Abstract
Photolithography has been a major enabling tool for miniaturisation of silicon devices that underpinned the electronics revolution. Rapid, high-resolution patterning of key material ...characteristics would, similarly, accelerate the advent of molecular electronics and photonics. Here we advance a versatile approach employing local diffusion of functional small-molecular compounds through a solution-processed ‘molecular gate’ interlayer. Diffusion is activated using laser light or solvent vapour jets―a process that can be finely modulated down to molecule-on-demand deposition precision with almost photolithographic resolution (<5 μm) and speeds (3 mm s
–1
). Examples of principal pattern types are presented including molecular conformation for integrated photonics; chain orientation for polarised security features and micro-engineered electronics; and doping with local conductivity values >3 S cm
–1
for improved electronic devices. Finally, we demonstrate the unique capability for one-step patterning of multiple functionalities by spatially modulating composition in ternary blends, leading to locally tunable photoluminescence from blue to red.
Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable ...and biointegrated electronics, is hampered by the use of bulky dispersive optics. Here, solution‐processed organic donor–acceptor blends are employed in a resonant optical cavity device architecture for wavelength‐tunable photodetection. While conventional photodetectors respond to above‐gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge‐transfer states of the intercalating poly2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno3,2‐bthiophene (PBTTT):6,6‐phenyl‐C61‐butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, near‐infrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors, is fabricated by using a blade‐coated PBTTT:PCBM thin film with a thickness gradient. As an application example, a measurement of the transmittance spectrum of water by this device is demonstrated.
Solution‐processed organic donor–acceptor blends are employed in a resonant optical cavity device architecture, providing tunable and narrowband photodetectors and miniature spectrometers. While conventional photodetectors respond to above‐gap excitation, these cavity devices exploit weak subgap absorption of intermolecular charge‐transfer states of intercalating bimolecular crystals.
While photovoltaic blends based on non‐fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic ...performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene‐based acceptors that readily co‐crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine‐grained ternary blend with nanometer‐sized domains that do not coarsen due to a high Tg ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high‐temperature processing protocols, which are needed for upscaling and high‐throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.
The two non‐fullerene acceptors 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno2,3‐d:2',3'‐d'‐s‐indaceno1,2‐b:5,6‐b'dithiophene (ITIC)‐4F and ITIC‐4Cl co‐crystallize, a process that is suppressed when blended with the donor polymer PTB7‐Th. As a result, the corresponding ternary devices display stable photovoltaic performance up to 170 °C, in contrast to the binary devices that suffer acceptor crystallization. This indicates that acceptor mixtures allow to fabricate devices with excellent thermal stability.
Organic solar cells incorporating non‐fullerene acceptors (NFAs) have reached remarkable power conversion efficiencies of over 18%. Unlike fullerene derivatives, NFAs tend to crystallize from ...solutions, resulting in bulk heterojunctions that include a crystalline acceptor phase. This must be considered in any morphology‐function models. Here, it is confirmed that high‐performing solution‐processed indacenodithienothiophene‐based NFAs, i.e., ITIC and its derivatives ITIC‐M, ITIC‐2F, and ITIC‐Th, exhibit at least two crystalline forms. In addition to highly ordered polymorphs that form at high temperatures, NFAs arrange into a low‐temperature metastable phase that is readily promoted via solution processing and leads to the highest device efficiencies. Intriguingly, the low‐temperature forms seem to feature a continuous network that favors charge transport despite of a poorly order along the π–π stacking direction. As the optical absorption of the structurally more disordered low‐temperature phase can surpass that of the more ordered polymorphs while displaying comparable—or even higher—charge transport properties, it is argued that such a packing structure is an important feature for reaching highest device efficiencies, thus, providing guidelines for future materials design and crystal engineering activities.
Non‐fullerene acceptors used in organic solar cells often crystallize when they are solution processsed. ITIC and its derivatives ITIC‐M, ITIC‐2F, and ITIC‐Th, exhibit various polymorphs, including a metastable form that is readily promoted via solution processing and leads to the highest device efficiencies despite exhibiting poor order along the π–π stacking direction.
While multi‐junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative ...spectral splitting device concept in which various individual semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW. Each lateral sub‐cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub‐cell. Using the resulting design rules, three systems are selected, with narrow, medium, and wide effective bandgaps, and their potential as sub‐cells in this geometry is experimentally investigated. With the aid of a custom‐built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub‐cell. Finally, a working proof‐of‐concept monolithic device consisting of two sub‐cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept.
RAINBOW solar cells consist of two or more junctions placed next to each other and illuminated by spectrally spread sunlight. Guidelines for materials selection are given for this novel geometry through device simulations. Moreover, experimental efficiencies greater than 46% with respect to the best sub‐cell are demonstrated for a monolithic two‐junction device based on organic semiconducting absorbers.
Functionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the ...thermoelectric properties of a chemically doped conjugated polymer is studied. The in‐plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 µm wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight‐fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll‐off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices.
Electrically programmed doping gradients are found to enhance the thermoelectric power factor of a chemically doped conjugated polymer. The in‐plane drift of counterions in moderate electric fields is used to create lateral doping gradients in doped polymer films, resulting in a decrease of the effective electrical conductivity with bias voltage but an overall increase in Seebeck coefficient.
Organic solar cells based on ternary active layers can lead to higher power conversion efficiencies than corresponding binaries, and improved stability. The parameter space for optimization of ...multicomponent systems is considerably more complex than that of binaries, due to both, a larger number of parameters (e.g., two relative compositions rather than one) and intricate morphology–property correlations. Most experimental reports to date reasonably limit themselves to a relatively narrow subset of compositions (e.g., the 1:1 donor/s:acceptor/s trajectory). This work advances a methodology that allows exploration of a large fraction of the ternary phase space employing only a few (<10) samples. Each sample is produced by a designed sequential deposition of the constituent inks, and results in compositions gradients with ≈5000 points/sample that cover about 15%–25% of the phase space. These effective ternary libraries are then colocally imaged by a combination of photovoltaic techniques (laser and white light photocurrent maps) and spectroscopic techniques (Raman, photoluminescence, absorption). The generality of the methodology is demonstrated by investigating three ternary systems, namely PBDB‐T:ITIC:PC70BM, PTB7‐Th:ITIC:PC70BM, and P3HT:O‐IDFBR:O‐IDTBR. Complex performance‐structure landscapes through the ternary diagram as well as the emergence of several performance maxima are discovered.
A novel methodology to efficiently screen ternary blends for organic photovoltaics is presented. Ternary blend libraries based on lateral gradients are generated. These are then, colocally and hyperspectrally imaged in terms of performance and composition. This ultrafast approach uncovers complex efficiency landscapes and helps to navigate the ternary phase space.
Crystallization and phase segregation during thermal annealing lead to the increase of power‐conversion efficiency in poly(3‐hexylthiophene) (P3HT):6,6‐phenyl C61‐butyric acid methyl ester (PCBM) ...bulk‐heterojunction solar cells. An understanding of the length and time scale on which crystallization and phase segregation occur is important to improve control of the nanomorphology. Crystallization is monitored by means of grazing incidence X‐ray diffraction in real time during thermal annealing. Furthermore, the change in film density is monitored by means of ellipsometry and the evolution of carrier mobilities by means of field effect transistors, both during annealing. From the combination of such measurements with those of device performance as a function of annealing time, it is concluded that the evolution of microstructure involves two important time windows: i) A first one of about 5 minutes duration wherein crystallization of the polymer correlates with a major increase of photocurrent; ii) a second window of about 30 minutes during which the aggregation of PCBM continues, accompanied by an increase in the fill factor.
The evolution of morphology in poly(3‐hexylthiophene) (P3HT):6,6‐phenyl C61‐butyric acid methyl ester (PCBM) bulk‐heterojunction solar cells during thermal annealing is studied by in situ grazing incidence X‐ray diffraction, ellipsometry, and field‐effect‐transistor measurements. We conclude that the solar‐cell performance initially improves thanks to the crystallization of the polymer chains, and then to the diffusion and aggregation of fullerene molecules.
UV‐induced switching from p‐ to n‐type character is demonstrated during deposition of carbon‐nanotube–conjugated polymer composites. This opens the possibility to photopattern n‐type regions within ...an otherwise p‐type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.
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