Cathode Interfacial Layer Research Articles

Efficiency exceeding 10% for inverted polymer solar cells with a ZnO/ionic liquid combined cathode interfacial layer

With the ZnO/IL interfacial layer, PTB7-Th : PC71BM i-PSCs can exhibit a champion PCE of 10.15%.

Correlation of structure and photovoltaic performance of benzo[1,2-b:4,5-b′]dithiophene copolymers alternating with different acceptors

The PBDTT–TTz/PC61BM (1 : 2) devices with PFN as the cathode interfacial layer delivered a power conversion efficiency of 2.60% for polymer solar cells.

Interface Engineering of Perovskite Hybrid Solar Cells with Solution-Processed Perylene–Diimide Heterojunctions toward High Performance

Perovskite hybrid solar cells (pero-HSCs) were demonstrated to be among the most promising candidates within the emerging photovoltaic materials with respect to their power conversion efficiency (PCE) and inexpensive fabrication. Further PCE enhancement mainly relies on minimizing the interface losses via interface engineering and the quality of the perovskite film. Here, we demonstrate that the PCEs of pero-HSCs are significantly increased to 14.0% by incorporation of a solution-processed perylene–diimide (PDINO) as cathode interface layer between the [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) layer and the top Ag electrode. Notably, for PDINO-based devices, prominent PCEs over 13% are achieved within a wide range of the PDINO thicknesses (5–24 nm). Without the PDINO layer, the best PCE of the reference PCBM/Ag device was only 10.0%. The PCBM/PDINO/Ag devices also outperformed the PCBM/ZnO/Ag devices (11.3%) with the well-established zinc oxide (ZnO) cathode interface layer. This enhanced performa...

Impact of sputtered ZnO interfacial layer on the S-curve in conjugated polymer/fullerene based-inverted organic solar cells

The impact of crystalline structure changes of sputtered ZnO interfacial layer on performances of inverted organic solar cells (OSCs) has been investigated. We find that the structural modification of the ZnO cathode interfacial layer, obtained by thermal annealing, plays a crucial role in the origin and solving of the S-curve in conjugated polymer/fullerene photovoltaics. Our results show that the crystallization (i.e. crystallites size) of poly(3-hexylthiophene) (P3HT) evolves as a function of that of ZnO according to the annealing temperature. This evolution can directly impact the interfacial orientation and organization of the chains of P3HT at the ZnO buried interface. Such an ordered profile favors the vertical phase segregation and raises the carrier mobility, which explains the disappearance of the S-shape observed in current density-voltage device characteristics for annealing temperatures above 200°C. These results adequately address recent research and provide an important insight into the interfacial layers of inverted OSCs.

Printable highly conductive conjugated polymer sensitized ZnO NCs as cathode interfacial layer for efficient polymer solar cells.

We report a facile way to produce printable highly conductive cathode interfacial layer (CIL) for efficient polymer solar cells (PSCs) by sensitizing ZnO nanocrystals (NCs) with a blue fluorescent conjugated polymer, poly(9, 9-bis-(6'-diethoxylphosphorylhexyl) fluorene) (PFEP). Herein, PFEP plays dual distinctive roles in the composite. Firstly, PFEP chains can effectively block the aggregation of ZnO NCs, leading to uniform and smooth film during solution processing via assembly on ZnO NC surfaces through their pending phosphonate groups. Secondly, PFEP can greatly improve the conductivity of ZnO NCs by charge transfer doping, that is the charge transfer from the sensitizer driven by electron-chemical potential equilibrium, which could be even more pronounced under light illumination because of light excitation of PFEP sensitizer. The increased conductivity in ZnO-PFEP layer renders more efficient electron transport and extraction compared to pristine ZnO layer. This ZnO-PFEP CIL was successfully applied to PSCs based on three polymer donor systems with different band-gaps, and efficiency enhancements from 44 to 70% were observed compared to those PSCs with pristine ZnO CIL. The highest efficiency of 7.56% was achieved in P(IID-DTC):PC70BM-based PSCs by using ZnO-PFEP film as CIL. Moreover, the enhanced conductivity due to the charge-transfer doping effect allows thick ZnO-PFEP film to be used as CIL in high-performance PSCs. Both the high conductivity and good film-forming properties of ZnO-PFEP CIL are favorable for large-scale printable PSCs, which is also verified by high-efficiency PSCs with ZnO-PFEP CIL fabricated using doctor-blading, a large-scale processing technique. The work provides an efficient printable cathode interfacial material for efficient PSCs.

Interface Modification of ZnO-Based Inverted PTB7:PC71BM Organic Solar Cells by Cesium Stearate and Simultaneous Enhancement of Device Parameters

We used cesium stearate (CsSt) to modify the interface of the electron-extracting contact in inverted organic solar cells. Surface microstructure, optical properties, and electrical characterization as well as exciton generation rate and dissociation probability were investigated to understand the impact of CsSt on the interface contact. The results indicated that by incorporation of CsSt, the surface morphology and energy level as well as conductivity of a zinc oxide (ZnO) film were improved. On the basis of the above properties, highly efficient inverted organic solar cells have been demonstrated by using a ZnO nanoparticle film and CsSt stacked bilayer structure as the cathode interfacial layer. The insertion of a CsSt layer between the ZnO film and active layer improved the electron extraction efficiency, and a high power conversion efficiency (PCE) of 8.69% was achieved. The PCE was improved by 20% as compared to the reference device using a ZnO-only electron extraction layer.

Influence of the Ionic Functionalities of Polyfluorene Derivatives as a Cathode Interfacial Layer on Inverted Polymer Solar Cells

In this work, we synthesized water-soluble polyfluorene derivatives (WPFs) with anionic and/or cationic side chains, which were used as an indium tin oxide (ITO) cathode interfacial layer in inverted polymer solar cells. Three WPFs (WPFN+, WPFZW, and WPFS-) were obtained via Suzuki coupling reactions. Their solubility in polar solvents allowed the WPFs to be used as interfacial layers in inverted polymer solar cells (I-PSCs). Among the WPF-modified ITO electrodes, WPFN+ (with ammonium side chains)-modified ITO can be used as a cathode for electron extraction, while WPFS- (with sulfonate side chains)-modified ITO cannot extract electrons in I-PSCs based on poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM). The electron extraction of WPF-modified ITO can mainly be attributed to the different dipole formations at the WPF/ITO interfaces, based on the types of ionic groups on the side chains of the polyfluorene. In addition, we observed that the extent of ITO work-function modification was not always exactly correlated with the device performance based on the results obtained using a WPFZW (with ammonium and sulfonate side chains)-modified ITO electrode.

Enhanced efficiency of inverted polymer solar cells using two-step sputtered ZnO as cathode interfacial layer

Efficient inverted polymer solar cells (PSCs) with two-step deposited zinc oxide (ZnO) cathode interfacial layer are developed by magnetron sputtering method. The first-step ZnO layer sputtered with Ar gas has lower bulk resistance due to increasing oxygen vacancy. The second-step ZnO layer sputtered with O2/Ar ratio (1:7) gas can obtain lower contact barrier and superior interface properties at ZnO/photoactive layer interface. The power conversion efficiencies of inverted poly(3-hexylthiophene) and phenyl-C61-butyric acid methylester (P3HT:PCBM) based PSCs with 11 nm/4 nm and 50 nm/10 nm ZnO layers are, respectively, 4.08% and 3.13%, which are obviously higher than that of corresponding PSCs with O2/Ar gas sputtered 15 nm and 60 nm ZnO layers. The conduction band level of ZnO film with O2/Ar sputtering gas is higher, 0.14 eV, than that of ZnO film with only Ar layer derived from ultraviolet photoelectron spectroscopy and optical band gap of ZnO films. The capacitive behavior of PSCs with 30 nm and 60 nm layers indicates that two-step sputtered ZnO layer can efficiently enhance electron transport ability of bulk ZnO layer and attenuate electron accumulation at ZnO/P3HT:PCBM interface. Lower reverse saturation current and higher shunt resistant of PSCs with two-step sputtered ZnO layer demonstrated that the defects penetrating into the whole ZnO layer can be inhibited.

Solution-processed nanostructured zinc oxide cathode interfacial layers for efficient inverted organic photovoltaics

Inverted organic photovoltaic (OPV) cells based on poly(3-hexylthiophene) (P3HT) as an electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor, were fabricated and characterized. To improve the photovoltaic performance, interface control using either dense or nanostructured ZnO films as cathode buffer layers for effective electron transport was demonstrated, while an under-stoichiometric transition metal oxide, such as MoOx, was employed as the anode buffer layer for efficient hole extraction. Incorporation of a nanostructured ZnO interlayer enhanced electron–hole dissociation by enabling a larger interfacial contact with the active layer, that results in increased short-circuit current density (Jsc) and eventually contributing to higher power conversion efficiency (PCE).

Dark currents in bulk heterojunction devices for imaging applications: The effect of a cathode interfacial layer

While sol–gel-processed metal oxides are widely used as an electron transport layer to enhance photovoltaic performances, their effect on photodetector application was not studied. We found sol–gel-processed titanium oxide deteriorated dark current characteristics in reverse biases by almost two orders of magnitude, whereas bare Al cathodes exhibited ideal dark current characteristics. Increased dark current came from space charge limited currents in microscopic p-i-p metal-semiconductor-metal configurations. The spatial variation of workfunction values was believed to form local leakage paths by partial filling of traps on the surface of sol–gel titanium oxide.

Laminated Polymer Solar Cells with PEDOT:PSS Film as Anode

A prefabricated conductive polymer film of polymer poly(3,4-ethylenedioxy-thiophene):poly (styrene-sulfonate) (PEDOT:PSS) is developed and is used as the anode in an inverted polymer solar cell (PSC) through a lamination process. The geometry structure of the PSC is indium tin oxide/interface layer/P3HT:PCBM/PEDOT:PSS. The PEDOT:PSS electrode is 5 μm and the sheet resistance is 10Ω/sq. The device fabrication process is vacuum-free and extremely simple. Lithium carbonate (Li2CO3) and cesium carbonate (Cs2CO3) are used as the cathode interface layers, respectively, and the result shows that Li2CO3 can enhance the open-circuit voltage (Voc) and fill factor distinctly, and the power conversion efficiency (PCE) can reach 2.1%.

Solution processable interface materials for nanoparticulate organic photovoltaic devices

Nanoparticulate zinc oxide can be prepared at low temperatures from solution processable zinc acetylacetonate. The use of this material as a cathode interfacial layer in nanoparticulate organic photovoltaic devices results in comparable performances to those based on reactive calcium layers. Importantly, the enhanced degradation stability and full solution processability make zinc oxide a more desirable material for the fabrication of large area printed devices.

A futuristic approach towards interface layer modifications for improved efficiency in inverted organic solar cells

Inverted polymer Solar Cells of the classical poly (3-hexylthiophene) (P3HT):(6,6)-phenyl-C61butyric acid methyl ester (PC61BM) blend on indium tin oxide substrates were fabricated, which shows improved device performance, by using a facile solution–processed ZnO-polyelectrolytes [poly (diallyldimethylammonium chloride) (PDADMAC), Poly (acrylic acid sodium salt) (PAS), poly (4-styrenesulfonic acid) (PSS), and Polyvinylpyrrolidone (PVP)] nanocomposite as a cathode interface layer compared to devices using pristine ZnO as cathode buffer layer in ambient conditions. The devices with different combinations of polyelectrolyte with ZnO show different improvements in the device efficiency. The combinations of ZnO with PVP and PDADMAC show highest amount of improvements in the efficiency by a factor of ∼17–19. The improvement of the efficiency may be due to various phenomena, such as the passivation of ZnO surface as well as bulk traps, work function modification, improved energy level alignment, improved electronic coupling of the inorganic/organic interface, improved light harvesting, and decrease of surface as well as bulk charge recombination in the device. The introduction of polyelectrolyte into ZnO inhibits the aggregation of ZnO nanoparticles yielding the large area ZnO nanoclusters; and hence, forming the uniform film of ZnO resulting in the modifications of morphology as well as electronic structure of ZnO-polyelectrolyte nano-composite favouring better electronic coupling between cathode and active layer and hence enhancing the current and, consequently, the efficiency. This simple low temperature ZnO-polyelectrolyte nanocomposite based protocol proposed for cathode interface layer modification may be very much useful for roll to roll industrial manufacturing of organic solar cells.

Alcohol-soluble polyfluorenes containing dibenzothiophene-S,S-dioxide segments for cathode interfacial layer in PLEDs and PSCs

A series of alcohol-soluble amino-functionalized polyfluorene derivatives (PF-N-S, PF-N-SC8 and PF-N-SOC8) comprising various ratios of dibenzothiophene-S,S-dioxide segments (S/SC8/SOC8) in the main chains, respectively, were synthesized and utilized as cathode interfacial layer (CIL) in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) with high-work-function Al (or Au) electrode. The polymers possess LUMO/HOMO levels at −2.78 to −3.53eV/−5.69 to −6.32eV. Multilayer PLEDs and PSCs with device configurations of ITO/PEDOT:PSS (40nm)/P-PPV or PFO-DBT35:PCBM=1:2 (80nm)/CIL (3–15nm)/Al (or Au) (100nm) were fabricated. The PF-N-S-10/Al (or Au) cathode PLEDs displayed maximum luminous efficiency of 24.4cdA−1 (or 11.9cdA−1), significantly higher than bare Al (or Au) cathode device, exceeding well-known Ba/Al and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)/Al (or PFN/Au) cathode devices. The enhanced open-circuit voltages (Vocs), electron reflux and reduced work functions clarify that the electron injection barrier from the Al (or Au) electrode can be lowered by inserting the polymers as CIL. The resulted PSCs also show device performances exceeding Al and PFN/Al cathode devices. The results indicate that PF-N-S, PF-N-SC8 and PF-N-SOC8 are excellent CIL materials for PLEDs and PSCs with high-work-function Al or Au electrode.

Highly Efficient Organic Photovoltaics via Incorporation of Solution-Processed Cesium Stearate as the Cathode Interfacial Layer

Highly efficient organic solar cells were successfully demonstrated by incorporating a solution-processed cesium stearate between the photoactive layer and metal cathode as a novel cathode interfacial layer. The analysis of surface potential change indicated the existence of an interfacial dipole between the photoactive layer and metal electrode, which was responsible for the power conversion efficiency (PCE) enhancement of devices. The significant improvement in the device performance and the simple preparation method by solution processing suggested a promising and practical pathway to improve the efficiency of the organic solar cells.

Work-function tuneable and aqueous solution-processed Cs2CO3 for high-performance polymer solar cells

The authors demonstrate an aqueous solution-processed Cs2CO3 thin film with an adjustable work function via MoO3 and/or Na2WO4 doping. The doped Cs2CO3 as a cathode interfacial layer is successfully used in poly(3-hexyl-thiophene) (P3HT)/indene-C60 bisadduct (IC60BA) heterojunction based solar cells with improved open-circuit voltage and unaffected short-circuit current density. X-ray photoelectron spectroscopy (XPS) evaluation was conducted to verify the formation of the new composites of W–O–Cs and Mo–O–Cs after doping of MoO3 and/or Na2WO4 into Cs2CO3. The change of the work function of MoO3- and/or Na2WO4-doped Cs2CO3 was further confirmed by ultraviolet photoelectron spectroscopy (UPS) measurements.

Porphyrin oriented self-assembled nanostructures for efficient exciton dissociation in high-performing organic photovoltaics

Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor–acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance.

A work-function tunable polyelectrolyte complex (PEI:PSS) as a cathode interfacial layer for inverted organic solar cells

Incorporation of PSS into a PEI interfacial-layer improves the inverted-OPV, and it is further enhanced by using a robust TiOx–PEI:PSS combination.

All-Organic Sulfonium Salts Acting as Efficient Solution Processed Electron Injection Layer for PLEDs

Herein we introduce the all-organic triphenylsulfonium (TPS) salts cathode interfacial layers (CILs), deposited from their methanolic solution, as a new simple strategy for circumventing the use of unstable low work function metals and obtaining charge balance and high electroluminescence efficiency in polymer light-emitting diodes (PLEDs). In particular, we show that the incorporation of TPS-triflate or TPS-nonaflate at the polymer/Al interface improved substantially the luminous efficiency of the device (from 2.4 to 7.9 cd/A) and reduced the turn-on and operating voltage, whereas an up to 4-fold increase in brightness (∼11 250 cd/m(2) for TPS-triflate and ∼14 682 cd/m(2) for TPS-nonaflate compared to ∼3221 cd/m(2) for the reference device) was observed in poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1',3-thiadiazole)] (F8BT)-based PLEDs. This was mainly attributed to the favorable decrease of the electron injection barrier, as derived from the open-circuit voltage (Voc) measurements, which was also assisted by the conduction of electrons through the triphenylsulfonium salt sites. Density functional theory calculations indicated that the total energy of the anionic (reduced) form of the salt, that is, upon placing an electron to its lowest unoccupied molecular orbital, is lower than its neutral state, rendering the TPS-salts stable upon electron transfer in the solid state. Finally, the morphology optimization of the TPS-salt interlayer through controlling the processing parameters was found to be critical for achieving efficient electron injection and transport at the respective interfaces.

Tuning indium tin oxide work function with solution-processed alkali carbonate interfacial layers for high-efficiency inverted organic photovoltaic cells

Selective electron collection by an interfacial layer modified indium tin oxide cathode is critically important for achieving high-efficiency inverted structure organic photovoltaic (OPV) cells. Here, we demonstrate that solution-processed alkali carbonates, such as Li2CO3, Na2CO3, K2CO3, Rb2CO3, Cs2CO3, are good interfacial layer materials. Both carbonate concentration and annealing conditions can affect cathode work function and surface roughness. By proper optimization, different alkali carbonates can be almost equally effective as the cathode interfacial layer. Furthermore, good device performance can be achieved at a low annealing temperature (<50 ° C), which allows for potential applications in solution-processed inverted OPV cells on plastic substrates. This work indicates that alkali carbonates, not just cesium carbonate, are valid choices as the cathode interlayer in inverted OPV devices.