The Effect of Poly(3‐hexylthiophene) Molecular Weight on Charge Transport and the Performance of Polymer:Fullerene Solar Cells

The time‐of‐flight method has been used to study the effect of P3HT molecular weight (M_n = 13–121 kDa) on charge mobility in pristine and PCBM blend films using highly regioregular P3HT. Hole mobility was observed to remain constant at 10^?4 cm²V^?1s^?1 as molecular weight was increased from 13–18 kDa, but then decreased by one order of magnitude as molecular weight was further increased from 34–121 kDa. The decrease in charge mobility observed in blend films is accompanied by a change in surface morphology, and leads to a decrease in the performance of photovoltaic devices made from these blend films.     

Photocrosslinkable liquid-crystalline polythiophenes with oriented nanostructure and stabilization for photovoltaics

A novel stable and photocrosslinkable electron-donor material, liquid-crystalline polythiophene containing cyano-biphenyl mesogenic pendant, namely, poly{3-[6-(4′-cyano-biphenyloxy)hexyl]thiopheneylenethiophene-alt-3-(6-bromohexyl)thiophene} (PTcbpTT), was designed and synthesized. The structural anisotropy originating from cyano-biphenyl mesogens can induce the PTcbpTT to assemble into a well ordered morphology and consequently lead to the red-shift absorption, enhanced photoluminescence. The thermal treatment drives further development of the morphology of the copolymer and its blend films mixed with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), towards a state of microphase separation in the nanometer scale. Furthermore, the bulk heterojunction devices based on the PTcbpTT:PCBM (1:1 wt.%) active layer have been constructed. Without extensive optimization, the LC annealing device yields an enhancement of power conversion efficiency from 0.5% to 1.2%, showing a significantly increased J_sc and FF with respect to its untreated counterpart, thanks to the ordered microphase separation channels for charge transportation. The high V_oc value of 0.731 V is due to the low HOMO level of PTcbpTT. Unlike devices prepared from PTcbpTT:PCBM blend without UV treatment, photocrosslinked PTcbpTT:PCBM devices are stable even when annealed for two days at the elevated temperature of 150 °C, implying that the photocrosslinked structure dramatically suppresses largescale phase segregation.     

Morphological Stabilization by In Situ Polymerization of Fullerene Derivatives Leading to Efficient,Thermally Stable Organic Photovoltaics

The successful design and synthesis of two styryl‐functionalized fullerene derivatives, [6,6]‐phenyl‐C₆₁‐butyric acid styryl dendron ester (PCBSD) and [6,6]‐phenyl‐C₆₁‐butyric acid styryl ester (PCBS) is presented. The polymerizable PCBS or PCBSD materials are incorporated into a poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blend to form an active layer of ternary blend. The blending systems are first thermally annealed at 110 C for 10 min to induce optimal morphology, followed by heating at 150 C for 10 min to trigger the in situ polymerization of styrene groups. Through chemical crosslinking of PCBSD, the initial morphology of the blend (P3HT:PCBM:PCBSD = 6:5:1 in weight) can be effectively fixed and stably preserved. The device based on this blend shows extremely stable device characteristics, delivering an average power conversion efficiency (PCE) of 3.7% during long‐term thermal treatment. By molecular engineering to reduce the insulating portion, PCBS with higher C₆₀ content (71 wt%) possesses better electron‐transport properties than PCBSD (58 wt%). Encouragingly, at a low doping concentration of PCBS in the blend (P3HT:PCBM:PCBS = 6:5:1 in weight), linear‐polymerized PCBS can stabilize the morphology against thermal heating. This device exhibits more balanced charge mobility to achieve an average PCE of 3.8% over 25 h heating at 150 °C.     

Room temperature solution-processed electron transport layer for organic solar cells

We present a new recipe for a solution-processed titanium oxide (TiO_x) based electron transport layer at room temperature. Due to its high chemical compatibility with all types of organic blends (semi-crystalline or amorphous) and it is good adhesion to both surfaces of glass/ITO substrate and the active layer (blend), the buffer layer is suitable for use in organic solar cell devices with conventional, inverted or multi-junction structures. The main goal of this recipe is producing with easiness an repeatable and stable precursor that will leads to titanium oxide buffer layer each time with the same quality. Since the processing of the titanium oxide layer itself does not require any initial or additional treatment before and after the coating, and can even be carried in air as well as under protective atmosphere, our room temperature solution-processed electron transport layer is highly versatile and very promising for cost effective mass production of organic solar cells.     

Random Copolymers Outperform Gradient and Block Copolymers in Stabilizing Organic Photovoltaics

Recent advances have led to conjugated polymer‐based photovoltaic devices with efficiencies rivaling amorphous silicon. Nevertheless, these devices become less efficient over time due to changes in active layer morphology, thereby hindering their commercialization. Copolymer additives are a promising approach toward stabilizing blend morphologies; however, little is known about the impact of copolymer sequence, composition, and concentration. Herein, the impact of these parameters is determined by synthesizing random, block, and gradient copolymers with a poly(3‐hexylthiophene) (P3HT) backbone and side‐chain fullerenes (phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM)). These copolymers are evaluated as compatibilizers in photovoltaic devices with P3HT:PC₆₁BM as the active layer. The random copolymer with 20 mol% fullerene side chains and at 8 wt% concentration in the blend gives the most stable morphologies. Devices containing the random copolymer also exhibit higher and more stable power conversion efficiencies than the control device. Combined, these studies point to the random copolymer as a promising new scaffold for stabilizing bulk heterojunction photovoltaics.     

Organic Field‐Effect Devices as Tool to Characterize the Bipolar Transport in Polymer‐Fullerene Blends: The Case of P3HT‐PCBM

In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field‐effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer‐fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance–voltage (C–V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current–voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o‐Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT‐PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M_w value, affects the performances of thick o‐Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space‐charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow‐carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built‐in electric field originated in the donor‐acceptor oBHJ.     

Dual structure modifications to realize efficient polymer solar cells with low fullerene content

The molecular structures of both donor polymer and acceptor fullerene were adjusted and their effect on the donor/acceptor blend ratio in the polymer solar cells was investigated. We found that increasing the side-chain rigidity of the donor polymer or bulkiness of the fullerene can both effectively reduce the fullerene intercalation into polymer side chains, realizing efficient electron transport at higher polymer/fullerene ratio. Especially, by using a bulkier fullerene molecule, all the three adopted polymers exhibit remarkably stable device performance over a wide range of blend ratio (from 1:0.5 to 1:1.0), which can't be achieved by using conventional PC₆₁BM. Moreover, using bulky PC₆₁BAd together with a high side-chain density polymer PTP8, the optimal device performance could be obtained at a surprisingly low blend ratio of 1:0.6, which may lead to more thermally stable and cost-effective devices.     

Solution‐Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells

The detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA)₂(OAc)]⁺ ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni³⁺ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.     

Fast Photoresponse from 1T Tin Diselenide Atomic Layers

Atomically layered 2D crystals such as transitional metal dichalcogenides (TMDs) provide an enchanting landscape for optoelectronic applications due to their unique atomic structures. They have been most intensively studied with 2H phase for easy fabrication and manipulation. 1T phase material could possess better electrocatalytic and photocatalytic properties, while they are difficult to fabricate. Herein, for the first time, the atomically layered 1T phase tin diselenides (SnSe₂, III‐IV compound) are successfully exfoliated by the method of mechanical exfoliation from bulk single crystals, grown via the chemical vapor transport method without transport gas. More attractively, the high performance atomically layered SnSe₂ photodetector has been first successfully fabricated, which displays a good responsivity of 0.5 A W^?1 and a fast photoresponse down to ≈2 ms at room temperature, one of the fastest response times among all types of 2D photodetectors. It makes SnSe₂ a promising candidate for high performance optoelectronic devices. Moreover, high performance bilayered SnSe₂ field‐effect transistors are also demonstrated with a mobility of ≈4 cm² V^?1 s^?1 and an on/off ratio of 10³ at room temperature. The results demonstrate that few layered 1T TMD materials are relatively stable in air and can be exploited for various electrical and optical applications.     

Influence of side chain symmetry on the performance of poly(2,5-dialkoxy-p-phenylenevinylene): fullerene blend solar cells

We report on studies of poly-(2,5-dihexyloxy-p-phenylenevinylene) (PDHeOPV), a symmetric side-chain polymer, as a potential new donor material for polymer:fullerene blend solar cells. We study the surface morphology of blend films of PDHeOPV with PCBM, the transport properties of the blend films, and the performance of photovoltaic devices made from such blend films, all as a function of PCBM content. In each case, results are compared with those obtained using the asymmetric side chain polymer, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), in order to investigate the influence of polymer side chain symmetry on solar cell performance. AFM images show that large PCBM aggregates appear at lower PCBM content (50 wt.% PCBM) for PDHeOPV:PCBM than for MDMO-PPV:PCBM (67 wt.% PCBM) blend films. Time-of-Flight (ToF) mobility measurements show that charge mobilities depend more weakly on PCBM content in PDHeOPV:PCBM than in MDMO:PPV:PCBM, with the result that at high PCBM content the mobilities in PDHeOPV:PCBM are significantly lower than in MDMO:PPV:PCBM blend films, despite the higher mobilities in pristine PDHeOPV compared to pristine MDMO-PPV. Photovoltaic devices show significantly lower power conversion efficiency (～0.93%) for PDHeOPV:PCBM (80 wt.% PCBM) blend films than for MDMO-PPV:PCBM (2.2% at 80 wt.% PCBM) blends. This is attributed to the relatively poor transport properties of the PDHeOPV:PCBM blend, which limit the optimum thickness of the photoactive layer in PDHeOPV:PCBM blend devices. The behaviour is tentatively attributed to a higher tendency for the symmetric side-chain polymer chains to aggregate, resulting in poorer interaction with the fullerene and poorer network formation for charge transport.     

Sensitive and Ultrabroadband Phototransistor Based on Two‐Dimensional Bi2O2Se Nanosheets

Bi₂O₂Se, a high‐mobility and air‐stable 2D material, has attracted substantial attention for application in integrated logic electronics and optoelectronics. However, achieving an overall high performance over a wide spectral range for Bi₂O₂Se‐based devices remains a challenge. A broadband phototransistor with high photoresponsivity (R) is reported that comprises high‐quality large‐area ( ≈ 180 µm) Bi₂O₂Se nanosheets synthesized via a modified chemical vapor deposition method with a face‐down configuration. The device covers the ultraviolet (UV), visible (Vis), and near‐infrared (NIR) wavelength ranges (360–1800 nm) at room temperature, exhibiting a maximum R of 108 696 A W^?1 at 360 nm. Upon illumination at 405 nm, the external quantum efficiency, R, and detectivity (D*) of the device reach up to 1.5 × 10⁷%, 50055 A W^?1, and 8.2 × 10¹² Jones, respectively, which is attributable to a combination of the photogating, photovoltaic, and photothermal effects. The devices reach a ?3 dB bandwidth of 5.4 kHz, accounting for a fast rise time (τ_rise) of 32 µs. The high sensitivity, fast response time, and environmental stability achieved simultaneously in these 2D Bi₂O₂Se phototransistors are promising for high‐quality UV and IR imaging applications.     

Locking the morphology with a green,fast and efficient physical cross-linking approach for organic electronic applications

Molecular packing and stability play crucial roles in determining the performance of organic electronic devices. To optimize the morphology of active layer, thus to improve the performance, especially the stability of devices, cross-linking technique is a viable approach that has been extensively applied to stabilize the morphology. In this work, we demonstrate a green, fast and efficient physical approach using hyperthermal hydrogen induced cross-linking (HHIC) to lock the morphology of organic electronic materials. By controlling the kinetic energy of the hyperthermal hydrogen (H₂) molecules, we can efficiently cleave the C–H bonds and induce cross-linking in a conjugated polymer. The cross-linking can be achieved in 1 min at room temperature, and the cross-linked films have excellent thermal stability and high resistance to organic solvents. Organic field effect transistors fabricated with HHIC treated poly (3-hexylthiophene) (P3HT) has comparable charge carrier mobility and superb stability than the untreated devices. Compared to the conventional chemically driven cross-linking approach, HHIC does not require additional modification in molecular structure, and the fast and non-destructive advantages have high potential for wide applications of highly stable organic electronic devices.     

Atomic layer deposited Al2O3 as a capping layer for polymer based transistors

The strong sensitivity of organic/polymeric semiconductors to the exposure to O₂ and H₂O atmospheres makes the use of capping layers mandatory for the realization of stable devices based on such materials. In this paper we explore the realization of inorganic capping layers by atomic layer deposition (ALD) that provides smooth and pinhole-free films with a great potential as passivation layer for organic based devices. We show that the deposition of Al₂O₃ on transistors based on poly-3 hexyltiophene (P3HT) allows to obtain air stable devices. Whereas the growth of Al₂O₃ directly on the P3HT layer leads to a rough interface and significant intermixing between the oxide and the polymer, which results in a deterioration of transistor performances, an interlayer of a poly-alcohol such as poly-vinylphenol interposed between the Al₂O₃ and the P3HT gives a well defined Al₂O₃/polymer interface without degradation of the hole mobility. Transistors capped with Al₂O₃/PVP are very stable in air, with no appreciable differences in the electrical characteristics when measured in vacuum or in air. In addition no significant degradation of the transistors electrical properties was detected even after one month of air exposure.     

Air stable C60 based n-type organic field effect transistor using a perfluoropolymer insulator

Air stable n-type organic field effect transistors (OFETs) based on C₆₀ are realized using a perfluoropolymer as the gate dielectric layer. The devices showed the field-effect mobility of 0.049 cm²/V s in ambient air. Replacing the gate dielectric material by SiO₂ resulted in no transistor action in ambient air. Perfluorinated gate dielectric layer reduces interface traps significantly for the n-type semiconductor even in air.     

Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V_oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E_loss) of 0.521 eV. Such small E_loss is the best record for polymer solar cells with PCEs over 16% to date.     

Efficient oligothiophene:fullerene bulk heterojunction organic photovoltaic cells

Novel small-molecule bulk heterojunction photovoltaic (PV) cells consisting of oligothiophen (alpha-sexithiophen: 6T) and fullerene (C₆₀) have been developed. Oligothiophen is well known as a good hole-transport material, and by changing the number of thiophen rings and making chemical modifications or substitutions, its characteristics relevant to PV applications (such as carrier mobility, energy level, packing, and ordered structure) can be controlled. Thus far it has been difficult to fabricate films of oligothiophene--fullerene blends with suitable morphology by using the common co-evaporation method, because oligothiophene crystallizes easily during film deposition. The present study found that the morphology of 6T:C₆₀ blends strongly depending on the composition of 6T:C₆₀. Suitable morphology was obtained only for films deposited with the co-evaporation of excess C₆₀. It is likely that excess C₆₀ prevents the crystallization of 6T. By successfully controlling the film morphology, we were able to demonstrate good PV performance in oligothiophene:fullerene bulk heterojunction PV cells for the first time. Moreover, it was found that PV performance could be further improved by inserting a C₆₀ layer between the blend layer and exciton blocking layer.     

High mobility strained Si0.5Ge0.5/SSOI short channel field effect transistors with TiN/GdScO3 gate stack

Short channel p-type metal-oxide-semiconductor field effect transistors (MOSFETs) with GdScO₃ gate dielectric were fabricated on a quantum well strained Si/strained Si_0.5Ge_0.5/strained Si heterostructure on insulator. Amorphous GdScO₃ layers with a dielectric constant of 24 show small hysteresis and low density of interface states. All devices show good performance with a threshold voltage of 0.585 V, commonly used for the present technology nodes, and high I_on/I_off current ratios. We confirm experimentally the theoretical predictions that the drive current and the transconductance of the biaxially strained (1 0 0) devices are weakly dependent on the channel orientation. The transistor’s hole mobility, extracted using split C-V method on long channel devices, indicates an enhancement of 90% (compared to SiO₂/SOI transistors) at low effective field, with a peak value of 265 cm²/V s. The enhancement is however, only 40% at high electrical fields. We demonstrate that the combination of GdScO₃ dielectric and strained SiGe layer is a promising solution for gate-first high mobility short channel p-MOSFETs.     

Design and synthesis of indole-substituted fullerene derivatives with different side groups for organic photovoltaic devices

A series of indole-substituted fulleropyrrolidine derivatives with different side groups on a pyrrolidine rings, including methyl (OIMC60P), benzyl (OIBC60P), 2,5-difluoroinebenzyl (OIB2FC60P), and 2,3,4,5,6-pentafluoroinebenzyl (OIB5FC60P), have been synthesized and used as electron acceptor in the active layer of polymer-fullerene solar cells to investigate the effect of various substitute groups on the electronic structures, morphologies, and device performances. Optical absorption, electrochemical properties and solubility of the fullerene derivatives have been explored and compared. The inverted photovoltaic devices with the configuration ITO/ZnO/Poly(3-hexylthiophene)(P3HT):[60]fullerene derivatives/MoO₃/Ag have been prepared including the reference cell based on the P3HT: methyl [6,6]-phenyl-C61-butylate (PCBM) blend films. All the devices properties were measured in air without encapsulation. We also investigated the effect of the thermal annealing on the crystallinity and morphology of the active layer and the device performance. The device based on the blend film of P3HT and OIBC60P showed a power conversion efficiency of 2.46% under illumination by AM1.5G (100 mW/cm²) after the annealing treatment at 120 °C for 10 min in air.