18% Efficiency organic solar cells

We theoretically investigate the effect of the incoherent glass substrate on the absorption efficiency of organic solar cells (OSCs) at oblique incidence. The light propagation in the mixed incoherent glass substrate and coherent multilayer is calculated based on the transfer matrix method with a simple correction factor, which is derived at oblique incidence by taking multiple reflections inside the glass substrate. The validity of the proposed method is demonstrated by comparing the calculation results with those obtained by the generalized transfer matrix method. We investigate how the multiple reflections within the incoherent glass substrate affect the light absorption efficiency of thin-film OSCs, depending on the incidence angle and polarization.

Organic solar cells (OSCs) have reached an outstanding certified power conversion efficiency (PCE) of over 19% in single junction and 20% in tandem architecture design. Such high PCEs have emerged with outstanding Y-shaped Y6 non-fullerene acceptors (NFAs), together with PM6 electron donor polymers. PCEs are on the rise for small-area OSCs. However, large-area OSC sub-modules are still unable to achieve such high PCEs, and the highest certified PCE reported so far is ∼12% having an area of 58 cm2. To fabricate efficient large-area OSCs, new custom-designed NFAs for large-area systems are imminent along with improvements in the sub-module fabrication platforms. Moreover, the search for stable yet efficient OSCs is still in progress. In this review, progress in small-area OSCs is presented with reference to the advancement in the chemical structure of NFAs and donor polymers. Finally, the life-cycle assessment of OSCs is presented and the energy payback time of the efficient and stable OSCs is discussed and lastly, an outlook for the OSCs is given.

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.

Organic solar cells (OSCs) are attractive as an alternative to inorganic devices for their easy fabrication and solution-processability. A major and unsolved problem with bulk heterojunction devices remains the optimization of the network morphology. Here, we discuss the influence of the 1,8-diiodooctane (DIO) solvent additive on the efficiency of OSCs and show that by selectively controlling the crystallization of the organic material, the power conversion efficiency (PCE) can be increased by about 30%. For P3HT:PCBM-based devices, the power conversion efficiency (PCE) was increased from 3.7% to 4.9% for PCPDTBT:P3HT:PCBM-based devices from 3.2% to 4.1%. This improvement is due to the higher I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> , which is in agreement with the higher external quantum efficiency (EQE) observed on the devices fabricated with DIO. We correlate this to an increase of the surface roughness observed with atomic force microscopy (AFM) analysis. We demonstrate that the effect of the DIO additive is equivalent to a high-temperature thermal annealing.

Physical processes in organic solar cells

To enhance the light trapping of organic solar cells (OSCs), metallic (e.g. Au, Ag) nanoparticles (NPs) have been incorporated into the polymer layers conveniently in solution process. Although power conversion efficiency (PCE) of OSCs has been shown to improve by incorporating metallic NPs in either the buffer layer such as poly-(3,4-ethylenedioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS)[1] or the active layer[2], the understanding on the changes is still not quite clear. Moreover, there are very limited studies on incorporating metallic NPs in more than one organic layer and investigating their effects on the optical and electrical properties as well as the performances of OSCs. In this work, monofunctional poly(ethylene glycol) (PEG)-capped Au NPs of sizes 18 nm and 35 nm are doped in the PEDOT:PSS and poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) layers respectively, leading to an improvement of PCE by ~22% compared to the optimized control device. We will firstly identify the impact of NPs in each polymer layer on OSC characteristics by doping Au NPs in either the PEDOT:PSS or P3HT:PCBM layer. Then, we will investigate Au NPs incorporated in all polymer layers. We demonstrate that the accumulated benefits of incorporating Au NPs in all organic layers of OSCs can achieve larger improvements in OSC performances.

The power conversion efficiencies of organic solar cells fabricated with Ag and Ti nanoparticle arrays using nanosphere lithography are studied. The Ag nanoparticle array exhibits a broad absorption spectrum peaked at 420nm, which spectrally overlaps with the absorption band of the organic absorbing layer centered at 520nm, while no peak presented for the Ti nanoparticle array. Power conversion efficiencies by the solar cells with Ag and Ti nanoparticle arrays are 2.42% and 1.68%, respectively. This efficiency improvement is proposed to originate from the strong surface plasmon resonant scattering of visible light by Ag nanoparticle arrays.

Ternary organic solar cells based on non-fullerene acceptors: A review

Organic solar cells (OSCs) are widely studied for their advantages such as simple production, low cost, good flexibility, and large‐area printing. The buffer layer between electrodes and the photoactive layer has a significant impact on the efficiency and stability of OSCs. In order to lower the energy barrier height at the interface, provide an Ohmic contact with lower series resistance, and increase the charge collection efficiency of the corresponding electrodes for either holes or electrons, suitable electrode buffer materials can be chosen. The selection of electrode buffer layer has an important effect on the photoelectric conversion efficiency. Therefore, the study of buffer layer characteristics has certain guiding significance for the improvement of device structure and performance optimization. At present, the buffer layer materials used in OSCs are mainly organic polymer materials, organic small molecule materials, metal fluoride, metal oxides, and so on. Herein, the structure and working principle of OSCs are introduced first. Then, the properties and function of buffer layer in OSCs are summarized and discussed. Finally, the application of common buffer layer materials in OSCs is summarized, and how to improve the performance of the device is described.

Thermally-induced wrinkles on PH1000/graphene composite electrode for enhanced efficiency of organic solar cells

For commercialization of organic solar cells, achieving high power conversion efficiency and prolonged thermal stability remains critical. We systematically investigated the thermal durability of PM6:Y6‐based organic solar cells incorporating PDINN and PFN‐Br as organic electron transport layers. PDINN‐based organic solar cells achieved an exceptional power conversion efficiency of 17.06% at room temperature and remarkably maintained &gt;15% power conversion efficiency even under harsh 110 °C thermal treatment. In contrast, PFN‐Br‐based devices initially showed 15.26% power conversion efficiency but exhibited significantly reduced performance with increasing processing temperatures. To elucidate the contrasting thermal behaviors, we conducted a comprehensive comparative analysis of both organic electron transport layer films and their effects on the PM6:Y6 active layer through advanced thermal analysis, optical spectroscopy, surface morphological characterization, and detailed charge dynamics investigations. Our findings reveal that PDINN‐based devices demonstrated superior charge transport efficiency and effectively suppressed recombination processes under thermal stress, primarily attributed to strong hydrogen bonding interactions between PDINN's amine groups and Y6 acceptor molecules. Conversely, PFN‐Br‐based organic solar cells exhibited poor thermal durability due to detrimental bromide ion migration and accumulation at the silver electrode interface. This study demonstrates the critical importance of strategic interfacial engineering for simultaneously improving both efficiency and thermal stability of organic solar cells, providing insights for next‐generation thermally resistant organic photovoltaics.

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.

Typical organic photovoltaic materials show high Urbach energies (ca. 25-50 meV), which is considerably higher than those of their inorganic counterparts and limits further improvement in the device efficiency of organic solar cells (OSCs). In this study, we introduce a facile method of selenium substitution to reduce the Urbach energy of organic photovoltaic materials to 20.4 meV (Y6Se), which is the lowest value reported for high-performance organic photovoltaic materials and very close to those (ca. 15 meV) of typical inorganic/hybrid semiconductors, such as crystalline silicon, gallium nitride, and lead-halide perovskite. Next, OSCs based on Y6Se showed 17.7% efficiency, which is among the best results for OSCs and the record efficiency of as-cast single junction OSCs to date.

The ternary strategy is an effective method to improve the efficiency of organic solar cells (OSCs). Herein, high‐performance OSCs with over 18% efficiency using PM6 as donor and alloy‐like acceptor containing two highly structurally similar acceptors (Y6 and Y6‐1O) is obtained. The spectral overlap of Y6 and Y6‐1O can increase the collection of photons via enhancing the absorption in near infrared region, which is conducive to improve the short‐circuit current density (J SC). Meanwhile, beneficial electron transport channels are established by the construction of cascaded energy levels of Y6 and Y6‐1O in the ternary films. In addition, compared with Y6‐ and Y6‐1O‐based binary devices, enhanced charge mobility and suppressed charge recombination are observed in the optimal ternary OSCs, contributing to better performance. The improved performance of ternary devices based on the introduction of Y6‐1O is also attributed to the enhancement of photon capture and improved charge extraction as well as optimized blend morphology. A very promising ternary strategy is presented with two highly compatible acceptors to synergize the device performance in the development of high‐efficient OSCs.

The localized surface plasmon resonance (LSPR) effects of nanoparticles (NPs) are effective for enhancing the power conversion efficiency (PCE) of organic solar cells (OSCs). In this study, spiky durian-shaped Au@Ag core-shell NPs were synthesized and embedded in the hole transport layer (HTL) (poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)) of PTB7:PC71BM bulk-heterojunction OSCs. Different volume ratios of PEDOT:PSS-to-Au@Ag NPs (8%, 10%, 12%, 14%, and 16%) were prepared to optimize synthesis conditions for increased efficiency. The size properties and surface morphology of the NPs and HTL were analyzed using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). UV–Vis spectroscopy and current density–voltage (J-V) analysis were used to investigate the electrical performance of the fabricated OSCs. From the results, we observed that the OSC with a volume ratio of 14% (PEDOT:PSS–to–Au@Ag NPs) performed better than others, where the PCE was improved from 2.50% to 4.15%, which is a 66% increase compared to the device without NPs.