Kuan Sun

Solution-Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices

The conductivity of PEDOT:PSS films was significantly enhanced from 0.3 S cm(-1) to 3065 S cm(-1) through a treatment with dilute sulfuric acids. PEDOT:PSS films with a sheet resistance of 39 Ω sq(-1) and transparency of around 80% at 550 nm are obtained. These PEDOT:PSS films with conductivity and transparency comparable to ITO can replace ITO as the transparent electrode of optoelectronic devices.

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

Ferroelectricity of CH3NH3PbI3 Perovskite.

Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.

Conducting polymer/carbon nanotube composite as counter electrode of dye-sensitized solar cells

This letter reports dye-sensitized solar cells with a thin film of multiwall carbon nanotube/conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite as the counterelectrode. The composite thin film was prepared by spin coating the aqueous solution of the composite. The devices exhibited high performance with the energy conversion efficiency of 6.5%, short-circuit current of 15.5mAcm−2, open-circuit voltage of 0.66V, and fill factor of 0.63. This performance is close to the devices using conventional platinum as the counterelectrode and is significantly higher than the ones using a thin film of multiwall carbon nanotube/poly(styrenesulfonate acide) composite as the counterelectrode.

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) films treated with an amphiphilic fluoro compound as the transparent electrode of polymer solar cells

Flexible transparent electrode materials are strongly needed for optoelectronic devices. We report a novel method to significantly enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films through treatment with a fluoro compound, hexafluoroacetone (HFA). HFA hydrolyzes with water into a geminal diol, 1,1,1,3,3,3-hexafluoropropane-2,2-diol (HFP2OH) that has two –OH groups connected to the middle carbon atom. The conductivity increased from 0.3 to 1164 and 1325 S cm−1 after the treatment with HFA once and four times, respectively. The highly conductive HFA-treated PEDOT:PSS films can have a sheet resistances of 46 Ω □−1 and a transparency of around 83% at 550 nm. These values are comparable to those of indium tin oxide (ITO) on polyethylene terephthalate (PET). The conductivity enhancement is attributed to the HFP2OH-induced phase segregation of some hydrophilic PSSH chains from PEDOT:PSS and the conformational change of the conductive PEDOT chains, driven by the interactions between amphiphilic HFP2OH and PEDOT:PSS. The hydrophobic –CF3groups of HFP2OH preferentially interact with the hydrophobic PEDOT chains of PEDOT:PSS, while the hydrophilic –OH groups preferentially interact with hydrophilic PSS chains. The highly conductive PEDOT:PSS films were used to replace ITO as the transparent anode of polymer solar cells. Polymer solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exhibited a photovoltaic efficiency of 3.57% under simulated AM1.5G illumination, comparable to the control devices with ITO as the anode.

Review on application of PEDOTs and PEDOT:PSS in energy conversion and storage devices

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most successful conducting polymer in terms of practical application. It possesses many unique properties, such as good film forming ability by versatile fabrication techniques, superior optical transparency in visible light range, high electrical conductivity, intrinsically high work function and good physical and chemical stability in air. PEDOT:PSS has wide applications in energy conversion and storage devices. This review summarizes its applications in organic solar cells, dye-sensitized solar cells, supercapacitors, fuel cells, thermoelectric devices and stretchable devices. Approaches to enhance the material/device performances are highlighted.

The role of solvent vapor annealing in highly efficient air-processed small molecule solar cells

We demonstrate highly-efficient, solution-processed small molecule solar cells with the best power conversion efficiency (PCE) of more than 5%. The active layer consists of a diketopyrrolopyrrole-based donor molecule (DPP(TBFu)2) and a fullerene derivative (PC71BM) that is spin cast and subsequently treated with solvent vapor annealing (SVA) in air. We find not all solvent vapors lead to the best PCE. Solvents of high vapor pressures and medium donor solubilities, such as tetrahydrofuran or carbon disulfide, are most suitable for SVA in the context of organic solar cell application. On the other hand, acceptor solubility plays an insignificant role in such a treatment. An active layer treated with ideal solvent vapors develops desirable phase separation in both lateral and vertical directions, as revealed by AFM, TEM and TEM tomography. The SVA also leads to enhanced hole mobility. We believe the fast SVA treatment performed in air is a viable way to tune the active layer morphology for printed solar cells.

Perovskites for photovoltaics: a combined review of organic–inorganic halide perovskites and ferroelectric oxide perovskites

Over the past few years, very interestingly, two subclasses of perovskites — organic–inorganic halide perovskites and ferroelectric oxide perovskites, have simultaneously become the hotspots in the research field of photovoltaics. Organic–inorganic halide perovskites have launched a new era of low-cost, high-efficiency solar cells, due to their easy solution processability and superior optical and electrical properties for the photovoltaic effect. More recently, a so-called giant switchable photovoltaic effect has been demonstrated in organic–inorganic halide perovskites, thus promising a new memristive functionality. On the other hand, the recent renaissance of ferroelectric oxide perovskites for photovoltaics is caused by their fundamentally new photovoltaic mechanisms, which can produce a photovoltage far beyond the bandgap and may even lead to a boost of energy conversion efficiency. In addition, the combination of photovoltaic properties with the ferroic orders may create many novel functionalities for ferroelectric oxide perovskites. Toward the common goals of developing high-efficiency photovoltaics and novel opto-electronic functional devices, these two different subclasses of perovskites shall be brought together into a combined review. In this context, we review both organic–inorganic halide perovskites and ferroelectric oxide perovskites for photovoltaics, focusing on the material nature and the photovoltaic mechanisms. We also discuss their respective unresolved issues, along with useful suggestions for future research.

Transparent Conductive Oxide-Free Perovskite Solar Cells with PEDOT:PSS as Transparent Electrode

UNLABELLED Perovskite solar cells (PSCs) have been attracting considerable attention because of their low fabrication cost and impressive energy conversion efficiency. Most PSCs are built on transparent conductive oxides (TCOs) such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), which are costly and rigid. Therefore, it is significant to explore alternative materials as the transparent electrode of PSCs. In this study, highly conductive and highly transparent poly(3,4-ethylenedioxythiophene):polystyrenesulfonate ( PEDOT PSS) films were investigated as the transparent electrode of both rigid and flexible PSCs. The conductivity of PEDOT PSS films on rigid glass or flexible poly(ethylene terephthalate) (PET) substrate is significantly enhanced through a treatment with methanesulfonic acid (MSA). The optimal power conversion efficiency (PCE) is close to 11% for the rigid PSCs with an MSA-treated PEDOT PSS film as the transparent electrode on glass, and it is more than 8% for the flexible PSCs with a MSA-treated PEDOT PSS film as the transparent electrode on PET. The flexible PSCs exhibit excellent mechanical flexibility in the bending test.

Highly efficient, inverted polymer solar cells with indium tin oxide modified with solution-processed zwitterions as the transparent cathode.

Polymer solar cells (PSCs) with inverted structure can greatly improve photovoltaic stability. This paper reports a novel method to lower the work function of indium tin oxide (ITO) through the modification with a thin layer of zwitterions which have both positive and negative charges in the same molecule. Zwitterions have a strong dipole moment due to the presence of the two types of charges and are immobile under electric field. Zwitterions with both conjugated and saturated structure were investigated. A zwitterion thin layer is formed on ITO by spin coating a methanol solution of the zwitterion. The zwitterion-modified ITO sheets can be used as the cathode for the electron collection of inverted PSCs. The inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC(61)BM) PSCs can exhibit photovoltaic efficiency as high as 3.98% under simulated AM1.5G illumination (100 mW cm(-2)), which is comparable to that of PSCs with normal architecture. The effective electron collection by the zwitterion-modified ITO sheets is attributed to the reduction of the work function of ITO as a result of the dipole moment by the zwitterions. The zwitterion modification can lower the work function of ITO by up to 0.97 eV. The photovoltaic performance of PSCs and the reduction in the work function of ITO strongly depend on the chemical structure of the zwitterions.