responsive Organic Dyes for Dye-Sensitized Solar Cells: Co-sensitization with Squaraine Dye for Panchromatic IPCE Response

Efficient phenothiazine-ruthenium sensitizers with high open-circuit voltage (Voc) for high performance dye-sensitized solar cells

Herein, we designed and synthesized two highly effective phenothiazine-ruthenium sensitizers JR1 and JR2 to use in dye-sensitized solar cells. The structure of N719 was further optimized, 1,10-phenanthroline modified by phenothiazine was used instead of 2,2′-bipyridine-4,4′-dicarboxylic acid to as the electron donor. Compared with N719, these two ruthenium dyes show better optical properties, both the spectral response range and the molar absorption coefficient of dye molecules have been significantly improved. The IPCE response of JR1 is obviously improved, the response range of photocurrent is widened to 800 nm, the overall response value is also higher. JR2 shows the widest and highest IPCE response curve, in the region of 410–600 nm, the IPCE value of JR2 is more than 70%, and at 530 nm, the IPCE value has reached 79%, so JR2 shows the highest Jsc of 17.18 mA cm−2. Further optimization of molecular structure not only improves the light-harvesting performance of the device, but also reduces the charge recombination behaviour and improves the electron lifetime, so the Voc is greatly increased. Especially for JR2, the introduction of the second long carbon chain not only increases the solubility of the dye, but also further suppresses the dye aggregation and reduces the charge recombination. The phenothiazine-ruthenium dye JR2 shows a high Voc of 841 mV, while the theoretical Voc of DSSCs based on I−/I3− electrolyte is about 900 mV. JR2 exhibits a superior PCE of 9.12%, which exceeds N719 (8.03%) about 14%.

Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: Panchromatic Light Absorption and Modulated Open Circuit Potential.

The design of dyes for panchromatic light absorption has attracted much attention in the field of dye-sensitized solar cells (DSSCs). An approach to enhance panchromatic light absorption utilizes mixtures of complementary light-absorbing dyes as well as dyes with specific anchoring groups that facilitate interfacial charge transfer with TiO2 . Dipole-dipole interactions between the dye molecules on the surface broaden the spectrum, which results in decreased DSSC device performance. However, controlled aggregation of dyes results in broadening the spectral profile along with enhanced photocurrent generation. To control the dye-dye interaction, dimeric dyes with different dipole lengths D1 -Dsq , Dsq -Dsq were systematically designed and synthesized. The photophysical and electrochemical properties were evaluated and the EHOMO and ELUMO levels were determined; these energy levels determines the electron injection from ELUMO of the dye to ECB of TiO2 and regeneration of oxidized dye by the electrolyte, respectively. The absorption spectra of Dsq -Dsq , D1 -Dsq were broadened in solution compared to model dye Dsq ; this indicates that the dye-dye interaction is prominent in solution. In D1 -Dsq excitation energy transfer between photoexcited D1 and Dsq was explained by using Förster resonance energy transfer (FRET). The homodimeric dye showed a device performace of 2.8 % (Voc 0.607, Jsc 6.62 mA/cm2 , ff 69.3 %),whereas the heterodimeric dye D1 -Dsq showed a device performance of 3.9 % (Voc 0.652 V, Jsc 8.89 mA/cm2 , ff 68.8 %). The increased photocurrent for D1 -Dsq is due to the panchromatic IPCE response compared to Dsq -Dsq . The increased Voc is due to the effective passivation of the TiO2 surface by the spirolinker, and the effective dipole moment that shifts the conduction band on TiO2 . Hence, the open circuit potential, Voc , for the devices prepared from Dsq , D1 -Dsq and Dsq -Dsq were systematically modulated by controlling the intermolecular π-π and intramolecular dipole-dipole interactions of the dimeric dyes.

Effect of substitution position on photoelectronic properties of indolo[3,2-b]carbazole-based metal-free organic dyes

Two new indolo[3,2-b]carbazole-based metal-free organic dyes (D1 and D2) were synthesized by utilizing triphenylamine as the first donor which were connected to the second donor indolo[3,2-b]carbazole via 9-position and 8-position, respectively. The dyes were characterized by UV–visible absorption spectra and electrochemical analyses. The both two dyes showed intensive absorption spectra in the wavelength of 300–600 nm and narrow band gap of 2.29 eV for D1 and 2.16 eV for D2. The cyclic voltammetry indicated that the oxidative potentials and reductive potentials of the dyes were well matched with the energy levels of the I−/I3− redox couple and TiO2 conduction band, respectively. The substituted positions on the indolo[3,2-b]carbazole played important roles in tuning the photoelectronic properties. The dye D2 with 2, 8-substituted indolo[3,2-b]carbazole core shows wider absorption spectrum and lower dye aggregation than D1 with 3,9-substituted isomer. The best power conversion efficiency (PCE) of 6.34% is obtained by the dye D2 due to its wider absorption spectra and strong steric hindrance, which can effectively suppress the undesirable dye aggregation. The co-sensitization of D1 and D2 enhances IPCE response in the visible light region and further improves the PCE up to 7.03%.

Dye-sensitized solar cells based on (D−π−A)3L2 phenothiazine dyes containing auxiliary donors and flexible linkers with different length of carbon chain

Four (D−π−A)3L2 phenothiazine dyes (PTZTB1−4) and a reported reference dye (R−SB) with a mono D−π−A chain were utilized as photosensitizers for dye-sensitized solar cells. In the skeleton of the dyes PTZTB1−4, three D−π−A chains (D−π−A)3 were connected by flexible linker of n-hexyl or n-dodecyl group (L2) to build the (D−π−A)3L2 configuration. The effect of both the auxiliary donors and linkers with different length of carbon chains upon the photophysical, electrochemical as well as photovoltaic properties of the (D−π−A)3L2 dyes were further investigated. Results reveal that the dyes PTZTB3−4 containing the auxiliary donor of 4-octyloxyphenyl group show a higher molar extinction coefficient, a broader IPCE response region, and a higher HOMO value than those of PTZTB1−2 and R−SB. Among all of the devices, the cells based on PTZTB2 and PTZTB4 exhibit a higher open circuit voltage due to that the charge recombination is blocked significantly by the n-dodecyl linkers. Meanwhile, a maximal power conversion efficiency of 6.66% is achieved by the device fabricated with PTZTB3. Besides, it is noteworthy that the (D−π−A)3L2 dyes possess a superior adsorbed stability on TiO2 interface than that of the reference dye as a result of the three anchors in a molecule of the former.

Wide-Range Near-Infrared Sensitizing 1 H-Benzo[ c, d]indol-2-ylidene-Based Squaraine Dyes for Dye-Sensitized Solar Cells.

NIR absorbing squaraine dyes SQ1-SQ7 having 1 H-benzo[ c, d]indol-2-ylidene as a donor moiety were designed for application in DSSCs. Annulation of the benzene ring to an 3 H-indolium-based anchor moiety led to a red-shifted and broadened absorption band on TiO2 film, which were reflected in the improved short-circuit current density of SQ2 (6.22 mA cm-2) compared to the nonbenzene fused derivative SQ1 (4.39 mA cm-2). Although the introduction of a butoxy (SQ4: 806 nm) or dialkylamino group (SQ5-SQ7: 815-820 nm) to the 1 H-benzo[ c, d]indol-2-ylidene-based donor moiety resulted in red-shifted absorption maxima in ethanol compared to the nonsubstituted derivative SQ2 (784 nm), the HOMO energy level of SQ4-SQ7 gave rise to an undesirable approximation to the redox potential of I-/I3-. Thus, the butoxy (SQ4: 0.56) and dialkylamino (SQ5-SQ7: 0.25-0.30) derivatives had relatively lower conversion efficiencies. Since the 2-ethylhexyl derivative SQ3 exhibited red-shifted absorption (λmax: 796 nm), suitable HOMO and LUMO energy levels, and relatively efficient restriction of charge recombination, this dye achieved the highest conversion efficiency (1.31%), along with a high IPCE response of over 20% over a wide range from 640 to 860 nm and an onset of IPCE at 1000 nm.

Novel diyne-bridged dyes for efficient dye-sensitized solar cells

Three new metal free organic dyes (FSD101-103) were synthesized to investigate the influence of diyne unit on dye molecules. FSD101 and FSD102 with diyne unit and FSD103 with monoyne unit were applied as sensitizers in the dye-sensitized solar cells (DSSCs). The optical and electrochemical properties, theoretical studies, and photovoltaic parameters of DSSCs sensitized by these dyes were systematically investigated. By replacing the monoyne unit with a diyne unit, FSD101 exhibited broader absorption spectrum, lower IP, higher EA, lower band gap energy, higher oscillator strength, more efficient electron injection ability, broader IPCE response range and higher τe in comparison with FSD103. Hence, DSSC sensitized by FSD101 showed higher Jsc and Voc values, and demonstrated a power conversion efficiency of 3.12%, about 2-fold as that of FSD103 (1.55%). FSD102 showed similar results as FSD101, with a power conversion efficiency of 2.98%, despite a stronger electron withdraw cyanoacrylic acid group was introduced. This may be due to the lower efficiency of the electron injection from dye to TiO2 and lower τe of FSD102 than that of FSD101. These results indicate that the performance of DSSCs can be significantly improved by introducing a diyne unit into this type of organic dyes.

Systematic Investigations on the Roles of the Electron Acceptor and Neighboring Ethynylene Moiety in Porphyrins for Dye-Sensitized Solar Cells.

Cyanoacrylic and carboxyl groups have been developed as the most extensively used electron acceptor and anchoring group for the design of sensitizers for dye-sensitized solar cells. In terms of the photoelectric conversion efficiency, each of them has been demonstrated to be superior to the other one in certain cases. Herein, to further understand the effect of these two groups on cell efficiencies, a series of porphyrin sensitizers were designed and synthesized, with the acceptors systematically varied, and the effect of the neighboring ethynylene unit was also investigated. Compared with the sensitizer XW5 which contains a carboxyphenyl anchoring moiety directly linked to the meso-position of the porphyrin framework, the separate introduction of a strongly electron-withdrawing cyanoacrylic acid as the anchoring group or the insertion of an ethynylene unit can achieve broadened light absorption and IPCE response, resulting in higher Jsc and higher efficiency. Thus, compared with the efficiency of 4.77% for XW5, dyes XW1 and XW6 exhibit higher efficiencies of 7.09% and 5.92%, respectively. Simultaneous introduction of the cyanoacrylic acid and the ethynylene units into XW7 can further broaden light absorption and thus further improve the Jsc. However, XW7 exhibits the lowest Voc value, which is not only related to the floppy structure of the cyanoacrylic group but also related to the aggravated dye aggregation effect due to the extended framework. As a result, XW7 exhibits a relatively low efficiency of 5.75%. These results indicate that the combination of the ethynylene and cyanoacrylic groups is an unsuccessful approach. To address this problem, a cyano substituent was introduced to XW8 at the ortho position of the carboxyl group in the carboxyphenyl acceptor. Thus, XW8 exhibits the highest efficiency of 7.59% among these dyes. Further cosensitization of XW8 with XS3 dramatically improved the efficiency to 9.31%.

Benzo[a]carbazole-Based Donor-π-Acceptor Type Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells.

A novel class of metal-free organic dyes based on benzo[a]carbazole have been designed, synthesized, and used in dye-sensitized solar cells for the first time. These types of dyes consisted of a cyanoacrylic acid moiety as the electron acceptor/anchoring group and different electron-rich spacers such as thiophene (JY21), furan (JY22), and oligothiophene (JY23) as the π-linkers. The photophysical, electrochemical, and photovoltaic properties, as well as theoretical calculations of these dyes were investigated. The photovoltaic performances of these dyes were found to be highly relevant to the π-conjugated linkers. In particular, dye JY23 exhibited a broad IPCE response with a photocurrent signal up to about 740 nm covering the most region of the UV-visible light. A DSSC based on JY23 showed the best photovoltaic performance with a Jsc of 14.8 mA cm(-2), a Voc of 744 mV, and a FF of 0.68, achieving a power conversion efficiency of 7.54% under standard AM 1.5 G irradiation.

A Near-Infrared Dithieno[2,3-a:3′,2′-c]phenazine-Based Organic Co-Sensitizer for Highly Efficient and Stable Quasi-Solid-State Dye-Sensitized Solar Cells

A novel near-infrared (NIR) organic sensitizer FNE53 with a strong electron-withdrawing unit, dithieno[2,3-a:3',2'-c]phenazine, has been designed and synthesized for quasi-solid-state dye-sensitized solar cells (DSSCs). By simply fusing the two thiophene rings on quinoxaline unit in sensitizer FNE48, the intramolecular charge transfer (ICT) band bathochromically shifts from 542 nm for FNE48 to 629 nm for FNE53 in toluene solution. The absorption spectrum of sensitizer FNE53 covers the whole visible region and extends to the NIR region, which exhibits complementary absorption profile to another organic dye FNE46 based on quinoxaline. When FNE46 and FNE53 are used as cosensitizers for metal-free cocktail-type quasi-solid-state DSSCs, sensitizer FNE53 not only extends the photoresponse range but also suppresses the intermolecular interactions among the dye molecules. Therefore, the cocktail-type quasi-solid-state DSSC displays much higher IPCE value compared with that for the DSSC sensitizer based on FNE53 and a broader IPCE response in comparison to that for the DSSC sensitizers based on FNE46, respectively. After the molar ratio between the two cocktail dyes is optimized, the highest energy conversion efficiency of 8.04% is achieved in a metal-free quasi-solid-state DSSC cosensitized with FNE46 and FNE53, which exhibits good long-term stability after continuous light soaking for 1000 h.

Photo-stable substituted dihydroindolo[2,3-b]carbazole-based organic dyes: tuning the photovoltaic properties by optimizing the π structure for panchromatic DSSCs

Three dihydroindolo[2,3-b]carbazole (DDC) donor based organic dyes (DDC6–DDC8) with long conjugated systems were successfully synthesized and characterized by optical, electrochemical, computational analysis, and photovoltaic methods. It was found that the elongated π system as well as the high molar extinction coefficients (4.2–5.3 M−1 cm−1) sufficiently reinforced the light harvesting ability with the IPCE response of one dye DDC8 extended to 850 nm, which resulted in large Jsc of these dyes based devices (high as 16.41 mA cm−2). The attached alkyl chains suppress the dark current to some extent, resulting the DDC6 and DDC7 devices in high Voc, while DDC8 displayed low Voc because BTD in DDC8 could attract the iodine ion, which could accelerate the charge recombination. Among the three dyes, DDC7 displayed the best performance with an η value 6.53% under 4 μm thick scattering layer (condition 1+1), which was boosted to 7.49% under 8 μm thick scattering layer (condition 1+2). The photo-stability measurements indicated that all the three dyes are fairly photo-stable.

Influence of different methylene units on the performance of rhodanine organic dyes for dye-sensitized solar cells

In D–π–A structured organic dyes, rhodanine acetic acid and its derivates have served as electron acceptors successfully. In this work, two single rhodanine organic dyes with different methylene units have been synthesized and applied in dye-sensitized solar cells (DSSCs). The photophysical, electrochemical and photovoltaic properties have been studied systematically. The results show that dye with double methylene units produces a higher photon-to-electron conversion efficiency (η) of 4.5% than that of a single methylene unit dye with a η of 4.1%. Also, it is interesting to notice that the lengthened distance between electron acceptor and TiO2 surface does not decrease the short-circuit current density (Jsc) and IPCE response for both structured dyes. On the contrary, rhodanine dye bearing double methylene units exhibits a better Jsc and IPCE response than the single methylene unit. The explanation for this trend is probably due to the fact that increased methylene units could suppress regeneration between the injected electron and the oxidized dye effectively.

Iridium (III) complexes with 5,5-dimethyl-3-(pyridin-2-yl)cyclohex-2-enone ligands as sensitizer for dye-sensitized solar cells

The three iridium complexes based on 5,5-dimethyl-3-(pyridine-2-yl)cyclohex-2-enone ligand and pyridine-2,4-dicarboxyl acid (5b) or 2,2′-bipyridine-4,4′-dicarboxyl acid (5c) as ancillary anchoring ligands were synthesized and characterized as potential photosensitizer for dye-sensitized solar cells (DSSC). Using of cyclohexenone derivates as ligands extended the absorption response of the iridium complex to low energy band near 550nm and shifted the maximum emission peak to deep red (near the 680nm). The theoretical molecular orbital calculations shows that the HOMO orbitals of all the complexes are contributed by the combination of orbitals on Ir atom (about 50%) and π orbitals located on 5,5-dimethyl-3-(pyridine-2-yl)cyclohex-2-enone. While auxiliary ligands with anchoring group exclusively contribute to electron density of the LUMO orbital, accounting for 96.33% in 5b and 95.51% in 5c. It is beneficial for electron injection in DSSC application. Applying them to DSSC, the IPCE response of the DSSCs covered a wide visible spectral range from 350 to 675nm and the cells presented an open-circuit voltage of 645mV, a power conversion efficiency of 1.03%.

Enhancement of dye-sensitized solar cells performances by improving electron density in conduction band of nanostructure TiO2 electrode with using a metalloporphyrin as additional dye

A zinc(II)-porphyrin dye with four carboxyphenyl moiety of ancillary (ZnTCPP) was studied as a sensitizer in combination with a ruthenium complex (N719) in co-sensitized solar cells. The high molar extinction coefficient (ɛ) of porphyrin dyes leads to high light absorption in the dye-sensitized TiO2 electrode. In spite of the high ɛ of porphyrin dyes, they usually have a narrow absorption band and also to suffer from dye aggregation due to their planar structural nature. This causes lower efficiencies of the DSSCs for the porphyrins than the ruthenium complexes. Co-sensitization of two or more dyes with complementary absorption spectra on TiO2 film is an important method to further enhance the IPCE response and energy conversion efficiency of dye-sensitized solar cells. Interestingly, when the ZnTCPP electrode was used to assemble a co-sensitized solar cell by additional adsorption of N719 dye, the efficiency improved to 6.35% (in comparison to N719 that the efficiency was 4.74%). The results indicated that the co-sensitized device shows enhancements of photovoltaic performance not only in short-circuit current density (JSC) but also in open-circuit voltage (VOC). In the present study we have been shown that co-sensitization of a zinc(II)-porphyrin with N719 dye changes the energy levels of the TiO2 electrode and in result produces further improvement for its device performance.

Facile synthesis of a bulky BPTPA donor group suitable for cobalt electrolyte based dye sensitized solar cells

An efficient synthesis of N-(2′,4′-bis(hexyloxy)-[1,1′-biphenyl]-4-yl)-N-(4-bromophenyl)-2′,4′-bis(hexyloxy)-[1,1′-biphenyl]-4-amine (BPTPA donor, 5) was developed based on a new 4-step synthetic route. The BPTPA donor is used to quickly access two new sensitizers: NT35 and G220, which both exhibit greater IPCE response with cobalt-based redox shuttles, compared to the classical iodide/triiodide electrolyte. A reference sensitizer, G221, comprising only a TPA unit as the donor and cyanoacrylic acid as the acceptor/anchor unit, was also realized for comparative purposes. The devices consisting of the sensitizers bearing the BPTPA donor and cobalt-based redox shuttles showed the highest Jsc, Voc and IPCE spectra response compared to G221. The BPTPA donor is shown to efficiently suppress the electron recombination between the TiO2 surface and the redox shuttle yielding high Voc values. Among these sensitizers, G220 showed the best photovoltaic performance having a short-circuit photocurrent density (Jsc) of 14.8 mA cm−2, an open-circuit photovoltage (Voc) of 0.87 V, and a fill factor (FF) of 0.71, corresponding to an overall conversion efficiency of 9.06% under standard global AM 1.5 solar light conditions. These results demonstrate that organic sensitizers based on this bulky BPTPA donor are promising candidates for cobalt-based DSSCs.

Y-shaped dyes based on triphenylamine for efficient dye-sensitized solar cells

Three new triphenylamine-based dyes with Y-shaped conformation bearing triphenylamino-vinyl, 10-octyl-10H-phenothiazine-vinyl and 9-octyl-9H-carbazole-vinyl as arms (TT, TP, and TC) have been synthesized. From electrochemical investigations it is found that they can be employed in DSSCs due to the balanced HOMO and LUMO energy levels. Notably, the photo-to-electrical conversion efficiency of the DSSCs sensitized with branched TT, TP, and TC reach 5.12%, 4.84%, and 3.63%, which are higher than that sensitized with T (2.79%), and the DSSC sensitized with TT shows higher IPCE response and better photovoltaic performances (Jsc=12.37mA/cm2, Voc=0.72V and ff=0.58) than others. These results reveal that the introduction of branched Y-shaped extended π-conjugated donors to D-π-A dyes cannot only enlarge the spectral response range, but also suppress the molecular aggregation on TiO2 films to a certain extent, which would enhance the performance of DSSCs.

Optimization of dye-sensitized solar cells prepared by compression method

Nanostructured TiO 2 films have been deposited onto conducting glass and on flexible conducting plastic substrates using a compression technique. Dye-sensitized solar cells prepared from Degussa P25 TiO 2 powder, red dye (Ru(dcbpy) 2(SCN) 2) or black dye (Ru(tcterpy)(SCN) 3) and an electrolyte containing LiI and I 2 in 3-methoxypropionitrile were tested using standard photoelectrochemical techniques. The average overall efficiency of small open cells sensitized with the red dye on plastic substrates was 4.5% (100 W m −2). In a direct comparison, red and black dye gave about the same efficiencies. For both dyes, addition of 4- tert-butylpyridine to the electrolyte resulted in a decreased IPCE response in the whole spectral range, with a specific decrease in the red region, which is attributed to a shift in the conduction band edge of the nanostructured TiO 2.