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Organic photovoltaics will become 30 years old relatively soon. In spite of the impressive development achieved throughout these years, especially in terms of reported power conversion efficiencies, there are still important technological and fundamental obstacles to circumvent before they can be implemented into reliable and long-lasting applications. Regarding device processing, the synthesis of highly soluble polymeric semiconductors first, and fullerene derivatives then, was initially considered as an important breakthrough that would definitely change the fabrication of photovoltaics once for all. Nowadays, the promise of printing solar cells by low-cost and high throughput mass production techniques still stands. However, the potential and the expectation raised by this technology is such that it is considerably difficult to keep track of the most significant progresses being now published in different and even monographic journals. There is therefore the need to compile the most remarkable advances in well-documented reviews than can be used as a reference for future ideas and works. In this letter, we review the development of polymeric solar cells from its origin to the most efficient devices published to date. After analyzing their fundamental limits, we separate these achievements into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: Active materials, strategies -fabrication/processing procedures- that can mainly modify the active film morphology and result in improved efficiencies for the same starting materials, and all the different cell layout/architectures that have been used in order to extract as high photocurrent as possible from the Sun. The synthesis of new donors and acceptors, the use of additives and post-processing techniques, buffer interlayers, inverted and tandem designs are some of the most important aspects that are in detailed reviewed in this letter. All have equally contributed to develop this technology and leave it at doors of commercialization.

In this work, diketopyrrolopyrrole-based polymer bulk heterojunction solar cells with inverted and regular architecture have been investigated. The influence of the polymer:fullerene ratio on the photoactive film nanomorphology has been studied in detail. Transmission Electron Microscopy and Atomic Force Microscopy reveal that the resulting film morphology strongly depends on the fullerene ratio. This fact determines the photocurrent generation and governs the transport of free charge carriers. Slight variations on the PCBM ratio respect to the polymer show great differences on the electrical behavior of the solar cell. Once the polymer:fullerene ratio is accurately adjusted, power conversion efficiencies of 4.7% and 4.9% are obtained for inverted and regular architectures respectively. Furthermore, by correlating the optical and morphological characterization of the polymer:fullerene films and the electrical behavior of solar cells, an ad hoc interpretation is proposed to explain the photovoltaic performance as a function of this polymer:blend composition.

This work describes a patterning technique for the photoactive layer of organic photovoltaic modules. We demonstrate the fabrication of efficient poly[3-(hexyl)thiophene-2,5-diyl]:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based organic photovoltaic modules through a specific surface treatment, based on the deposition of a fluorinated self assembled monolayer (SAM) on top of the bottom electric contact. Direct self-patterning of the photoactive layer is achieved by the high contact angle between the SAM and the polymer solution, while a smooth topography is created by combining two solvents with different surface tensions and boiling points in the polymer:PCBM solution. The resolution of the patterning is approximately 400 μm for modules based on a conventional cell architecture and 120 μm for an inverted architecture. As a result, we show 25 cm2 P3HT:PCBM based organic photovoltaic modules with 10 series-connected cells, fabricated via roll-to-roll compatible deposition and patterning techniques.

We report a detailed study on the operating principles of silver nanowire (Ag NW) networks as a transparent conducting electrode. Current limitations to achieve high conversion efficiencies are overcome with the combination of Ag NW and specifically designed ZnO nanoparticles (NP). The new transparent electrode presents improved optoelectronic properties with respect to the most widely used ITO—indium tin oxide. We demonstrate that there are non-conducting void spaces left between neighboring nanowires onto the holding substrate as a consequence of processing the Ag NW film from a solvent dispersion. Moreover, the fabrication of efficient ITO-free organic solar cells (OSC) based on Ag NW relies on filling the resulting void spaces with a highly selective and conductive supporting thin layer. This slight modification of the Ag NW film enables the great majority of photogenerated charge carriers – that would otherwise be recombined at voids – to be efficiently collected by the nanowires. In this way, we report – to our knowledge – one of the highest ITO-free OSC (3.8%) also fabricated onto flexible substrates (3.2%) with commercially available Poly (3-Hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). This breakthrough could once and for all bring the potential of flexible low-cost organic photovoltaics into reality.

Here the role of metal oxide (ZnO) electron-selective layers in the operating mechanisms of bulk-heterojunction polymer−fullerene solar cells is addressed. Inverted as well as regular structures containing ZnO layers at the cathode contact have been analyzed using capacitance methods in the dark and impedance spectroscopy under illumination. We systematically observed that the open-circuit voltage Voc at 1 sun illumination results higher for inverted cells than that achieved by regular structures in ΔVoc ≈ 30−50 mV. This shift correlates with the displacement of the flat-band potential Vfb extracted from Mott−Schottky capacitance analysis. A coherent picture is provided that states the hole Fermi level of the polymer highest occupied molecular orbital as an energy reference for both Voc and Vfb. The study connects the position of the hole Fermi level to the p-doping character of the active layer that is influenced by the film morphology through vertical phase segregation.

We report organic photovoltaic devices in which the standard ITO transparent electric contact has been substituted by lower cost ultrathin metallic electrodes. Solution and vacuum processable n and p-type semiconductors provide the electrode with the rectifying behavior of the diode. We are in this way able to invert the built-in electric field at wish and make the device deliberately either top or bottom sensitive with the same efficiency depending on the application. Taking advantage of these new generation electrodes we furthermore report devices with fill factors over 70%—to our knowledge, the largest published to date for an organic photovoltaic cell—and power conversion efficiencies over the state-of-art with 3.5% in inverted P3HT:PCBM devices, ITO free designs over 2.5% and (semi)transparent photovoltaic devices with conversion efficiencies close to 2.6%. This breakthrough could once and for all trigger the fabrication of organic tandem solar cells and photovoltaic windows.

We present a detailed study and experimental recipes to tune the outcoming light of single layer polymeric diodes fabricated with commercially available polyfluorene. The standard blue emission peaking at 440 nm can be tuned to obtain even white emission by controlling the deliberate oxidation of the material. Furthermore, with the addition of inorganic quantum dots was not only possible to redshift the emission as a function of the blend composition but also a fine tuning of the emitted light as a function of the external applied voltage. Merging these two effects we fabricated PFO and PFO:QD light emitting diodes that practically cover the entire visible spectrum. The same device exhibits emission from blue to red depending on the applied bias, from blue to green depending on the oxidation degree of the polymer and from white to blue and/or red combining both. The door is already open to cover the whole color gamut with solely one device.

Charge carrier diffusion and recombination in an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) with indium tin oxide (ITO) and aluminium contacts have been analyzed in the dark by means of impedance spectroscopy. Reverse bias capacitance exhibits Mott-Schottky-like behavior indicating the formation of a Schottky junction (band bending) at the P3H:PCBM-Al contact. Impedance measurements show that minority carrier (electrons) diffuse out of the P3HT:PCBM-Al depletion zone and their accumulation contributes to the capacitive response at forward bias. A diffusion-recombination impedance model accounting for the mobility and lifetime parameters is outlined. Electron mobility results to be 2 × 10-3 cm2 V-1 s-1 and lifetime lies within the milliseconds timescale. © 2008 Elsevier B.V. All rights reserved.

In this paper, a new method for obtaining poly(3,4-ethylene-dioxythiophene) (PEDOT/PSS)/gold nanocomposites is described. In a first step, PEDOT/PSS gold nanoparticle aqueous dispersions were obtained by simultaneous chemical synthesis of PEDOT and gold nanoparticles in the presence of PSS that acts as a stabilizer. In a second step, these PEDOT/PSS gold nanoparticle dispersions were used to formulate nanocomposites by mixing the initial dispersion with commercially available PEDOT/PSS aqueous dispersion. Nanocomposite thin films, obtained by casting these dispersions, present an intimate contact between the inorganic and organic components.

Poly(3,4-ethylenedioxithiophene)/poly(styrene sulphonate) (PEDOT/PSS) aqueous dispersions were mixed with aqueous gold nanoparticle and aqueous silver nanoparticle colloids. PEDOT/gold nanoparticles (Au NP) and PEDOT/silver nanoparticles (Ag NP) films were obtained by solvent casting the corresponding aqueous solutions. The nanocomposite films showed the optical characteristics associated with both the surface plasmon absorption resonance of the metal nanoparticles and the excitation of the bipolaron band of the conducting polymer. As an interesting application we demonstrate the use of metal nanoparticles to tune the color of PEDOT based electrochromic films from blue to violet in the case of Au NP or green in the case of Ag NP.

We present a study of dark air-exposure degradation of organic solar cells based on photoactive blends of the conjugated polymer, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photovoltaic devices were fabricated on indium tin oxide (ITO) glass with or without a layer of poly (3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), and were studied without encapsulation. Photovoltaic performance characteristics were measured as a function of time for different ambient conditions (under white light irradiation and in the dark, and under air, dry oxygen and humid nitrogen atmospheres). It was found that a key cause of degradation under air exposure is light independent and results from water adsorption by the hygroscopic PEDOT:PSS layer. Measurements of the charge mobility and hole injection after air exposure showed that the degradation increases the resistance of the PEDOT:PSS/blend layer interface.

A study of the photo-oxidation of films of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar-cell performance is degraded primarily through loss in short-circuit current density, JSC. The effect of the same photodegradation treatment on the optical-absorption, charge-recombination, and charge-transport properties of the active layer is studied. It is concluded that the loss in JSC is primarily due to a reduction in charge-carrier mobility, owing to the creation of more deep traps in the polymer during photo-oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo-bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.

We present a study of charge transfer in polyfluorene: [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films. After photoexciting the conjugated polymer, electrons are very quickly transferred onto the PCBM leaving positive charges in the polymer. The presence of new photogenerated species in both polymer and PCBM can be demonstrated by photoinduced optical absorption studies. Using ultrafast spectroscopic experiments, not only are we able to demonstrate the charge transfer process in such composites but also we can time resolve the dynamics of the process. Charge transfer is, in this way, estimated to take place ps after photoexcitation. After ps all charges have already been transferred and the process concludes. The resulting separated charges have the potential to produce a photocurrent if they are successfully driven through the bulk and collected by suitable electrodes.

The photosensitivity of semiconducting polymers can be enhanced by blending donor (D) and acceptor (A) polymers to optimise photoinduced charge separation. The photoluminescence of two such polymers, an acceptor poly(9,9-dioctylfluorene-co-benzothidiadiozole) (BT) and a donor poly(9,9-dioctylfluorene-co-bis- N , N ′-(4-butylphenyl)-bis- N , N ′-phenylbenzidine) (BFB) are quenched in their mutual blend indicative of rapid and efficient separation of photogenerated electron–hole pairs with electrons in the BT acceptor and holes in the BFB donor. In order to be useful for photovoltaic applications, the excitons have to reach a D/A interface before they decay radiatively or non-radiatively. Whereas charge separation seems to be efficient in these polymer networks, charge transport still remains a limiting factor due to the imperfect network structures and low charge carrier mobilities. This charge transport can be improved by controlling the morphology of the blend. Although the initial power conversion efficiency (PCE) was low for our composites (0.12%), the blends show promising photovoltaic characteristics; the efficiency is 220 times higher than for diodes made with pure BFB. We report work aimed at improving the efficiency via control of the blend morphology and the use of polymeric anodes.

Accurate characterization and reporting of organic photovoltaic (OPV) device performance remains one of the important challenges in the field. The large spread among the efficiencies of devices with the same structure reported by different groups is significantly caused by different procedures and equipment used during testing. The presented article addresses this issue by offering a new method of device testing using >suitcase sample> approach combined with outdoor testing that limits the diversity of the equipment, and a strict measurement protocol. A round robin outdoor characterization of roll-to-roll coated OPV cells and modules conducted among 46 laboratories worldwide is presented, where the samples and the testing equipment were integrated in a compact suitcase that served both as a sample transportation tool and as a holder and test equipment during testing. In addition, an internet based coordination was used via plasticphotovoltaics.org that allowed fast and efficient communication among participants and provided a controlled reporting format for the results that eased the analysis of the data. The reported deviations among the laboratories were limited to 5% when compared to the Si reference device integrated in the suitcase and were up to 8% when calculated using the local irradiance data. Therefore, this method offers a fast, cheap and efficient tool for sample sharing and testing that allows conducting outdoor measurements of OPV devices in a reproducible manner., This work has been supported by the EUDP (j.no. 64012-0202), the Danish National Research Foundation, the Eurotech Universities Alliance project “Interface science for photovoltaics (ISPV)”, the European Research Infrastructure (SOPHIA), the European Energy Research Alliance (EERA), the Danish Ministry of Science, Innovation and Higher Education under a Sapere Aude Top Scientist Grant no.( DFF – 1335-00037A), an Elite Scientist Grant no.( 11-116028), MICINN-Spain(Grant # MAT2010-21267-C02-02) including FEDER funds, the UK Department for Business, Innovation & Skills, the Royal Society and the EPSRC for grants EP/G031088, EP/J500021 and EP/K030671. MN and SAC acknowledge co-funding by the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Strategic Infrastructure Project ΝΕΑ ΥΠΟΔΟΜΗ/ΣΤΡΑΤΗ/0308/06). A. E. G., R. K. M. and E.A.K. thank a financial support from the European Commission׳s Seventh Framework Program under Grant agreement no. 261936.

In this work we compare the photovoltaic performance of different cell designs, standard and inverted, for one of the most promising systems to achieve power conversion efficiencies over 10% in polymer:fullerene single cells, namely PTB7:PC70BM. Impedance spectroscopy, charge extraction and transient photovoltage are used in order to assign the electrical losses initially observed in the current density–voltage curve and understand the main limitation of every design. While inverted devices show competitive performance in terms of charge generation, transport of carriers and also for charge collection at electrodes, standard devices present additional resistive losses that are assigned to charge transfer issues at the active layer/anode interface. This additional resistance increase the overall series resistance of devices, lowers the fill factor and it is the ultimate responsible for the observed reduced device performance of standard cells in comparison to inverted ones. In this way, devices over 7.2% are reported with ZnO and MoO3 as interlayer electrodes that act as improved highly selective and extracting contacts in comparison to standard PEDOT:PSS and Ca/Ag. Contacts are thus electrically optimized. Additional improvement of device performance must consider enhancement of intrinsic recombination properties of the blend. Lower molecular weights and/or any residual catalyst impurities with respect to other batches are the only limitation to reach record efficiencies as those shown in recent works.

Tandem solar cells are the best approach to maximize the light harvesting and adjust the overall absorption of the cell to the solar irradiance spectrum. Usually, the front and back subcells are connected in series in two-terminal device (2T) designs which require a current matching between both subcells in order to avoid potential losses. Alternatively, they can also be connected in parallel giving rise to a three terminal connection (3T). In principle, both designs have their assets and drawbacks in terms of device performance, design and materials¿ characterization. In this letter, we theoretically and experimentally confront both designs with each other (2T and 3T). Theoretical estimations show a maximum PCE of 15% for 2T and about 13% for 3T structures with ideal bandgaps for the front and back cell. However, 3T tandem devices can yield higher efficiencies than 2T for some specific material combinations whose theoretical values are between 10% and 12%. Therefore, other aspects related to the fabrication feasibility are studied in order to analyze the most convenient approach for module development. The need of a conducting interlayer restricts the width of the cell and causes a 3% reduction in the geometrical fill factor of the module in comparison to the 2T approach. The R2R processing of modules with 3T cells would also require an additional laser step. Finally, a couple of existing material combinations have been experimentally implemented into 2T and 3T tandem devices. The limitation imposed by their specific and non-ideal bandgaps restricts the efficiency to around 7%, considerably below the ideal case.

In this work we study the different electrical loss pathways occurring during the operation of bulk heterojunction solar cells by using a variety of electrical and optical characterization techniques beyond the current density–voltage curve (J–V): Impedance Spectroscopy (IS), Charge Extraction (CE) and Transient Photovoltage (TPV). Two sets of devices are analyzed: the first is based on the donor polymer P3HT, known to provide efficient cells using thick active layers (i.e. 270 nm), and the recently developed PTB7 which offers maximum efficiencies for devices with thinner layers (i.e. 100 nm). Devices fabricated with P3HT:PC60BM are not limited by transport of carriers and large active layer thickness may be used. Importantly, increasing the active layer thickness does not modify the contact selectivity. This is supported by analysis of the diode curve measured in the dark (similar leakage currents) and by capacitance–voltage measurements (similar fullerene content covering the cathode). Under these conditions the current density curve under illumination is mainly defined by the recombination processes taking place in the bulk of the active layer. In contrast, transport of carriers and contact selectivity are both limiting factors for the PTB7:PC60BM system. In this case, best efficiencies are obtained with a low active layer thickness and a high fullerene ratio. Reduced active layer thickness minimizes undesired electrical resistances related to carrier transport through the bulk of the active layer. High fullerene content enhances the amount of fullerene molecules at the cathode leading to decreased leakage currents. Then, the overall device efficiency will be a combination of the recombination kinetics in the bulk of the active layer, undesired resistance to transport of carriers and leakage current present due to low selectivity of the contact. The use of additives has also been explored which enhances charge generation and extraction. Overall, this work provides a comprehensive guide on how to interpret results obtained from some of the most widely used optoelectronic techniques employed to analyse operating devices.

We present a study of charge transport in polyfluorene:[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films. The electron and hole mobilities, measured via time-of-flight photocurrent experiments, show a strong dependence on the PCBM concentration. As a result, ambipolar transport can be obtained in blends of two otherwise unipolar materials. Both the magnitude of the mobility and the shape of the current transient are strongly affected by the presence of PCBM molecules in the film. Highly dispersive electron and hole transport for the pure polymer becomes nondispersive when PCBM is added.

Quantum dots (QDs) are inorganic semiconductor particles that exhibit size and shape dependent optical and electronic properties (Alivisatos, 1996; Smith & Nie, 2010). Due to the typical dimension in the range of 1-100 nm, the surface-to-volume ratios of the materials become large and their electronic states become discrete. Moreover, due to the fact that the size of the semiconductor nanocrystal is smaller than the size of the exciton, charge carriers become spatially confined, which raises their energy (quantum confinement). Thus, the size and shape-dependent optoelectronic properties are attributed to the quantum confinement effect. Because of this effect, light emission from these particles can be tuned, throughout the ultraviolet, visible and near infrared spectral ranges.

Energetics and kinetics of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester solar cells, varying the temperature. The analysis reveals the presence of defect bands (shallow acceptors) centered at E0 = 35 meV above the highest-occupied molecular orbital level of P3HT. The total density of defects results of order 1016 cm− 3. Characteristic frequency is obtained to be situated within the range of 1–10 Hz. Defect bands acting as negatively charged levels are responsible for the p-doping of the active layer and the band-bending near the cathode contact, as derived from Mott–Schottky capacitance–voltage analysis.

1-Dimensional nanostructured ZnO electrodes have been demonstrated to be potentially interesting for their application in solar cells. Herein, we present a novel procedure to control the ZnO nanowire optoelectronic properties by means of surface modification. The nanowire surface is functionalized with ZnO nanoparticles in order to provide an improved contact to the photoactive P3HT:PCBM film that enhances the overall power conversion efficiency of the resulting solar cell. Charge extraction and transient photovoltage measurements have been used to successfully demonstrate that the surface modified nanostructured electrode contributes in enhancing the exciton dissociating ratio and in enlarging the charge lifetime as a consequence of a reduced charge recombination. Under AM1.5G illumination, all these factors contribute to a considerably large increase in photocurrent yielding unusually high conversion efficiencies over 4% and external quantum efficiencies of 87% at 550 nm for commercially available P3HT:PCBM based solar cells. The same approach might be equally used for polymeric materials under development to overcome the record reported efficiencies.

Organic polymer solar cells (OPSCs) have been prepared using TiO(x) metal oxides as selective electrodes for electron collection. The interfacial charge transfer reactions, under working conditions, that limit the energy conversion efficiency of these devices have been measured and compared to the standard OPSC geometry which collects the electrons through a low work function metal contact.

We have measured the charge transfer processes on near IR small molecule solution processes solar cells under working conditions. The results show that there are two different behavior, for the device under working conditions, controlled by the intrinsic properties of the organic bulk-heterojunction phase to store charges at the different organic materials. Furthermore, our recombination kinetic studies show the origin of the photovoltage on these thin organic devices (80 nm thick) and the reason for the difference between the observed open circuit cell potential and the theoretical expected value based on the LUMOPCBM–HOMO phthalocyanine difference.

Since the demonstration of efficient hybrid photovoltaic devices using CdSe/P3HT, a number of different QD/polymer molecular solar cells have been investigated. However, considerably less attention has been paid to the photo-induced processes occurring in these devices, in particular the strong dependence that device efficiencies show upon QD concentration. This study aims to shed more light on this dependence by monitoring these processes occurring at the QD/polymer interface, namely, exciton formation, charge separation and undesired charge recombination.

In order to get a deeper understanding of the conduction mechanisms limiting the electrical characteristics of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells, dark current-voltage measurements at different temperatures were analyzed using a compact electrical equivalent circuit previously used in p/n junctions. Between 0.2 V and 0.6 V, the current-voltage characteristic is modeled by an exponential term which can be described by Multi-Tunneling Capture Emission process. For larger voltage, the model takes into account Space-Charge Limited process and series resistance. In addition, the model is useful to calculate the built in potential of the solar cell using only dark current- voltage-temperature measurements.

In spite of the impressive development achieved by organic photovoltaics throughout the last decades, especially in terms of reported power conversion efficiencies, there are still important technological and fundamental obstacles to circumvent before they can be imple- mented into reliable and long-lasting applications. Regarding device processing, the synthesis of highly soluble polymeric semiconductors first, and then fullerene derivatives, was initially considered as an important breakthrough that would definitely change the fabrication of photo- voltaics once and for all. The potential and the expectation raised by this technology is such that it is very difficult to keep track of the most significant progresses being now published in differ- ent and even monographic journals. In this paper, we review the development of polymeric solar cells from its origin to the most efficient devices published to date. We separate these achieve- ments into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: active materials, strategies—fabrication/process- ing procedures—that can mainly modify the active film morphology, and all the different cell layout/architectures that have been used in order to extract as high a photocurrent as possible from the Sun. The synthesis of new donors, the use of additives and postprocessing techniques, buffer interlayers, inverted and tandem designs are some of the most important aspects that are reviewed in detail in this paper. All have equally contributed to develop this technology and bring it at the doors of commercialization. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.JPE.5.057214)

We present a study of charge recombination in polyfluorene:[6,6]-Phenyl C 61 -butyric acid methyl ester (PCBM) blend photovoltaic cells. The recombination kinetics of photogenerated charge carriers are investigated at room temperature by frequency and time domain photoinduced absorption. Monomolecular and bimolecular recombination processes are discussed in detail. Finally, a discussion on what kind of recombination can be dominant in our photovoltaic cells is addressed.

We present a study of photocurrent generation and charge transport in poly fluorene/PCBM blend films. Diode rectification ratios of 10 6 and photocurrent (PC) enhancements of four orders of magnitude over the dark response make the polyfluorene/PCBM blend system a sensitive photodiode material and one of the most promising polymer blend systems for use in a solar cell. We have also studied the effect of varying PCBM concentration, optical excitation intensity and the unusual. strong dependence of the short circuit current on the operating temperature.

Quantum dot-hydrotalcite layered nanoplatforms were successfully prepared following a one-pot synthesis. The process is very fast and a priori delamination of hydrotalcite is not a prerequisite for the intercalation of quantum dots. The novel materials were extensively characterized by X-ray diffraction, thermogravimetry, infrared spectroscopy, transmission electron microscopy, true color fluorescence microscopy, photoluminescence, and nitrogen adsorption. The quantum dot-hydrotalcite nanomaterials display extremely high stability in mimicking physiological media such as saline serum (pH 5.5) and PBS (pH 7.2). Yet, quantum dot release from the solid structure is noted. In order to prevent the leaking of quantum dots we have developed a novel strategy which consists of using tailor made double layered hydrotalcites as protecting shells for quantum dots embedded into silica nanospheres without changing either the materials or the optical properties.

In this study, full coincidence between impedance spectroscopy and transient photovoltage techniques in measuring recombination kinetics of photogenerated charge carriers in inverted polymer:fullerene organic solar cells with ZnO hole-blocking layer is reported. Carrier lifetime exhibits values at illumination intensities near 1 sun within the microseconds time scale. Photogenerated charge carrier density attains values within 1015–1016 cm−3. Decay kinetics is analyzed by means of a bimolecular recombination law with a recombination coefficient slightly dependent on the charge density, which lies within the order of k ∼ 10−12 cm3 s−1. It is also demonstrated that inverted-processed cells exhibit capacitance, recombination resistance, and lifetime parameters comparable to those extracted from regular cells, despite the great differences between the contact structures of these kinds of devices.