Bulk Heterojunction Photovoltaics Research Articles

Deuteration-enhanced neutron contrasts to probe amorphous domain sizes in organic photovoltaic bulk heterojunction films

An organic photovoltaic bulk heterojunction comprises of a mixture of donor and acceptor materials, forming a semi-crystalline thin film with both crystalline and amorphous domains. Domain sizes critically impact the device performance; however, conventional X-ray scattering techniques cannot detect the contrast between donor and acceptor materials within the amorphous intermixing regions. In this study, we employ neutron scattering and targeted deuteration of acceptor materials to enhance the scattering contrast by nearly one order of magnitude. Remarkably, the PM6:deuterated Y6 system reveals a new length scale, indicating short-range aggregation of Y6 molecules in the amorphous intermixing regions. All-atom molecular dynamics simulations confirm that this short-range aggregation is an inherent morphological advantage of Y6 which effectively assists charge extraction and suppresses charge recombination as shown by capacitance spectroscopy. Our findings uncover the amorphous nanomorphology of organic photovoltaic thin films, providing crucial insights into the morphology-driven device performance.

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.

Uncovering the out-of-plane nanomorphology of organic photovoltaic bulk heterojunction by GTSAXS

The bulk morphology of the active layer of organic solar cells (OSCs) is known to be crucial to the device performance. The thin film device structure breaks the symmetry into the in-plane direction and out-of-plane direction with respect to the substrate, leading to an intrinsic anisotropy in the bulk morphology. However, the characterization of out-of-plane nanomorphology within the active layer remains a grand challenge. Here, we utilized an X-ray scattering technique, Grazing-incident Transmission Small-angle X-ray Scattering (GTSAXS), to uncover this new morphology dimension. This technique was implemented on the model systems based on fullerene derivative (P3HT:PC71BM) and non-fullerene systems (PBDBT:ITIC, PM6:Y6), which demonstrated the successful extraction of the quantitative out-of-plane acceptor domain size of OSC systems. The detected in-plane and out-of-plane domain sizes show strong correlations with the device performance, particularly in terms of exciton dissociation and charge transfer. With the help of GTSAXS, one could obtain a more fundamental perception about the three-dimensional nanomorphology and new angles for morphology control strategies towards highly efficient photovoltaic devices.

V OC variation with different molecular weight fractions in highly efficient organic photovoltaic bulk heterojunctions

Tuning molecular weight fractions is one of the most important strategies for optimizing the morphology of bulk heterojunctions (BHJs) to obtain high-performance organic solar cells. Here, we investigate the effects of molecular weight fractions on the performance of state-of-the-art PM6:Y6 BHJ solar cells. A remarkable blueshift in the photoluminescence spectra can be observed with the introduction of the PM6 component. The electroluminescent and morphological analyses indicate that this observation can be attributed to increased charge-transfer state energy and an intermolecular interaction between donor and acceptor, leading to open circuit voltage (V OC) variations in the photovoltaic device. The transient absorption spectra confirm that the optimized donor:acceptor (D:A) ratio can suppress the generation of polarons and achieve high efficiency exciton dissociation. The results of atomic force microscopy and neutron reflectometry reveal that higher acceptor contents promote the formation of a desirable interpenetrating network and enable high fill factors and short-circuit currents to be obtained.

Organic bulk heterojunction photovoltaics incorporating cyclopenteno[60]fullerene monoadducts as n‐type materials display superior power conversion efficiency than with PC61BM

AbstractOrganic bulk heterojunction photovoltaics, merely incorporating monoadducts of cyclopenteno[60]fullerenes (CPFs) as n‐type materials and P3HT as p‐type materials, display superior power conversion efficiency up to 4.6 ± 0.12%, superseding that with PC61BM/P3HT (3.8 ± 0.20%) for ca. 20%, under AM 1.5G irradiation―primarily attributed to the lack of homo‐conjugation on CPFs and their higher LUMO energy levels.

Effect of Solvent Additives on the Morphology and Device Performance of Printed Nonfullerene Acceptor Based Organic Solar Cells.

Printing of active layers of high-efficiency organic solar cells and morphology control by processing with varying solvent additive concentrations are important to realize real-world use of bulk-heterojunction photovoltaics as it enables both up-scaling and optimization of the device performance. In this work, active layers of the conjugated polymer with benzodithiophene units PBDB-T-SF and the nonfullerene small molecule acceptor IT-4F are printed using meniscus guided slot-die coating. 1,8-Diiodooctane (DIO) is added to optimize the power conversion efficiency (PCE). The effect on the inner nanostructure and surface morphology of the material is studied for different solvent additive concentrations with grazing incidence small-angle X-ray scattering (GISAXS), grazing incidence wide-angle X-ray scattering (GIWAXS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Optical properties are studied with photoluminescence (PL), UV/vis absorption spectroscopy, and external quantum efficiency (EQE) measurements and correlated to the corresponding PCEs. The addition of 0.25 vol % DIO enhances the average PCE from 3.5 to 7.9%, whereas at higher concentrations the positive effect is less pronounced. A solar cell performance of 8.95% is obtained for the best printed device processed with an optimum solvent additive concentration. Thus, with the large-scale preparation method printing similarly well working solar cells can be realized as with the spin-coating method.

The Crucial Role of Quaternary Mixtures of Active Layer in Organic Indoor Solar Cells

A bulk heterojunction (BHJ) consisting of more than one donor/acceptor is one plausible way to improve the charge transport and/or the spectral absorption range in organic solar cells. Ternary and quaternary solar cells have shown promise in this regard. However, quaternary structures have not yet been intensively tested under indoor lighting conditions. A finite-difference time-domain (FDTD)-based simulation was used to solve for the electric field intensity distribution inside a quaternary photovoltaic device illuminated by 500 lx indoor white light emitting diodes (LEDs). We found that quaternary indoor photovoltaics (IPVs) showed peculiarly high oscillations in the simulated ideal short-circuit current density (Jsc,ideal). Here, we simulated the electric field intensity inside the photovoltaic, compared it to single BHJ photovoltaics, and deduced that the electric field intensity inside the active layer of the IPV was highly sensitive to its thickness due to interference between the incident light and the light reflecting from the back electrode. We also found that Poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) acted as the primary light absorber in the quaternary blend while poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) acted primarily as a cascade energy level and secondarily as a supplementary light absorber.

Random Copolymers Outperform Gradient and Block Copolymers in Stabilizing Organic Photovoltaics

AbstractRecent advances have led to conjugated polymer‐based photovoltaic devices with efficiencies rivaling amorphous silicon. Nevertheless, these devices become less efficient over time due to changes in active layer morphology, thereby hindering their commercialization. Copolymer additives are a promising approach toward stabilizing blend morphologies; however, little is known about the impact of copolymer sequence, composition, and concentration. Herein, the impact of these parameters is determined by synthesizing random, block, and gradient copolymers with a poly(3‐hexylthiophene) (P3HT) backbone and side‐chain fullerenes (phenyl‐C61‐butyric acid methyl ester (PC61BM)). These copolymers are evaluated as compatibilizers in photovoltaic devices with P3HT:PC61BM as the active layer. The random copolymer with 20 mol% fullerene side chains and at 8 wt% concentration in the blend gives the most stable morphologies. Devices containing the random copolymer also exhibit higher and more stable power conversion efficiencies than the control device. Combined, these studies point to the random copolymer as a promising new scaffold for stabilizing bulk heterojunction photovoltaics.

BODIPY-Based Semiconducting Materials for Organic Bulk Heterojunction Photovoltaics and Thin-Film Transistors.

The rapid emergence of organic (opto)electronics as a promising alternative to conventional (opto)electronics has been achieved through the design and development of novel π-conjugated systems. Among various semiconducting structural platforms, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) π-systems have recently attracted attention for use in organic thin-films transistors (OTFTs) and organic photovoltaics (OPVs). This Review article provides an overview of the developments in the past 10 years on the structural design and synthesis of BODIPY-based organic semiconductors and their application in OTFT/OPV devices. The findings summarized and discussed here include the most recent breakthroughs in BODIPYs with record-high charge carrier mobilities and power conversion efficiencies (PCEs). The most up-to-date design rationales and discussions providing a strong understanding of structure-property-function relationships in BODIPY-based semiconductors are presented. Thus, this review is expected to inspire new research for future materials developments/applications in this family of molecules.

Numerical modelisation of ZnO interfacial layer on P3HT:PCBM based organic photovoltaic bulk heterojunction devices

The interface layers thickness effect has been studied for inverted organic solar cells based on Poly(3-hexylthiophene):[6,6] phenyl-C61-butyric Acid Methyl Ester (P3HT :PCBM). We used the SCAPS simulation tool from which we reproduced and predicted the properties of the components introduced into the composition of the inverted structure photovoltaic device in order to model the cell parameters. We varied the cathode layer thickness used as electron transport layer (ETL) and calculated the J-V characteristics of the solar cells and their external quantum efficiency (EQE). The highest energy conversion efficiency of 4.88% was obtained for 20 nm of buffer layer thickness. However, an increase in interfacial layer thickness leads to a significant decrease in efficiency as well as other parameters. The results showed that there was agreement with devices parameter measurements found in the literature.

Low-Vapor-Pressure Solvent Additives Function as Polymer Swelling Agents in Bulk Heterojunction Organic Photovoltaics

Bulk heterojunction (BHJ) photovoltaics based on blends of conjugated polymers and fullerenes require an optimized nanoscale morphology. Casting BHJ films using solvent additives such as 1,8-diiodooctane (DIO), 1,8-octanedithiol (ODT), chloronapthalene (CN), or diphenyl ether (DPE) often helps achieve this proper morphology: adding just a few volume percent of additive to the casting solution can improve polymer/fullerene mixing or phase separation, so that solvent additives have become staples in producing high-efficiency BHJ solar cells. The mechanism by which these additives improve BHJ morphology, however, is poorly understood. Here, we investigate how these additives control polymer/fullerene mixing by taking advantage of sequential processing (SqP), in which the polymer is deposited first and then the fullerene is intercalated into the polymer underlayer in a second processing step using a quasi-orthogonal solvent. In this way, SqP isolates the role of the additives’ interactions with the polymer an...

Revisiting the Impact of Interfacial Transport Layers on Organic Bulk Heterojunction Systems

The utilization of interfacial transport layers is critically important for extensively elaborating organic photovoltaic efficiencies. Despite some fundamental knowledge regarding their electrical and optical properties, a full realization of their impact on photoactive layers is still lacking. In this work, we report a new insight into the impact of the interfacial transport layers on physical properties of an organic bulk heterojunction (BHJ) system comprising ITO(glass)/PEDOT:PSS/PIDTDTQx:PC70BM/PFN/Al. A combination of multiple measurements was performed either at steady states or at working conditions aiming to deeply understand the role of the interfacial transport layers in the organic BHJ photovoltaics. Experimental results validated our assumption that the optoelectronic properties of the organic blends, such as PIDTDTQx:PC70BM, can be remarkably influenced due to the presence of the electron/hole transport layers. We have looked into this issue by examining the organic blend PIDTDTQx:PC70BM-based BHJ photovoltaic devices from the interfacial trap-related density of states, electron–hole recombination resistance (Rrecomb), recombination lifetime (τrecomb), excitons lifetime (τ), photoinduced electrical polarization, energetic disorder characteristic, and magneto-photocurrent, respectively. Our concept should guide people to reconsider the critical role of the interfacial transport layer in the organic BHJ system, in particular to compensate with two controversial assumptions: (i) properties of the organic blend are nearly unaltered due to the presence of the interfacial transport layers, and (ii) they mainly facilitate the charge carrier extraction and injection. By highlighting the factors that eliminate the performance of the organic photovoltaics, we thus believe this work offers scientifically valuable information for device physicists because the optoelectronic, dielectric, and electronic structure properties of the organic blend are firmly bound to the presence of the interfacial transport layer.

Bulk heterojunction photovoltaics with improved efficiencies using stem-leaf, shish-kebab and double-fibrillar nano-hybrids based on modified carbon nanotubes and poly(3-hexylthiophene)

For improving the photovoltaic efficiencies, the conductive ordered and disordered nano-hybrids were developed as morphology mediator based on multi-walled carbon nanotubes (CNTs), their chemically surface modified derivatives, regioregular poly(3-hexyl thiophene) (RR-P3HT) and non-regioregular poly(3-dodecyl thiophene) (PDDT). In unmodified CNTs and their derivatives functionalized with 2-hydroxymethyl thiophene (CNT-f-COOTh, FCNT) and grafted with poly(3-dodecylthiophene) (CNT-g-PDDT, GCNT), double-fibrillar, shish-kebab, and stem-leaf nanostructures were decorated. When the pre-developed FCNT/P3HT and GCNT/P3HT nano-hybrids were applied in thin active layers, a progress was detected in photovoltaic characteristics, in particular for P3HT:GCNT/P3HT devices via larger phase separations. The current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) values ranged in 8.14–8.66 mA/cm2, 49–51%, and 2.55–2.78% for P3HT:GCNT/P3HT systems, respectively. The best photovoltaic features were detected for P3HT:GCNT/P3HT:phenyl-C71-butyric acid methyl ester (PC71BM) devices (Jsc = 9.91–10.22 mA/cm2, FF = 57–59%, and PCE = 3.50–3.74%). The peak values around 69–71% and 52–54% in the external quantum efficiency (EQE) curves were detected at about 550 nm for P3HT:GCNT/P3HT:PCBM and P3HT:GCNT/P3HT devices, respectively.

Solution‐Processed In2O3/ZnO Heterojunction Electron Transport Layers for Efficient Organic Bulk Heterojunction and Inorganic Colloidal Quantum‐Dot Solar Cells

We report the development of a solution‐processed In2O3/ZnO heterojunction electron transport layer (ETL) and its application in high efficiency organic bulk‐heterojunction (BHJ) and inorganic colloidal quantum dot (CQD) solar cells. Study of the electrical properties of this low‐dimensional oxide heterostructure via field‐effect measurements reveals that electron transport along the heterointerface is enhanced by more than a tenfold when compared to the individual single‐layer oxides. Use of the heterojunction as the ETL in organic BHJ photovoltaics is found to consistently improve the cell's performance due to the smoothening of the ZnO surface, increased electron mobility and a noticeable reduction in the cathode's work function, leading to a decrease in the cells’ series resistance and a higher fill factor (FF). Specifically, non‐fullerene based organic BHJ solar cells based on In2O3/ZnO ETLs exhibit very high power conversion efficiencies (PCE) of up to 12.8%, and high FFs of over 70%. The bilayer ETL concept is further extended to inorganic lead‐sulphide CQD solar cells. Resulting devices exhibit excellent performance with a maximum PCE of 8.2% and a FF of 56.8%. The present results highlight the potential of multilayer oxides as novel ETL systems and lay the foundation for future developments.

Novel N-heteroacene small molecules as electron donors for organic bulk heterojunction photovoltaics

Abstract Two N-heteroacene small molecules, P2BT and P2B2 T, with 2,1,3-benzodiathiazole end-capped N-heteroacene unit as the central building block have been designed and synthesized for bulk-heterojunction solar cells. Single-junction devices based on P2B2 T:PC71BM have achieved an impressive power conversion efficiency of 7.16% without any special treatment, which is the record efficiency among the N-heteroacene-based photovoltaics.

Computational approach to the study of morphological properties of polymer/fullerene blends in photovoltaics

Abstract Organic solar cells have the ability to transform solar energy efficiently and have a promising energy balance. Producing these cells is economical and makes use of methods of printing using inks built on solvents that are well-matched with a variety of cheap materials like flexible plastic or paper. The primary materials used to manufacture organic solar cells include carbon-based semiconductors, which are good light absorbers and efficient charge generators. In this article, we review previous research of interest based on morphology of polymer blends used in bulk heterojunction (BHJ) solar cells and introduce their basic principles. We further review computational models used in the analysis of surface behavior of polymer blends in BHJ as well as the trends in the field of polymer surface science as applied to BHJ photovoltaics. We also give in brief, the opportunities and challenges in the area of polymer blends on BHJ organic solar cells.

Gram-Scale Syntheses and Conductivities of [10]Cycloparaphenylene and Its Tetraalkoxy Derivatives.

[10]Cycloparaphenylene ([10]CPP) and its tetraalkoxy derivatives were synthesized on the gram scale in 7 steps starting from 1,4-benzoquinone or 2,5-dialkoxy-1,4-benzoquinone. The key steps involve the highly cis-selective bis-addition of 4-bromo-4'-lithiobiphenyl to the quinones to produce a five-ring unit containing cyclohexa-1,4-diene-3,6-diol moiety, the platinum-mediated dimerization of the five-ring unit, and the H2SnCl4-mediated reductive aromatization of cyclohexadienediol. The tetraalkoxy substituents increased the solubility of [10]CPP in common organic solvents. The carrier-transport properties of thin films of [10]CPP and its derivatives were measured for the first time and indicated that [10]CPP derivatives could rival phenyl-C61-butyric acid methyl ester, which is used widely as an n-type active layer in bulk heterojunction photovoltaics.

Hole-Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials.

Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC → donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.

Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics.

Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beam-induced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p-n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n+-n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths.

Source materials grain size effect on electrode microstructure and its effect on conventional bulk hetero-junction photovoltaics

In this work, the effect of silver cathode on the polymer diode and organic photovoltaic device (OPV) performance was investigated. The electron collecting contacts in diode and OPV device have been deposited from both silver pellet and nanoparticles form. Diode was fabricated by sandwiching poly(3-hexylthiophane-2,5-diyl) between ITO and Ag electrodes. It is observed that diode fabricated using Ag nanoparticles showed significant current density improvement. The enhancement in diode performance was studied by C-V measurements. Further, OPV cells fabricated using Ag nanoparticles showed improvement in current density which results in improved power conversion efficiency (PCE). The observed performance enhancements in diode and OPV device have been correlated with electrode microstructure and its interface properties. The performance enhancement in diode and OPV cells is evaluated by various electrical parameters such carrier concentration (N), density of trap states, trap distribution width (ω) and current density. These data indicate that the electrode grain size matching with the semiconductor morphology across metal/semiconductor interface is more critical for better charge transfer. Matching the roughness of the interface across the junction by use of appropriate starting material particle/grain size can help in extracting the maximum performance of an organic device.