The photochemical stability of the active layer blend for organic solar cells was explored by introducing electron withdrawing cyano groups into the backbone of poly-3-hexylthiophene (P3HT). Random copolymerization of 2-bromo-3-hexyl-5-trimethylstannylthiophene and 2-bromo-3-cyano-5-trimethylstannylthiophene enabled introduction of the cyanogroups along the polythiophene backbone. The percentage of the cyano groups was 10%. The photochemical stability of poly(3-hexylthiophene-co-3-cyanothiophene) (CN-P3HT) was shown to be significantly better than pristine P3HT and the addition of CN-P3HT to P3HT also increased the photochemical stability of the blend. The photochemical stability of bulk heterojunction mixtures of the polymers and their blends with the fullerene phenyl-C61-butyric acid methyl ester ([60]PCBM) were then studied and it was found that [60]PCBM had a significantly more stabilizing effect on P3HT than CN-P3HT and that the stabilization of the bulk heterojunction mixture was dominated by the fullerene. The mixture comprising both fullerene and CN-P3HT, however, demonstrated the highest degree of photochemical stability supporting earlier observations that the stabilizing effects are additive. Finally, the blends were explored in fully printed flexible ITO-free roll coated inverted devices (with an active area of 0.8 cm2) using two different back PEDOT:PSS electrode compositions and the operational stability of the devices was studied under ISOS-L-2 conditions. The pure P3HT:PCBM devices were found to be the most stable in operation demonstrating that photochemical stability alone is not necessarily the dominant factor for overall device stability.
The dynamics of mobile charge carrier generation in polymer bulk heterojunction films is of vital importance to the development of more efficient organic photovoltaics. As with conventional semiconductors, the optical signatures of mobile carriers lie in the far-infrared (1-30 THz) although the electrodynamics deviate strongly from the Drude model. The key time scales for the process are sub-100 fs to picoseconds, and is a challenge to perform low energy spectroscopy on these time scales as it is less than the period of oscillation for the probing light. In this work, we demonstrate sub-100 fs spectroscopy of a polymer bulk heterojunction film P3HT:PCBM using a single-cycle, phase-locked and coherently detected multi-THz transient as a probe pulse following femtosecond excitation at 400 nm. By observing changes to the reflected THz transients from the film surface following photoexcitation, we can extract the complex optical conductivity spectrum for the film in snapshots of 40 fs following photoexcitation. We find that for our excitation conditions mobile charges are created in less than 120 fs and are characterized by a spectrum which is characteristic of a two dimensional delocalized polaron. A large fraction of mobile carriers relax to a localized state on a 1 ps time scale. Pump energy dependent photon-to- mobile carrier conversion efficiency supports hot exciton dissociation as a mechanism for such fast mobile carrier generation.
This work is part of the inter-laboratory collaboration to study the stability of seven distinct sets of state-of-the-art organic photovoltaic (OPVs) devices prepared by leading research laboratories. All devices have been shipped to and degraded at the Danish Technical University (DTU, formerly RISO-DTU) up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. In this work we present a summary of the degradation response observed for the NREL sample, an inverted OPV of the type ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag/Al, under full sun stability test. The results reported from the combination of the different characterization techniques results in a proposed degradation mechanism. The final conclusion is that the failure of the photovoltaic response of the device is mainly due to the degradation of the electrodes and not to the active materials of the solar cell.
Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration
planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to DTU and characterized
simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun
simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally
identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for
a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the
devices. Characterization includes current-voltage curves, light beam induced current (LBIC) imaging, dark lock-in thermography (DLIT),
photoluminescence (PL), electroluminescence (EL), in situ incident photon-to-electron conversion efficiency (IPCE), time of flight secondary
ion mass spectrometry (TOF-SIMS), cross sectional electron microscopy (SEM), UV visible spectroscopy, fluorescence microscopy, and
atomic force microscopy (AFM). Over 100 devices with more than 300 cells were used in the study. We present here design of the device
sets, results both on individual devices and uniformity of device sets from the wide range of characterization methods applied at different
stages of aging under the three illumination conditions. We will discuss how these data can help elucidate the degradation mechanisms as well
as the benefits and challenges associated with the unprecedented size of the collaboration.
The work focuses on the degradation of performance induced by both water and oxygen in an inverted geometry organic photovoltaic device with emphasis on the accumulated barrier effect of the layers comprising the layer stack. By studying the exchange of oxygen in the zinc oxide (ZnO) layer, the barrier effect is reported in both a dry oxygen atmosphere and an oxygen-free humid atmosphere. The devices under study are comprised of a bulk heterojunction formed by poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester sandwiched between a layer of zinc oxide (electron transporting layer) and a layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (hole transport layer) and the two electrodes indium tin oxide and silver. Time-of-flight secondary ion mass spectrometry is employed to characterize the accumulated barrier effect. A pronounced barrier effect is observed in the humid atmosphere, correlating well with a long observed lifetime in the same atmosphere
KEYWORDS: Polymers, Photovoltaics, Absorption, Organic photovoltaics, Magnesium, Visible radiation, Solar cells, Chromatography, Polymerization, Solar energy
Two new conjugated polymers consisting of the donors 1,5-bis(2-hexyldecyloxy)naphthalene, thiophene, or bithiophene and the acceptor benzothiadiazole has been synthesized and their optical and photovoltaic properties have been characterized. The two polymers were compared with earlier synthesized and characterized polymers containing benzene instead of naphthalene. The two polymers absorb light in the visible spectrum (400 to 700 nm). The naphthalene containing polymers had blueshifted absorption spectra compared to the benzene containing polymers and also higher band gaps. In photovoltaic devices the bithiophene containing polymer gave the best efficiency of 0.6%, whereas the single thiophene only showed efficiency of 0.005%. This is lower than the best benzene incorporated polymer that showed efficiency up to 2.2%.
We report ultrafast carrier dynamics in hybrid CdSe nanorod/poly(3-hexythiophene) (P3HT) bulk heterojunction films
measured by time-resolved terahertz spectroscopy, and compare to the well studied P3HT/phenyl-C61-butyric acid
methyl ester (PCBM) blend. Both films show an improved peak photoconductivity compared to P3HT alone, consistent
with efficient charge transfer. The photoconductivity dynamics show fast, picosecond trapping or recombination in the
hybrid blend while the all-organic film shows no such loss of mobile charge over ns time scales. The ac conductivity for
all samples is well described by a Kramers-Kronig compatible Jonscher-type power law with exponent between 0.5 and
1 suggesting that interchain hopping in the polymer or between nanorods occurs at frequencies higher than 3 THz
immediately after photoexcitation.
Polymer solar cells have many advantages such as light weight, flexibility, environmental friendliness, low thermal
budget, low cost and most notably very fast modes of production by printing techniques. Production experiments have
shown that it is highly feasible with existing technology to mass produce polymer solar cells at a very low cost. We have
employed state-of-the-art analytical techniques to address the challenging issues of degradation and stability of R2R
manufactured devices. We have specifically studied the relative effect of oxygen and water on the operational devices in
regard to degradation.
This book is intended to be a practical guide in the laboratory for the experimental solar-cell scientist whether he or she is involved with synthesis, device preparation, processing, or device characterization. Useful to all scientists working practically in the field, the book presents the process of creating a polymer solar-cell device beginning with a description of materials, including how they are made and characterized, followed by how the materials are processed into devices and films, and how these are characterized. From there, the status of two emerging fields of polymer solar cells are described: degradation and stability and large-scale processing.
Insufficient lifetimes of organic photovoltaics are manifested in a reduced photovoltaic response, which is a consequence of physical and chemical degradation of the photovoltaic device. To prevent degradation it is vital to gain detailed insight into the degradation mechanisms. This is possible by utilizing state-of-the-art characterization techniques such as TOF-SIMS, XPS, AFM, SEM, interference microscopy and fluorescence microscopy as well as isotopic labeling (18O2 and H218O). By a combination of lateral and vertical analyses of the devices we obtain in-depth and in-plane information on the reactions and changes that take place in the various layers and interfaces. Examples will be presented that describe the advantages and disadvantages of various characterization techniques in relation to obtaining information on the degradation behavior of complete photovoltaic devices.
The synthesis of copolymers based on thiophene, benzothiadiazole and benzo-bis-thiadiazole are described. The
polymers were obtained by employing Stille cross coupling polymerization. The polymers were characterized by NMR,
size exclusion chromatography, UV-vis and ultraviolet photoelectron spectroscopy. The results obtained from UV-vis
and ultraviolet photoelectron spectroscopy showed band gaps of 2.1-1.7 eV for polymers based on benzothiadiazole and
0.7 eV for polymers based on benzo-bis-thiadiazole. Furthermore the results showed that the band gap decreases with an
increase in the number, n, of thiophenes in the polymer repeating unit (n= 1-4). Large area photovoltaic devices were
prepared and the results of these devices are described.
In this paper we would like to address the key role of fabrication in the performance and lifetime of organic photovoltaics. The realization of a complete process line for the construction of large area organic photovoltaics (250 x 400 mm) is described. Among many of the factors that influence organic solar cell lifetime, oxygen and water exposure is the most important. Multiple processes have to be performed under controlled atmosphere and a glove box (or glove boxes), which involves more volume than commercially available glove boxes, needs to house different instruments. The processes housed in the glove boxes were spin coating, evaporation, lamination/sealing and testing, under an inert atmosphere. The main strategy employed multiply connected glove boxes with one load lock. The first glove box was used for spin coating and lamination/sealing, the second will house a screen printer and the third one accommodate an evaporator completely build in house. The evaporator has 2 thermal evaporation sources and 2 e-beams with 4 and 1 crucibles. The process line should allow the entire device realization from substrate coating, to electrode evaporation including the sealing process avoiding air and water exposure. Organic solar cells from small test cells on ITO glass to big modules (250 x 400 mm) of 91 connected cells on ITO PET substrates were fabricated and characterized.
The synthesis of two new poly(dialkylstilbenevinylene)s obtained through a palladium-catalyzed polymerization with a controlled molecular weight and a terpyridine moiety in the backbone is presented. Assembly using ruthenium complexation led to coordination polymers with a ruthenium complex in the middle. The coordination homo and copolymers were characterized using NMR, UV-vis and were processed into thin films for solar cells applications. The best photoresponse was obtained for the device prepared from the ruthenium homopolymer bearing cyano substitutents with a maximum output power of 0.86 μW cm-2 and a fill factor of 26% under illumination at 1000 W m-2 AM1.5. A blend of this compound with a zinc porphyrin was also investigated and gave a lower performance.
The sidechains that are added to conjugated polymer materials convey desirable solubility and film forming properties but they generally lead to a low glass transition temperature that is comparable or slightly higher than the operational temperature for the devices. The use of thermo cleavable sidechains allow for solution processing and film forming while a subsequent thermal treatment leads to efficient removal of the sidechains yielding a dense insoluble film with in principle a much higher glass transition temperature making diffusion phenomena much slower. This approach has been shown to improve the operational lifetime using accelerated lifetime testing with an incident light intensity of 1000 W m-2 (AM1.5) at 72oC and opens up the possibility for the formation of multilayer structures by sequential film forming and thermal cycles. The synthesis of regiorandom poly(2,5-thienyl-co-3'-(1''-valeryloxy-1''-ethyl)-2',5'-thienyl) (4) a polythiophene based on a copolymer of thiophene and the valeric acid ester of 3-hydroxyethylthiophene was demonstrated to thermo cleave valeric acid efficiently at temperatures above 200oC leaving vinyl groups on the polythiophene backbone giving regiorandom poly(2,5-thienyl-3'-vinyl-2',5'-thienyl) (5). The thermocleaved film was insoluble in common organic solvents. Thermocleavage experiments using spincoated films of a 1:1 (w/w) mixture of 4 and the soluble fullerene derivative 6,6-phenyl-C61-butyric acid methylester (PCBM) gave films of 5 and PCBM in a ratio of 2:3 (w/w). Photovoltaic devices were prepared and devices based on 5 and PCBM gave significantly improved lifetimes for devices operated in the atmosphere while the efficiency for the devices was lowered by a factor of 20 upon thermo cleavage for this system.
The synthesis of conjugated low band-gap copolymers based on thiophene and benzothiadiazole is described. The synthesis was carried out by oxidative ferric chloride polymerization or Stille cross coupling polymerization. The solubility of the polymer based on quarterthiophene and benzothiadiazole was tested with hexyl, 2-ethyl-hexyl and dodecyl as side chains on the thiophene. It was found that 2-ethyl-hexyl substituents gave high molecular weight polymer products with good film forming ability and good solubility. The polymers based on di-thiophene and benzothiadiazole were applied in photovoltaic devices and the coupling of the alkylthiophene showed no effect on the maximum photovoltaic performance. Band-gaps were estimated to be 2 eV for polymers based on di-thiophene and benzothiadiazole and 1.8 eV for polymers based on quarterthiophene and benzothiadiazole. Attempts to synthesize the polymers with a benzo-bis-thiadiazole unit are also described.
The fabrication of very large area polymer based solar cell modules with a total aperture area of 1000 cm2 has been accomplished. The substrate was polyethyleneterephthalate (PET) foil with a pre-etched pattern of indium-tin-oxide (ITO) anodes. The module was constructed as a matrix of 91 devices comprising 7 rows connected in parallel with each row having 13 individual cells connected in series. The printing of the organic layer employed screen printing of a chlorobenzene solution of the active material that consisted of either poly-1,4-(2-methoxy-5-ethylhexyloxy) phenylenevinylene (MEH-PPV) on its own or a 1:1 mixture (w/w) of MEH-PPV and [6,6]-phenyl-C61-butanoic acid methyl ester (PCBM). Our first results employed e-beam evaporation of the aluminium cathode directly onto the active layer giving devices with very poor performance that was discouragingly lower than expected by about three orders of magnitude. We found that e-beam radiation leads to a much poorer performance and thermal evaporation of the aluminium using a basket heater improved these values by an order of magnitude in efficiency for the geometry ITO/MEH-PPV/C60/Al. Finally the lifetimes (τ1/2) of the modules were established and were found to improve significantly when a sublimed layer of C60 was included between the polymer and the aluminium electrode. Values for the half life of 150 hours were typically obtained. This short lifetime is linked to reaction between the reactive metal electrode (aluminium) and the constituents of the active layer.
The design of light harvesting systems based on zinc-porphyrin linked polyphenyleneethynylene systems is demonstrated through two different synthetic procedures. The use of the Sonogashira cross-coupling reaction was found to be problematic in several aspects and it was only possible for one of the synthetic strategies to incorporate a single porphyrin molecule in each polymer chain. The polymer materials were separated into fractions using preparative SEC and subjected to photophysical studies in both the solution and in the solid state. In the solid state light harvesting by the pendent polyphenyleneethynylene and energy transfer to the zinc-porphyrin was found to be high whereas it was low in solution. This observation was confirmed when making electroactive devices. I/V characteristics were measured on fabricated solar cells of the polymers. Material properties for the native polyphenyleneethynylene were determined using ultra violet photoelectron spectroscopy on thin films and carrier mobility studies were performed using pulse radiolysis time resolved microwave conductivity.
Porphyrins have attracted a lot of interest as potential light harvesting dyes in polymer solar cells due to a broad absorption range, wherein the porphyrin serves as a synthetic surrogate for chlorophyll. Asymmetric porphyrins are essential building blocks in one of our polymer solar cell projects. Synthesizing these asymmetric porphyrins on large scale, in good yield, with few scrambling byproducts and without the necessity for chromatographic workup proved to be a challenge. Here we present different approaches to the synthesis of asymmetric trans-A2B2-porphyrins and trans-AB2C-porphyrins on a large scale and detail problems associated with the synthetic work. The products were characterized using SEC, MALDI-TOF, UV-vis and NMR.
Light harvesting and energy transfer in two oligomer-dye assemblies has been investigated. In both cases the oligomer was a poly(terphenylenecyanovinylene) derivative while two different dyes was used, a porphyrin and an ionic dye. It is well known that the efficiency of solar cells consisting of a single homopolymer is limited. To increase overall efficiency different strategies have been used. One possible strategy aims at covalently linking different domains. With careful design, this type of assemblies is envisaged to show improved charge separation and charge transport properties. We have shown how photophysical measurements can be used to determine what happens to an exciton formed on any of the domains. From fluorescence and absorption measurements on the assemblies, along with model compounds, it was possible to quantify the number of excitons that are emitted (fluorescence), transferred between domains or lost in internal transfer processes. Both steady state and lifetime measurements were performed in solution and on solid films. The effect of acid was investigated in the cases of the oligomer-porphyrin assembly. We found that in solution the effect of acid was an increase in the time of energy transfer, probably due to acid induced structural change of the porphyrin moiety. It was possible to make LB-films of the ionic dye-assembly, which made it possible to investigate a monolayer of the assembly.
A series of oligo phenylene vinyles (OPVs) have been prepared using a generic step-wise and uni-directional synthesis from stilbene type monomers containing masked aldehyde and benzylic phosphonate ester functionalities. In the course of this investigation six different monomers with alkyl or alkoxy substituents and with benzene, thiophene or benzothiadiazole groups were developed and prepared. Trimer OPVs were assembled and their optical spectra investigated. Systematic end-group modification gave a series of donor-wire-acceptor OPVs that were used to prepare simple large area photovoltaic cells (3 cm2) without any fullerene derivatives. The efficiency of the devices were measured and compared based on the short circuit current ISC. Two materials were found to perform 10-100 fold better than standard PPV materials and the other OPVs investigated. Another type of end capping with a terpyridine moiety was realized to prepare an example of an OPV-ruthenium dye. A series of devices with mixtures of OPVs with the soluble fullerene derivative PCBM were made and characterized. A maximum efficiency of 0.8 % under AM1.5 conditions were found for a thiophene containing OPV trimer mixed with PCBM (1:4).
In this paper we report on an attempt to substitute the liquid-electrolyte in Dye Sensitized Solar Cells (LC) by quasi-solid-state constructions (SC) adopting organic/inorganic gels as well as a novel dye comprised of a conjugated polymer covalently linked to a ruthenium complex that can be bound to a TiO2 anatase electrode. Gel polymer electrolytes are prepared by incorporating liquid electrolytes into a polymer matrix such as poly methyl methacrylate (PMMA) using a gelling solvent such as propylene carbonate (PC). Dye Sensitized Solar Cell (DSSC) fabricated using the former gel electrolytes and standard sensitizing dye such as cis-bis(thiocyano) ruthenium(II)-bis-2,2'-bipyridine-4,4'-dicarboxylate (N3) exhibit an encouraging short circuit current densitie (Jsc) of 4.45 mA cm-2 with open circuit voltages (Voc) of 495 mV. In the novel dye the conjugated polymer provides light harvesting and hole conduction while the ruthenium complex binds to the anatase electrode providing efficient charge carrier separation and injection into the anatase electrode.
The degradation mechanisms of conjugated polymer materials used in organic photovoltaic cells were studied. To elucidate the parts of the degradation mechanisms induced by molecular oxygen, isotopic labeling was employed in conjunction with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Devices that were kept in the dark were compared with devices that had been subjected to illumination under simulated sunlight. It was found that molecular oxygen diffuses into the device causing oxygen-containing species to be generated throughout the active layers. The isotopic labeling combined with TOF-SIMS depth profiling and imaging allowed mapping of the oxidation processes by measuring the vertical and lateral distribution of oxygen-containing species. The exact pinpointing of the parts of the device that are susceptible to oxidation allows for a mechanism to be proposed that partly explains the device failure manifested in the insufficient life times of the organic photovoltaics.
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