Cathode Interfacial Materials Research Articles

An alcohol-soluble small molecule as efficient cathode interfacial layer materials for polymer solar cells

Abstract Interface modification makes an important contribution to improving the optoelectronic performance and stability of polymer solar cells (PSCs). Here, the hexamethylenetetramine (C6H12N4) was successfully introduced into PSCs as cathode interface layers (CILs) using environmental-friendly methanol as solvent. Compared to the PSCs without CILs, the open circuit voltage (VOC) and short-circuit current density (JSC) of the device with C6H12N4 CILs was increased from 0.44 V to 12.73 mA cm−2 to 0.77 V and 15.89 mA cm−2, respectively. More importantly, under the same experimental conditions, the optimal power conversion efficiency (PCE) of the device with C6H12N4 CILs was improved to 7.79%, which is 185% higher than that of the device without C6H12N4 CILs (2.73%). We found that the active layer PTB7-Th:PC71BM showed a smoother interface morphology and better hydrophilicity after insertion of CILs, which facilitated enhance the physical contact between active layers and cathode as well as electron extraction and transport. The results show that the introduction of C6H12N4 as CILs provides an effective method for preparing high-performance PSCs.

Alcohol-soluble fluorene derivate functionalized with pyridyl groups as a high-performance cathode interfacial material in organic solar cells

The synthesis and application as a cathode interlayer in organic photovoltaics of a fluorene derivative with pyridyl functional chains are presented.

Recent advances on aromatic-diimide based cathode interfacial materials for polymer solar cells

As an emerging photovoltaic technology, polymer solar cells (PSCs) have attracted broad interest for both academic and industry communities due to their outstanding advantages, such as lightweight, low cost and easy to process in large areas. In recent years, the power conversion efficiency of PSCs has been improved rapidly, exceeding 17% for the state-of-the-art devices. For device optimization, it has been demonstrated that the efficiency and stability can be significantly improved by constructing efficient organic cathode interfacial materials. Specially, aromatic-diimide based cathode interfacial materials have attracted much attention due to their advantages of excellent electron transport, easily chemical modification and suitable energy levels. In this review, along with the briefly introduction on the basic functions of cathode interlayer, recent progress of cathode interlayer based on perylene diimides and naphthalene diimides was summarized. At last, the perspective and challenge were proposed according to the requirements of practical application.

3D surfactant-dispersed graphenes as cathode interfacial materials for organic solar cells

Graphene dispersions in low-boiling-point green solvents have wide applications in coatings, conducting inks, batteries, electronics and solar cells. Two three-dimensional (3D) cathode interfacial materials (CIMs) (1,3,5,7,9,11,13,15-octa-(9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-vinylpentacyclo-octasiloxane) (POSSFN) and (1,3,5,7-tetra-(9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-adamantane) (ADMAFN) are excellent surfactants for dispersing graphene in ethanol at the concentration of 0.97–1.18 mg mL−1, in agreement with their calculated large adsorption energies on graphene. The results of electron spin resonance, Raman, scanning Kelvin probe microscopy and X-ray photoelectron spectroscopy measurements indicate that the amino groups could n-dope graphene or form dipole interaction with graphene. The two 3D-surfactant-based graphene composites (POSSFN-G and ADMAFN-G) can work as high-performance CIMs in organic solar cells (OSCs), which improve the power conversion efficiency (PCE) of the OSCs based on PM6:Y6 to 15.9%–16.1%. ADMAFN forms dipole interaction with graphene in ADMAFN-G and the composite CIM delivers high PCE of 16.11% in the OSCs, while POSSFN forms n-doped composition with graphene in POSSFN-G which works well as thicker CIM film in the OSCs.

Cathode engineering with perylene-diimide interlayer enabling over 17% efficiency single-junction organic solar cells

In organic solar cells (OSCs), cathode interfacial materials are generally designed with highly polar groups to increase the capability of lowering the work function of cathode. However, the strong polar group could result in a high surface energy and poor physical contact at the active layer surface, posing a challenge for interlayer engineering to address the trade-off between device stability and efficiency. Herein, we report a hydrogen-bonding interfacial material, aliphatic amine-functionalized perylene-diimide (PDINN), which simultaneously down-shifts the work function of the air stable cathodes (silver and copper), and maintains good interfacial contact with the active layer. The OSCs based on PDINN engineered silver-cathode demonstrate a high power conversion efficiency of 17.23% (certified value 16.77% by NREL) and high stability. Our results indicate that PDINN is an effective cathode interfacial material and interlayer engineering via suitable intermolecular interactions is a feasible approach to improve device performance of OSCs.

Efficient Cathode Interfacial Materials Based on Triazine/Phosphine Oxide for Conventional and Inverted Organic Solar Cells

Cathode interfacial layers (CIL) have been applied in organic solar cells (OSCs) for the enhancement of photovoltaic characteristics. Most of them are employed in either conventional organic solar cells (COSCs) or inverted organic solar cells (IOSCs) only. Herein, we have designed and synthesized two cathode interfacial materials, namely, 3-(4,6-bis(4-bromophenoxy)-1,3,5-triazin-2-yl)-2,6-difluorophenyl)diphenylphosphine oxide (Br-PO-TAZ) and 4,4′-((6-(3-(diphenylphosphoryl)-2,4-difluorophenyl)-1,3,5-triazine-2,4-diyl)bis(oxy))dibenzonitrile (CN-PO-TAZ), and utilized them as CILs for both COSCs and IOSCs. The incorporation of our new CIL layers significantly enhanced the photovoltaic performance compared to COSCs and IOSCs without the CILs. The CN-PO-TAZ exhibited a power conversion efficiency (PCE) of 8.19% for COSCs and 8.33% for IOSCs, whereas Br-PO-TAZ yielded a PCE of 8.15% for COSCs and 8.23% for IOSCs, respectively. The improved performance was attributed to the multiple favorable factors: significantly reducing leakage current, decreasing series resistance, suppressing recombination, efficient charge transport and collection. Moreover, the CIL layers helped for sustaining device stability because they served as an internal shield against humidity.

Water–Alcohol-Soluble Hyperbranched Polyelectrolytes and Their Application in Polymer Solar Cells and Photocatalysis

We report here a series of water–alcohol-soluble hyperbranched polyelectrolytes, which can be used as both cathode interfacial materials (CIMs) in polymer solar cells (PSCs) and organic photocataly...

Poly[2,7-(9,9-dihexylfluorene)]-block-poly[2-(dimethylamino)ethylmethacrylate] as resilient cathode interlayers in polymer solar cells: the effect of block ratios

A series of alcohol-soluble diblock copolymers PF15-b-PDMAEMA975 are applied as cathode interlayers in the PSCs based on PTB7-Th:PC71BM. Herein, the PF units are fixed at 15, and the PDMAEMA units vary as 60, 150 and 975. The device results show that varying the length of PDMAEMA block has a little effect on the performance of the devices, and all the devices show comparable and even higher performances in comparison to the devices based on the well-known PFN-Br. Control experiment indicates that the PF unit is necessary for the high performances. The improvement of the performance should be attributed to the reduction of work function and balance of charge carrier mobilities upon addition of the cathode interlayers. We believe that this type of copolymers should be resilient to the block ratios, and may be developed to a potential cathode interfacial material in highly efficient organic solar cells.

Molecular Engineering on Bis(benzothiophene-S,S-dioxide)-Based Large-Band Gap Polymers for Interfacial Modifications in Polymer Solar Cells.

The development of effectively universal interfacial materials for both conventional and inverted polymer solar cells (PSCs) plays a very crucial role in achieving highly photovoltaic performance and feasible device engineering. In this study, two novel alcohol-soluble conjugated polymers (PBSON-P and PBSON-FEO) with bis(benzothiophene-S,S-dioxide)-fused aromatics (FBTO) as the core unit and amino as functional groups are synthesized. They are utilized as universal cathode interfacial layers for both conventional and inverted PSCs simultaneously. Ascribing to the enlarged conjugated planarity and higher electron affinity for an FBTO unit, both PBSON-P and PBSON-FEO exhibit versatile electron-transporting abilities. They show wide band gaps that are important for light absorption in inverted PSCs, at which point PBSON-P and PBSON-FEO are more progressive than some of the reported small band gap cathode interfacial materials. Importantly, PBSON-P and PBSON-FEO display deep highest occupied molecular orbital energy levels, which can block holes at the cathode and thus increase the fill factor. As a result, both conventional and inverted PSCs using PBSON-P and PBSON-FEO as cathode interlayers realize high photovoltaic performance. Therefore, this series of novel polymers are amphibious cathode interfacial materials for high-performance conventional and inverted PSCs.

Triarylphosphine Oxide as Cathode Interfacial Material for Inverted Perovskite Solar Cells

AbstractMetal halide perovskite solar cells (PSCs) in the inverted planar p‐i‐n configuration often employ phenyl‐C61‐butyric acid methyl ester (PC61BM) as electron transport layer, onto which Ag is deposited as outer electrode. However, the energy offset between PC61BM and Ag imposes an energy barrier for electron extraction. In this work, to improve the contact quality of this stack, a small organic molecule (2‐(1,10‐phenanthrolin‐3‐yl)naphth‐6‐yl)diphenylphosphine oxide (DPO) as a cathode interfacial material (CIM), inserted between PC61BM and Ag, is introduced. In devices with the indium tin oxide (ITO)/NiOx/methylammonium lead iodide (MAPbI3)/PC61BM/CIM/Ag configuration, it is found that this results in fill factor (FF) and short‐circuit current density values (JSC) that are up to ≈34% and ≈1 mA cm−2 higher, respectively, compared to DPO‐free devices. Inserting additional thin ZnO nanoparticle layers further improves the contact quality, leading to a power conversion efficiency of 18.2%. Semitransparent PSCs, utilizing DPO as an interlayer buffer layer are also realised. Resultant devices exhibit improved performance compared to DPO‐free devices. This proves that DPO withstands the sputtering of ITO, and may thus find application in perovskite‐based tandem devices. It is concluded that DPO acts as an excellent cathode modifier, opening new device‐engineering opportunities for p‐i‐n PSCs, especially in their semitransparent implementation.

Intermolecular n-Doping Nonconjugated Polymer Cathode Interfacial Materials for Organic Solar Cells

Three alcohol-soluble nonconjugated polymers poly [p-(N,N-dimethylamino)styrene] (PSN), poly[p-(N,N-dimethylamino)styrene] oxide (PSO), and poly[p-(N,N-dimethylamino)styrene] mesylate (PSM) were devised and synthesized for the application as cathode interlayers in the organic solar cells (OSCs) based on PBDB-T:IT-M. All of the polymers contain a highly polarized amino or ammonium group at the side chains, which can improve electron collection ability of the cathode interfacial layer of the OSCs. Both the amino and ammonium group with a negative ion of oxygen allow PSN and PSO to possess intermolecular n-doping effect with the IT-M acceptor at the interface. The understanding of the relationship between the n-doping effect and the cathode interlayer structures may offer additional insights to further develop novel cathode interfacial materials for OSCs.

Benzothiadiazole-based conjugated polyelectrolytes for interfacial engineering in optoelectronic devices

Abstract Polar semiconducting polymers based on a conjugated polymer backbone endowed with chemically anchored polar groups on the side chains have proved to be particularly interesting as optimization layer at organic/cathode interface in optoelectronic devices. In particular, the pendant phosphonate groups impart water-alcohol solubility allowing easy solution processing, and improve electron injection thanks to both a favorable interfacial dipole of phosphonate groups and an intense coordination interaction between the phosphonate groups and Al cathode. In this work we synthesize alternating fluorene-benzothiadiazole copolymers by proposing a post-polymerization reaction to insert the phosphonate groups. Thanks to this approach it is possible to use standard Suzuki coupling conditions, simplifying the process of synthesis, purification and characterization. The polymer Poly[9,9-bis(6′-diethoxylphosphorylhexyl)-alt-benzothiadiazole] (P2), is tested in conventional organic solar cells as cathode interfacial layers showing, with respect to the control device, an increasing of all the photovoltaic parameters, with a final power conversion efficiency that reaches 5.35% starting from 4.6%. The same trend is observed for multilayered polymer light-emitting diodes with an external quantum efficiency of the P2-based PLED enhanced of 1.5 times with respect to the basic devices with bare Al cathode, and negligible roll-off efficiency. The synergic effects of energy gap modulation and of polar phosphonated pendant functionalities of P2 are compared with the corresponding fluorene-based polar homopolymer. Our results show that, not only a proper selection of side functionalities, but also the tailoring of the energy gap of cathode interfacial materials (CIMs) is a possible effective strategy to engineer cathode of different optoelectronic devices and enhance their performance.

Perylene diimide-based cathode interfacial materials: adjustable molecular structures and conformation, optimized film morphology, and much improved performance of non-fullerene polymer solar cells

Through the regulation of PDI derivatives, the optimized photovoltaic performance was achieved by the adjustable molecular packing and film morphology.

Solution-processable n-doped graphene-containing cathode interfacial materials for high-performance organic solar cells

Solution-processable n-doped graphene-containing cathode interfacial material with a low work function demonstrates 16.52% power conversion efficiency in organic solar cells.

A bifunctional conjugated polyelectrolyte for the interfacial engineering of polymer solar cells

In this work a novel combination of side chain functionalities, alkyl-phosphonate (EP) and alkyl-ammonium bromide (NBr) groups, on a polyfluorene backbone (PF-NBr-EP) was studied as cathode interfacial material (CIM) in polymer-based solar cells. The devices were made with a conventional geometry, with PTB7:PC71 BM as active layer and aluminum as metal electrode. The CIM showed good solubility in ethanol and film forming ability onto the active layer so that its deposition could be finely tuned. The interface engineering imparted by this CIM was assessed and discussed through kelvin probe force microscopy (KPFM), impedance spectroscopy, charge recombination and electron transport characterizations. To discriminate between the interfacial modifications imparted by the interlayer and its solvent, we included in this study a surface ethanol treated device. In the optimized conditions an average power conversion efficiency of 7.24% was obtained, which is about 60% higher when compared to devices made with bare Al and 26% when compared to devices made with a standard calcium/aluminum cathode. Besides performances, some insights about the devices shelf life stability are also presented. A good persistency through aging was found for the cathode interfacial engineering capabilities of PF-NBr-EP.

8‐Hydroquinolatolithium as a Highly Effective Solution‐Processable Cathode Interfacial Material in Inverted Perovskite Solar Cells with an Efficiency Over 19%

The solution‐processed 8‐hydroquinolatolithium (Liq) has first been demonstrated as a highly effective cathode interfacial layer (CIL) in inverted perovskite solar cells (PSCs). Compared with the control device without a CIL, the Liq‐based device possesses a faster charge transport rate, a lower surface work function of Ag electrode, a larger built‐in potential, and a higher charge recombination resistance, probably due to the formed interface dipole between [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and Ag induced by Liq, which eventually results in a great improvement of device performance. After optimization, the Liq‐based PSC has achieved a power conversion efficiency higher than 19%. These results will assuredly expand the present limited range of available solution‐processable cathode interfacial materials for low‐cost high‐performance PSCs.

Electron transport layer-free polymer solar cells show 40% higher efficiency than using ZnO transparent cathode

The poor compatibility of inorganic materials (electron transport layer) with the active layer and an ultrathin film of conjugated polymers becomes the great obstacle to producing high-quality polymer solar cells with high-throughput roll-to-roll (R2R) method. Novel electron transport layer-free polymer solar cells have been constructed by integrating the conjugated aminoalkyl-functionalized polymer, poly[3-(5-(9,9-bis(3-(dimethylamino)propyl)-7-methyl-9H-fluoren-2-yl)thiophen-2-yl)-2,5-bis(2-butyloctyl)-6-(5-methylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PDTFN) into the active layer. PDTFN was synthesized by different Suzuki reaction conditions to gain different molecular weights- small molecular weight (PDTFN-L) and high molecular weight (PDTFN-H). It is noteworthy that PDTFN-H can significantly enhance the power conversion efficiency of the solar cells that incorporate a photoactive layer composed of poly[(3-hexylthiophene)-2,5-diyl] (P3HT) and the fullerene acceptor [6,6-phenyl-C71-butyric acid methyl ester] (PC71BM). The power conversion efficiency varies from 2.5% for ZnO transparent cathode based solar cells to 3.6% for PDTFN-H based electron transport layer-free solar cells. This improved performance can be attributed to the following reasons: the PDTFN-H can enhance cathode electron transporting by introducing the PDTFN-H into active layer without extra coating the cathode interfacial material; besides, a favorable vertical phase separation of active layer was formed due to the PDTFN-H excellent compatibility with the bulk-heterojunction. These experimental results revealed that the electron transport layer-free polymer solar cells based on PDTFN-H can be a promising novel effective fabrication with simplified manufacturing process and lower cost.

Alcohol-Soluble Conjugated Polymers as Cathode Interlayers for All-Polymer Solar Cells

Conjugated polymers with polar side-chain components have successfully been used as cathode interfacial materials (CIMs) to improve the performances of polymer solar cells with fullerenes as acceptors. However, their uses and functions in all-polymer solar cells (all-PSCs) have not been well-investigated. Therefore, in this work, four conjugated polymers bearing different functional side chains, including dimethylamine, diethanolamine, and imidazole, were used as CIMs to study their effects on all-PSCs. With a combination of high-performing acceptor and donor polymers as active layers, the performances of the devices based on the four CIMs were compared with conventional devices with LiF/Al and Al as cathode. Compared to the devices with only Al as cathode, the devices comprising conjugated polymers as CIMs presented large improvement in power conversion efficiency from 2.7% to around 5.3%, which is comparable to the devices with LiF/Al as cathode. The encouraging results demonstrated that the use of conj...

High-Efficiency and Stable Organic Solar Cells Enabled by Dual Cathode Buffer Layers.

Various cathode interface materials have been used in organic solar cells (OSCs) to realize high performance. However, most cathode interface materials have their respective weaknesses in maximizing the efficiency or stability of OSCs. Herein, three kinds of alcohol-soluble cathode interfacial materials are combined with bathocuproine (BCP) to serve as multifunctional bilayer cathode buffers for the regular OSCs, and thus greatly enhanced power conversion efficiencies over 10.11% and significantly improved device stability have been achieved. By utilizing double interlayers, both light absorption and light distribution in active layer are improved. Furthermore, double interlayers offer favorable energy-level alignment, alcohol treatment, and duplicate protection of active layer, resulting in significantly reduced leakage current, suppressed recombination, and efficient charge collection. The improved device stability is related to the blocking effect of the complex formed between BCP and the metal electrode and the additional protection effect of the underlying alcohol-soluble materials. In view of the universal use of alcohol-soluble organic electrolyte as cathode buffer layers and by courtesy of the superiority of the double cathode layers relative to the monolayer controls, the double interlayer strategy demonstrated here opens a new way to fully exploiting the potential of OSCs and is believed to be extended to a wider application.

Substituent position engineering of phosphine oxide functionalized triazine-based cathode interfacial materials for flexible organic and perovskite solar cells

Abstract The grand challenge involved in inverted organic solar cell (IOSC) and perovskite solar cell (PSC) technology is to control the interfacial losses at the interface of the photoactive layer with the electron extracting electrode. Herein, three organic small molecules, namely o-(F)-PO-TAZ, m-(F)-PO-TAZ and p-(F)-PO-TAZ, consisting of phosphine oxide (P O) functionalized 1,3,5-triazine (TAZ) with a fluorine atom ( F) at the ortho (o)/meta (m)/para (p) positions of the phenoxy group attached to the TAZ unit are developed via a rational design approach and used as cathode interfacial layers (CILs) in flexible IOSCs and PSCs. We realized that the F position particularly affects the glass transition temperature (Tg) of the PO-TAZ derivatives, which in turn exerts a positive effect by maintaining robust and smooth morphological features. This gives the IOSC and PSC devices an F position-dependent performance. Particularly, ZnO/p-(F)-PO-TAZ-based flexible IOSC and PSC devices display not only high power conversion efficiencies (PCEs) of 7.68% and 14.64% but also long-term stability that are superior to those of the control and o-(F)- and m-(F)-PO-TAZ-based solar cell devices. The current work offers new findings on constructing photoactive (organic or perovskite) layer/CIL interfaces and assists in the rational design of CILs to facilitate efficient photovoltaic devices.