Bilayer Organic Solar Cells Research Articles

Controlling the concentration gradient in sequentially deposited bilayer organic solar cells via rubbing and annealing.

We elucidate the formation mechanism of adequate vertical concentration gradients in sequentially deposited poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) bilayer solar cells. Using advanced analytical techniques, we clarify the origins of the enhanced photovoltaic performances of as-deposited and annealed bilayer P3HT/PCBM organic solar cells upon P3HT layer rubbing prior to PCBM deposition. Energy-dispersive X-ray spectroscopy reveals the individual effects of rubbing and annealing on the formation of adequate concentration gradients in the photoactive layers. Repetitive rubbing of P3HT strongly affects the active layer nanomorphology, forming an intermixed layer in the as-deposited devices which is retained after the annealing process. Infrared p-polarized multiple-angle incidence resolution spectrometry measurements indicate that rubbing induces a minor reorganization of the P3HT molecules in the polymer-only thin films towards face-on orientation. However, the deposition of the upper PCBM layer reverts the P3HT molecules back to their original orientation. These findings suggest that the formation of an adequate concentration gradient upon rubbing corresponds to the dominant contribution to the improved photovoltaic characteristics of rubbed bilayer organic solar cells. Using the reference low bandgap copolymer PCDTBT, we demonstrate that rubbing can be successfully applied to increase the photovoltaic performances of PCDTBT/PCBM organic solar cells. We also demonstrate that rubbing can be an efficient and versatile strategy to improve the power conversion efficiency of non-fullerene solar cells by using the reference materials in the field, PBDB-T and ITIC.

A built-in electric field induced by ferroelectrics increases halogen-free organic solar cell efficiency in various device types

In principle, an electric field via ferroelectric materials can affect the photovoltaic properties, although there is not yet a complete mechanistic understanding. Herein, a built-in electric field without a poling processing step was established by introducing developed PVDF-based ferroelectric additives within active-layer matrices of organic solar cells (OSCs). Upon the existence of the ferroelectric polarization induced by the ferroelectric additives in o-xylene/N-methylpyrrolidone pair featuring halogen-free processing system, high efficiencies of 11.02% and 11.76% are achieved in fullerene and non-fullerene acceptor bulk-heterojunction OSCs, respectively. A comparative study exploring the role of the ferroelectric polarization surrounding the active-layer matrix was also performed using structural, electrical, and morphological techniques, to shed light on the underlying ferroelectric polarization effects on OSCs. Furthermore, the use of the ferroelectric additive is extended to p-n like bilayer OSC to access a rich understanding of the complex enhancement mechanisms afforded by it, demonstrating a highly efficient (11.83%) bilayer device. The above results are fairly comparable to the highest value reported for the recently developed state-of-the-art OSCs processed from halogen-free systems. The use of the ferroelectric additives in the halogen-free system is promising in related organic-semiconductor fields for reasons extending beyond the enhancement of efficiency and the environment-friendly manufacturing.

Satisfying both interfacial- and bulk requirements for organic photovoltaics: Bridged-triphenylamines with extended π-conjugated systems as efficient new molecules

Abstract In the domain of organic photovoltaic (OPV) devices, the design of new materials faces the extraordinary challenge to satisfy two opposite requirements: the new materials should exhibit weak intermolecular interactions at the donor-acceptor interface, but strong intermolecular interactions and good charge transport properties in the bulk. In an effort to progress in this direction, here we present three new diphenylethenyl substituted derivatives of methylene-bridged triphenylamine, synthesized by condensation of dimethylmethylene-bridged triphenylamines with 2,2-diphenylacetaldehyde. The synthesized compounds were found to be capable of glass formation with glass transition temperatures in the range of 65–130 °C. The ionization potentials of these derivatives were found to be in the range of 5.29–5.68 eV. The time-of-flight hole drift mobilities measured at room temperature well exceeded 10−2 cm [2]/Vs for all the synthesized compounds. By combining the compounds with fullerene C70 acceptor in bilayer organic solar cells, a power conversion efficiency of 1.9% was obtained. Density functional methods-based calculations indicate that this result can be explained by considering edge-on orientations at the donor-acceptor interface. In this case, the designed architecture of the compounds has an effect to sufficiently “hide” the donor HOMO from a direct and easy contact with C70 LUMO(s), thus maintaining low level of geminate donor-acceptor charge recombination, without losing hole-transport properties in the donor bulk. These two design strategies operate consequently in different parts of the molecules, which is central to the success of the new compounds. Our study shows consequently a possible strategy for a simultaneous improvement of both interfacial- and bulk properties of molecules for OPV applications.

Bilayer–Ternary Polymer Solar Cells Fabricated Using Spontaneous Spreading on Water

AbstractA new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk‐heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin‐coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near‐infrared bias light illumination. Drift‐diffusion calculations confirm that near‐infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.

Impact of Phosphorescent Sensitizers and Morphology on the Photovoltaic Performance in Organic Solar Cells

AbstractPhosphorescent sensitizers (PSs) are considered as a promising alternative for increasing the internal quantum efficiency (IQE) in organic solar cells (OSCs). By converting short‐lifetime singlet into long‐living triplet excitons, enhanced exciton diffusion and dissociation have been reported previously. However, only a limited increase in the OSC performance has been achieved. In this work, the interplay of the PS with both singlet and triplet excitons within organic blends is examined using kinetic Monte Carlo simulations including a comprehensive model of excitonic processes. Different morphologies of the conventional P3HT:PCBM solar cell are simulated, and the excitonic properties and their influence on the photovoltaic performance under doping are studied. The use of phosphorescent sensitization ensures high intersystem crossing and enlarges the diffusion length. An increase in the IQE of 34% is observed for a bilayer OSC. The increasing decay of triplets in proximity to the PS due to a strong spin‐orbit coupling limits the IQE. Unlike expected, triplet‐triplet annihilation does not provide a significant loss of excitons. A doped planar‐mixed molecular heterojunction outperforms an undoped bulk‐heterojunction OSC due to the enhanced exciton diffusion. A further study of optimal PS parameters predicts an increase in the IQE within bilayer solar cells by about 100%.

Design of MoO3 buffer layer for plasmonic organic solar cell

Metal nanoparticles (MNPs) are employed in thin film solar cells for increased light harvesting due to their plasmonic effects, so as to achieve higher device power conversion efficiency. In organic solar cells (OSCs), the MNPs are typically placed next to the photovoltaic layer separated by a buffer layer, the thickness of which affects the optical as well as electrical characteristics of the OSC. Single layer copper phthalocyanine and bilayer CuPC-C60 (buckminsterfullerene) OSC with silver (Ag) MNPs and a Molybdenum oxide (MoO3) buffer layer of different thicknesses have been fabricated and characterised. The proximity of MNP with the photovoltaic layer enhances the short circuit current density (JSC) from 2.56 mA cm−2 to 5.75 mA cm−2 in bilayer OSC. The presence of MNP near the active layer results in lowered shunt resistance and open circuit voltage (VOC), but the introduction of the thin MoO3 buffer layer mitigates the effect. The analysis of the dark characteristics of the OSC reveals the change in built-in potential due to MNP. Photocurrents are analysed to understand the extent of recombination of charge carriers due to the presence of MNP. Both these factors seem to play roles in VOC reduction due to MNP incorporation, but the decrease is offset to some extent by the introduction of the buffer layer in the devices.

Thermally induced crystallization, hole-transport, NLO and photovoltaic activity of a bis-diarylamine-based push-pull molecule

The synthesis of a molecule constituted of two diarylamine-based push-pull chromophores covalently linked via their nitrogen atom is described. Comparison of the electronic properties with the parent monomer shows that dimerization has negligible influence on the electronic properties of the molecule but exerts a dramatic impact on the capacity of the material to self-reorganize. Application of thermal annealing to thin films induces the crystallization under original morphologies, a process accompanied by a partial bleaching of the absorption in the visible range and by a huge increase of hole-mobility. X-ray diffraction data on single crystals reveal the presence of π-stacked organization with a non-centrosymmetric co-facial arrangement of the dipoles which leads to intrinsic 2nd order bulk NLO properties of thin films as evidenced by second harmonic generation under 800 nm laser light. The implications of this thermally induced crystallization on the photovoltaic properties of the material are discussed on the basis of preliminary results obtained on simple bilayer organic solar cells.

Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells.

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.

Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands

In organic solar cells (OSCs), the energy of the charge‐transfer (CT) complexes at the donor–acceptor interface, E CT, determines the maximum open‐circuit voltage (V OC). The coexistence of phases with different degrees of order in the donor or the acceptor, as in blends of semi‐crystalline donors and fullerenes in bulk heterojunction layers, influences the distribution of CT states and the V OC enormously. Yet, the question of how structural heterogeneities alter CT states and the V OC is seldom addressed systematically. In this work, we combine experimental measurements of vacuum‐deposited rubrene/C60 bilayer OSCs, with varying microstructure and texture, with density functional theory calculations to determine how relative molecular orientations and extents of structural order influence E CT and V OC. We find that varying the microstructure of rubrene gives rise to CT bands with varying energies. The CT band that originates from crystalline rubrene lies up to ≈0.4 eV lower in energy compared to the one that arises from amorphous rubrene. These low‐lying CT states contribute strongly to V OC losses and result mainly from hole delocalization in aggregated rubrene. This work points to the importance of realizing interfacial structural control that prevents the formation of low E CT configurations and maximizes V OC.

True nature of active layers in organic solar cells fabricated by sequential casting of donor and acceptor layers

The operation characteristics of nominal bilayer (BL) organic solar cells (OSCs), the active layers (ALs) of which consisted of sequentially casted bottom P3HT donor and top ICBA acceptor layers, resembled those of OSCs with bulk heterojunction (BHJ) ALs. Optical analysis and device simulations showed that such resemblance can be attributed to a similarity in the micromorphology of ALs; as‐deposited BL‐type ALs transformed spontaneously into BHJ‐type ALs. The inclusion of P3HT nanowires (NWs) in the donor layers resulted in different AL micromorphology and consequently a larger power conversion efficiency. Separate assessment of the exciton generation and charge–carrier transport and/or extraction showed that the contribution of P3HT NWs was more prominent in optical effects. magnified imageTransformation of sequentially casted P3HT and ICBA layers into a bulk heterojunction layer accounts for good agreement between measured and simulated J–V curves.

Monomolecular and Bimolecular Recombination of Electron–Hole Pairs at the Interface of a Bilayer Organic Solar Cell

While it has been argued that field‐dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT‐type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p‐DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV‐characteristics as a function of intensity and charge‐extraction‐by‐linearly‐increasing‐voltage‐type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p‐DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s‐shape of the JV‐curves and the concomitant additional decrease of the FF with increasing layer thickness.

Fabrication of efficient graphene-doped polymer/fullerene bilayer organic solar cells in air using spin coating followed by ultrasonic vibration post treatment

In this work, in an attempt to improve the performance and lifetime of organic solar cells, P3HT photon absorbing polymer was doped with graphene (G) nano-sheets, to make light harvesting G-P3HT composite thin film. The composite this film was then employed as the donor of a bilayer organic solar cell with the structure of glass/ITO/PEDOT:PSS/G-P3HT/C60/Al. The reference P3HT:PCBM bulk heterojunction solar cell was also fabricated for comparison. All solution-processed layers were made by spin coating in humid air (Shanghai, China); C60 and Al were deposited by thermal evaporation. An effective mechanical treatment approach developed by the authors, i.e. the application of forced ultrasonic vibration on the wet spun-on films, was used to improve the dispersion of graphene in G-P3HT composite films to obtain a uniform nanostructure. This mechanical method eliminates tedious and expensive chemical steps, currently performed to engineer the structure of organic solar cells. It is evidenced that the G-P3HT composite thin films, post treated by ultrasonic vibration at the optimum vibration duration, possess superior electrical conductivity, charge carrier mobility and density, uniform surface potential distribution, and lower surface roughness, compared to those of P3HT and G-P3HT thin films made without vibration. The results show significant improvement in the power conversion efficiency (PCE) of vibration-treated G-P3HT/C60 cell (PCE = 5.17%, the highest reported for this structure), substantiating the strong positive effect of using graphene and forced vibration for the fabrication of P3HT active layer in the bilayer cell structure.

Reappraising the validity of poly(3-hexylthiophene) nanostructures in interdigitated bilayer organic solar cells

Interdigitated nanostructures can increase power conversion efficiency (PCE) by providing large interfaces for the dissociation of excitons and arrays for carrier transportation without recombination. Here, we reappraise the validity of donor–acceptor nanostructures composed of well-aligned porphyrin–fullerene nanopillars based on their contribution to the PCE compared with bulk heterojunction (BHJ) and bilayer (BL) organic photovoltaic solar cells (OPVs) with high efficiencies. Contrary to expectations, the results showed that poly(3-hexylthiophene) (P3HT):[6, 6]-phenyl C61 butyric acid methyl ester (PCBM) interdigitated nanostructures hardly enhanced the PCE. We suggested three reasons focused on the morphological characteristics: the misbalanced carrier transit time caused by differences in the travel distances and carrier mobilities, detrimental effects on carrier transportation due to the overlap of the pathways to the electrodes, and the dependence of the PCE on the shapes and sizes of the nanostructures.

Role of the Charge-Transfer State in Reduced Langevin Recombination in Organic Solar Cells: A Theoretical Study.

Reduced Langevin recombination has been observed in organic solar cells (OSCs) for many years, but its origin is still unclear. A recent work by Burke et al. (Adv. Energy Mater.2015, 5, 1500123-1) was inspired by this reduced Langevin recombination, and they proposed an equilibrium model of charge-transfer (CT) states that correlates the open-circuit voltage of OSCs with experimentally available device parameters. In this work, we extend Burke et al.’s CT model further and for the first time directly correlate the reduced Langevin recombination with the energetic and dynamic behavior of the CT state. Recombination through CT states leads in a straightforward manner to a decrease in the Langevin reduction factor with increasing temperature, without explicit consideration of the temperature dependence of the mobility. To verify the correlation between the CT states and reduced Langevin recombination, we incorporated this CT model and the reduced Langevin model into drift-diffusion simulations of a bilayer OSC. The simulations not only successfully reproduced realistic current–voltage (J–V) characteristics of the bilayer OSC, but also demonstrate that the two models consistently lead to same value of the apparent Langevin reduction factor.

Modeling of optimum size and shape for high photovoltaic performance of poly(3-hexylthiophene) nanopore in interdigitated bilayer organic solar cells

The interdigitated design for donor–acceptor in solar cell has been studied in some detail, but the optimum size and shape leading to direct enhancement in nanopore (or nanopillar) structure is still not well understood. Here, we demonstrate a modeling method to forecast the optimum size and shape for poly(3-hexylthiophene) (P3HT) nanopores in interdigitated P3HT: [6, 6]-phenyl C61 butyric acid methyl ester (PCBM) photovoltaic device, based on experimental results of P3HT:PCBM bilayer solar cell. In our analysis, the energy generated at unit nanopore is supposed to the same as the one generated at infinite point of P3HT:PCBM bilayer solar cell with variable layer thickness. A definitive function in terms of a radius of unit nanopore with various shapes is established, substituting a regression function derived from the results of power conversion efficiency in bilayer solar cell. Interpreting the function, we finally showed that the effective radius for P3HT nanopores with rectangular or cylinder, cut-cone, cone shape should be less than 135, 53, 2 nm respectively.

High-Performance Small Molecule/Polymer Ternary Organic Solar Cells Based on a Layer-By-Layer Process.

PSS), and then the mixed solution of polymer and fullerene derivative was spin-coated on top of a small molecule layer. In this method, the small molecules in crystalline state were effectively mixed in the active layer. Without further optimization of the morphology of the ternary blend, a high power conversion efficiency (PCE) of 8.76% was obtained with large short-circuit current density (Jsc) (17.24 mA cm(-2)) and fill factor (FF) (0.696). The high PCE resulted from not only enhanced light harvesting but also more balanced charge transport by incorporating small molecules.

Influence of chemically p-type doped active organic semiconductor on the film thickness versus performance trend in cyanine/C60 bilayer solar cells

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C60 bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study.

Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye/fullerene organic solar cells enables one to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron-hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers' generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant.

Effect of molecular electrical doping on polyfuran based photovoltaic cells

The electronic, optical, and morphological properties of molecularly p-doped polyfuran (PF) films were investigated over a wide range of doping ratio in order to explore the impact of doping in photovoltaic applications. We find evidence for integer-charge transfer between PF and the prototypical molecular p-dopant tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and employed the doped polymer in bilayer organic solar cells using fullerene as acceptor. The conductivity increase in the PF films at dopant loadings ≤2% significantly enhances the short-circuit current of photovoltaic devices. For higher doping ratios, however, F4TCNQ is found to precipitate at the heterojunction between the doped donor polymer and the fullerene acceptor. Ultraviolet photoelectron spectroscopy reveals that its presence acts beneficial to the energy-level alignment by doubling the open-circuit voltage of solar cells from 0.2 V to ca. 0.4 V, as compared to pristine PF.

Simulation Studies on Semiconducting Single Walled Carbon Nanotube Based Bilayer and Bulk Hetero Junction Organic Solar Cells

The characteristics of the bilayer and bulk hetero junction (BHJ) model organic solar cell have been modeled and simulated using Silvaco Atlas TCAD. The cell structure used is Graphene/S-SWCNT:C60/Al. The simulations are carried out to obtain the I-V characteristics of an OSC for both the models. The parameters obtained from I-V characteristics are open circuit voltage (Voc) and short circuit current (Isc), maximum voltage (Vm) and maximum current (Im). Using those parameters short circuit current density (Jsc), fill factor (FF) and efficiency are calculated. In bilayer model OSC, efficiency decreased (1.6 to 0.27%) by increasing the thickness of active layer. It shows poor agreement between the exciton diffusion and absorption lengths. Therefore, BHJ model OSC is designed to overcome the exciton diffusion length limitations in bilayer model. It is found that, the efficiency increases with an increase in active layer thickness (5% to 10.4%) for BHJ model. The simulations are carried out to study the effect of active layer thickness on those parameters.