Polymer-fullerene Solar Cells Research Articles

Ternary Polymer–Perylenediimide–Carbon Nanotube Photovoltaics with High Efficiency and Stability under Super-Solar Irradiation

Polymer solar cells (PSCs) have achieved power conversion efficiencies exceeding 10%, but their performance has been limited under concentrated sunlight because of poor stability and recombination processes despite their potential for low-cost concentrated solar power. Recently, ternary polymer solar cell blends have been explored as a strategy to improve PSC performance; however, this approach has been demonstrated only for polymer–fullerene solar cells with organic ternary additives and has not addressed stability issues under supersolar irradiation. Here, we present the first polymer solar cells comprising ternary blends of high efficiency polymers, nonfullerene perylenediimide acceptors, and semiconducting single-walled carbon nanotube additives. We find that the addition of carbon nanotubes reduces efficiency-degrading recombination and improves performance and photostability, most notably under concentrated sunlight exceeding 10 suns. The utilization of carbon nanomaterials as ternary additives in o...

Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells.

In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.

Neutron polarisation analysis of Polymer:Fullerene blends for organic photovoltaics

The photogeneration process in polymer-fullerene organic solar cells relies strongly on the nanostructure and on the nano/picosecond dynamics occurring in these complex blends. Elastic and inelastic neutron scattering techniques are valuable tools with which to investigate those features in the appropriate time and space domains. In particular, quasi-elastic neutron scattering (QENS) connects useful structural and dynamical information by the measurement of dynamical incoherent (single particle) fluctuations in soft materials as a function of lengthscale. Extraction of these fluctuation rates can, however, be hampered by the presence of coherent contributions, originating from elastic scattering, and/or inelastic scattering modes which overlap in the space/time domain with the incoherent single-particle motions. As we have already seen in a previous study [1], this happens in poly(3-hexylthiophene) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) solid blends, in which the coherent contribution arising from the PCBM crystalline phase seems to affect the interpretation of the polymer dynamics. Here, we utilise neutron polarisation analysis as an effective tool to separate coherent and incoherent contributions and make QENS data analysis of these blends more reliable.

Improved efficiency of polymer-fullerene bulk heterojunction solar cells by the addition of Cu(II)-porphyrin-oligothiophene conjugates

Ternary organic photovoltaic devices were prepared through the addition of small amounts of metalloporphyin-terthiophenes to two established low bandgap polymer:fullerene blends. The methodology afforded a PCDTBT:PC71BM-based device that displayed an initial power conversion efficiency of 5.1%, an increase of 16% when compared to the binary blend. Among the range of metalloporphyrins considered in this study, the Cu(II)-porphyrin derivatives were found to result in the most significant efficiency improvements.

Development of polymer–fullerene solar cells

Abstract Global efforts and synergetic interdisciplinary collaborations on solution-processed bulk-heterojunction polymer solar cells (PSCs or OPVs) made power conversion efficiencies over 10% possible. The rapid progress of the field is credited to the synthesis of a large number of novel polymers with specially tunable optoelectronic properties, a better control over the nano-morphology of photoactive blend layers, the introduction of various effective interfacial layers, new device architectures and a deeper understanding of device physics. We will review the pioneering materials for polymer–fullerene solar cells and trace the progress of concepts driving their development. We discuss the evolution of morphology control, interfacial layers and device structures fully exploring the potential of photoactive materials. In order to guide a further increase in power conversion efficiency of OPV, the current understanding of the process of free charge carrier generation and the origin of the photovoltage is summarized followed by a perspective on how to overcome the limitations for industrializing PSCs.

Criteria for validating polaron pair dissociation in polymer-fullerene bulk heterojunction solar cells

The dissociation of polaron pairs into free charge carriers in organic bulk heterojunction solar cells is a fundamental step in generating photocurrent and is still in debate. In this study, we propose two simple criteria that can be used to test the validity of any polaron pair dissociation model for polymer-fullerene bulk heterojunction solar cells. The first criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density should increase as a function of applied voltage. The second criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density at short circuit should not be larger than 1. We apply these criteria to test the validity of the widely used Onsager-Braun model by using the experimental current-voltage data of poly[2-methoxy-5-(3′-7′-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells. We find that our numerical analysis is not suitable to employ these criteria. Our analytical analysis, on the other hand, clearly demonstrates that the Onsager-Braun model simply cannot fulfill the first criteria. The reason is because the polaron pair dissociation given by the Onsager-Braun model is too strongly influenced by the electric field (i.e., decreases too rapidly as the electric field decreases). The analysis provides a further evidence against the widely used Onsager-Braun model. The proposed criteria can help us to determine the correct model for polaron pair dissociation by serving as a guideline on how strongly the electric field is allowed to influence the polaron pair dissociation.

Cathode deposition, paramagnetic defect formation and performance degradation in polymer–fullerene solar cells

Although changes in morphology and defect distribution at the cathode/active layer interface are presumed to limit the efficiency of sputtered cathodes for organic solar cells, little information is available on the nature of defects involved and on possible strategies to mitigate these effects. We demonstrate the sputter-induced formation of paramagnetic centers associated to the change in conformation and amorphization of P3HT in the proximity of the cathodes, which strongly affects the performance of polymer–fullerene bulk heterojunction photovoltaics. Similarity in g-values and X-ray diffraction analysis suggest that defects produced at the cathode/organic interface by sputtering are of the same nature as those produced by thermal annealing at 150–180°C, and that thin calcium layers thermally evaporated prior to cathode sputtering act as thermal sinks mitigating the defect formation by impinging sputtered ions.

Comparing molybdenum oxide thin films prepared by magnetron sputtering and thermal evaporation applied in organic solar cells

We have presented an organic polymer-fullerene solar cell with molybdenum oxide (MoOx) film prepared by direct current magnetron sputtering (sMoOx) and thermal evaporation (eMoOx) as a hole transport layer (HTL), respectively. The result shows that best power conversion efficiency (PCE) of the device using sMoOx as HTL is 3.84 %, while the PCE of the devices with eMoOx film is 3.33 %. The intrinsic mechanisms behind the PCE difference were studied by analyzing the morphology, electrical and optical properties of MoOx thin films prepared by the different methods. The higher PCE obtained in the cells with sMoOx may be due to the smoother interface and higher absorption of the active layer.

Band energy control of molybdenum oxide by surface hydration

The application of oxide buffer layers for improved carrier extraction is ubiquitous in organic electronics. However, the performance is highly susceptible to processing conditions. Notably, the interface stability and electronic structure is extremely sensitive to the uptake of ambient water. In this study we use density functional theory calculations to asses the effects of adsorbed water on the electronic structure of MoOx, in the context of polymer-fullerene solar cells based on PCDTBT. We obtain excellent agreement with experimental values of the ionization potential for pristine MoO3 (010). We find that IP and EA values can vary by as much as 2.5 eV depending on the oxidation state of the surface and that adsorbed water can either increase or decrease the IP and EA depending on the concentration of surface water.

Inverted polymer fullerene solar cells exceeding 10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer layers.

Polymer solar cells have been spotlighted due to their potential for low-cost manufacturing but their efficiency is still less than required for commercial application as lightweight/flexible modules. Forming a dipole layer at the electron-collecting interface has been suggested as one of the more attractive approaches for efficiency enhancement. However, only a few dipole layer material types have been reported so far, including only one non-ionic (charge neutral) polymer. Here we show that a further neutral polymer, namely poly(2-ethyl-2-oxazoline) (PEOz) can be successfully used as a dipole layer. Inclusion of a PEOz layer, in particular with a nanodot morphology, increases the effective work function at the electron-collecting interface within inverted solar cells and thermal annealing of PEOz layer leads to a state-of-the-art 10.74% efficiency for single-stack bulk heterojunction blend structures comprising poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] as donor and [6,6]-phenyl-C71-butyric acid methyl ester as acceptor.

Semiconducting Carbon Nanotubes for Improved Efficiency and Thermal Stability of Polymer–Fullerene Solar Cells

The effects of the incorporation of semiconducting single‐walled nanotubes (sc‐SWNTs) with high purity on the bulk heterojunction (BHJ) organic solar cell (OSC) based on regioregular poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (rr‐P3HT:PCBM) are reported for the first time. The sc‐SWNTs induce the organization of the polymer phase, which is evident from the increase in crystallite size, the red‐shifted absorption characteristics and the enhanced hole mobility. By incorporating sc‐SWNTs, OSC with a power conversion efficiency (PCE) as high as 4% can be achieved, which is ≈8% higher than our best control device. A novel application of sc‐SWNTs in improving the thermal stability of BHJ OSCs is also demonstrated. After heating at 150 °C for 9 h, it is observed that the thermal stability of rr‐P3HT:PCBM devices improves by more than fivefold with inclusion of sc‐SWNTs. The thermal stability enhancement is attributed to a more suppressed phase separation, as shown by the remarkable decrease in the formation of sizeable crystals, which in turn can be the outcome of a more controlled crystallization of the blend materials on the nanotubes.

Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells.

In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.

Flexible, highly efficient all-polymer solar cells.

All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices.

(Invited) Understanding the Role of Organic Alloys in Polymer-Fullerene Solar Cells

While the efficiency of organic solar cells increases beyond 10%, it is still clear that there is much room for improvement in efficiency, lifetime, and cost-effective synthetic and fabrication approaches. We are investigating ternary blend solar cells based on two donor components and one acceptor component (or one donor and two acceptors), which have been recognized as a potential route to increase the absorption breadth of a solar cell and consequently the short-circuit current density. Recently, using a three-component system, we demonstrated for the first time that the open-circuit voltage of ternary blend solar cells is composition dependent and can be tuned across the full range defined by the corresponding limiting binary blends without negatively impacting the fill factor or the short circuit current. As a result, with judicious choice of components, the attainable product of short circuit current and open circuit voltage (and by extension the efficiency) in a single-layer ternary blend solar cell could be higher than is achievable with a standard binary blend solar cell. We have successfully demonstrated higher efficiencies in ternary blends based on two donor polymers and one fullerene acceptor than could be achieved in either of the limiting binary blends. It has become apparent in the systems we have investigated that the formation of so-called organic alloys is critical to the synergistic function of components in these ternary blends. Here, the electronic and physical structure of these organic alloys will be presented and the broader ramifications of organic alloys will be discussed. Efforts toward developing a deeper understanding of the mechanism of operation in these ternary blends will also be discussed.

A model for studying the performance of P3HT:PCBM organic bulk heterojunction solar cells

In the study of bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), P3HT/PCBM device performance is strongly depending on the thickness of active region, materials in use and fabrication methods. In such devices, optoelectronic behaviors such as charge carrier generation and recombination, photocurrent generation and charge transport mechanism are different in devices of different fabrication methods. An electrical model accounting for study of the performance of P3HT:PCBM organic bulk heterojunction solar cells is developed to achieve the device parameter for high performance of the polymer-fullerene bulk heterojunction solar cells. In this model, by solving the drift–diffusion equations by considering the boundary condition and uniform potential energy, the effects of the active region thickness, potential energy and photocurrent generation on the performance of P3HT:PCBM bulk heterojunction solar cell have been studied. Simulated current–voltage characteristics as a function of active layer thickness reveal relatively good agreement between the model's predictions and published modeling and experimental reports.

Characteristics of Polymer-Fullerene Solar Cells with ZnS Nanoparticles

Characteristics of Polymer-Fullerene Solar Cells with ZnS Nanoparticles

A real-time study of the benefits of co-solvents in polymer solar cell processing.

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. Controlling the degree of phase separation between these components is crucial to reach efficient solar cells. In solution-processed polymer-fullerene solar cells, small amounts of co-solvents are commonly used to avoid the formation of undesired large fullerene domains that reduce performance. There is an ongoing discussion about the origin of this effect. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large fullerene-rich domains form via liquid-liquid phase separation at ~20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studied.

Contrasting performance of donor-acceptor copolymer pairs in ternary blend solar cells and two-acceptor copolymers in binary blend solar cells.

Here two contrasting approaches to polymer-fullerene solar cells are compared. In the first approach, two distinct semi-random donor-acceptor copolymers are blended with phenyl-C61-butyric acid methyl ester (PC61BM) to form ternary blend solar cells. The two poly(3-hexylthiophene)-based polymers contain either the acceptor thienopyrroledione (TPD) or diketopyrrolopyrrole (DPP). In the second approach, semi-random donor-acceptor copolymers containing both TPD and DPP acceptors in the same polymer backbone, termed two-acceptor polymers, are blended with PC61BM to give binary blend solar cells. The two approaches result in bulk heterojunction solar cells that have the same molecular active-layer components but differ in the manner in which these molecular components are mixed, either by physical mixing (ternary blend) or chemical "mixing" in the two-acceptor (binary blend) case. Optical properties and photon-to-electron conversion efficiencies of the binary and ternary blends were found to have similar features and were described as a linear combination of the individual components. At the same time, significant differences were observed in the open-circuit voltage (Voc) behaviors of binary and ternary blend solar cells. While in case of two-acceptor polymers, the Voc was found to be in the range of 0.495-0.552 V, ternary blend solar cells showed behavior inherent to organic alloy formation, displaying an intermediate, composition-dependent and tunable Voc in the range from 0.582 to 0.684 V, significantly exceeding the values achieved in the two-acceptor containing binary blend solar cells. Despite the differences between the physical and chemical mixing approaches, both pathways provided solar cells with similar power conversion efficiencies, highlighting the advantages of both pathways toward highly efficient organic solar cells.

Semitransparent polymer-based solar cells with aluminum-doped zinc oxide electrodes.

With the use of two transparent electrodes, organic polymer-fullerene solar cells are semitransparent and may be combined to parallel-connected multijunction devices or used for innovative applications like power-generating windows. A challenging issue is the optimization of the electrodes, to combine high transparency with adequate electric properties. In the present work, we study the potential of sputter-deposited aluminum-doped zinc oxide as an alternative to the widely used but relatively expensive indium tin oxide (ITO) as cathode material in semitransparent polymer-fullerene solar cells. Concerning the anode, we utilized an insulator-metal-insulator structure based on ultrathin Au films embedded between two evaporated MoO3 layers, with the outer MoO3 film (capping layer) serving as a light coupling layer. The performance of the ITO-free semitransparent polymer-fullerene solar cells was systematically studied as dependent on the thickness of the capping layer and the active layer as well as the illumination direction. These variations were found to have strong impact on the obtained photocurrent densities. We performed optical simulations of the electric field distribution within the devices using the transfer-matrix method, to analyze the origin of the current density variations in detail and provide deep insight into the device physics. With the conventional absorber materials studied here, optimized ITO-free and semitransparent devices reached 2.0% power conversion efficiency and a maximum optical transmission of 60%, with the device concept being potentially transferable to other absorber materials.

Charge transfer state energy in ternary bulk-heterojunction polymer–fullerene solar cells

In ternary bulk heterojunction solar cells based on a semiconducting biphenyl-dithienyldiketopyrrolopyrrole copolymer donor and two different fullerene acceptors that distinctly differ in electron affinity, the open-circuit voltage is found to depend in a slightly sublinear fashion on the relative ratio of the two fullerenes in the blend. Similar effects have previously been observed and have been attributed to the formation of an alloyed fullerene phase possessing electronic levels that are the weighted average of the two components. By analyzing the contribution of the charge transfer (CT)-state absorption to the external quantum efficiency of the ternary blend solar cells as a function of composition, we find no evidence for a CT state formed between the polymer and an alloyed fullerene phase. Rather, the results are consistent with the presence of two distinct CT states, one for each polymer–fullerene combination. The two-state CT model does not, however, explain the sublinear behavior of the open-circuit voltage as a function of the blend composition.