Organic thin-film solar cells: Devices and materials

In recent years, the performance of organic thin-film solar cells has gained rapid progress, of which the power conversion efficiencies (ηp) of 3%-5% are commonly achieved, which were difficult to obtain years ago and are improving steadily now. The ηp of 7.4% was achieved in the year 2010, and ηp of 9.2% was disclosed and confirmed at website of Mitsubishi Chemical in April, 2011. The promising future is that the ηp of 10% is achievable according to simulation results. Apparently, these are attributed to material innovations, new device structures, and also the better understanding of device physics. This article summarizes recent progress in organic thin-film solar cells related to materials, device structures and working principles. In the device functioning part, after each brief summary of the working principle, the methods for improvements, such as absorption increment, organic/electrode interface engineering, morphological issues, are addressed and summarized accordingly. In addition, for the purpose of increasing exciton diffusion efficiency, the benefit from triplet exciton, which has been proposed in recent years, is highlighted. In the active material parts, the chemical nature of materials and its impact on device performance are discussed. Particularly, emphasis is given toward the insight for better understanding device physics as well as improvements in device performance either by development of new materials or by new device architecture.     

Synthesis and photovoltaic properties of polymeric metal complexes containing 8-hydroxyquinoline as dye sensitizers for dye-sensitized solar cells

Four new donor–acceptor type polymeric metal complexes (P1, P2, P3, and P4) with the same Cd(II) complex in side chain and different conjugated backbone structures were synthesized by Yamamoto coupling and applied in dye-sensitized solar cells (DSSCs) as photosensitizers. The photophysical, electrochemical, and thermal properties were investigated in detail, showing that conjugated backbone containing fluorene improved intramolecular charge transfer and increased generation of photocurrent. The highest power conversion efficiency of 0.56% (J _sc?=?1.63?mA?cm^?2, V _oc?=?0.69?V, FF?=?0.50) was obtained with a DSSC based on P3 under simulated air mass 1.5 G solar irradiation, which shows a new strategy to design photosensitizers for DSSCs.     

Anionic benzothiadiazole containing polyfluorene and oligofluorene as organic sensitizers for dye-sensitized solar cells

Anionic polyfluorene and oligofluorene derivatives were synthesized and utilized as organic dye sensitizers in dye sensitized solar cells to show a maximum power conversion efficiency of 1.39%.     

Structural and electronic properties of CuSbS2 and CuBiS2: potential absorber materials for thin-film solar cells

As the demand for photovoltaics rapidly increases, there is a pressing need for the identification of new visible light absorbing materials for thin-film solar cells that offer similar performance to the current technologies based on CdTe and Cu(In,Ga)Se(2). Metal sulphides are the ideal candidate materials, but their band gaps are usually too large to absorb significant fractions of visible light. However, by combining Cu(+) (low binding energy d(10) band) and Sb(3+)/Bi(3+) (low binding energy s(2) band), the ternary sulphides CuSbS(2) and CuBiS(2) are formed, which have been gathering recent interest for solar cell applications. Using a hybrid density functional theory approach, we calculate the structural and electronic properties of these two materials. Our results highlight the stereochemical activity of the Sb and Bi lone pair electrons, and predict that the formation of hole carriers will occur in the Cu d(10) band and hence will involve oxidation of Cu(I).     

Amphiphilic ruthenium sensitizers and their applications in dye-sensitized solar cells

Amphiphilic ligands 4,4'-bis(1-adamantyl-aminocarbonyl)-2,2'-bipyridine (L(1)), 4,4'-bis[5-[N-[2-(3beta-cholest-5-en-3-ylcarbamate-N-yl)ethyl]aminocarbonyl]]-2,2'-bipyridine (L(2)), 4,4'-bis[5-[N-[2-(3beta-cholest-5-en-3-ylcarbamate-N-yl)propyl]aminocarbonyl]]-2,2'-bipyridine (L(3)), and 4,4'-bis(dodecan-12-ol)-2,2'-bipyridine (L(4)) and their heteroleptic ruthenium(II) complexes of the type [Ru(II)LL(1)(NCS)(2)] (5), [Ru(II)LL(2)(NCS)(2)] (6), [Ru(II)LL(3)(NCS)(2)] (7), and [Ru(II)LL(4)(NCS)(2)] (8) (where L = 4,4'-bis(carboxylic acid)-2,2'-bipyridine) have been synthesized starting from dichloro(p-cymene)ruthenium(II) dimer. All the ligands and the complexes were characterized by analytical, spectroscopic, and electrochemical techniques. The performance of these complexes as charge-transfer photosensitizers in nanocrystalline TiO(2)-based solar cells was studied. When complexes 5-8 anchored onto a 12 + 4 microm thick nanocrystalline TiO(2) films, very efficient sensitization was achieved (85 +/- 5% incident photon-to-current efficiencies in the visible region, using an electrolyte consisting of 0.6 M butylmethylimidazolium iodide, 0.05 M I(2), 0.1 M LiI, and 0.5 M tert-butyl pyridine in 1:1 acetonitrile + valeronitrile). Under standard AM 1.5 sunlight, the complex 8 yielded a short-circuit photocurrent density of 17 +/- 0.5 mA/cm(2), the open-circuit voltage was 720 +/- 50 mV, and the fill factor was 0.72 +/- 0.05, corresponding to an overall conversion efficiency of 8.8 +/- 0.5%.     

Effect of degradation on electronic properties of polymer solar cells

We present here the effect of degradation on electronic properties of polymer solar cells. Investigations were performed on two types of solar cells based on the bulk‐heterojunction network of poly(3‐hexylthiophene) and phenyl [6,6] C₆₁ butyric acid methyl ester, one with slow degradation whereas other with faster degradation. Samples were prepared in identical conditions with controlled atmosphere, but for faster degradation, one of the samples was exposed to ambient air (rich in O₂ and H₂O molecules) before deposition of top metal electrode. The sample with slow degradation showed linear degradation in short circuit current density (J_sc), whereas the sample with faster degradation exhibited exponential degradation in J_sc. Linear degradation happens due to degradation in the active layer only whereas the exponential degradation is because of through degradation of the solar cell. The effect of degradation is investigated on different diode parameters. Because of different degradation processes in the two samples, the variations in diode parameters with time are different. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and photovoltaic properties of organic sensitizers containing electron-deficient and electron-rich fused thiophene for dye-sensitized solar cells

Diverse fused thiophenes with electron-rich and electron-deficient blocks have been synthesized and employed as the π-conjugated spacers of organic dyes for the dye-sensitized solar cells (DSSCs). The effects of these fused thiophenes were investigated by their absorption spectra, electrochemical and photovoltaic properties. For a typical device a maximum power conversion efficiency of 6.11% was obtained under simulated AM 1.5 irradiation (100 mW cm^?2): a short-circuit current (J_SC) of 14.47 mA cm^?2, an open-circuit voltage (V_OC) of 670 mV, and a fill factor (FF) of 0.63.     

Synthesis and characterization of organic dyes containing 4,5-diazafluorene as efficient sensitizers for dye-sensitized solar cells

Two dyes which are 4,5-diazafluoren-9-one-derived diimine ligands and their corresponding Ru(II) bipyridine complexes were synthesized. The structures of all compounds were determined by FTIR, UV–Vis, ¹H-NMR, ¹C-NMR, and MS spectroscopic data. The photovoltaic and electrochemical properties of these compounds were investigated and the applicability in DSSCs as photosensitizers was studied. The photovoltaic cell efficiencies (PCE) of the devices were 0.36–1.26% under simulated AM 1.5 solar irradiation of 100 mW/cm², and the highest open-circuit voltage (V_oc) reached 0.34 V. When comparing the photovoltaic performance of DSSC devices, efficiency increases L2 < L2-Ru < L1 < L1-Ru. The PCE value of 1.26% was obtained with a DSSC based on L1-Ru under AM irradiation (100 mW/cm²). DSSC based on L1-Ru produced efficiency of 1.26%, whereas DSSC-based L1 exhibited the device performance with efficiency of 0.84% under illumination. These results show that the availability of a π-conjugated bridge and a richer electron donor for photovoltaic performance of the DSSC provides increased efficiency.     

Electronic structures and absorption properties of three kinds of ruthenium dye sensitizers containing bipyridine-pyrazolate for solar cells

The geometries, electronic structures and the electronic absorption spectra of three kinds of ruthenium complexes, which contain tridentate bipyridine-pyrazolate ancillary ligands, were studied using density functional theory (DFT) and time-dependent DFT. The calculated results indicate that: (1) the strong conjugated effects are formed across the pyrazoalte-bipyridine groups; (2) the interfacial electron transfer between electrode and the dye sensitizers is an electron injection processes from the excited dyes to the conduction band of TiO2; (3) the absorption bands in visible region have a mixed character of metal-to-ligand charge transfer and ligand-to-ligand charge transfer, but the main character of absorption bands near UV region ascribe to π→π* transitions; (4) introducing pyrazolate and -NCS groups are favorable for intra-molecular charge transfer, and they are main chromophores that contribute to the sensitization of photon-to-current conversion processes, but introducing -Cl and the terminal group -CF3 are unfavorable to improve the dye performance in dye sensitized solar cells.     

Structural and electronic properties of heptachlor

In this work, the molecular geometry of heptachlor is investigated using ab initio HF, DFT, LDA, and GGA methods. The natural bond orbital (NBO) analysis is performed at the B3LYP/6-311++G(d,p) level of theory. The first order hyperpolarizability β_total, the mean polarizability Δα, the anisotropy of the polarizability Δα, and the dipole moment μ, are calculated by B3LYP/6-311++G(d,p) and HF/6- 311++G(d,p) methods. The first order hyperpolarizability (β_total) is calculated based on the finite field approach. UV spectral parameters along with HOMO, LUMO energies for heptachlor are determined in vacuum and the solvent phase using HF, DFT, and TD-DFT/B3LYP methods implemented with the 6-311++G(d,p) basis set. Atomic charges and electron density of heptachlor in vacuum and ethanol are calculated using DFT/B3LYP and TD-DFT/B3LYP methods and the 6-311++G(d,p) basis set. In addition, after the frontier molecular orbitals (FMOs), the molecular electrostatic potential (MEP), the electrostatic potential (ESP), the electron density (ED), and the solvent accessible surface of heptachlor are visualized as a results of the B3LYP/6-311++G(d,p) calculation. Densities of states (DOS), the external electric field (EF) effect on the HOMO-LUMO gap, and the dipole moment are investigated by LDA and GGA methods.     

Novel carboxylated oligothiophenes as sensitizers in photoelectric conversion systems

Novel carboxylated oligothiophenes with different thiophene units were designed and synthesized as photosensitizers in dye-sensitized solar cells (DSSCs) for efficient opto-electric materials. The introduction of -COOH into thiophene molecules can lead to a red shift of UV-visible absorption, increase light-harvesting efficiency, and enhance photoinduced charge transport by forming efficient covalent bonds to the substrate surface. A red shift of the absorption spectrum of oligothiophene is also achieved by the increase in the number of thiophene units. The DSSCs based on the oligomers have excellent photovoltaic performances. Under 100 mW cm(-2) irradiation a short-circuit current of 10.57 mA cm(-2) and an overall energy conversion efficiency of 3.36 % is achieved when pentathiophene dicarboxylated acid was used as a sensitizer. The incident photo-to-current conversion efficiency (IPCE) has a maximum as high as 80 %. In addition, photovoltage and photocurrent transients show that slow charge recombination in DSSCs is important for efficient charge separation and excellent photoelectric conversion properties of the oligomers. These initial and promising results suggest that carboxylated oligothiophenes are efficient photosensitizers.     

Molecular adjustment of the electronic properties of nanoporous electrodes in dye-sensitized solar cells

Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V(oc)) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I(sc)). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V(oc) in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I(sc) shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current.     

A comparison of carboxypyridine isomers as sensitizers for dye-sensitized solar cells: assessment of device efficiency and stability

Three novel organic dyes (DF13A–C) carrying regioisomeric carboxypyridine anchoring groups were synthesized by means of a multistep synthetic sequence involving a Pd-catalyzed Stille coupling as the key step. The new compounds underwent full spectroscopic, electrochemical, and computational characterization, and their properties were compared with those of a reference compound endowed with a classic cyanoacrylic acid acceptor (DF15). Photovoltaic measurements showed that dye-sensitized solar cells built with dyes DF13A–C as photosensitizers yielded power conversion efficiencies corresponding to 54–63% of those obtained with the reference compound. Determination of desorption pseudo-first order rate constants indicated that isomers DF13B–C, having the nitrogen atom in neighboring position relative to the carboxylic moiety, were removed from TiO₂ more slowly than isomer DF13A or cyanoacrylic derivative DF15, suggesting a possible cooperative effect of the two functional groups on semiconductor binding: such hypothesis was supported by device stability tests carried out on transparent, larger area cells.     

Synthesis and photosensitizing properties of fluoroalkoxyl phthalocyanine metal complexes

Nine fluoroalkoxyl phthalocyanine metal complexes (Zn, AlCl, Mg, Co, Cu, FeCl) were synthesized from 4-(polyfluoroalkoxyl) phthalic anhydride. The fat-soluble phthalocyanines were characterized by elemental analysis, IR, ¹H NMR and fast-atom-bombardment mass spectroscopy. Zinc and aluminum chloride complexes show higher photooxidation ability in solution containing 20% perfluorocarbons than in hydrocarbon solvents.     

Structural and electronic properties of borazine cyclacenes

The structural and electronic properties of borazine cyclacenes (BNn) have been investigated by performing semiempirical molecular orbital self-consistent field calculations at the level of AM1 method within the RHF formulation. It has been found that as the number of borazine rings increases in the arenoid belt the structures become more exothermic.     

Novel organic dye sensitizers containing fluorenyl and biphenyl moieties for solar cells

Three novel triarylamine dyes(AFL1-AFL3) containing fluorenyl and the biphenyl moieties have been designed and synthesized for application in dye-sensitized solar cells.The light-harvesting capabilities and photovoltaic performance of these dyes were investigated systematically through comparison of different π-bridges.The dye with a furan linker exhibited a higher open-circuit voltage(V_(OC)) and monochromatic incident photon-to-current conversion efficiency(IPCE) compared to thiophene and benzene linker.Thus,AFL3 containing a furan linker exhibited the maximum overall conversion efficiency of 5.81%(V_(OC) = 760 mV,J_(SC) = 11.36 mA cm~2 and ff=0.68) under standard global AM 1.5 G solar condition.     

高分子功能材料图案化制备及其在光电领域的应用

基于半导体高分子功能材料的新型光电器件因其兼容性好、成本低、加工处理方便等特点逐渐走入人们的视野.加工集成高性能高分子光电器件需要用到可控图案化技术,这也将成为未来物联网、光通信、智能社会的基础核心技术.近年来,许多致力于此方面的研究取得了重要的成果.本文总结了各类高分子材料图案化技术,包括光刻法、模具诱导法、印刷法、浸润性调控组装法的技术特点,然后归纳了高分子图案化技术在场效应晶体管、光电探测器、气体传感器、电致发光二极管、光伏器件等光电器件领域的研究进展,最后对未来这一领域发展的挑战与机遇进行了展望.     

Structural and electronic properties of endohedral metallofullerenes

This account presents an overview of our achievements in structural and chemical understanding of endohedral metallofullerenes (EMFs), a new class of metal‐carbon hybrid materials formed by encapsulation of metals inside fullerene cavities. Structural determination of EMFs is of fundamental importance for understanding their intrinsic properties and the formation mechanism, and for broadening their applications. We have developed an effective method for determining the structures of paramagnetic EMFs, and also succeeded in observing the motion of cluster in a di‐metal EMF for the first time. Recently, we unambiguously established the structures of some carbide EMFs which had been wrongly assumed as conventional EMFs previously. More importantly, we have obtained some insoluble EMF species which had never been explored or even expected before. Meanwhile, the chemical properties of various EMFs with different cage structures or different metallic cores have been systematically investigated by means of both covalent and supramolecular considerations, yielding many fascinating results relating to the dictating effect of internal metals. It is noteworthy that all these achievements are based on unambiguous X‐ray results of pristine or functionalized EMFs. DOI 10.1002/tcr.201100038