Bulk-heterojunction Cells Research Articles

Long‐Range Charge Carrier Transport in Planar Polymer Bulk‐Heterojunction Photovoltaic Cells

One‐dimensional scanning optical beam‐induced current (OBIC) measurements have been carried out on polymer bulk heterojunction (BHJ) photovoltaic cells with a planar, or lateral configuration. The planar P3HT:PCBM cells have parallel aluminum or gold electrodes that are 390 to 560 micrometers apart. When a focused laser beam is scanned across the electrode gap, photocurrent or photovoltage is recorded as a function of beam position along with the transmission of the excitation beam. Despite the large electrode gap size, cells with symmetric Al/Al electrodes exhibit significant photocurrent and photovoltage which are the highest at the electrode interfaces and null at the cell center. The OBIC in these large planar polymer BHJ cells is attributed to the metal/BHJ blend Schottky junction. The larger Schottky barrier of the Al/BHJ junction gives rise to a stronger OBIC response than the Au/BHJ junction. The photocurrent and photovoltage always have opposite signs and are antisymmetric about the cell center. In asymmetric Al/Au cells, the electrode work function difference contributes an additional built‐in field/potential drop and significantly modifies the photocurrent and photovoltage profiles. The depletion width of the Al/BHJ Schottky junction is 110–120 μm, while the minority electron diffusion length is determined to be 43.8 μm.

Novel photoactive material and fabrication techniques for solar cells application: nanocellulose-based graphene oxide CdS composite

Abstract In recent times, solar energy has become one of the largest available sources of renewable energy at our disposal. However, the design of highly efficient solar cells is increasingly becoming crucial as there has been a surge for economically viable alternative energy sources with the lowest cost. Significant advances have been made through different routes to make photovoltaic (PV)/solar technologies economically viable, eco-friendly and consequently scalable. As a result, cellulose nanomaterials have become one of the emerging technologies in this regard because of the advantages of high-value bio-based nanostructured materials, such as their abundance and sustainability. Nanocellulose-based photoactive nanocomposite materials can be made by integrating conducting photoactive and electroconductive materials with hydrophilic biocompatible cellulose. Inorganic nanoparticles, such as graphene/reduced graphene oxide cadmium sulphide quantum dots, amongst others, can be introduced into the nanocellulose matrix and can be applied either as charge transporters or photoactive materials in different types of solar cells. Thus, in this review, we highlight the optoelectronic properties of different photoactive materials, particularly nanocellulose-based graphene nanocomposites; their efficiencies and drawbacks were X-rayed. The effect of doping each PV material on the PV performance is also discussed. It is anticipated that the novel material would result in a reduction in the cost of solar cells, jointly enhancing their efficacy in generating environmentally friendly electricity. Since the fabrication techniques and equipment play a crucial role in the development of solar cells, the fabrication techniques of bulk-heterojunction (BHJ) cells containing a nanocellulose-based graphene composite and case studies of already fabricated BHJ PV cells with nanocellulose-based graphene composite are discussed.

Recent Advances in Graphene-Enabled Materials for Photovoltaic Applications: A Comprehensive Review.

Graphene's two-dimensional structural arrangement has sparked a revolutionary transformation in the domain of conductive transparent devices, presenting a unique opportunity in the renewable energy sector. This comprehensive Review critically evaluates the most recent advances in graphene production and its employment in solar cells, focusing on dye-sensitized, organic, and perovskite devices for bulk heterojunction (BHJ) designs. This comprehensive investigation discovered the following captivating results: graphene integration resulted in a notable 20.3% improvement in energy conversion rates in graphene-perovskite photovoltaic cells. In comparison, BHJ cells saw a laudable 10% boost. Notably, graphene's 2D internal architecture emerges as a protector for photovoltaic devices, guaranteeing long-term stability against various environmental challenges. It acts as a transportation facilitator and charge extractor to the electrodes in photovoltaic cells. Additionally, this Review investigates current research highlighting the role of graphene derivatives and their products in solar PV systems, illuminating the way forward. The study elaborates on the complexities, challenges, and promising prospects underlying the use of graphene, revealing its reflective implications for the future of solar photovoltaic applications.

Organic Solar Cells with Nanofibers in the Active Layer Obtained by Coaxial Electrospinning

The high demand for energy and the intensification of climate change have led to the need to improve renewable energy sources, such as solar energy. Organic bulk heterojunction (BHJ) cells using conjugated polymers as the electron donor in the active layer are an alternative because they are easy processability, flexible, have the absorption spectrum corresponding to the structure of the polymer, and can be synthesized in an endless variety of chemical structures. One of the limiting factors for large-scale applications was the relatively low conversion of solar energy into electricity, but this has been practically overcome recently, as efficiencies of around 20% have been achieved for small-area devices and around 16% for larger devices, approaching conventional silicon cells. The device’s stability remains the most restraining factor, as it has to last at least ten years in sun and weather exposure. This review presents works that improve the efficiency of photovoltaic devices by addressing the development of materials used in the active layer (semiconductor polymers and acceptor materials). Additionally, the use of nanofibers in the active layer obtained by the electrospinning technique is discussed. The nanofiber quality parameters (solute properties, solution properties, processing conditions, and environmental conditions) are discussed, including coaxial electrospinning.

Photosensitization of asymmetric molecular, and bimolecular aliphatic-μ-bridged-meso-phenyl porphyrin

The desired asymmetric metal-free porphyrin derivatives were prepared by condensing freshly prepared the phenyl dipyrraomethane and triphenyl tetrapyrrane building blocks with the different molar ratios of acetaldehyde ([Formula: see text]CHO), and glutaraldehyde (OHC([Formula: see text]CHO, Glu). The obtained asymmetric [3+1] ([Formula: see text]Por, [Formula: see text]Por-[Formula: see text]-([Formula: see text]Por) and [2+2] ([Formula: see text]Por, [Formula: see text]-([Formula: see text] oligomer) porphyrin derivatives were characterized by UV-Vis, FT-IR, and 1H NMR. Their photoelectric properties were investigated by examining their current density-voltage (J-V) curves in dark, and under artificial sunlight, illumination using standard bulk heterojunction cell arrangements. The [Formula: see text]-V plots show that they display photosensitive semiconducting properties in the range of ([Formula: see text] = 1.7–14.5 [Formula: see text]S/cm) with negligible photovoltaic effects. All data were compared to those of symmetric metal-free [Formula: see text]Por([Formula: see text] = 33.8 [Formula: see text]S/cm). Density functional theory (DFT) is employed to validate the ground-state molecular geometry.

Semi-Transparent, Chiral Organic Photodiodes with Incident Direction-Dependent Selectivity for Circularly Polarized Light.

Detection of the circular polarization of light is possible using chiral semiconductors, yet the mechanisms remain poorly understood. Semi-transparent chiral photodiodes allow for a simple experiment to investigate the basis of their selectivity: changing the side from which the diode is illuminated. A reversal of circular selectivity is observed in photocurrent generation when changing the direction of illumination on organic, bulk-heterojunction cells. The change in selectivity can be explained by a space-charge limitation on the collection of photocarriers in combination with preferential absorption of one of the circular polarizations of near-infrared light by the chiral non-fullerene acceptor. The space-charge limitation is supported by detailed measurements of frequency and intensity dependence of dc and ac photocurrents.

Nanocrystalline CaWO4 and ZnWO4 Tungstates for Hybrid Organic-Inorganic X-ray Detectors.

Hybrid materials combining an organic matrix and high-Z nanomaterials show potential for applications in radiation detection, allowing unprecedented device architectures and functionality. Herein, novel hybrid organic-inorganic systems were produced using a mixture of tungstate (CaWO4 or ZnWO4) nanoparticles with a P3HT:PCBM blend. The nano-tungstates with a crystallite size of 43 nm for CaWO4 and 30 nm for ZnWO4 were synthesized by the hydrothermal method. Their structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The hybrid systems were used to fabricate direct conversion X-ray detectors able to operate with zero bias voltage. The detector performance was tested in a wide energy range using monochromatic synchrotron radiation. The addition of nanoparticles with high-Z elements improved the detector response to X-ray radiation compared with that of a pure organic P3HT:PCBM bulk heterojunction cell. The high dynamic range of our detector allows for recording X-ray absorption spectra, including the fine X-ray absorption structure located beyond the absorption edge. The obtained results suggest that nanocrystalline tungstates are promising candidates for application in direct organic-inorganic X-ray detectors.

Importance of structural hinderance in performance–stability equilibrium of organic photovoltaics

Power conversion efficiency and long-term stability are two critical metrics for evaluating the commercial potential of organic photovoltaics. Although the field has witnessed a rapid progress of efficiency towards 19%, the intrinsic trade-off between efficiency and stability is still a challenging issue for bulk-heterojunction cells due to the very delicate crystallization dynamics of organic species. Herein, we developed a class of non-fullerene acceptors with varied side groups as an alternative to aliphatic chains. Among them, the acceptors with conjugated side groups show larger side-group torsion and more twisted backbone, however, they can deliver an efficiency as high as 18.3% in xylene-processed cells, which is among the highest values reported for non-halogenated solvent processed cells. Meanwhile, decent thermal/photo stability is realized for these acceptors containing conjugated side groups. Through the investigation of the geometry–performance–stability relationship, we highlight the importance of side-group steric hinderance of acceptors in achieving combined high-performance, stable, and eco-friendly organic photovoltaics.

Heat transfer in binary and ternary bulk heterojunction solar cells

Ternary strategy is one of the most commonly used methods to boost the performance of organic solar cells (OSCs) from a binary blend of donor and acceptor. Fullerene derivatives are popular choices for the ternary component as they could benefit the electrical property. However, the ternary component could also affect other physical properties of the bulk-heterojunction (BHJ). Among these properties, heat transfer has rarely been reported, despite its relevance for thermal durability of OSCs. Here, we employ scanning photothermal deflection technique to study thermal diffusion properties of binary PM6:Y7 and ternary PM6:Y7:X BHJs, where X = PC71BM, ICBA, and N2200. It is found that fullerene derivatives deteriorate the thermal diffusivity (D) of blend films and the device thermal durability, despite enhancing the electrical and device performance. In contrast, when an n-type conjugated polymer N2200 is used as the ternary component, both the electrical and thermal properties are enhanced, with improved power conversion efficiency and prolonged device thermal durability. These results offer a perspective on how to choose a favorable third component. Fullerene derivatives are not necessarily the optimal choice for ternary component for BHJ cells because of the inferior thermal properties.

Mechanism of the Photodegradation of A‐D‐A Acceptors for Organic Photovoltaics**

AbstractHerein, we elucidate the photodegradation pathway of A‐D‐A‐type non‐fullerene acceptors for organic photovoltaics. Using IT‐4F as a benchmark example, we isolated the photoproducts and proved them isomers of IT‐4F formed by a 6‐e electrocyclic reaction between the dicyanomethylene unit and the thiophene ring, followed by a 1,5‐sigmatropic hydride shift. This photoisomerization was accelerated under inert conditions, as explained by DFT calculations predicting a triplet‐mediated reaction path (quenchable by oxygen). Adding controlled amounts of the photoproduct P1 to PM6:IT‐4F bulk heterojunction cells led to a progressive decrease in photocurrent and fill factor attributed to its poor absorption and charge transport properties. The reaction is a general photodegradation pathway for a series of A‐D‐A molecules with 1,1‐dicyanomethylene‐3‐indanone termini, and its rate varies with the structure of the donor and acceptor moiety.

Enhanced Charge Separation in Ternary Bulk-Heterojunction Organic Solar Cells by Fullerenes.

Carrier generation dynamics in binary PTB7-Th:COi8DFIC (1:1.5) and ternary PTB7-Th:COi8DFIC:PC71BM (1:1.05:0.45) composites were investigated to identify the origins of high power conversion efficiencies (PCEs) in ternary bulk-heterojunction (BHJ) organic solar cells. Steady-state photoluminescence and time-resolved photoinduced absorption spectroscopic analyses revealed that the ternary composite exhibited faster hole transfer from COi8DFIC to PTB7-Th (8 ps compared to 21 ps in the binary composite), which led to an improved exciton separation yield in COi8DFIC (94% compared to 68% in the binary composite). Improved intermixing of the component materials and efficient electron transfer from COi8DFIC to PC71BM facilitated enhancement in the hole transfer rate. The COi8DFIC-to-PC71BM electron transfer promoted an electron transport cascade over PTB7-Th, COi8DFIC, and PC71BM, which efficiently deactivated back-electron transfer (carrier recombination loss) from COi8DFIC to PTB7-Th at ∼160 ps and assisted in improving the PCE of the ternary BHJ cell (13.4% compared to 10.5% in the binary BHJ cell).

Characterization and Performance of PAni-TiO2 Photovoltaic Cells Treated by RF Plasma

Severe research attempts are still in progress to improve the performance of polyaniline (PAni) based photoactive layers as one of the cheapest materials used for manufacturing organic solar cells. Herein, polymer solar cells were fabricated with ITO/(PAni-TiO2)/Au system. The photoactive layers (PAni-TiO2) were treated with a hydrogen-plasma discharge for low processing time of 0, 3 and 5 min to enhance the synthesized solar cells efficiency. The morphology, micro structure and optical properties of the prepared samples and plasma treated nanocomposite layers were investigated and discussed. The performance of bulk heterojunction (BHJ) cell samples have been systematically investigated before and after plasma treatment. The absorption and optical band gap energy is increased after the treated PAni-TiO2 photoactive layers. It is found that, the efficiency was enhanced to 0.7% after 5 min of hydrogen plasma process compared to 0.36% for the pristine cell. The efficiency increase is ascribed to a structural change that accompanied by a rapid increase in surface roughness, which leaded to a decrease in the reflected photons and in turn an increase in the produced charge carriers.

Highly‐Transparent and True‐Colored Semitransparent Indoor Photovoltaic Cells

AbstractOrganic photovoltaic (OPV) cells are promising indoor light energy harvesters because organic materials absorb strongly in the visible range. An indoor photovoltaic (IPV) device is an effective tool for the remote off‐grid wireless charging. However, as the indoor light fluxes are much weaker than the 1‐Sun condition, high‐performance indoor cells should have large areas in order to generate appreciable energies. They would then appear as flat, but expansive and dark objects if deployed indoors. Their presence would then alter the indoor lighting environment and affect visual perceptions. This work addresses the lighting and perception issues of IPV cells in three ways. i) A concept is proposed such that a high‐efficiency, semitransparent indoor OPV cell should possess an absorbance which is mismatched with the emission spectra of the light sources. ii) It is demonstrated that bulk heterojunction (BHJ) OPV solar cells with porphyrin donors can serve as high‐transparency and high‐efficiency indoor light harvesters. iii) Quantitative assessment criteria are presented for the transparency and chromaticity of an indoor semitransparent OPV cell, demonstrating that porphyrin‐based P2:PC71BM semitransparent BHJ cells can achieve a power conversion efficiency (PCE) exceeding 10%, and an illuminance transparency ~65%, while preserving the color perception of the light sources.

Quantifying charge carrier density in organic solar cells by differential charging techniques

Accurate determination of charge carrier density in organic solar cells under light irradiation is essential because charge carrier density is directly related to the bimolecular recombination rate and open-circuit voltage of the cells. We investigate the robustness of transient photovoltage/current (TPV/C) and impedance spectroscopy (IS) to interference from the geometric capacitance of the cells (Cgeo) during quantification of the charge carrier density. TPV/C and IS accurately quantify the charge carrier density of bulk heterojunction cells with small Cgeo. For planar heterojunction cells with a larger Cgeo contribution, IS fails to separate the charge carriers in the organic layer from those in the electrodes. In contrast, TPV/C eliminates the effect of Cgeo and gives a reasonable estimation of the charge carrier density in the organic layer with the planar heterojunction, demonstrating that TPV/C is more robust than IS to interference from Cgeo of the cells.

Design and characterisation of graphene‐based nano‐photodiode array device for photo‐stimulation of subretinal implant

Electrically stimulating the retina of the patients suffering due to extreme retinal disorders such as age related macula degeneration, retinitis pigmentosa is quickly turning into a feasible accomplishment through retinal implants. The objective of this work is to optimise the design, develop and test an organic photovoltaic based Nano Photo Diode Array (NPDA) device for retinal prosthesis application to restoring vision to patients blinded by degenerative retinal diseases. NPDA for subretinal stimulation has been fabricated based on Bulk heterojunction (BHJ) solar cell technology. Each pixel is composed by a graphene alone bottom anode, a semiconducting single wall carbon nanotube (S-SWCNT):C60 blend, and a top cathode in aluminum (Al) of 100 µm or 150 diameter. This NPDA device photovoltaically converts incident light into electric current to stimulate nearby retinal neurons without wired power connections. The fabricated device is able to provide a photo current density of 245 µA/cm 2 and photovoltage of 0.31 V. These results suggest that BHJ cell employed photodiode array used in this work make it better than POLYRETINA and SCNC. The significant improvement in photocurrent density paves the way for scaling up density of NPDA array in implant. Increased number of electrodes will indeed enhance visual acuity to greater extent.

Observing electron transport and percolation in selected bulk heterojunctions bearing fullerene derivatives, non-fullerene small molecules, and polymeric acceptors

We systematically study how electrons achieve percolation pathways in a bulk-heterojunction (BHJ) blend as the weight fraction of the electron acceptor is altered. Three different classes of BHJs are explored: BHJs that contain (1) fullerene-based acceptors, (2) non-fullerene small molecules, and (3) polymer acceptors. The nano-topologies of acceptors in (1) to (3) can be approximately associated with spheres, oblates (plates) and prolates (long rods), respectively. We discover that the weight fraction of the acceptor for electron percolation to complete is successively reduced in the order of fullerenes (0.32), non-fullerene (NF) small molecules (0.25), and polymers (0.11). A new parameter, namely, the effective volume fraction, χ, is introduced to quantify the fraction of the acceptor moieties that can contribute to electron transport in the BHJs. The electron percolation regions can be fitted with a model that considers the shapes of the acceptors and their χ in the BHJs. An all-polymer BHJ containing PTB7-Th:N2200 exhibits the largest χ (0.6–0.8), indicating N2200 acceptors are the most effective in transportation of electrons. All-polymer BHJs can better preserve their electron transport capability arising from composition variations, and they are immuned from the introduction of inert materials or the exposure of moisture, in stark contrast to fullerene-based BHJs. Our results correlate the electron transport behaviors to the nano-topologies of acceptors for BHJ cells, and offer guidance to tune the compositions and understand the nano-morphology of BHJ cells using different classes of acceptors.

Increasing Quantum Efficiency of Polymer Solar Cells with Efficient Exciton Splitting and Long Carrier Lifetime by Molecular Doping at Heterojunctions

Optimizing the photovoltaic processes directly by electric technique attracts the exploration of molecular doping in organic photovoltaics (OPVs). However, the inappropriate and inhomogeneous dopant distribution in the bulk heterojunction (BHJ) film inhibits the performance improvement and mechanism understanding in doped OPVs. A strategy to solve these critical problems is reported here. By employing a planar heterojunction (PHJ) device structure, the role of dopant location and photovoltaic performance is clarified. Dopants tris(pentafluorophenyl)borane (BCF) distributing at the heterojunction rather than in the single component (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) or 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC)) produces the best performance with quantum efficiency (QE) enhancement. Mechanism studies reveal the facilitated exciton splitting and suppressed bimolecular recombination as the causes for device improvement. The sequential coating procedure is further developed for the doped BHJ cells, and the devices based on this method exhibit a short-circuit current (Jsc) increase of 1.3 mA/cm2, which is an impressive value compared with the previous reports.

Discriminating bulk versus interface shunts in organic solar cells by advanced imaging techniques

AbstractAn important aspect when upscaling organic photovoltaics from laboratory to industrial scale is quality control. Established imaging techniques like lock‐in thermography or luminescence imaging are frequently used for this purpose. While these techniques allow for the lateral detection of defects, they cannot provide information on the vertical position of the defect in the OPV stack. Here, we present an approach to overcome this limitation. A femtosecond‐laser is deployed to introduce well‐defined artificial calibration defects selectively into both the interface and the bulk active layer of inverted P3HT:PCBM bulk heterojunction cells during device fabrication. The defective cells are then characterized using J‐V analysis and several nondestructive imaging methods (dark lock‐in thermography, photoluminescence, and electroluminescence imaging). The distinct response for each defect in the different imaging methods enables us to uniquely distinguish between bulk and interface defects. This allows to study surface recombination under most controlled conditions.

Chlorophyll-based organic solar cells with improved power conversion efficiency

Abstract Chlorophylls (Chls), and associated chlorophyll derivatives, are one of the oldest, most versatile organic semiconductors found in nature. Herein, we present two easily semi-synthesized chlorophyll derivatives, namely, chlorin e6 trimethyl ester (Ce6Me3) and its copper complex (Cu–Ce6Me3), as the p-type dopants for organic semiconductors and their impact in organic solar cells (OSCs). In our study, both Chls showed intense Soret and Qy bands in the UV-visible spectra, leading to an effect means for capturing solar light and energy. Chls also exhibited high carrier mobility owing to the partial formation of aggregates through the spin-coating process. Using Chls, we fabricated OSCs in both planar-heterojunction (PHJ) and bulk-heterojunction (BHJ) solar cell configurations, together with C70/PC70BM as electron acceptors. In PHJ solar cells, we received solar power conversion efficiencies (PCEs) of only 0.85% and 0.93% for Cu–Ce6Me3- and Ce6Me3-based devices, respectively, with the thickness of the donor layer at 5 nm. In BHJ cells, we achieved much higher PCEs of 1.53% and 2.05% for Cu–Ce6Me3:PC70BM and Ce6Me3:PC70BM respectively, where both blending ratios were set to 1:8. The improvement on PCE in BHJ cells may be attributed to the better charge separation increase at the donor–acceptor interface.

Rationalizing device performance of perylenediimide derivatives as acceptors for bulk-heterojunction organic solar cells

Abstract This study examines the correlation between the structure of four perylenediimide (PDI) derivatives and their optoelectronic behaviors. Three of them with one, two or four PDI moieties conjugated peripherally to porphyrin core by phenylene linker, denoted as nPDI-ZnP (n = 1,2,4), are investigated as acceptors on the charge transport property and device performance in non-fullerene (NF) bulk-heterojunction (BHJ) organic photovoltaic cells with a commercial polymer PTB7-Th as the donor material. Besides, the parent PDI molecule is studied as a control. Detailed charge carrier transport analysis is carried out on these PTB7-Th:Acceptor BHJs. Among all acceptor molecules, PTB7-Th:4PDI-ZnP film possesses the most desirable transport properties among the NF BHJs, and the best device performance with a power conversion efficiency (PCE) of 8.35%. To quantify the transport behaviors, the charge imbalance factor Δ is introduced to describe the global electron and hole balance in different electric fields. We discover that PTB7-Th:4PDI-ZnP has the least Δ and thereby gives rise to a BHJ cells with the highest fill factor and PCE.