Promising Power Conversion Efficiency Research Articles

Novel High Pressure Exfoliated Graphene-Based Semitransparent Stable DSSCs for Building Integrated Photovoltaics

Integrating dye-sensitized solar cells (DSSCs) with a building’s architecture is required for its commercialization. Coupling semitransparent designer DSSCs with windows has the dual benefit of providing daylighting and power generation. To achieve this, we report a low-cost, novel, high-pressure exfoliation technique for graphene and utilize it as a transparent counter electrode for fabrication of semitransparent DSSCs. By adopting environmentally friendly and economic exfoliated graphene instead of conventional platinum, the overall device cost comes down. The electrocatalytic behavior of fabricated transparent graphene counter electrode was assessed using cyclic voltammetry, Tafel plot in symmetry cell configuration, and impedance spectroscopy. We have fabricated DSSC with >70% transmittance in the visible spectrum, which gives promising power conversion efficiency of 3.19%. The fabricated cells were stable for more than 500 h under constant illumination with no significant efficiency drop. Also, we ha...

N-doped carbon@Cu nanocomposites as counter electrode catalysts in quantum dot-sensitized solar cells

To improve the catalytic activity of carbons in counter electrodes (CEs), Cu nanoparticles (NPs) were introduced into mesoporous N-doped carbons (N-C) derived from metal-organic frameworks (MOFs) to prepare the composites of N-C and Cu NPs (N-C@Cu) via impregnation and subsequent reduction method in this work. The obtained N-C@Cu composites with 2–4 nm Cu NPs uniformly distributing in N-C were deposited onto fluorine doped tin oxide (FTO) substrates by screen-printing to fabricate CEs in quantum dot solar cells (QDSCs). The N-C@Cu CEs exhibit higher electrocatalytic activity than both pristine N-C/FTO and commonly used Cu2S/FTO CEs, which could be ascribed to the synergistic effect between CuxS NPs with the excellent electrocatalytic ability and MOF derived N-C with high graphitization, large hydrophilic surface as well as the abundant dopant active sites on the reduction of polysulfide. The superior performance of N-C@Cu composites results in an improved fill factor and a high short circuit current density. Based on optimized N-C@Cu CEs and Zn-Cu-In-Se quantum dot sensitizers, the QDSCs obtained present a promising power conversion efficiency of 9.23%, significantly superior to those of pristine N-C/FTO (7.20%) and Cu2S/FTO (8.89%) CEs based QDSCs.

Lewis Acid–Base Interaction-Induced Porous PbI2 Film for Efficient Planar Perovskite Solar Cells

The quality of perovskite film is an important factor influencing the photovoltaic properties of solar cells, which is greatly affected by deposition methods. Here, Lewis acid–base interaction-induced porous PbI2 film is obtained by 4-tert-butylpyridine (TBP) vapor treatment method. X-ray diffraction, Raman spectroscopy, and Fourier transform infrared characteristics indicate that a new complex including both TBP and PbI2 molecules was formed by Lewis acid–base reaction. A continuous, uniform, and PbI2-free perovskite film with large grains was obtained from the porous PbI2 reacted completely with CH3NH3I. As a result, a promising power conversion efficiency of ∼18% is achieved in planar-heterojunction perovskite solar cells. Furthermore, via controlling the vapor treatment time, the porosity and thickness of PbI2 film can be readily adjusted, and the unexpected mesoporous perovskite film was first fabricated. Our work demonstrates the preparation of porous PbI2 films by Lewis base vapor treatment for hig...

Wide-Bandgap Conjugated Polymers Based on Alkylthiofuran-Substituted Benzo[1,2-b:4,5-b′]difuran for Efficient Fullerene-Free Polymer Solar Cells

The alkythiofuran-based benzo[1,2-b:4,5-b′]difuran (BDFS) unit presents the planar and rigid structure with favorable inter/intramolecular interactions and is expected to be a promising building block to construct the highly efficient photovoltaic polymers. Herein, we reported two novel wide-bandgap conjugated polymers, PBDFS-Bz and PBDFS-fBz, which were based on BDFS and benzo[d][1,2,3]triazole (Bz) units, and investigated their photovoltaic performances in fullerene-free polymer solar cells (PSCs). PBDFS-Bz and PBDFS-fBz show similar optical properties with the wide bandgaps of ∼1.88 eV. The HOMO energy level of PBDFS-Bz is at −5.30 eV, whereas it is downshifted to −5.45 eV for PBDFS-fBz due to the strong electron-withdrawing fluorine substitutes. The optimized fullerene-free PSC based on PBDFS-Bz:ITIC achieved a power conversion efficiency (PCE) of 8.07% with a Voc of 0.82 V, a Jsc of 15.14 mA cm–2, and a FF of 65%. Under the same conditions, PBDFS-fBz:ITIC device won a promising PCE of 9.00% with an e...

Enhancement of the Photovoltaic Properties of Dye‐Sensitized Solar Cells Using Y0.80Yb0.18Er0.02OF Nanorods

AbstractThe photoanode plays a critical role in influencing the performance of dye‐sensitized solar cells (DSSCs). In this work, Y0.80Yb0.18Er0.02OF nanorods (YYEOF NRs) are synthetized by a simple hydrothermal method and incorporated into the TiO2 film to form a bi‐layer photoanode consisting of nanoparticles (NPs) on the bottom and nanorods on top. Owing to the improved light collection efficiency and electron transport rate, the resultant YYEOF NRs are advantageous as photoanode dopants for DSSCs. Following assembly into an integrated DSSC, an enhanced photocurrent and power conversion efficiency (PCE) are observed. Benefiting from the p‐type doping effect of the YYEOF NRs, the photovoltage of the DSSCs is elevated. The cell performance can be maximized through optimization of the fraction of YYEOF NRs, and the results demonstrate that a promising PCE of 7.74 % is determined for the DSSC with 5 wt % YYEOF NRs, yielding 32.5 % enhancement in comparison to pristine TiO2 NPs‐based solar cells. This work provides a promising strategy to explore photoanode materials, which will lead to further efficiency improvements for solar cells.

Near-Infrared Harvesting Fullerene-Free All-Small-Molecule Organic Solar Cells Based on Porphyrin Donors

Fullerene-free all-small-molecule organic photovoltaic cells (SM-OPVs) have attracted considerable attention because of well-defined molecular structures with low batch-to-batch variation. Porphyrin derivatives have recently emerged as one of the promising conjugated building blocks for the small-molecule (SM) donors. Herein, we first report fullerene-free SM-OPVs employing porphyrin-based donors. Three zinc porphyrin (PZn)-based SM donors, which have strong bimodal absorption in the visible region and near-infrared region, are synthesized. Constructing bulk-heterojunction (BHJ) active layers using the PZn donors and a SM acceptor, IDIC, which have complementary absorption, achieved panchromatic photon-to-current-conversion from 400 to 900 nm. The manipulation of side chains in the PZn donors considerably influenced the molecular ordering and nanomorphology of the BHJ active layers. PZn-based fullerene-free SM-OPV devices with promising power conversion efficiency of 6.13% were achieved, which also offers...

Heterojunction Hybrid Solar Cells by Formation of Conformal Contacts between PEDOT:PSS and Periodic Silicon Nanopyramid Arrays.

Surface nanotexturing with excellent light-trapping property is expected to significantly increase the conversion efficiency of solar cells. However, limited by the serious surface recombination arising from the greatly enlarged surface area, the silicon (Si) nanotexturing-based solar cells cannot yet achieve satisfactory high efficiency, which is more prominent in organic/Si hybrid solar cells (HSCs) where a uniform polymer layer can rarely be conformably coated on nanotextured substrate. Here, the HSCs featuring advanced surface texture of periodic upright nanopyramid (UNP) arrays and hole-conductive conjugated polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), are investigated. The tetramethylammonium hydroxide etching is used to smooth the surface morphologies of the Si-UNPs, leading to reduced surface defect states. The uniform Si-UNPs together with silane chemical-incorporated PEDOT:PSS solution enable the simultaneous realization of excellent broadband light absorption as well as enhanced electrical contact between the textured Si and the conducting polymer. The resulting PEDOT:PSS/Si HSCs textured with UNP arrays show a promising power conversion efficiency of 13.8%, significantly higher than 12.1% of the cells based on the-state-of-the-art surface texture with random pyramids. These results provide a viable route toward shape-controlled nanotexturing-based high-performance organic/Si HSCs.

Engineering the interconnecting layer for efficient inverted tandem polymer solar cells with absorption complementary fullerene and nonfullerene acceptors

Applying materials with complementary optical absorption spectra to construct tandem solar cell can fulfill further improvement of photovoltaic performance, since tandem solar cell can harvest more photons and enhance the open-circuit voltage of the devices. For tandem solar cells, an interconnecting layer (ICL), which physically and electrically connects the front and the rear subcells, is essential to minimize the energy losses. Herein, we adopt MoO3/Ag/PFN-Br as interconnecting layer to join front subcell of PBDTBDD:PC70BM and rear subcell of PTB7-Th:IEICO-4F for construction the inverted tandem solar cell with structure of ITO/TIPD/PBDTBDD:PC70BM/MoO3/Ag/PFN-Br/PTB7-Th:IEICO-4F/MoO3/Al. The tandem devices demonstrate strong and wide light response in the wavelength between 300 and 1000 nm. A promising power conversion efficiency (PCE) of 9.74% has been achieved with an open circuit voltage (Voc) of 1.503 V. The PCE of tandem cell is 41.36% higher than that (6.89%) of front subcell and 27.82% higher than that (7.62%) of rear subcell.

New Strategy for Two‐Step Sequential Deposition: Incorporation of Hydrophilic Fullerene in Second Precursor for High‐Performance p‐i‐n Planar Perovskite Solar Cells

AbstractIn p‐i‐n planar perovskite solar cells (pero‐SCs) based on methylammonium lead iodide (MAPbI3) perovskite, high‐quality MAPbI3 film, perfect interfacial band alignment and efficient charge extracting ability are critical for high photovoltaic performance. In this work, a hydrophilic fullerene derivative [6,6]‐phenyl‐C61‐butyric acid‐(3,4,5‐tris(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB‐OEG) is introduced as additive in the methylammonium iodide precursor solution in the preparation of MAPbI3 perovskite film by two‐step sequential deposition method, and obtained a top‐down gradient distribution with an ultrathin top layer of PCBB‐OEG. Meanwhile, a high‐quality perovskite film with high crystallinity, less trap‐states, and dense‐grained uniform morphology can well grow on both hydrophilic (poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonic acid)) and hydrophobic (polytriarylamine, PTAA) hole transport layers. When the PCBB‐OEG‐containing perovskite film (pero‐0.1) is prepared in a p‐i‐n planar pero‐SC with the configuration of ITO/PTAA/pero‐0.1/[6,6]‐phenyl‐C61‐butyric acid methyl ester/Al, the device delivers a promising power conversion efficiency (PCE) of 20.2% without hysteresis, which is one of the few PCE over 20% for the p‐i‐n planar pero‐SCs. Importantly, the pero‐0.1‐based device shows an excellent stability that can retain 98.4% of its initial PCE after being exposed for 300 h under ambient atmosphere with a high humidity, and the flexible pero‐SCs based on pero‐0.1 also demonstrate a promising PCE of 18.1%.

Conformation Locking on Fused‐Ring Electron Acceptor for High‐Performance Nonfullerene Organic Solar Cells

AbstractIn this work, sidechain engineering on conjugated fused‐ring acceptors for conformation locking is demonstrated as an effective molecular design strategy for high‐performance nonfullerene organic solar cells (OSCs). A novel nonfullerene acceptor (ITC6‐IC) is designed and developed by introducing long alkyl chains into the terminal electron‐donating building blocks. ITC6‐IC has achieved definite conformation with a planar structure and better solubility in common organic solvents. The weak electron‐donating hexyl upshifts the lowest unoccupied molecular orbital level of ITC6‐IC, resulting in a higher VOC in comparison to the widely used ITIC. The OSCs based on PBDB‐T:ITC6‐IC reveal a promising power conversion efficiency of 11.61% and an expected high VOC of 0.97 V. The weaker π–π stacking induced by steric hindrance affords ITC6‐IC with enhanced compatibility with polymer donors. The blend film treated with suitable thermal annealing exhibits a fibril crystallization feature with a good bicontinuous network morphology. The results indicate that the molecular design approach of ITC6‐IC can be inspirational for future development of nonfullerene acceptors for high efficiency OSCs.

Organic photovoltaic cells – promising indoor light harvesters for self-sustainable electronics

Organic photovoltaic (OPV) cells using BTR:PC71BM show promising power conversion efficiency of >28% under 1000 lux generating 78.2 μW cm−2, outperforming Si based PV cells and comparable to GaAs PV cells. This result suggests that OPV cells have excellent potential for indoor applications.

Carbazole-based twin molecules as hole-transporting materials in dye-sensitized solar cells

Three new carbazole-based twin molecules with non-conjugated linear alkyl chains of various length as linker were synthesized in a one-step reaction. The new hole-transporting materials TCz-C3, TCz-C6, and TCz-C12 exhibit molecular glass behavior and the glass transition temperature can be tuned via the length of the linker. The optical and electrochemical properties are not significantly altered by the twin molecular structure, while the glass transition temperature increases and the hole mobility is strongly increased with reduced chain length by up to almost one order of magnitude, as determined from OFET measurements in saturation regime, demonstrating that conjugated linkers are not necessarily required in the design of twin molecules. The molecule with the shortest chain length (C3), namely TCz-C3, exhibited a promising power conversion efficiency of 2.21% in solid-state DSSC using the D102 dye as organic sensitizer.

Environmentally Friendly Solvent-Processed Organic Solar Cells that are Highly Efficient and Adaptable for the Blade-Coating Method.

The power conversion efficiencies (PCEs) of state-of-the-art organic solar cells (OSCs) have increased to over 13%. However, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical production. Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this study, a highly efficient PBTA-TF:IT-M-based device processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, is fabricated; a high PCE of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. More importantly, a blade-coated non-fullerene OSC processed with THF/IPA is demonstrated for the first time to obtain a promising PCE of 11.7%; even for the THF/IPA-processed large-area device (1.0 cm2 ) made by blade-coating, a PCE of 10.6% can still be maintained. These results are critical for the large-scale production of highly efficient OSCs in future studies.

Approaches for Selective Synthesis of Ullazine Donor–Acceptor Systems

Methods for effective synthesis for the four possible isomeric 3,9-diphenylullazine carboxaldehydes and reactive halogen intermediates are described. Ullazine donor-acceptor (D-A) dyes were studied using UV/Vis, photoluminescence (PL) spectroscopy and cyclic voltammetry. X-ray single crystal diffraction analysis independently confirmed the structures of two key intermediates. A D-A dye based on ullazine with dihexylmalonate acceptor was tested as a dopant-free hole-transporting material (HTM) in a perovskite solar cell, exhibiting promising power conversion efficiency (PCE) reaching 13.07 %.

The Effect of Donor and Nonfullerene Acceptor Inhomogeneous Distribution within the Photoactive Layer on the Performance of Polymer Solar Cells with Different Device Structures.

Due to the inhomogeneous distribution of donor and acceptor materials within the photoactive layer of bulk heterojunction organic solar cells (OSCs), proper selection of a conventional or an inverted device structure is crucial for effective exciton dissociation and charge transportation. Herein, we investigate the donor and acceptor distribution within the non-fullerene photoactive layer based on PBDTTT-ET:IEICO by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) and scanning Kelvin probe microscopy (SKPM), indicating that more IEICO enriches on the surface of the photoactive layer while PBDTTT-ET distributes homogeneously within the photoactive layer. To further understand the effect of the inhomogeneous component distribution on the photovoltaic performance, both conventional and inverted OSCs were fabricated. As a result, the conventional device shows a power conversion efficiency (PCE) of 8.83% which is 41% higher than that of inverted one (6.26%). Eventually, we employed nickel oxide (NiOx) instead of PEDOT:PSS as anode buffer layer to further enhance the stability and PCE of OSCs with conventional structure, and a promising PCE of 9.12% is achieved.

Uniform, high crystalline, (100) crystal orientated perovskite films without PbI2 residue by controlling the nanostructure of PbI2

The photovoltaic performance and stability of perovskite solar cells (PVSCs) is deeply dependent on the morphology of the perovskite film and the amount of lead iodide (PbI2) residue. However, the morphology of perovskite film fabricated through two-step sequential deposition method usually suffers from poor film coverage and low crystallinity due to the ungovernable crystallization kinetics. Moreover, owing to the compact stacking of PbI2 crystals, the conversion of PbI2 is incomplete as well. In this work, we propose for the first time to control the nanostructure of PbI2 to get high crystallinity, identical (100) crystal orientation and PbI2 residue-free film. Porous PbI2 with plate structure was obtained by selected solvent annealing (SSA). The reaction rate between PbI2 and methylammonium iodide (MAI) was accelerated because the contact area between PbI2 and MAI was enlarged due to the porous nanostructure of PbI2. Thus, uniform, high crystalline perovskite film without PbI2 residue was obtained. Furthermore, (100) crystal orientation of perovskite was obtained due to the PbI2 shape changing from grain to plate, which originated from the changing growth frontier of PbI2 crystals. As a result, a promising power conversion efficiency of 16.61% is achieved in planar-heterojunction PVSCs due to improved photoabsorption and carrier extraction efficiency. This work for the first time establishes the relationship between PbI2 structure and the corresponding structure of perovskite in detail, and shows a reasonable design of PbI2 nanostructure is important to improve the perovskite morphology.

Thiophene-Fused Naphthalene Diimides: New Building Blocks for Electron Deficient π-Functional Materials

Abstract Development of novel π-conjugated building blocks that can be integrated into molecular or macromolecular systems is key to the evolution of new superior organic semiconductors utilized as the active materials in organic electronics devices such as organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and organic thermoelectric (TE) devices. This review affords a brief overview of thiophene-fused naphthalene diimide (NDI), namely naphtho[2,3-b:6,7-b′]dithiophene diimide (NDTI) and naphtho[2,3-b]thiophene diimide (NTI), recently developed as novel electron deficient building blocks for n-type and ambipolar organic semiconductors. These thiophene-fused NDI building blocks had not been known until 2013 owing to their synthetic difficulty; more precisely, the difficulty in attaching fused-thiophene ring(s) on the NDI core. We have successfully established a thiophene-annulation reaction on ethyne-substituted NDI derivatives, which allows us to elaborate various NDTI and NTI derivatives. The key features of these building blocks are low-lying energy levels of lowest unoccupied molecular orbitals (LUMO, 3.8–4.1 eV below the vacuum level) and easy functionalizability of the thiophene α-positions, which allows their derivatives and polymers to conjugate efficiently with additional π- and co-monomer units. These features make the NDTI- and NTI-derivatives and polymers promising n-type and ambipolar materials for OFETs and acceptors for OPVs. In fact, various useful materials have already been derived from the NDTI and NTI building blocks: air-stable n-type small molecules and polymers with high electron mobility (∼0.8 cm2 V−1 s−1), ambipolar oligomers and polymers with well-balanced hole and electron mobilities, doped n-type semiconductors affording bulk conductors applicable to n-type TE materials, and electron acceptor molecules and polymers for OPVs showing promising power conversion efficiencies of up to 9%. These impressive and diversified device performances testify the usefulness of thiophene-fused NDI building blocks in the development of new electron deficient π-functional materials.

A double-layered photoanode made of ZnO/TiO2 composite nanoflowers and TiO2 nanorods for high efficiency dye-sensitized solar cells

We present a simple sol-gel hydrothermal process for the fabrication of a double-layered structure composed of a TiO2 nanorod overlayer and TiO2 nanoparticle-embedded ZnO nanoflower (ZNFs@TNPs-TNRs) underlayer. The ZNFs@TNPs-TNRs was used as a photoanode in dye-sensitized solar cells (DSSCs) and their photovoltaic performance was analyzed. The ZNFs@TNPs-TNRs can enhance the adsorption of N719 dyes, charge transport, and light scattering. The cell performances can be maximized by optimizing thickness ratio and total thickness of the double-layered photoanode, and the preliminary results demonstrate that a promising power conversion efficiency (PCE) of 8.01% is determined on the DSSC with ZNFs@TNPs-TNRs anode, yielding a 28.9% enhancement in the PCE in comparison to pristine TiO2–P25 nanoparticle-based DSSC.

Efficiency Rating of Various PV Technologies under Different Indoor Lighting Conditions

We used the RTOS (real-time one-sweep) method for I-V measurement, which derives the results with better accuracy by eliminating the capacitance effect in real time. We also used this method to compare emerging PV cells exhibiting promising power conversion efficiency and no hysteresis behavior under indoor lighting simulator with solar simulator. In this study, we defined performance ratios to compare indoor lighting with solar illumination. The indoor/outdoor ratios show different trends depending on the PV device type, which could highlight the specific type of PV that harvests better under indoor lighting and is preferable to indoor applications.

Efficient and Stable Inverted Planar Perovskite Solar Cells Employing CuI as Hole‐Transporting Layer Prepared by Solid–Gas Transformation

AbstractThe inorganic p‐type semiconductor CuI possesses several unique characteristics such as high transparency, low‐production cost, high hole mobility, and good chemical stability and is a promising hole‐transporting material candidate that can be explored in solar‐cell devices. Herein, we adopt a simple solid–gas reaction method to fabricate a uniform CuI film by exposing a thermally evaporated copper film to iodine vapor and apply it as a hole‐transporting layer (HTL) in inverted planar perovskite solar cells (PSCs). The optimized devices display a promising power conversion (PCE) efficiency of 14.7 %, with an open‐circuit voltage of 1.04 V, a short‐circuit current density of 20.9 mW cm−2, and a fill factor of 0.68. This is one of the highest PCE values reported so far for CuI‐based HTL in PSCs. Moreover, the devices studied also exhibit good long‐term stability at ambient atmosphere, arising from the hydrophobicity of CuI HTL. The results highlight that CuI fabricated using the simple and low‐temperature processing method presented here holds great promise as low‐cost alternative HTL material for the development of efficient and stable inverted planar PSCs in the future.