Triplet Exciton Diffusion Length Research Articles

Organic solar cells and their applications Shah, Razon Oda, F. Hazuan

Enhancing the power conversion efficiency (PCE) is the ma- jor task in the development of solar cells. In 1961, Willi- am Shockley and Hans J. Queisser pointed out that the highest PCE for a single-junction solar cell is limited to 31% due to the thermalization and transmission losses. Singlet fission (SF) is an appealing carrier-multiplication approach to break the Shockley-Queisser limit, since it converts one high- energy singlet exciton into two low-energy triplet excitons. Thus, one absorbed photon generates two electron–hole pairs [1]. SF phenomenon was first observed in anthracene single crystals in 1965. It attracted attention again in 2006. In organic semiconductors, a singlet exciton (S1) forms after the ab- sorption of a photon. If the energy of this singlet exciton is greater than twice of that for triplet exciton (T1), namely E(S1) > 2E(T1), then spin-allowed fission (S1 → 2T 1) could occur. The general kinetic model for SF process is S0 is the ground state, S1 is the excited singlet state, 1(TT) is the intermediate state of a correlated triplet pair, and T1 + T1 are two independent triplet states (Figure 1(a)). The application needs materials with high fission efficiency, suitable bandgap for capturing high-energy photons, appropriate triplet energy and high chemical stability. Owing to the existence of competitive pathways (i.e. S1 to S0 relaxation in picosecond–nanosecond timescale; excimer formation in femtosecond timescale; charge transfer (CT) process in femtosecond timescale, SF needs to occur in pico- second or sub-picosecond timescale. Thus, SF requires strong intermolecular coupling. The intermolecular coupling can be realized via Van der Waals force, weak non-bonded interactions and so on. The coupling distance not only affects SF rate and triplet yield, but also influences the lifetime and diffusion length of triplet excitons [2].

Efficient Long-Range Triplet Exciton Transport by Metal-Metal Interaction at Room Temperature.

Efficient and long-range exciton transport is critical for photosynthesis and opto-electronic devices, and for triplet-harvesting materials, triplet exciton diffusion length ( ) and coefficient ( ) are key parameters in determining their performances. Herein, we observed that PtII and PdII organometallic nanowires exhibit long-range anisotropic triplet exciton LD of 5-7 μm along the M-M direction using direct photoluminescence (PL) imaging technique by low-power continuous wave (CW) laser excitation. At room temperature, via a combined triplet-triplet annihilation (TTA) analysis and spatial PL imaging, an efficient triplet exciton diffusion was observed for the PtII and PdII nanowires with extended close M-M contact, while is absent in nanowires without close M-M contact. Two-dimensional electronic spectroscopy (2DES) and calculations revealed a significant contribution of the delocalized 1/3 [dσ*(M-M)→π*] excited state during the exciton diffusion modulated by the M-M distance.

Efficient Long‐Range Triplet Exciton Transport by Metal–Metal Interaction at Room Temperature

AbstractEfficient and long‐range exciton transport is critical for photosynthesis and opto‐electronic devices, and for triplet‐harvesting materials, triplet exciton diffusion length ( ) and coefficient ( ) are key parameters in determining their performances. Herein, we observed that PtII and PdII organometallic nanowires exhibit long‐range anisotropic triplet exciton LD of 5–7 μm along the M−M direction using direct photoluminescence (PL) imaging technique by low‐power continuous wave (CW) laser excitation. At room temperature, via a combined triplet–triplet annihilation (TTA) analysis and spatial PL imaging, an efficient triplet exciton diffusion was observed for the PtII and PdII nanowires with extended close M−M contact, while is absent in nanowires without close M−M contact. Two‐dimensional electronic spectroscopy (2DES) and calculations revealed a significant contribution of the delocalized 1/3[dσ*(M−M)→π*] excited state during the exciton diffusion modulated by the M−M distance.

J-Aggregation Enhances the Electroluminescence Performance of a Sky-Blue Thermally Activated Delayed-Fluorescence Emitter in Nondoped Organic Light-Emitting Diodes.

A pivotal thermally activated delayed-fluorescence (TADF) emitter, DspiroAc-TRZ, was developed, and it exhibits greatly enhanced electroluminescence performance in nondoped organic light-emitting diodes (OLEDs) owing to the concurrent manipulation of aggregation behavior and monomolecular structure. The delicate non-planar packing pattern in the DspiroAc-TRZ crystal can not only lead to highly efficient solid-state luminescence but also form a loose intermolecular packing pattern, greatly decreasing the HOMO or LUMO overlaps in dimers and shortening the triplet exciton diffusion length. In addition, the rigid donor and acceptor moieties in DspiroAc-TRZ can rigidify the molecular backbone, resulting in a tiny geometric vibrational relaxation in the excited state. Impressively, high photoluminescent quantum yields of 78.5 and 83.7% were achieved for the DspiroAc-TRZ single crystal and nondoped film. A high external quantum efficiency (EQE) of 25.7% was achieved in a nondoped sky-blue TADF OLED, which is higher than any reported EQE value of nondoped sky-blue TADF OLEDs so far.

Dependence of Exciton Diffusion Length and Diffusion Coefficient on Photophysical Parameters in Bulk Heterojunction Organic Solar Cells

Recently, the dependence of exciton diffusion length $$ (L_{D} ) $$ on some photophysical parameters of organic solids has been experimentally demonstrated, however no systematic theoretical analysis of this phenomenon has been carried out. We have conducted a theoretical study by using the Forster resonance energy transfer and Dexter carrier transfer mechanisms together with the Einstein–Smoluchowski diffusion equation to derive analytical models for the diffusion lengths $$ (L_{D} ) $$ and diffusion coefficients $$ (D) $$ of singlet $$ (S) $$ and triplet $$ (T) $$ excitons in organic solids as functions of spectral overlap integral $$ (J) $$ , photoluminescence (PL) quantum yield $$ (\phi_{D} ) $$ , dipole moment $$ (\mu_{T} ) $$ and refractive index $$ (n) $$ of the photoactive material. The exciton diffusion lengths and diffusion coefficients in some selected organic solids were calculated, and we found that the singlet exciton diffusion length $$ (L_{D}^{S} ) $$ increases with $$ \phi_{D} $$ and J, and decreases with n. Also, the triplet exciton diffusion length $$ (L_{D}^{T} ) $$ increases with $$ \phi_{D} $$ and decreases with $$ \mu_{T} $$ . These may be achieved through doping the organic solids into broad optical energy gap host materials as observed in previous experiments. The calculated exciton diffusion lengths are compared with experimental values and a reasonably good agreement is found between them. The results presented are expected to provide insight relevant to the synthesis of new organic solids for fabrication of bulk heterojunction organic solar cells characterized by better power conversion efficiency.

Effect of exciton blocking layers on the color-tunable properties of organic light-emitting devices

A series of voltage-controlled color-tunable organic light-emitting devices (OLEDs) were fabricated. Four charge transport materials with different characteristics serving as the exciton blocking layers (EBLs) were inserted between yellow and blue emissive layers, respectively, and the color-tunable properties of OLEDs were studied in detail. Carrier transport and electroluminescence (EL) mechanisms of the color-tunable devices were investigated by analyzing the EL spectra and device efficiencies. The results showed that a color-tunable range of blue, white, and yellow light had been realized by tuning the film thicknesses of EBLs. The device with 16nm N,N′-dicarbazolyl-3,5-benzene hole transport material as the EBL could shift the color coordinates from (0.21, 0.32) to (0.38, 0.41), which exhibited the broadest color tuning range. Meanwhile, the device based on 18nm 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene electron transport material had the opposite color-tuning sequence and slightly higher tuning voltages, comparing with other three devices. The difference of color-tunable properties in OLEDs based on various EBLs was attributed to the variation of carrier mobility and energy level of EBL materials. Moreover, to effectively harvest the excitons for color-tunable light emission, the EBL thickness should be comparable to the triplet exciton diffusion length of EBLs.

Two Birds with One Stone: Tailoring Singlet Fission for Both Triplet Yield and Exciton Diffusion Length.

By direct imaging of singlet and triplet populations with ultrafast microscopy, it is shown that the triplet diffusion length and singlet fission yield can be simultaneously optimized for tetracene and its derivatives, making them ideal structures for application in bilayer solar cells.

Defect density dependent photoluminescence yield and triplet diffusion length in rubrene

We investigate how excitonic processes in rubrene single crystals are affected by a deliberately implanted defect gradient induced by proton irradiation. Spatially resolved measurements show a gradually reduced photoluminescence quantum yield and triplet exciton diffusion length along the defect gradient. Both effects are caused by a decrease in the average triplet lifetime due to interaction with the implanted defects. The triplet lifetime was reduced by almost two orders of magnitude at the highest implanted defect density of 1017 cm–3. The strong sensitivity of the photoluminescence quantum yield to the defect density that is observed already at moderate excitation densities is caused by the combination of two effects: the dominant contribution of triplet-fusion to the observed photoluminescence and the long-range diffusion of triplet excitons.

Understanding the effect of triplet sensitizers in organic photovoltaic devices

The effect of PtOEP as a dopant on the performance of MEH-PPV/C60 photovoltaic devices was studied. Bilayer heterojunction devices with various compositions and layer structures were used to determine the possible pathways by which the photogeneration efficiency is enhanced. A key finding is that photocurrent generation enhancement always occurs in the MEH-PPV absorption region, regardless of the PtOEP dopant concentration or the MEH-PPV layer thickness. This result suggests that the presence of PtOEP in the donor MEH-PPV layer is primarily responsible for increasing the triplet exciton diffusion length of MEH-PPV by acting as a triplet sensitizer, rather than as an additional absorber for direct photogeneration. Values obtained from simulation show that the enhancement of exciton diffusion length of MEH-PPV can be more than a factor of 2 with optimal PtOEP concentrations. Further support for the role of PtOEP as a triplet sensitizer in MEH-PPV was obtained in experiments incorporating a blocking layer between MEH-PPV and C60, whereby the various exciton transfer processes can be differentiated.

Optimization of hybrid blue organic light-emitting diodes based on singlet and triplet exciton diffusion length

The hybrid blue organic light-emitting diodes (HB OLEDs) with triplet harvesting (TH) structures within an emitting layer (EML) are fabricated with fluorescent and phosphorescent EMLs. The TH is to transfer triplet excitons from fluorescence to phosphorescence, where they can decay radiatively. Remarkably, the half-decay lifetime of a hybrid blue device with fluorescent and phosphorescent EML thickness of 5 and 25 nm, measured at an initial luminance of 500 cd/m2, has improved twice than that of using a conventional structure. Additionally, the blue device’s efficiency improved. We attribute this improvement to the efficient triplet excitons energy transfer and the optimized distribution of the EML which depends on singlet and triplet excitons diffusion length that occurs within each the EML.

Diffusion length and Langevin recombination of singlet and triplet excitons in organic heterojunction solar cells.

We derived new expressions for the diffusion length of singlet and triplet excitons by using the Föster and Dexter transfer mechanisms, respectively, and have found that the diffusion lengths of singlet and triplet excitons are comparable. By using the Langevin recombination theory, we derived the rate of recombination of dissociated free charges into their excitonic states. We found that in some organic polymers the probabilities of recombination of free charge carriers back into the singlet and triplet states are approximately 65.6 and 34.4 %, respectively, indicating that Langevin-type recombination into triplet excitons in organic semiconductors is less likely. This implies that the creation of triplet excitons may be advantageous in organic solar cells, because this may lead to dissociated free charge carriers that can be collected at their respective electrodes, which should result in better conversion efficiency.

Triplet Exciton Diffusion in Platinum Polyyne Films

A time-resolved photoluminescence quenching approach is developed for determining the triplet exciton diffusion coefficient and diffusion length (D and LD, respectively) of phosphorescent conjugated polymers. This method is applied to a solid-state organometallic conjugated polymer with the structure [−Pt(PBu3)2–CC–C6H4–CC−]n (where Bu = n-butyl and −C6H4– is 1,4-phenylene). The approach relies on analysis of the lifetime quenching of the polymer’s phosphorescence by a series of three different quenchers that are dispersed into the polymer phase at varying concentration. The lifetime quenching data are analyzed by using a modified Stern–Volmer quenching expression to determine the diffusion-controlled quenching rate constant, kq, which is then related to the exciton diffusivity, D, and diffusion length, LD. The diffusion coefficients that are determined using the three quenchers are consistent, D ≈ 4 × 10–6 cm2 s–1, and combined with the triplet exciton lifetime of the pristine polymer (τ = 1.05 μs) give an exciton diffusion length LD ≈ 22 nm. The results are compared to literature studies of singlet exciton diffusion in conjugated polymers and triplet exciton diffusion in molecular materials.

Efficient and color-stable white organic light-emitting diodes based on exciton management and phosphorescent sensitization

We report efficient and color-stable white organic light-emitting diodes (WOLEDs) by an approach combining the concepts of exciton management and phosphorescent sensitization to achieve 100% internal quantum efficiency and reduce efficiency roll-off. The approach employs a layer of 4-bis-(1-naphthyl-N-phenylamino)-biphenyl (NPB) as a fluorescent blue emitter to collect all singlet excitons for blue emission, and fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) as a phosphorescent emitter and sensitizer in other layer to harvest all triplet excitons for generating green light and sensitizing a fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran (DCJTB) for orange–red emission. By carefully adjusting the concentration of DCJTB and the thickness of the NPB layer, efficiency roll-off has been reduced and efficient balanced white light has been produced from the device ITO/PEDOT:PSS (30nm)/poly(vinylcarbazole) (PVK):Ir(ppy)3:DCJTB (100:5:0.4 in wt.) (60nm)/NPB (4nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10nm)/tris(8-hydroxyquinoline)aluminum (Alq3) (20nm)/LiF (1nm)/Al. The operating principles have been studied and the results show that the long diffusion length of triplet excitons and long-range nonradiative energy transfer between the triplets of the phosphorescent emitter and the singlets of the fluorescent dye are crucial to realize white light emission.

Roles of binding energy and diffusion length of singlet and triplet excitons in organic heterojunction solar cells

AbstractThe influence of binding energy and diffusion length on the dissociation of excitons in organic solids is studied. The binding energy and excitonic Bohr radius of singlet and triplet excitons are calculated and compared using the dissociation energy of 0.3 eV, which is provided by the lowest unoccupied molecular orbital offset in heterojunction organic solar cells. A relation between the diffusion coefficient and diffusion length of singlet and triplet excitons is derived using the Förster and Dexter transfer processes and are plotted as a function of the donor‐acceptor separation. The diffusion length reduces nearly to a zero if the distance between donor and acceptor is increased to more than 1.5 nm. It is found that the donor‐acceptor separation needs to be ≤ 1.5 nm for easy dissociation on singlet excitons leading to better conversion efficiency in heterojunction organic solar cells. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct Measurement of the Triplet Exciton Diffusion Length in Organic Semiconductors

We present a new method to measure the triplet exciton diffusion length in organic semiconductors. N,N'-di-[(1-naphthyl)-N,N'-diphenyl]-1,1'-biphenyl)-4,4'-diamine (NPD) has been used as a model system. Triplet excitons are injected into a thin film of NPD by a phosphorescent thin film, which is optically excited and forms a sharp interface with the NPD layer. The penetration profile of the triplet excitons density is recorded by measuring the emission intensity of another phosphorescent material (detector), which is doped into the NPD film at variable distances from the injecting interface. From the obtained triplet penetration profile we extracted a triplet exciton diffusion length of 87±2.7 nm. For excitation power densities >1 mW/mm(2) triplet-triplet annihilation processes can significantly limit the triplet penetration depth into organic semiconductor. The proposed sample structure can be further used to study excitonic spin degree of freedom.

Optimization of yellow phosphorescent organic light-emitting devices based on triplet exciton diffusion length

Study on the diffusion of triplet excitons from light emitting layer (EML) in organic light-emitting devices doped with yellow phosphorescent material to the adjacent hole transporting layer (HTL) is carried out, in which a non-doped red phosphorescent ultra-thin layer as an exciton-sensing layer is set in various position of HTL. A diffusion length of triplet exciton between 3 nm and 5 nm is inferred from the observed and disappeared red light emission in electroluminescent spectra. For further device optimization, either 5 nm 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) or 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as a exciton blocking layer is introduced to block the exciton diffusion. The maximum current efficiency and power efficiency of the optimized device are reached to 24.6 cd/A and 16.3 lm/W, respectively. The high performance is ascribed to confined diffusion of triplet excitons from light-emitting zone and higher radiation efficiency of the triplet exciton.

Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals

We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.

Measuring the exciton diffusion length of C60 in organic planar heterojunction solar cells

AbstractPlanar copper phthalocyanine (CuPc)/C60 heterojunction solar cells with a 2 nm layer of bathocuproine (BCP) inserted into the C60 layer were fabricated and characterized. The 2 nm BCP layer in the devices was used as an electronically selective sieve allowing the electron current through but blocking the excitons in the C60 layer. By combining the experimental results with the optical modeling, the effective triplet exciton diffusion length in C60 was confirmed to be 30–35 nm under the device working condition. We demonstrate a simple, useful method to determine the exciton diffusion lengths of organic electron acceptors.

Study on triplet exciton diffusion length of mCP in phosphorescent organic light-emitting devices using electroluminescent spectra

Electroluminescent (EL) spectra was employed to probe the triplet exciton diffusion length ( L T ) of a commonly used host material of N,N′-dicarbazolyl-3,5-benzene (mCP) in phosphorescent organic light-emitting devices (OLEDs). By varying the film thickness of bis [2-(4-tertbutylphenyl) benzothiazolato-N,C 2], iridium (acetylacetonate) [(t-bt) 2Ir(acac)] phosphor doped layer within 30 nm thick mCP layer, a series of devices were fabricated to investigate the EL characteristics. The results showed that with the increasing doped layer thickness ( d), both (t-bt) 2Ir(acac) emission peaks at 562 nm and mCP emission centered at 403 nm were observed. Moreover, the relationship between mCP EL intensity and d was detected. The L T was induced by an abrupt decrease in variation of mCP EL intensity when d is increased from 10 to 15 nm, and the reason to cause this phenomenon was investigated. The L T of mCP approximately to 15 nm was perfectly consistent to the result of 16±1 nm, which was calculated by the traditional steady-state diffusion model.

Diffusion length of triplet excitons in organic semiconductors

The exciton diffusion length in organic semiconductors strongly impacts the efficiency of energy harvesting organic optoelectronics. We demonstrate an experimental technique to estimate the diffusion length of triplet excitons in a ladder-type conjugated organic semiconductor from quenching of the triplet-triplet absorption in the presence of acceptor molecules. The diffusion lengths are consistent with a phenomenological random-walk model. Experiments on two different molecular weights of the polymer sample show that the diffusion length of triplet excitons is in the few micrometers range and depends upon the size of the $\ensuremath{\pi}$-conjugated system.