Direct Comparison of Electron Transport and Recombination Behaviors of Dye-Sensitized Solar Cells Prepared Using Different Sintering Processes

Flexible dye-sensitized solar cells on plastic substrates have achieved a conversion efficiency of 8.6% with the hot compression technique (<150 °C). However, the value of efficiency is only 70% of that achieved using glass substrates with high-temperature sintering technique (500 °C). Investigating the origin of this difference is a critical step for further improving the performance of plastic dye-sensitized solar cells. In this study, an optimized ternary viscous titania paste without the addition of organic binders enables the fabrication of efficient dye-sensitized solar cells with a low-temperature process. Therefore, the electron-transport behavior of dye-sensitized solar cells can be directly compared with those prepared with the high-temperature sintering technique. In addition to the structural and optical differences, the hot compressed photoanode of dye-sensitized solar cells have an electron diffusion coefficient that is 2 times smaller and a recombination time that is 6 times shorter than those of the high-temperature sintered cells, suggesting inadequate interparticle connections and more recombination events. These results indicate that electron transport and recombination are still the key factors governing the performance of low-temperature fabricated dye-sensitized solar cells. Eventually, the flexible cell with an efficiency of 6.81% has been achieved on flexible indium tin oxide/polyethylene naphthalate substrate. Further improvements in advanced low-temperature processing or novel materials with minimized defect or grain boundaries are required

Novel D‑π‑A Organic Dyes with Thieno[3,2-<i>b</i>]thiophene-3,4-ethylenedioxythiophene Unit as a π‑Bridge for Highly Efficient Dye-Sensitized Solar Cells with Long-Term Stability

This paper reports on new D-π-A organic dyes for application in dye-sensitized solar cells (DSSCs), which were developed by incorporating thieno­[3,2-b]­thiophene-thiophene (M9) and thieno­[3,2-b]­thiophene-EDOT (M10) as π-bridges. These dyes exhibited relatively small highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gaps in spite of the short π-conjugation lengths, resulting in broad spectral responses. As photosensitizers in DSSCs, M10 showed a broader spectral response than M9, leading to a greater short-circuit photocurrent (Jsc). In addition, M10 exhibited higher open-circuit voltage (Voc) compared to M9, because of the greater electron lifetime of the photoanode. The impedance analysis revealed that the greater electron lifetime of the photoanode with M10 was attributed to the lower electron recombination rate caused by the blocking effect of the bulky EDOT unit. As a result, M10 showed much higher conversion efficiency (η = 7.00%) than M9 (η = 5.67%) under one sun condition (AM 1.5 G, 100 mW/cm2). This conversion efficiency was comparable to that of the conventional Ru-based dye N719 (η = 7.24%) under the same condition. In addition, M10 exhibited a remarkable long-term stability, i.e., 95% of the initial conversion efficiency was maintained after light soaking for 45 days (1080 h)

Image1_Hot-injection synthesis of lead-free pseudo-alkali metal-based perovskite (TlSnX3) nanoparticles with tunable optical properties.JPEG

The commercialization of organo-inorganic hybrid perovskite materials for optoelectronic applications is limited owing to the restriction of lead (Pb) usage in consumer electronics and the instability of organic cations in the perovskite structure. To address these challenges, we synthesize TlSnX3 (X = Cl, Br, and I) perovskite nanoparticles (NPs) with high crystallinity and uniformity using the hot-injection method. The optical properties of TlSnX3 NPs are fine-tuned by substituting the halide ions of TlSnX3. In addition, the oxidation of Sn in TlSnX3 NPs is effectively prevented by the strong reducing ligands such as dioleamide (DOA) and trioctylphosphine (TOP). Furthermore, TlSnX3 NPs-based perovskite solar cells (PSCs) are fabricated by a spin-coating method; they exhibited a high open-circuit voltage (∼1.4 V). These results demonstrate that TlSnX3 NPs can be an attractive candidate for solution-processable optoelectronic devices.</p

Enhancing Stability of Perovskite Solar Cells to Moisture by the Facile Hydrophobic Passivation

In this study, a novel and facile passivation process for a perovskite solar cell is reported. Poor stability in ambient atmosphere, which is the most critical demerit of a perovskite solar cell, is overcome by a simple passivation process using a hydrophobic polymer layer. Teflon, the hydrophobic polymer, is deposited on the top of a perovskite solar cell by a spin-coating method. With the hydrophobic passivation, the perovskite solar cell shows negligible degradation after a 30 day storage in ambient atmosphere. Suppressed degradation of the perovskite film is proved in various ways: X-ray diffraction, light absorption spectrum, and quartz crystal microbalance. This simple but effective passivation process suggests new kind of approach to enhance stability of perovskite solar cells to moisture

Enhancing the Efficiency of Electron Conduction in Spray-Coated Anode of Photoelectrochemical Cell Using Oxygenated Multi-Walled Carbon Nanotubes

A multiwalled carbon nanotube (MWNT) was physically cured with oxygen plasma treatment, and the as-prepared oxygenated MWNT (OMWNT) was incorporated into TiO₂ nanopowders to prepare a spray-coatable OMWNT–TiO₂ composite suspension. The composite layer was directly formed on a fluorinated tin oxide surface by spray coating and served as a photoanode of a photoelectrochemical cell (PEC). The cell performance was optimized in terms of the plasma treatment time and compared with a conventional PEC, showing 37% increased energy conversion efficiency. The efficiency improvement confirmed by the electrochemical impedance spectra was related to the reduced charge-transfer resistance and efficient electron transport through the OMWNT network

Wide-Band-Gap (2.0 eV) Perovskite Solar Cells with a <i>V</i>_OC of 1.325 V Fabricated by a Green-Solvent Strategy

Perovskite-based tandem solar cells are promising candidates for next-generation photovoltaic devices. However, the defects caused by ion migration cause a large deficit of open-circuit voltage (VOC) in conventional wide-band-gap perovskite films. Here, we present a new strategy that employs nontoxic acetic acid and isopropanol as solvents to deposit a perovskite film with a 2.0 eV band gap in an ambient atmosphere. The in situ formed acetate anions strongly stabilize the intrinsic defects in perovskite films. These features effectively improve the phase stability of 2.0 eV Cs0.2FA0.8PbI0.9Br2.1 perovskite, allowing the VOC to reach 1.325 V and the corresponding power conversion efficiency to reach 10.62%, which is close to the state-of-art performance of perovskite solar cells employing perovskite around a 2.0 eV band gap

Importance of 4-<i>tert</i>-Butylpyridine in Electrolyte for Dye-Sensitized Solar Cells Employing SnO₂ Electrode

The photovoltaic performance of dye-sensitized solar cells (DSSCs) employing SnO2 electrodes was investigated while increasing the content of 4-tert-butylpyridine (TBP) in the conventional liquid-type electrolyte. As the added TBP content increased, the open circuit voltage (Voc) and conversion efficiency were highly enhanced while the short circuit current (Jsc) was not much affected. With the electrolyte of 2.0 M TBP, the Voc and conversion efficiency were increased by 26 and 33%, respectively, compared with the conventional electrolyte (0.5 M TBP). The electrochemical impedance spectra revealed that the enhancement of Voc resulted from the negative shift of the SnO2 conduction band potential and the increase in resistance of electron recombination by 1 order of magnitude. It is noteworthy that the optimized concentration of TBP for the SnO2 electrode is greatly larger than that for the TiO2 electrode. This may be due to the much faster electron recombination rate and more positive conduction band potential of the SnO2 electrode. The SnO2 electrode modified with TiO2 shell showed only slightly enhanced performance due to the similar effects of shell layer and those of the TBP. In contrast to the SnO2, TiO2 electrodes did not show performance enhancement with the electrolyte of TBP concentration larger than 0.5 M. The impedance spectra of symmetric dummy cells employing Pt counter electrodes indicated that the catalytic effect of Pt was deteriorated, and the resistance of electrolyte diffusion was increased by the higher concentration of TBP. This brings up the need for development of a counter electrode that TBP is not easily adsorbed on, and alternative additives to TBP which are not highly viscous

Synthesis and Charge Transport Properties of Conjugated Polymers Incorporating Difluorothiophene as a Building Block

A series of conjugated copolymers, PDPPFT and PNDIFT, were developed using difluoroterthiophene and DPP or NDI as the cobuilding block. We obtained two different molecular weight polymers for each polymer type by changing the conditions for the Stille coupling reaction and studied their optoelectrochemical properties and charge-transport behavior in organic field-effect transistors (OFETs). Both the lower molecular weight polymers, PDPPFT­(L) and PNDIFT­(L), showed better long-range ordered structures in films, whereas the polymers with higher molecular weights were less long-range ordered and showed a more preferential face-on orientation. By virtue of their favorable polymer packing structures, PDPPFT­(L) and PNDIFT­(L) exhibited much higher hole mobilities compared with their higher molecular weight counterparts, PDPPFT­(H) and PNDIFT­(H). By contrast, both PDPPFT and PNDIFT maintained good n-channel properties independent of their molecular weights, thus their long-range ordering in a film. The strong electron-withdrawing fluorine groups are favorable for stabilizing electrons on the polymer chain and would enable the polymer to transport electrons efficiently even in the case of a less-ordered packing structure with an unfavorable face-on orientation

Unbiased Sunlight-Driven Artificial Photosynthesis of Carbon Monoxide from CO₂ Using a ZnTe-Based Photocathode and a Perovskite Solar Cell in Tandem

Solar fuel production, mimicking natural photosynthesis of converting CO₂ into useful fuels and storing solar energy as chemical energy, has received great attention in recent years. Practical large-scale fuel production needs a unique device capable of CO₂ reduction using only solar energy and water as an electron source. Here we report such a system composed of a gold-decorated triple-layered ZnO@ZnTe@CdTe core–shell nanorod array photocathode and a CH₃NH₃PbI₃ perovskite solar cell in tandem. The assembly allows effective light harvesting of higher energy photons (>2.14 eV) from the front-side photocathode and lower energy photons (>1.5 eV) from the back-side-positioned perovskite solar cell in a single-photon excitation. This system represents an example of a photocathode–photovoltaic tandem device operating under sunlight without external bias for selective CO₂ conversion. It exhibited a steady solar-to-CO conversion efficiency over 0.35% and a solar-to-fuel conversion efficiency exceeding 0.43% including H₂ as a minor product