Organic Photovoltaic Applications Research Articles

Effect of different topological structures (D-π-D and D-π-A-π-D) on the optoelectronic properties of benzo[2,1-B:3,4-B́]dithiophene based donor molecules toward organic solar cells

As substantial building units, different shapes and topologies enhance optical and electronic properties. Hence, in this paper, twelve anisotropic molecules of different shapes (star or S, π and H) based on D-π-D and D-π-A-π-D topologies have been designed and investigated theoretically applying a double overlapping wave band strategy for organic photovoltaic applications. This analysis indicates that stronger electron withdrawing A1 fragment, D-π-A-π-D topology, star and π shaped molecules impart toward lower LUMO and smaller Eg values than those of D-π-D type and X or H- shaped molecules. According to TD-DFT calculations, start-shaped molecules show broader spectra with intense peaks in both topologies covering not only visible region but also IR region of spectrum. Furthermore, the optical analysis also demonstrates that π-π* absorptions in short and middle wavelength region are because of central anisotropic multibranched donor (Bi)n-2DF-(Bi)n moiety, where A components exhibit absorptions in the ML wavelength region of spectrum. In General, smaller λe and λh values are exhibited for star and π-shaped molecules because of better dimensionality, whereas H and X-shaped (D1b) display more or less similar values. The predicted electron and hole mobility values for DAD1-S are as; µe = 2.46 (cm2/Vs) and µh = 0.0540 (cm2/Vs), respectively. Thus, results of these calculation prove that our theoretical studies provide a deep insights to confirm that investigated donor compounds would be appropriate vs PDIs acceptors for OSCs.

Azadipyrromethene-based near-IR dyes with styryl substituents at the pyrrolic positions for organic photovoltaic applications

Azadipyrromethene dyes are attractive building blocks for the development of near-IR dyes, with applications ranging from biosensing, photodynamic therapy and solar energy conversion. In particular, Zn(II) bis(2,8-diphenylethynyl 1,3,7,9 tetraphenylazadipyrromethene) complexes are promising n-type non-fullerene electron acceptors for organic photovoltaics. Here, the triple bond in phenylethynyl pyrrolic substituents is replaced with a double bond. The UV–Vis absorption spectra were not significantly different, indicating that both the double and triple bond similarly extend the conjugation of ADP. Interestingly, the Zn(II) complex with styryl substituents (Zn(WS6)2) showed emission in the near IR, with a λmax of 773 nm. The BF2+ chelate also emitted in the near IR with a λmax of 815 nm, but the emission was very weak. Cyclic voltammetry revealed that double bond compounds were easier to oxidize than un-substituted ADP analogues, and unlike with triple bond, were not easier to reduce. In organic photovoltaics, the Zn(WS6)2 did not perform as well as the Zn(WS3)2 analogue, showing unfavorable film morphology when blended with P3HT and low blend electron mobility of 2 × 10−8 cm2V−1s−1 by SCLC, compared to 1 × 10−7 cm2V−1s−1 in neat film. The hole mobility of Zn(WS6)2 in neat films is 6 × 10−5 cm2V−1s−1, higher than its electron mobility, suggesting its potential as p-type material as well. Further work in finding an appropriate n-type polymer acceptor is required to confirm this hypothesis. Using a triple bond at the pyrrolic positions is therefore better than a double bond for the development of n-type materials based on ADP dyes.

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

Push-pull molecules bearing a hydrazonocyclopentadiene acceptor moiety: from the synthesis to organic photovoltaic applications

First representatives of a novel class of compounds containing an electron-accepting hydrazonocyclopentadiene moiety, π-conjugated phenylene spacer, and electron-donating triphenylamine moiety have been synthesized. Investigation of their optical, electrochemical and photovoltaic properties revealed a high potential of the hydrazonocyclopentadiene acceptor moiety in the design of donor-acceptor compounds for organic photovoltaics.

Surface Self-Assembly, Film Morphology, and Charge Transport Properties of Semiconducting Triazoloarenes.

Surface-assisted molecular self-assembly is a powerful strategy for forming molecular-scale architectures on surfaces. These molecular self-assemblies have potential applications in organic electronics, catalysis, photovoltaics, and many other technologies. Understanding the intermolecular interactions on a surface can help predict packing, stacking, and charge transport properties of films and allow for new molecular designs to be tailored for a required function. We have previously studied a molecular platform, tris( N-phenyltriazole) (TPT), that exhibits planar stacking through >20 molecular layers through donor-acceptor-type intermolecular π-π contacts between the electron-deficient tris(triazole) core and electron-rich peripheral phenyl units. Here, we investigate an expanded family of TPT-based molecules with variations made on the peripheral aryl groups to modulate the molecular electron distribution and examine the impact on molecular packing and charge transport properties. Molecular-resolution scanning tunneling microscopy was used to compare the molecular packing in the monolayer and to investigate the effects that the structural and electronic modifications have on the stacking in subsequent layers. Conductivity measurements were made using the four-point probe van der Pauw technique to demonstrate charge transport properties comparable to pentacene. Although molecular packing is clearly impacted by the chemical structure, we find that the charge transport efficiency is quite tolerant to small structural variations.

Solution assisted roll-coating of transparent conducting oxide thin films for organic photovoltaic applications

Transparent conduction oxide (TCO) thin films are one of the key material in optoelectronic applications. These materials are usually produced by vacuum-based methods such as magnetron sputtering, both for laboratory work and for commercial purposes. Alternative methods for producing large-scale TCO films are needed. In this work, a non-vacuum coating technique for preparation of TCO thin films on large area glass substrates was presented. Tin doped indium oxide (ITO) thin films were successively coated on glass substrates by non-vacuum based roll coating method. A low electrical resistivity of ~1.3 × 10−3 Ωcm and high optical transmittance of 84% in the visible region were obtained for ITO thin film samples. Furthermore, an ITO/PEDOT PSS/P3HT:PCBM/LiF/Al structured organic photovoltaic cell was also prepared on non-vacuum roll coated ITO thin films. The device with the roll coated ITO thin film exhibits a power conversion efficiency of 1.83%, average short-circuit current density (Jsc) of 7.4 mA/cm2, open-circuit voltage (Voc) of 0.590 V, and fill factor (FF) of 0.42.

Thiadiazoloquinoxaline and benzodithiophene bearing polymers for electrochromic and organic photovoltaic applications

Two novel thiadiazoloquinoxaline and benzodithiophene (BDT) bearing copolymers were designed and synthesized. Different BDT units (alkoxy and thiophene substituted) were used as donor materials and the effect of alkoxy and thiophene substitution on the electrochemical, spectroelectrochemical and photovoltaic properties were investigated. Both polymers exhibited low oxidation potentials at around 0.90 V and low optical band gaps at around 1.00 eV due to the insertion of electron poor thiadiazoloquinoxaline unit into the polymer backbone. Both P1 (poly-6,7-bis(3,4-bis(decyloxy)phenyl)-4-(4,8-bis(nonan-3-yloxy)benzo[1,2-b:4,5-b']dithiophen-2-yl)-[1, 2, 5]thiadiazolo[3,4-g]quinoxaline) and P2 (poly- 4-(4,8-bis(5-(nonan-3-yl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-6,7-bis(3,4-bis(decyloxy)phenyl)-[1, 2, 5]thiadiazolo[3,4-g]quinoxaline) exhibited multichromic behavior with different tones of greenish yellow and gray in the neutral and fully oxidized states, respectively. In addition, both polymers revealed very high optical contrasts (∼87%) in the NIR region which make these promising polymers good candidates for NIR applications. Finally, in order to explore the organic photovoltaic performances, P1 and P2 were mixed with PC71BM in the active layer of organic solar cells (OSCs) by conventional device structure. As a result P1 and P2 based devices revealed power conversion efficiencies (PCEs) of 0.33% and 0.60% respectively. However, the additive treatment enhanced PCE from 0.49 to 0.73% for P2 based devices.

Trap-Free Space-Charge-Limited Hole Transport in a Fullerene Derivative

Fullerenes and their derivatives are well-known electron-transport materials for organic (opto)electronic devices, but the extent to which they can transport holes is strongly disputed. To resolve the matter, this study quantifies bulk hole transport in a fullerene derivative. Remarkably, hole transport here is trap-free, with a high mobility equal to that for electrons. This demonstration that fullerenes can exhibit balanced, bipolar charge transport, which is unusual for organic semiconductors, is interesting for application in organic photovoltaics and ambipolar transistors.

Potentially self-dopable poly(3-hexylthiophene) block copolymers/carbon nanotube nanocomposites for enhanced processibility and electrical properties

Development of new hole-transport layer (HTL) materials for organic photovoltaic (OPV) applications is one crucial issue to mitigate such limitations of low power efficiency, processibility, and stability derived from poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a common HTL material. To this end, poly(3-hexylthiophene) (P3HT)-based diblock copolymers including the different ratios between polystyrene (PSty) or/and poly(neopentyl styrene sulfonate) (PNSS) as a second block segment were designed and then used for incorporating with single wall carbon nanotubes (SWCNTs) to form potentially self-dopable nanocomposites. The structure of fabricated P3HT-based diblock copolymer/SWCNT nanocomposites was examined by microscopic and spectroscopic characterizations, exhibiting a charge transfer behavior between P3HT and SWCNT, and growth of the P3HT crystalline phase along the SWCNT surface with a form of the conducting path. The acquired SWCNT nanocomposites exhibited good solubility and dispersion in various organic solvents including toluene, chlorobenzene, THF, chloroform, and DMF, which are good solvents for the PSty or/and PNSS neutral second block. The concept for the neutralized second block by protecting groups can prevent the poisoning ITO substrate by water contamination as shown in the conventional aqueous PEDOT-PSS solution. Notably, the PNSS constituent in the nanocomposite was merely acidified to give in-situ sulfonic acid groups in PSty by simple thermolysis process at 160 °C for 30 min after a solution process for fabrication of a thin film. The thermolysis step provided efficient doping of P3HT by PSS, in which the thiophene groups worked as Brønsted base are protonated by taking protons from sulfonic acid groups and then lead to a form of ionic bonds between cationic thiophene and anionic sulfonate groups. In addition, the formed ionic bonds in the nanocomposites led to an insoluble thin film, which has a good advantage for the further solvent process without substantial damages. The beneficial features of the conducting path along SWCNT and the efficient doping of P3HT by PSS in the acquired nanocomposites led to ca. 5-fold higher electrical conductivity (3.16 S cm−1) and similar work function (5.1 eV) in comparison to water contained commercial PEDOT:PSS (0.6 S cm−1 and 5.0 eV, respectively). As such, the rationally designed potentially self-dopable SWCNT nanocomposites can be one of the promising alternatives beyond the commercial PEDOT: PSS HTL material.

Photovoltaic donor-acceptor conjugated polymers with minimally substituted acceptor moieties

Abstract A novel family of conjugated donor-acceptor (D-A) copolymers designed with comparatively small methyl substituted acceptor units was synthesized for use as donor phase materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) applications. These polymers make use of common acceptor moieties thieno [3,4-c]pyrrole-4,6-dione (TPD), diketopyrrolopyrrole (DPP), and isoindigo (iI) with methyl side chains to provide sterically unhindered sites for enhanced interactions with a molecular acceptor (MA), either PC71BM or ITIC in this study. The resulting polymers P(T4-TPD-M), P(T4-DPP-M), and P(T4-iI-M) were utilized in BHJ OPV devices where the highest performing D-A polymer, P(T4-TPD-M), obtained a maximum PCE of 7.5% with PC71BM and 4.0% with ITIC. These results indicate that minimally-sized methyl side chain derivatives of TPD, DPP, and iI can be useful acceptor moieties in D-A polymer systems, and the impact of sterics on the acceptor moiety can be elucidated in future studies through modification of the methyl side chain to bulkier aliphatic substituents.

Photostability of Fullerene and Non-Fullerene Polymer Solar Cells: The Role of the Acceptor.

Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.

Evaluation of the daylighting potential of OPV windows for deep-plan rooms

Edifícios de escritório com múltiplos pavimentos, em sua maioria, possuem grandes superfícies envidraçadas que possibilitam aos usuários usufruir de luz e ventilação natural, além do contato do observador com o meio externo. Entretanto, sem um devido tratamento, essas superfícies podem possibilitar aumento/redução de carga térmica para o edifício, assim como causar ofuscamento nos trabalhadores. Neste sentido, a janela fotovoltaica poderia ser utilizada tanto para melhorar a distribuição da iluminação natural quanto para suplementar energia para iluminação artificial quando necessária. O objetivo dessa pesquisa é avaliar o potencial da aplicação de Dispositivos Orgânicos Fotovoltaicos (OPV) em janelas laterais de edifícios de escritórios com múltiplos pavimentos e salas profundas, a fim de verificar a sua contribuição em relação à qualidade da iluminação natural no ambiente interno. Realizou-se a avaliação por meio de um experimento com um modelo em escala reduzida de uma sala de escritório genérica, iluminada por uma janela lateral, a qual corresponde a 100% da área da fachada Norte. Compararam-se três tipos de materiais para a janela sob condição de céu real em Curitiba-PR: Cenário A - vidro simples 3 mm; Cenário B – vidro e OPV; e, Cenário C – vidro com aplicação de película solar. Apesar da redução da iluminância com a utilização dos materiais dos cenários B e C em relação ao material do cenário A, constatou-se uma melhor distribuição da iluminação natural no interior da sala, sendo que no cenário B existe a possibilidade de utilizar a energia gerada na janela fotovoltaica para a iluminação artificial.

Distinct Routes of Singlet Fission and Triplet Fusion: A Fluorescence Kinetic Study of Rubrene

Singlet fission of organic molecules has attracted recent attention owing to its potential advantages in organic photovoltaic and electroluminescence applications. Its microscopic mechanism however...

Symmetric pyrrolic squaraines and their application to organic photovoltaics

We report the material characterization and single crystal Xray analysis of bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-olate (PSQ) and a derivative of this symmetric squaraine dye with a thiophene substituent (NSP) that has high solubility and high absorptivity (ε = 5.2 × 105 M−1 cm−1) in the 500–800 nm region of the solar spectrum, making it a potential candidate for application in organic photovoltaics (OPVs). The squaraine derivatives show markedly broader absorption in the solid state than in solution. Photoluminescent quantum yields for PSQ and NSP are low (ΦPL = 0.014 and 0.047 respectively). OPVs were fabricated using the following architecture: ITO/ MoO3 (120 Å)/ Squaraine (80–85 Å)/ C60(400 Å)/ BCP (100 Å)/ Al (1000 Å), BCP = bathocuproine. Devices fabricated with thin films of PSQ give higher fill factors (FF), photocurrent and better performance under 1 sun, AM1.5 G simulated illumination: open-circuit voltage Voc = 0.43 V, short-circuit current Jsc = 5.17 mA/cm2, FF = 0.52, and power conversion efficiency η = 1.15%, compared with NSP based OPVs: Voc = 0.56 V, short-circuit current Jsc = 4.34 mA/cm2, FF = 0.42, and power conversion efficiency η = 1.02%.

Synthesis and photovoltaic application of NIR-emitting perylene-monoimide dyes with large Stokes-shift.

An efficient Sonogashira coupling protocol is developed for tetra-alkynylation at the bay and peri-positions of the perylene-monoimide (PMI) dye in its PMI(Br)4 form. The absorption band for these PMI dyes covered from the visible to Near-infrared (NIR) region and PMI(NMe2)4 is found to exhibit a record-breaking Stokes-shifted NIR emission with good photovoltaic properties.

Energy level tuning of ‘Z’-shaped small molecular non-fullerene electron acceptors based on a dipyrrolo[2,3-b:2′,3′-e]pyrazine-2,6(1H,5H)-dione acceptor unit for organic photovoltaic applications: a joint experimental and DFT investigation on the effect of fluorination

Three dipyrrolo[2,3-b:2′,3′-e]pyrazine-2,6(1H,5H)-dione based small molecule non-fullerene acceptors with various end-capped fluorine units have been investigated.

Adsorption of tetracyanoquinodimethane and tetrathiafulvalene on aluminium (100) surface – a first principle study of structural and electronic properties

ABSTRACTPossible adsorption configurations and electronic properties, such as charge analysis, density of states, work function and Schottky barrier height of tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) on Aluminium (100) surface is studied by using density functional theory methods with local density approximation (LDA), generalised gradient approximation (GGA), PBE and PBE-D2 methods. TCNQ is strongly adsorbed on Al(100) with adsorption energy of −3.66 eV. The charge is transferred from Al(100) to TCNQ and charge transfer occurs mainly through cyano group of TCNQ. Adsorption on Al(100) surface leads to downshift in energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TCNQ by 2.5 eV as result of hybridisation of p orbital of carbon and nitrogen atoms of TCNQ and p orbital of surface Al atoms. Compared to TCNQ adsorption, TTF adsorption on Al(100) surface is having less adsorption energy and type of interaction is physisorption. The charge transfer from TTF to Al(100) surface leads to decrease in work function of Al by 0.24 eV. The n-type Schottky barrier height of TCNQ/Al(100) and p-type Schottky barrier height of TTF/Al(100) is 0.68eV and 1.97 eV, respectively showing that TCNQ and Al(100) are suitable for organic photovoltaic and electrochemical applications.

Elucidating the impact of N-arylanilino substituents of squaraines on their photovoltaic performances

Abstract Squaraines have attracted increasing research attention for organic photovoltaic applications because of their facile and low-cost synthesis, and intense and broad absorption in the visible and near-infrared (Vis-NIR) regions. Herein, three new squaraines, N-phenylanilino substituted SQ-EP, N-2-naphthylanilino substituted SQ-EN, and N-fluorenylanilino substituted SQ-EF, were developed for improving the hole mobility of aniline-based squaraines. The substitution effect of N-arylanilino groups on the optoelectronic properties of squaraines was investigated. These squaraines exhibited similar absorbance spectra, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, and hole mobilities in their neat films. However, compared to their neat films, the hole mobility of the SQ-EP-based BHJ blend film showed the smallest decrease when blending with PC71BM ([6,6]-phenyl C71 butyric acid methyl ester). Besides, morphological studies have shown that a phase separation of 20–30 nm can only be observed in the SQ-EP/PC71BM blend film. Consequently, a solution-processed bulk-heterojunction (BHJ) organic photovoltaic (OPV) device fabricated with the as-cast SQ-EP/PC71BM blend film exhibited obviously higher power conversion efficiency (PCE: 5.4%) than those of the SQ-EN/PC71BM (PCE: 4.3%) or SQ-EF/PC71BM systems (PCE: 2.8%). This is one of the highest recorded PCE values in aniline-based squaraines single-junction BHJ-OPV devices. This much-enhanced PCE can be attributed to the significant increases in short-circuit current density (Jsc) and fill factor (FF) of the SQ-EP/PC71BM-based OPV device.

Exhibition of Förster resonance energy transfer from CdSe/ZnS quantum dots to zinc porphyrazine studied in solution

Molecular level interaction between CdSe/ZnS core-shell quantum dots (QD) and zinc porphyrazine (1) is studied in solution by means of UV–vis spectroscopy, steady state emission and time-resolved fluorescence decay measurements. Appreciable value of red shift of the Soret absorption band of 1 (~4 nm) in presence of QD is indicative of ground state interaction between these two species in solution. Photoluminescence (PL) of QD is quenched considerably in presence of 1 upon formation of QD–1 system and average value of lifetime of QD is reduced considerably following the addition of 1 in increasing concentration. The PL quenching mechanism may be identified as Förster resonance energy transfer from QD to 1 in solution. Evaluation of bimolecular quenching constant (kq) value of QD–1 system (kq (av) ~ 3.2 × 1012 dm3·mol−1·sec−1) establishes that static quenching mechanism predominates over diffusion controlled pathway behind quenching of PL of QD in presence of 1 in solution. The results demonstrate that 1 may be selectively employed as an energy accepting unit in combination with quantum dots for possible application in organic photovoltaics.

Facilitate charge transfer at donor/acceptor interface in bulk heterojunction organic photovoltaics by two-dimensional nanoflakes

Recently, two-dimensional (2D) materials have been applied in organic photovoltaics (OPVs) as electrode modifier or third component in bulk due to their high charge carrier mobility, various electronic band structure, and facile fabrication via liquid phase exfoliation. Herein, solution exfoliated few layers black phosphorus (BP) and tungsten diselenide (WSe2) were used as third component in bulk heterojunction OPVs, resulting in improved power conversion efficiencies (PCEs) from 8.60% for reference device, to 9.20% and 9.24% for devices based on BP and WSe2, respectively. Thorough study has been executed to look into the bulk morphology change and intermolecular charge transfer behavior within the OPV device by means of atomic force microscopy, grazing-incidence small-angle X-ray scattering, photoluminescence spectroscopy, and electrochemical impedance spectroscopy. Results show that a neat BHJ morphology and low energetic loss donor/acceptor (D/A) interfaces were achieved when incorporated with BP nanoflakes or WSe2 nanofalkes, leading to suppressed exciton recombination and enhanced charge transport property. Besides, more balanced bipolar carrier mobility within the bulk also helped boost the performance of OPVs. The storage and operating stabilities of the devices are also improved after incorporating with BPNFs or WSNFs. This work suggests the potential application of 2D materials as third component in OPV applications.