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1.
    
The emerging field of stacked layers (double‐ and even multi‐layers) in organic photovoltaic cells is reviewed. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single‐layer bulk heterojunction photovoltaic cell. Furthermore, the low charge‐carrier mobilities of most organic materials limit the thickness of the active layer. Consequently, only part of the intensity of the incident light at the absorption maximum is absorbed. A tandem or multi‐junction solar cell, consisting of multiple layers each with their specific absorption maximum and width, can overcome these limitations and can cover a larger part of the solar flux. In addition, tandem or multi‐junction solar cells offer the distinct advantage that photon energy is used more efficiently, because the voltage at which charges are collected in each sub‐cell is closer to the energy of the photons absorbed in that cell. Recent developments in both small‐molecule and polymeric photovoltaic cells are discussed, and examples of photovoltaic architectures, geometries, and materials combinations that result in tandem and multi‐junction solar cells are presented.  相似文献   

2.
    
Evidence is presented for the formation of a weak ground‐state charge‐transfer complex in the blend films of poly[9,9‐dioctylfluorene‐co‐N‐(4‐methoxyphenyl)diphenylamine] polymer (TFMO) and [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM), using photothermal deflection spectroscopy (PDS) and photoluminescence (PL) spectroscopy. Comparison of this polymer blend with other polyfluorene polymer/PCBM blends shows that the appearance of this ground‐state charge‐transfer complex is correlated to the ionization potential of the polymer, but not to the optical gap of the polymer or the surface morphology of the blend film. Moreover, the polymer/PCBM blend films in which this charge‐transfer complex is observed also exhibit efficient photocurrent generation in photovoltaic devices, suggesting that the charge‐transfer complex may be involved in charge separation. Possible mechanisms for this charge‐transfer state formation are discussed as well as the significance of this finding to the understanding and optimization of polymer blend solar cells.  相似文献   

3.
The photovoltaic behavior of three hexa‐peri‐hexabenzocoronene (HBC) derivatives has been investigated with respect to the influence of the alkyl side chains. Upon increasing the side chain length, the HBC chromophore becomes diluted, thus decreasing the amount of light absorbed. Differential scanning calorimetry and powder X‐ray analysis reveal that the HBC with the 2‐ethyl‐hexyl side chain is in a crystalline state at room temperature, while the other two HBCs containing 2‐hexyl‐decyl and 2‐decyl‐tetradecyl substituents in so‐called plastic crystalline state. The HBC with the shortest side chain is proven to be the best donor for perylenediimide, showing a highest external quantum efficiency of 12 %. Furthermore, scanning electron microscopy imaging suggested an important role of the morphology of the active film in determining the performance of the device.  相似文献   

4.
    
Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral‐mismatch factors for different test‐cell/reference‐cell combinations. The test devices include both polymer/fullerene‐based bulk‐heterojunction solar cells and small‐molecule‐based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light‐bias intensity is reported. The current–voltage curves are measured under 100 mW cm–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094‐1) generated from a source set with a reference cell and corrected for spectral error.  相似文献   

5.
    
Ultra‐thin films of subphthalocyanine (SubPc) were grown onto Si/SiO2 substrates by organic molecular beam deposition and the complex refractive index has been characterized by spectroscopic ellipsometry. The peak maximum in the extinction coefficient is determined to be 1.6 at 590 nm and the dielectric constant equals 3.9 in the limit of long wavelength. These values are extraordinary high when compared to the well‐known metal‐phthalocyanines and will be beneficial for the performance in a photovoltaic cell. The amorphous SubPc structure on top of indium‐tin‐oxide (ITO) as well as quartz glass is imaged by atomic force microscopy and scanning electron microscopy and we have characterized the nearly flat surface topology. Next, subphthalocyanine films in combination with buckminsterfullerene (C60) have been studied in a planar bilayer donor/acceptor heterojunction by current density‐voltage characterization under AM 1.5 simulated illumination at various light intensities. A power conversion efficiency of 3.0 % under 1 sun was measured. Finally, the external and internal quantum efficiencies demonstrated peak maxima at 590 nm of 46 % and 55 %, respectively. Considering the abrupt junction at the donor/acceptor interface, the electron transfer from SubPc to the acceptor material is thus determined to be highly efficient.  相似文献   

6.
    
A series of poly(3‐hexylthiophene)s (P3HTs) and poly(3‐butylthiophene)s (P3BTs) with predetermined molecular weights and varying polydispersities are prepared using a simplified Grignard metathesis chain‐growth polymerization. Techniques were elaborated to prepare extremely high molecular weight P3HT (number‐average molecular weight of around 280 000 g mol–1) with a low polydispersity (< 1.1) without resorting to fractionation. Optimization of the annealing of a series of solar cells based on blends of poly(3‐alkylthiophene)s (P3ATs) and [6,6]‐phenyl C61 butyric acid methyl ester indicates that the polydispersities, molecular weights, and degrees of conjugation of the P3ATs all have an important impact not only on cell characteristics but also on the most effective annealing temperature required. The results indicate that each cell requires annealing treatments specific to the type of polymer and its molecular weight distribution.  相似文献   

7.
    
Polymer‐based photovoltaic devices have been fabricated by blending the conjugated polymer, poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV) with the buckminsterfullerene, C60. The photo‐induced current and the open‐circuit voltage show a strong dependence on the polymer processing conditions. It was found that the photovoltaic devices fabricated with tetrahydrofuran or chloroform (non‐aromatic solvents) have smaller photocurrents under same reverse bias as well as higher open circuit voltages than the devices fabricated with xylene, dichlorobenzene, or chlorobenzene (aromatic solvents). The device performance dependence on the processing solvent is attributed to the different solvation‐induced polymer morphology.  相似文献   

8.
    
The power conversion efficiency of organic and hybrid solar cells is commonly reduced by a low open‐circuit voltage (VOC). In these cases, the VOC is significantly less than the energy of the lowest energy absorbed photon, divided by the elementary charge q. The low photovoltage originates from characteristically large band offsets between the electron donor and acceptor species. Here a simple method is reported to systematically tune the band offset in a π‐conjugated polymer–metal oxide hybrid donor–acceptor system in order to maximize the VOC. It is demonstrated that substitution of magnesium into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the VOC of poly(3‐hexylthiophene) (P3HT)–ZnMgO planar devices. The VOC is seen to increase from 500 mV at x = 0 up to values in excess of 900 mV for x = 0.35. A concomitant increase in overall device efficiency is seen as x is increased from 0 to 0.25, with a maximum power‐conversion efficiency of 0.5 % obtained at x = 0.25, beyond which the efficiency decreases because of increased series resistance in the device. This work provides a new tool for understanding the role of the donor–acceptor band offset in hybrid photovoltaics and for maximizing the photovoltage and power‐conversion efficiency in such devices.  相似文献   

9.
    
Easily adjustable parameters such as area and design can affect the determination of the efficiency of donor–acceptor organic solar cells. Devices with crossing electrodes and unpatterned (semi)conducting organic layers can collect a non‐negligible current from regions usually not considered as part of the photovoltaic element, a fact that might lead to an overestimation of the power conversion efficiency.  相似文献   

10.
    
We have used Grignard metathesis polymerization to successfully synthesize a series of regioregular polythiophene copolymers that contain electron‐withdrawing and conjugated phenanthrenyl‐imidazole moieties as side chains. The introduction of the phenanthrenyl‐imidazole moieties onto the side chains of the regioregular polythiophenes increased their conjugation lengths and thermal stabilities and altered their bandgap structures. The bandgap energies, determined from the onset of optical absorption, could be tuned from 1.89 eV to 1.77 eV by controlling the number of phenanthrenyl‐imidazole moieties in the copolymers. Moreover, the observed quenching in the photoluminescence of these copolymers increases with the number of phenanthrenyl‐imidazole moieties in the copolymers, owing to the fast deactivation of the excited state by the electron‐transfer reaction. Both the lowered bandgap and fast charge transfer contribute to the much higher external quantum efficiency of the poly(3‐octylthiophene)‐side‐chain‐tethered phenanthrenyl‐imidazole than that of pure poly(3‐octylthiophene), leading to much higher short circuit current density. In particular, the short circuit current densities of the device containing the copolymer having 80 mol % phenanthrenyl‐imidazole, P82 , improved to 14.2 mA cm–2 from 8.7 mA cm–2 for the device of pure poly(3‐octylthiophene), P00 , an increase of 62 %. In addition, the maximum power conversion efficiency improves to 2.80 % for P82 from 1.22 % for P00 (pure P3OT ). Therefore, these results indicate that our polymers are promising polymer photovoltaic materials.  相似文献   

11.
    
We have shown recently that the use of high‐aspect‐ratio inorganic nanorods in conjunction with conjugated polymers is a route to obtaining efficient solar cells processed from solution. Here, we demonstrate that the use of binary solvent mixtures in which one of the components is a ligand for the nanocrystals is effective in controlling the dispersion of nanocrystals in a polymer. By varying the concentration of the solvent mixture, phase separation between the nanocrystal and polymer could be tuned from micrometer scale to nanometer scale. In addition, we can achieve nanocrystal surfaces that are free of surfactant through the use of weak binding ligands that can be removed through heating. When combined, the control of film morphology together with surfactant removal result in nanorod–polymer blend photovoltaic cells with a high external quantum efficiency of 59 % under 0.1 mW cm–2 illumination at 450 nm.  相似文献   

12.
    
The effects of anode/active layer interface modification in bulk‐heterojunction organic photovoltaic (OPV) cells is investigated using poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and/or a hole‐transporting/electron‐blocking blend of 4,4′‐bis[(p‐trichlorosilylpropylphenyl)‐phenylamino]biphenyl (TPDSi2) and poly[9,9‐dioctylfluorene‐co‐N‐[4‐(3‐methylpropyl)]‐diphenylamine] (TFB) as interfacial layers (IFLs). Current–voltage data in the dark and AM1.5G light show that the TPDSi2:TFB IFL yields MDMO‐PPV:PCBM OPVs with substantially increased open‐circuit voltage (Voc), power conversion efficiency, and thermal stability versus devices having no IFL or PEDOT:PSS. Using PEDOT:PSS and TPDSi2:TFB together in the same cell greatly reduces dark current and produces the highest Voc (0.91 V) by combining the electron‐blocking effects of both layers. ITO anode pre‐treatment was investigated by X‐ray photoelectron spectroscopy to understand why oxygen plasma, UV ozone, and solvent cleaning markedly affect cell response in combination with each IFL. O2 plasma and UV ozone treatment most effectively clean the ITO surface and are found most effective in preparing the surface for PEDOT:PSS deposition; UV ozone produces optimum solar cells with the TPDSi2:TFB IFL. Solvent cleaning leaves significant residual carbon contamination on the ITO and is best followed by O2 plasma or UV ozone treatment.  相似文献   

13.
The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. Morphological parameters, such as the extent and the composition of donor- and acceptor-rich domains, influence both the charge generation and the charge transport throughout the active layer. This work focuses on a polymer:fullerene system based on a small bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (PDPP5T) mixed with [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) that is capable of efficiencies higher than 6%. By changing the processing conditions, the morphology can be varied from a coarse separated morphology, with fullerene domains (blobs) embedded in a polymer-rich matrix, to a completely mixed layer.  相似文献   

14.
    
GaInN/GaN solar cells made without p-type material are demonstrated using an oxidized Ni/Au Schottky barrier design to collect photo-generated carriers. The best devices exhibit a short-circuit current density of 0.065 mA/cm2 with an open-circuit voltage of 0.4 V under AM0 (1-Sun) illumination. Preliminary computer simulations are in reasonable agreement with experimental results, giving a pathway to improve device performance via iterative redesign and testing.  相似文献   

15.
    
Light‐induced generation of charges into an electron acceptor–donor phase‐segregated blend is studied. The blend is made of highly ordered nanoscopic crystals of 3″‐methyl‐4″‐hexyl‐2,2′:5′,2″:5″,2?:5?,2″″‐quinquethiophene‐1″,1″‐dioxide embedded into a regioregular poly(3‐hexylthiophene) matrix, acting as acceptor and donor materials, respectively. Kelvin probe force microscopy investigations reveal a tendency for the acceptor nanocrystals to capture the generated electrons whereas the donor matrix becomes more positively charged. The presence of particular positively charged defects, i.e., nanocrystals, is also observed within the film. The charging and discharging of both materials is studied in real time, as well as the effect of different acceptor–donor ratios. Upon prolonged thermal annealing at high temperatures the chemical structure of the blend is altered, leading to the disappearance of charge separation upon light irradiation. The obtained results allow a better understanding of the correlation between the nanoscopic structure of the photoactive material and solar‐cell performance.  相似文献   

16.
    
Two new ruthenium complexes [Ru(dcbpy)(L)(NCS)2], where dcbpy is 4,4′‐dicarboxylic acid‐2,2′‐bipyridine and L is 3,8‐bis(4‐octylthiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P1) or 3,8‐bis(4‐octyl‐5‐(4‐octylthiophen‐2‐yl)thiophen‐2‐yl)‐1,10‐phenanthroline (CYC‐P2), are synthesized, characterized by physicochemical and semiempirical computational methods, and used as photosensitizers in nanocrystalline dye‐sensitized solar cells. It was found that the difference in light‐harvesting ability between CYC‐P1 and CYC‐P2 is associated mainly with the location of the frontier orbitals, in particular the highest occupied molecular orbital (HOMO). Increasing the conjugation length of the ancillary ligand decreases the energy of the metal‐to‐ligand charge transfer (MLCT) transition, but at the same time reduces the molar absorption coefficient, owing to the HOMO located partially on the ancillary ligand of the ruthenium complex. The incident photon‐to‐current conversion efficiency curves of the devices are consistent with the MLCT band of the complexes. Therefore, the overall efficiencies of CYC‐P1 and CYC‐P2 sensitized cells are 6.01 and 3.42 %, respectively, compared to a cis‐di(thiocyanato)‐bis(2,2′‐bipyridyl)‐4,4′‐dicarboxylate ruthenium(II)‐sensitized device, which is 7.70 % using the same device‐fabrication process and measuring parameters.  相似文献   

17.
    
The self‐organization of the polymer in solar cells based on regioregular poly(3‐hexylthiophene) (RR‐P3HT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) is studied systematically as a function of the spin‐coating time ts (varied from 20–80 s), which controls the solvent annealing time ta, the time taken by the solvent to dry after the spin‐coating process. These blend films are characterized by photoluminescence spectroscopy, UV‐vis absorption spectroscopy, atomic force microscopy, and grazing incidence X‐ray diffraction (GIXRD) measurements. The results indicate that the π‐conjugated structure of RR‐P3HT in the films is optimally developed when ta is greater than 1 min (ts ～ 50 s). For t s < 50 s, both the short‐circuit current (JSC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < ts < 55 s, the JSC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered π‐conjugated structure of RR‐P3HT to 1.2 % for a less‐ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon‐to‐electron conversion efficiency spectrum of the less‐ordered solar cell shows a clear loss in both the overall magnitude and the long‐wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open‐circuit voltage (VOC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.  相似文献   

18.
    
Oligomers and regioregular copolymers based on fluorenone subunits are synthesized and used in bulk‐heterojunction photovoltaic cells. These are 2,7‐bis(5‐[(E)‐1,2‐bis(3‐octylthien‐2‐yl)ethylene])‐fluoren‐9‐one (TVF), the product of its oxidative polymerization, that is, (poly[(5,5′‐(bis‐(E)‐1,2‐bis(3‐octylthien‐2‐yl)ethylene]‐alt‐(2,7‐fluoren‐9‐one)]) (PTVF), and an alternate copolymer of fluoren‐9‐one and di‐n‐alkylbithiophene, namely poly[(5,5′‐(3,3′‐di‐n‐octyl‐2,2′‐bithiophene))‐alt‐(2,7‐fluoren‐9‐one)] (PDOBTF). The interpenetrating networks of active layers consisting of these new compounds as electron donors and of methanofullerene [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) as an acceptor exhibit an extended absorption band in the visible part of the spectrum with an absorption edge close to 700 nm. The external power conversion efficiencies (EPCEs) and the external quantum efficiency of the various TVF‐, PTVF‐, and PDOBTF‐based photovoltaic cells have been determined. EPCE values of up to 1 % have been achieved, which demonstrate the potential of fluorenone‐based materials in solar cells. It has also been demonstrated that fluorenone subunits are efficient photon absorbers for the conversion. Interestingly, some cell parameters such as, for example, the fill factor, have been improved as compared to photovoltaic cells with a “classical” poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene]/PCBM active layer, fabricated and studied under the same experimental conditions.  相似文献   

19.
    
Electron donors based on Ru(bpy)3 (bpy = 2,2′‐bipyridine) are covalently attached to the walls of γ‐ZrP and acceptor species based on viologen are arranged side by side following quite simple experimental protocols. The resulting materials are characterized by using the usual techniques and their luminescence and electrochemical properties are assessed. Strong evidence is presented of the efficient occurrence of photoinduced electron transfer in the solid state among the active species attached to the transparent inorganic matrix. The prepared materials may find applications in the clean conversion of light into useful energy.  相似文献   

20.
    
A new method for the preparation of active layers of polymeric solar cells without the need for thermal post‐treatment to obtain optimal performance is presented. Poly(3‐hexylthiophene) (P3HT) nanofibers are obtained in highly concentrated solutions, which enables the fabrication of nanostructured films on various substrates. Here, the preparation of these fibers along with their characterization in solution and in the solid state is detailed. By mixing these nanofibers with a molecular acceptor such as [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) in solution, it is possible to obtain in a simple process a highly efficient active layer for organic solar cells with a demonstrated power conversion efficiency (PCE) of up to 3.6 %. The compatibility of the room‐temperature process developed herein with commonly used plastic substrates may lead to applications such as the development of large‐area flexible solar cells.  相似文献   

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