Bulk Heterojunction Architecture Research Articles

Organic photovoltaics: The current challenges.

Organic photovoltaics are remarkably close to reaching a landmark power conversion efficiency of 20%. Given the current urgent concerns regarding climate change, research into renewable energy solutions is crucially important. In this perspective article, we highlight several key aspects of organic photovoltaics, ranging from fundamental understanding to implementation, that need to be addressed to ensure the success of this promising technology. We cover the intriguing ability of some acceptors to undergo efficient charge photogeneration in the absence of an energetic driving force and the effects of the resulting state hybridization. We explore one of the primary loss mechanisms of organic photovoltaics-non-radiative voltage losses-and the influence of the energy gap law. Triplet states are becoming increasingly relevant owing to their presence in even the most efficient non-fullerene blends, and we assess their role as both a loss mechanism and a potential strategy to enhance efficiency. Finally, two ways in which the implementation of organic photovoltaics can be simplified are addressed. The standard bulk heterojunction architecture could be superseded by either single material photovoltaics or sequentially deposited heterojunctions, and the attributes of both are considered. While several important challenges still lie ahead for organic photovoltaics, their future is, indeed, bright.

High-speed printing of a bulk-heterojunction architecture in organic solar cells films

A polymer self-doping strategy can improve the high-speed processability of the active layer without compromising performance to reduce minimum sustainable price, which guide the lab-to-fab of the high coating speed printing of organic solar cells.

Theoretical study of carbon nanotubes as candidates for active layer in solar cells

We present results of theoretical calculations of electronic and optical properties for 35 finite Carbon NanoTubes (CNTs). The quantities calculated are: HOMO and LUMO energies, energy gaps, and, most important, visible and infrared spectra. Calculations were performed in DFT and TDDFT formalism. The motivation of this study was examination of possibility of application of CNTs as an active layer in organic solar cells (OSCs). We propose to use CNTs as an active layer for their exceptional property, namely, large exciton diffusion length. This opportunity would allow to construct an OSC in a layer architecture, where thickness of the layer could be sufficient for an absorption of significant part of light and, at the same time, large exciton diffusion length would preserve effectiveness of excitons dissociation into charges. An important aspect is that CNTs should be parallelly ordered in order to maximize the advantage of large exciton length. This way, we hope to radically improve the OSC efficiency and to overcome Bulk HeteroJunction (BHJ) architecture limitations, where – besides last achievements – the efficiency is still substantially lower than theoretical predictions. Our calculations show that overall absorption of CNTs is large in visible and infrared regions. This opportunity imply that necessary condition for construction of layer high-efficient CNT-based OSCs is fulfilled.

Development of regio-regular poly(3-hexylthiophene) by direct arylation using Fagnou's conditions.

Semiconducting polymers are widely studied due to their applications in photovoltaic devices, the predominant conjugated polymer used in organic solar cells (OSCs) is semicrystalline poly(3-hexylthiophene) P3HT, which can be produced by easy, low-cost and environmentally friendly methods such as direct arylation polymerization (DArP) (N.S. Gobalasingham et al. 2018). Due to its solution processing, deposition on different types of substrates, even flexible, with technologies such as spin coating and roll to roll, it is a feasible option for large-scale production. In this work we present the synthesis, physicochemical characterization of a regio- regular P3HT by the direct arylation method using Fagnou conditions and its evaluation on the active layer of OSCs with bulk heterojunction architecture.

Submolecular Resolution Imaging of P3HT:PCBM Nanostructured Films by Atomic Force Microscopy: Implications for Organic Solar Cells.

The efficiency of organic bulk-heterojunction (BHJ) solar cells depends greatly on both the bulk and surface structure of the nanostructured bicontinuous interpenetrating network of materials, known as the active layer. The morphology of the top layer of a coated film is often resolved at the scale of a few nanometers, but fine details of the domains and the order within them are more difficult to identify. Here, we report a high-resolution atomic force microscopy (AFM) investigation of various stoichiometries of the well-studied poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) active layer mixture. Images of the surface were obtained using AC-mode AFM exciting higher-order resonance frequencies of a standard silicon probe, a promising technique for acquiring real-space images of organic-based thin films with nanoscale and even submolecular resolution. We provide firm evidence of the nanoscale organization of the P3HT polymer and of the P3HT:PCBM stoichiometric mixtures at the surface-air interface of the BHJ architecture. Our study shows the characteristic periodicity of the regioregular P3HT identified in the nanoscale domain areas with submolecular resolution. Such areas are then distorted in place when adding different quantities of PCBM forming stoichiometric mixtures. When the samples were exposed to ambient light, the morphologies were very different, and submolecular resolution was not achieved. This approach is shown to provide a precise view of the active layer's nanostructure and will be useful for studies of other materials as a function of various parameters, with particular attention to the role of the acceptor in tuning morphology for understanding optimum performance in organic photovoltaic devices.

Active Layer Thickness Optimization for Maximum Efficiency in Bulk Heterojunction Solar Cell

Organic solar cell (OSC) is one of the best promising candidate for harvesting energy mainly due to their simple and economic fabrication process, the reduced manufacturing cost, and easy integration to other products. Bulk heterojunction solar cell in which the active layer is a blend of donor –acceptor materials are one of the best organic photovoltaic device with highest efficiency and a significant improvement in the device performance occur over last years. Bulk heterojunction architecture gives a high interfacial surface area for efficient charge dissociation. In this study, bulk heterojunction solar cell is simulated using General Purpose Photovoltaic Device Model Software. A donor-acceptor blend of Zinc pthalocyanine (ZnPc) fullerene C60 is used as the active layer. The power conversion efficiency for various thickness of the active layer is studied. Optimization of active layer thickness for maximum power conversion efficiency are done. The dependence of various electrical parameters such as short circuit current density(Jsc), Open circuit voltage(Voc), fill factor (FF), average carrier mobility on power conversion efficiency are also studied.

17.6%-Efficient Quasiplanar Heterojunction Organic Solar Cells from a Chlorinated 3D Network Acceptor.

Bulk heterojunction (BHJ) organic solar cells (OSCs) have achieved great success because they overcome the shortcomings of short exciton diffusion distances. With the progress in material innovation and device technology, the efficiency of BHJ devices is continually being improved. For some special photovoltaic material systems, it is difficult to manipulate the miscibility and morphology of blend films, and this results in moderate, even poor device performance. Quasiplanar heterojunction (Q-PHJ) OSCs have been proposed to exploit the excellent photovoltaic properties of these materials. An OSC with BTIC-BO-4Cl has a 3D interpenetrating network structure with multiple channels that can facilitate the exciton diffusion and charge transport, and BTIC-BO-4Cl is therefore a good candidate for Q-PHJ OSCs. In this work, a D18:BTIC-BO-4Cl-based Q-PHJ device is fabricated. The exciton diffusion lengths of D18 and BTIC-BO-4Clare in accord withthe requirements of the Q-PHJ device and the efficiency of Q-PHJ device is as high as 17.60%. This study indicates that the Q-PHJ architecture can replace the BHJ architecture to produce excellent OSCs for certain unique donors and acceptors, providing an alternative approach to photovoltaic material design and device fabrication.

Predicting the substituent effects in the optical and electrochemical properties of N,N\u2032-substituted isoindigos

Isoindigo, the structural isomer of the well-known dye indigo, has seen a major revival recently because of the increasing interest of its use as a potential drug core structure and for the development of organic photovoltaic materials. Highly beneficial for diverse applications are its facile synthesis, straightforward functionalisation and the broad absorption band in the visible range. Moreover, its intrinsic electron deficiency renders isoindigo a promising acceptor structure in bulk heterojunction architectures. Here we present new insights into the substituent effects of N-functionalised isoindigos, developing a reliable and fast in silico screening approach of a library of compounds. Using experimental UV–Vis and electrochemical data increased the accuracy of the TD-DFT method employed. This procedure allowed us to accurately predict the optical and electrochemical properties of N-functionalised isoindigos and the elucidation of the relationship between substituent effects and electronic properties.Graphic abstract

Pseudo-bilayer architecture enables high-performance organic solar cells with enhanced exciton diffusion length

Solution-processed organic solar cells (OSCs) are a promising candidate for next-generation photovoltaic technologies. However, the short exciton diffusion length of the bulk heterojunction active layer in OSCs strongly hampers the full potential to be realized in these bulk heterojunction OSCs. Herein, we report high-performance OSCs with a pseudo-bilayer architecture, which possesses longer exciton diffusion length benefited from higher film crystallinity. This feature ensures the synergistic advantages of efficient exciton dissociation and charge transport in OSCs with pseudo-bilayer architecture, enabling a higher power conversion efficiency (17.42%) to be achieved compared to those with bulk heterojunction architecture (16.44%) due to higher short-circuit current density and fill factor. A certified efficiency of 16.31% is also achieved for the ternary OSC with a pseudo-bilayer active layer. Our results demonstrate the excellent potential for pseudo-bilayer architecture to be used for future OSC applications.

Effects on Photovoltaic Characteristics by Organic Bilayer- and Bulk-Heterojunctions: Energy Losses, Carrier Recombination and Generation

We investigate the photovoltaic characteristics of organic solar cells (OSCs) for two distinctly different nanostructures, by comparing the charge carrier dynamics for bilayer- and bulk-heterojunction OSCs. Most interestingly, both architectures exhibit fairly similar power conversion efficiencies (PCEs), reflecting a comparable critical domain size for charge generation and charge recombination. Although this is, at first hand, surprising, a detailed analysis points out the similarity between these two concepts. A bulk-heterojunction architecture arranges the charge generating domains in a 3D ensemble across the whole bulk, while bilayer architectures arrange the specific domains on top of each other, rather than sharp bilayers. Specifically, for the polymer PBDB-T-2F, we find that the enhanced charge generation in a bulk composite is partially compensated by reduced recombination in the bilayer architecture, when nonfullerene acceptors (NFAs) are used instead of a fullerene acceptor. Overall, we demonstrate that bilayer-heterojunction OSCs with NFAs can reach competitive PCEs compared to the corresponding bulk-heterojunction OSCs because of reduced nonradiative open-circuit voltage losses, and suppressed trap-assisted recombination, as a result of a vertically separated donor-to-acceptor nanostructure. In contrast, the bilayer-heterojunction OSCs with the fullerene acceptor exhibited poor photovoltaic characteristics compared to the corresponding bulk devices because of highly aggregated acceptor molecules on top of the polymer donor. Although free carrier generation is reduced in a in a bilayer-heterojunction, because of reduced donor/acceptor interfaces and a limited exciton diffusion length, more favorable transport pathways for unipolar charge collection can partially compensate the aforementioned disadvantages. We propose that the unique properties of NFAs may open a technical venue for the bilayer-heterojunction as a great and easy alternative to the bulk heterojunction.

Strategic Halogen Substitution to Enable High‐Performance Small‐Molecule‐Based Tandem Solar Cell with over 15% Efficiency

AbstractSmall molecules have been recently highlighted as active materials owing to their facile synthesisis method, well‐defined molecular structure, and highly reproducible performance. In particular, optimizing bulk heterojunction (BHJ) morphologies is important to achieving high performance in solution‐processable small molecule solar cells (SM‐SCs). Herein, a series of benzodithiophene‐based active materials with different halogen atoms substituted at the end‐group, are reported, as well as how these halogen atoms affect the morphology of BHJ architectures through microstructure analyses. Materials with chlorine atoms show a well‐mixed morphology and interpenetrating networks when blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester, facilitating effective charge transportation. This controlled morphology helps attain excellent performance with a power conversion efficiency (PCE) of 10.5% and a highest fill factor of 78.0% without additives. In addition, it can be applied to two‐terminal (2T)‐tandem solar cells, attaining an outstanding PCE of up to 15.1% with complementary absorption in the field of the 2T‐tandem solar cells introducing the SM‐SCs. These results suggest that tailoring interactions with halogen atoms is an effective way to control BHJ architectures, thereby achieving remarkable performance in SM‐SCs.

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

Millimeter-Scale Unipolar Transport in High Sensitivity Organic-Inorganic Semiconductor X-ray Detectors.

Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 μm due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1-1 mm thickness regime is demonstrated, having a high sensitivity of ∼160 μC mGy-1 cm-3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.

Fluorous Fullerene Acceptors in Vacuum‐Deposited Photovoltaic Cells

The first successful example of vacuum‐deposited organic photovoltaic (OPV) cells based on a fullerene derivative and a small‐molecule donor is reported. A series of thermally robust fluorous fullerene acceptors with experimental gas‐phase electron affinities ranging from 2.8 to 3.3 eV are paired with a new dicyanovinyl thiophene‐based molecular donor to enable direct comparison of their performance in planar and bulk heterojunction architectures in the cells fabricated by vacuum deposition. Unprecedented insights into the role of the acceptor intrinsic molecular and electronic structures are obtained, which are not obscured by solvent and additive effects as in the typical solution‐processed fullerene‐based OPV cells.

Ambipolar thin-film transistors based on organic semiconductor blend

Abstract Ambipolar thin-film transistors are fabricated employing p-channel (2-decyl-7-phenylbenzo[b]benzo [4,5]thieno[2,3-d]thiophene; Ph-BTBT-C10) and n-channel (N,N'-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide; PTCDI-C8) semiconductors in bilayer and bulk heterojunction architecture. Bilayer organic semiconductors with small thickness of the first layer exhibit ambipolar device characteristics with relatively poor electrical performance. On the other hand, bulk heterojunction organic thin-film transistors (OTFTs) with optimized blend ratio exhibited higher ambipolar charge transport properties with carrier mobility as high as 0.22 and 0.038 cm2 V−1s−1 for hole and electron, respectively. Complementary-like inverters fabricated based on optimized bulk heterojunction OTFTs show high transfer gain of 96.

Organic photovoltaic cell analysis through quantum efficiency and scanning tunneling microscopy of the donor/blend as an active film

In this work is reported a comparison of the film morphology, film molecular ordering and X-ray diffraction pattern between two of the most common and efficient donor polymers used in organic photovoltaic (OPV) cells: PTB7 and PTB7-Th. These comparisons indicate that PTB7-Th film chains are somewhat thicker and less spaced than those for PTB7; also, PTB7-Th films have a slightly better organized structure and higher co-planarity, which could provide a possible better electrical charge transport. On the order hand, an analysis of the external/internal quantum efficiency (EQE/IQE) of OPVs, based on PTB7-Th, as a function of the active layer thickness ranging from 40 to 165 nm was carried out. It was used the bulk heterojunction architecture to fabricate OPVs cells under the configuration glass/ITO/PEDOT:PSS/PTB7-Th:PC71BM/PFN/FM (Field’s Metal: eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn by weight that melts at 62 °C). IQE spectra were determined by using the active layer absorption calculated through the transfer matrix method (TMM). Our results show a significant reduction of IQE when increasing the active layer thickness above 120 nm. IQE decreases, and consequently EQE and PCE, mainly due to the reduction in charge carriers collection probability. On the reversed side, when the active layer is very thin (< 70 nm), there exists also a decrease in the IQE values. A comparison between the experimental measurements and theoretical simulations (by TMM) is discussed in order to have better understanding of the OPVs performance.

A Spectral Density Function Approach for Active Layer Design of Organic Photovoltaic Cells

Organic photovoltaic cells (OPVCs), having received significant attention over the last decade, are yet to be established as viable alternatives to conventional solar cells due to their low power conversion efficiency (PCE). Complex interactions of several phenomena coupled with the lack of understanding regarding the influence of fabrication conditions and nanostructure morphology have been major barriers to realizing higher PCE. To this end, we propose a computational microstructure design framework for designing the active layer of P3HT:PCBM based OPVCs conforming to the bulk heterojunction (BHJ) architecture. The framework pivots around the spectral density function (SDF), a frequency space microstructure characterization, and reconstruction methodology, for microstructure design representation. We validate the applicability of SDF for representing the active layer morphology in OPVCs using images of the nanostructure obtained by cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). SDF enables a low-dimensional microstructural representation that is crucial in formulating a parametric-based microstructure optimization scheme. A level-cut Gaussian random field (GRF, governed by SDF) technique is used to generate reconstructions that serve as representative volume elements (RVEs) for structure–performance simulations. A novel structure–performance (SP) simulation approach is developed using a physics-based performance metric, incident photon to converted electron (IPCE) ratio, to account for the impact of microstructural features on OPVC performance. Finally, a SDF-based computational IPCE optimization study incorporating only three design variables results in 36.75% increase in IPCE, underlining the efficacy of the proposed design framework.

Bulk Heterojunction Solar Cells: The Role of Alkyl Side Chain on Nanoscale Morphology of Sulfur Over-rich Regioregular Polythiophene/Fullerene Blends

Regioregular HH-TT poly(3,3′-thioalkylbithiophene)s-bearing branched or linear alkyl side-chain substituents (PT2SR) have been synthesized and characterized to investigate their behavior, when used as electron-donor components in blend with a fullerene derivative [[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)] as an electron acceptor, in air-processed photovoltaic solar cells with bulk heterojunction architecture. The optoelectronic characteristics, energy gap, nanoscale morphology, and crystallinity of the blends (PT2SR/PCBM) were examined by ultraviolet–visible spectroscopy, cyclic voltammetry, Kelvin probe force microscopy (KPFM), and X-ray diffraction (XRD). We demonstrate that thioalkyl substituents are able to influence the PCBM self-assembly and the morphology of the polymeric film, important parameters to maximize the efficiency of the solar cell. In particular, the presence of chemical branching in the side chain of the sulfur over-rich polythiophene backbone favors the formation of PCBM clus...

Self-Assembled Organic Materials for Photovoltaic Application.

Organic photovoltaic cells based on bulk-heterojunction architecture have been a topic of intense research for the past two decades. Recent reports on power conversion efficiency surpassing 10% suggest these devices are a viable low-cost choice for a range of applications where conventional silicon solar cells are not suitable. Further improvements in efficiency could be achieved with the enhanced interaction between the donor and acceptor components. Effective utilization of supramolecular interactions to tailor and manipulate the communication between the components in the blend is a good strategy towards this end. Literature reports suggest that the long-term stability of organic solar cells, a major hurdle for commercial applications, can also be partially addressed by generating stable supramolecular nanostructures. In this review, we have made an attempt to summarize advances in small molecule, oligomer and polymer based systems, wherein supramolecular interactions such as hydrogen-bonding, pi-pi stacking, and dipole-dipole are explored for realizing stable and efficient bulk-heterojunction solar cells.

Temperature-dependent Schottky barrier in high-performance organic solar cells

Organic solar cells (OSCs) have attracted great attention in the past 30 years, and the power conversion efficiency (PCE) now reaches around 10%, largely owning to the rapid material developments. Meanwhile with the progress in the device performance, more and more interests are turning to understanding the fundamental physics inside the OSCs. In the conventional bulk-heterojunction architecture, only recently it is realized that the blend/cathode Schottky junction serves as the fundamental diode for the photovoltaic function. However, few researches have focused on such junctions, and their physical properties are far from being well-understood. In this paper based on PThBDTP:PC71BM blend, we fabricated OSCs with PCE exceeding 10%, and investigated temperature-dependent behaviors of the junction diodes by various characterization including current-voltage, capacitance-voltage and impedance measurements between 70 to 290 K. We found the Schottky barrier height exhibits large inhomogeneity, which can be described by two sets of Gaussian distributions.