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

Development of Highly Crystalline Donor–Acceptor-Type Random Polymers for High Performance Large-Area Organic Solar Cells

We developed donor–acceptor (D–A)-type random polymers based on 3,3′-difluoro-2,2′-bithiophene with various relative amounts of 5,6-difluoro-4,7-bis­(5-bromo-(2-decyl­tetradecyl)­thiophen-2-yl)-2,1,3-benzothiadiazole (2FBT) and 5,6-difluoro-4,7-bis­(5-bromo-(2-octyldodecyl)­thiophen-2-yl)-2-(3,4-dichloro­benzyloxybutyl)-2<i>H</i>-benzo­[<i>d</i>]­[1,2,3]­triazole (DCB-2FBTZ). Introducing small relative amounts of DCB-2FBTZ into the polymer was found to effectively enhance its solar cell performance, resulting in a power conversion efficiency of 9.02%, greater than the 7.29% that resulted from the PFBT-FTh copolymer. Moreover, when the active area of the BHJ film was increased to 1 cm², the solar cell reproducibly showed a high performance, here with an efficiency of 8.01% even when the thickness of the active layer was 313 nm. Our studies revealed that including the DCB-2FBTZ group in the polymer simultaneously improved the solution processability and crystallinity of the polymer. These improvements resulted in the formation of highly homogeneous BHJ films throughout large areas with only minor amounts of defects resulting from overaggregation and hence with appropriate morphologies for effective charge generation and transport

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

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

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

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

Highly Efficient Monolithic Dye-Sensitized Solar Cells

Monolithic dye-sensitized solar cells (M-DSSCs) provide an effective way to reduce the fabrication cost of general DSSCs since they do not require transparent conducting oxide substrates for the counter electrode. However, conventional monolithic devices have low efficiency because of the impediments resulting from counter electrode materials and spacer layers. Here, we demonstrate highly efficient M-DSSCs featuring a highly conductive polymer combined with macroporous polymer spacer layers. With M-DSSCs based on a PEDOT/polymer spacer layer, a power conversion efficiency of 7.73% was achieved, which is, to the best of our knowledge, the highest efficiency for M-DSSCs to date. Further, PEDOT/polymer spacer layers were applied to flexible DSSCs and their cell performance was investigated

Hierarchical Nanoflake Surface Driven by Spontaneous Wrinkling of Polyelectrolyte/Metal Complexed Films

A mechanical or physical change observed in nanocomposite thin films has recently offered new opportunities to generate intriguing nanostructures. In this study, we present a novel means of creating a hierarchically developed nanoflake structure by exploiting surface wrinkles that occur during the incorporation process of metallic nanoparticles into layer-by-layer assembled polyelectrolyte multilayer (PEM) thin films. The PEM film composed with linear polyethylenimine (LPEI) and poly(acrylic acid) (PAA) allows for facilitated cationic exchange reaction within the film even after the electrostatic complexation and chemical cross-linking reaction. The subsequent reduction process induces an <i>in situ</i> complexation of metallic nanoparticles with a PEM matrix, causing an accumulation of lateral compressive stress for surface wrinkling. The wrinkling characteristics of the complexed films can be theoretically interpreted by employing the gradationally swollen film model, whereby a gradual change in the elastic property along the axial direction of the film can be appropriately reflected. In addition, wrinkled surfaces are further processed to form vertically aligned and hierarchically ordered nanoflakes after selective removal of the PEM matrix with plasma ashing. Consequently, superhydrophobic surface properties (water contact angle = 170°, sliding angle <1°) can be attained from the hierarchical nanoflake structure. The method presented here is advantageous in that large-scale preparation can be readily implemented by a stepwise dipping process without resorting to specific patterning or a serially applied complex structuring process, which can provide a promising platform technique for various surface engineering applications

Highly Efficient Copper–Indium–Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Copper–indium–selenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO₂ electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date

Solution-Processed Ultrathin TiO₂ Compact Layer Hybridized with Mesoporous TiO₂ for High-Performance Perovskite Solar Cells

The electron transport layer (ETL) is a key component of perovskite solar cells (PSCs) and must provide efficient electron extraction and collection while minimizing the charge recombination at interfaces in order to ensure high performance. Conventional bilayered TiO₂ ETLs fabricated by depositing compact TiO₂ (c-TiO₂) and mesoporous TiO₂ (mp-TiO₂) in sequence exhibit resistive losses due to the contact resistance at the c-TiO₂/mp-TiO₂ interface and the series resistance arising from the intrinsically low conductivity of TiO₂. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO₂ layer hybridized with mp-TiO₂, which is fabricated by performing one-step spin-coating of a mp-TiO₂ solution containing a small amount of titanium diisopropoxide bis­(acetylacetonate) (TAA). By using electron microscopies and elemental mapping analysis, we establish that the optimal concentration of TAA produces an ultrathin blocking layer with a thickness of ∼3 nm and ensures that the mp-TiO₂ layer has a suitable porosity for efficient perovskite infiltration. We compare PSCs based on mesoscopic ETLs with and without compact layers to determine the role of the hole-blocking layer in their performances. The hybrid ETLs exhibit enhanced electron extraction and reduced charge recombination, resulting in better photovoltaic performances and reduced hysteresis of PSCs compared to those with conventional bilayered ETLs