Balanced Charge Carrier Transport Research Articles

Charge carrier transport and nanomorphology control for efficient non-fullerene organic solar cells

Single junction organic photovoltaic devices (OPVs) have exceeded 15% power conversion efficiency (PCE) with the help of fused ring based low-bandgap non-fullerene acceptors (NFAs). As a major type of NFA, the indacenodithiophene derivative NFA (IDTBR) has been shown to have superior OPV stability with outstanding VOC, but the efficiency is relatively lower compared to the reported OPV champion devices. Further improvements towards high efficiencies in this OPV system remains challenging due to the relatively poor charge carrier transport properties in the bulk heterojunction film, particularly the electron transport in small molecule non-fullerene acceptor network. Here we conducted detailed study on the dependence of carrier transport on BHJ donor–acceptor (D–A) composition. Our results show that the nano-morphology or phase aggregation of non-fullerene acceptor (NFA) molecules can be tuned via D–A composition in bulk heterojunction layer, and the improvement of electron mobility was shown to be enhanced by almost one order – from 1.23 × 10−6 cm2/V (D:A = 1:1 by weight) to 1.02 × 10−5 cm2/V (D:A = 1:2) – due to the improved connectivity of electron transport pathways. Further increase of NFA component content, however, has led to over-sized phase segregation, deteriorating the photovoltaic performance of organic soar cells. The optimized BHJ cell shows more balanced charge carrier transport and phase segregation, which yields a PCE of 10.79%. Furthermore, it shows a VOC as high as 1.03 V, which is ascribed to the significantly suppressed radiative and non-radiative recombination losses with bandgap-VOC offset Eg/q-VOC of only 0.55 V.

Reduced Efficiency Roll-Off in White Phosphorescent Organic Light-Emitting Diodes Based on Double Emission Layers.

We demonstrate high-efficiency white phosphorescent organic light-emitting diodes with low efficiency roll-off. The feature of the device concept is employing two phosphorescent emission layers (EMLs) separated by a mixed interlayer. Both the EMLs are doped by two phosphorescent dyes. The resulting white device with the optimized doping concentration shows a maximum efficiency of 31.0 cd/A with extremely low efficiency roll-off of 30.7 cd/A at 1000 cd/m2, 27.2 cd/A at 5000 cd/m2, and 25.5 cd/A at 10,000 cd/m2, respectively, without any outcoupling structures. This is enabled by the balanced charge carrier transport in EMLs, leading to broader exciton recombination zone.

Preferential Orientation of Tetrahedral Silicon-Based Hosts in Phosphorescent Organic Light-Emitting Diodes.

Vacuum-processed diphenylbis(3-(pyridine-2-yl)phenyl)silane (2PTPS), diphenylbis(3-(pyridine-3-yl)phenyl)silane (3PTPS), and diphenylbis(3-(pyridine-4-yl)phenyl)silane (4PTPS) have been used as electron-transporting host materials combined with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the hole-transporting host, which induce balanced charge carrier transport for high-efficiency phosphorescent organic light-emitting diodes. The 4PTPS-based organic light-emitting diodes with tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3] dopant showed highest current efficiency and external quantum efficiency of 53.54 cd/A and 15.61%, compared to 2PTPS (40.75 cd/A, 11.84%) and 3PTPS (29.35 cd/A, 8.54%). These results were attributed to the well-aligned structure with preferential horizontal orientation of the emitting material layer by the diffraction intensity distribution as a function of azimuthal angle in two-dimensional grazing incidence X-ray diffraction analysis. The molecular orientation of TCTA:4PTPS material with a narrow azimuthal intensity distribution had better priority to the horizontal direction than the other TCTA:2PTPS and TCTA:3PTPS materials, which is related to the charge transport as well as the device efficiency. We found that the preferential horizontal orientation of the co-host material with a balanced charge carrier was not affected by Ir(ppy)3 dopant with a homoleptic structure and bis-[2-(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium [Firpic] dopant with a heteroleptic structure in the co-host/dopant system.

Improving electroluminescent performance of blue light-emitting poly(fluorene-co-dibenzothiophene-S,S-dioxide) by end-capping

In this manuscript, we introduced an effective strategy to improve the electroluminescent performance of poly[(fluorene)-co-dibenzothiophene-S,S-dioxide]s (PFSO) derivatives by end-capping, which was carried out by introducing N-([1,1'-biphenyl]-4-yl)-N-(4-phenyl)-9,9-dimethyl-9H-fluoren-2-amine moiety (TF) at the end of the polymerisation. Compared with the pristine copolymer PFSO that end-capped by phenyl unit, the TF-end-capped copolymers exhibit improved highest occupied molecular orbital levels, which can facilitate hole injection and thus lead to more balanced charge carrier transport in the emissive layer. Of particular importance is that the resulting copolymers can be used to fabricate single-layer devices without hole transport layer, which dramatically reduced driving voltage and obviously increased luminous efficiency relative to device fabricated from the copolymer PFSO that end-capped by phenyl unit. The best performance is obtained from device based on PFSO-TF16, which exhibits an unprecedented turn-on voltage of 2.7 V and a maximum luminous efficiency of 4.45 cd A−1, with CIE coordinates of (0.15, 0.13). These results demonstrate that our end-capping strategy has great potential for the development of highly efficient blue light-emitting polymers for single-layer light-emitting devices.

High-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure

We demonstrate high-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure. Excellent performance is realized by using step-graded p- and n-type doping structure in orange homojunction device. The resulting orange homojunction device exhibits a maximum current efficiency of 30.0 cd/A and low efficiency roll-off. The improvements are mainly attributed to the utilization of step-graded doped profile, which facilitates balanced charge carrier injection and transport. Moreover, one optimized white homojunction device based on two complementary colors shows a maximum efficiency of 15.4 cd/A, and superior color-stability in a wide range of luminance.

Improving the charge carrier transport of organic solar cells by incorporating a deep energy level molecule.

Tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), a strong molecular acceptor, has been proved to be an excellent candidate to achieve the p-type doping effect. When F4-TCNQ is incorporated into a poly(3-hexylthiophene) (P3HT): indene-C60 bisadduct (ICBA) active layer, superior behavior upon inducing polymer donor excited electron transport is demonstrated due to the addition of a deep-lying lowest unoccupied molecular orbital (LUMO) from F4-TCNQ, leading to the realization of organic solar cells (OSCs) with an improved power conversion efficiency (PCE) of 5.83%, accounting for 29.6% enhancement. In the system of active layer, the low LUMO of F4-TCNQ can easily accept electrons, remarkably reducing electron/hole recombination, which contributes to the enhancement of the photoconductivity and charge carrier mobility, resulting in higher short-circuit current density (Jsc), and achieving a more balanced charge carrier transport, as well as an ideal fill factor (FF).

Highly efficient ternary polymer solar cells by optimizing photon harvesting and charge carrier transport

Ternary strategy exhibits some apparent advantages to improve the performance of polymer solar cells (PSCs), such as using two donors or two acceptors to enhance photon harvesting and maintain the simple cell fabrication process. The power conversion efficiency (PCE) of PSCs was increased from 4.89% to 5.54% by doping 4wt% SMPV1 into Si-PCPDTBT:PC71BM binary system. The PCE improvement should be attributed to the enhanced photon harvesting in the visible light range and optimized charge carrier transport by doping appropriate SMPV1 in the active layers. The champion PCE values were also increased from 4.89% to 5.73% or from 5.54% to 6.44% for binary or the optimized ternary PSCs by CHCl3 solvent vapor annealing treatment. The positive effect of appropriate SMPV1 doping ratio should be attributed to the efficient energy transfer from SMPV1 to Si-PCPDTBT and the more balanced charge carrier transport in the ternary active layers. The energy transfer from SMPV1 to Si-PCPDTBT can be clearly confirmed from the absorption, photoluminescence (PL) spectra and time-resolved transient PL spectra of pure and blend films. The more balanced charge carrier transport can be further demonstrated from the corresponding hole-only and electron-only devices.

Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4-b]thiophene Moiety for High-Performance Solar Cells.

By replacing the central thiophene of STDR, a sepithiophene terminated with two 3-ethylrhodanine moieties, with 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate, an A-D-Q-D-A-type small molecule has been developed for high-performance organic solar cells with improved photocurrent. STDR-TbT exhibits a significant bathochromic shift with a low optical bandgap of approximately 1.60 eV in the thin film. Accordingly, STDR-TbT shows broad external quantum efficiency spectral response up to 800 nm. A high short circuit current (Jsc) of 10.90 mA cm(-2) was achieved for STDR-TbT:PC71BM-based devices; this is significantly higher than that of STDR:PC71BM-based devices, Jsc: 5.61 mA cm(-2), with a power-conversion efficiency (PCE) of 5.05%. Compared with STDR-based devices, STDR-TbT-based devices show balanced charge carrier transport, better thin-film morphology, and favorable charge separation/collection.

Improving working lifetime and efficiency of phosphor doped organic light-emitting diodes

Long working lifetime and high efficient phosphorescent organic light-emitting diode (PHOLED) in which mixed host composed of wide-band-gap based 4, 7-diphenyl-1, 10-phenanthroline (Bphen) and (4,4'-bis(carbazol-9-yl)-biphenyl) (CBP) was demonstrated. The PHOLED with structure of ITO/MoO(3)/CBP:MoO(3) (15 v%, 30 nm)/CBP(10 nm)/([50v%:50v% CBP:Bphen]: 6v% Ir(ppy)(3))(30 nm)/Bphen (40 nm)/LiF (1 nm)/Al offers a peak power efficiency of 41.6 lm/W (a peak current efficiency of 39.8 cd/A)) at a low driving voltage of 3 V which increases by 55% and 27% compared to that of corresponding single-host (SH) and double emitting layer (DML) devices, respectively. Especially very long work lifetime (3530 hs) at an initial luminance of 500 cd/m(2) of the mixed hosted device is exhibited, rising by about 4.1 and 2.46 times relative to that of corresponding SH and DML devices. High efficiency and longer working lifetime was attributed to the absence of heterojunction and balanced charge carrier transport characteristics in the mixed host based OLED structure. The more detail mechanism was also presented.

Mechanisms of the improved organic photovoltaic performance by using electron-transporting CuPc derivative as anode modifying layer

The authors demonstrate the mechanisms of the improved performance of the organic photovoltaic (OPV) cells by using an electron-transporting copper phthalocyanine (CuPc) derivative, hexadecafluoro-copper-phthalocyanine (F16CuPc) as the anode buffer layer. The power conversion efficiency and long-term lifetime of the OPV cells with 3nm F16CuPc as the anode buffer layer are increased by 50% and 165%, respectively, compared to the reference cell. X-ray diffraction analysis suggests that increased crystalline of the CuPc layer is found, leading to increased hole mobility and exciton diffusion length when it is deposited on F16CuPc. As a result, a balanced charge carrier transport between CuPc and C60, decreased charge carrier recombination probability, and increased exciton dissociation efficiency can be obtained. Meanwhile, the built-in potential of the cell is also increased, which further promotes the extraction of the photogenerated charge carriers and increases the open circuit voltage of the cells. And the prolonged long-term lifetime of the cells is attributed to the suppressed formation of the reactive −O species on ITO surface and the prevented penetration of them into the organic layers.

Novel green light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide-arylamine derivatives

Green light-emitting polyfluorenes were synthesized by Suzuki polycondensation via introducing dibenzothiophene-S,S-dioxide-(di-)tri-phenylamine (G1 and G2) moieties and dibenzothiophene-S,S-dioxide (SO) unit into the polyfluorene backbone, respectively. PF–SO–G copolymers show a high thermal stability and a moderate photoluminescence quantum yield in the range of 20–40%. The lowest unoccupied molecular orbital (LUMO) levels reduce with increasing the content of SO unit in the polymers. The efficient energy transfer from fluorene segment to G1 or G2 unit occurred in the PL process, and the EL emission peaked at about 510 nm was exclusively from G1 or G2 unit. Incorporating SO unit into the polymer backbone makes the device performances improved. The maximal luminous efficiency of 9.0 cd A−1 with the CIE coordinates of (0.27, 0.56) was obtained for PF–SO15-G25 based on a single-layered device of ITO/PEDOT:PSS/polymer/CsF/Al. And the polymers (PF–SO15-G5)s exhibited a dramatic LE stability at high current densities, even though at the current density of 200 mA cm−2, the luminous efficiencies only dropped 10%. SO unit lowers the LUMO level, balances the injection and transportation of both electron and hole in the polymers, and therefore improves the device performances. The hole- and electron-only devices show that the hole and electron flux are well balanced, which demonstrates that (PF-SO15-G5)s are bipolar polymers with a balanced charge carrier transport.

Highly efficient green-emitting electrophosphorescent hyperbranched polymers using a bipolar carbazole-3,6-diyl-co-2,8-octyldibenzothiophene-S,S-dioxide-3,7-diyl unit as the branch

Green-emitting hyperbranched polymers with a bipolar 3,6-carbazole-co-2,8-dioctyl-dibenzothiophene-S,S-dioxide-3,7-diyl [Cz-co-DOSO] as the branch and fac-tris(2-phenylpyridine)iridium[Ir(ppy)3] as the core were synthesized by Suzuki polycondensation. The Cz-co-DOSO branch with a triplet energy level above 2.5 eV can effectively prevent energy feedback from the phosphorescent core. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the copolymers reduce dramatically by over 0.6 and 0.8 eV with increasing DOSO content up to 45.5 mol% in the polymers, respectively. A maximum luminous efficiency of 50.5 cd A−1 and a maximum external quantum efficiency of 15.3% were obtained for PCzDOSO40Ir3 based on a single layer device of ITO/PEDOT:PSS/polymer/CsF/Al, which were improved by over 13 times compared with PCzIr3. The hole- and electron-only devices show that the hole and electron flux are well balanced, which demonstrates that PCzDOSO40Ir3 is a bipolar polymer with a balanced charge carrier transport.

Evolved Phase Separation toward Balanced Charge Transport and High Efficiency in Polymer Solar Cells

Understanding effect of morphology on charge carrier transport within polymer/fullerene bulk heterojunction is necessary to develop high-performance polymer solar cells. In this work, we synthesized a new benzodithiophene-based polymer with good self-organization behavior as well as favorable morphology evolution of its blend films with PC(71)BM under improved processing conditions. Charge carrier transport behavior of blend films was characterized by space charge limited current method. Evolved blend film morphology by controlling blend composition and additive content gradually reaches an optimized state, featured with nanoscale fibrilla polymer phase in moderate size and balanced mobility ratio close to 1:1 for hole and electron. This optimized morphology toward more balanced charge carrier transport accounts for the best power conversion efficiency of 3.2%, measured under simulated AM 1.5 solar irradiation 100 mW/cm(2), through enhancing short circuit current and reducing geminate recombination loss.

Role of balanced charge carrier transport in low band gap polymer:Fullerene bulk heterojunction solar cells

AbstractLowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylenevinylene] and 6,6‐phenyl C61‐butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Multiscale molecular simulations of the nanoscale morphologies of P3HT:PCBM blends for bulk heterojunction organic photovoltaic cells

In this study, we developed a multiscale molecular simulation framework including coarse-grained (CG) molecular simulation, reverse-mapping, and morphology evaluation schemes to investigate the nanoscale morphologies of bulk heterojunction (BHJ) blend films comprising poly(3-hexylthiophene) (P3HT) and the methanofullerene derivative PCBM. A stable and phase-separated blend film with the fibrillar P3HT structure was observed after CG simulation of the thermal annealing process, and by the reverse-mapping technique the atomistic details—showing strong π–π interaction between thiophene rings—were retrieved. To evaluate the morphologies of P3HT:PCBM blends, a spatial-discretization scheme was developed. With such a scheme, we estimated the average domain sizes, interface-to-volume ratios, and percolation ratios of the blends at different P3HT:PCBM weight ratios. The average domain sizes determined through these simulations were in excellent agreement with those reported experimentally. Moreover, our simulations indicated that blend films having weight ratios close to 1 : 1 would have the highest interface-to-volume ratio and the most balanced charge carrier transport in both the P3HT and PCBM phases, consistent with the experimental observation that a 1 : 1 weight ratio is optimal for P3HT:PCBM blends. The multiscale molecular simulation framework proposed herein can be extended to investigating the morphologies of other photoactive layers of organic photovoltaic cells.

Design of organic tandem solar cells using PCPDTBT:PC61BM and P3HT:PC71BM

We conducted optical and electrical simulations with the goal of determining the optimal design for conjugated polymer-fullerene tandem solar cells using poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT): [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as a bottom cell and poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as a top cell. The effects of photon density, absorption, balanced and unbalanced charge carrier transport, and bimolecular recombination in the two subcells were incorporated into the simulations. We found that the maximum energy conversion efficiency (η) is 9% when charge carrier mobilities in both top and bottom cells are balanced. However, the efficiency drops significantly if the carrier mobilities are unbalanced in either the top or bottom cell. In addition, we found that unbalanced carrier mobilities in the top cell require a reduction in the thickness of the bottom cell whereas unbalanced bottom cell mobilities require an increase in the thickness of the bottom cell to compensate for the reduced current.

Enhancement of spectral stability and efficiency on blue light-emitters via introducing dibenzothiophene- S, S-dioxide isomers into polyfluorene backbone

Dibenzothiophene- S, S-dioxide isomer (FSO) monomers were copolymerized with extensively employed 9,9-dioctyl-2,7-fluorene monomer by Suzuki polycondensation. By introducing FSO unit into polyfluorene backbone, the spectral stability and efficiency of the blue-emitting polyfluorenes are significantly improved. All of the obtained copolymers and their devices exhibit stable photoluminescence (PL) and electroluminescence (EL) spectra, respectively, upon change of thermal annealing temperature and current densities. Furthermore green emission which is usually associated with excimer/aggregation or defects is absent in PL and EL spectra. In fact, the FSO unit acts as a deep trap for electron in the polyfluorene backbone, which could suppress the long wavelength emission. Moreover, the FSO unit lowers LUMO energy levels of the polyfluorenes, thus can maintain a balanced charge carrier injection and transport in the device based on the polymers, and therefore improves the device efficiencies. The device based on PF-2,8FSO1 and PF-3,7FSO5 show an external quantum efficiency (EQE) of 3.6% and a luminance efficiency (LE) of 3.7 cd/A with CIE coordinate of (0.16, 0.07), and an EQE of 3.8% and a LE of 4.6 cd/A with CIE coordinate of (0.15, 0.12), respectively. The results indicate that poly (dibenzothiophene- S, S-dioxide -co-9,9-dioctyl-2,7-fluorene)s could be a promising candidate for blue-emitting polymers with spectral stability and high efficiency.

Balanced ambipolar charge carrier mobility in mixed layers for application in hybrid white organic light-emitting diodes

We investigate the electron and hole mobility in mixed layers of N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine and bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum with different mix ratios, using both space-charge limited currents of single-carrier devices with electrically doped charge transport layers and time-of-flight measurements. Both experimental methods yield consistent results. The 1:1 blend shows balanced ambipolar charge carrier transport, which is advantageous for the application as exciton blocking interlayer in hybrid white organic light-emitting diodes: The electroluminescence spectrum is rather stable against changes in interlayer thickness and driving current. Moreover, the external quantum efficiency is enhanced by a factor of 2.5 as compared to a device without interlayer.