Bilayer Solar Cells Research Articles

Effect of donor\u2013acceptor molecular orientation on charge photogeneration in organic solar cells

The relative orientation of an electron donor and electron acceptor, which significantly affects charge photogeneration in an organic solar cell, is investigated here. The effects of the molecular orientations at the donor–acceptor heterojunction are examined using bilayer solar cells comprising a fixed acceptor layer and donor polymer layers that assume a variety of orientations. The orientation of the conjugated polymer is controlled during film formation using solvents with slow or fast drying rates. Although the donor polymer layers show similar light-harvesting and nongeminate recombination dynamics, photocurrent generation is more efficient at the face-on donor–acceptor interface than at the edge-on interface. Photophysical analysis reveals that the efficient charge generation at the face-on interface originates from enhanced exciton diffusion toward the donor–acceptor interface and reduced geminate recombination of charge pairs. These findings offer clear evidence that the separation efficiency of an interfacial charge pair is affected by the relative orientations of the donor and acceptor molecules. This orientation should be controlled to maximize the PCE of an organic solar cell.

Facile synthesis and optical properties of extended TPA-Benzodifuran derivatives connected by cyano-vinylene junctions

Two new conjugated molecules based on triphenylamine – benzo[1,2-b:4,5-b’]difuran moieties and integrating cyanovinyl bonds have been synthesized by using the new benzodifuran-acetonitrile synthon, easily and rapidly prepared in two steps from 2,2’-(2,5-dihydroxy-1,4-phenylene)diacetic acid. The electronic properties of the molecules were analysed by UV-Vis absorption and emission spectroscopies and cyclic voltammetry. The potential use of the molecules as donor materials for photovoltaic conversion were evaluated in simple bilayer solar cells using C60 as the acceptor materials. The comparison between the two derivatives reveals that the extension of the conjugated system with the insertion of furan cycles leads to a narrowing band gap, a better emission property, a direct access to dication state and an enhancement of the photovoltaic characteristics.

Self‐Doping Fullerene Electrolyte‐Based Electron Transport Layer for All‐Room‐Temperature‐Processed High‐Performance Flexible Polymer Solar Cells

AbstractTo achieve high‐performance large‐area flexible polymer solar cells (PSCs), one of the challenges is to develop new interface materials that possess a thermal‐annealing‐free process and thickness‐insensitive photovoltaic properties. Here, an n‐type self‐doping fullerene electrolyte, named PCBB‐3N‐3I, is developed as electron transporting layer (ETL) for the application in PSCs. PCBB‐3N‐3I ETL can be processed at room temperature, and shows excellent orthogonal solvent processability, substantially improved conductivity, and appropriate energy levels. PCBB‐3N‐3I ETL also functions as light‐harvesting acceptor in a bilayer solar cell, contributing to the overall device performance. As a result, the PCBB‐3N‐3I ETL‐based inverted PSCs with a PTB7‐Th:PC71BM photoactive layer demonstrate an enhanced power conversion efficiency (PCE) of 10.62% for rigid and 10.04% for flexible devices. Moreover, the device avoids a thermal annealing process and the photovoltaic properties are insensitive to the thickness of PCBB‐3N‐3I ETL, yielding a PCE of 9.32% for the device with thick PCBB‐3N‐3I ETL (61 nm). To the best of one's knowledge, the above performance yields the highest efficiencies for the flexible PSCs and thick ETL‐based PSCs reported so far. Importantly, the flexible PSCs with PCBB‐3N‐3I ETL also show robust bending durability that could pave the way for the future development of high‐performance flexible solar cells.

Improved photovoltaic performance of bilayer small molecular solar cells via geometrical rearrangement of active materials

Nowadays, non-fullerene electron transport material for the fabrication of organic photovoltaic devices has become one of extensive issue of research. But unfortunately, it has been observed that the replacements of the fullerene acceptors with non-fullerene acceptor materials typically led down the device efficiency. In this article, improvement in photovoltaic response of bilayer solar cells based on small molecular Zinc Phthalocyanine (ZnPc) and non-fullerene acceptor material (Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)) is reported via their geometrical rearrangement. The structural and optical property of ZnPc and PTCDA thin films were investigated using SEM, AFM, XRD and UV–vis absorption spectroscopy and their role on device performance is discussed in detail. It has been observed that the photovoltaic performance of the devices in PTCDA/ZnPc layer sequence exhibits 8 times higher value of JSC with almost double VOC that collectively shows 15 times higher value of the device efficiency as compared to the devices with ZnPc/PTCDA layer sequence. The results show that ZnPc and PTCDA exhibit complimentary absorption spectra but it would be efficient only if the materials are deposited in right sequence.

Toward Sustainable Organic Semiconductors from a Broad Palette of Green Reactions

New conjugated materials, based on the triphenylamine‐thiophene moiety and integrating azomethine bonds with a dibenzofuran unit or a cyanovinyl bond with a phenylthiophene or bithiophene unit, have been synthesized by using a wide range of green reactions such as direct heteroarylation coupling reactions, Knoevenagel and Schiff‐base condensations and Stille cross‐coupling reactions using ionic‐liquid‐supported thiophenylstannane. The electronic properties of the new molecules were analyzed by UV/Vis spectroscopy and cyclic voltammetry. The potential use of the molecules as donor materials for photovoltaic conversion were evaluated in simple bilayer solar cells using C60as the acceptor material.

Efficient Charge Separation of Cold Charge-Transfer States in Organic Solar Cells Through Incoherent Hopping.

We demonstrate that efficient and nearly field-independent charge separation of electron-hole pairs in organic planar heterojunction solar cells can be described by an incoherent hopping mechanism. Using kinetic Monte Carlo simulations that include the effect of on-chain delocalization as well as entropic contributions, we simulate the dissociation of the charge-transfer state in polymer-fullerene bilayer solar cells. The model further explains experimental results of almost field independent charge separation in bilayers of molecular systems with fullerenes and provides important guidelines at the molecular level for maximizing the efficiencies of organic solar cells. Thus, utilizing coherent phenomena is not necessarily required for highly efficient charge separation in organic solar cells.

Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells.

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.

High efficiency CH3NH3PbI3:CdS perovskite solar cells with CuInS2 as the hole transporting layer

Abstract The CH3NH3PbI3:CdS composite films are prepared by a newly developed precursor blending solution method, which are further used to fabricate CH3NH3PbI3:CdS perovskite solar cells. Our experimental results demonstrate that the introduced CdS effectively improves the light absorption property of the ITO/CuInS2/Al2O3/CH3NH3PbI3:CdS film stack and decreases the charge recombination in the prepared solar cells due to the formation of CH3NH3PbI3/CdS bulk heterojunction. Furthermore, the formed CdS/CuInS2 heterojunction also contributes to the enhanced efficiency. As a consequence, the CH3NH3PbI3/CdS bulk heterojunction perovskite solar cells exhibit a maximum power conversion efficiency of (16.5 ± 0.2)%, which is 1.35 times the best efficiency of 12.2% of previously reported CdS/CH3NH3PbI3 bilayer solar cell. In addition, this efficiency is a 59% improvement compared with the efficiency of (10.4 ± 0.2)% for the ITO/CuInS2/Al2O3/CH3NH3PbI3/PC60BM/Ag cell without CdS.

Liquid crystals in photovoltaics: a new generation of organic photovoltaics

This article presents an overview of the developments in the field of organic photovoltaics (PVs) with liquid crystals (LCs). A brief introduction to the PV and LC fields is given first, followed by application of various LCs in organic PVs. Details of LCs used in bilayer solar cells, bulk heterojunction solar cells and dye-sensitized solar cells have been given. All the liquid crystalline materials used in PVs are structured and the efficiency of solar cells is tabulated. Finally, an outlook into the future of this newly emerging, fascinating and exciting field of self-organizing supramolecular LC PV research is provided. Liquid crystals (LCs) have recently gained significant importance in organic photovoltaics (PVs). Power-conversion efficiency up to about 10% has reached in solar cells incorporating LCs. This review presents an overview of the developments in the field of organic PVs with LCs. Comprehensive details of LCs used in bilayer solar cells, bulk heterojunction solar cells and dye-sensitized solar cells have been given. An outlook into the future of this newly emerging, fascinating and exciting field of self-organizing supramolecular LC PV research is provided.

Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C60 and PCBM

In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by ...

Two Birds with One Stone: Tailoring Singlet Fission for Both Triplet Yield and Exciton Diffusion Length.

By direct imaging of singlet and triplet populations with ultrafast microscopy, it is shown that the triplet diffusion length and singlet fission yield can be simultaneously optimized for tetracene and its derivatives, making them ideal structures for application in bilayer solar cells.

Thieno[2,3-b]indole-Based Small Push-Pull Chromophores: Synthesis, Structure, and Electronic Properties.

Small push-pull molecules were synthesized in high yields by connecting a N-methyl or N-phenyl substituted thieno[2,3-b]indole electron-donating block directly to a 2,2-dicyanovinyl or (1-(dicyanomethylene)-3-oxo-1-inden-2-ylidene)methyl electron-withdrawing group. The effects of the N-substitution on thieno[2,3-b]indole and the nature of the electron-accepting group on the electrochemical, optical, and charge-transport properties were investigated by cyclic voltammetry, UV-vis spectroscopy, and the space-charge-limited current method, respectively. These results, together with the 1% power conversion efficiency of a bilayer solar cell prepared with the smallest compound of the series, show the potential of thieno[2,3-b]indole for organic electronics.

Investigating the effect of solvent boiling temperature on the active layer morphology of diffusive bilayer solar cells

Using chlorobenzene as a base solvent for the deposition of the poly(3-hexylthiophene-2,5-diyl) (P3HT) layer in P3HT:phenyl-C61-butyric acid methyl ester diffusive bilayer solar cells, we investigate the effect of adding of small amounts of high-boiling-point solvents with similar chemical structures on the resulting active layer morphologies. The results demonstrate that the crystallinity of the P3HT films as well as the vertical donor–acceptor gradient in the active layer can be tuned by this approach. The use of high-boiling-point solvents improved all photovoltaic parameters and resulted in a 32% increase in power conversion efficiency.

Modeling of optimum size and shape for high photovoltaic performance of poly(3-hexylthiophene) nanopore in interdigitated bilayer organic solar cells

The interdigitated design for donor–acceptor in solar cell has been studied in some detail, but the optimum size and shape leading to direct enhancement in nanopore (or nanopillar) structure is still not well understood. Here, we demonstrate a modeling method to forecast the optimum size and shape for poly(3-hexylthiophene) (P3HT) nanopores in interdigitated P3HT: [6, 6]-phenyl C61 butyric acid methyl ester (PCBM) photovoltaic device, based on experimental results of P3HT:PCBM bilayer solar cell. In our analysis, the energy generated at unit nanopore is supposed to the same as the one generated at infinite point of P3HT:PCBM bilayer solar cell with variable layer thickness. A definitive function in terms of a radius of unit nanopore with various shapes is established, substituting a regression function derived from the results of power conversion efficiency in bilayer solar cell. Interpreting the function, we finally showed that the effective radius for P3HT nanopores with rectangular or cylinder, cut-cone, cone shape should be less than 135, 53, 2 nm respectively.

High-Performance Small Molecule/Polymer Ternary Organic Solar Cells Based on a Layer-By-Layer Process.

PSS), and then the mixed solution of polymer and fullerene derivative was spin-coated on top of a small molecule layer. In this method, the small molecules in crystalline state were effectively mixed in the active layer. Without further optimization of the morphology of the ternary blend, a high power conversion efficiency (PCE) of 8.76% was obtained with large short-circuit current density (Jsc) (17.24 mA cm(-2)) and fill factor (FF) (0.696). The high PCE resulted from not only enhanced light harvesting but also more balanced charge transport by incorporating small molecules.

Impact of Molecular Orientation and Spontaneous Interfacial Mixing on the Performance of Organic Solar Cells

A critically important question that must be answered to understand how organic solar cells operate and should be improved is how the orientation of the donor and acceptor molecules at the interface influences exciton diffusion, exciton dissociation by electron transfer, and recombination. It is exceedingly difficult to probe the orientation in bulk heterojunctions because there are many interfaces and they are arranged with varying angles with respect to the substrate. One of the best ways to study the interface is to make bilayer solar cells with just one donor–acceptor interface. Zinc phthalocyanine is particularly interesting to study because its orientation can be adjusted by using a 2 nm-thick copper iodide seed layer before it is deposited. Previous studies have claimed that solar cells in which fullerene acceptor molecules touch the face of zinc phthalocyanine have more current than ones in which the fullerenes touch the edge of zinc phthalocyanine because of suppressed recombination. We have more thoroughly characterized the system using in situ X-ray photoelectron spectroscopy and X-ray scattering and found that the interfaces are not as sharp as previous studies claimed when formed at room temperature or above. Fullerenes have a much stronger tendency to mix into the face-on films than into the edge-on films. Moreover we show that almost all of the increase in the current with face-on films can be attributed to improved exciton diffusion and to the formation of a spontaneously mixed interface, not suppressed recombination. This work highlights the importance of spontaneous interfacial molecular mixing in organic solar cells, the extent of which depends on molecular orientation of frontier molecules in donor domains.

Energy Transfer Directly to Bilayer Interfaces to Improve Exciton Collection in Organic Photovoltaics

Ternary blends and energy cascades are gaining popularity as ways to engineer absorption as well as exciton and charge collection in organic solar cells. Here, we use kinetic Monte Carlo simulations to investigate energy cascade designs for improving exciton collection in bilayer solar cells via a Forster energy transfer mechanism. We determine that an interfacial monolayer (C) between the donor and acceptor with a D → A → C energy cascade will lead to good exciton collection, allowing for >90% collection, even for energy donor layers up to 75 nm thick. We further examine how roughening the interface, increasing the exciton diffusion length, and using other energy cascade designs affect the enhancement from the energy transfer. We also propose using the inherent charge transfer states at the interfaces as energy acceptors and estimate that the Forster radius could be as large as 3.4 nm, leading to nearly 70% improvement in exciton collection, without the need for a third material.

Sea urchin-like TiO2 microspheres as scattering layer of nanosized TiO2 film-based dye-sensitized solar cell with enhanced conversion efficiency

Sea urchin-like TiO2 microspheres are synthesized through a facile one-step solvothermal process by using titanium tetrabutoxide (TBOT) as titanium source. The obtained TiO2 microspheres with ∼3 μm diameter display anatase phase and hierarchical structures composed of TiO2 nanoribbons containing secondary nanoparticles with an average particle size of ∼15 nm. By using as photoanode material, the anatase TiO2 microspheres film-based dye-sensitized solar cells (DSSCs) show higher short-circuit current but slightly lower conversion efficiency than the TiO2 nanoparticles (P25) film-based one; while the bilayer film-based solar cell by using the TiO2 microspheres as overlayer of the P25 film exhibits significantly improved photovoltaic performance with an efficiency up to 7.14%, improved by 26.6% as compared to the P25 film-based one (5.64%). The anatase TiO2 microspheres as overlayer can not only enhance the light harvesting capability due to their scattering effect of the sea urchin-like hierarchical structures, but also contribute to the larger dye-loading amount, which lead to a decrease in the charge transfer resistance, charge recombination and dark current, and then resulting in the improved photovoltage and photovoltaic performance. The simple fabrication method of the bilayer solar cell, which contains a photoanode composed of P25 film as underlayer and sea urchin-like TiO2 hierarchical microspheres as overlayer, demonstrates a strategy for the development of the low cost and high efficiency solar cells through tuning the photoanode's component and structure.

A Combined Theoretical and Experimental Study of Dissociation of Charge Transfer States at the Donor-Acceptor Interface of Organic Solar Cells.

The observation that in efficient organic solar cells almost all electron-hole pairs generated at the donor-acceptor interface escape from their mutual coulomb potential remains to be a conceptual challenge. It has been argued that it is the excess energy dissipated in the course of electron or hole transfer at the interface that assists this escape process. The current work demonstrates that this concept is unnecessary to explain the field dependence of electron-hole dissociation. It is based upon the formalism developed by Arkhipov and co-workers as well as Baranovskii and co-workers. The key idea is that the binding energy of the dissociating "cold" charge-transfer state is reduced by delocalization of the hole along the polymer chain, quantified in terms of an "effective mass", as well as the fractional strength of dipoles existent at the interface in the dark. By covering a broad parameter space, we determine the conditions for efficient electron-hole dissociation. Spectroscopy of the charge-transfer state on bilayer solar cells as well as measurements of the field dependence of the dissociation yield over a broad temperature range support the theoretical predictions.

(Invited) Impact of Indiumand Gallium Doping on the Photovoltaic Performance of CdSe Quantum Dot Hybrid Solar Cells

Colloidal CdSe quantum dots show great promise for fabrication of hybrid solar cells with enhanced power conversion efficiency, yet controlling the doping on the atomic length scale is challenging. Here, we demonstrate that gallium-, indium-, tin-doped CdSe quantum dots show significantly improved conductivity and charge carrier density, and also temperature dependent behavior. Furthermore, the doped CdSe hybrid solar cells greatly enhance photocurrent and photovoltage, in which the gallium doped CdSe quantum dots and P3HT bi-layer heterojunction solar cells leads to a maximum power conversion efficiency of 2.0% at elevated temperatures under AM 1.5 solar illumination. All the doped samples exhibit inverted temperature dependent power conversion of the photovoltaic cells, which could be effectively utilized in solar concentrators. The approach presented can be applied to a wide range of doped quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general tactic for improving the efficiency of quantum dot based solar cells.