Bulk Heterojunction Devices Research Articles

Oriented thin films of mixture of a low-bandgap polymer and a fullerene derivative prepared by friction-transfer method

We have been studying oriented thin films of polymers fabricated by the friction-transfer method, which allows the alignment of a variety of conjugated polymers into highly oriented films. In this study, we prepared oriented blend films of a mixture of a low-bandgap polymer, poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7), and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), which is a promising combination for application in organic solar cells. We obtained oriented blend films of PTB7 and PC71BM by the friction-transfer method from a solid block. Polarized UV–visible spectra show that the PTB7 chains were aligned parallel to the friction direction in the blend films. Grazing-incidence X-ray diffraction (GIXD) studies with synchrotron radiation suggested that the preferred orientation of PTB7 crystallites was face-on in the blend films. The GIXD results also showed the high uniaxial orientation of PTB7 chains in blend films. Photovoltaic devices were fabricated using the friction-transferred blend films of the PTB7 and PC71BM. These bulk heterojunction devices showed better performance than planar heterojunction devices fabricated using pure friction-transferred PTB7 films.

Generating a three-dimensional non-fullerene electron acceptor by combining inexpensive spiro[fluorene-9,9′-xanthene] and cyanopyridone functionalities

A spiro[fluorene-9,9′-xanthene]-functionalized non-fullerene acceptor A1 [D : A1 = 1 : 1.2; P3HT(D) = 5.84%, PTB7(D) = 7.21%].

Melt-processing of small molecule organic photovoltaics via bulk heterojunction compatibilization.

Melt-processing of organic semiconductors (OSCs) is a promising environmentally-friendly technique that can alleviate dependence on toxic chlorinated solvents. While melt-processed single-component OSC devices (e.g. field-effect-transistors) have been demonstrated, multi-component bulk heterojunctions (BHJs) for organic photovoltaics (OPVs) remain a challenge. Herein, we demonstrate a strategy that affords tunable BHJ phase segregation and domain sizes from a single-phase homogeneous melt by employing strongly-crystalline small-molecule OSCs together with a customized molecular compatibilizing (MCP) additive. An optimized photoactive BHJ with 50 wt% MCP achieved a device power conversion efficiency of ca. 1% after melting the active layer at 240 °C (15 min, followed by slow cooling) before deposition of the top electrode. BHJ morphology characterization using atomic force and Kelvin probe microscopy, X-ray diffraction, and photo-luminescence measurements further demonstrate the trade-off between free charge generation and transport with respect to MCP loading in the BHJ. In addition, a functional OPV was also obtained from the melt-processing of dispersed micron-sized solid BHJ particles into a smooth and homogeneous thin-film by using the MCP approach. These results demonstrate that molecular compatibilization is a key prerequisite for further developments towards true solvent-free melt-processed BHJ OPV systems.

An efficient non-fullerene acceptor based on central and peripheral naphthalene diimides.

Through the coupling of central and terminal naphthalene diimide functionalities, a unique non-fullerene electron acceptor, coded as N10, was designed, synthesized, characterized and applied in solution-processable bulk-heterojunction devices. The target N10 displayed good solubility, excellent thermal stability and energy levels complementing those of the conventional donor polymer poly(3-hexyl thiophene) (P3HT). An excellent power conversion efficiency of 7.65% was obtained in simple BHJ devices (P3HT : N10 1 : 1.2), which is the highest observed so far for NDI core-based non-fullerene acceptors.

The Impact of Graphene on the Fabrication of Thin Film Solar Cells: Current Status and Future Prospects.

Commercial solar cells have a power conversion efficiency (PCE) in the range of 10–22% with different light absorbers. Graphene, with demonstrated unique structural, physical, and electrical properties, is expected to bring the positive effects on the development of thin film solar cells. Investigations have been carried out to understand whether graphene can be used as a front and back contacts and active interfacial layer in solar cell fabrication. In this review, the current progress of this research is analyzed, starting from the graphene and graphene-based Schottky diode. Also, the discussion was focused on the progress of graphene-incorporated thin film solar cells that were fabricated with different light absorbers, in particular, the synthesis, fabrication, and characterization of devices. The effect of doping and layer thickness of graphene on PCE was also included. Currently, the PCE of graphene-incorporated bulk-heterojunction devices have enhanced in the range of 0.5–3%. However, device durability and cost-effectiveness are also the challenging factors for commercial production of graphene-incorporated solar cells. In addition to the application of graphene, graphene oxides have been also used in perovskite solar cells. The current needs and likely future investigations for graphene-incorporated solar cells are also discussed.

Additive‐Morphology Interplay and Loss Channels in “All‐Small‐Molecule” Bulk‐heterojunction (BHJ) Solar Cells with the Nonfullerene Acceptor IDTTBM

AbstractAchieving efficient bulk‐heterojunction (BHJ) solar cells from blends of solution‐processable small‐molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well‐defined SM donors—DR3TBDTT (DR3), SMPV1, and BTR—used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All‐SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution‐processing additive 1,8‐diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as‐cast” BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal‐insulator‐semiconductor carrier extraction” methods, and systematic recombination examinations by light‐dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase‐separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall.

From Semi- to Full-Two-Dimensional Conjugated Side-Chain Design: A Way toward Comprehensive Solar Energy Absorption

Two polymers with fully two-dimensional (2D) conjugated side chains, 2D-PTB-Th and 2D-PTB-TTh, were synthesized and characterized through simultaneously integrating the 2D-TT and the 2D-BDT monomers onto the polymer backbone. Resulting from the synergistic effect from the conjugated side chains on both monomers, the two polymers showed remarkably efficient absorption of the sunlight and improved π–π intermolecular interactions for efficient charge carrier transport. The optimized bulk heterojunction device based on 2D-PTB-Th and PC71BM shows a higher PCE of 9.13% compared to PTB7-Th with a PCE of 8.26%, which corresponds to an approximately 10% improvement in solar energy conversion. The fully 2D-conjugated side-chain concept reported here developed a new molecular design strategy for polymer materials with enhanced sunlight absorption and efficient solar energy conversion.

Aqueous‐Processed Polymer/Nanocrystal Hybrid Solar Cells with Double‐Side Bulk Heterojunction

AbstractAqueous‐solution‐processed solar cells (ASCs) are promising candidates of the next‐generation large‐area, low‐cost, and flexible photovoltaic conversion equipment because of their unique environmental friendly property. Aqueous‐solution‐processed polymer/nanocrystals (NCs) hybrid solar cells (AHSCs) can effectively integrate the advantages of the polymer (e.g., flexibility and lightweight) and the inorganic NCs (e.g., high mobility and broad absorption), and therefore be considered as an ideal system to further improve the performance of ASCs. In this work, double‐side bulk heterojunction (BHJ), in which one BHJ combines the active material with electron transport material and the other combines the active material with hole transport material, is developed in the AHSCs. Through comparing with the single‐side BHJ device, promoted carrier extraction, enhanced internal quantum efficiency, extended width of the depletion region, and prolonged carrier lifetime are achieved in double‐side BHJ devices. As a result, power conversion efficiency exceeding 6% is obtained, which breaks the bottleneck efficiency around ≈5.5%. This work demonstrates a device architecture which is more remarkable compared with the traditional only donor–acceptor blended BHJ. Under conservative estimation, it provides instructive architecture not only in the ASCs, but also in the organic solar cells (SCs), quantum dot SCs, and perovskite SCs.

Synthesis and studies of bay‐substituted perylene diimide‐based D–A–D‐type SM acceptors for OSC and antimicrobial applications

In this study, the synthesis of a series of bay-substituted donor–acceptor–donor (D–A–D) type perylene diimide derivatives (3a–3d) has been reported as an acceptor for the small-molecule-based organic solar cells (SM-OSCs) by the Suzuki coupling method. It has been evaluated for the antimicrobial activity against some of the bacteria and fungi. The synthesised SMs were confirmed by Fourier transform-infrared spectroscopy, nuclear magnetic resonance (NMR), and high resolution mass spectroscopy (HR-MS). The SMs showed absorption up to 750 nm, which eventually reduced the optical band gap E g opt to < 2 eV. SMs showed thermal stability up to 400 °C. In the SM-OSC, the SMs showed a power conversion efficiency of < 1% with the P3HT donor in bulk hetero-junction device structure. Additionally, the new SMs showed antimicrobial activity against Gram-negative bacteria such as Escherichia coli Gram-positive bacteria such as Bacillus subtilis and antifungal activity against the Candida albicans, and Aspergillus niger. Cytotoxicity studies were carried out against the breast cancer cell lines MCF-7 using MTT assay method. The results revealed that the SMs was able to inhibit the cancer cells. LD50s calculated for the SMs 3a–3d were between 200 and 400 µg/ml.

Exploitation of inimitable properties of CuInS2 quantum dots for energy conversion in bulk heterojunction hybrid solar cell

Quantum dots (QDs) are the suitable material for solar cell devices owing to its distinctive optical, electrical and electronic properties. Currently, the most efficient devices have employed the toxic QDs which cause destructive impact on environment. In the present article, we have used environment benign CuInS2 QDs as an acceptor material in bulk heterojunction device of P3HT and QDs. The energy level positions corroborated from UPS spectra substantiates the acceptor property of CuInS2. We scrutinized the hybrid solar cell by tailoring the acceptor content in active layer. The increased acceptor content intensifies the performance of device. The enhancement in photovoltaic parameters is mainly due to the fast dissociation and extraction of photogenerated excitons which occurs with the larger wt% of acceptor QDs. Current density–voltage characteristics describes the greater Voc and Isc in the 60 wt% CuInS2 QDs based solar cell as compared to the low wt% of QDs in the active layer.

Novel thienoisoindigo-based dyes for near-infrared organic photovoltaics - A combination of theoretical and experimental study

Abstract A series of thienoisoindigo (TII)-based dyes (TII 1–4) using different electron-donating moieties (D) and π-linkers were synthesized. The experimental results show that the maximum absorption band of TII 1–4 was located at 611–749 nm. Moreover, their absorption onset was extended to 741–901 nm that covers the near infrared (NIR) region. The bulk-heterojunction solar cell device fabricated by using TII 3/PC61BM as the active layer exhibited a power conversion efficiency (PCE) of 0.82%, showing the potential of TII-based dyes for NIR organic photovoltaics (OPVs). In order to study the structural effect of dyes on the PCE of solar cells, we further designed four TII-based dyes (TII 5–8) and the photophysical and photovoltaic properties for TII 1–8 were investigated through theoretical calculations. Calculated results demonstrate that carbazole group, whose has strong electron-donating ability than D moieties in this study, leads to generate long lifetime of the first excited state and whose generate large short-circuit current (Jsc). Thiophene group, whose has weak resonance energy than other π-linkers, can improve the π delocalization effect throughout whole molecule and generate large open-circuit voltage (Voc). According to these results, TII 3 and TII 8 with the carbazole as D moiety and thiophene as π-linker should be useful in development of a new SM-OPV device.

Novel brominated compounds using in binary additives based organic solar cells to achieve high efficiency over 10.3%

Abstract Solvent additives have been considered as a simple and efficient method to increase the performance of bulk-heterojunction (BHJ) organic solar cells, in which, the morphology of the active layer could obtain further improvements by using the binary solvent additives. In this paper, a series of brominated compounds, 1-Bromo-4-butylbenzene (Brbb), 1-Bromo-4- n -hexylbenzene (Brbh) and 1-Bromo-4- n -octylbenzene (Brbo), have been respectively incorporated with 1, 8-diiodooctane (DIO) and regarded as binary solvent additives to fabricate highly efficient bulk heterojunction (BHJ) organic solar cells (OSCs). Compared with the BHJ film based on single additive, the binary additives contained BHJ film shows increased optical absorption, efficient charge transport and better active layer morphology, leading to an enhancement of short-circuit current (J SC ) together with a higher achieved fill factor (FF). The conventional BHJ device using PTB7: PC 71 BM or PTB7-th: PC 71 BM with the binary solvent additives exhibit enhanced PCE of 8.13% and 10.31%, respectively, which is much higher than that of single additive based devices (7.04% for PTB7 and 8.73% for PTB7-th). The optimized performance of BHJ devices indicates that these brominated compounds are promising additives to improve device efficiency.

Interface Electrode Morphology Effect on Carrier Concentration and Trap Defect Density in an Organic Photovoltaic Device.

Formation of Schottky barrier contact (SBC) leads reconstruction of charges at the metal/semiconductor (MS) interface because of the wave function overlap between semiconductor and metal contact. Not only is the Schottky barrier contact formation a signature of the material's work function, but also it is sensitive to the interface trap states, the crystal orientation of the interacting materials, and other interface properties. In this work, the effect of aluminum cathode morphology on the polymer Schottky diode and bulk heterojunction (BHJ) photovoltaic device performance is studied. The electron collecting contacts in Schottky diode and BHJ device have been deposited using aluminum in pellet and nanoparticle forms. Devices fabricated by using Al nanoparticle showed improvement in dark as well as photocurrent density. Significant enhancement in JSC leads to overall improved power conversion efficiency. Enhanced performance in Schottky structured diode and OPV device have been correlated with electrode microstructure and its interface properties such as improved electrically active contact and enhanced charge transport. Electrical conductivity is discussed based on enhanced electrical coherence across organic semiconductor and electrode interface. Therefore, the contribution of electrical enhancement leads to improvement in short-circuit current density (JSC) in BHJ solar cell which is due to reduced trap density. Further, PCE was correlated with the density of interface trap states studied by drive level capacitance profiling technique.

Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-2-one functionality for efficient bulk-heterojunction devices

Two simple semiconducting donor–acceptor–acceptor (D–A1–A) modular, small molecule, non-fullerene electron acceptors, 2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (P3), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of P2 and P3 were directly compared with those of a structural analogue, 3-((4-((4-(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one (P1), which was designed based on a D–A format. All of these new materials comprised an electron rich triphenylamine (TPA) donor core (D) and electron deficient chromen-2-one terminal core (A). In the simple D–A system, TPA and chromenone were the terminal functionalities, whereas in the D–A1–A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping the D/A units constant. The inclusion of A1 was primarily done to induce cross-conjugation within the molecular backbone and hence to generate low band gap targets. The physical and optoelectronic properties were characterized by ultraviolet–visible (UV–Vis), thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited broadened absorption spectra, for instance panchromatic absorbance in case of P3, excellent solubility and thermal stability, and energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with P1, P2 and P3 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: P3-based device showed an impressive enhanced power conversion efficiency of 4.21%, an increase of around two-fold with respect to the efficiency of the best P3HT: P1-based device (2.28%). Our results clearly demonstrate that the D–A1–A type small molecules are promising non-fullerene electron acceptors in the research field of organic solar cells.

Polymer Main‐Chain Substitution Effects on the Efficiency of Nonfullerene BHJ Solar Cells

Abstract“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high‐efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main‐chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide‐bandgap polymer donor analogues composed of benzo[1,2‐b:4,5‐b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4‐difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine‐ and ring‐substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.

Benzo[1,2‐b:4,5‐b′]Dithiophene–6,7‐Difluoroquinoxaline Small Molecule Donors with &gt;8% BHJ Solar Cell Efficiency

AbstractSolution‐processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk‐heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self‐assembly patterns, optimum thin‐film morphologies, and high carrier mobilities in optimized donor–acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant‐group substitution promoting structural order and mediating morphological effects. In this paper, the rational pendant‐group substitution in benzo[1,2‐b:4,5‐b′]dithiophene–6,7‐difluoroquinoxaline SMs is shown to be an effective approach to narrowing the optical gap (Eopt) of the SM donors (SM1 and SM2), without altering their propensity to order and form favorable thin‐film BHJ morphologies with PC71BM. Systematic device examinations show that power conversion efficiencies &gt;8% and open‐circuit voltages (VOC) nearing 1 V can be achieved with the narrow‐gap SM donor analog (SM2, Eopt = 1.6 eV) and that charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap‐free recombination. Detailed device simulations, light intensity dependence, and transient photocurrent analyses emphasize how carrier recombination impacts BHJ device performance upon optimization of active layer thickness and morphology.

Recent Development of Quinoxaline Based Polymers/Small Molecules for Organic Photovoltaics

AbstractAmong the various molecular designs developed for the synthesis of conjugated polymers and small molecules for optoelectronic applications, the donor: acceptor (D–A) approach is the most widely explored method over the past decades. Through the covalent linkage of electron‐rich and electron‐deficient units, a plethora of medium‐low band gap materials has been developed and tested in organic photovoltaic devices. In particular, the quinoxaline aromatic structure and its derivatives are among the most studied electron deficient aromatic units used in D–A structures. Quinoxaline based materials are endowed with characteristics that are useful for large scale production in real world applications, such as easy synthetic procedures and excellent stability in air. Moreover, the use of quinoxaline based polymers/small molecules in bulk heterojunction (BHJ) devices led to power conversion efficiencies over 9%. Considering the potential of quinoxaline based materials, this review gathers together quinoxaline based polymers and small molecules reported in the literature during the last 5 years, summarizing and discussing the structure‐properties relationships for this class of organic semiconductors, aiming to serve as a background and to promote efforts for the further development of new quinoxaline derivatives with improved and advanced properties for future applications.

Cross‐Linkable and Dual Functional Hybrid Polymeric Electron Transporting Layer for High‐Performance Inverted Polymer Solar Cells

A cross-linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino-functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n-type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7-Th:PC71 BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.

Polymers with Alkylsulfanyl Side Chains for Bulk Heterojunction Solar Cells: Toward a Greener Strategy

The synthesis and properties of three copolymers obtained through a Knoevenagel polycondensation are reported. They are soluble in organic solvents, filmable, and solvatochromic. They are characterized through gel permeation chromatography, NMR and UV–vis spectroscopies, cyclic voltammetry, and atomic force microscopy. They display some proneness to form π-stacks, broad absorptions up to about 700 nm, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies that fit those of [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM). The polymers are incorporated in blends with PCBM within bulk heterojunction devices. The best results are found for polymer solar cells obtained from P1 (2.7% power conversion efficiency). The insertion of alkylsulfanyl chains improves the photovoltaic performances, with respect to those of alkyl-substituted analogous polymers. The Jsc of the devices are greatly enhanced, supporting the employment of (alkylsulfanyl)thienyl units as base units in photoactive materials.

The influence of intramolecular noncovalent interactions in unsymmetrical squaraines on material properties, film morphology and photovoltaic performance

Three unsymmetrical squaraines (USQs) with different number of hydroxyl groups (BIISQ bearing two hydroxyl substituents, BIISQ-MOH bearing mono-one, and BIISQ-NOH bearing none) have been synthesized to understand of the insightful molecular structure-property relationship of photovoltaic materials. Our research shows that the introduction of strong intramolecular hydrogen bonding (HB) interactions between donor (D) and acceptor (A) units of squaraines can affect the whole π-conjugated properties, such as deepening the highest occupied molecular orbital (HOMO) energy level and thus enhancing the open-circuit voltage (Voc), increasing the dipole moment and improving hole mobility, weakening the electron-withdrawing ability of centric four-membered rings (as A unit in squaraines) and then improving the compatibility between USQ and [6,6]-phenyl-C71butyric acid methyl ester (PC71BM). Eventually, the bulk-heterojunction organic photovoltaic device (BHJ-OPV) based on BIISQ has obtained the best performance, which is approximately three times higher than that of BIISQ-NOH-based one. Interestingly, our research also finds that thin layer chromatography (TLC) analysis might be a simple, cheap, and rapid way to preliminarily forecast the compatibility between donor materials and PC71BM when the molecular skeletons of donor materials are relatively similar.