Solution-processed organic films are a facile route to high-speed, low cost, large-area deposition of electrically functional components (transistors, solar cells, emitters, etc.) that can enable a ...diversity of emerging technologies, from Industry 4.0, to the Internet of things, to point-of-use heath care and elder care. The extreme sensitivity of the functional performance of organic films to structure and the general nonequilibrium nature of solution drying result in extreme processing–performance correlations. In this Review, we highlight insights into the fundamentals of solution-based film deposition afforded by recent state-of-the-art in situ measurements of functional film drying. Emphasis is placed on multimodal studies that combine surface-sensitive X-ray scattering (GIWAXS or GISAXS) with optical characterization to clearly define the evolution of solute structure (aggregation, crystallinity, and morphology) with film thickness.
Efficient injection of charge carriers from the contacts into the semiconductor layer is crucial for achieving high-performance organic devices. The potential drop necessary to accomplish this ...process yields a resistance associated with the contacts, namely the contact resistance. A large contact resistance can limit the operation of devices and even lead to inaccuracies in the extraction of the device parameters. Here, we demonstrate a simple and efficient strategy for reducing the contact resistance in organic thin-film transistors by more than an order of magnitude by creating high work function domains at the surface of the injecting electrodes to promote channels of enhanced injection. We find that the method is effective for both organic small molecule and polymer semiconductors, where we achieved a contact resistance as low as 200 Ωcm and device charge carrier mobilities as high as 20 cm
V
s
, independent of the applied gate voltage.
One of the most inspiring and puzzling developments in the organic electronics community in the last few years has been the emergence of solution-processable semiconducting polymers that lack ...significant long-range order but outperform the best, high-mobility, ordered semiconducting polymers to date. Here we provide new insights into the charge-transport mechanism in semiconducting polymers and offer new molecular design guidelines by examining a state-of-the-art indacenodithiophene-benzothiadiazole copolymer having field-effect mobility of up to 3.6 cm(2) V(-1) s(-1) with a combination of diffraction and polarizing spectroscopic techniques. Our results reveal that its conjugated planes exhibit a common, comprehensive orientation in both the non-crystalline regions and the ordered crystallites, which is likely to originate from its superior backbone rigidity. We argue that charge transport in high-mobility semiconducting polymers is quasi one-dimensional, that is, predominantly occurring along the backbone, and requires only occasional intermolecular hopping through short π-stacking bridges.
Solution processing via roll-to-roll (R2R) coating promises a low cost, low thermal budget, sustainable revolution for the production of solar cells. ...Poly(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3' ''-di(2-octyldodecyl)-2,2'; 5',2''; 5'',2'''-quaterthiophen-5 ,5-diyl), PffBT4T-2OD, has recently been shown to achieve high power conversion efficiency (>10%) paired with multiple acceptors when thick films are spun-coat from hot solutions. We present detailed morphology studies of PffBT4T-2OD based bulk heterojunction films deposited by the volume manufacturing compatible techniques of blade-coating and slot-die coating. Significant aspects of the film morphology, the average crystal domain orientation and the distribution of the characteristic phase separation length scales, are remarkably different when deposited by the scalable techniques vs. spun-coat. Yet, we find that optimized blade-coated devices achieve PCE > 9.5%, nearly the same as spun-coat. These results challenge some widely accepted propositions regarding what is an optimal BHJ morphology and suggest the hypothesis that diversity in the morphology that supports high performance may be a characteristic of manufacturable systems, those that maintain performance when coated thicker than approximately 200 nm. In situ measurements reveal the key differences in the solidification routes for spin- and blade-coating leading to the distinct film structures.
Spin-dependent nonlinear processes in organic materials such as singlet-fission and triplet-triplet annihilation could increase the performance for photovoltaics, detectors, and light emitting ...diodes. Rubrene/C
light emitting diodes exhibit a distinct low voltage (half-bandgap) threshold for emission. Two origins for the low voltage turn-on have been proposed: (i) Auger assisted energy up-conversion, and (ii) triplet-triplet annihilation. We test these proposals by systematically altering the rubrene/C
interface kinetics by introducing thin interlayers. Quantitative analysis of the unmodified rubrene/C
device suggests that higher order processes can be ruled out as the origin of the sub-bandgap turn-on. Rather, band-to-band recombination is the most likely radiative recombination process. However, insertion of a bathocuproine layer yields a 3-fold increase in luminance compared to the unmodified device. This indicates that suppression of parasitic interface processes by judicious modification of the interface allows a triplet-triplet annihilation channel to be observed.
A consensus is emerging that mixed phases are present in bulk heterojunction organic photovoltaic (OPV) devices. Significant insights into the mixed phases have come from bilayer stability ...measurements, in which an initial sample consisting of material pure layers of donor and acceptor is thermally treated, resulting in swelling of one layer by the other. We present a comparative study of the stability of polymer/fullerene bilayers using two common OPV polymer donors poly(3-hexylthiophene), P3HT, and polyN-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole), PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl ester, phenyl-C71-butyric acid methyl ester, 60PCBM bis-adduct, and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry to characterize the quasi-steady state behavior of the films, we find that the polymer glass transition temperature (T g) is critical to the bilayer stability, with no significant changes occurring below T g of the high T g PCDTBT. Above the polymer T g, we find the behavior is irreversible and most consistent with swelling of the polymer by the fullerene, constrained by tie chains in the polymer network and influenced by the rubbery dynamics of the mixed region. The swelling varies significantly with the nature of the fullerene and the polymer. Across the eight systems studied, there is no clear relationship between swelling and OPV device performance. The relationship between the observed swelling and the underlying fullerene–polymer miscibility is explored via Flory–Rehner theory.
Engineering two-/three-dimensional (2D/3D) perovskite solar cells is nowadays a popular strategy for efficient and stable devices. However, the exact function of the 2D/3D interface in controlling ...the long-term device behavior is still obscure. Here, we reveal a dynamical structural mutation of the 2D/3D interface: the small cations in the 3D cage move towards the 2D layer, which acts as an ion scavenger. If structurally stable, the 2D layer physically blocks the ion movement at the interface boosting the device stability. Otherwise, the 2D layer embeds them, dynamically self-transforming into a quasi-2D structure. The judicious choice of the 2D constituent is decisive in controlling the 2D/3D kinetics and improving the device lifetime, opening a new avenue for perovskite interface design.
We reveal that 2D/3D interfaces are dynamical in nature which is detrimental for long term perovskite solar cells stability.
Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge ...extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.
An in situ back‐contact passivation strategy is adopted to optimize the photovoltaic performance of n–i–p planar perovskite solar cells. Devices with a flat‐band alignment between the perovskite and polymer passivation layer achieve a high photovoltage of 1.15 V and fill factor of 83% with 1.53 eV bandgap perovskite, leading to a stabilized power conversion efficiency of 21.6%.
A novel method of strain‐aligning polymer films is introduced and applied to regioregular poly(3‐hexylthiophene) (P3HT), showing several important features of charge transport. The polymer backbone ...is shown to align in the direction of applied strain resulting in a large charge‐mobility anisotropy, where the in‐plane mobility increases in the applied strain direction and decreases in the perpendicular direction. In the aligned film, the hole mobility is successfully represented by a two‐dimensional tensor, suggesting that charge transport parallel to the polymer backbone within a P3HT crystal is strongly favored over the other crystallographic directions. Hole mobility parallel to the backbone is shown to be high for a mixture of plane‐on and edge‐on packing configurations, as the strain alignment is found to induce a significant face‐on orientation of the originally highly edge‐on oriented crystalline regions of the film. This alignment approach can achieve an optical dichroic ratio of 4.8 and a charge‐mobility anisotropy of 9, providing a simple and effective method to investigate charge‐transport mechanisms in polymer semiconductors.
A novel method of strain‐aligning polymer films is introduced and applied to the semiconducting polymer, regioregular poly(3‐hexylthiophene) (P3HT). Detailed morphological and electronic characterization is performed on the aligned film, showing several important features of charge transport. The polymer backbone is shown to align in the direction of applied strain resulting in a mobility anisotropy as high as 9. Reported results indicate that charge transport parallel to the polymer backbone within a P3HT crystal is strongly favored over the other crystallographic directions. (OTFT = organic thin‐film transistor)
Colloidal quantum dots (CQDs) are of interest in light of their solution‐processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the ...properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p‐type hole‐transport layer (HTL) implemented using 1,2‐ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic‐acid‐crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).
A chemically orthogonal hole transport layer for lead sulfide colloidal quantum dot (CQD) solar cells is introduced. By substituting the 1,2‐ethanedithiol‐treated CQDs with malonic‐acid‐treated CQDs, the surface chemistry of the active layer is preserved. This increases the charge diffusion length by 1.4×, enabling near‐unity charge extraction efficiency at the back electrode, achieving 13.0% efficiency.
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