Billy

The rapid development of the modern electronics gives rise to higher demands of flexible and wearable energy resources. Flexible transparent conducting electrodes (TCEs) are one of the essential components for flexible/wearable thin-film solar cells (SCs). In this regard, commercial indium tin oxide (ITO) on plastics has demonstrated superior optoelectronic performance although some drawbacks, i.e., the low abundance, film brittleness, low infrared transmittance, and poor chemical stability remain. On the other hand, several other transparent conducting oxide (TCO)-free transparent conductive materials, such as carbon nanotubes (CNTs), graphene, metallic nanowires (NWs), and conducting polymers, have experienced a rapid development to address these issues. In this feature article, an overview over the latest development of several flexible TCO-free thin film SCs, i.e., organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), perovskite solar cells (pero-SCs), and fiber/wire-shaped SCs is provided. Three groups of flexible TCO-free thin film solar cells can be categorized according to their configurations: (i) front-side illuminated planar configuration; (ii) back-side illuminated planar configuration, and (iii) fiber-shaped solar cells (FSSCs). The article is focused on flexible TCO-free TCEs, including CNTs, graphene, metallic NW/nanotroughs, metallic grids, conducting polymers, metallic fiber, and carbon based fibers.

Three groups of flexible transparent conducting oxide (TCO)-free thin film solar cells can be categorized according to their configurations: (i) front-side illuminated planar configuration; (ii) back-side illuminated planar configuration; and (iii) fiber-shaped solar cells. This article is focused on flexible TCO-free transparent conducting electrodes, including carbon nanotubes, graphene, metallic nanowires/nanotroughs, metallic grids, conducting polymers, metallic fiber, and carbon-based fibers.

Two different nonfullerene acceptors and one copolymer are used to fabricate ternary organic solar cells (OSCs). The two acceptors show unique interactions that reduce crystallinity and form a homogeneous mixed phase in the blend film, leading to a high efficiency of ≈10.3%, the highest performance reported for nonfullerene ternary blends. This work provides a new approach to fabricate high-performance OSCs.

Side-chain fluorination of polymers is demonstrated as a highly effective strategy to improve the efficiency of all-polymer solar cells from 2.93% (nonfluorinated P1) to 7.13% (fluorinated P2). This significant enhancement is achieved by synergistic improvements in open-circuit voltage, charge generation, and charge transport, as fluorination of the donor polymer optimizes the band alignment and the film morphology.

Polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. The recent development of semicrystalline polymers with strongly improved charge transport properties forces a review of the current understanding of the charge transport mechanisms and how they relate to the polymer's chemical and structural properties. Here, the charge density dependence of field effect mobility in semicrystalline polymer semiconductors is studied. A simultaneous increase in mobility and its charge density dependence, directly correlated to the increase in average crystallite size of the polymer film, is observed. Further evidence from charge accumulation spectroscopy shows that charges accumulate in the crystalline regions of the polymer film and that the increase in crystallite size affects the average electronic orbitals delocalization. These results clearly point to an effect that is not caused by energetic disorder. It is instead shown that the inclusion of short range coulomb repulsion between charge carriers on nanoscale crystalline domains allows describing the observed mobility dependence in agreement with the structural and optical characterization. The conclusions that are extracted extend beyond pure transistor characterization and can provide new insights into charge carrier transport for regimes and timescales that are relevant to other optoelectronic devices.

The charge density dependence of the charge carrier mobility in semicrystalline polymers is shown to be directly correlated to the increase in crystallite size in the polymer film. The effect is not caused by energetic disorder and can be explained if the morphology of semicrystalline polymers and the effect of electron electron interaction is explicitly taken into account.

A novel naphtho[1,2-c:5,6-c′]bis([1,2,5]­thiadiazole)-based narrow-bandgap π-conjugated polymer is designed for application in polymer solar cells. Remarkable power conversion efficiencies over 10% can be achieved based on both conventional and inverted device architectures with thick photoactive layers, which are processed by using chlorinated or nonhalogenated solvents, suggesting its great promise toward practical applications based on high-throughput roll-to-roll processing.

Small-molecule nonfullerene-based tandem organic solar cells (OSCs) are fabricated for the first time by utilizing P3HT:SF(DPPB)₄ and PTB7-Th:IEIC bulk heterojunctions as the front and back subcells, respectively. A power conversion efficiency of 8.48% is achieved with an ultrahigh open-circuit voltage of 1.97 V, which is the highest voltage value reported to date among efficient tandem OSCs.

All-polymer solar cells (all-PSCs) utilizing p-type polymers as electron-donors and n -typepolymers as electron-acceptors have attracted a great deal of attention, and their efficiencies have been improved considerably. Here, five polymer donors with different molecular orientations are synthesized by random copolymerization of 5-fluoro-2,1,3-benzothiadiazole with different relative amounts of 2,2′-bithiophene (2T) and dithieno[3,2-b;2′,3′-d]thiophene (DTT). Solar cells are prepared by blending the polymer donors with a naphthalene diimide-based polymer acceptor (PNDI) or a [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor and their morphologies and crystallinity as well as optoelectronic, charge-transport and photovoltaic properties are studied. Interestingly, charge generation in the solar cells is found to show higher dependence on the crystal orientation of the donor polymer for the PNDI-based all-PSCs than for the conventional PC₇₁BM-based PSCs. As the population of face-on-oriented crystallites of the donor increased in PNDI-based PSC, the short-circuit current density (J_SC) and external quantum efficiency of the devices are found to significantly improve. Consequently, device efficiency was enhanced of all-PSC from 3.11% to 6.01%. The study reveals that producing the same crystal orientation between the polymer donor and acceptor (face-on/face-on) is important in all-PSCs because they provide efficient charge transfer at the donor/acceptor interface.

Five polymer donors showing different molecular orientations are synthesized by carrying out random copolymerization, and their photovoltaic properties are investigated by fabricating all-polymer solar cells using a PNDI polymer acceptor. As compared with PC₇₁BM-based devices, charge generation in the PNDI-based devices is found to be highly dependent on the orientation of the polymer donor.

Perovskite solar cells (PSCs) are demonstrating great potential to compete with second generation photovoltaics. Nevertheless, the key issue hindering PSCs full exploitation relies on their stability. Among the strategies devised to overcome this problem, the use of carbon nanostructures (CNSs) as hole transporting materials (HTMs) has given impressive results in terms of solar cells stability to moisture, air oxygen, and heat. Here, the use of a HTM based on a poly(3-hexylthiophene) (P3HT) matrix doped with organic functionalized single walled carbon nanotubes (SWCNTs) and reduced graphene oxide in PSCs is proposed to achieve higher power conversion efficiencies (η = 11% and 7.3%, respectively) and prolonged shelf-life stabilities (480 h) in comparison with a benchmark PSC fabricated with a bare P3HT HTM (η = 4.3% at 480 h). Further endurance test, i.e., up to 3240 h, has shown the failure of all the PSCs based on undoped P3HT, while, on the contrary, a η of ≈8.7% is still detected from devices containing 2 wt% SWCNT-doped P3HT as HTM. The increase in photovoltaic performances and stabilities of the P3HT-CNS-based solar cell, with respect to the standard P3HT-based one, is attributed to the improved interfacial contacts between the doped HTM and the adjacent layers.

Improved photovoltaic efficiencies and stabilities over prolonged times are demonstrated for perovskite solar cells based on a poly-3(hexylthiophene) layer doped with organic functionalized single-walled carbon nanotubes and reduced graphene oxide with respect to a reference device based on the undoped polymer.

Organic solar cells (OSCs) and organic-inorganic metal halide perovskite solar cells (pero-SCs) have been regarded as two promising photovoltaic technologies. The recent advances with power conversion efficiency over 10% and 20% have been realized in OSCs and pero-SCs, respectively. The fullerene derivatives play important role as acceptor materials in OSCs and cathode buffer layer (CBL) materials in OSCs and pero-SCs. Here, we provide a comprehensive overview of recent progresses and perspectives of the functional fullerene derivatives as acceptor materials and CBLs for OSCs and pero-SCs.

Fullerene derivatives play a very important role as acceptor materials in organic solar cells (OSCs), cathode buffer layer materials in OSCs and perovskite solar cells (pero-SCs). Here, a comprehensive overview of recent progresses and perspectives of functional fullerene derivatives as acceptor materials and buffer layer materials for OSCs and pero-SCs is provided.

A ternary-blend strategy is presented to surmount the shortcomings of both fullerene derivatives and nonfullerene small molecules as acceptors for the first time. The optimal ternary device shows a high power conversion efficiency (PCE) of 10.4%. Moreover, a significant enhancement in PCE (≈35%) relative to both of the binary reference devices, which has never been achieved before in high-efficiency ternary devices, is demonstrated.

Bis(thiophen-2-yl)-diketopyrrolopyrrole (DPP) dyes bearing various alkyl substituents at the amide positions (n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl) and chlorine (Cl), bromine (Br), or cyano (CN) substituents at the thiophene positions have been synthesized and investigated with regard to their molecular and semiconducting properties. Intense absorption, strong fluorescence, and reversible oxidation and reduction processes are common to all of these dyes. Their characterization as organic semiconductors in vacuum-processed thin-film transistors reveals p-channel operation with field-effect mobilities ranging from 0.01 to 0.7 cm² V⁻¹ s⁻¹. The highest mobility is found for the DPP dyes bearing the 2-ethylhexyl substituents, which is surprising, considering that as a result of the chiral substituents, this material is a mixture of (R,R), (S,S), and (R,S) stereoisomers. The high carrier mobility in the films of the DPPs bearing stereoisomerically inhomogeneous ethylhexyl groups is rationalized here by single-crystal X-ray diffraction (XRD) analysis in combination with XRD and atomic force microscopy studies on thin films, which reveal the presence of slightly different 2D layer arrangements for the n-alkyl and the 2-ethylhexyl derivatives. For the cyano-substituted DPPs possessing the lowest LUMO levels, ambipolar transport characteristics are observed.

Twenty-four diketopyrrolopyrrole dyes with branched and linear solubilizing alkyl chains and electron-withdrawing substituents at the aromatic core are investigated as the semiconductor in organic thin-film transistors. Remarkably, layers of stereoisomeric mixtures of the 2-ethylhexyl-substituted derivatives outperform the devices of dyes with n-alkyl chains with carrier mobilities as high as 0.7 cm² V⁻¹ s⁻¹.

A very high hole mobility of 15 cm² V⁻¹ s⁻¹ along with negligible hysteresis are demonstrated in transistors with an organic–inorganic perovskite semiconductor. This high mobility results from the well-developed perovskite crystallites, improved conversion to perovskite, reduced hole trap density, and improved hole injection by employing a top-contact/top-gate structure with surface treatment and MoO_x hole-injection layers.

The aggregation of conjugated polymers is found to have a significant influence on the surface organization of deposited films. Difluorobenzothiadiazole-based polymers show a strong pre-aggregation in solution, but the addition of 1,2,4-trichlorobenzene efficiently reduces such aggregates, leading to the transition of the surface organization from edge- to face-on orientation in deposited films.

Fine energy-level modulations of small-molecule acceptors (SMAs) are realized via subtle chemical modifications on strong electron-withdrawing end-groups. The two new SMAs (IT-M and IT-DM) end-capped by methyl-modified dicycanovinylindan-1-one exhibit upshifted lowest unoccupied molecular orbital (LUMO) levels, and hence higher open-circuit voltages can be observed in the corresponding devices. Finally, a top power conversion efficiency of 12.05% is achieved.

A highly efficient fullerene-free polymer solar cell (PSC) based on PDCBT, a polythiophene derivative substituted with alkoxycarbonyl, achieves an impressive power conversion efficiency of 10.16%, which is the best result in PSCs based on polythiophene derivatives to date. In comparison with a poly(3-hexylthiophene):ITIC-based device, the photovoltaic and morphological properties of the PDCBT:ITIC-based device are carefully investigated and interpreted.

A highly sensitive flexible magnetic sensor based on the anisotropic magnetoresistance effect is fabricated. A limit of detection of 150 nT is observed and excellent deformation stability is achieved after wrapping of the flexible sensor, with bending radii down to 5 mm. The flexible AMR sensor is used to read a magnetic pattern with a thickness of 10 μm that is formed by ferrite magnetic inks.

In organic solar cells, a major source of energy loss is attributed to nonradiative recombination from the interfacial charge transfer states to the ground state. By taking pentacene–C₆₀ complexes as model donor–acceptor systems, a comprehensive theoretical understanding of how molecular packing and charge delocalization impact these nonradiative recombination rates at donor–acceptor interfaces is provided.

Stretchable electronics are essential for the development of intensely packed collapsible and portable electronics, wearable electronics, epidermal and bioimplanted electronics, 3D surface compliable devices, bionics, prosthesis, and robotics. However, most stretchable devices are currently based on inorganic electronics, whose high cost of fabrication and limited processing area make it difficult to produce inexpensive, large-area devices. Therefore, organic stretchable electronics are highly attractive due to many advantages over their inorganic counterparts, such as their light weight, flexibility, low cost and large-area solution-processing, the reproducible semiconductor resources, and the easy tuning of their properties via molecular tailoring. Among them, stretchable organic semiconductor devices have become a hot and fast-growing research field, in which great advances have been made in recent years. These fantastic advances are summarized here, focusing on stretchable organic field-effect transistors, light-emitting devices, solar cells, and memory devices.

Stretchable organic semiconductor devices are essential for the development of low-cost, large-area collapsible and portable electronics, wearable electronics, epidermal and bioimplanted electronics, 3D surface compliable devices, bionics, prosthesis, and robotics. Great advances have been made in this field in recent years, which are summarized, focusing on stretchable organic field-effect transistors, light-emitting devices, solar cells, and memory devices.