Polyimide-Based Covalent Organic Framework as a Photocurrent Enhancer for Efficient Dye-Sensitized Solar Cells.

Covalent organic frameworks (COFs) are of great interest in the energy and optoelectronic fields due to their high porosity, superior thermal stability, and highly ordered conjugated architecture, which are beneficial for charge migration, charge separation, and light harvesting. In this study, polyimide COFs (PI-COFs) are synthesized through the condensation reaction of pyromellitic dianhydride (PMDA) with tris(4-aminophenyl) amine (TAPA) and then doped in the TiO2 photoelectrode of a dye-sensitized solar cell (DSSC) to co-work with N719 dye to explore their functionality. As a benchmark, the pristine DSSC without the doping of PI-COFs exhibits a power conversion efficiency of 9.05% under simulated one sun illumination. The doping of 0.04 wt % PI-COFs contributes an enhanced short-circuit current density (JSC) from 17.43 to 19.03 mA/cm2, and therefore, the cell efficiency is enhanced to 9.93%. The enhancement of JSC is attributed to the bifunctionality of PI-COFs, which enhances the charge transfer/injection and suppresses the charge recombination through the host (PI-COF)-guest (N719 dye) interaction. In addition, the PI-COFs also function as a cosensitizer and contribute a small quantity of photoinduced electrons upon sunlight illumination. Surface modification of oxygen plasma improves the hydrophilicity of PI-COF particles and reinforces the heterogeneous linkage between PI-COF and TiO2 nanoparticles, giving rise to more efficient charge injection. As a result, the champion cell exhibits a high power conversion efficiency of 10.46% with an enhanced JSC of 19.43 mA/cm2. This methodology of increasing solar efficiency by modification of the photoelectrode with the doping of PI-COFs in the TiO2 nanoparticles is promising in the development of DSSCs.

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.

Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination.

Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π-π interaction, self-aggregation of dyes leads to reduced cell performance. In this report, a series of homodimeric (D1-D1 and D2-D2) and heterodimeric (D1-D2 and D2-D4) donor/acceptor (D/A) dyes containing spiroBiProDOT π-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and correlates the device performance with monomeric dyes (D1 and D2). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye D1-D1 showed higher power conversion efficiency (PCE), of 7.6% with a Voc and Jsc of 0.672 V and 16.16 mA/cm2, respectively. On the other hand, the monomeric D1 exhibited a PCE of 3.2% (Voc of 0.64 V and Jsc of 7.2 mA/cm2), which is lower by 2.4 fold compared to dimeric analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO2 due to four sp3-centers, which arrest the molecular motions after chemisorption. This study shows a new molecular approach to incorporate two chromophores in the dimeric dye possessing complementary absorption characteristics toward panchromatic absorption. The attenuated charge recombination at TiO2/Dye/redox couple interface in case of D1-D1, owing to better passivation of TiO2 surface, was elucidated through impedance analysis. The FT-IR spectrum of D1-D1 adsorbed on TiO2 film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO2.

Effect of Out-of-Plane Alkyl Group's Position in Dye-Sensitized Solar Cell Efficiency: A Structure-Property Relationship Utilizing Indoline-Based Unsymmetrical Squaraine Dyes.

Squaraine dyes are promising chromophores to harvest visible and near-infrared (NIR) photons. A series of indoline-based unsymmetrical squaraine (SQ) dyes that contain alkyl chains at sp3 C- and N- atoms of indoline moieties with a carboxylic acid anchoring group were synthesized. The optical and electrochemical properties of the SQ dyes in solution were nearly identical as there was no change in the D-A-D SQ framework; however, remarkable changes with respect to the power conversion efficiencies (PCE) were observed depending upon the position of alkyl groups in the dye. Introduction of alkyl groups to the indoline unit that was away from anchoring unit were helped in more dye loading with controlled organization of dyes on surface, increased charge transfer resistance, long electron lifetime, and hence higher PCE than that of the corresponding isomer in which the alkyl groups funtionalized indoline unit contains the carboxylic acid anchoring group. Careful analysis of incident photon-to-current conversion efficiency (IPCE) profiles indicated the presence of aggregated structure on the TiO2 surface that contributes to the charge injection in the presence of a coadsorbent. A dye-sensitized solar cell (DSSC) device made out of SQ5 was achieved an efficiency of 9.0%, with an open-circuit potential (Voc) of 660 mV and short-circuit current density (Jsc) of 19.82 mA/cm2, under simulated AM 1.5G illumination (100 mW/cm2). The IPCE profile of SQ5 shows an onset near to 750 nm with a good quantum efficiency (>80%) in the range of 550-700 nm, indicating the importance of self-organization of dyes on the TiO2 surface for an efficient charge injection. This present investigation revealed the importance of position of alkyl groups in the squaraine-based dyes for the better PCE.