Synergistic side-chain engineering and random terpolymerization strategy toward optimized molecular aggregation for efficient organic solar cells

Performance investigation of ZnO/PVA nanocomposite film for organic solar cell

Physical processes in organic solar cells

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Side-chain engineering has been an effective strategy in tuning electronic energy levels, intermolecular interaction, and aggregation morphology of organic photovoltaic materials, which is very important for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, two D-A copolymers, PBQ5 and PBQ6, are designed and synthesized based on bithienyl-benzodithiophene (BDTT) as the donor (D) unit, difluoroquinoxaline (DFQ) with different side chains as the acceptor (A) unit, and thiophene as the π-bridges. PBQ6 with two alkyl-substituted fluorothiophene side chains on the DFQ units possesses redshifted absorption, stronger intermolecular interaction, and higher hole mobility than PBQ5 with two alkyl side chains on the DFQ units. The blend film of the PBQ6 donor with the Y6 acceptor shows higher and balanced hole/electron mobilities, less charge carrier recombination, and more favorable aggregation morphology. Therefore, the OSC based on PBQ6:Y6 achieves a PCE as high as 17.62% with a high fill factor of 77.91%, which is significantly higher than the PCE (15.55%) of the PBQ5:Y6-based OSC. The PCE of 17.62% is by far one of the highest efficiencies for the binary OSCs with polymer donor and Y6 acceptor.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Solar energy is considered as one of the most promising green energy due to clean, safe, long life, and renewable advantages. Solar cell is an electrical device that converts solar energy directly into electric power on the basis of photovoltaic effect. Fritts built the first solar cell in 1883. Although the initial efficiency was only 1%, it has been exciting to know that the power conversion efficiency of solar cells is endlessly improved since it was reported. Up to now, the efficiency of commercial silicon solar cell is between 10%–18%. The latest research shows that the best efficiency of perovskite solar cell has been improved to be over 20% within several years. As it is well known, the maximum power conversion efficiency of single-junction solar cells are only around 33% according to the Shockley- Queisser limit. With the development of new materials and high technologies, one issue is emerging: What is the ultimate efficiency of solar cells? The highest efficiency of solar cell is possible to break the Schockley-Queisser limit? How to further enhance the efficiency of solar cell? All the questions are hard to answer at present. Surrounding these concerns, this article presents a brief review about the efficiency limits of different solar cells. It might help to understand the ultimate efficiency of solar cells. The details address the basic principle, advantages and disadvantages of single-junction, multi-junction and other new concept solar cells regarding the power conversion efficiency. The review also includes the solar cell materials involving silicon, compound, perovskite, quantum dot, hybrid materials, etc. Finally, we look ahead into the prospect of new concept solar cells and the maximum power conversion efficiency.

Gold nano-island structures were fabricated on the light-illumination side of an organic solar cell device to investigate how the light scattering by localized surface plasmon resonance influences the quantum efficiency of an organic solar cell. A light beam from a solar simulator experiences multiple interaction processes with the gold nanostructures before reaching the organic active material, which may include the scattering, the reflection, and the absorption by the gold nano-islands. However, only the scattering process may partially contribute to the enhancement of the conversion efficiency. The reflection and absorption processes make the gold nanostructures act as “blockers” and prevent the light from reaching the solar cell device. Even the scattering process may not always play positive roles in improving the performance of the device. Thus, experimental studies in this work intend to find out a balance between the loss and the enhancement mechanisms, so that the efficiency of the solar cell can be improved. Our experimental results found a possibly optimized configuration of the gold nano-island structures, which leads to enhancement of the conversion efficiency of the solar cell device.

Thin-film blends or bilayers of donor- and acceptor-type organic semiconductors form the core of heterojunction organic photovoltaic cells. Researchers measure the quality of photovoltaic cells based on their power conversion efficiency, the ratio of the electrical power that can be generated versus the power of incident solar radiation. The efficiency of organic solar cells has increased steadily in the last decade, currently reaching up to 6%. Understanding and combating the various loss mechanisms that occur in processes from optical excitation to charge collection should lead to efficiencies on the order of 10% in the near future. In organic heterojunction solar cells, the generation of photocurrent is a cascade of four steps: generation of excitons (electrically neutral bound electron-hole pairs) by photon absorption, diffusion of excitons to the heterojunction, dissociation of the excitons into free charge carriers, and transport of these carriers to the contacts. In this Account, we review our recent contributions to the understanding of the mechanisms that govern these steps. Starting from archetype donor-acceptor systems of planar small-molecule heterojunctions and solution-processed bulk heterojunctions, we outline our search for alternative materials and device architectures. We show that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport. As a result, the donor layer needs to be ultrathin, and all layers of the device have to be tuned to account for optical interference effects. Using these optimization techniques, we illustrate cells with 3.1% efficiency for the non-planar chloroboron subphthalocyanine donor. Molecules offering a better compromise between absorption and carrier mobility should allow for further improvements. We also propose a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets. By doping a polymer with a phosphorescent molecule, we demonstrate an increase in the exciton diffusion length of a polymer from 4 to 9 nm. If researchers can identify suitable phosphorescent dopants, this method could be employed with other materials. The carrier transport from the junction to the contacts is markedly different for a bulk heterojunction cell than for planar junction cells. Unlike for bulk heterojunction cells, the open-circuit voltage of planar-junction cells is independent of the contact work functions, as a consequence of the balance of drift and diffusion currents in these systems. This understanding helps to guide the development of new materials (particularly donor materials) that can further boost the efficiency of single-junction cells to 10%. With multijunction architectures, we expect that efficiencies of 12-16% could be attained, at which point organic photovoltaic cells could become an important renewable energy source.

ABX3 Perovskites for Tandem Solar Cells

Charge transfer and charge extraction mechanisms are two prevalent issues in the growing field of organic solar cells. Due to their complexity in nature, new methods need to be involved in addressing the fundamental properties associated with organic polymer solar cells. This dissertation has focused on developing a new method to estimate the charge collection lengths and surface recombination lengths of organic polymer solar cells. Photocurrent spectra have been analyzed systematically to observe the dependence on thickness of the active material. A red shift of the peak of the normalized photocurrent with respect to the device thickness has been further analyzed for two major material systems used in organic polymer solar cells, namely MDMO-PPV: PCBM and P3HT: PCBM. A theoretical model that measures the charge extraction of bulk hetero junction solar cell structures has been used taking into account of three main parameters including charge carrier collection length, absorption variation and surface recombination. This model has led to estimate two important parameters associated with charge transfer, recombination and extraction of organic solar cells which will provide opportunities for improvements in the performance of organic electronic devices. Key results are summarized as follows. A complete analysis of photocurrent spectra has been done to see its variation with active material thickness of well-known two material systems of bulk heterojunction organic solar cells. Results of these preliminary measurements suggest that peak of the photocurrent for both systems red shift with increasing thickness. Charge extraction model is introduced to explain the initial red shift of the photocurrent. This model fits well with the experimental results. Further analysis of the model suggests that the charge collection lengths can be estimated for organic polymer structures. Theoretical model gives higher collections lengths for MDMO-PPV solar cells while a lower collection length for P3HT solar cells. This model also has the capability to estimate the surface recombination length of organic bulk heterojunction solar cells. Different interfacial layers have been used to fit to the model calculation. These results suggest that the least surface recombination lengths were achieved with solar cells of PEDOT-PSS. This method can be used to optimize the interfacial layers to improve the efficiency in organic solar cells. AC photocurrent measurements have been carried out to observe the frequency dependence of organic solar cells. Main results show that increasing response time from the light source increases the performance of the solar cells. Further analysis of these

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Molecular photovoltaic devices, in a broad sense, are devices made by using molecules or molecular materials that are capable of converting sunlight into electrical current and voltage. Since the use of low molecular weight organic molecules by Tang,[1] the sensitization of TiO2 with dyes coupled to the use of iodine/iodide red/ox couple by Grätzel and O'Regan,[2] the fabrication of bulk-heterojunction polymer/fullerene-based films by Sariciftci et al.,[3] to the latest breakthrough using hybrid lead based perovskite semiconductor materials by Miyasaka and collaborators,[4] there has been an amazing journey harnessing solar power. The dream for cheap, globally available solar energy is closer than ever with a great deal of excellent research being done in laboratories worldwide. This special collection of research articles in Solar RRL presents a flavor of the research done recently in hot research topics from dye sensitized solar cells, organic solar cells, and perovskite solar cells and serves as an example about how proactive the research in molecular photovoltaic devices is. Photochromic thin films based on Grätzel solar cells represent the ultimate frontier in the application of transparent devices that hold the promise for efficient translucid solar cells for applications in building photovoltaics. The dye design still holds the key for the optimization of the solar cell efficiency, when the primary role of the solar cell is not just the efficiency but also its transparency and its operational stability. In organic solar cells, the design of molecules and organic materials with alike goal, transparency, is also key. Moreover, taking into account the general low open circuit voltage in organic solar cells, it is also crucial to understand and reduce the possible pathways for voltage losses, which leads to better solar cell efficiencies. In fact, the efficiency of organic solar cells has exponentially increased during the last few years due to the synthesis and use of non-fullerene electron acceptor molecules and molecular materials. Further research on non-fullerene electron acceptors, as well as organic doping of semiconductors, will remain a hot topic in the near future with the challenge to match the current efficiency of their perovskite-based counterparts. Perovskite semiconductor materials for photovoltaic applications have been, without doubt, a major achievement in recent research. Applications ranging from energy conversion devices, artificial lighting, sensors and photonic applications, have rapidly appeared in the scientific literature since 2009. Indeed, scientific challenges remain, such as the passivation of defects to boost the solar cell efficiencies and the replacement of lead by a less harmful element, but the gap between laboratory and industrial deployment is closing rapidly. For their industrial application, better materials for grids, low-cost manufacture contacts, and treatments to increase the device stability are goals that will be the focus of present and future intense research. Emilio Palomares studied Biology at the UVEG. After graduating, he joined Prof. Hermenegildo García's group at the UPV where he got his PhD. In 2009, he was awarded an ERC Starting Grant to work on quantum dots for energy conversion devices and an ERC PoC in 2015. In 2019, he was awarded Energy & Environmental Solutions International Chair (E2S) by the Université de Pau et des Pays de l'Adour (UPPA). Later in 2020, he was elected ICIQ Director for the next 5 years. His research is focused on several aspects of light induced electron transfer reactions in supramolecular structures and nanostructured inorganic materials, which has evolved towards the control and improvement of the reactions that govern the efficiency on devices such as molecular solar cells, the creation of new hybrid nanomaterials for hydrogen production, and molecular based sensing devices to detect toxic substance in the environment. He is also involved in promoting science and education in society through chemistry workshops for primary and secondary schools.

Non-fullerene acceptors are currently widely used as the acceptors of organic thin film solar cells due to their high efficiency. However, the molecular mechanism is still unclear. We have studied the excited states in the film states of non-fullerene acceptors together with experimental collaborators by using theoretical methods. In this presentation, we will present recent progress of our research. For example, we will talk about the excited states in the film states of ITIC. ITIC is a popular acceptor-donor-acceptor-type non-fullerene acceptor. Recently, our collaborator, Prof. Yamakata at Okayama University, Japan, found that the charge separation can take place in ITIC acceptor film even without donor molecules. We investigated this phenomenon by using MD simulations and quantum chemical calculations. MD simulations of the ITIC film showed that the acceptor parts of ITIC are stacked with each other. The dominant angle between two stacked ITIC was found to be ~180 degrees, making J-type dimer. On the other hand, it was also found that there are quite a few V-type dimers with the angles less than 90 degrees. Quantum chemical calculations revealed that the dipole moment in the excited state of J-type dimer is almost zero whereas that of V-type dimer is quite large, indicating strong charge-transfer excited state. Therefore, it is considered that the charge separation of ITIC film can take place in the V-type dimers. We also investigated the excited states in the film states of ITIC analogs. Recently, our collaborator, Prof. Ie at Osaka University, Japan, synthesized several ITIC analogs with different sidechains and found the different efficiency of solar cells. We theoretically investigated the excited states in the film sates of ITIC analogs as in the case of ITIC. It is found that the modification of side chains can change the distributions of angles between two stacked molecules and excited-state properties. These findings highlight the importance of controlling stacking structures in the film state for the efficiency of organic thin film solar cells.

In this work, we report on efficient heterojunction organic solar cells containing a new oligothiophene derivative &#945;,&#945;'-bis-(2,2-dicyanovinyl)-quinquethiophene (DCV5T) as donor (D) and fullerene C₆₀ as acceptor (A). The oligothiophene carries electron withdrawing substituents which increase the ionization energy and even more strongly the electron affinity. In thin films, the absorption is significantly broadened compared to solution and the optical gap is reduced to 1.77 eV. Nevertheless, the material shows strong fluorescence with low Stokes shift (peak at 1.71 eV), i.e. low energy loss upon reorganisation in the excited state. At the heterointerface between the low band-gap oligothiophene and fullerene C₆₀, photogenerated excitons from both materials are efficiently separated into electrons on the LUMO of C₆₀ and holes on the low-lying HOMO of the oligothiophene. This step involves only low energetic losses since both the HOMO and the LUMO offset of the two materials are below 0.6 eV, close to the expected exciton binding energy. We can thus reach high open circuit voltages of up to 1.0 V. The most efficient solar cells with power efficiencies around 4 % are obtained when the photoactive heterojunction is embedded between a p-doped hole transport layer on the anode side and a combination of a thin exciton blocking layer and aluminium on the cathode side. However, due to the high ionization energy of the oligothiophene (approx. (5.6 ± 0.1) eV), hole injection from any anode or hole transport layer is difficult and the IV curves thus show a characteristic S-shape which reduces the fill factor FF. It is found that the actual FF sensitively depends on the work function of the p-doped hole transport layer, that can be influenced by doping.

Developing a high-performance donor polymer is critical for achieving efficient non-fullerene organic solar cells (OSCs). Currently, most high-efficiency OSCs are based on a donor polymer named PM6, unfortunately, whose performance is highly sensitive to its molecular weight and thus has significant batch-to-batch variations. Here we report a donor polymer (named PM1) based on a random ternary polymerization strategy that enables highly efficient non-fullerene OSCs with efficiencies reaching 17.6%. Importantly, the PM1 polymer exhibits excellent batch-to-batch reproducibility. By including 20% of a weak electron-withdrawing thiophene-thiazolothiazole (TTz) into the PM6 polymer backbone, the resulting polymer (PM1) can maintain the positive effects (such as downshifted energy level and reduced miscibility) while minimize the negative ones (including reduced temperature-dependent aggregation property). With higher performance and greater synthesis reproducibility, the PM1 polymer has the promise to become the work-horse material for the non-fullerene OSC community.