Optimization of Si NC/P3HT Hybrid Solar Cells

Silicon nanocrystals (Si NCs) are shown to be an electron acceptor in hybrid solar cells combining Si NCs with poly(3‐hexylthiophene) (P3HT). The effects of annealing and different metal electrodes on Si NC/P3HT hybrid solar cells are studied in this paper. After annealing at 150 °C, Si NC/P3HT solar cells exhibit power conversion efficiencies as high as 1.47%. The hole mobility in the P3HT phase extracted from space‐charge‐limited current measurements of hole‐only devices increases from 2.48 × 10^?10 to 1.11 × 10^?9 m² V^?1 s^?1 after annealing, resulting in better transport in the solar cells. A quenching of the open‐circuit voltage and short‐circuit current is observed when high work function metals are deposited as the cathode on Si NC/P3HT hybrid devices.     

Chloride and Indium‐Chloride‐Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in Solution and Doping of Nanocrystal Solids

Here, the surface functionalization of CdSe and CdSe/CdS core/shell nanocrystals (NCs) with compact chloride and indium‐chloride‐complex ligands is reported. The ligands provide not only short interparticle distances but additionally control doping and passivation of surface trap states, leading to enhanced electronic coupling in NC‐based arrays. The solids based on these NCs show an excellent electronic transport behavior after heat treatment at the relatively low temperature of 190 °C. Indeed, the indium‐chlorido‐capped 4.5 nm CdSe NC based thin‐film field‐effect transistor reaches a saturation mobility of μ = 4.1 cm² (V s)^?1 accompanied by a low hysteresis, while retaining the typical features of strongly quantum confined semiconductor NCs. The capping with chloride ions preserves the high photoluminescence quantum yield ( ≈ 66%) of CdSe/CdS core/shell NCs even when the CdS shell is relatively thin (six monolayers). The simplicity of the chemical incorporation of chlorine and indium species via solution ligand exchange, the efficient electronic passivation of the NC surface, as well as their high stability as dispersions make these materials especially attractive for wide‐area solution‐processable fabrication of NC‐based devices.     

Coordinatable and High Charge‐Carrier‐Mobility Water‐Soluble Conjugated Copolymers for Effective Aqueous‐Processed Polymer–Nanocrystal Hybrid Solar Cells and OFET Applications

A water‐soluble conjugated polymer (WCP) poly[(3,4‐dibromo‐2,5‐thienylene vinylene)‐co‐(p‐phenylene‐vinylene)] (PBTPV), containing thiophene rings with high charge‐carrier mobility and benzene rings with excellent solubility is designed and prepared through Wessling polymerization. The PBTPV precursor can be easily processed by employing water or alcohols as the solvents, which are clean, environmentally friendly, and non‐toxic compared with the highly toxic organic solvents such as chloroform and chlorobenzene. As a novel photoelectric material, PBTPV presents excellent hole‐transport properties with a carrier mobility of 5 × 10^?4 cm² V^?1 s^?1 measured in an organic field‐effect transistor device. By integrating PBTPV with aqueous CdTe nanocrystals (NCs) to produce the active layer of water‐processed hybrid solar cells, the devices exhibit effective power conversion efficiency up to 3.3%. Moreover, the PBTPV can form strong coordination interactions with the CdTe NCs through the S atoms on the thiophene rings, and effective coordination with other nanoparticles can be reasonably expected.     

Interface Engineering for Both Cathode and Anode Enables Low‐Cost Highly Efficient Solution‐Processed CdTe Nanocrystal Solar Cells

Low‐cost solution‐processed CdTe nanocrystal (NC) solar cells always suffer from a high interface energy barrier and unbalanced hole/electron transport as well as anisotropic atom diffusion on the CdTe surface due to the limited amount of hole/electron interface materials or the difficulty in interface processing. In this work, a novel strategy is first adopted with gradient electron transport layer (CdS/CdSe) modification in the cathode and a new crosslinkable hole transport polymer (P‐TPA) implantation in the anode. The carrier recombination at interfaces is greatly decreased and thus the carrier collection is increased. Moreover, the light harvesting is improved both in short and long wavelength regions, making J_sc and V_oc increase simultaneously. A champion solar cell shows a very high power conversion efficiency of 9.2% and an outstanding J_sc of 25.31 mA cm^?2, which are among the highest values for all solution‐processed CdTe NC solar cells with a superstrate structure, and the latter value is even higher than that of traditional thick CdTe thin‐film solar cells (2 µm) via the high temperature close space sublimation method. This work demonstrates that facile surface modifications in both the cathode and anode with stepped extraction and organic–inorganic hybridization are very promising in constructing next‐generation highly efficient NC photovoltaic devices.     

The Impact of Polymer Regioregularity on Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices

The charge transport in pristine poly(3‐hexylthiophene) (P3HT) films and in photovoltaic blends of P3HT with [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) is investigated to study the influence of charge‐carrier transport on photovoltaic efficiency. The field‐ and temperature dependence of the charge‐carrier mobility in P3HT of three different regioregularities, namely, regiorandom, regioregular with medium regioregularity, and regioregular with very high regioregularity are investigated by the time‐of‐flight technique. While medium and very high regioregularity polymers show the typical absorption features of ordered lamellar structures of P3HT in the solid state even without previous annealing, films of regiorandom P3HT are very disordered as indicated by their broad and featureless absorption. This structural difference in the solid state coincides with partially non‐dispersive transport and hole mobilities µ_h of around 10^?4 and 10^?5 cm² V^?1 s^?1 for the high and medium regioregularity P3HT, respectively, and a slow and dispersive charge transport for the regiorandom P3HT. Upon blending the regioregular polymers with PCBM, the hole mobilities are typically reduced by one order of magnitude, but they do not significantly change upon additional post‐spincasting annealing. Only in the case of P3HT with high regioregularity are the electron mobilities similar to the hole mobilities and the charge transport is, thus, balanced. Nonetheless, devices prepared from both materials exhibit similar power conversion efficiencies of 2.5%, indicating that very high regioregularity may not substantially improve order and charge‐carrier transport in P3HT:PCBM and does not lead to significant improvements in the power‐conversion efficiency of photovoltaic devices.     

Carrier Modulation of Ambipolar Few‐Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping

Semiconducting molybdenum ditelluride (2H‐MoTe₂), a fast‐emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field‐effect transistors and is the focus of substantial research interest, showing hole‐dominated ambipolar characteristics. Here, carrier modulation of ambipolar few‐layer MoTe₂ transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe₂ layers, the carrier polarity of MoTe₂ transistors from p‐type to n‐type can be reversely controlled. The electron mobility of MoTe₂ is significantly enhanced from 0.1 to 20 cm² V^?1 s^?1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high‐performance complementary inverters with high DC gain of >25 and photodetectors based on few‐layer MoTe₂ flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition‐metal dichalcogenide semiconductors.     

Highly Efficient Charge Transport in a Quasi‐Monolayer Semiconductor on Pure Polymer Dielectric

The study of monolayer organic field‐effect transistors (MOFETs) provides an effective way to investigate the intrinsic charge transport of semiconductors. To date, the research based on organic monolayers on polymeric dielectrics lays far behind that on inorganic dielectrics and the realization of a bulk‐like carrier mobility on pure polymer dielectrics is still a formidable challenge for MOFETs. Herein, a quasi‐monolayer coverage of pentacene film with orthorhombic phase is grown on the poly (amic acid) (PAA) dielectric layer. More significantly, charge density redistribution occurs at the interface between the pentacene and PAA caused by electron transfer from pentacene to the PAA dielectric layer, which is verified by theoretical simulations and experiments. As a consequence, an enhanced hole accumulation layer is formed and pentacene‐based MOFETs on pure polymer dielectrics exhibit bulk‐like carrier mobilities of up to 13.7 cm² V^?1 s^?1 from the saturation region at low V_GS, 9.1 cm² V^?1 s^?1 at high V_GS and 7.6 cm² V^?1 s^?1 from the linear region, which presents one of the best results of previously reported MOFETs so far and indicates that the monolayer semiconductor growing on pure polymer dielectric could produce highly efficient charge transport.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

The Impact of Carrier Transport Confinement on the Energy Transfer Between InGaN/GaN Quantum‐Well Nanorods and Colloidal Nanocrystals

The energy transfer (ET) between InGaN/GaN multiple‐quantum‐well (MQW) nanorods (NRs) and semiconductor nanocrystals (NCs) for efficient color conversion is studied. An exceptional contribution of carrier transport confinement to the ET mechanisms is observed in the proximal side‐wall coupling system, which consists of InGaN/GaN NRs and CdSe NCs. Under relatively low or high excitation, the ET rate shows different carrier‐density dependence, resulting from different electron‐hole configurations, i.e., bound excitons and free carriers. In the localized exciton regime, the ET rate decreases when increasing temperature from 20 K to 200 K. However, in the free‐carrier regime, the ET rate varies insignificantly in the same temperature range. The temperature dependence in this NR‐NC coupling system is different from that in the previously studied planar MQW‐NC coupling system. It is suggested that the carrier transport confinement in NRs is a major factor for these divergences. The highly efficient ET with efficiency up to 80% shows a promising potential of using such NR‐NC coupled structures for ET‐pumped, NC‐based, light‐emitting devices.     

Solution‐Processed High‐Performance Tetrathienothiophene‐Based Small Molecular Blends for Ambipolar Charge Transport

Four soluble dialkylated tetrathienoacene ( TTAR) ‐based small molecular semiconductors featuring the combination of a TTAR central core, π‐conjugated spacers comprising bithiophene ( bT ) or thiophene ( T ), and with/without cyanoacrylate ( CA ) end‐capping moieties are synthesized and characterized. The molecule DbT‐TTAR exhibits a promising hole mobility up to 0.36 cm² V^?1 s^?1 due to the enhanced crystallinity of the microribbon‐like films. Binary blends of the p‐type DbT‐TTAR and the n‐type dicyanomethylene substituted dithienothiophene‐quinoid ( DTTQ‐11 ) are investigated in terms of film morphology, microstructure, and organic field‐effect transistor (OFET) performance. The data indicate that as the DbT‐TTAR content in the blend film increases, the charge transport characteristics vary from unipolar (electron‐only) to ambipolar and then back to unipolar (hole‐only). With a 1:1 weight ratio of DbT‐TTAR DTTQ‐11 in the blend, well‐defined pathways for both charge carriers are achieved and resulted in ambipolar transport with high hole and electron mobilities of 0.83 and 0.37 cm² V^?1 s^?1, respectively. This study provides a viable way for tuning microstructure and charge carrier transport in small molecules and their blends to achieve high‐performance solution‐processable OFETs.     

Tuning the Charge Carrier Polarity of Organic Transistors by Varying the Electron Affinity of the Flanked Units in Diketopyrrolopyrrole‐Based Copolymers

Fine‐tuning of the charge carrier polarity in organic transistors is an important step toward high‐performance organic complementary circuits and related devices. Here, three new semiconducting polymers, namely, pDPF‐DTF2, pDPSe‐DTF2, and pDPPy‐DTF2, are designed and synthesized using furan, selenophene, and pyridine flanking group‐based diketopyrrolopyrrole cores, respectively. Upon evaluating their electrical properties in transistor devices, the best performance has been achieved for pDPSe‐DTF2 with the highest and average hole mobility of 1.51 and 1.22 cm² V^?1 s^?1, respectively. Most intriguingly, a clear charge‐carrier‐polarity change is observed when the devices are measured under vacuum. The pDPF‐DTF2 polymer exhibits a balanced ambipolar performance with the µ_h/µ_e ratio of 1.9, whereas pDPSe‐DTF2 exhibits p‐type dominated charge carrier transport properties with the µ_h/µ_e ratio of 26.7. Such a charge carrier transport change is due to the strong electron‐donating nature of the selenophene. Furthermore, pDPPy‐DTF2 with electron‐withdrawing pyridine flanking units demonstrates unipolar n‐type charge transport properties with an electron mobility as high as 0.20 cm² V^?1 s^?1. Overall, this study demonstrates a simple yet effective approach to switch the charge carrier polarity in transistors by varying the electron affinity of flanking groups of the diketopyrrolopyrrole unit.     

High Hole Mobility and Thickness‐Dependent Crystal Structure in α,ω‐Dihexylsexithiophene Single‐Monolayer Field‐Effect Transistors

Monolayer‐thickness two‐dimensional layers of α,ω‐dihexylsexithiophene (α,ω‐DH6T) exhibit field‐effect hole mobility of up to 0.032 cm² V^?1 s^?1, higher than previously reported for monolayers of other small‐molecule organic semiconductors. In situ measurements during deposition show that the source‐drain current saturates rapidly after the percolation of monolayer‐high islands, indicating that the electrical properties of α,ω‐DH6T transistors are largely determined by the first molecular monolayer. The α,ω‐DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In‐plane lattice constants measured using synchrotron grazing‐incidence diffraction are larger in monolayer‐thickness films than the in‐plane lattice constants of several‐monolayer films and of previously reported thick‐film structures. Near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in‐plane lattice constant of single‐monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω‐DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.     

Versatile α,ω‐Disubstituted Tetrathienoacene Semiconductors for High Performance Organic Thin‐Film Transistors

Facile one‐pot [1 + 1 + 2] and [2 + 1 + 1] syntheses of thieno[3,2‐b]thieno[2′,3′:4,5]thieno[2,3‐d]thiophene (tetrathienoacene; TTA) semiconductors are described which enable the efficient realization of a new TTA‐based series for organic thin‐film transistors (OTFTs). For the perfluorophenyl end‐functionalized derivative DFP‐TTA , the molecular structure is determined by single‐crystal X‐ray diffraction. This material exhibits n‐channel transport with a mobility as high as 0.30 cm²V^?1s^?1 and a high on‐off ratio of 1.8 × 10⁷. Thus, DFP‐TTA has one of the highest electron mobilities of any fused thiophene semiconductor yet discovered. For the phenyl‐substituted analogue, DP‐TTA , p‐channel transport is observed with a mobility as high as 0.21 cm²V^?1s^?1. For the 2‐benzothiazolyl (BS‐) containing derivative, DBS‐TTA , p‐channel transport is still exhibited with a hole mobility close to 2 × 10^?3 cm²V^?1s^?1. Within this family, carrier mobility magnitudes are strongly dependent on the semiconductor growth conditions and the gate dielectric surface treatment.     

Role of PbSe Structural Stabilization in Photovoltaic Cells

Semiconductor nanocrystals are promising materials for printed optoelectronic devices, but their high surface areas are susceptible to forming defects that hinder charge carrier transport. Furthermore, correlation of chalcogenide nanocrystal (NC) material properties with solar cell operation is not straightforward due to the disorder often induced into NC films during processing. Here, an improvement in long‐range ordering of PbSe NCs symmetry that results from halide surface passivation is described, and the effects on chemical, optical, and photovoltaic device properties are investigated. Notably, this passivation method leads to a nanometer‐scale rearrangement of PbSe NCs during ligand exchange, improving the long‐range ordering of nanocrystal symmetry entirely with inorganic surface chemistry. Solar cells constructed with a variety of architectures show varying improvement and suggest that triplet formation and ionization, rather than carrier transport, is the limiting factor in singlet fission solar cells. Compared to existing protocols, our synthesis leads to PbSe nanocrystals with surface‐bound chloride ions, reduced sub‐bandgap absorption and robust materials and devices that retain performance characteristics many hours longer than their unpassivated counterparts.     

Transistor Paint: Environmentally Stable N‐alkyldithienopyrrole and Bithiazole‐Based Copolymer Thin‐Film Transistors Show Reproducible High Mobilities without Annealing

New solution processable 4‐(2‐hexyldecan)‐4H‐bisthieno[2,3‐d:3′,2′‐b]pyrrole and 4,4′‐dialkyl‐2,2′‐bithiazole‐based copolymers (PBTzDTPs) are synthesized with excellent FET performance. These novel copolymers have considerable potential in printable electronics as they have high charge carrier mobilities, excellent air stability, good solution processibility, and no requirement for post‐deposition thermal annealing, all requirements for this field of application. The thin film transistors fabricated from PBTzDTPs achieve field effect mobilities as high as 0.14 cm² V^?1 s^?1 with current on/off ratios up to 10⁶ without thermal annealing. In addition, the devices exhibit stable performance in air, showing no significant degradation over 60 days. Moreover, the polymers described here provide an excellent example of the systems in which higher mobility performance does not require higher crystalline, long‐range ordered structures. Such a system appears to be particularly promising for rapid fabrication techniques, where kinetic conditions usually prevent the development of long‐range order.     

Organic and Inorganic Blocking Layers for Solution‐Processed Colloidal PbSe Nanocrystal Infrared Photodetectors

A PbSe solution‐processed nanocrystal‐based infrared photodetector incorporating carrier blocking layers is demonstrated, and significant reduction of dark current is achieved. Detectivity values close to 10¹² Jones are achieved by using poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butyl))diphenylamine)] (TFB) and ZnO nanocrystals (NC) as the electron blocker and hole blocker, respectively. An improvement in lifetime is also observed in the devices with the ZnO NCs hole blocking layer.     

Synergistic Use of Pyridine and Selenophene in a Diketopyrrolopyrrole‐Based Conjugated Polymer Enhances the Electron Mobility in Organic Transistors

To achieve semiconducting materials with high electron mobility in organic field‐effect transistors (OFETs), low‐lying energy levels (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) and favorable molecular packing and ordering are two crucial factors. Here, it is reported that the incorporation of pyridine and selenophene into the backbone of a diketopyrrolopyrrole (DPP)‐based copolymer produces a high‐electron‐mobility semiconductor, PDPPy‐Se. Compared with analogous polymers based on other DPP derivatives and selenophene, PDPPy‐Se features a lower LUMO that can decrease the electron transfer barrier for more effective electron injection, and simultaneously a lower HOMO that, however, can increase the hole transfer barrier to suppress the hole injection. Combined with thermal annealing at 240 °C for thin film morphology optimization to achieve large‐scale crystallite domains with tight molecular packing for effective charge transport along the conducting channel, OFET devices fabricated with PDPPy‐Se exhibit an n‐type‐dominant performance with an electron mobility (μ_e) as high as 2.22 cm² V^?1 s^?1 and a hole/electron mobility ratio (μ_h/μ_e) of 0.26. Overall, this study demonstrates a simple yet effective approach to boost the electron mobility in organic transistors by synergistic use of pyridine and selenophene in the backbone of a DPP‐based copolymer.     

Anisotropy of Charge Transport in a Uniaxially Aligned and Chain‐Extended,High‐Mobility,Conjugated Polymer Semiconductor

Charge transport in the ribbon phase of poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT)—one of the most highly ordered, chain‐extended crystalline microstructures available in a conjugated polymer semiconductor—is studied. Ribbon‐phase PBTTT has previously been found not to exhibit high carrier mobilities, but it is shown here that field‐effect mobilities depend strongly on the device architecture and active interface. When devices are constructed such that the ribbon‐phase films are in contact with either a polymer gate dielectric or an SiO₂ gate dielectric modified by a hydrophobic, self‐assembled monolayer, high mobilities of up to 0.4 cm² V^?1 s^?1 can be achieved, which is comparable to those observed previously in terrace‐phase PBTTT. In uniaxially aligned, zone‐cast films of ribbon‐phase PBTTT the mobility anisotropy is measured for transport both parallel and perpendicular to the polymer chain direction. The mobility anisotropy is relatively small, with the mobility along the polymer chain direction being higher by a factor of 3–5, consistent with the grain size encountered in the two transport directions.     

Bithienopyrroledione‐Based Copolymers,Versatile Semiconductors for Balanced Ambipolar Thin‐Film Transistors and Organic Solar Cells with V oc > 1 V

Conjugated polymer semiconductors P1 and P2 with bithienopyrroledione (bi‐TPD) as acceptor unit are synthesized. Their transistor and photovoltaic performances are investigated. Both polymers display high and balanced ambipolar transport behaviors in thin‐film transistors. P1‐ based devices show an electron mobility of 1.02 cm² V^?1 s^?1 and a hole mobility of 0.33 cm² V^?1 s^?1, one of the highest performance reported for ambipolar polymer transistors. The electron and hole mobilities of P2 transistors are 0.36 and 0.16 cm² V^?1 s^?1, respectively. The solar cells with PC₇₁BM as the electron acceptor and P1/P2 as the donor exhibit a high V _oc about 1.0 V, and a power conversion efficiency of 6.46% is observed for P1‐ based devices without any additives and/or post treatment. The high performance of P1 and P2 is attributed to their crystalline films and short π–π stacking distance (<3.5 Å). These results demonstrate (1) bi‐TPD is an excellent versatile electron‐deficient unit for polymer semiconductors and (2) bi‐TPD‐based polymer semiconductors have potential applications in organic transistors and organic solar cells.     

Effect of Donor Molecular Structure and Gate Dielectric on Charge‐Transporting Characteristics for Isoindigo‐Based Donor–Acceptor Conjugated Polymers

This study investigates the effect of the molecular structure of three different donor units, naphthalene (Np), bithiophene (BT), and thiophene–vinylene–thiophene (TVT), in isoindigo (IIG)‐based donor –acceptor conjugated polymers (PIIG‐Np, PIIG‐BT and PIIG‐TVT) on the charge carrier mobility of organic field‐effect transistors (OFETs). The charge transport properties of three different IIG‐based polymers strongly depend on donor units. PIIG–BT OFETs showed 50 times higher hole mobility (0.63 cm² V^?1 s^?1) than PIIG–TVT and PIIG–Np ones of ≈ 0.01 cm² V^?1 s^?1 with CYTOP dielectric though the BT units have less planarity than the TVT and Np units. The reasons for the different mobility in IIG‐based polymers are studied by analyzing the energy structure by absorption spectra, calculating transport levels by density functional theory, investigating the in‐ and out‐of‐plane crystallinity of thin film by grazing‐incidence wide‐angle X‐ray scattering, and extracting key transport parameters via low‐temperature measurements. By combining theoretical, optical, electrical, and structural analyses, this study finds that the large difference in OFET mobility mainly originates from the transport disorders determined by the different microcrystal structure, rather than the intrinsic transport properties in isolated chains for different polymers.