Bulk Heterojunction Solar Cells Research Articles

Unraveling the device performance differences between bulk-heterojunction and single-component polymer solar cells

A new conjugated-block copolymer PBDB-YTCl-2 was synthesized. The photovoltaic performance, morphological characteristics and physical mechanisms of PBDB-YTCl-2 were compared with those of the bulk-heterojunction PBDB-T:PYCl-2 system.

Non-fused BODIPY-based acceptor molecules for organic photovoltaics

A new NFA design based on the unusual BODIPY unit as the central electron accepting component is described. All derivatives exhibit low optical bandgaps, high extinction coefficients and LUMO levels deep enough to be used as NFAs.

An electron acceptor with an intrinsic quinoidal core for bulk-heterojunction organic solar cells and photodetectors.

An electron acceptor based on a quinoidal dipyrrolopyrazinedione core was synthesized for organic solar cells and photodetectors. A power conversion efficiency of 6.7% and a specific detectivity of 4.1 × 1013 Jones at 800 nm have been obtained, suggesting the promising prospects of quinoidal molecules for optoelectronic devices.

An analytical model for organic bulk heterojunction solar cells based on Saha equation for exciton dissociation.

The power conversion efficiencies of organic solar cells (OSCs) have been greatly improved in recent years. However, latest experimental data of high efficiency OSCs, the sublinear relationship between the short circuit current density (Jsc) and light intensity (Pin), and the effects of energetic disorder in bulk heterojunction organic solar cells have not been understood. An analytical model for high-efficiency OSCs is proposed, which takes most physical factors into account that have been ignored in most previous models, including practical solar spectra and absorption spectra, degeneracy effect, exciton effect, space charge limited current, and unified mobility expression dependent on temperature, electric field and density, etc. Three analytical iterative methods are proposed to solve the strong non-linear Poisson equation and the drift-diffusion equations. The method for the drift-diffusion equations involves introducing two constant coefficients and determining their values self-consistently by demanding the space averages of approximate drift and diffusion currents equal to the averages of accurate ones. The theoretical results for five high-efficiency OSCs are in good agreement with experimental data, including current-voltage curves, light intensity-dependent Jsc and open-circuit voltage (Voc) curves. The effects of energetic disorder in bulk heterojunction organic solar cells, and the sublinear relationship Jsc ∝ Pαin (α < 1) can be well explained. The Saha equation for exciton dissociation and the space-charge-limited-current (SCLC) effect are important for modelling high-efficiency OSCs. The Voc ∼ Pin relationship can be influenced by many factors. But, the Jsc ∼ Pin relationship can be mainly and slightly influenced by the exciton effect and energetic disorder, respectively. When aiming to realize higher performance OSCs, one should decrease six material parameters, including the energetic disorder, exciton mass, deep level impurity concentration, the ratios of electron and hole mobilities, densities of states for electrons and holes, and potential barriers at the anode and cathode. The performance parameters of 15 triad compounds are predicted by using ab initio Eg and absorption spectra from the literature along with other input parameters taken from previous optimized values, and the efficiency of two compounds was found to exceed 35%.

Challenging PM6-like donor polymers for pairing with a Y-type state-of-the-art acceptor in binary blends for bulk heterojunction solar cells.

Being fluorine-free and a high performance material as a small organic acceptor molecule, BTP-eC9 has been well mixed with BDT-based PM6 donor polymers for providing satisfactory photovoltaic properties, especially towards future large scale/large area solar cell production. However, as one of the key electrical outputs from such binary active layer materials, the open circuit voltage (VOC) was limited to ca. 0.84 V, which needs to be further improved for BTP-eC9 to have a bright future. This paper focuses on the molecular design of alkylthio- and alkoxy-phenyl flanked benzo[1,2-b:4,5-b']dithiophene-based conjugated polymers (PBDT-PS-ttTPD or P10 for short and PBDT-PO-ttTPD or P11), which were successfully synthesized and applied as donor materials for pairing with BTP-eC9 in organic photovoltaic (OPV) devices. By fine-tuning the side chains of the benzodithiophene (BDT) moiety, such non-fullerene OPV devices with normal configuration demonstrate an attractively high open circuit voltage (VOC) of 0.89 and 0.87 V in P10/BTP-eC9 and P11/BTP-eC9 based binary single bulk heterojunction OPV devices, while still maintaining an excellent JSC of 22.7 and 20.0 mA cm-2 with a final power conversion efficiency (PCE) of 12.93% and 9.37%, respectively. The alkylthio-phenyl chain substituted BDT polymer exhibits better photovoltaic performance in all aspects than the alternative with alkoxy chains due to the synergistic effect of the alkylthio-phenyl flanked BDT, TPD, and π-bridge (thieno[3,2-b]thiophene).

Nonpolymer Organic Solar Cells: Microscopic Phonon Control to Suppress Nonradiative Voltage Loss via Charge-Separated State.

Recent remarkable developments on nonfullerene solar cells have reached a photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this regard, understanding the impact of small donor molecules on nonpolymer solar cells is essential. Here, we systematically investigated mechanisms of solar cell performance using diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP) conjugates of C4-DPP-H2BP and C4-DPP-ZnBP, where C4 represents the butyl group substituted at the DPP unit as small p-type molecules, while an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester is employed. We clarified the microscopic origins of the photocarrier caused by phonon-assisted one-dimensional (1D) electron-hole dissociations at the donor-acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking. This ensures carrier transport through stacking molecular conformations to suppress nonradiative voltage loss capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. We show that, while disordered lattice motions by the π-π stackings via zinc ligation are essential to enhance the entropy for charge dissociations at the interface, too much ordered crystallinity causes the backscattering phonon to reduce the open-circuit voltage by geminate charge-recombination.

Recent Advances and Challenges toward Efficient Perovskite/Organic Integrated Solar Cells

Recently, emerging third-generation photovoltaic technologies have shown rapid progress in device performance; the power conversion efficiencies (PCEs) of organic bulk heterojunction (BHJ) and perovskite solar cells (PSCs) are now surpassing 19% and 25%, respectively. Despite this dramatic enhancement, their efficiencies are theoretically limited based on the detailed balance model which accounts for inevitable loss mechanisms under operational conditions. Integrated solar cells, formed by monolithically integrating two photoactive layers of perovskite and BHJ with complementary absorption, provide a promising platform for further improvement in solar cell efficiency. In perovskite/BHJ integrated solar cells (POISCs), high bandgap perovskite offers high open-circuit voltage with minimal losses while low bandgap organic BHJ extends absorption bandwidth by covering the near-infrared region, resulting in additional photocurrent gain. Different from conventional tandem solar cells, integrated solar cells contain merged photoactive layers without the need for complicated recombination layers, which greatly simplifies fabrication processes. In this review, we summarize the recent progress in POISCs, including operational mechanism and structural development, and remaining challenges on the road toward efficient devices.

Two multifunctional benzoquinone derivatives as small molecule organic semiconductors for bulk heterojunction and perovskite solar cells

Two novel quinone derivatives (NN3 and NN4) were synthesized in this work and they were characterized to be used as small organic semiconductor molecules in different types of photovoltaic applications. To make accessible compounds, three simple steps were followed to prepare NN3 and NN4 compounds. Furthermore, energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined for the computationally optimized models of the investigated compounds. The obtained optical and electrochemical results of this work indicated that NN3 and NN4 compounds were good candidates for application in the fields of bulk heterojunction (BHJ) and perovskite solar cells. Indeed, investigating new energy resources has been seen an important topic of research for producing clean energies and portable storage systems.

Molecular engineering of new electron acceptor for highly efficient solution processable organic solar cells using state-of-the-art polymer donor PffBT4T-2OD

Two molecularly engineered fullerene-based electron acceptors, coded as BITh-C60 and BITh-C70, were designed, synthesized and fully characterized. The molecules have LUMO energy levels at −3.62 and-3.65 eV; and HOMO energy levels at −5.62 and −5.60 eV, respectively as confirmed by cyclic voltammetry. The acceptor BITh-C70 shows comparatively broader and stronger absorption with respect to BITh-C60. A narrow band gap polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl) 2,2′;5′,2″;5″,2′′′-quaterthiophen-5,5′′′-diyl)] (PffBT4T-2OD) was used to construct the solution processed bulk-heterojunction solar cells. Under optimized fabrication and evaluation conditions, BITh-C60 and BITh-C70 showed a highest power conversion efficiency (PCE) of 8.1 % and 8.8 %, respectively, which are comparatively higher than the PCEs obtained from the respective reference solar cells. The PffBT4T-2OD:BITh-C70 based BHJ solar cell shows a nearly flat EQE spectra of 75 % between 380 and 700 nm.

The Synergy of Lead Chalcogenide Nanocrystals in Polymeric Bulk Heterojunction Solar Cells.

Photoactive polymer and quantum dots (QDs)/nanocrystals (NCs)-based bulk heterojunction (BHJ) solar cells have the combined positivity of organic semiconductors and inorganic components, which can enable a high carrier mobility and absorption coefficient. Additionally, the NCs also provide the opportunity to tune the band gap to obtain enhanced absorption in a broad solar spectrum. Among the semiconductors, lead chalcogenide NCs are of particular interest due to their good photosensitivity in the near-infrared (NIR) region of the solar spectrum. These NCs have large exciton Bohr radii (18, 46, and 150 nm for PbS, PbSe, and PbTe, respectively) and tunable sizes depending on the optical bandgaps between 0.3 and 1.5 eV. Independently, lead chalcogenide NCs have been studied extensively for different applications; however, uses in polymer-NC-based bulk heterojunction solar cells are limited. This Review has been structured on the lead chalcogenide NCs incorporated in polymer composite-based bulk heterojunction solar cells covering the material, properties, and solar cell performance to find the issues and explore future opportunities.

Quantifying the effect of energetic disorder on organic solar cell energy loss

•Correlation between non-radiative energy loss and CT-state energetic disorder •D-A interface optimization strategy for reduced non-radiative energy loss •Universal energy loss metric incorporating disorder Understanding the factors affecting energy loss in organic photovoltaics (OPVs) is imperative to achieve further improvements in their efficiency and to establish design rules for the development of new materials. Here, we provide direct experimental evidence supporting correlation between charge-transfer (CT) state static disorder and energy loss. Specifically, upon studying several planar and bulk heterojunction solar cells, we demonstrate that the non-radiative energy loss component quadratically increases with increasing Gaussian CT-state disorder. We also show that by defining the total energy loss in terms of the peak of the CT-state distribution, obtained from temperature-dependent external quantum efficiency measurements, the effect of disorder on OPV performance can be unambiguously identified, offering a universal metric for quantifying energy loss across various devices. Understanding the factors affecting energy loss in organic photovoltaics (OPVs) is imperative to achieve further improvements in their efficiency and to establish design rules for the development of new materials. Here, we provide direct experimental evidence supporting correlation between charge-transfer (CT) state static disorder and energy loss. Specifically, upon studying several planar and bulk heterojunction solar cells, we demonstrate that the non-radiative energy loss component quadratically increases with increasing Gaussian CT-state disorder. We also show that by defining the total energy loss in terms of the peak of the CT-state distribution, obtained from temperature-dependent external quantum efficiency measurements, the effect of disorder on OPV performance can be unambiguously identified, offering a universal metric for quantifying energy loss across various devices.

Increasing the Strength, Hardness, and Survivability of Semiconducting Polymers by Crosslinking

AbstractCrosslinking is a ubiquitous strategy in polymer engineering to increase the thermomechanical robustness of solid polymers but has been relatively unexplored in the context of π‐conjugated (semiconducting) polymers. Notwithstanding, mechanical stability is key to many envisioned applications of organic electronic devices. For example, the wide‐scale distribution of photovoltaic devices incorporating conjugated polymers may depend on integration with substrates subject to mechanical insult—for example, road surfaces, flooring tiles, and vehicle paint. Here, a four‐armed azide‐based crosslinker (“4Bx”) is used to modify the mechanical properties of a library of semiconducting polymers. Three polymers used in bulk heterojunction solar cells (donors J51 and PTB7‐Th, and acceptor N2200) are selected for detailed investigation. In doing so, it is shown that low loadings of 4Bx can be used to increase the strength (up to 30%), toughness (up to 75%), hardness (up to 25%), and cohesion of crosslinked films. Likewise, crosslinked films show greater physical stability in comparison to non‐crosslinked counterparts (20% vs 90% volume lost after sonication). Finally, the locked‐in morphologies and increased mechanical robustness enable crosslinked solar cells to have greater survivability to four degradation tests: abrasion (using a sponge), direct exposure to chloroform, thermal aging, and accelerated degradation (heat, moisture, and oxygen).

Improving morphology of P3HT:PCBM bulk heterojunction solar cells with anisotropic shaped silica nanoparticles

Using coarse-grained molecular dynamics simulations we study blends of Poly(3-hexylthiophene-2,5-diyl) (P3HT), [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) and Silica nanoparticle (SiNP) to understand the effect of adding SiNP on morphology of P3HT:PCBM in Bulk heterojunction (BHJ) solar cells. We use an approximately 3 nm anisotropic shaped SiNP and predicted the morphology of BHJ upon its incorporation. The SiNP arrange themselves into anisotropic structures depending on the concentration of P3HT, PCBM and SiNP respectively creating a network like morphology. PCBM molecules utilize the surface energy of SiNP and gather at its surface forming a morphology which is beneficial for device efficiency. Our results suggest that an optimum weight fraction of all the three components leads to higher surface area of contact, optimum domain size and high percolation of domains throughout the system. The effective control of all the morphological parameters help in improving the charge generation, extraction and transport to electrodes, thereby improving the performance of BHJ solar cells.

Phenothiazine-Based Donor-Acceptor Polymers as Multifunctional Materials for Charge Storage and Solar Energy Conversion.

The increasing energy demand for diverse applications requires new types of devices and materials. Multifunctional materials that can fulfill different roles are of high interest as they can allow fabricating devices that can both convert and store energy. Herein, organic donor-acceptor redox polymers that can function as charge storage materials in batteries and as donor materials in bulk heterojunction (BHJ) photovoltaic devices are investigated. Based on its reversible redox chemistry, phenothiazine is used as the main building block in the conjugated copolymer design and combined with diketopyrrolopyrrol and benzothiadiazole as electron-poor comonomers to shift the optical absorption into the visible region. The resulting polymers show excellent cycling stability as positive electrode materials in lithium-organic batteries at discharge potentials of 3.6-3.7V versus Li/Li+ as well as good performances in BHJ solar cells with up to 1.9% power conversion efficiency. This study shows that the design of such multifunctional materials is possible, however, that it also faces challenges, as essential properties for good device function can lead to diametrically opposite requirements in materials design.

Electronic Materials: An Antiaromatic Propeller Made from the Four‐Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105 M−1 cm−1 in CH2Cl2) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Electronic Materials: An Antiaromatic Propeller Made from the Four-Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides.

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105 M-1 cm-1 in CH2 Cl2 ) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Phenothiazine functionalized fulleropyrrolidines: synthesis, charge transport and applications to organic solar cells.

A series of phenothiazine-C60/70 dyads containing fulleropyrrolidine tethered to C-3 position (C60-PTZ and C70-PTZ) or to the heteroatom N-position via either phenyl (C60-Ph-PTZ and C70-Ph-PTZ) or phenoxyethyl linkers (C60-PhOEt-PTZ and C70-PhOEt-PTZ) of the phenothiazine were synthesized and light-induced electron transfer events were explored. Optimized studies suggested that the highest molecular orbital (HOMO) resides on donor phenothiazine moiety while lowest molecular orbital (LUMO) on the acceptor fulleropyrrolidine moiety of the dyads. Optical and electrochemical properties suggested no electronic communication between the donor and acceptor moieties in the ground state. However, steady-state emission studies in solvents of varied polarity, involving selective excitation of C60/C70, disclosed that the emission intensity of C60/C70 was quenched in the dyads in the increasing order, C60/70-PTZ > C60/70-Ph-PTZ > C60/70-PhOEt-PTZ as a consequence of the donor-acceptor distance resulted due to spacer lengths. Also, the emission quenching is more pronounced in polar solvents such as DMF compared to a non-polar solvent, toluene. With the support of parallel electrochemical studies, the emission quenching is attributed to intramolecular photo-induced electron transfer occurring from PTZ to (C60/C70)* generating a radical ion pair, PTZ+⋅-C60-⋅/PTZ+⋅-C70-⋅. Finally, bulk heterojunction (BHJ) solar cells devices inverted fashion prepared by employing the dyads as acceptors, and PTB7 as donor, suggested that the devices prepared from C70 derivatives i.e., PTB7:C70-PTZ and PTB7:C70-PhOEt-PTZ exhibited better power conversion efficiency of 2.66% and 2.15%, respectively over C60 derivatives i.e., PTB7:C60-PTZ and PTB7:C60-PhOEt-PTZ with the efficiencies of 1.80 and 1.72%, respectively. AFM studies revealed that the poor performance of PTB7:C60-PTZ- and PTB7:C60-PhOEt-PTZ-based devices can be ascribed to the lower solubility of the dyads in 1,2-DCB solvent leading to rough morphology.

An inverted architecture P3HT:CdSe bulk-heterojunction hybrid solar cell utilizing a quantum junction with high open circuit voltage and efficiency

Exploring CdSe QDs as a substitute for [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) in Poly (3-hexylthiophene) (P3HT) based photovoltaic devices received much attention due to its properties such as tunable band gap over the visible range, better band alignment with P3HT and inexpensiveness. But, the VOC values of the P3HT:CdSe QDs bulk-heterojunction (BHJ) hybrid solar cells are still inferior, which happens to be the main performance-limiting factor for these devices. We have resolved this issue in this study by the application of an interfacial buffer layer (BL) in an inverted architecture utilizing zinc oxide (ZnO) as an electron transporting layer. The BL was also formed of CdSe QDs. It was found that the application of the interfacial BL reduces the interface recombination which resulted in a VOC value of 0.96 V. To the best of our knowledge, this is the best VOC value reported for the P3HT:CdSe QDs BHJ system so far. A quantum junction (QJ) was also formed between the QDs in the BL and the QDs in the BHJ by passivating them with Iodine and 3-Mercaptopropionic acid (MPA), respectively. The QJ induces an extended electric field in the BHJ which benefits charge extraction and ultimately resulting in an enhanced short circuit current (JSC).

Tetracyanobutadiene Bridged Push-Pull Chromophores: Development of New Generation Optoelectronic Materials.

This review describes the design strategies used for the synthesis of various tetracyanobutadiene bridged donor-acceptor molecular architectures by a click type [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction sequence. The photophysical and electrochemical properties of the tetracyanobutadiene bridged molecular architectures based on various moieties including diketopyrrolopyrrole, isoindigo, benzothiadiazole, pyrene, pyrazabole, truxene, boron dipyrromethene (BODIPY), phenothiazine, triphenylamine, thiazole and bisthiazole are summarized. Further, we discuss some important applications of the tetracyanobutadiene bridged derivatives in dye sensitized solar cells, bulk heterojunction solar cells and photothermal cancer therapy.

Optimized perovskite active layer morphologies and crystallization by doping cesium iodide for high-performance bulk heterojunction solar cells and X-ray detectors

Organic-inorganic hybrid methylammonium lead iodide perovskite (MAPbI 3 ) has attracted widespread attention for its potential use in a range of optoelectronic devices. In this paper, we describe the fabrication of inverted-type perovskite/fullerene planar heterojunction perovskite solar cells and X-ray detectors where a small amount of cesium iodide (CsI) was added to the MAPbI 3 precursor solution. The appearance and crystal structure of the constructed perovskite active layer were carefully studied by microscope and X-ray diffraction measurements, respectively. The results reveal that the CsI additive can control the size and aggregation behavior of the perovskite crystals, helping to form a smoother perovskite film. Thus, the constructed perovskite solar cell with its Cs 0.1 MA 0.9 PbI 3 active layer (AC) revealed a power conversion efficiency (PCE) of 11.16 ± 0.13%, which is a significant improvement over conventional planar heterojunction devices that use pristine MAPbI 3 AC (PCE 9.12 ± 0.11%). We also fabricated an X-ray detector using the same Cs 0.1 MA 0.9 PbI 3 , this achieved a maximum sensitivity of 3.26 mA/Gy cm 2 . Density functional theory calculations investigate the origin of the enhancement of device performance characteristics. Interestingly, the results show that perovskite surfaces constructed with a suitable amount of CsI can effectively enhance charge extraction and reduce charge carrier recombination. This work provides an efficient way to improve carrier transfer, which will inspire other perovskites optoelectronic applications. • We have synthesized smooth and highly crystalline perovskite thin films by incorporating an appropriate amount of CsI. • We fabricated inverted-type perovskite/PCBM hybrid solar cells and X-ray detectors using cesium-doped perovskites. • Competitive efficiency of 11.16 ± 0.13% was achieved for constructed perovskite solar cell with Cs 0.1 MA 0.9 PbI 3 AC. • Invented X-ray detector with Cs 0.1 MA 0.9 PbI 3 AC exposed a high sensitivity of 3.26 mA/Gy·cm 2 .