Heterojunction Organic Photovoltaic Devices Research Articles

Effect of fluorine on the photovoltaic properties of 2,1,3-benzothiadiazole-based alternating conjugated polymers by changing the position and number of fluorine atoms.

Fluorination is one of the most effective ways to manipulate molecular packing, optical bandgap and molecular energy levels in organic semiconductor materials. In this work, different number of fluorine atoms was introduced into the acceptor moiety 2,2'-dithiophene linked 2,1,3-benzothiadiazole, utilizing the alkylthiophene modified dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b] (DTBDT) as the donor unit, three polymers: PDTBDT-0F-BTs, PDTBDT-2F-BTs and PDTBDT-6F-FBTs were synthesized. With the number of fluorine atoms in each repeat unit of polymers varying from 0 to 2 and then up to 6, PDTBDT-0F-BTs, PDTBDT-2F-BTs and PDTBDT-6F-FBTs exhibited gradually downshifted energy levels and improved dielectric constants (εr) from 3.4 to 4.3 to 5.8, further successively increased charge transport mobilities. As a result, the power conversion efficiency (PCE) of the bulk heterojunction organic photovoltaic devices (BHJ-OPV) from the blend films of aforementioned polymers paired with PC71BM were gradually increased from 1.69 for PDTBDT-0F-BTs to 1.89 for PDTBDT-2F-BTs and then to 5.28 for PDTBDT-6F-FBTs. The results show that the continuous insertion of fluorine atoms into the repeating units of the benzothiadiazole conjugated polymer leads to the deepening of HOMO energy level, the increase of εr and the increase of charge mobility, which improve the efficiency of charge transfer and electron collection, thus improving the photovoltaic performance of BHJ-OPV.

Flexible organic solar cell to power modern cardiac pacemakers: Versatile for all age groups, skin types and genders

Bio-medical electronic components execute an vital part in medical services. Powering these devices is a task. Thus, the biomedical electronic devices which are able to self-harvest and store power are in huge demand. Present pacemakers are powered by batteries which have limited volume for energy packing and are compulsory to be changed. This needs a surgical intervention and is costly, with attachment of complications and risk. The objective of this paper is to validate if a subdermal PPV-PCBM [poly (2-methoxy-5-{3′,7′-dimethyloctyloxy}-p-phenylene vinylene) and {6,6}-phenyl C61—butyric acid methyl ester] active layer bulk heterojunction (BHJ) organic photo-voltaic (OPV) device could power a cardiac pacemaker. Power yield of 0.05 milliWatts (mW), 0.45 milliWatts & 2.1 milliWatts for African, Asian & Caucasian skin tones are gained at 2-millimeter implementation depth, acceptable to operate cardiac pacemaker demanding approximate power of 10 microWatts. Additionally, results correspondingly display higher output power is generated if the skin is thinner and brighter.

Carbazole and Diketopyrrolopyrrole-Based D-A π-Conjugated Oligomers Accessed via Direct C-H Arylation for Opto-Electronic Property and Performance Study.

Five carbazole and diketopyrrolopyrrole-based donor-acceptor (D-A) new π-conjugated oligomers (π-COs) with gradually elongated lengths are facilely synthesized via a single pot of direct C-H arylation with merits of atom- and step-economy. The structure-property-performance correlations of these π-COs and their parent polymer are studied in detail by opto-electronic characterizations and bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. It is found that the π-COs having longer lengths enable better performance in OPVs owing to the enhanced intermolecular interaction with the elongation of the conjugations. The above results not only highlight the powerful synthetic strategy here provided, but also reveal that π-COs with unique properties might find promising application in OPVs.

Metal-Organic Framework Nanosheets as Templates to Enhance Performance in Semi-Crystalline Organic Photovoltaic Cells.

Optimizing the orientation, crystallinity, and domain size of components within organic photovoltaic (OPV) devices is key to maximizing their performance. Here a broadly applicable approach for enhancing the morphology of bulk heterojunction OPV devices using metal–organic nanosheets (MONs) as additives is demonstrated. It is shown that addition of porphyrin‐based MONs to devices with fully amorphous donor polymers lead to small improvements in performance attributed to increased light absorption due to nanosheets. However, devices based on semi‐crystalline polymers show remarkable improvements in power conversion efficiency (PCE), more than doubling in some cases compared to reference devices without nanosheets. In particular, this approach led to the development of PffBT4T2OD‐MON‐PCBM device with a PCE of 12.3%, which to the authors’ knowledge is the highest performing fullerene based OPV device reported in literature to date. Detailed analysis of these devices shows that the presence of the nanosheets results in a higher fraction of face‐on oriented polymer crystals in the films. These results therefore demonstrate the potential of this highly tunable class of two‐dimensional nanomaterials as additives for enhancing the morphology, and therefore performance, of semi‐crystalline organic electronic devices.

Increased Efficiency of Organic Solar Cells by Seeded Control of the Molecular Morphology in the Active Layer

The performance of non‐fullerene, polymer bulk heterojunction (BHJ) organic photovoltaic devices has a significant correlation with the molecular morphology of the donor and acceptor. The authors show that small organic molecules coordinated to a metal oxide, an electron transport seed layer (ETSL), can profoundly modify the donor:acceptor molecular morphology of inverted organic photovoltaic (OPV) devices. Using grazing incidence wide angle X‐ray scattering (GIWAXS), the authors show that a PTB7‐Th:IEICO‐4F BHJ active layer has a higher degree of face‐on molecular alignment on ETSL‐1 (biphenyl‐4,4′‐dicarboxylic acid, coordinated to ZnO), whilst for naphthalene‐2,6‐dicarboxylic acid coordinated to ZnO (ETSL‐2), it is reduced. Devices of PTB7‐Th:IEICO‐4F BHJ prepared on ETSL‐1 had a 19.91% increase in the average power conversion efficiency (PCE), a 1.56% increase in the fill factor (FF), and a 16.66 ± 0.2% enhancement in the short circuit current density. The observed improvements are believed to be due to significant modifications to the oxide‐BHJ interfacial region of ETSL‐1, namely the elimination of nano‐ridges and defect centers, along with an enhanced wettability. These factors can be correlated with the enhanced device performances, leading to the conclusion that the modulation of the molecular morphology of donor:acceptor blends by ETSL‐1 has a broad impact on improving OPV cell efficiencies.

Dipolar self-assembled monolayers grafted on ZnO for the tuning of electronic properties of the poly (3-hexylthiophène)- [6,6]-phenyl C61-butyric acid methylester blend

A promising solution to improve the performance of Bulk heterojunction organic photovoltaic devices is the insertion of functionalized self-assembled monolayers (SAMs) at the interface between the Electron Transporting Layer (ETL) and the active layer. By grafting such dipolar SAMs based on Y-PP-COOH (P = Phenyl, YY= CH3O, F)) on a ZnO ETL, both the electronic and the chemical properties of the interface are modified. The growth of the active blend the poly(3-hexylthiophène)- [6,6]-phenyl C61-butyric acid methylester blend deposited onto the SAMs is also altered. The dipole with a component oriented perpendicularly to the ZnO surface modifies the ZnO work function whereas the surface energy of the SAM influences the microstructuration of the active material. When we analyze the inverted photovoltaic cell, we find that the two effects may be distinguished: the CH3O moiety favorably affects the electrical transfer of carriers with mainly an improvement of the open-circuit voltage Voc whereas fluorine disfavors both electron extraction and blend microstructure quality, with a lower Voc and electrical current density Jsc.

Synthesis and Characterization of a 2,3-Dialkoxynaphthalene-Based Conjugated Copolymer via Direct Arylation Polymerization (DAP) for Organic Electronics.

Poly[(5,5’-(2,3-bis(2-ethylhexyloxy)naphthalene-1,4-diyl)bis(thiophene-2,2′-diyl))-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PEHONDTBT) was synthesized for the first time and through direct arylation polymerization (DAP) for use as p-donor material in organic solar cells. Optimized reaction protocol leads to a donor-acceptor conjugated polymer in good yield, with less structural defects than its analog obtained from Suzuki polycondensation, and with similar or even higher molecular weight than other previously reported polymers based on the 2,3-dialkoxynaphthalene monomer. The batch-to-batch repeatability of the optimized DAP conditions for the synthesis of PEHONDTBT was proved, showing the robustness of the synthetic strategy. The structure of PEHONDTBT was corroborated by NMR, exhibiting good solubility in common organic solvents, good film-forming ability, and thermal stability. PEHONDTBT film presented an absorption band centered at 498 nm, a band gap of 2.15 eV, and HOMO and LUMO energy levels of −5.31 eV and −3.17 eV, respectively. Theoretical calculations were performed to understand the regioselectivity in the synthesis of PEHONDTBT and to rationalize its optoelectronic properties. Bilayer heterojunction organic photovoltaic devices with PEHONDTBT as the donor layer were fabricated to test their photovoltaic performance, affording low power-conversion efficiency in the preliminary studies.

Measurement and Theoretical Interpretation of Exciton Diffusion as a Function of Intermolecular Separation for Squaraines Targeted for Bulk Heterojunction Solar Cells

The efficiency of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices depends significantly upon absorption of photons and the migration of the photogenerated excited state to the heteroju...

Tailoring Morphology Compatibility and Device Stability by Adding PBDTTPD-COOH as Third Component to Fullerene-Based Polymer Solar Cells

The crystallinity and morphology of polymer and fullerene have a profound influence on the performance of bulk heterojunction (BHJ) organic photovoltaic devices. The poor compatibility of donor and...

Dual Role of Graphene Quantum Dots in Active Layer of Inverted Bulk Heterojunction Organic Photovoltaic Devices.

Graphene quantum dots (GQDs) have shown broad application prospects in the field of photovoltaic devices due to their unique quantum confinement and edge effects. Here, we prepared GQDs by a photon-Fenton reaction as reported in our previous work, which has great advantage in the preparation scale. The photoelectric properties of the inverted hybrid solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methylester (PCBM):GQDs and P3HT:GQDs with different contents of GQDs as the active layers are demonstrated, as well as their morphology and structure by atomic force microscopy images. Then, the different roles of GQDs played in the ternary (P3HT:PCBM:GQDs) and binary (P3HT:GQDs) hybrid solar cells are studied systematically. The results indicate that the GQDs provide an efficient excition separation interface and charge transport channel for the improvement of hybrid solar cells. The preliminary exploration and elaboration of the role of GQDs in hybrid solar cells will be beneficial to understand the interfacial procedure and improve device performance in the future.

Homocoupling Defects of a Small Donor Molecule for Organic Photovoltaics: Quantification of the Eutectic State Diagram by Rapid Heat–Cool Differential Scanning Calorimetry

The conjugated small donor molecule p-DTS(FBTTh2)2, leading to efficient bulk heterojunction organic photovoltaic devices with PC71BM, shows a eutectic phase behavior in combination with its homoco...

Surface analysis and surface doping of graphene on indium-tin-oxide

Surface doped indium tin oxide (ITO)/graphene systems as a hole transporting transparent front electrode in electro-optical applications are potential alternatives to conventional ITO with wet chemically prepared polymers.In this paper we investigate, firstly, chemical impurities of chemical vapor deposition prepared graphene transferred onto glass/ITO substrates under ambient conditions using x-ray photoemission spectroscopy (XPS) at the oxygen and carbon peak positions. For each of the two elements the intensity contribution of the graphene layer could be extracted from a mathematical simulation of the respective XPS spectra. Impurities from the supporting poly(methyl methacrylate) layer for the transfer were identified. Secondly, we study effects of graphene surface-doping on the work function of such transparent front electrodes using ultraviolet photoemission spectroscopy data. As doping material we have deposited ultra-thin layers of low-cost electron acceptors, iron chloride, vanadium oxide, molybdenum oxide (MoOx) or tetracyano-2,3,5,6-tetrafluoroquinodimethane in order to increase its initial low surface work function of Φ = 4.3 eV up to Φ = 6.3 eV.Finally, we fabricated under vacuum conditions a bulk heterojunction organic photovoltaic device based on a Zn-phtalocyanine:C60 absorber layer thermally deposited on an ITO/graphene front electrode, which has been electronically adjusted to the Zn-phthalocyanine donor (Φ = 5.2 eV) by applying the matching dopant MoOx. The current-voltage characteristics of the device shows improved open circuit voltage and fill factor, hence improved power conversion efficiency compared to that with pristine ITO/graphene.

(Invited) Isomer Effects of Nanocarbons in Organic Solar Cells

Fullerene derivatives are excellent electron-accepting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. The spherical cage structure of fullerenes endows the small reorganization energy in electron transfer, thereby yielding superior electron-accepting and electron-transporting properties. A typical fullerene derivative is [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM). Its [70]fullerene analogue ([70]PCBM) has been frequently employed in high-performance OPVs over [60]PCBM because of better light-harvesting ability in the visible region and solubility in common organic solvents arising from the rugby ball-shape with lower symmetry. High power conversion efficiencies (PCEs) of 8–11% have been achieved using [70]PCBM in combination with various low-bandgap conjugated polymers. However, the lower symmetric C70 cage possesses four distinct types of double bonds (α, β, γ, and δ), and mono-addition to C70 generally leads to a mixture of regioisomer products. In fact, [70]PCBM prepared by a conventional method using 4-benzoylbutyrate tosylhydrazone and C70 contains α-type (80-90%) and β-type (10-20%) isomers. Despite its prevalence use in OPV devices, [70]PCBM is usually used as the isomer mixture, which veils the device performances depending on the individual isomers of [70]PCBM. In this talk, I will give an overview of the isomer effects of nanocarbons on BHJ OPV devices. In particular, I will focus on the comprehensive regioisomer and diastereomer separation effect of the most prevalent electron-acceptor ([70]PCBM) on the structures and photovoltaic properties of the blend films with a high-performance conjugated polymer (i.e., PffBT4T-2OD). In spite of the strong tendency of PffBT4T-2OD to form a bicontinuous structure with fullerene derivatives, one of the diastereomers of the β-isomer (β1-[70]PCBM) showed an extraordinary cohesion nature in the blend film, deteriorating the OPV device performance. OPV devices based on the other isomers (i.e., α-[70]PCBM (8.4%) and β2-[70]PCBM (8.4%)) rival or slightly surpass the device with the as-synthesized isomer mixture (mix-[70]PCBM, 8.2%) because of the existence of the poor performance β1-[70]PCBM. The results demonstrate the large impact of the substituent position and direction on a C70 surface on the photovoltaic properties. [1] S. Kitaura, K. Kurotobi, M. Sato, Y. Takano, T. Umeyama, and H. Imahori, Chem. Commun., 48, 8550-8552 (2012). [2] R. Tao, T. Umeyama,K. Kurotobi, and H. Imahori, ACS Appl. Mater. Interface, 6, 17313-17322 (2014). [3] R. Tao, T. Umeyama, T. Higashino, T. Koganezawa, and H. Imahori, Chem. Commun., 51, 8199-8388 (2015). [4] R. Tao, T. Umeyama, T. Higashino, T. Koganezawa and H. Imahori, ACS Appl. Mater. Interfaces, 7, 16676-16685 (2015). [5] T. Umeyama, T. Miyata, A. C. Jakowetz, S. Shibata, K. Kurotobi, T. Higashino, T. Koganezawa, M. Tsujimoto, , S. Gélinas, W. Matsuda, S. Seki, R. H. Friend, and H. Imahori, Chem. Sci., 8, 181-188 (2017). [6] T. Umeyama, S. Shibata, T. Miyata, K. Igarashi, T. Koganezawa, and H. Imahori, RSC Advance, 7, 45697-45704 (2017). [7] T. Umeyama, K. Igarashi, D. Sakamaki, S. Seki, and H. Imahori, Chem. Commun., 54, 405-408 (2018).

Silicon Phthalocyanines as Acceptor Candidates in Mixed Solution/Evaporation Processed Planar Heterojunction Organic Photovoltaic Devices

Silicon phthalocyanines (SiPc) are showing promise as both ternary additives and non-fullerene acceptors in organic photovoltaics (OPVs) as a result of their ease of synthesis, chemical stability and strong absorption. In this study, bis(3,4,5-trifluorophenoxy) silicon phthalocyanine ((345F)2-SiPc)) and bis(2,4,6-trifluorophenoxy) silicon phthalocyanine ((246F)2-SiPc)) are employed as acceptors in mixed solution/evaporation planar heterojunction (PHJ) devices. The donor layer, either poly(3-hexylthiophene) (P3HT) or poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), was spin coated followed by the evaporation of the SiPc acceptor thin film. Several different donor/acceptor combinations were investigated in addition to investigations to determine the effect of film thickness on device performance. Finally, the effects of annealing, prior to SiPc deposition, after SiPc deposition, and during SiPc deposition were also investigated. The devices which performed the best were obtained using PCDTBT as the donor, with a 90 nm film of (345F)2-SiPc as the acceptor, followed by thermal annealing at 150 °C for 30 min of the entire mixed solution/evaporation device. An open-circuit voltage (Voc) of 0.88 V and a fill factor (FF) of 0.52 were achieved leading to devices that outperformed corresponding fullerene-based PHJ devices.

Enhanced exciton harvesting in a planar heterojunction organic photovoltaic device by solvent vapor annealing

The singlet exciton diffusion length was measured in a small molecule electron donor material DR3TBDTT using fluorescence quenching at a planar interface with a cross-linked fullerene derivative. The one-dimensional exciton diffusion length was increased from ~16 to ~24 nm by annealing the film in carbon disulfide solvent vapor. Planar heterojunction solar cells were fabricated using bilayers of these materials and it was found that solvent vapor annealing increased the short circuit current density by 46%. This can be explained by improved exciton harvesting in the annealed bilayer.

High-Performance Near-Infrared Absorbing n-Type Porphyrin Acceptor for Organic Solar Cells.

While the outstanding charge transport and sunlight-harvesting properties of porphyrin molecules are highly attractive as active materials for organic photovoltaic (OPV) devices, the development of n-type porphyrin-based electron acceptors has been challenging. In this work, we developed a high-performance porphyrin-based electron acceptor for OPVs by substitution of four naphthalene diimide (NDI) units at the perimeter of a Zn-porphyrin (PZn) core using ethyne linkage. Effective π-conjugation between four NDI wings and the PZn core significantly broadened Q-band absorption to the near infrared region, thereby achieving the narrow band gap of 1.33 eV. Employing a windmill-structured tetra-NDI substituted PZn-based acceptor ( PZn-TNDI) and mid-band gap polymer donor (PTB7-Th), the bulk heterojunction OPV devices achieved a power conversion efficiency (PCE) of 8.15% with an energy loss of 0.61 eV. The PCE of our PZn-TNDI-based device was the highest among the reported OPVs using porphyrin-based acceptors. Notably, the amorphous characteristic of PZn-TNDI enabled optimization of the device performance without using any additive, which should make industrial fabrication simpler and cheaper.

Network-Stabilized Bulk Heterojunction Organic Photovoltaics

Bulk heterojunction organic photovoltaic (OPV) devices are multilayer organic devices that can be fabricated using low-cost and scalable solution processing methods, but current devices exhibit poor mechanical stability and degrade under deformation due to cracking and delamination. Recent approaches to improve mechanical durability involve modifying the side-chain or main-chain structures of conjugated polymers in the active layer, but in general it is difficult to simultaneously optimize electronic properties, morphology, and mechanical stability. Here, we present a general approach to improve the mechanical stability of bulk heterojunction active layers through incorporation of an internal elastic network. Network-stabilized bulk heterojunction OPVs are prepared using reactive small molecular additives that are rapidly cross-linked through thiol–ene coupling after processing the active layer. Thiol–ene reactions catalyzed by a base or initiated through short exposure to UV light produce insoluble, elas...

Improved Performance of Thick Films Based Binary and Ternary Bulk Heterojunction Organic Photovoltaic Devices Incorporated with Electrospinning Processed Nanofibers

AbstractNanofibers (average diameter of ≈150–160 nm) consisting of both binary and ternary bulk heterojunction organic photovoltaic (BHJ‐OPV) donor‐acceptor pairs poly(3‐hexylthiophene):phenyl‐C61‐butyric acid methyl ester (P3HT:PC61BM) and (P3HT/PTB7:PC61BM) are produced in a coaxial electrospinning process. Thick (≈260 nm) BHJ‐OPV devices are fabricated by incorporating the nanofibers. Compared to thin (140 nm) P3HT:PC61BM binary devices, thick devices with P3HT:PC61BM nanofibers achieve an improved performance, i.e., an improvement from 65.15 to 73.24% in filling factor (FF), from 3.32 to 4.71% in power conversion efficiency (PCE), and from 8.51 to 11.09 mA cm−2 in short‐circuit current density (Jsc); while compared to thin P3HT/PTB7:PC61BM ternary devices, thick devices with P3HT/PTB7:PC61BM nanofibers show an improvement from 5.1 to 5.68% in PCE, from 12.51 to 14.74 mA cm−2 in Jsc, and a decrease from 66.83 to 63.55% in FF. The improved performance is attributed to an enhanced optical absorption induced by prolonged light absorption path due to scattering from nanofibers, a favorable nanomorphology with optimum crystallinity of P3HT/PTB7 and their good mixing with PC61BM, as well as an increased in‐plane alignment of P3HT and PTB7 polymer chains for efficient exciton dissociation and carrier extraction, and improved charge transport in the vertical direction.

The Fabrication of Bulk Heterojunction P3HT: PCBM Organic Photovoltaics

Bulk heterojunction Organic photovoltaic (OPV) devices are gaining a lot of interest due to their potential for ease of processing and lower manufacturing cost sustainable energy generation. In consequence, the number of studies into the properties and characteristics of organic solar cell devices has been increased to improving their power conversion. A further advancement over past decade has shown that improved efficiency could be obtained by mixed of poly(3 - hexylthiophene) (P3HT) and [1] – phenyl - C61-butyric acid methyl ester (PCBM) as an active layer. A series of optimizations of this P3HT: PCBM blends, such as the mixture ratio variation, the annealing treatments, and solvent treatment, have been emerged to improve the efficiency of the OPV. As a result, significant improvements were achieved. Here, we report the fabrication heterojunction devices of 2.9 % efficiency. This result has been achieved using the configuration of a typical heterojunction solar cell modules consists of layered glass/ITO/PEDOT: PSS/active layer/cathode interlayer

Synergistic Effect of Fluorine Substitution and Thio-Alkylation on Photovoltaic Performances of Alternating Conjugated Polymers Based on Alkylthio-Substituted Benzothiadiazole-Quaterthiophene

Three low bandgap polymers (PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT) based on fluorine and alkylthio-substituted benzothiadiazole-quaterthiophene alternating units are synthesized and applied in polymer solar cells. The effects of fluorination and thio-alkylation on photophysical, electrochemical, and photovoltaic properties of the polymers are comparatively investigated. The results indicate that PSDTfBT-DFDT and PSDTffBT-DFDT show better light-harvesting abilities, lower HOMO energy levels, and better morphology than that of PSDTBT-DFDT. Bulk heterojunction (BHJ) organic photovoltaic devices based on the three polymers as donor materials were fabricated. The power conversion efficiencies (PCE) of 6.94% for PSDTfBT-DFDT and 7.50% for PSDTffBT-DFDT were attained, respectively, both of which are higher than that of PSDTBT-DFDT (4.48%) due to the increase in the numbers of fluorine atoms. This work demonstrates that both fluorine substitution and thio-alkylation are efficient to improve photovoltaic pe...