Bulk Heterojunction Active Layer Research Articles

Titanium Nitride as an Alternative Plasmonic Material for Plasmonic Enhancement in Organic Photovoltaics

This paper investigates TiN for its potential to enhance light-harvesting efficiency as an alternative material to Au for nanoscale plasmonic light trapping in thin-film solar cells. Using nanosphere lithography (NSL), plasmonic arrays of both Au and TiN are fabricated and characterized. Later, the fabricated TiN and Au arrays are integrated into a thin-film organic photovoltaic (OPV) device with a PBDB-T:ITIC-M bulk heterojunction (BHJ) active layer. A comparative study between these Au and TiN nanostructured arrays evaluates their fabrication process and plasmonic response, highlighting the advantages and disadvantages of TiN compared to a conventional plasmonic material such as Au. The effect of the fabricated arrays when integrated into an OPV is presented and compared to understand the viability of TiN. As one of the first experimental studies utilizing TiN arrays for the plasmonic enhancement of photovoltaics, the results offer valuable insight that can guide future applications and decisions in design.

Temperature-dependent charge transport measurements unveil morphological insights in non-fullerene organic solar cells

The morphological analysis of bulk heterojunction (BHJ) active layer stands as a critical imperative for advancing the performance of future organic solar cells. Conventional characterization tools employed for morphological investigation often require substantial resources, both in cost and physical space, thereby imposing restraints on research endeavors in this domain. Here, we extend the application of charge carrier transport characterization beyond conventional mobility assessments, utilizing it as a table-top method for preliminary morphological screening in organic thin films. The investigation focuses on several high-performance BHJ systems that utilize typical “Y” non-fullerene acceptors. It involves in-depth transport studies, including temperature- and field-dependent transport characterizations. The resulting transport data are analyzed in detail using the Gaussian disorder model to extract key transport parameters, specifically the high-temperature limited mobility (μ∞) and positional disorder (∑). Integrating these transport parameters with morphological insights obtained through various characterization tools—including x-ray scattering, sensitive spectroscopy, and quantum chemistry simulation—provides a deep understanding of the intricate interplay between charge transport properties and morphological characteristics. The results reveal explicit relationships, associating μ∞ with the degree of molecular stacking in BHJs and ∑ with the structural disorder in molecule skeleton. Our findings point to the promising potential of utilizing a simple transport characterization technique for the early stage evaluation of thin film packing and geometric properties of organic materials.

Cover Picture: Novel low‐band gap non‐fullerene acceptors based on IDIC core as potential photovoltaic materials (BKCS 9/2024) Radhiha Ravindran, Inchan Kim, Yun‐Hi Kim, Soon‐Ki Kwon

Organic photovoltaics (OPVs) attracted a lot of research attention owing to its advantages such as convenient processing technique, unsubstantial weight, low cost, flexibility and use of environmentally benign materials. In general, these are bulk heterojunction (BHJ) OPVs obtained through a solution process and the large‐area OPV production is achieved via roll‐to‐roll process. In OPVs, the BHJ active layer consists of electron donor (D) and electron acceptor (A) materials blended together by dissolving them in a common organic solvent. The new low band gap non‐fullerene based acceptors could be used as potential candidates for the fabrication of OPVs in combination with suitable donor polymers. More details are available in the article by Radhiha Ravindran, Inchan Kim, Yun‐Hi Kim, Soon‐Ki Kwon image

N-Doping Donor-Dilute Semitransparent Organic Solar Cells to Weaken Donor: Acceptor Miscibility and Consolidate Donor-Phase Continuity.

Lightweight and semi-transparent organic solar cells (ST-OSCs) offer bright promise for applications such as building integrated photovoltaics. Diluting donor content in bulk-heterojunction active layers to allow greater visible light transmittance (AVT) effectively enhances device transparency, yet the ineluctable compromise of the donor-phase continuity is challenging for efficient charge transport. Herein, a trace amount of n-type N-DMBI dopant is incorporated, which facilitates the donor:acceptor (D:A) de-mixing by strengthening both acceptor polarity and D/A crystallization. With the diminution of component inter-mixing, the limited number of donors increasingly self-aggregate to establish the more continuous phases. For the benchmark PM6:Y6-based ST-OSCs, when the donor content is reduced from regular 45 to optimal 30 wt.%, the device AVT is remarkably raised by more than a quarter, accompanied by a marginal drop in power conversion efficiency from 13.89% to 13.03%. This study reveals that by decreasing the donor content to <30 wt%, acceptor excitons induced by Förster resonance energy transfer are prone to severe radiative recombination. This is nonetheless mitigated by dopant inclusion within the acceptor phase by providing extra energy offset and prolonging charge transfer state lifetime to assist exciton dissociation.

Improved performance of ternary organic solar cells based on both bulk and pseudo-planar heterojunction active layer

The high photovoltaic conversion efficiency (PCE) of ternary organic solar cells (OSCs) based on PTB7-Th, PC71BM and IEICO-4F material is obtained, the short-circuit current density (J SC), fill factor (FF), and open-circuit voltage is 25.90 mA cm−2, 73.20% and 0.71 V, respectively. PCE is as high as 13.53%. It is the highest PCE of ternary OSCs based on PTB7-Th, PC71BM and IEICO-4F materials for all we know. The narrow bandgap material of IEICO-4F is deposited on PTB7-Th:PC71BM bulk heterojunction (BHJ) by layer-by-layer process. We constructed the dual bandgap active layer both BHJ and pseudo-planar heterojunction (P-PHJ), it could be defined as ternary BHJ/P-PHJ of OSCs. This type of OSCs is not only the complementary bandgap material of the active layer, but also increasing the donor/acceptor (D/A) interface. The excitons generation and collection of the device are increased leading a higher J SC and FF. The semitransparent OSCs (ST-OSCs) is prepared by varying the thickness of Ag electrode and PCE can reach 9.70%, and the average visible light transmittance and light use efficiency of ST-OSCs are improved effectively.

Crystallization‐Driven Optimization of Morphology and Performance in Near‐Infrared Organic Photodetectors via Alkyl Side Chain Tuning of Narrow Bandgap Non‐Fullerene Acceptors

AbstractNarrow bandgap non‐fullerene acceptors (NBG NFAs) are crucial in advancing near‐infrared organic photodetectors (NIR OPDs). However, the polymorphous behavior of NFAs introduces energetic disorder and charge‐trapping sites in the bulk heterojunction (BHJ) active layers, thereby hindering the NIR performance of OPDs. This study demonstrates the design and optimization of the molecular structures of alkyl side chains in NFAs to morphologically address these electrical limitations and shed light on how side chains affect the crystallization‐driven control of the morphogenesis process in BHJ layers. The structure, linearity, and length of the alkyl side chain molecules are strategically designed and controlled to synthesize a series of NBG NFAs, namely COT‐R (R = EH, BO, Oct, or Dod). The in‐depth quantitative investigations into the morphological and electrical factors reveal that side‐chain optimization effectively enhances various morphological properties, such as the molecular alignment and compatibility between components in the BHJ layer, resulting in the mitigation of energetic disorders and charge traps in the BHJ layers. Therefore, the optimized BHJ system, comprising COT‐Oct blended with PTB7‐Th, achieves a reduced dark current and increased responsivity, contributing to the development of high‐performance NIR OPDs with an impressive specific detectivity of 1.49 × 1012 Jones at 1000 nm under ‐0.5 V.

Enhancing Photocurrent Efficiency in Filter‐Less NIR Organic Photodetectors Through a Photomultiplication‐Inducing Perovskite Quantum Dot Interlayer

AbstractA unique charge‐injection‐narrowing organic photodetector (CIN OPD) is demonstrated for near‐infrared (NIR) range operation. In conventional CIN OPDs, photomultiplication (PM) is achieved within a bulk heterojunction (BHJ) active layer with a donor:acceptor ratio of 100:1. Unfortunately, this approach encounters an issue in which the narrowband detection wavelength is determined solely by an excessive donor quantity. Consequently, the conventional devices face a critical limitation in that their detection range cannot extend to longer wavelengths. However, this limitation has been overcome through a novel mechanism and have fabricated a device that responds at λ = 940 nm. When a perovskite quantum dot interlayer is inserted as the PM‐inducing layer, PM occurs outside the active layer. Therefore, the BHJ active layer is structured with a donor:acceptor ratio of 50:50, resulting in an extension of the detection wavelength to the absorption tail of the narrow‐bandgap acceptor. The device demonstrates a remarkable external quantum efficiency (EQE) of 39 000% with a narrow full‐width at half‐maximum (FWHM) of 98 nm at 60 V. It also has a high specific detectivity and a short response time. The device represents a significant advancement in filter‐free, high‐efficiency photodetectors suitable for optical sensor applications such as light detection and ranging (LiDAR).

Synthesis and Characterization of Benzobisoxazole-based Conjugated Polymers for Organic Photodetectors

Benzo[1,2-d:4,5-d’]bis(oxazole) (BBO) is a conjugated building block that can be easily synthesized for the development of organic optoelectronic polymers. Here, the authors synthesized BBO-based polymers, P1 and P2, by coupling BBO with thiophenes for use in organic photodetectors (OPDs). The optical characteristics of P1 and P2 make them suitable for OPDs, as they selectively absorb the green light with a narrow bandwidth in the range of 500–600 nm. The OPD devices were fabricated by making a bulk heterojunction active layer between the synthesized polymers and a nonfullerene acceptor, IDIC. P1-based devices showed slightly higher responsivity (R) of 0.169 A/W than 0.112 A/W of P2-based devices. However, the P2-based device was higher than D* value of the P1-based under 100 μW/cm2, which was due to the influence of excessive thiophene units that had low dark current density.

Spin-dependent recombination affected by post-annealing of organic photovoltaic devices

Organic photovoltaic devices (OPVs) are attracting attention because of recent rapid enhancement of their power conversion efficiency. For further improvement, optimization of fabrication processes is one useful path to a solution. During OPV fabrication, particularly of the bulk heterojunction active layer, annealing treatments contribute to the device performance. Many studies have examined annealing-related properties. However, further research must clarify how paramagnetic species in the devices play their roles by annealing. Using well-known OPVs, we investigated the relation between spin-dependent recombination (SDR) current and the paramagnetic species, which vary the numbers by post-annealing with active layers consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). A simultaneous detection method of electron spin resonance (ESR) and electrically detected magnetic resonance (EDMR), which we originally developed, was applied to OPVs for the first time ever reported. Results show that PC61BM anion radicals generated by post-annealing of P3HT:PC61BM OPVs with a lithium fluoride (LiF)/aluminum (Al) electrode do not contribute to the SDR current at the interface and that P3HT cation radicals enhance the SDR current. By contrast, devices with an Al electrode without LiF decrease the total SDR current, although the quantities of cation radical molecules do not vary. This finding suggests that changes of the hole blocking layer in the devices caused by the annealing treatment affect the size of capture cross sections of P3HT cation radicals. Our new method of quantitative observation of the EDMR changes through the ESR signals is expected to be useful for investigating the capture cross sections in OPVs.

Dual Polymerized Y-Acceptors of Distinct-Dimensionality Create Neuron-Like Interpenetrating Hierarchical Network towards Efficient and Stable All-Polymer Solar Cells.

All-polymer solar cells have garnered particular attention thanks to their superior thermal, photo, and mechanical stabilities for large-scale manufacturing, yet the performance enhancement remains largely restrained by inherent morphological challenges of the bulk-heterojunction active layer. Herein, a 3D Y-branched polymerized small-molecule acceptor named PYBF, characteristic of high molecular weight and glass transition temperature, is designed and synthesized by precisely linking C3h-symmetric benzotrifuran with Y6 acceptors. In comparison to the benchmark thiophene-bridged linear PYIT acceptor, an optical blue-shift absorption is observed for PYBF yet a slightly higher power conversion efficiency (PCE) of 15.7% (vs 15.14%) is obtained when paired with polymer donor PM6, which benefit from the more crystalline and face-on-oriented PYBF domains. However, the star-like bulky structure of PYBF results in the nucleation-growth dominant phase-separation in polymeric blends, which generates stumpy droplet-like acceptor fibrils and impairs the continuity of acceptor phases. This issue is however surprisingly resolved by incorporating a small amount of PYIT, which leads to the formation of the more interconnective neuron-like dual-acceptor domains by long-chain entanglements of linear acceptors and alleviates bimolecular recombination. Thus, the champion device realizes a respectable PCE of up to ≈17% and importantly exhibits thermal and storage stabilities superior to the linear counterpart.

Single-component organic solar cells-Perspective on the importance of chemical precision in conjugated block copolymers.

Organic photovoltaics (OPV) present a promising thin-film solar cell technology with particular benefits in terms of weight, aesthetics, transparency, and cost. However, despite being studied intensively since the mid 90's, OPV has not entered the mass consumer market yet. Although the efficiency gap with other thin-film photovoltaics has largely been overcome, active layer stability and performance reproducibility issues have not been fully resolved. State-of-the-art OPV devices employ a physical mixture of electron donor and acceptor molecules in a bulk heterojunction active layer. These blends are prone to morphological changes, leading to performance losses over time. On the other hand, in "single-component" organic solar cells, the donor and acceptor constituents are chemically connected within a single material, preventing demixing and thereby enhancing device stability. Novel single-component materials affording reasonably high solar cell efficiencies and improved lifetimes have recently emerged. In particular, the combination of donor and acceptor structures in conjugated block copolymers (CBCs) presents an exciting approach. Nevertheless, the current CBCs are poorly defined from a structural point of view, while synthetic protocols remain unoptimized. More controlled synthesis followed by proper structural analysis of CBCs is, however, essential to develop rational structure-property-device relations and to drive the field forward. In this perspective, we provide a short overview of the state-of-the-art in single-component organic solar cells prepared from CBCs, reflect on their troublesome characterization and the importance of chemical precision in these structures, give some recommendations, and discuss the potential impact of these aspects on the field.

PC71BM as Morphology Regulator for Highly Efficient Ternary Organic Solar Cells with Bulk Heterojunction or Layer-by-Layer Configuration.

The ternary strategy is one of the effective methods to regulate the morphology of the active layer in organic solar cells (OSCs). In this work, the ternary OSCs with bulk heterojunction (BHJ) or layer-by-layer (LbL) active layers are prepared by using the polymer donor PM6 and the non-fullerene acceptor L8-BO as the main system and the fullerene acceptor PC71BM as the third component. The power conversion efficiencies (PCEs) of BHJ OSCs and LbL OSCs are increased from 17.10% to 18.02% and from 17.20% to 18.20% by introducing PC71BM into the binary active layer, respectively. The in situ UV-vis absorption spectra indicate that the molecular aggregation and crystallization process can be prolonged by introducing PC71BM into the PM6:L8-BO or PM6/L8-BO active layer. The molecular orientation and molecular crystallinity in the active layer are optimized by introducing the PC71BM into the binary BHJ or LbL active layers, which can be confirmed by the experimental results of grazing incidence wide-angle X-ray scattering. This study demonstrates that the third component PC71BM can be used as a morphology regulator to regulate the morphology of BHJ or LbL active layers, thus effectively improving the performance of BHJ and LbL OSCs.

Overcoming the voltage losses caused by the acceptor‐based interlayer in laminated indoor OPVs

AbstractHarvesting indoor light to power electronic devices for the Internet of Things has become an application scenario for emerging photovoltaics, especially utilizing organic photovoltaics (OPVs). Combined liquid‐ and solid‐state processing, such as printing and lamination used in industry for developing indoor OPVs, also provides a new opportunity to investigate the device structure, which is otherwise hardly possible based on the conventional approach due to solvent orthogonality. This study investigates the impact of fullerene‐based acceptor interlayer on the performance of conjugated polymer–fullerene‐based laminated OPVs for indoor applications. We observe open‐circuit voltage (VOC) loss across the interface despite this arrangement being presumed to be ideal for optimal device performance. Incorporating insulating organic components such as polyethyleneimine (PEI) or polystyrene (PS) into fullerene interlayers decreases the work function of the cathode, leading to better energy level alignment with the active layer (AL) and reducing the VOC loss across the interface. Neutron reflectivity studies further uncover two different mechanisms behind the VOC increase upon the incorporation of these insulating organic components. The self‐organized PEI layer could hinder the transfer of holes from the AL to the acceptor interlayer, while the gradient distribution of the PS‐incorporated fullerene interlayer eliminates the thermalization losses. This work highlights the importance of structural dynamics near the extraction interfaces in OPVs and provides experimental demonstrations of interface investigation between solution‐processed cathodic fullerene layer and bulk heterojunction AL.

Hexanary blends: a strategy towards thermally stable organic photovoltaics

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 °C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells.

Suppressing Bimolecular Charge Recombination and Energetic Disorder with Planar Heterojunction Active Layer Enables 18.1% Efficiency Binary Organic Solar Cells

The performance of organic solar cells has been dramatically enhanced due to the use of bulk heterojunction active layer structure. However, blending donor and acceptor in the bulk volume results in a large extent of bimolecular charge recombination and increasing energetic disorder. Herein, we fabricated organic solar cells with polymer donor PM6 and nonfullerene acceptor N3 in bilayer structure to investigate the impact of planar heterojunction configurations on charge recombination and energetic landscapes. We find superior crystallinity features in the bilayer composed of pure donor and acceptor layers. The bimolecular charge recombination and energetic disorder are effectively suppressed compared with the bulk heterojunction counterparts. Thus, a power conversion efficiency as high as 18.1% (17.8% averaged) is achieved, which stands among the top values of planar heterojunction organic solar cells. In addition, improved device shelf stability is observed because of the fewer donor–acceptor interfaces in the active layer. Our results suggest that a planar heterojunction structure could efficiently suppress the bimolecular charge recombination and energetic disorder, providing an alternative pathway for developing solution-processed organic solar cells.

N-Annulated Perylene Diimide Non-Fullerene Acceptors for Organic Photovoltaics

This work covers the development of non-fullerene acceptors for use in organic photovoltaics built using the N-annulated perylene diimide dye. The classic perylene diimide dye has been extensively used to construct non-fullerene acceptors, leading to device power conversion efficiencies of over 10%. Strong visible light absorption and deep frontier molecular energy levels have made such materials (both molecular and polymeric) near ideal for pairing with narrow-gap conjugated polymers in bulk-heterojunction active layers. The N-annulation of the dye provides an extra site for side-chain engineering and alters the electronic structure of the polycyclic aromatic core. In addition, N-annulation allows for selective bromination of the perylene core, leading to building blocks that are useful for the construction of large molecular frameworks using the atom-economical direct heteroarylation cross-coupling method. Herein, we detail a series of molecules developed by our team that are based on the N-annulated perylene diimide in the form of dimers with different cores (both electron-rich and electron-deficient); dimers with varied side chains; tetramers with varying geometries; and large, asymmetric molecules with internal energy cascades. The use of these molecules as non-fullerene acceptors in organic photovoltaic devices (binary and ternary blends, outdoor and indoor light applications, and spin-coated vs. slot-die-coated photoactive layers) is presented.

Physical mechanisms on FRET and ICT for efficient Y6:PM6 as bulk heterojunction active layers.

Organic solar cells (OSCs) have emerged as a promising new energy technology because of their advantages of efficient light harvesting, comprehensive material sources, and flexible and translucent devices. In this study, fluorescence resonance energy transfer (FRET) and intermolecular charge transfer (ICT) in the donor-acceptor system for efficient OSCs of the Y6:PM6 heterostructure are investigated with ultrafast pump-probe transient absorption spectroscopy, time-resolved fluorescence spectroscopy, steady absorption, and fluorescence spectroscopy, which is significantly supported by theoretical results. The physical mechanisms of FRET and ICT in the donor-acceptor system for efficient OSCs of the Y6:PM6 heterostructure are investigated theoretically and experimentally. FRET can decrease the recombination of electron-hole in the donor's fluorescence and enhance the acceptor's fluorescence. Our study contributes to a better understanding of FRET and ICT and provides valuable references for the rational design of FRET- and ICT-based OSCs.

Promoting the stability of organic photovoltaics by planar heterojunction optimization

In organic photovoltaics (OPVs) with a bulk heterojunction (BHJ) active layer, the donor and acceptor materials have a metastable nanoscale phase mixture, where the uncontrollable morphology greatly reduces the stability of the device.

Low-Cost Silicon Phthalocyanine as a Non-Fullerene Acceptor for Flexible Large Area Organic Photovoltaics.

We demonstrate large-area (1 cm2) organic photovoltaic (OPVs) devices based on bis(tri-n-butylsilyl oxide) silicon phthalocyanine (3BS)2-SiPc as a non-fullerene acceptor (NFA) with low synthetic complexity paired with poly(3-hexylthiophene) (P3HT) as a donor polymer. Environment-friendly nonhalogenated solvents were used to process large area OPVs on flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates. An alternate sequentially (Alt-Sq) blade-coated active layer with bulk heterojunction-like morphology is obtained when using (3BS)2-SiPc processing with o-xylene/1,3,5-trimethylbenzene solvents. The sequential (Sq) active layer is prepared by first blade-coating (3BS)2-SiPc solution followed by P3HT coated on the top without any post-treatment. The conventional sequentially (Sq) blade-coated active layer presents very low performance due to the (3BS)2-SiPc bottom layer being partially washed off by processing the top layer of P3HT. In contrast, alternate sequentially (Alt-Sq) blade-coated layer-by-layer film shows even better device performance compared to the bulk heterojunction (BHJ) active layer. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and atomic force microscopy (AFM) reveal that the Alt-Sq processing of the active layer leads to a BHJ-like morphology with a well-intermixed donor-acceptor component in the active layer while providing a simpler processing approach to low-cost and large-scale OPV production.

Microscopic, Spectroscopic, and Electrochemical Characterization of Novel Semicrystalline Poly(3-hexylthiophene)-Based Dendritic Star Copolymer.

In this study, electron-donating semicrystalline generation 1 poly(propylene thiophenoimine)-co-poly(3-hexylthiophene) star copolymer, G1PPT-co-P3HT was chemically prepared for the first time. Copolymerization was achieved with high molecular weight via facile green oxidative reaction. 1H NMR analyses of the star copolymer demonstrated the presence of 84% regioregular (rr) head-to-tail (HT) P3HT, which accounts for the molecular ordering in some grain regions in the macromolecule’s morphology, as revealed by the high-resolution scanning electron microscopy (HRSEM) and Selected Area Electron Diffraction (SAED) images, and X-ray diffraction spectroscopy (XRD) measurements. The star copolymer also exhibited good absorption properties in the ultraviolet-visible (UV-Vis) and the near infrared (NIR) spectral regions, which give rise to an optical energy bandgap value as low as 1.43 eV. A HOMO energy level at −5.53 eV, which is below the air-oxidation threshold, was obtained by cyclic voltammetry (CV). Electrochemical impedance spectroscopy (EIS) ascertained the semiconducting properties of the macromolecule, which is characterized by a charge transfer resistance, Rct, value of 3.57 kΩ and a Bode plot-phase angle value of 75°. The combination of the EIS properties of G1PPT-co-P3HT and its highly electron-donating capability in bulk heterojunction (BHJ) active layer containing a perylene derivative, as demonstrated by photoluminescence quenching coupled to the observed Förster Resonance charge transfer, suggests its suitability as an electron-donor material for optoelectronic and photovoltaic devices.