Hole Collection Layer Research Articles

Structural and electronic properties of 2,2′,6,6′-tetraphenyl-dipyranylidene and its use as a hole-collecting interfacial layer in organic solar cells

The accumulation of positive charges at the anodic interface considerably limits the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs). Interfacial layers (IFLs) such as PEDOT:PSS improve charge injection but have no effect on the unbalanced electron/hole transport across the BHJ. We report here the use of 2,2′,6,6′-tetraphenyl-dipyranylidene (DIPO-Ph4), a planar quinoïd compound, as an efficient anodic IFL in organic solar cells based on BHJs made of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM). When deposited under vacuum onto a glass substrate, DIPO-Ph4 thin films are constituted of densely packed and vertically aligned crystalline needles. Current-sensing atomic force microscopy (CS-AFM) reveals a considerable increase of the hole-carrying pathways in DIPO-Ph4 thin films as compare to PEDOT:PSS, thus revealing their hole transporting/electron blocking properties. Inserting a 10 nm thick IFL of DIPO-Ph4 in combination with a 5 nm thick PEDOT:PSS between the ITO electrode and the P3HT:PCBM film leads to photocurrent densities up to 11.5 mA/cm2 under AM 1.5G and conversion efficiencies up to 4.6%, that is substantially higher than PEDOT:PSS-only devices.

Cylindrical Ultra-Thin a-Si:H Photovoltaic Cell With No Doped Layers

ABSTRACTIn order to collect the hot electrons, light trapping using cylindrical quartz substrates for ultra-thin a-Si:H photovoltaic cells with no doped layers is introduced. The photovoltaic cell has the structure of 2nmMoOx(hole collection layer) – 10nmintrinsic a-Si:H (photoactive layer) – 1.5nmLiF / 300nmAl (electron collection layer and back electrode), all deposited in that order onto a cylindrical quartz substrate covered with a 100nmITO layer, which is acting as the transparent front electrode for the cell. By rotating the cell with respect to the incoming light, the angle of incidence of the incoming light for which the cell efficiency is at its highest value, is determined.

Influence of Weak Base Addition to Hole-Collecting Buffer Layers in Polymer:Fullerene Solar Cells.

We report the effect of weak base addition to acidic polymer hole-collecting layers in normal-type polymer:fullerene solar cells. Varying amounts of the weak base aniline (AN) were added to solutions of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The acidity of the aniline-added PEDOT:PSS solutions gradually decreased from pH = 1.74 (AN = 0 mol %) to pH = 4.24 (AN = 1.8 mol %). The electrical conductivity of the PEDOT:PSS-AN films did not change much with the pH value, while the ratio of conductivity between out-of-plane and in-plane directions was dependent on the pH of solutions. The highest power conversion efficiency (PCE) was obtained at pH = 2.52, even though all devices with the PEDOT:PSS-AN layers exhibited better PCE than those with the pristine PEDOT:PSS layers. Atomic force microscopy investigation revealed that the size of PEDOT:PSS domains became smaller as the pH increased. The stability test for 100 h illumination under one sun condition disclosed that the PCE decay was relatively slower for the devices with the PEDOT:PSS-AN layers than for those with pristine PEDOT:PSS layers.

Ferroelectric polarization enhancement of photovoltaic effects in BaTiO3/BiFeO3/TiO2 heterostructure by introducing double-functional layers

Multilayer Pt/BaTiO3/BiFeO3/TiO2 heterostructure was fabricated on FTO substrates by sol-gel technique. This heterostructure shows strong light absorption, sensitive optical switching response, and excellent photovoltaic properties, which are effectively modulated by the ferroelectric polarization. The short circuit current density and open circuit voltage of the multilayer heterostructure are significantly enhanced compared with that of the single layer BiFeO3 thin film under the initial and positive polarization states. Analysis on the schematic energy band diagrams suggests that the excellent photovoltaic properties can be attributed to the interface Schottky barrier regulated by ferroelectric polarization, in which the TiO2 layer acts as the electron collection layer, and the ferroelectric BaTiO3 layer acts as a hole collection layer and also an auxiliary photovoltaic absorption layer to enhance optical absorption properties, thus improve the light absorption efficiency and the separation of electron-hole pairs. These results provide a promising method to optimize the performance of ferroelectric photovoltaic devices by using multilayer heterostructure.

Nanocrystalline Silicon Carrier Collectors for Silicon Heterojunction Solar Cells and Impact on Low-Temperature Device Characteristics

Silicon heterojunction solar cells typically use stacks of hydrogenated intrinsic/doped amorphous silicon layers as carrier selective contacts. However, the use of these layers may cause parasitic optical absorption losses and moderate fill factor ( FF ) values due to a high contact resistivity. In this study, we show that the replacement of doped amorphous silicon with nanocrystalline silicon is beneficial for device performance. Optically, we observe an improved short-circuit current density when these layers are applied to the front side of the device. Electrically, we observe a lower contact resistivity, as well as higher FF . Importantly, our cell parameter analysis, performed in a temperature range from −100 to +80 °C, reveals that the use of hole-collecting p-type nanocrystalline layer suppresses the carrier transport barrier, maintaining FF s in the range of 70% at −100 °C, whereas it drops to 40% for standard amorphous doped layers. The same analysis also reveals a saturation onset of the open-circuit voltage at −100 °C using doped nanocrystalline layers, compared with saturation onset at −60 °C for doped amorphous layers. These findings hint at a reduced importance of the parasitic Schottky barrier at the interface between the transparent electrodes and the selective contact in the case of nanocrystalline layer implementation.

Atomic Layer Deposition of TiO2 for a High-Efficiency Hole-Blocking Layer in Hole-Conductor-Free Perovskite Solar Cells Processed in Ambient Air.

In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO2 as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. First, uniform, pinhole-free TiO2 films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO2 films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode were fabricated in ambient air, and the effect of thickness of TiO2 compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The best-performance devices based on optimized TiO2 compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCEs) of as high as 7.82%. Furthermore, they can maintain over 96% of their initial PCE after 651 h (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability.

Solution-processed pH-neutral conjugated polyelectrolytes with one-atom variation (O, S, Se) as a novel hole-collecting layer in organic photovoltaics

We present pH-neutral hole transporting layers processed from a solution at room temperature for organic photovoltaics (OPVs). Three new conductive conjugated polyelectrolytes (CPEs), identified as PFF, PFT and PFSe, containing aromatic five-membered heterocyclic compounds (furan, thiophene, and selenophene, respectively), are designed and synthesized using the Stille-coupling reaction. These polymers dissolve in water/alcohol as they are comprised of a conjugated backbone with hydrophilic side chains of sodium butane sulfonates. The CPEs with heterocyclic rings having stronger dipole moments decrease the work function of ITO electrodes and the surface potential between the active layer and hole-transporting layer (HTL) effectively. The device with PFSe as a HTL exhibits the best performance (7.2%), with JSC of 14.4mAcm−2, VOC of 0.677V and FF of 69.0%. The device also exhibits improvements in air stability under room temperature due to its neutral nature. Also, 70% of its initial power conversion efficiency (PCE) is maintained upon exposure to ambient air for ∼480h, whereas the initial PCE with PEDOT:PSS decreases by more than 50%. Our results indicate that it is possible to improve the performance of OPVs using pH-neutral CPEs as efficient HTLs. We determine the way of molecular design is used to decrease the energy barrier using a heteroatom with a large dipole moment.

Bulk-heterojunction polymer solar cells with polyaniline-silica nanocomposites as an efficient hole-collecting layer

At first, bulk-heterojunction polymer solar cells (PSCs) with conventional configuration: ITO/PEDOT:PSS/P3HT:C60/Al, containing different blend ratios of poly(3-hexylthiophene):fullerene, (P3HT):C60 as active layer have been fabricated. The effect of replacement of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by the prepared polyaniline-fumed silica (PANI-SiO2) nanocomposites as the hole-collecting layer (HCL) on the performance of the fabricated PSC with the optimized blending ratio of P3HT:C60 was examined in detail. According to the obtained results, it was found that the fabricated PSC with PANI-SiO2 nanocomposite containing 10% SiO2 (PANI-10% SiO2) as the HCL and P3HT:C60 with the optimized blending ratio (P3HT:33% C60) as active layer exhibited best performance with a fill factor (FF) of 0.35, compared to the PSC containing conventional PEDOT:PSS HCL with an FF of 0.32. Our demonstration suggests that PANI-SiO2 nanocomposites could be promising HCL replacing PEDOT:PSS in PSCs as well as other organic electronic devices.

MoOx Hole Collection Layer for a-Si:H Based Photovoltaic Cells

ABSTRACTAn experimental investigation to verify the suitability of MoOx as the hole collection layer for a-Si:H based thin film photovoltaic cell is carried out. The photovoltaic cell investigated has the structure of MoOx (hole collection layer) / intrinsic a-Si:H (photoactive layer) / phosphorus doped a-Si:H (electron collection layer) / Ag (back reflector electrode); all deposited in that order onto an Asahi glass (type U) substrate, which is also acting as the transparent front electrode for the cell. The effects of different post deposition annealing temperatures are investigated. The highest efficiency values are obtained for the cells annealed at 120°C. For the photovoltaic cell with 100 nm thick photoactive layer, the highest efficiency is measured to be 6.46 % with an open current voltage (Voc) of 827 mV and a short current density of (Jsc) of 10.44 mA/cm2. For the photovoltaic cell with 300 nm thick photoactive layer, the highest efficiency is measured to be 7.93 % with Voc of 818 mV and Jsc of 13.24 mA/cm2. The efficiency measurements are carried out under AM1.5 test conditions. Jsc values are corrected according to the external quantum efficiency measurements of the cells in the AM1.5 photovoltaic spectrum region between 270 nm and 800 nm. Compared to the reference cell with boron doped μ-SiOx layer acting as the hole collection layer, the cell with MoOx hole collection layer has similar FF, lower Voc, higher Jsc for wavelength up to the green light region of the AM1.5 spectrum and lower Jsc for the longer wavelengths.

Low-temperature preparation of a molybdenum oxide hole collection layer by using a peroxo precursor for polymer solar cells

Amorphous molybdenum oxide (MoOx) as a hole collection layer (HCL) in polymer solar cells (PSCs) was prepared by annealing of a peroxo precursor. The power conversion efficiency of PSCs based on a blend of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester was improved to 3.19% by using an MoOx layer prepared by annealing at 200°C in N2 as an alternative for poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). X-ray photoelectron spectra of MoOx layers showed that the peroxide precursor was converted into MoOx by annealing at 100–150°C and that annealing at a high temperature in N2 under unaerated conditions promotes the reduction of Mo6+ species into Mo5+ species. In contrast, ultraviolet photoelectron spectra of an MoOx layer showed that the electric structure of MoOx depends on not the amount of Mo5+ species but the presence or absence of Mo5+ species. These results indicated that an appropriate amount of Mo5+ species in the MoOx layer is important for an efficient MoOx HCL. A stability test revealed the PSCs with MoOx layers had relatively high device stability in air compared to that of PSCs with PEDOT:PSS.

Triple-stacked hole-selective layers for efficient solution-processable organic semiconducting devices.

PSS). The use of the triple-stacked GO/MoO3/ PSS layers as hole-injecting layers (HILs) in solution-processable organic light-emitting diodes (OLEDs) resulted in a considerable improvement of device performance in terms of brightness (maximum brightness: 47,000 cd/m2) as well as efficiency (peak efficiency: 31.5 cd/A), exceeding those of an OLED with a conventional single PSS HIL. Furthermore, polymer solar cells (PSCs) with these triple-stacked layers used as hole-collecting layers (HCLs) showed a considerable improvement in power conversion efficiency (6.62%), which was also higher than that (5.65%) obtained using the single PSS HCL. These results clearly indicate the benefits of using triple-stacked GO/MoO3/ PSS layers, which provide better hole-injection/collection, electron-blocking, and improved stability for high performance solution-processable OLEDs and PSCs.

(Invited) Single-Walled Carbon Nanotubes and Graphene As Highly Efficient Hole Collecting and Transport Layer for Solar Cells

We investigate the application of single-walled carbon nanotubes (SWNTs) and graphene as hole extraction and transport layer in various kinds of solar cells including structured SWNTs-Si solar cell [1], dry-deposited SWNTs-Si solar cell [2], graphene-Si solar cells, organic solar cell [3] and perovskite solar cells. Using millimeter-scale monocrystalline single-layer graphene and honeycomb-structured SWNT network, the nanocarbon-Si solar cell demonstrated the air-stable power conversion efficiency (PCE) of 11.6% before any intentional doping process. For organic solar cells (OSC), the SWNT/MoOx/PEDOT:PSS nanocomposite was proposed and developed as hole extracting/electron blocking layer and electrode replacing ITO. Using PTB7/PC71BM mixture as active materials, the PCE of 6% was obtained for on glass substrate and 3.89% on flexible PET substrate. The hole collecting and transport function of SWNTs was also demonstrated in perovskite solar cells with high PCE. We expect that the carbon nanomaterials is very promising in the application solar cells.

Hole-selective and impedance characteristics of an aqueous solution-processable MoO3 layer for solution-processable organic semiconducting devices

We herein report an investigation of aqueous solution-processable molybdenum-oxide (MoO3) hole-selective layers fabricated for solution-processable organic semiconducting devices. A homogeneous MoO3 layer was successfully deposited via spin-coating using aqueous solutions of ammonium heptamolybdate as a MoO3 precursor. The use of the solution-processable MoO3 layer as a hole-injecting layer (HIL) on an indium-tin-oxide (ITO) anode in solution-processable organic light-emitting diodes (OLEDs) resulted in excellent device performance in terms of the brightness (maximum brightness of 37,000 cd m−2) and the efficiency (peak efficiency of 25.2 cd A−1), comparable to or better than those of a reference OLED with a conventional poly(ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS) HIL. Such good device performance is attributed to the water-processable MoO3 hole-selective layers, which allowed the formation of a high-quality film and provided good matching of the energy levels between adjacent layers with improved hole-injecting properties, impedance characteristics, and stability. Furthermore, polymer solar cells (PSCs) with a MoO3 layer used as a hole-collecting layer (HCL) showed improved power conversion efficiency (3.81%), which was higher than that obtained using the PEDOT:PSS HCL. These results clearly indicate the benefits of using a water-processable MoO3 layer, which effectively acts as a hole-selective layer on an ITO anode and provides good hole-injection/collection, electron-blocking and energy-level-matching properties, and improved stability. They, therefore, offer considerable promise as an alternative to a conventional PEDOT:PSS layer in the production of high-performance solution-processable organic semiconducting devices.

Solution-processed nickel compound as hole collection layer for efficient polymer solar cells

We demonstrated efficient bulk heterojunction polymer solar cells (PSCs) by inserting a solution-processable hole collection layer (HCL) between the indium tin oxide (ITO) electrode and photoactive layer. The HCL was prepared by spin-coating nickel acetylacetonate (Ni(acac)2) isopropanol solution on ITO, and then baking in air at 180 °C for 10 min followed by UV ozone treatment, which was marked as a-Ni(acac)2. The a-Ni(acac)2 HCL shows suitable energy levels, high hole mobility of 4.09 × 10−3 cm2 V−1·s−1, and high transparency with light transmittance better than poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) in the wavelength range 550–800 nm. The PSCs with a-Ni(acac)2 HCL showed improved performance compared with the PSCs without or with traditional PEDOT:PSS HCL. The power conversion efficiency of the PSC based on PBDTTT-C-T:PC70BM with a-Ni(acac)2 HCL reached 7.84% under the illumination of AM 1.5 G, 100 mW cm−2.

Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells.

Organic-inorganic perovskite solar cells have recently emerged at the forefront of photovoltaics research. Power conversion efficiencies have experienced an unprecedented increase to reported values exceeding 19% within just four years. With the focus mainly on efficiency, the aspect of stability has so far not been thoroughly addressed. In this paper, we identify thermal stability as a fundamental weak point of perovskite solar cells, and demonstrate an elegant approach to mitigating thermal degradation by replacing the organic hole transport material with polymer-functionalized single-walled carbon nanotubes (SWNTs) embedded in an insulating polymer matrix. With this composite structure, we achieve JV scanned power-conversion efficiencies of up to 15.3% with an average efficiency of 10 ± 2%. Moreover, we observe strong retardation in thermal degradation as compared to cells employing state-of-the-art organic hole-transporting materials. In addition, the resistance to water ingress is remarkably enhanced. These are critical developments for achieving long-term stability of high-efficiency perovskite solar cells.

A low-temperature processed environment-friendly full-organic carrier collection layer for polymer solar cells

We constructed a concept of the full-organic carrier collection layer (CCL) used for polymer solar cells. The CCL is composed of dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile as hole collection layer (HCL) and chlorine-free solvents (formic acid (FA)) processed 4,7-Diphenyl-1,10-phenanthroline (Bphen) as electron collection layer, exhibiting good solubility, and environmental protection. The FA based device shows ideal power conversion efficiency (3.75%), which is higher than that of control device (3.6%). Besides, the HCL shows a different mechanism in hole extraction by functioning as a charge recombination zone for electrons injected from anode and holes extracted from the donor materials.

Organic photovoltaic performance improvement using atomic layer deposited ZnO electron-collecting layers

Inverted organic photovoltaic (OPV) cells based on poly(3-hexylthiophene (P3HT) as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor, were fabricated and characterized. To improve the photovoltaic performance, ZnO films were used as electron collection layers, while an under-stoichiometric molybdenum oxide MoOx was employed as the hole collection layer. Two types of ZnO layers were employed; one deposited by atomic layer deposition (ALD-ZnO) and another deposited using the sol–gel method (sg-ZnO). OPV cells with a 20nm thick ALD-ZnO layer exhibited significant efficiency enhancement compared with those based on the sg-ZnO layer with the same thickness. The ALD-ZnO film exhibited reduced defect/trap concentration compared with the sg-ZnO counterpart, as confirmed by steady state photoluminescence spectroscopy, showing a promising interface layer for efficient organic photovoltaic devices exhibiting also improved temporal stability. By employing capacitance–voltage measurements we were able to identify a downward shift of the conduction band edge of ALD-ZnO film (or equivalently, an upward shift of the conduction band minimum of the sg-ZnO film), verified also by ultraviolet photoelectron spectroscopy measurements. This resulted in a significant decrease in the electron extraction barrier at the ALD-ZnO/organic active layer interface, as was also demonstrated by the increased current in unipolar (electron only) devices. This work highlights the importance of using the ALD method to develop conformal and defect free ZnO electron collection layers for high performance organic photovoltaics.

Development of bifacial inverted polymer solar cells using a conductivity-controlled transparent PEDOT:PSS and a striped Au electrode on the hole collection side

An inverted bifacial polymer solar cell was developed using a conductivity-controlled transparent poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS) as a hole collection layer and a striped Au electrode with a large open aperture ratio (Rap) as a hole collection electrode. We investigated the performance of the device by varying the interelectrode distance of the striped Au electrode and the sheet resistance of the PEDOT:PSS film. The device using untreated Clevios P (PEDOT:PSS) showed a maximum electric output (Pw) in the Rap range of 50 to 65%. When alcohol-treated Clevios P (Clevios P+) with a lower electrical resistance was used, the maximum Pw increased by 40% compared with that of the device using Clevios P. The maximum Pw was obtained in the Rap range of 84% as the hole collection efficiency of the striped Au electrode improved with the decreased sheet resistance of the PEDOT:PSS.

Role of Electron- and Hole-Collecting Buffer Layers on the Stability of Inverted Polymer: Fullerene Photovoltaic Devices

Systematic device performance and air stability comparison of inverted architecture polythiophene:fullerene photovoltaic cells with eight different electron-collecting layers (ECLs) and two hole-collecting layers are presented in this study. Regardless of the ECL, we achieved an efficiency of over 3.5% and lifetime of over 1000 h. These results indicate the relative interchangeability of various solution-processed ECLs. Long-term (>5000 h) air exposure revealed a secondary failure mechanism of inverted cells, which is assigned to hindered exciton harvesting. Notably, devices with a polymeric hole-collecting layer and Ag/Al electrode exhibited the longest lifetime (defined as 80% of the initial performance) of 4000 h, compared with 3000 h for MoO 3/Ag/Al.

The annealing effects of tungsten oxide interlayer based on organic photovoltaic cells

We investigated the effect of tungsten oxide (WO3) interlayer as a hole collection layer on the performance of organic photovoltaic cells according to the thickness and temperature of the interlayer. The characteristics of organic photovoltaic cells such as fill factor, current density, and open circuit voltage are continuously improved by increasing the temperature of the WO3 interlayer. The surface of a treated WO3 film promotes the crystallization of P3HT because a treated WO3 film is more hydrophobic than a pristine WO3 film. Furthermore, the energy barrier between P3HT and the WO3 interlayer is minimized since the work function of the WO3 film after annealing progressively increases until a hole can be smoothly transferred. Therefore, organic photovoltaic cells using an interlayer of treated WO3 film have higher hole mobility and better efficiency. The efficiency of an organic photovoltaic cell with a 40nm-thick WO3 interlayer is significantly enhanced from 0.94 to 3.04% as the temperature changes from room temperature to 350°C under AM 1.5G illumination.