Acid Methyl Ester Bulk Heterojunction Research Articles

Efficiency enhancement of polymer solar cells with Ag nanoparticles incorporated into PEDOT:PSS layer

In this study, large-sized silver nanoparticles (Ag NPs) (average size: 80 nm) have been introduced into the anodic buffer poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer (thickness: about 55 nm) of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester bulk heterojunction polymer solar cells. The results showed that the short-circuit current density can increase from 8.73 to 11.36 mA/cm2, and power conversion efficiency increases from 2.28 to 2.65 % when 0.1 wt% Ag NPs was incorporated in PEDOT:PSS layer, corresponding to an efficiency improvement of 16.2 %. Absorption spectrums of the active layers indicate that large-sized Ag NPs have no clear contribution to optical absorption improvement. By measuring the conductivity of PEDOT:PSS films without and with Ag NPs and analyzing device structure of this polymer solar cell, it was founded that the improvements in power conversion efficiency was originated from higher conductivity of PEDOT:PSS layer incorporated with Ag NPs and the shorter routes for holes to travel to the anode.

Improving efficiencies by modulating the central metal ion in porphyrin-oligothiophene-mediated P3HT/PCBM organic solar cells

The power conversion efficiencies of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction organic solar cells were increased through the addition of small amounts (1–10%) of metalloporphyrin-oligothiophene conjugates. The effectiveness of these porphyrin additives was found to increase in the order free-base<Co<Zn<Ni<Cu, with the efficiency being enhanced by up to 17% for the terthiophene-substituted Cu(II)-porphyrin, pointing to an important effect of the metal center.

Optimisation of thermal annealing parameters for different thickness of active layers based on polymer/fullerene bulk heterojunction solar cells

Thin film polymer solar cells have been fabricated using regioregular poly(3-hexylthiophene) (P3HT) within [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction active layer. The influence of the thermal annealing temperature and the time duration on the performance of P3HT/PCBM bulk heterojunction solar cells based on different thickness of active layers has been investigated. The results demonstrate that thermal annealing is a thermally activated kinetic process. The effect is almost the same for high temperature annealing for a short time and low temperature annealing for a long time at a temperature greater than the glass temperature of P3HT. The device with a thicker active layer requires a higher temperature or a longer time annealing treatment to achieve the optimised phase separation for efficient charge separation and transport and thereby leads to optimal performance. The device with a thinner active layer requires a lower temperature or a shorter time annealing treatment to achieve the optimal performance.

Effect of a-Si:H–polymer interface on the behavior of hybrid solar cells

Organic photovoltaics (OPVs) are primary candidates for economical and flexible next-generation solar electricity conversion devices. However, efficiencies of the OPVs are limited, among other reasons, by poor charge transport and limited acceptor materials. To overcome this, hybrid organic–inorganic solar cells have been proposed. In this study, the authors investigated the photovoltaic characteristics of hybrid amorphous silicon (a-Si:H) films with varying doping nature with poly(3-hexylthiophene) (P3HT) only and P3HT:phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction–based solar cells. Both intrinsic and doped a-Si:H films were used. Despite the higher short circuit current density of P3HT/intrinsic a-Si:H device, the charge collection and open circuit voltage in P3HT/n+ a-Si:H device were the highest. This was attributed to favorable band bending and high built-in electric field in P3HT. Previously proposed model for photovoltaic mechanism in polymer/a-Si:H devices was revisited, and a revision was proposed. P3HT:PCBM cells on the three types of cells largely followed the same trend as the P3HT-only devices, except that intrinsic device did not have a photocurrent contribution from the organic layer.

High Efficiency of Poly(3-hexylthiophene)/[6,6]-phenyl C61 Butyric Acid Methyl Ester Bulk Heterojunction Solar Cells through Precrystallining of Poly(3-hexylthiophene) Based Layer

An facile approach for improving device efficiency of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) bulk heterojunction solar cells is presented. This method is used by simply precasting a tiny thin P3HT layer with high crystallinity between PEDOT:PSS and photoactive P3HT:PC61BM layers. The high crystalline thin P3HT layers are casted from three different solvents such as dichloromethane (DCM), dichlorobenzene (o-DCB), and tetrahydrofuran (THF). It is demonstrated that THF used for thin P3HT layer preparation is a suitable solvent for yielding a high crystalline film, which is unreadily washed away during the solution processing of the active layer. The results indicate that the morphology of P3HT:PC61BM active layers strongly depend on the formation of P3HT buffer layer. A great morphology difference of P3HT:PC61BM is caused from crystallinity of P3HT buffer layers prepared by different solvents. The thin P3HT layer with high crystallinity can improve the crystalline degree of P3HT in the active layer, subsequently inducing the whole active layer to form a well self-assembled pathway for efficient charge transfer and transportation to their respective electrodes. Therefore, a dramatically enhanced short-circuit current density of the device is resulted. After optimization of thickness of the P3HT buffer layer, an improvement of the power conversion efficiency is obtained from 2.98% to 5.14%.

Small molecule dye rubrene doped organic bulk heterojunction solar cells

Poly(3-hexylthiphene) (P3HT):[6,6]-phenyl C60-butyric acid methyl ester (PCBM) bulk heterojunction solar cells doped with a small molecule dye of rubrene were fabricated. The effect of different doping concentration on the performance of solar cells was investigated. The results showed that by doping 3.5wt% rubrene, the short circuit current and the power conversion efficiency (PCE) of the cell were improved by 23.1% and 23.4%, respectively. The increased light absorption and effective charge separation of blend films contributed to the enhancement of PCE in doping devices. Moreover, by simulating the photocurrent of solar cells using Onsager–Braun theory, it was found that doping rubrene was responsible for the enhancement of charge transfer generation and dissociation, thereby improving the performance of P3HT:PCBM solar cells.

High-performance organic solar cells based on a low-bandgap poly-thienothiophene-benzodithiophene polymer and fullerene composite prepared by using the airbrush spray-coating technique

In the present work, we have demonstrated high-performance organic solar cells with spray coated active layers. The influence of the nanomorphology on the power conversion efficiency of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl-C71-butyric acid (PC71BM) methyl ester bulk heterojunction solar cells is presented. Active layers were prepared using a chlorobenzene solvent containing a small volume of 1,8-diiodooctane as an additive by spray coating with conventional handheld airbrushes. The surface morphology of the active layers deposited for various spray-coating times was examined using atomic force microscopy. The resulting devices were measured under AM 1.5G (100 mW/cm2) conditions in an ambient atmosphere. The optimized spray-coated PTB7:PC71BM film showed a high solar cell performance with a short-circuit current density of 14.20 ± 0.41 mA/cm2, a fill factor of 56.00 ± 0.02% and a power conversion efficiency of 5.96 ± 0.15%.

Morphological characterization of plastic solar cells using polarized neutron reflectivity

Thermal annealing is known to significantly improve the photovoltaic performance of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. We have used polarized neutron reflectometry to compare the PCBM depth profiles of the same P3HT:PCBM film before and after annealing at conditions know to improve the photovoltaic efficiencies of similarly prepared devices. Contrary to previous findings, annealing is shown to have little effect on the vertical PCBM concentration profile. We attribute the improved efficiency from thermal annealing to increased P3HT crystallinity and/or change in crystal orientation, which inhibits exciton recombination.

Discrepancy of Optimum Ratio in Bulk Heterojunction Photovoltaic Devices: Initial Cell Efficiency vs Long-Term Stability

Organic photovoltaic devices are difficult to commercialize because of their vulnerability to chemical degradation related with oxygen and water and to physical degradation with aging at high temperatures. We investigated the photophysical degradation behaviors of a series of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PC60BM) bulk heterojunctions (BHJs) as a model system according to the donor-acceptor ratio. We found that the optimum P3HT:PC60BM ratio in terms of long-term stability differs from that in terms of initial cell efficiency. On the basis of cell performance decays and time-resolved photoluminescence measurements, we investigated the effects of oxygen and material self-aggregation on the stability of an organic photovoltaic device. We also observed the changes in morphological geometry and analyzed the surface elements to verify the mechanisms of degradation.

Slot-die coating parameters of the low-viscosity bulk-heterojunction materials used for polymer solarcells

Slot-die coating is one of the promising methods for fabricating large-area polymer solar cells (PSCs) due to its compatibility with roll-to-roll printing and one-dimensional direct patterning. In this study, we investigated correlations between conformal film morphology of intermixed poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction (BHJ) material and universal control factors such as shim length, substrate temperature, and coating speed. The observed correlation equation is better matched to the meniscus coating parameter equation rather than to the traditional high-viscosity extrusion slot-die coating parameter equation. Based on our findings regarding the governing factors of the thin film formation of bulk-heterojunction materials, we demonstrated the development of uniform large-area polymer solar cells with power conversion efficiency (PCE) of 3.07% under Air Mass 1.5condition and the possibility of high-throughput production.

Conjugated polyelectrolyte and zinc oxide stacked structure as an interlayer in highly efficient and stable organic photovoltaic cells

A highly efficient and stable inverted organic photovoltaic cell has been demonstrated by incorporating a zinc oxide (ZnO)/conjugated polyelectrolyte (CPE) stacked structure as an electron-transporting layer. The CPE not only improved the electron collection efficiency in inverted devices but also smoothened the ZnO surfaces, thereby reducing the leakage current and improving the device reliability. In this work, two CPEs, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and poly[3-(6-trimethylammoniumhexyl)thiophene] (PTMAHT), are used, and three different interlayers, ZnO, ZnO/PFN, and ZnO/PTMAHT, are systematically studied. By coating PFN onto the ZnO surface, a high power conversion efficiency of 8.54% (with a fill factor of 73%) and ∼17% efficiency improvement are achieved in thieno[3,4-b]-thiophene/benzodithiophene copolymer, PTB7, and [6,6]-phenyl C71 butyric acid methyl ester bulk heterojunction solar cells, along with excellent stability.

Assembly and organization of poly(3-hexylthiophene) brushes and their potential use as novel anode buffer layers for organic photovoltaics

Buffer layers that control electrochemical reactions and physical interactions at electrode/film interfaces are key components of an organic photovoltaic cell. Here the structure and properties of layers of semi-rigid poly(3-hexylthiophene) (P3HT) chains tethered at a surface are investigated, and these functional systems are applied in an organic photovoltaic device. Areal density of P3HT chains is readily tuned through the choice of polymer molecular weight and annealing conditions, and insights from optical absorption spectroscopy and semiempirical quantum calculation methods suggest that tethering causes intrachain defects that affect co-facial π-stacking of brush chains. Because of their ability to modify oxide surfaces, P3HT brushes are utilized as an anode buffer layer in a P3HT-PCBM (phenyl-C₆₁-butyric acid methyl ester) bulk heterojunction device. Current-voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT-PCBM solar cells.

Efficiency enhancement in P3HT-based polymer solar cells with a NaYF4:2% Er3+, 18% Yb3+ up-converter

The commonly used donor material poly(3-hexylthiophene) (P3HT) confines the power conversion efficiency (PCE) in P3HT-based polymer solar cells due to its relatively large bandgap of ∼1.9 eV and the resultant limited absorption wavelength region of less than 650 nm. In this communication, the highly efficient up-conversion (UC) material NaYF4:2% Er3+, 18% Yb3+, converting near-infrared radiation into green and red emissions, is introduced into a P3HT/P3HT:[6,6] phenyl C61 butyric acid methyl ester (PC61BM) bulk heterojunction solar cell, referred to as a “bilayer cell”, to compensate for the non-absorbable wavelength region of P3HT. With an optimal UC doping concentration of 11.7% (weight ratio of UC to P3HT) in the P3HT matrix, the short-circuit current density and PCE for UC-doped bilayer cell are as high as 10.89 mA cm−2 and 3.62%, about 16.6% and 10.7% higher than the P3HT/P3HT:PC61BM bilayer cell and 22.4% and 16.4% higher than the standard P3HT:PC61BM bulk heterojunction one, respectively, although the fill factor in the UC-doped bilayer cell shows a slight decrease. The research result demonstrates that both the emission and the scattering of UC nanoparticles are beneficial to the enhancement of the solar cell's electrical performances.

Correlation of polymeric compatibilizer structure to its impact on the morphology and function of P3HT:PCBM bulk heterojunctions

The impact of various polymeric compatibilizers, including end-functionalized P3HTs and diblock copolymers containing P3HT, on the structure and function of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions is presented. Careful analyses of small angle neutron scattering curves provide a measure of the miscibility of PCBM in P3HT, the average PCBM domain size, and the interfacial area between PCBM and the P3HT-rich phase in the uncompatibilized and compatibilized systems. Differential scanning calorimetry (DSC) also provides information regarding the changes in the crystallinity of P3HT due to the presence of the compatibilizer. Results show that most compatibilizers cause the domain sizes to decrease and the P3HT crystallinity to increase; however, some cause an increase in domain size, suggesting that they are not effective interfacial modifiers. The correlation of morphology with photovoltaic activity shows that the decreased domain size, increased crystallinity and increased interfacial area do not always result in improved power conversion efficiency (PCE). It appears that the introduction of an insulating molecule at the PCBM:P3HT interface as a compatibilizer results in a decrease in PCE. Thus, the presence of the compatibilizer at this interface dominates the photovoltaic activity, rather than the morphological control.

Characterization of indigo-doped poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction solar cells

Poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester bulk hetero-junction photovoltaic cells doped with indigo dye as a p-type solar collector were studied. Doping indigo increases the absorption of the visible light spectrum, as determined using ultraviolet–visible spectrophotometry analysis. The photocurrent was measured in air under a solar simulator with AM 1.5G illumination (100mW/cm2). The best photovoltaic efficiency obtained was 2.55% after doping indigo into the cell.

Manipulating the Morphology of P3HT–PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing

Using grazing incidence X-ray scattering, we observe the effects of solvent vapors upon the morphology of poly(3-hexylthiophene)–phenyl-C61-butyric acid methyl ester (P3HT–PCBM) bulk heterojunction thin film blends in real time; allowing us to observe morphological rearrangements that occur during this process as a function of solvent. We detail the swelling of the P3HT crystallites upon the introduction of solvent and the resulting changes in the P3HT crystallite morphology. We also demonstrate the ability for tetrahydrofuran vapor to induce crystallinity in PCBM domains. Additionally, we measure the nanoscale phase segregated domain size as a function of solvent vapor annealing and correlate this to the changes observed in the crystallite morphology of each component. Finally, we discuss the implications of the morphological changes induced by solvent vapor annealing on the device properties of BHJ solar cells.

Temperature Dependence of Open-Circuit Voltage and UPS Study for P3HT:PCBM Organic Solar Cells

In this paper, the open-circuit voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> of a poly(3-hexylthiophene-2,5-diyl) (P3HT) and a [6,6]-phenyl C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">61</sub> -butyric acid methyl ester (PCBM) bulk heterojunction organic solar cell device was measured at temperatures ranging from 15 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 145 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The temperature dependence of the vacuum shift and of the highest occupied molecular orbital (HOMO) energy level of P3HT and PCBM were measured by ultraviolet photoelectron spectroscopy in the same temperature range. The temperature dependence of the absorption edge was also studied in the same temperature range to obtain the temperature variation of the optical band gap energy <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> . The measured <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> of the devices shows a clear decreasing trend with increasing operating temperature, and the total decrease is found to be about 0.1 V. However, by utilizing the measured values of the HOMO for the P3HT (donor) and of the determined lowest unoccupied molecular orbital (LUMO) for the PCBM (acceptor), we have found that the calculated values of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> and its temperature dependence do not agree with the measured <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> values. This is a clear indication that the expression (HOMO-LUMO)-exciton energy does not hold and that other factors are impacting the value of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> .

Shockley equation parameters of P3HT:PCBM solar cells determined by transient techniques

The Shockley equation (SE), originally derived to describe a p--n junction, was frequently used in the past to simulate current--voltage (j/V) characteristics of organic solar cells (OSC). In order to gain a more detailed understanding of recombination losses, we determined the SE parameters, i.e. the ideality factor and the dark saturation current, from temperature dependent static j/V-measurements on poly(3-hexylthiophene-2,5-diyl)(P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) bulk heterojunction solar cells. As we show here, these parameters are directly related to charge carrier recombination and become also accessible by transient photovoltage and photocurrent methods in the case of field-independent charge carrier generation. Although determined in very different ways, both SE parameters were found to be identical. The good agreement of static and transient approaches over a wide temperature range demonstrates the validity of the Shockley model for OSC based on material systems satisfying the requirement of field-independent polaron-pair dissociation. In particular, we were able to reproduce the photocurrent at various light intensities and temperatures from the respective nongeminate recombination rates. Furthermore, the temperature dependence of the dark saturation current $j_0$ allowed determining the effective band gap of the photoactive blend perfectly agreeing with the literature values of the energy onset of the photocurrent due to charge transfer absorption. We also present a consistent model directly relating the ideality factor to recombination of free with trapped charge carriers in an exponential density of tail states. We verify this finding by data from thermally stimulated current measurements.

Utilizing insulating nanoparticles as the spacer in laminated flexible polymer solar cells for improved mechanical stability

Roll-to-roll lamination is one promising technique to produce large-area organic electronic devices such as solar cells with a large through output. One challenge in this process is the frequent electric point shorting of the cathode and anode by the excess or concentrated applied stress from many possible sources. In this paper, we report a method to avoid electric point shorting by incorporating insulating and hard barium titanate (BaTiO3) nanoparticles (NPs) into the active layer to work as a spacer. It has been demonstrated that the incorporated BaTiO3 NPs in poly(3-hexylthiophene):[6,6]-phenyl-c-61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells cause no deleterious effect to the power conversion process of this type of solar cell. The resulting laminated devices with NPs in the active layer display the same efficiency as the devices without NPs, while the laminated devices with NPs can sustain a ten times higher lamination stress of over 6 MPa. The flexible polymer solar cell device with incorporated NPs shows a much smaller survivable curvature radius of 4 mm, while a regular flexible device can only sustain a bending curvature radius of 8 mm before fracture.

Effect of a thermally evaporated bis (2-methyl-8-quninolinato)-4-phenylphenolate cathode buffer layer on the performance of polymer photovoltaic cells

We investigated the device characteristics of polymer photovoltaic (PV) cells based on a poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61 butyric acid methyl ester (PCBM) bulk heterojunction with a cathode buffer layer of thermally evaporated bis (2-methyl-8-quninolinato)-4-phenylphenolate (BAlq). A power conversion efficiency (PCE) of 2.46% was obtained with the insertion of a 4-nm-thick BAlq, which was ∼118% increase over that for the cell without a BAlq layer, under Air Mass 1.5 Global (AM 1.5 G) illumination, 100 mW/cm2. Moreover, we examined the charge carrier transport property, and found that the hole mobility of the cell was enhancement due to the insertion of a BAlq layer with a thickness of less than 4 nm, which accounted for the improved in the photocurrent and fill factor (FF) due to the better balance of charge carriers. Finally, the BAlq buffer layer was also demonstrated as an optical spacer that improved the optical absorption of the P3HT:PCBM layer, which accounted for the Jsc enhancement of the device.