Vertical Phase Separation Research Articles

Achieving Balanced Crystallization Kinetics of Donor and Acceptor by Sequential‐Blade Coated Double Bulk Heterojunction Organic Solar Cells

AbstractSequential deposition has great potential to achieve high performance in organic solar cells due to the resulting well‐controlled vertical phase separation. In this work, double bulk heterojunction organic solar cells are fabricated by sequential‐blade cast in ambient conditions. Probed by the in situ grazing incidence X‐ray diffraction and in situ UV–vis absorption measurements, the seq‐blade system exhibits a different tendency from each of the binary films during the film formation process. Due to the extensive aggregation of FOIC, the binary PBDB‐T:FOIC film displays a strong and large phase separation, resulting in low current density (Jsc) and unsatisfactory power conversion efficiency. In the seq‐blade cast system, the bottom layer PBDB‐T:IT‐M produces many crystal nuclei for the top layer PBDB‐T:FOIC, so the PBDB‐T molecules are able to crystallize easily and quickly. Balanced crystallization kinetics between polymer and small molecule and an ideal percolation network in the film are observed. In addition, the balanced crystallization kinetics are favorable toward realizing lower recombination loss through charge transport processes.

Single-Step Formation of a Low Work Function Cathode Interlayer and n-type Bulk Doping from Semiconducting Polymer/Polyethylenimine Blend Solution.

The use of polyethylenimine (PEI) as a thin interlayer between cathodes and organic semiconductors in order to reduce interfacial Ohmic losses has become an important approach in organic electronics. It has also been shown that such interlayers can form spontaneously because of vertical phase separation when spin-coating a blended solution of PEI and the semiconductor. Furthermore, bulk doping of semiconducting polymers by PEI has been claimed. However, to our knowledge, a clear delineation of interfacial from bulk effects has not been published. Here, we report a study on thin films formed by spin-coating blended solutions of PEI and poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} [P(NDI2OD-T2)] on indium tin oxide. We observed the vertical phase separation in such films, where PEI accumulates at the bottom and the top, sandwiching the semiconductor layer. The PEI interlayer on ITO reduces the electron injection barrier to the minimum value determined by Fermi level pinning, which, in turn, reduces the contact resistance by 5 orders of magnitude. Although we find no evidence for doping-induced polarons in P(NDI2OD-T2) upon mixing with PEI from optical absorption, more sensitive electron paramagnetic resonance measurements provide evidence for doping and an increased carrier density, at a very low level. This, in conjunction with an increased charge carrier mobility due to trap filling, results in an increase in the mixed polymer conductivity by 4 orders of magnitude relative to pure P(NDI2OD-T2). Consequently, both interfacial and bulk effects occur with notable magnitude in thin films formed from blended semiconductor polymer/PEI solution. Thus, this facile one-step procedure to form PEI interlayers must be applied with attention, as modification of the bulk semiconductor polymer (here doping) may occur simultaneously and might go un-noticed if not examined carefully.

Vertical Phase Separated Cesium Fluoride Doping Organic Electron Transport Layer: A Facile and Efficient “Bridge” Linked Heterojunction for Perovskite Solar Cells

AbstractIn perovskite solar cells (PSCs), the interfaces of the halide perovskite/electron transport layer (ETL) and ETL/metal oxide electrode (MOE) always attract and trap free carriers via the surface electrostatic force, altering quasi‐Fermi level (EFq) splitting of contact interfaces, and significantly limit the charge extraction efficiency and intrinsic stability of devices. Herein, a graded “bridge” is first reported to link the MOE and perovskite interfaces by self vertical phase separation doping (PSD), diminishing the side effect of notorious ionic defects via both reinforced interface Ebi and the vacancies filling. Experimental and theoretical results prove that the inhomogeneous distribution of CsF in the bulk or surface of PC61BM would not only form metal–oxygen (M–O) dipole on MOE, reinforcing the interface Ebi, but also create a graded energy bridge to alleviate the disadvantage of band offset raised by the enhanced interface Ebi, which significantly avoid the carrier accumulation and recombination at defective interfaces. Employing PSD, the power conversion efficiency of the devices approaches 21% with a high open‐circuit voltage (1.148 V) and delivers a high stability of 89% after aging 60 days in atmosphere without encapsulation, which is the highest efficiency of organic electron transport layers for n–i–p PSCs.

In situ Measuring Film-Depth-Dependent Light Absorption Spectra for Organic Photovoltaics.

Organic donor–acceptor bulk heterojunction are attracting wide interests for solar cell applications due to solution processability, mechanical flexibility, and low cost. The photovoltaic performance of such thin film is strongly dependent on vertical phase separation of each component. Although film-depth-dependent light absorption spectra measured by non-in situ methods have been used to investigate the film-depth profiling of organic semiconducting thin films, the in situ measurement is still not well-resolved. In this work, we propose an in situ measurement method in combination with a self-developed in situ instrument, which integrates a capacitive coupled plasma generator, a light source, and a spectrometer. This in situ method and instrument are easily accessible and easily equipped in laboratories of the organic electronics, which could be used to conveniently investigate the film-depth-dependent optical and electronic properties.

13.2% Efficiency of Organic Solar Cells by Controlling Interfacial Resistance Resulting from Well-Distributed Vertical Phase Separation

Two strategies were investigated to improve the efficiency of organic solar cells (OSCs) with the aim of controlling the interfacial resistance in the devices: the use of a ternary active layer and...

Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells.

Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4, 5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Low-voltage tunable liquid crystal lens fabricated with self-assembled polymer gravel arrays.

A novel approach for fabricating liquid crystal (LC) lenses is presented. The approach involves the use of a photocurable prepolymer dispersed in a cell fabricated with vertically aligned substrates. A radial gradient UV irradiation intensity distribution is produced using a radial variable neutral density filter. Under UV irradiation, the prepolymer diffuses and is then polymerized on the substrate surfaces owing to vertical phase separation. After polymerization, the diameter of the self-assembled polymer gravel on the substrates has a radial gradient distribution, causing a radial gradient pretilt angle (RGPA) distribution on the substrates and producing LC lenses. By numerical simulation, RGPA LC lens has significantly lower supplied voltage than conventionally hole-patterned electrode (HPE) LC lens, and higher lens power. In the experiment, the fabricated RGPA LC lens with aperture size of 5 mm possesses a simple planar electrode structure, low operation voltage (< 4 V), small root mean square wavefront error (< 0.08 λ), and acceptable focusing quality. By the overdriving scheme, the switched-off time of the fabricated RGPA LC lens reaches 0.27 s. With the novel approach, low-voltage LC lenses with different optical aperture sizes can be easily fabricated.

Phase segregation effect on TIPS pentacene crystallization and morphology for organic thin film transistors

In this study, we report that the vertical phase separation between a small-molecule organic semiconductor and a polymer additive can be utilized to drive semiconductor crystallization, enhance thin film morphology, and improve device performance of solution-processed organic thin film transistors (OTFTs). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was demonstrated as a benchmark semiconductor material to blend with a polyacrylate polymer additive poly(2-ethylhexyl acrylate) (P2EHA), it was found that a vertical phase segregation occurred between TIPS pentacene and P2EHA, providing a confinement of anisotropic semiconductor crystallization and resulting in an elevated TIPS pentacene concentration at the charge transport interface. Accordingly, distinctive TIPS pentacene thin film morphology in terms of areal coverage, grain width, and crystal orientation was obtained by varying the loading ratio of the P2EHA polymer additive. Bottom-gate and bottom-contact OTFTs were fabricated to test charge transport and a hole mobility of up to 0.27 cm2/Vs was demonstrated with 80% loading ratio of P2EHA. The effective experimental method and important results as we showcased in this work can be applied to drive crystallization and optimize film morphology of small-molecule organic semiconductors other than TIPS pentacene.

Fluorosurfactant-Improved Wettability of Low-Viscosity Organic Semiconductor Solution on Hydrophobic Surfaces for High-Mobility Solution-Processed Organic Transistors

Solution processibility is a unique superiority of organic semiconductors, which provides tremendous potential for the low-cost production of wearable and implantable electronics. However, the solution affinity to the substrate surface remains a serious dilemma due to the difficulty of liquid manipulation on hydrophobic surfaces, but the use of such surfaces is essential to improve device performance. Here, we applied a new-type additive, non-ionic fluorosurfactant, to add into the organic semiconductor solution. It not only can effectively improve the wettability of the low-viscosity solution on the hydrophobic surface, but also enhance the field-effect performance of the fabricated poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithophen-2-yl)-alt-[1, 2, 5]- thiadiazolo[3,4-c]pyridine] organic thin-film transistors (OTFTs) by forming vertical phase separation. Combined with the nanogrooved octadecyltrichlorosilane/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the obtained OTFTs exhibit excellent field-effect characteristics with mobility as high as 4.02 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , which far surpasses that of modern industrial amorphous silicon. This work provides a simple strategy to prepare continuous and uniform organic semiconductor thin film on the hydrophobic surface, indicating promising potential for future high-performance solution-processed organic electronic devices.

Improvement of field-effect transistor performance with highly oriented, vertically phase separated TIPS-pentacene/polystylene blends on high-k metal oxide films by using meniscus coating

We report on the quick deposition technique of the highly oriented 6, 13-bis (trisopropylsilylethynyl) pentacene (TIPS-pentacene) crystalline films from the polystylene (PS)/TIPS-pentacene blend solution onto the oxide dielectric films at an elevated temperature for fabricating organic field-effect transistors (FETs). A vertical phase separation between TIPS-pentacene and PS films on high-k metal oxide films leads to the reduction of operating voltage and the interfacial trap density at the gate insulator interface. The polarized optical microscopy image and in-plane X-ray diffraction analysis suggest the pi-stacking of TIPS-pentacene molecules along the direction parallel to the sweeping direction of meniscus coating. The highly oriented TIPS-pentacene crystalline films were obtained at 0.75–2.0 mm s−1 at 70 °C which was much faster than room temperature (0.01 mm s−1). These effects eventually lead to both the improvement of FET properties and device fabrication processes.

Intensification of Vertical Phase Separation for Efficient Polymer Solar Cell via Piecewise Spray Assisted by a Solvent Driving Force

Piecewise spray has the advantage of controlling the component distribution, i.e., constructing vertical phase‐separated active layers, to improve the charge generation efficiency of large‐scale polymer solar cells. However, it brings a connection problem because of the drastic difference in the donor/acceptor ratio of continuous coatings using the conventional solvent. Assisted by 1,3,5‐trimethylbenzene‐based solvent engineering, a solvent driving force is formed, resulting in a continuous interface for piecewise spray‐coated active layers. The blend film forms a bicontinuous interpenetrating network in the active layer and provides efficient percolation pathways for charge carrier transport, resulting in a considerable improvement in the photovoltaic performance. When the donor/acceptor ratio is 1:2 in the first coating and 2:1 in the second coating, and the thickness of each coating is 90 nm, the performance of the PBDB‐T‐2Cl:IT4F photovoltaic device shows a notable conversion efficiency of 12.29% with a high short‐circuit current density of 21.55 mA cm−2. Importantly, this piecewise spray technique can be used for large‐scale module fabrication.

Sequential Blade‐Coated Acceptor and Donor Enables Simultaneous Enhancement of Efficiency, Stability, and Mechanical Properties for Organic Solar Cells

AbstractAs a predominant fabrication method of organic solar cells (OSCs), casting of a bulk heterojunction (BHJ) structure presents overwhelming advantages for achieving higher power conversion efficiency (PCE). However, long‐term stability and mechanical strength are significantly crucial to realize large‐area and flexible devices. Here, controlling blend film morphology is considered as an effective way toward co‐optimizing device performance, stability, and mechanical properties. A PCE of 12.27% for a P‐i‐N‐structured OSC processed by sequential blade casting (SBC) is reported. The device not only outperforms the as‐cast BHJ devices (11.01%), but also shows impressive stability and mechanical properties. The authors corroborate such enhancements with improved vertical phase separation and purer phases toward more efficient transport and collection of charges. Moreover, adaptation of SBC strategy here will result in thermodynamically favorable nanostructures toward more stable film morphology, and thus improving the stability and mechanical properties of the devices. Such co‐optimization of OSCs will pave ways toward realizing the highly efficient, large‐area, flexible devices for future endeavors.

Flexible organic field-effect transistors based on 6,13-bis(triisopropylsilylethynyl) pentacene/polystyrene blend film prepared by electrostatic spray deposition

In this study, we fabricated flexible organic field-effect transistors (OFETs) (bottom-gate and top-contact architecture) based on the blend film composed of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) as a π-conjugated small-molecule semiconductor and polystyrene (PS) as a polymer insulator. Blend solutions including both the TIPS pentacene and PS molecules were electrosprayed onto bottom-gate electrodes which were patterned on polyethylene naphthalate films. The devices exhibited superior electrical characteristics due to well-defined vertical phase separation between the TIPS pentacene crystalline layer (top) and PS insulator (bottom). This study will contribute to a facile one-step printing process to produce high-performance flexible OFETs.

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.

Phase transitions and magnetic domain coexistence in Nd0.5Sr0.5MnO3 thin films

We present a study of the physical properties of perovskite oxide Nd0.5Sr0.5MnO3 (NSMO) thin films grown on (110)-oriented SrTiO3 substrates. In bulk form, NSMO displays coupled magnetic and electronic transitions from paramagnetic/insulator to ferromagnetic (FM)/metal and then to antiferromagnetic (AFM)/charge-ordered insulator with decreasing temperature. In thin films, the AFM ordering only occurs when the films exist in an anisotropic strain state such as those obtained on (110)-oriented cubic substrates. In this work, resonant X-ray reflectivity, soft X-ray photoemission electron microscopy (X-PEEM), and magnetometry measurements showed that the NSMO film displays both vertical and lateral magnetic phase separation. Specifically, the film consists of three layers with different density and magnetic properties. The FM and AFM properties of the main NSMO layer were probed as a function of temperature using soft X-ray magnetic spectroscopy, and the coexistence of lateral FM and AFM domains was demonstrated at 110 K using X-PEEM.

Phase Separation of P3HT/PMMA Blend Film for Forming Semiconducting and Dielectric Layers in Organic Thin-Film Transistors for High-Sensitivity NO2 Detection.

Formation of the semiconductor/dielectric double-layered films via vertical phase separations from polymer blends is an effective method to fabricate organic thin-film transistors (OTFTs). Here, we introduce a simple one-step processing method for the vertical phase separation of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) blends in OTFTs and their applications for high-performance nitrogen dioxide (NO2) sensors. Compared to the conventional two-step coated OTFT sensors, one-step processed devices exhibit a great enhancement of the responsivity from 116 to 1481% for 30 ppm NO2 concentration and a limit of detection of ∼0.7 ppb. Studies of the microstructures of the blend films and the electrical properties of the sensors reveal that the devices formed by the one-step vertical phase separation have better capability for the adsorption of NO2 molecules. Moreover, a careful adjustment of the blend ratio between P3HT and PMMA can further improve the performance of the NO2 sensors, ranging from sensitivity to selectivity and to the ability of recovery. This simple one-step processing method demonstrates a potential possibility for developing high-performance, low-cost, and large-area OTFT gas sensors.

High Performance Thick‐Film Nonfullerene Organic Solar Cells with Efficiency over 10% and Active Layer Thickness of 600 nm

AbstractDeveloping efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.

One-Step Coating Processed Phototransistors Enabled by Phase Separation of Semiconductor and Dielectric Blend Film.

Fabrication of organic thin-film transistors (OTFTs) via high throughput solution process routes have attracted extensive attention. Herein, we report a simple one-step coating method for vertical phase separation of the poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) blends as semiconducting and dielectric layers in OTFTs. These OTFTs can be used as phototransistors for ultraviolet (UV) light detection, where the phototransistors exhibited great photosensitivity of 597.6 mA/W and detectivity of 4.25 × 1010 Jones under 1 mW/cm2 UV light intensity. Studies of the electrical properties in these phototransistors suggested that optimized P3HT contents in the blend film can facilitate the improvement of film morphology, and therefore form optimized vertical phase separation of the PMMA and P3HT. These results indicate that the simple one-step fabrication method creates possibilities for realizing high throughput phototransistors with great photosensitivity.

Face-on orientation and vertical phase separation of p-DTS(FBTTh2)2/PC70BM induced by epitaxial crystallization of polymer interface layer

Abstract It is important to arrange the donor and acceptor phases and molecules within those phases in a way that optimizes relevant optoelectronic processes required for efficient operation of bulk-heterojunction (BHJ) organic solar cells (OSCs). 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) tends to adopt edge-on orientation with respect to PEDOT/PSS substrate, which is adverse to charge and exciton transport in the vertical direction. Besides, [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) tends to deposit on PEDOT/PSS substrate, while p-DTS(FBTTh2)2 is inclined to accumulate at surface, which blocks charge transport and increases injection barrier of charge. Here, we tried to optimize both the molecular orientation and vertical phase separation of p-DTS(FBTTh2)2/PC70BM with proper crystal size and bicontinuous network structure. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate], (PTB7-Th) was used as the interfacial layer to induce p-DTS(FBTTh2)2 to adopt face-on orientation because of epitaxial crystallization. At the same time, vertical phase separation can be improved due to that p-DTS(FBTTh2)2 tends to deposit on PTB7-Th. Consequently, the efficiency of exciton separation of devices with PTB7-Th as the interfacial layer is improved and both the hole and electron mobility were increased and became more balanced. As a result, the power conversion efficiency (PCE) increased from 5.47% to 6.74%.

Fullerene derivative layer induced phase separation and charge transport improvement for inverted polymer solar cells

The phase separation of donor and acceptor in bulk-heterojunction plays an important role on the exciton dissociation and charge carrier transport, which decides the performance of organic solar cells (OSCs). In this paper, the thin [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) layer was incorporated as interlayer between active layer and the polyethylenemine electron transport layer of OSCs to realize high efficiency. Both a high short-circuit current density and fill factor were achieved, resulting in a power conversion efficiency up to 9.17% for the optimal device. The incorporation of PC71BM layer not only induced vertical phase separation of active layer to improve exciton dissociation and charge transfer but also modified surface morphology to increase light absorption of the active layer. This study offers a simple and lost cost strategy to improve performance of OSCs.