Hole-collecting Buffer Layer Research Articles

Performance Enhancement of P3HT: ZnO Solar Cells by Incorporating Metal Oxide Fe2O3 Nanoparticles

The ferric oxide nanoparticles (Fe2O3) which are deposited at interface which is related to hole collecting buffer layer [poly(3,4-ethyl-enedioxythiophene): poly(styrene-sulfonate) (PEDOT: PSS)] as well as regioregular poly(3-hexyl-thiophene): Zinc oxide nanoparticles (P3HT): (ZnO) active layer have been considerable increasing the performance of solar cell. Also, the solar cell devices have been fabricated with a weight ratio of 1:0.7, 1:0.8, 1:0.9 and 1:1 of P3HT and ZnO, respectively. In addition, photo physical characteristics regarding such devices with different value of the weight ratio were examined. This work is indicating that the absorption spectrum related to blend will be broad with varying ratios that was extremely required for the devices of the organic solar cells. Furthermore, the film morphology was estimated via atomic force microscope (AFM). EQE (i.e. External quantum efficiency) and XRD patterns measures were achieved for the optimal device, while the improvement in the efficiency with regard to a device with 1:1 was more considerable compared to 1:0.90, 1:0.80 as well as 1:0.70 values of weight ratio of P3HT and ZnO. With different weight ratio values, a solar cell upon (1:1) provides PCE (i.e. Power Conversion Efficiency) of 4.1%, dissimilar to 3.92% for (1:0.9), 3.9% for (1:0.8) and 3:6% devices.

Nickel oxide and polytetrafluoroethylene stacked structure as an interfacial layer for efficient polymer solar cells

An efficient polymer solar cell (PSC) has been demonstrated by incorporating an ultrathin interfacial insulating organic layer of polytetrafluoroethylene (PTFE) between a photoactive layer and hole-collecting buffer layer (NiOx). The photoactive layer is made with bulk heterojunction composites of poly[N-9’’-hepta-decanyl-2,7-carbazolealt-5,5-(4′,7′-di-2-thienyl- 2′,1′,3′-ben-zothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). The PTFE layer not only improves the energy level alignment by forming an interfacial dipole at the interface but also reduces the inherent incompatibility between the hydrophobic active layer and hydrophilic NiOx layer, thereby benefiting to the charge extraction and transport in the solar cells. As a result, with the NiOx/PTFE stacked structure, all the photovoltaic performance parameters are significantly improved, leading to a higher power conversion efficiency (PCE) of up to 7.11% compared to the control device without PTFE layer (PCE 5.50%). The PTFE layer provides a superior alternative to interfacial engineering of the metal oxide/organic semiconductor interface in polymer solar cells and other organic electronic devices.

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.

Effect of size and morphology of Au nanostructures on boosting performance of organic photovoltaic devices: Plasmonic and non-plasmonic effects

We fabricated organic photovoltaic (OPV) devices containing various Au nanostructures mixed with hole-collecting buffer layer. The presence of the Au nanostructures results in enhancement of the external quantum efficiencies (EQE) at dissimilar wavelengths of visible light, which can be attributed to the modulated plasmonic absorption frequency of the Au nanostructures. In addition to this plasmonic effect induced by visible light absorption, an increase in the EQE was also found upon UV excitation, which can be attributed to scattering effects induced by Au particles. The optical response pattern of organic photovoltaic devices can be modulated in a wide range of visible and UV wavelengths, by controlling sizes and shapes of the Au nanostructures.

UV-ozone-treated MoO3 as the hole-collecting buffer layer for high-efficiency solution-processed SQ:PC71BM photovoltaic devices

The enhanced performance of a squaraine compound, with 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor, in solution-processed organic photovoltaic devices is obtained by using UV-ozone-treated MoO3 as the hole-collecting buffer layer. The optimized thickness of the MoO3 layer is 8 nm, at which the device shows the best power conversion efficiency (PCE) among all devices, resulting from a balance of optical absorption and charge transport. After being treated by UV-ozone for 10 min, the transmittance of the MoO3 film is almost unchanged. Atomic force microscopy results show that the treated surface morphology is improved. A high PCE of 3.99% under AM 1.5 G illumination (100 mW/cm2) is obtained.

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.

Wide range thickness effect of hole-collecting buffer layers for polymer:fullerene solar cells

Here we report the wide range thickness effect of hole-collecting buffer layers (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) on the performance of polymer:fullerene solar cells with blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The thickness of the PEDOT:PSS layers was controlled from 3nm to 625nm, followed by characterizations such as optical transmittance, electrical resistances in the in-plane and out-of-plane directions, work functions, contact angles, and device performances. Results showed that the optical transmittance was gradually decreased with the PEDOT:PSS thickness but a maximum value was measured for other properties in the thickness range of 10–30nm. The device performance was noticeably improved with only 3nm-thick PEDOT:PSS layer, while it was almost similar in the thickness range of 30–225nm in the presence of gradual decrease in the surface roughness. The similar device performance between 30nm and 225nm has been assigned to the compensation effect between the reduced electrical resistance (increased conductivity) and the decreased optical transmittance as the thickness of the PEDOT:PSS layer increased.

Effects of Hole-Collecting Buffer Layers and Electrodes on the Performance of Flexible Plastic Organic Photovoltaics

Here we report the influences of the sheet resistance (Rsheet) of a hole-collecting electrode (indium tin oxide, ITO) and the conductivity of a hole-collecting buffer layer (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS) on the device performance of flexible plastic organic photovoltaic (OPV) devices. The series resistance (RS) of OPV devices steeply increases with increasingRsheetof the ITO electrode, which leads to a significant decrease of short-circuit current density (JSC) and fill factor (FF) and power conversion efficiency, while the open-circuit voltage (VOC) was almost constant. By applying high-conductivity PEDOT:PSS, the efficiency of OPV devices with highRsheetvalues of 160 Ω/□ and 510 Ω/□ is greatly improved, by a factor of 3.5 and 6.5, respectively. These results indicate that the conductivities of ITO and PEDOT:PSS will become more important to consider for manufacturing large-area flexible plastic OPV modules.

Enhancing the performance of P3HT:ICBA based polymer solar cells using LiF as electron collecting buffer layer and UV–ozone treated MoO3 as hole collecting buffer layer

Efficient bulk-heterojunction (BHJ) polymer solar cells (PSCs) based on poly (3-hexylthiophene) (P3HT): indene-C60 bisadduct (ICBA) were fabricated with lithium fluoride (LiF) as electron collecting buffer layer and UV–ozone (UVO) treated molybdenum trioxide (MoO3) as hole collecting buffer layer. LiF is observed to be critical for the device performance, significantly enhancing the fill factor (FF) and the efficiency compared to the device without LiF layer. MoO3 layer is also critical to the device performance, efficiently extracting holes to prevent the exciton quenching and reducing the interfacial resistance because of alignment of energy band. After UVO treatment on MoO3 film, its surface morphology, transmittance and film quality can be improved, therefore, it is beneficial to the performance of the PSCs. The optimal thickness of MoO3 and UVO treatment time on MoO3 was 12nm and 15min, respectively. Based on these optimal parameters, a power conversion efficiency (PCE) of 6.43% was obtained under AM1.5G 100mW/cm2 illumination.

Effect of Inorganic Nanoparticle Addition to the Hole-Collecting Buffer Layers in Polymer Solar Cells

We investigated the influence of nickel oxide (NiO) nanoparticles that are incorporated into the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] on the performance of polymer:fullerene solar cells. To understand the optimum composition of NiO nanoparticles, the composition of NiO nanoparticles was varied from 0 wt% to 23 wt%. Results showed that the optical transmittance was gradually decreased as the NiO content increased. However, the device performance (short circuit current density, fill factor, series resistance, power conversion efficiency) exhibited a two stage trend in a boundary of approximately 9 wt% NiO content. This trend was in good agreement with the trend of sheet resistance in the presence of slight discrepancy owing to the different charge transport geometry.

Towards fabrication of high-performing organic photovoltaics: new donor-polymer, atomic layer deposited thin buffer layer and plasmonic effects

Using a novel polymer (polythienothiophene-co-benzodithiophenes 7 F-20) as a donor and phenyl-C71-butyric acid methyl ester as an acceptor of bulk heterojunction, inverted organic photovoltaics (OPVs) were fabricated. Wet-chemically prepared ZnO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were used as buffer layers. Particularly, for PEDOT:PSS deposition, no annealing step was employed. This inverted OPV showed a power conversion efficiency (PCE) of ∼7.0%, which is comparable to the hitherto reported highest efficiency of the inverted OPV with vacuum-deposited MoO3 as hole-collecting buffer layers without plasmonic enhancements. Incorporation of Au nanoparticles into PEDOT:PSS was performed for plasmonic enhancement of the electromagnetic field, whereas ZnO thin layers were deposited on ZnO using atomic layer deposition for quenching electron–hole recombination at surface defects of ZnO ripples. These additional treatments could be used for improving the performance of OPV, which ultimately resulted in a PCE of 7.9%.

Device Performance and Lifetime of Polymer:Fullerene Solar Cells with UV-Ozone-Irradiated Hole-Collecting Buffer Layers

We report the influence of UV-ozone irradiation of the hole-collecting buffer layers on the performance and lifetime of polymer:fullerene solar cells. UV-ozone irradiation was targeted at the surface of the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layers by varying the irradiation time up to 600 s. The change of the surface characteristics in the PEDOT:PSS after UV-ozone irradiation was measured by employing optical absorption spectroscopy, photoelectron yield spectroscopy, and contact angle measurements, while Raman and X-ray photoelectron spectroscopy techniques were introduced for more microscopic analysis. Results showed that the UV-ozone irradiation changed the chemical structure/composition of the surface of the PEDOT:PSS layers leading to the gradual increase of ionization potential with irradiation time in the presence of up-and-down variations in the contact angle (polarity). This surface property change was attributed to the formation of oxidative components, as evidenced by XPS and Auger electron images, which affected the sheet resistance of the PEDOT:PSS layers. Interestingly, device performance was slightly improved by short irradiation (up to 10 s), whereas it was gradually decreased by further irradiation. The short-duration illumination test showed that the lifetime of solar cells with the UV-ozone irradiated PEDOT:PSS layer was improved due to the protective role of the oxidative components formed upon UV-ozone irradiation against the attack of sulfonic acid groups in the PEDOT:PSS layer to the active layer.

Effect of nanoparticle stabilizing ligands and ligand-capped gold nanoparticles in polymer solar cells

Alkyl amine stabilized gold nanoparticles (Au-NPs), with diameters of 1–10 nm, deposited at the interface of the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)] and poly(3-hexylthiophene):[6,6]-phenyl C 61-butyric acid methyl ester (P3HT):(PCBM) active layer were found to significantly reduce solar cell performance. The reduced performance was attributed to interactions between the amine based organic stabilizers and the PEDOT:PSS buffer layer. This was confirmed by screening various organic stabilizing ligands at the interfacial layer and measuring the resulting solar cell performances. In addition, it was found that alkyl thiol capped Au-NPs (5–10 nm) incorporated at the buffer layer and their corresponding alkyl thiol stabilized ligands had a lesser impact on the overall performance of the polymer solar cells.

Influence of Controlled Acidity of Hole-Collecting Buffer Layers on the Performance and Lifetime of Polymer:Fullerene Solar Cells

We report the influence of the controlled acidity of the hole-collecting buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), on the performance and lifetime of polymer:fullerene solar cells. The acidity was controlled by adding a strong base (NaOH) to the pristine PEDOT:PSS solutions. The NaOH-modified PEDOT:PSS layers were used for fabricating polymer:fullerene solar cells with active layers made from blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The results showed that a small addition of NaOH (0.2 molar ratio) removed 23% of the sulfonic acid groups but did not change the device performance, even though further NaOH addition degraded the device performance owing to an increased sheet resistance and lowered work function, as well as a changed surface morphology. Storage lifetime tests showed that the device with the modified PEDOT:PSS layer (0.2 molar ratio NaOH) was almost not degraded, whereas the pristine PEDOT:PSS...

Influence of UV-Ozone Treatment to Hole-Collecting Buffer Layer on the Performance of Polymer Solar Cells

Here we report the influence of a UV-ozone treatment on the performance of polymer solar cells. As an initial attempt, we tried to treat the hole-collecting buffer layer, poly(3,4 ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS), with UV-ozone for a short time (5 seconds) before coating the active layer that consisted of poly(3-hexylthiophene) and soluble fullerene. To understand the effect of the UV-ozone treatment, the dark current density―voltage characteristics were measured and analyzed in terms of their leakage current. The results showed that the diode characteristics of the device with the UV-ozone-treated PEDOT:PSS layer were improved, particularly in the presence of a remarkably reduced base current at lower forward voltages and entire reverse bias. Due to the improved diode characteristics, the fill factor of the device with the treated buffer layer was improved, which led to the increased power conversion efficiency.

Effect of Temperature and Light Intensity on the Performance of Polymer/Fullerene Solar Cells with Titanium Dioxide Nanolayers

In this work, alternative architecture for polymer/fullerene solar cells has been explored using titanium dioxide nanolayers which invert the polarity of the cell and may relax the necessity to have a hole-collecting buffer layer poly(styrene sulfonate)-doped poly(ethylene dioxy-thiophene) (PEDOT:PSS). This work particularly focuses on the performance of the inverted devices with dense TiO 2 nanolayers as a function of temperature, illumination intensity and time. We find that both temperature and illumination intensity slightly influence the power conversion efficiency of devices with the PEDOT:PSS layer. However, the inverted solar cells without the PEDOT:PSS layer showed very different characteristics regarding the power conversion efficiency which increased significantly with the operating temperature from 30 °C to 65 °C. This was attributed to a consequence from the strong and positive temperature dependence of open-circuit voltage which may be due to a kink in the current―voltage characteristics near the open-circuit voltage.

Effect of strong base addition to hole-collecting buffer layer in polymer solar cells

We report a brief study on the effect of strong base addition to the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] on the performance of polymer solar cells made using blend films of poly(3-hexylthiophene) and soluble fullerene. A concentrated aqueous solution of sodium hydroxide (NaOH) was added to the PEDOT:PSS solution to decrease its acidity. The optical absorption spectra of modified buffer layers were measured to investigate the influence of NaOH addition on the spectral shape, while the surface of modified buffer layers was examined using atomic force microscopy. Results showed that the acidity of PEDOT:PSS solutions was remarkably reduced by adding the NaOH solution. However, the performance of solar cells was slightly degraded, which has been attributed to the decreased charge transportability as evidenced from the dark current–voltage characteristics.

Polymer Solar Cells with Polymer/Carbon Nanotube Composite Hole-Collecting Buffer Layers

We report the performance of polythiophene/fullerene solar cells with a hole-collecting buffer layer that was made using composite films of functionalized multi-walled carbon nanotube (f-MWCNT) and poly(3,4- ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS). The MWCNT was functionalized with carboxyl groups to bestow solubility in a weak base solvent for mixing with PEDOT:PSS. Results showed that the optical transmittance of the PEDOT:PSS/f-MWCNT composite layer coated substrate sample was slightly improved in some parts of visible and infrared regions. The polymer solar cells with the PEDOT:PSS/f-MWCNT buffer layer exhibited improved short circuit current density but other parameters became poorer than those of control device.