Fullerene-free Organic Photovoltaics Research Articles

Initial Engineering and Outdoor Stability Assessment of "Gray/Black" Fullerene-Free Organic Photovoltaics Based on Only Two Complementary Absorbing Materials: A Tetrabenzotriazacorrole and a Subphthalocyanine.

Broad absorption is a desired characteristic of materials employed in the photoactive layers of organic photovoltaic (OPV) devices. Here, we have identified tetrabenzotriazacorroles (Tbcs) as complementary absorbing chromophores and electron donors to the promising nonfullerene acceptors boron subphthalocyanines (BsubPcs). These two materials, which can be utilized as donor–acceptor pairs within fullerene-free OPVs, yield spectral coverage over the entire visible range of 300–750 nm. Oxy phosphorus Tbc derivative (POTbc) was employed as an electron donor and paired initially with multiple BsubPc derivatives having a distribution of highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels in planar heterojunction OPVs. These devices were “gray/black” due to the broad absorption across the visible spectrum. Upon screening, the partially halogenated chloro hexachloro BsubPc (Cl–Cl6BsubPc) showed the greatest promise for coupling with POTbc. The thickness ratio and total thickness of the active layer were then probed in order to identify the optical and electrical limitations on the POTbc/Cl–Cl6BsubPc-based OPV device. A maximum power conversion efficiency (PCE) of 2.13% was achieved at 60 nm total thickness of the active layer and 1 to 3 (POTbc to Cl–Cl6BsubPc) thickness ratio. Outdoor stability of the champion device was evaluated using protocols established by International Summits on OPV Stability and was found to be on par with an α-sexithiophene/Cl–Cl6BsubPc baseline OPV.

Potassium-Presenting Zinc Oxide Surfaces Induce Vertical Phase Separation in Fullerene-Free Organic Photovoltaics.

Bulk heterojunction (BHJ) structure based organic photovoltaics (OPVs) have recently showed great potential for achieving high power conversion efficiencies (PCEs). An ideal BHJ structure would feature large donor/acceptor interfacial areas for efficient exciton dissociation and gradient distributions with high donor and acceptor concentrations near the anode and cathode, respectively, for efficient charge extraction. However, the random mixing of donors and acceptors in the BHJ often suffers the severe charge recombination in the interface, resulting in poor charge extraction. Herein, we propose a new approach-treating the surface of the zinc oxide (ZnO) as an electron transport layer with potassium hydroxide-to induce vertical phase separation of an active layer incorporating the nonfullerene acceptor IT-4F. Density functional theory calculations suggested that the binding energy difference between IT-4F and the PBDB-T-2Cl, to the potassium (K)-presenting ZnO interface, is twice as strong as that for IT-4F and PBDB-T-2Cl to the untreated ZnO surface, such that it would induce more IT-4F moving toward the K-presenting ZnO interface than the untreated ZnO interface thermodynamically. Benefiting from efficient charge extraction, the best PCEs increased to 12.8% from 11.8% for PBDB-T-2Cl:IT-4F-based devices, to 12.6% from 11.6% for PBDB-T-2Cl:Y1-4F-based devices, to 13.5% from 12.2% for PBDB-T-2Cl:Y6-based devices, and to 15.7% from 15.1% for PM6:Y6-based devices.

Realizing Efficient Charge/Energy Transfer and Charge Extraction in Fullerene-Free Organic Photovoltaics via a Versatile Third Component.

Solution-processed organic photovoltaics (OPVs) based on bulk-heterojunctions have gained significant attention to alleviate the increasing demend of fossil fuel in the past two decades. OPVs combined of a wide bandgap polymer donor and a narrow bandgap nonfullerene acceptor show potential to achieve high performance. However, there are still two reasons to limit the OPVs performance. One, although this combination can expand from the ultraviolet to the near-infrared region, the overall external quantum efficiency of the device suffers low values. The other one is the low open-circuit voltage (VOC) of devices resulting from the relatively downshifted lowest unoccupied molecular orbital (LUMO) of the narrow bandgap. Herein, the approach to select and incorporate a versatile third component into the active layer is reported. A third component with a bandgap larger than that of the acceptor, and absorption spectra and LUMO levels lying within that of the donor and acceptor, is demonstrated to be effective to conquer these issues. As a result, the power conversion efficiencies (PCEs) are enhanced by the elevated short-circuit current and VOC; the champion PCEs are 11.1% and 13.1% for PTB7-Th:IEICO-4F based and PBDB-T:Y1 based solar cells, respectively.

Electrochemical Advances in Boron Subnaphthalocyanines (BsubNcs) for Organic Photovoltaic Applications: The Ambient Stability of Bsubnc Mixed Alloys; Their Separation and Electrochemical Properties; And Chemical Progresses Towards Pure Bsubncs to Yield Their Individual Electrochemical Evaluation

For some time, our group has been focused on the design, synthesis and application of derivatives of boron subphthalocyanines (BsubPcs). Our focal point has been and continues to be equally between the basic chemistry of BsubPcs and their application in organic electronic devices with a focus on two specific device applications, organic photovoltaics (OPVs)/solar cells[1,2] and organic light emitting diodes (OLEDs). More recently, we have taken an equal approach to derivatives similar to BsubPcs, boron subnaphthalocyanines (BsubNcs); BsubNcs being unique and beneficial materials for OPV application.[3] We have recently shown that BsubNcs actually become randomly chlorinated during their synthetic preparation and actually then form a mixed alloy composition of chlorinated materials, which we have designated as Cl-ClnBsubNcs.[4] The mixed alloy composition is unique, and has been determined to be a mixture of 24 (more or less) chlorinated BsubNcs that despite being a mixture form single crystals. The formation of single crystals is enabled by the chlorine atoms occupying vacancies within the solid state structure, the vacancies being the so-called “bay position” of the BsubNcs structure. After establishing the correlation of electrochemical characteristics and OPV performance for Cl-ClnBsubNcs, we have recently shown that phenoxylated versions of ClnBsubNc are also a mixed alloy and applicable in BHJ OPVs also as electron accepting materials.[5] We have also shown that Cl-ClnBsubNcs when applied as electron donating materials in OPVs have the same performance correlations to the amount of chlorination present.[6] We have also shown that BsubNcs are equally stable within an OPV in the ambient environment compared with BsubPcs.[7] This accumulation of properties led us to consider how to separate the alloyed mixtures and how to prepare pure BsubNcs. During this presentation I will describe a chemical pathway to enable the separation of the mixed alloys of BsubNcs. We found that not all 24 (more or less) isomers can be separated, yet this process has enabled a more detailed electrochemical characterization and correlation to amount of halogenation. I will also describe a chemical pathway to pure BsubNcs, avoiding halogenation/chlorination in the bay position. This pathway enables a detailed characterization of a pure BsubNc including its electrochemical properties. Finally, if completed by the presentation time, I will also describe a chemical pathway to enable complete halogenation of the bay-position of a BsubNc. This pathway then enabled the characterization and therefore the impact on the electrochemical and other physical properties of complete bay-position halogenation. Co-authors/investigators will be identified during this presentation. References. [1] “Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics.” Josey, D.; et al, ACS Energy Lett., 2017, 2 (3), 726–732. DOI: 10.1021/acsenergylett.6b00716. [2] “Boron Subphthalocyanines as Electron Donors in Outdoor Lifetime Monitored Organic Photovoltaic Cells.” Garner, R.K.; et al, Solar Energy Materials and Solar Cells, 2018 176, 331-335. DOI: 10.1016/j.solmat.2017.10.018 [3] “8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer” Cnops, K.; et al., Nature Comm., 5, Article number: 3406, DOI:10.1038/ncomms4406. [4] “The mixed and alloyed chemical composition of chloro-(chloro)n-boron subnaphthalocyanines dictates their physical properties and performance in organic photovoltaics.” Dang, J.D.; et al, J. Mat. Chem. A., 2016, 4, 9566-9577. [5] “Phenoxy-(chloro)n-boron subnaphthalocyanines; alloyed mixture, electron-accepting functionality, enhanced solubility for bulk heterojunction organic photovoltaics” Dang, J.D.; et al, ACS Omega, 2018, 3(2), 2093–2103. DOI: 10.1021/acsomega.7b01892. [6] “The Mixed Alloyed Chemical Composition of Chloro-(chloro)n-Boron Subnaphthalocyanines Dictates Their Performance as Electron-Donating and Hole-Transporting Materials in Organic Photovoltaics” Garner, R.K.; et al, ACS Appl. Energy Materials, 2017, 1(3), 1029-1036. DOI: 10.1021/acsaem.7b00180. [7] "Outdoor Stability of Chloro-(Chloro)n-Boron Subnaphthalocyanine and Chloro-Boron Subphthalocyanine as Electron Acceptors in Bilayer and Trilayer Organic Photovoltaics" Josey, D.; et al, ACS Applied Energy Materials, 2018, submitted. Figure 1

All-polymer solar cells based on photostable bis(perylene diimide) acceptor polymers

Fullerene-free organic photovoltaics have recently reached impressive power conversion efficiencies above 14% for single junctions, increasing their competitiveness with respect to alternative thin-film technologies. In most record devices, electron-donating conjugated polymers are combined with novel generation small molecule acceptors. All-polymer organic solar cells, on the other hand, still lag behind in efficiency, although they have specific advantages in terms of ink formulation and long-term operational stability. Another point of attention is the synthetic complexity of the active layer materials, notably on the side of the new acceptor molecules. Therefore, the present study focuses on the implementation of the stable and cost-effective perylene diimide structure as the key component of high-performance electron-accepting polymers. The synthesis, structural and optoelectronic characterization of four push-pull type copolymers containing the electron-deficient bis(perylene diimide) (bis-PDI) unit is reported, as well as the photovoltaic analysis of these acceptor materials in combination with a well-known donor polymer (PTB7-Th). The acceptor polymers differ in the electron-rich part of the alternating push-pull structure and their solar cell power conversion efficiencies range from 3.2 to 4.7%. The maximum efficiency - the best performance achieved with a bis-PDI polymer so far - is obtained for the structurally most simple polymer, containing merely thiophene as the electron-rich building block. Controlled degradation under blue light in air is monitored by the bleaching of the relevant UV–Vis absorption bands, demonstrating high stability for the bis-PDI-thiophene containing polymers as compared to some prototype small molecule acceptors (FBR and ITIC).

Enabling low voltage losses and high photocurrent in fullerene-free organic photovoltaics

Despite significant development recently, improving the power conversion efficiency of organic photovoltaics (OPVs) is still an ongoing challenge to overcome. One of the prerequisites to achieving this goal is to enable efficient charge separation and small voltage losses at the same time. In this work, a facile synthetic strategy is reported, where optoelectronic properties are delicately tuned by the introduction of electron-deficient-core-based fused structure into non-fullerene acceptors. Both devices exhibited a low voltage loss of 0.57 V and high short-circuit current density of 22.0 mA cm−2, resulting in high power conversion efficiencies of over 13.4%. These unconventional electron-deficient-core-based non-fullerene acceptors with near-infrared absorption lead to low non-radiative recombination losses in the resulting organic photovoltaics, contributing to a certified high power conversion efficiency of 12.6%.

Diketopyrrolopyrrole-based terpolymers with tunable broad band absorption for fullerene and fullerene-free polymer solar cells

Fullerene and fullerene-free organic photovoltaics.

Analyzing the efficiency, stability and cost potential for fullerene-free organic photovoltaics in one figure of merit

Organic semiconductors with low synthetic complexity, such as P3HT, would be the preferred choice for large-scale production and commercialization.

A Twisted Thieno[3,4-b]thiophene-Based Electron Acceptor Featuring a 14-π-Electron Indenoindene Core for High-Performance Organic Photovoltaics.

With an indenoindene core, a new thieno[3,4-b]thiophene-based small-molecule electron acceptor, 2,2'-((2Z,2'Z)-((6,6'-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-π-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated π-conjugation with 14 π-electrons and one more sp3 carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 × 105 m-1 cm-1 in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.

Impact of Fluorine Atoms on Perylene Diimide Derivative for Fullerene‐Free Organic Photovoltaics

The incorporation of fluorine atoms in organic semiconducting materials has attracted much attention recently due to its unique function to manipulate the molecular packing, film morphology and molecular energy levels. In this work, two perylenediimide (PDI) derivatives FPDI-CDTph and FPDI-CDTph2F were designed and synthesized to investigate the impact of fluorination on non-fullerene acceptors. Both FPDI-CDTph and FPDI-CDTph2F exhibited strong and broad absorption profiles, suitable lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels, and good electron transport ability. Compared with FPDI-CDTph, the fluorinated acceptor (FPDI-CDTph2F) afforded an optimal bulk heterojunction morphology with an interconnected and nanoscale phase separated structure that allowed more efficient exciton dissociation and balanced charge transport. Consequently, organic solar cells based on FPDI-CDTph2F showed a much higher power conversion efficiency (PCE) of 6.03 % than that of FPDI-CDTph based devices (4.10 %) without any post-fabrication treatment.

Morphological characterization of fullerene and fullerene-free organic photovoltaics by combined real and reciprocal space techniques

Abstract

Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics

The outdoor lifetime and performance of organic photovoltaics (OPVs) using boron subphthalocyanine (BsubPc) derivatives as electron-accepting materials is presented. The protocols followed are based on the most advanced level of outdoor testing established by the International Summit on OPV Stability (ISOS). The stability of each BsubPc is compared using three different sets of encapsulated planar heterojunction OPVs, with each set containing a different BsubPc as the electron-accepting layer. The performance and stability of each set is tested outdoors using an epoxy glue and a glass coverslip as protection from the ambient environment. Outdoor testing continued until the OPVs reached 80 or 50% of their original power conversion efficiency, as determined by frequent indoor characterization. OPVs utilizing chloro-BsubPc are shown to exhibit the highest stability and performance, while the stability of the other two BsubPc derivatives is reduced presumably as a result of their phenoxy or phenyl functionali...

Diketopyrrolopyrrole-based oligomers accessed via sequential C[sbnd]H activated coupling for fullerene-free organic photovoltaics

Exploring sustainable chemistry for renewable energy plays a key role in meeting the ever increasing energy demand without sacrificing the environment. In this study, two novel diketopyrrolopyrrol(DPP)-based π-conjugated oligomers (named as TPE-DPP4 and BP-DPP4) have been readily synthesized via a ligand-free Pd-catalyzed sequential activation of CH bond in two steps with good yields starting from simple building blocks. Poly(3-hexylthiophene) is employed as an electron donor to blend with the new DPP-derived electron acceptors for the fullerene-free bulk heterojunction organic photovoltaics. The power conversion efficiency of 2.49% has been achieved, corresponding with an open-circuit voltage of 1.16 V, which is among the highest open-circuit voltages for the single-junction organic photovoltaics. The facilely accessible electron acceptors blended with cost-effective poly(3-hexylthiophene) donor for fullerene-free organic photovoltaics opens a new pathway to access renewable solar energy via sustainable chemistry.

Enhanced Efficiency in Fullerene-Free Polymer Solar Cell by Incorporating Fine-designed Donor and Acceptor Materials

Among the diverse nonfullerene acceptors, perylene bisimides (PBIs) have been attracting much attention due to their excellent electron mobility and tunable molecular and electronic properties by simply engineering the bay and head linkages. Herein, guided by two efficient small molecular acceptors, we designed, synthesized, and characterized a new nonfullerene small molecule PPDI with fine-tailored alkyl chains. Notably, a certificated PCE of 5.40% is realized in a simple structured fullerene-free polymer solar cell comprising PPDI as the electron acceptor and a fine-tailored 2D-conjugated polymer PBDT-TS1 as the electron donor. Moreover, the device behavior, morphological feature, and origin of high efficiency in PBDT-TS1/PPDI-based fullerene-free PSC were investigated. The synchronous selection and design of donor and acceptor materials reported here offer a feasible strategy for realizing highly efficient fullerene-free organic photovoltaics.

An N-ethylated barbituric acid end-capped bithiophene as an electron-acceptor material in fullerene-free organic photovoltaics

A new evaporable electron acceptor material for organic photovoltaics based on N-ethyl barbituric acid bithiophene (EBB) has been demonstrated. Bilayer devices fabricated with this non-fullerene acceptor and boron subphthalocyanine chloride (SubPc) donor produce power conversion efficiencies as high as 2.6% with an extremely large open-circuit voltage approaching 1.4 V.

Fullerene-free organic photovoltaics based on unconventional material combination: a molecular donor and polymeric acceptors

Photovoltaic cells based on molecular donor/polymeric acceptors were investigated and 3.01% PCE was achieved by using DIO as an additive.