Rediscovering a key interface in dye-sensitized solar cells: guanidinium and iodine competition for binding sites at the dye/electrolyte surface

Investigation of Perovskite Solar Cells Using Guanidinium Doped MAPbI3 Active Layer

In this work, guanidinium (GA+) was doped into methylammonium lead triiodide (MAPbI3) perovskite film to fabricate perovskite solar cells (PSCs). To determine the optimal formulation of the resulting guanidinium-doped MAPbI3 ((GA)x(MA)1-xPbI3) for the perovskite active layer in PSCs, the perovskite films with various GA+ doping concentrations, annealing temperatures, and thicknesses were systematically modulated and studied. The experimental results demonstrated a 400-nm-thick (GA)x(MA)1-xPbI3 film, with 5% GA+ doping and annealed at 90 °C for 20 min, provided optimal surface morphology and crystallinity. The PSCs configured with the optimal (GA)x(MA)1-xPbI3 perovskite active layer exhibited an open-circuit voltage of 0.891 V, a short-circuit current density of 24.21 mA/cm2, a fill factor of 73.1%, and a power conversion efficiency of 15.78%, respectively. Furthermore, the stability of PSCs featuring this optimized (GA)x(MA)1-xPbI3 perovskite active layer was significantly enhanced.

GABr Post-Treatment for High-Performance MAPbI3 Solar Cells on Rigid Glass and Flexible Substrate

Perovskite solar cells have exhibited astonishing photoelectric conversion efficiency and have shown a promising future owing to the tunable content and outstanding optoelectrical property of hybrid perovskite. However, the devices with planar architecture still suffer from huge Voc loss and severe hysteresis effect. In this research, Guanidine hydrobromide (GABr) post-treatment is carried out to enhance the performance of MAPbI3 n-i-p planar perovskite solar cells. The detailed characterization of perovskite suggests that GABr post-treatment results in a smoother absorber layer, an obvious reduction of trap states and optimized energy level alignment. By utilizing GABr post-treatment, the Voc loss is reduced, and the hysteresis effect is alleviated effectively in MAPbI3 solar cells. As a result, solar cells based on glass substrate with efficiency exceeding 20%, Voc of 1.13 V and significantly mitigated hysteresis are fabricated successfully. Significantly, we also demonstrate the effectiveness of GABr post-treatment in flexible device, whose efficiency is enhanced from 15.77% to 17.57% mainly due to the elimination of Voc loss.

Stability of Perovskites at the Surface Analytic Level

Organic-inorganic halide perovskites have attracted considerable attention in the past few years because of their remarkable performance in optoelectronic devices. However, long-term stability of the materials and devices remains the biggest challenge for realistic implementation of perovskite solar cells. Although significant efforts have been carried out on the causes of degradation at the device level, few measurements have been made at the surface analytic level to reveal the degradation mechanisms. In this Perspective, we review the effects of environmental factors, such as O₂, water, and light, on the perovskite layer by monitoring the intrinsic electronic structure and compositional changes in different exposure tests. This work contributes in developing better understanding of the degradation mechanisms and improving the overall stability of perovskite solar cells.

Iodine binding with thiophene and furan based dyes for DSCs

Donor–π-bridge–acceptor dyes with thiophene have been shown to bind I₂ leading to diminished dye-sensitized solar cell performances relative to furan based analogues.

Time-periodic oscillation reaction in an organic-solvent dominated electrolyte

Time-periodic phenomena widely exist in natural life sustaining systems but are rarely reported in highly efficient artificial electrochemical energy-harvesting systems. Herein, we observed for the first time the periodic oscillation reaction in organic-solvent dominated electrolytes that could be also derived from some electrochemical energy harvesting cells. Owing to different reaction activity and acidity in the organic solvent, the oscillation reaction could occur under milder pH conditions and exhibit better durability. Its influence on electric output and related differences from an aqueous B-Z reaction are discussed from a mechanism point of view. Our findings may contribute to smart self-oscillation materials and new strategies for highly efficient long-term energy harvesting.

Gel Electrolytes with Polyamidopyridine Dendron Modified Talc for Dye-Sensitized Solar Cells

Organic-inorganic hybrid layered materials are proposed as additives in a quasi-solid gel electrolyte for dye-sensitized solar cells. Talcs could provide a low-cost and environmentally friendly, as well as abundant, option as gelators. Here, talcs were prepared by functionalizing an organotalc with three polyamidopyridine dendron generations, PAMPy-talc-Gn (n = 1, 2 and 3). PAMPy dendrons grow parallel to the lamellae plane and form an organized structure by intermolecular interactions. In addition, polyiodide-dendron charge-transfer complexes were prepared onto the organotalc by adsorption of iodine. In this work, the effect of the dendron generation of PAMPy-talc and the influence of polyiodide intercalation on solar cell performance and stability were investigated. The best results were reached with the use of lowest-generation PAMPy-talc (η = 4.5 ± 0.3%, V_OC = 710 ± 19 mV, J_sc = 10.4 ± 0.9 mA cm^-2, and FF = 61 ± 2%): 15% higher efficiency compared to similar liquid devices. While some previously studied talcs illustrate very strong absorption of the iodide from the electrolyte, in the case of PAMPy-talc such interfering effects were absent: In a 1000 h light soaking test, the PAMPy-talc cells both with and without polyiodide intercalation demonstrated stable performances. Furthermore, the color analysis of the electrolyte indicated that the color of the electrolyte remained stable after an initial period of stabilization, which is a good indication of the compound being stable and not absorbing charge carriers from the electrolyte. The performance and stability results indicate that PAMPy-talc has potential as a gelling method for electrolytes for dye solar cells.

CdTe nanoparticles decorated titania for dye sensitized solar cell: a novel co-sensitizer approach towards highly efficient energy conversion

A hybrid TiO₂–CdTe multi-layer matrix fabricated for validation in a dye sensitized solar cell (DSSC) operating with N3 dye as the sensitizer.

Unveiling iodine-based electrolytes chemistry in aqueous dye-sensitized solar cells

Aqueous dye-sensitized solar cells (DSSCs) have recently emerged as promising systems, which can combine low cost and environmental compatibility with appreciable efficiency, long-term durability and enhanced safety. In the present study, we thoroughly investigate the chemistry behind the iodide/triiodide-based redox mediator, which presents - in a completely aqueous environment - several differences when compared to the behavior observed in the conventionally used organic solvents. The speciation of ions, the effect of the concentration of the redox mediator and the type of counter-ion are characterized from the electrochemical, spectroscopic, photovoltaic and analytical viewpoints. Furthermore, we demonstrate that aqueous DSSCs, often assumed as unstable, hold the potential to assure unparalleled stability after five months of aging without any addition of stabilizers or gelling agents, thus envisaging the construction of eco-friendly photovoltaic devices free of expensive, flammable and toxic solvents.

Enhanced Conversion Efficiencies in Dye-Sensitized Solar Cells Achieved through Self-Assembled Platinum(II) Metallacages

Two-component self-assembly supramolecular coordination complexes with particular photo-physical property, wherein unique donors are combined with a single metal acceptor, can be utilized for many applications including in photo-devices. In this communication, we described the synthesis and characterization of two-component self-assembly supramolecular coordination complexes (SCCs) bearing triazine and porphyrin faces with promising light-harvesting properties. These complexes were obtained from the self-assembly of a 90° Pt(II) acceptor with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPyT) or 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). The greatly improved conversion efficiencies of the dye-sensitized TiO₂ solar cells were 6.79 and 6.08 respectively, while these SCCs were introduced into the TiO₂ nanoparticle film photoanodes. In addition, the open circuit voltage (Voc) of dye-sensitized solar cells was also increased to 0.769 and 0.768 V, which could be ascribed to the inhibited interfacial charge recombination due to the addition of SCCs.

Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells

Hybrid perovskites have shown astonishing power conversion efficiencies owed to their remarkable absorber characteristics including long carrier lifetimes, and a relatively substantial defect tolerance for solution-processed polycrystalline films. However, nonradiative charge carrier recombination at grain boundaries limits open circuit voltages and consequent performance improvements of perovskite solar cells. Here we address such recombination pathways and demonstrate a passivation effect through guanidinium-based additives to achieve extraordinarily enhanced carrier lifetimes and higher obtainable open circuit voltages. Time-resolved photoluminescence measurements yield carrier lifetimes in guanidinium-based films an order of magnitude greater than pure-methylammonium counterparts, giving rise to higher device open circuit voltages and power conversion efficiencies exceeding 17%. A reduction in defect activation energy of over 30% calculated via admittance spectroscopy and confocal fluorescence intensity mapping indicates successful passivation of recombination/trap centers at grain boundaries. We speculate that guanidinium ions serve to suppress formation of iodide vacancies and passivate under-coordinated iodine species at grain boundaries and within the bulk through their hydrogen bonding capability. These results present a simple method for suppressing nonradiative carrier loss in hybrid perovskites to further improve performances toward highly efficient solar cells.

Redox Active Compounds in Controlled Radical Polymerization and Dye-Sensitized Solar Cells: Mutual Solutions to Disparate Problems

Controlled radical polymerization (CRP) and dye-sensitized solar cells (DSSCs) are two fields of research that at an initial glance appear to have little in common. However, despite their obvious differences, both in application and in scientific nature, a closer look reveals a striking similarity between many of the compounds widely used as control agents in radical polymerization and as redox couples in dye-sensitized solar cells. Herein, we review the various redox active compounds used and examine the characteristics that give them the ability to perform this dual function. In addition we explore the advances in the understanding of the structural features that enhance their activity in both CRP and DSSCs. It is hoped that such a comparison will be conducive to improving process performance in both fields.

Boosting the Open Circuit Voltage and Fill Factor of QDSSCs Using Hierarchically Assembled ITO@Cu2S Nanowire Array Counter Electrodes

The key challenges in enhancing the power conversion efficiency (PCE) of a quantum dot-sensitized solar cell (QDSSC) are efficiently achieving charge separation at the photoanode and improving the charge transfer, which is limited by the interface between the electrolyte and the counter electrode (CE). Here, hierarchically assembled ITO@Cu2S nanowire arrays with conductive single-crystalline ITO cores and Cu2S nanocrystal shells were designed as efficient QDSSCs CEs. These arrays not only provided an efficient three-dimensional charge transport network but also allowed for the effective deposition of more Cu2S nanocrystals as active sites to catalyze the electrolyte reaction. This design considerably reduced the sheet and charge transfer resistance of the CE, thus decreasing the series resistance and increasing the shunt resistance of the QDSSC. As a result, QDSSCs with this CE exhibited an unprecedentedly high Voc of 0.688 V, a fill factor of 58.39%, and a PCE of 6.12%, which is 21.2% higher than that of the conventional brass/Cu2S CE.

A comparative computational study on the interactions of N719 and N749 dyes with iodine in dye-sensitized solar cells

The intermolecular interactions of the two most basic Ru(II) complex dyes for dye-sensitized solar cells (DSSCs), N719 and N749, with the iodine species are investigated using density functional theory (DFT). In addition to interactions with a single I2 molecule, multiple I2 interactions and simultaneous interactions of I2 and I(-) occur. N719 with two isothiocyanato (NCS) ligands interacts with two I2 molecules via the two terminal S atoms in the ground singlet electronic state, whereas N749 with three NCS ligands forms three S···I-I bonds. Irrespective of the NCS position and the number of I2 molecules, N749 has a stronger interaction with I2 than N719. Conversely, the interaction of I(-) with oxidized N749 via the terminal S atom of the NCS ligand is weaker than that with oxidized N719. However, simultaneous interactions of oxidized N749 with two I2 molecules promote the I(-) interaction, and the I(-) interaction with N749 becomes stronger than that with N719 bonded to both an I2 and I(-). The computational results of multiple interactions between the dye and iodine species suggest that the difference in DSSC performance between N719 and N749 dyes is explained by recombination related to the I2 interaction and regeneration of the oxidized dye by I(-).

Enhanced performance of dye-sensitized solar cells with dual-function coadsorbent: reducing the surface concentration of dye–iodine complexes concomitant with attenuated charge recombination

We introduce a dual-function coadsorbent in DSCs, that play a beneficial role in the recombination and iodine binds to the N719 dye.

Theoretical description of efficiency enhancement in DSSCs sensitized by newly synthesized heteroleptic Ru complexes

Recently, some new series of heteroleptic ruthenium-based dyes, the so-called RD dyes, were designed and synthesized showing better performances compared to the well-known homoleptic N719.

Enhancing the stability of porphyrin dye-sensitized solar cells by manipulation of electrolyte additives

The use of porphyrin-based photosensitizers with superior light-harvesting properties has enabled the power conversion efficiency of dye-sensitized solar cells (DSCs) to reach 13 % under full sun illumination. However, a major limitation of such devices corresponds to the volatility of the solvent used so far for the electrolyte, which prevents practical applications. In this work, we describe a porphyrin-ionic liquid DSC, which not only affords the highest efficiency reported to date, but is also stable for more than 300 h under continuous full sun illumination at 60 °C. Furthermore, we identify a previously unreported pathway for device degradation, and show that the addition of N-methylbenzimidazole and a thiocyanate salt to the electrolyte is critical to obtaining long-lived devices.

Intermolecular interactions between a Ru complex and organic dyes in cosensitized solar cells: a computational study

Intermolecular interactions in cyclometalated Ru complex dye (FT89) dimers, carbazole organic dye (MK-45 and MK-111) dimers, FT89-MK-45 complexes, and FT89-MK-111 complexes were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) to elucidate the improvement mechanism of dye-sensitized solar cell (DSSC) performance due to cosensitization with FT89 and MK dyes. All of the dimers and complexes form intermolecular cyclic hydrogen bonds via the carboxyl groups. The FT89 dimer and complexes with the TiO2Na model system promote intermolecular interactions with I2via the NCS ligand of the FT89 monomer. The computational results verify that MK-111 behaves not only as a sensitizer but also inhibits FT89 aggregation by effectively serving as a coadsorbent similar to deoxycholic acid (DCA) in the dye solution, suppressing recombination of the injected electrons in TiO2 with I2, improving DSSC performance.