Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine-Fullerene Dyads

Challenges in the design and synthesis of self-assembling molecules as selective contacts in perovskite solar cells

Self-assembling molecules (SAMs), as selective contacts, play an important role in perovskite solar cells (PSCs), determining the performance and stability of these photovoltaic devices. These materials offer many advantages over other traditional materials used as hole-selective contacts, as they can be easily deposited on a large area of metal oxides, can modify the work function of these substrates, and reduce optical and electric losses with low material consumption. However, the most interesting thing about SAMs is that by modifying the chemical structure of the small molecules used, the energy levels, molecular dipoles, and surface properties of this assembled monolayer can be modulated to fine-tune the desired interactions between the substrate and the active layer. Due to the important role of organic chemistry in the field of photovoltaics, in this review, we will cover the current challenges for the design and synthesis of SAMs PSCs. Discussing, the structural features that define a SAM, (ii) disclosing how commercial molecules inspired the synthesis of new SAMs; and (iii) detailing the pros- and cons- of the reported synthetic protocols that have been employed for the synthesis of molecules for SAMs, helping synthetic chemists to develop novel structures and promoting the fast industrialization of PSCs.

Oxidation State Tuning of Room Temperature Phosphorescence and Delayed Fluorescence in Phenothiazine and Phenothiazine-5,5-dioxide Dimers

Heterocyclic dimers consisting of combinations of butterfly-shaped phenothiazine (PTZ) and its chemically oxidized form phenothiazine-5,5-dioxide (PTZ(SO2 )) have been synthesized. A twist is imposed across the dimers by ortho-substituents including methyl ethers, sulfides and sulfones. X-ray crystallography, cyclic voltammetry and optical spectroscopy, underpinned by computational studies, have been employed to study the interplay between the oxidation state, conformational restriction, and emission mechanisms including thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). While the PTZ(SO2 ) dimers are simple fluorophores, the presence of PTZ induces triplet-mediated emission with a mixed PTZ-PTZ(SO2 ) dimer displaying concentration dependent hallmarks of both TADF and RTP.

Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization

A series of new naphthalimide and phenothiazine-based push-pull systems (NPI-PTZ1-5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.

Photoinduced charge-separated molecular probe for ultrasensitive spectrum analysis and rapid colorimetric detection of platinum ions

Increased utilization of platinum ions in chemicals and drugs escalates environmental pollution and toxicity associated with Pt ions. However, current analysis and detection strategies of Pt ions display limited sensitivity due to the similar inert metal nature of platinum to gold. Herein, a photoinduced charge-separated molecule (MTPA)₂Ab was synthesized as a probe for enhanced sensitive selection of Pt ions. Long-lived charge-separated states generated upon exposure to 365 nm light lead to a stable complex between (MTPA)₂Ab and PtCl₂/PtCl₄ with highly-selectivity via sequential photoinduced electron transfers. Owing to the linear relationship of complex characteristic absorption and fluorescence emission intensities to Pt²⁺/Pt⁴⁺ concentrations, ultrasensitive spectrum analysis of Pt ions is achieved with a detection limit of 14.2 nM (2.8 ppb) for Pt²⁺ and 12.6 nM (2.5 ppb) for Pt⁴⁺ by an absorption spectrometer and 9.8 nM (1.9 ppb) for Pt ions (Pt²⁺/Pt⁴⁺) by a fluorescence spectrometer, far less than the reported values. Furthermore, a portable test box is developed based on (MTPA)₂Ab test strips due to distinguishable color change with Pt²⁺/Pt⁴⁺ concentrations for rapid colorimetric detection of Pt ions. The results highlight the promise of photoinduced charge-separated molecular probe in ultrasensitive and rapid detection of Pt ions to overcome current limitations of detection strategies.

Phenothiazine functional materials for organic optoelectronic applications

Phenothiazine (PTZ) is one of the most extensively investigated S, N heterocyclic aromatic hydrocarbons due to its unique optical, electronic properties, flexibility of functionalization, low cost, and commercial availability. Hence, PTZ and its derivative materials have been attractive in various optoelectronic applications in the last few years. In this prospective, we have focused on the most significant characteristics of PTZ and highlighted how the structural modifications such as different electron donors or acceptors, length of the π-conjugated system or spacers, polar or non-polar chains, and other functional groups influence the optoelectronic properties. This prospective provides a recent account of the advances in phenothiazine derivative materials as an active layer(s) for optoelectronic (viz. dye sensitized solar cells (DSSCs), perovskite solar cells (PSCs), organic solar cells (OSCs), organic light-emitting diodes (OLEDs), organic field-effect transistor (OFETs), chemosensing, nonlinear optical materials (NLOs), and supramolecular self-assembly applications. Finally, future prospects are discussed based on the structure-property relationship in PTZ-derivative materials. This overview will pave the way for researchers to design and develop new PTZ-functionalized structures and use them for various organic optoelectronic applications.

Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine-C₆₀ dyads has been investigated. For this, two C₆₀-SiPc-C₆₀ dyads, 1 and 2, varying in their donor-acceptor distance, have been newly synthesized and characterized. In the case of C₆₀-SiPc-C₆₀ 1 where the SiPc and C₆₀ are separated by a phenyl spacer, faster electron transfer was observed with k_cs equal to 2.7×10⁹ s^-1 in benzonitrile. However, in the case of C₆₀-SiPc-C₆₀ 2, where SiPc and C₆₀ are separated by a biphenyl spacer, a slower electron transfer rate constant, k_cs=9.1×10⁸ s^-1, was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of ³SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3-20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc^.+-C₆₀ ^.- charge separated states (1.57 eV) populated the low-lying ³SiPc* (1.26 eV) prior returning to the ground state.

Phenothiazine-Based Hole Transport Materials for Perovskite Solar Cells

The promising photovoltaic solar cells based on the perovskite light-harvesting materials have attracted researchers with their outstanding power conversion efficiencies (over 23% certified). The perovskite work has geared up in just under a decade and is competing with well-established semiconductor technologies such as silicon (Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe). To commercialize the perovskite solar cells, their stability is the major concern. To address the stability issue, several factors need to be taken into account, and one of them is developing stable hole transport materials (HTMs), which are the essential building blocks. In this mini-review, we will discuss the important features of the HTMs, such as design and development of phenothiazine-based HTMs. Since phenothiazine is a low cost and stable molecule compared to the spiro-OMeTAD, it can be modified further via molecular engineering.

Molecular Semiconductor Surfactants with Fullerenol Heads and Colored Tails for Carbon Dioxide Photoconversion

The leaf is a prime example of a material converting waste (CO2 ) into value with maximum sustainability. As the most important constituent, it contains the coupled photosystems II and I, which are imbedded in the cellular membrane of the chloroplasts. Can key functions of the leaf be packed into soap? We present next-generation surfactants that self-assemble into bilayer vesicles (similar to the cellular membrane), are able to absorb photons of two different visible wavelengths, and exchange excited charge carriers (similar to the photosystems), followed by conversion of CO2 (in analogy to the leaf). The amphiphiles contain five dye molecules as the hydrophobic entity attached exclusively to one hemisphere of a polyhydroxylated fullerene (Janus-type). We herein report on their surfactant, optical, electronic, and catalytic properties. Photons absorbed by the dyes are transferred to the fullerenol head, where they can react with different species such as CO2 to give formic acid.

trans-A2B-corrole bearing 2,3-di(2-pyridyl)quinoxaline (DPQ)/phenothiazine moieties: synthesis, characterization, electrochemistry and photophysics

Singlet–singlet energy transfer and electron transfer processes in corrole–phenothiazine and corrole–DPQ dyads were demonstrated by using electrochemical and fluorescence (steady-state and time-resolved) spectral studies.

Phenothiazine-based small-molecule organic solar cells with power conversion efficiency over 7% and open circuit voltage of about 1.0 V using solvent vapor annealing

We have used two unsymmetrical small molecules with a D–A–D–π–D configuration as small molecule donors, along with PC₇₁BM as an acceptor, for solution processed bulk heterojunction solar cells.

Crystallisation-enhanced bulk hole mobility in phenothiazine-based organic semiconductors

A series of three novel donor-acceptor systems based on C(3)-malononitrile-substituted phenothiazines was synthesised in good overall yields and their thermal, spectroscopic, and electrochemical properties were characterised. The compounds were prepared through a sequence of Ullmann-coupling, Vilsmeier-Haack formylation and Knoevenagel-condensation, followed by Suzuki-coupling reactions for introduction of aryl substitutents at C(7) position of the phenothiazine. The introduction of a donor unit at the C(7) position exhibited a weak impact on the optical and electrochemical characteristics of the compounds and led to amorphous films with bulk hole mobilities in the typical range reported for phenothiazines, despite the higher charge delocalisation as attested by computational studies. In contrast, highly ordered films were formed when using the C(7)-unsubstituted 3-malononitrile phenothiazine, exhibiting an outstanding mobility of 1 × 10⁻³ cm² V⁻¹ s⁻¹, the highest reported for this class of compounds. Computational conformational analysis of the new phenothizanes suggested that free rotation of the substitutents at the C(7) position suppresses the ordering of the system, thereby hampering suitable packing of the new materials needed for high charge carrier mobility.

Enhanced Internal Quantum Efficiency in Dye-Sensitized Solar Cells: Effect of Long-Lived Charge-Separated State of Sensitizers

Effective charge separation is one of the key determinants for the photovoltaic performance of the dye-sensitized solar cells (DSSCs). Herein, two charge-separated (CS) sensitizers, MTPA-Pyc and YD-Pyc, have been synthesized and applied in DSSCs to investigate the effect of the CS states of the sensitizers on the device's efficiency. The CS states with lifetimes of 64 and 177 ns for MTPA-Pyc and YD-Pyc, respectively, are formed via the photoinduced electron transfer (PET) from the 4-styryltriphenylamine (MTPA) or 4-styrylindoline (YD) donor to the pyrimidine cyanoacrylic acid (Pyc) acceptor. DSSCs based on MTPA-Pyc and YD-Pyc exhibit high internal quantum efficiency (IQE) values of over 80% from 400 to 600 nm. In comparison, the IQEs of the charge transfer (CT) sensitizer cells are 10-30% lower in the same wavelength range. The enhanced IQE values in the devices based on the CS sensitizers are ascribed to the higher electron injection efficiencies and slower charge recombination. The results demonstrate that taking advantage of the CS states in the sensitizers can be a promising strategy to improve the IQEs and further enhance the overall efficiencies of the DSSCs.

Through-space electronic communication of zinc phthalocyanine with substituted [60]Fullerene bearing O2Nxaza-crown macrocyclic ligands

Two new macrocyclic ligands containing 17- and 19-membered O₂N₃-donor aza-crowns anchored to [60]Fullerene were synthesized and characterized by employing HPLC, electrospray ionization mass (ESI-MS), ¹H and ¹³C NMR, UV-vis, IR spectroscopies, as well as powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) in solid state. TGA measurements revealed that upon linking each of these macrocycle rings to [60]Fullerene, the decomposition point measured for [60]Fullerene moiety was increased, indicating on the promoted stability of [60]Fullerene backbone during binding to these macrocyclic ligands. Moreover, the ground state non-covalent interactions of [60]Fullerene derivatives of O₂N_x (x=2, and 3) aza-crown macrocyclic ligands namely, L¹-L⁴ with zinc phthalocyanine (ZnPc) were also investigated by UV-vis absorption, steady state and time resolved fluorescence spectrophotometry in N-methyl-2-pyrrolidone (NMP). The calculation of Stern-Volmer constants (K_SV) indicated on existence of an efficient quenching mechanism comprising of the excited singlet state of ZnPc in the presence of L¹-L⁴. The observation of an appropriate correlation between decrease in fluorescence intensity and lifetime parameters led us to propose the occurrence of a static mechanism for the fluorescence quenching of ZnPc in the presence of L¹-L³. The binding constants (K_BH) of L¹-L⁴/ZnPc were also determined applying the fluorescence quenching experiments. Meanwhile, the incompatibility of both K_SV and K_BH values found for L⁴ was also described in terms of structural features using DFT calculations using the B3LYP functional and 6-31G* basis set.

Regioisomer effects of [70]PCBM on film structures and photovoltaic properties of composite films with a crystalline conjugated polymer P3HT

The β-isomer of [70]PCBM induced a face-on P3HT packing, resulting in the superior hole mobility and photovoltaic properties.

Hypochlorite-promoted inhibition of photo-induced electron transfer in phenothiazine–borondipyrromethene donor–acceptor dyad: a cost-effective and metal-free “turn-on” fluorescent chemosensor for hypochlorite

APTZ-BODIPY based fluorescent chemosensor was designed and used for hypochlorite detection.