Effect and position of molecularly engineered bipropylenedioxythiophene-bridged donar/ acceptor dyes in dye-sensitized solar cells (DSSC): homo and hetero-dimeric Di-anchoring organic dyes for panchromatic light absorption with controlled dye aggregation and reduced charge recombination

Abstract

Chapter 1. Introduction a) Dye-sensitized solar cells: Dye-sensitized solar cells have emerged as potential device architecture to harvest the solar energy due to easy device fabrication and diversity in key components such as semiconductors, dyes, electrolytes and cathode materials. Ever since the discovery of homoleptic metallated ruthenium-based dyes with 7.12% device efficiency by O’Regan and Grätzel, last two and half decade research on the functional materials provided deeper understanding of device structure and various pathways associated with interfacial charge transfer and recombination processes. Dye design with panchromatic absorption, avoiding both charge recombination and dye aggregation achieved high device efficiencies by increasing the Jsc and Voc besides the choice of electrolyte having minimum loss-in-potentials. The structure-property relationship of organometallic and metal-free organic dyes have been carried out, so far the best conversion efficiency of about 13% was achieved for zinc-porphyrin dye and 12.5% for indenoperylene dye with cobalt (II/III) redox couple.

One of the advantages of having D/A metal free organic dyes is that it utilizes simple precursors to tune the optical and electrochemical properties and which developed high molar absorptivity dyes with strong intermolecular charge transfer (ICT). Dyes with D--A, and D-A--A configurations provided good harvesting efficiency in the visible and far-red regions, respectively, whereas few chromophores such as polymethine, porphyrin, and phthalocyanine dyes have been explored for photo-current generation in near infrared (NIR) region. DSSC device contains dye coated TiO2 as photo anode, platinized FTO as cathode with electrolyte (Figure 1). Upon photo-excitation, a photo-excited electron is injected from the excited state (LUMO) of the dye into the conduction band of the TiO2. The oxidized dye molecules are then regenerated by the electrolyte leaving the oxidized electrolyte in the system. The injected electron in TiO2 percolates through the film and collected at a transparent conductive layer of fluorine-doped tin oxide (FTO) glass substrate, on which the TiO2 film is formed. After passing through an external circuit, the electron is reintroduced into the DSSC device at the Pt cathode, where the oxidized electrolyte is reduced to regenerate the electrolyte. b) Statement of Problem: Out of various interfaces in DSSC device, dye-TiO2 interface plays an important role for charge injection and dye regeneration besides aggregation of dye on the TiO2 surface. Charge injection from aggregated structure that contributes to the device performance is varied and which can be modulated by co-adsorbing the dyes with optically transparent CDCA. Hence, controlling the dye-dye interaction on the TiO2 surface has become one of the challenging tasks for synthetic chemists besides having desired optical band gap of dyes. Dimeric dyes that are connected through spiro--spacer showed better device efficiency than corresponding monomeric units. Generally, for panchromatic absorption, two or more dyes with complementary absorption properties were adsorbed to enhance the light harvesting properties over solar spectrum. In the present approach, this critical problem is solved by connecting two structurally different dyes through spiro-linkage. Spiro-based -spacer spiroBiProDOT has been explored to synthesize dimeric dyes with orthogonal dye orientation.

Chapter 2. 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 chapter, a series of homodimeric (D1-D1 and D2-D2) and heterodimeric (D1-D2 and D2-D4) donor/acceptor (D/A) dyes containing spiroBiProDOT-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and compared the device performance with model monomeric dyes, D1 and D2 (Figure 2). 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 and16.16 mA/cm2, respectively. On the other hand, the monomeric D1 exhibited a PCE of 3.2 % (Voc of 0.64V and Jsc of 7.2 mA/cm2), which is lower by 2.2 fold compared to dimeric D1-D1 analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO2 surface due to four sp3-centers, which arrest the molecular motions after chemisorption. This study showed a new molecular approach to incorporate two different chromophores in the dimeric dye possessing complementary absorption characteristics towards panchromatic light absorption. The attenuated charge recombination at TiO2/Dye/redox couple interface in case of D1-D1, owing to better passivation of TiO2 surface due to the spiro spacer was elucidated through electrochemical impedance spectroscopy. 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.

Chapter 3. Symmetrical and Unsymmetrical Bi-Chromophoric Squaraine Based Sensitizer for Dye-Sensitized Solar Cells Dimeric derivatives of symmetrical (Dsq-Dsq) and unsymmetrical (D1-Dsq) spiro-squaraine dyes were synthesized by using direct arylation method (Figure 3). Photo-physical, electrochemical properties were evaluated for the feasibility of charge injection and dye regeneration processes which is required for utilizing the synthesized dyes as an active material for photovoltaic device fabrication. A model squaraine dye Dsq was also synthesized and characterized. In Dsq-Dsq dye, two Dsq monomeric units were attached through spiroBiProDOT unit. Similarly, D1-Dsq contains two different dyes namely Dsq and D1 (donor-- acceptor) connected through spiro-spacer. Excitation energy transfer between photoexcited D1 and Dsq was explained by using Förster resonance energy transfer (FRET). Compared to Dsq-Dsq, unsymmetrical spiro-squaraine dye, D1-Dsq, is more efficient in terms of circuit voltage Voc = 0.607 V, short-circuit current density Jsc = 6.6 mA/cm2, fill factor (ff) = 69% and power conversion efficiency (η) = 2.8 % under 1 sun, AM1.5G, whereas D1-Dsq dye showed the Voc = 0.612 V, Jsc = 9 mA/cm2, ff = 71% and η = 3.9%. From the IPCE profile of D1-Dsq, absence of response in the visible region, where D1 absorb the photons, indicates that efficient energy transfer from exited D1 to Dsq. A new dye D1-Dsq has been developed to improve the performance of DSSCs by Förster resonance energy transfer (FRET), in addition to retarding the charge recombination for the improved DSSC device performance.

Chapter 4. Effect of Position of Spiro-Bipropylenedioxythiophene-Spacer in D--A Dyes for Dye-Sensitized Solar Cell To understand the effect of position spiro-spacer in D--A dyes, spiro-spacer near (TT1) and away (T1T) from anchoring unit were designed, synthesized and characterized (Figure 4). The dimeric spiro-dye T1T showed higher power conversion efficiency (PCE), with 3.9 % with a Voc and Jsc of 0.593 V and 9.09 mA/cm2, respectively, under simulated AM 1.5G illumination (100 mW/cm2) which is 1.6 times higher than that of isomeric TT1. Further electrochemical impedance spectroscopy indicates that the charge transfer resistance for the T1T (8.62 ohm) is higher than that of TT1 (6.75 ohm). Accordingly, dye TT1T designed and synthesized which showed the PCE of 4.16 % (Voc and Jsc are 0.589 V and 9.79 mA/cm2 respectively). The positioning of branching spiroBiProDOT-spacer makes an impact on the device performance, which might be due to effective passivation, originates from strong interaction between the dye and TiO2 in the case of T1T.

Chapter 5. Orthogonally Functionalized Molecular Diad Containing D--A Dye and Hole Transport Material for Dye-Sensitized Solar Cell An orthogonally functionalized molecular diad containing D--A dye and hole transport material (D1-HTM)for dye-sensitized solar cell were synthesized (Figure 5) and characterized by electrochemical, photo physical methods and fabricating photovoltaic devices. The main objective of this work is to embarrass the charge recombination by appending a hydrophobic triarylamine moiety, which is orthogonally attached to that D--A part through spiroBiProDOT-spacer. Dye regeneration may be faster due to faster hole transfer from dye (D--A) to I-/ I3- system through HTM part. Fluorescence quenching experiments indicate that electron transfer from hole transporting material (PTS1, HTM-1) to excited D1-D1 and D1 dye. Cyclic voltammetry was carried out to evaluate the HOMO energy level of D1-HTM and LUMO level was calculated from the band gap. LUMO energy level of this dye is higher than that of the conduction band of TiO2 and at the same time HOMO is lower than that of the energy level of the electrolyte which is required for an efficient charge injection and dye regeneration processes. Photo-voltaic studies shown that the best efficiency (2.24 % with a Voc and Jsc of 0.590 V and 5.27 mA/cm2) was obtained of D1-HTM by using 5 equivalent of CDCA.