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Hybrid nanostructures, comprising of a metal core and a semiconductor shell layer, show great potential for a new generation of low-cost solar cells due to their unique electronic and optical properties. However, experimental results have fallen far short of the ultra-high efficiency (i.e. beyond Shockley-Queisser limit) predicted by theoretical simulations. This limits the commercial application of these materials. Here, a non-transparent organic solar cell with an array of Ag/ZnO nanowires has been experimentally fabricated to increase the internal quantum efficiency (IQE) by a factor of 2.5 compared to a planar counterpart. This result indicates a significant enhancement of charge collection efficiency due to the ultrafast Ag nanowire channels. This hybrid nanostructure can also serve as a perfect back reflector for semi-transparent solar cells, which can result in enhanced light absorption by a factor of 1.8 compared to the reference samples. The enhanced charge collection and light absorption can make these Ag/ZnO nanostructures available for the application of modern optoelectronic devices. Keywords: Vertically aligned nanowires, Silver, Core-shell, Charge collection efficiency, Light harvesting, Semi-transparent organic solar cells

Hybrid nanostructures, comprising of a metal core and a semiconductor shell layer, shows great potential for a new generation of low-cost solar cells, due to their unique electronic and optical properties. However, experimental results have fallen far short of unltra-high efficiency (i.e. beyond Shockley-Queisser limit) predicted by theoretical simulation, limiting commercial application. Here, we experimentally design a non-transparent organic solar cell containing an array of Ag/ZnO nanowires, increasing internal quantum efficiency (IQE) by a factor of 2.5 compared with the planar counterpart, indicating a great enhancement of charge collection efficiency, due to the ultrafast Ag nanowire channels. Furthermore, we exploit this hybrid nanostructure as a perfect back reflector for semi-transparent solar cells, resulting in enhanced light absorption by a factor of 1.8 compared with the planar counterparts. The ability to enhance the charge collection and light absorption makes these Ag/ZnO nanostructures a flexible platform for the development of modern optoelectronic devices.

Titanium dioxide (TiO2) semiconductors are widely used in energy conversion, energy storage, catalysis, and other electronic applications. Hydrothermally grown TiO2 nanowires are an alternative to mesoporous TiO2 nanostructures due to possible direct charge transport, but their full potential waits to be unleashed. Latest findings show a peculiar defect structure, consisting of small single-crystalline fingers and free internal surfaces, which supposedly promote a number of loss mechanisms. In this paper, the influence of these defects on charge transport is studied on the basis of hybrid solar cells featuring a TiO2/dye/polymer interface. Electrical, optical, and structural characterization identifies a number of loss mechanisms, which are inhibited by the introduction of particular annealing steps at specific processing points during fabrication. An increase in power-conversion efficiency of 35% is obtained, resulting in 2.71% and surpassing mesoporous films of the same material combination. These results suggest that caution has to be exercised when dealing with defect structures possibly present in metal oxides which appear single-crystalline by conventional analysis methods.

The macroscopic device performance of organic solar cells is governed by interface physics on a nanometer scale. A comb-like bilayer all-polymer morphology featuring a controlled enhancement in donor-acceptor interfacial area is employed as a model system to investigate the fundamental processes of exciton separation and polaron recombination in these devices. The different nanostructures are characterized locally by SEM/AFM, and the buried interdigitating interface of the final device architecture is statistically verified on a large area via advanced grazing incidence X-ray scattering techniques. The results show equally enhanced harvesting of photoexcitons in both donor and acceptor materials directly correlated to the total enhancement of interfacial area. Apart from this beneficial effect, the enhanced interface leads to significantly increased polaron recombination losses both around the open-circuit voltage and maximum power point, which is determined in complement with diode dark current characteristics, impedance spectroscopy, and transient photovoltage measurements. From these findings, it is inferred that a spatially optimized comb-like donor-acceptor nanonetwork alone is not the ideal morphology even though often postulated. Instead, the energetic landscape has to be considered. A perfect morphology for an excitonic solar cell must be spatially and energetically optimized with respect to the donor-acceptor interface.

Hybrid nanostructures have shown increasing potential as a replacement for Si solar cells due to the availability of low-cost material combinations. However, up to now, hybrid solar cells, where photon absorption occurs in a semiconducting polymer and charge separation occurs at a metal oxide-polymer interface, show limited efficiencies. One limitation is caused by a relative low charge carrier mobility in the metal oxide. Here we addressed this issue and describe the use of a Sn:TiO2|TiO2 core–shell nanowire array to increase the charge-carrier mobility in the core of the nanowires while decreasing the charge-carrier recombination at the metal oxide–polymer interface due to fast electron extraction from this interface, driven by a cascaded conduction band energy from shell to core of the nanowires. These doped cores with an undoped shell structure resulted in impressive efficiency improvement in hybrid solar cells of 33% over the reference TiO2-based device. Additionally, this device structure resulted i...

illtrafast, spatial atmosphe1ic atomic layer deposition, which does not involve vacuum steps and is compatible with roll-to-roll processing, is used to grow high quality Ti02 blocking layers for organic solar cells. Dense, uniform thin Ti02 films are grown at temperatures as low as 100 oc in only 37 s ( -20 nm/min growth rate). Incmporation of these films in P3HT-PCBM-based solar cells shows perfmmances comparable with cells made using Ti02 films deposited with much longer processing times and/or higher temperatures. Copyright © 2013 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article.

Arrays of silver nanowires have received increasing attention in a variety of applications such as surface-enhanced Raman scattering (SERS), plasmonic biosensing and electrode for photoelectric devices. However, until now, large scale fabrication of device-suitable silver nanowire arrays on supporting substrates has seen very limited success. Here we show the synthesis of free-standing silver nanowire arrays on indium-tin oxide (ITO) coated glass by pulsed electrodeposition into anodic aluminum oxide (AAO) templates. We use an in situ oxygen plasma cleaning process and a sputtered Ti layer to enhance the adhesion between the template and ITO glass. An ultrathin gold layer (2 nm) is deposited as a nucleation layer for the electrodeposition of silver. An unprecedented high level of uniformity and control of the nanowire diameter, spacing and length has been achieved. The absorption measurements show that the free-standing silver nanowire arrays possess tunable plasmonic resonances. published

In this work, we present resonant tail-state regime absorption enhanced organic photovoltaics. We combine periodically structured ${\mathrm{TiO}}_{2}$ bottom electrodes with P3HT-PCBM bulk-heterojunction solar cells in an inverted device configuration. The wavelength-scale patterns are transferred to the electron-selective bottom electrodes via direct laser interference patterning, a fast method compatible with roll-to-roll processing. Spectroscopic and optoelectronic device measurements suggest polarization-dependent absorption enhancement along with photocurrent generation unambiguously originating from the population of tail states. We discuss the effects underlying these absorption patterns with the help of electromagnetic simulations using the discontinuous Galerkin time domain method. For this, we focus on the total absorption spectra along with spatially resolved power loss densities. Our simulations stress the tunability of the absorption resonances towards arbitrary wavelength regions.

Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells. published

Anodized TiO2 nanotube arrays fabricated on a TiO2 thin film on conducting glass substrates can be readily implemented in diverse applications like hybrid solar cells. In this study, we concentrate on morphologies with inner tube diameter being around 30 nm which is in dimension of the exciton diffusion length of common organic hole conductors. Cross-sectional preparation of the intact tube array in correlation with transmission electron microscopy has been performed to get local information on the TiO2 nanotubes and their arrangements, depending on anodization voltage. Crystallites have been found to be anatase and in size of several hundred nanometers along tube walls with increasing length for increasing anodization voltages. Inter-tube connections with similar crystal orientations of adjacent tubes are found. These give rise to large areas of uniform orientation. Thus, the number of grain boundaries within the film is low compared to the reported values for different TiO2-polymer material systems. Using the arrays, hybrid TiO2 solar cells were fabricated, which show high fill factors indicating good electron transport. The results suggest high electron mobility and are encouraging for a utilization of the nanotube arrays in next generation photovoltaics.

We investigate thin blend films of phenyl-substituted hexa-peri-hexabenzocoronenes (HBC) with various alkyl side chain lengths ((CH 2 )n. n = 6. 8. 12 and 16)/perylenediimide (POl). These blends constitute the active layers in bulk-hetero junction organic solar cells we studi ed recently [1 J. Their structural properties are studied by both scanning electron microscopy and X-ray diffraction measurements. The results support the evidence for the formation of HBC donor-POI acceptor complexes in all blends regardless of the side chain length of the HBC molecule. These complexes are packed into a layered structure parallel to the substrate for short side chain HBC molecules (n = 6 and 8). The layered structure is disrupted by increasing the side chain length of the HBC molecule and eventually a disordered structure is formed for long side chains (n > 12). We attribute this behavior to the size difference between the aromatic parts of the HBC and POl molec ules. For short side chains. the size difference results in a room for the side chains of the two moleeules to fill in the space around the aromatic cores. For long side chains (n >12). the empty space will not be enough to accommodate this increase. leading to the disruption of the layered structure and a rather disordered structure is formed. Our results highlight the importance of the donor-acceptor interaction in a bulk heterojunction active layer as weil as the geometry of the two molecules and their role in determining the structure of the active layer and thus their photovoltaic performance. 1.lntroduction Bulk- heterojunction (BHJ) organic solar cells have recently attracted considerable attention owing to their potential low cost fabrication with currently maximum efficiency exceeds 7% [2-6]. The solution-processed active layer in these cells is formed by bien ding two self-assembling organic semiconductors: a hole-transporting donor (0). and an electron-transporting acceptor (A). Ideally, the two semiconductors have to vertically segregate while maintaining a large interfacial surface area. This ensures an unperturbed transport of positive and negative charge carriers to their respective electrodes and effective charge separation needed to achieve highest power conver­ sion efficiency. Therefore, the structure and nanoscale morphology of the active layer are of major importance for the performance of a BHJ organic solar cell. Another prerequisite of the semiconductors is high charge-carrier mobility. A promising candidate for such organic semi­ conductors is offered by conjugated discotic liquid crystals [7 - 10]. Their flat, disk-shaped aromatic, cores with strong Tl-TI interactions promote their self-assembly into columnar stacks that allow electric

We present the addition of an energy relay dye to fullerenes resulting in increased light harvesting and signifi cantly improved power conversion effi ciency for organic photovoltaic (OPV) devices. Although exhibiting excellent properties as electron acceptors, visible light absorption of fullerenes is limited. Strongly light absorbing donor materials are needed for effi cient light harvesting in the thin active layer of OPV devices. Therefore, photocurrent generation and thus power conversion effi ciency of this type of solar cell is confi ned by the overlap of the relatively narrow absorption band of commonly used donor molecules with the solar spectrum. Herein the concept of fullerene dye sensitization is presented, which allows increased light harvesting on the electron acceptor side of the heterojunction. The concept is exemplarily shown for an UV absorbing small molecule and a near infrared absorbing polymer, namely hexa- peri -hexabenzocoronene (HBC) and Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1b:3,4-b’]dithiophene-2,6-diyl]] (PCPDTBT), respectively. In both systems remarkably higher power conversion effi ciency is achieved via perylene sensitization of the fullerene acceptor. Steady state photoluminescence, transient absorption and transient photocurrent decay studies reveal pathways of the additionally generated excited states at the sensitizer molecule. The fi ndings suggest fl uorescence resonance energy transfer from the photo-excited dye to the fullerene enabling decoupling of light absorption and charge transport. The presented sensitization method is proposed as a viable new concept for performance enhancement in organic photovoltaic devices.

This Progress Report highlights recent developments in nanostructured organic and hybrid solar cells. The authors discuss novel approaches to control the fi lm morphology in fully organic solar cells and the design of nanostructured hybrid solar cells. The motivation and recent results concerning fabrication and effects on device physics are emphasized. The aim of this review is not to give a summary of all recent results in organic and hybrid solar cells, but rather to focus on the fabrication, device physics, and light trapping properties of nanostructured organic and hybrid devices.

We report on nanotubular TiO2 on top of a compact TiO2 layer on conducting glass substrates. Highly regular structures were obtained from anodization of DC sputtered Ti films of thicknesses between 0.2 and 2 μm in ammonium fluoride containing ethylene glycol solutions. The influence of different anodization parameters was systematically analyzed revealing that full control over tube length, diameter, spacing, and tube wall thickness is possible. Inner tube diameters and spacing between tubes can be controlled via the anodization field. Tube wall thickness can be tuned using different anodization bath temperatures. Tube length can be adjusted by sputtering appropriate layers of Ti. The resulting TiO2 structures can be readily used in poly(3-hexylthiophene)-TiO2 hybrid solar cells and solid state dye sensitized solar cells. In contrast to anodized Ti foils the presented geometry allows the fabrication of photovoltaic devices which can be frontside-illuminated (through the glass substrate).

Dye-sensitized metal oxide polymer hybrid solar cells deliver a promising basis in organic solar cell development due to many conceptual advantages. Since the power conversion efficiency is still in a noncompetitive state, it has to be understood how the photocurrent contribution can be maximized (i.e., which dye-polymer properties are most beneficial for efficient charge generation in hybrid solar cells). By the comparison of three model systems for hybrid solar cells with TiO2-dye-polymer interfaces, this paper was aimed at elucidating the role of the exact mechanism of charge generation. In the exciton dissociation (ED) case, an exciton that is generated in the polymer is split at the dye-polymer interface. Alternatively, this exciton can be transferred to the dye via an energy transfer (ET), upon which charge separation occurs between dye and TiO2. For comparison, the third case is included in which the high lowest unoccupied molecular orbital of the dye does not allow exciton separation or ET from the dye to the polymer, so that the dye only is responsible for charge generation. To separate effects owing to differences in energy levels of the involved materials from the impact of local order and disorder in the polymer close to the interface, this paper further comprises a detailed analysis of the polymer crystallinity based on the H-aggregate model. While the massive impact of the poly(3-hexylthiophene) crystallinity on device function has been outlined for bare metal oxide-polymer interfaces, it has not been considered for hybrid solar cells with dye-sensitized TiO2. The results presented here indicate that all dye molecules in general influence the polymer morphology, which has to be taken into account for future optimization of hybrid solar cells. Apart from that, it can be suggested that ED on the polymer needs an additional driving force to work efficiently; thus, energy transfer seems to be currently the most promising strategy to increase the polymer photocurrent contribution.

3D hierarchical Nb3O7(OH) mesocrystals can be formed by self-organization from nanometer sized building blocks. The present study focuses on the synthesis and detailed investigation of mesocrystals, which can be achieved from a one-step, template-free hydrothermal synthesis approach. The obtained cubic superstructures consist of a periodic nanowire-network and combine a large surface area, high crystallinity, with a band gap of 3.2 eV and photocatalytic activity. Their easy processability in combination with the named excellent properties makes them promising candidates for a large number of applications. These include photochemical and photophysical devices where the Nb3O7(OH) mesocrystals can be used as electrode material since they are semiconducting and possess a large surface area. Generally the forces involved in the self-organized formation of mesocrystals are not fully understood. In this regard, the assembly of the Nb3O7(OH) mesocrystals was investigated in-depth applying transmission electron microscopy, scanning electron microscopy, UV/Vis measurements and electron energy-loss spectroscopy. Based on the achieved results a formation mechanisms is proposed, which expands the number of mechanisms for mesocrystal formation reported in literature. In addition, our study reveals different types of nanowire junctions and investigates their role at the stabilization of the networks.

We introduce hybrid solar cells with an architecture consisting of an electrodeposited ZnO nanorod array (NRA) coated with a conformal thin layer (

To unleash the full potential of hybrid solar cells, it is imperative to get significant photocurrent contribution from both the sensitizing dye and the polymeric hole transporter. Here we report on the interfacial modifier 4-mercaptopyridine (4-MP), which induces controlled orientation of poly(3-hexylthiophene) (P3HT), the most widely used hole transporting polymer for hybrid solar cells, at the interface. 4-MP optimizes the charge separating interface between P3HT and a squaraine dye-decorated TiO2, inducing enhanced contribution to photocurrent generation by the polymer. In combination with 4-tert-butylpyridine, which enhances the open circuit potential in dye-sensitized and hybrid solar cells but reduces the photocurrent, a synergistic effect is observed and it is possible to enhance both open circuit voltage and photocurrent simultaneously. Similar effects on device performance are also found for two other commonly used dye molecules, a fullerene derivative and a common indoline dye.

Lately, a lot of attention has been paid to metal oxide-organic hybrid solar cells. In these devices, conjugated polymers replace the typically transparent hole transporter as usually used in solid-state dye-sensitized solar cells in order to maximize the photon absorption efficiency. However, to unleash the full potential of hybrid solar cells it is imperative to push the photocurrent contribution of the absorbing polymer.

Novel, nanostructured electrode materials comprising porous ZnO films with aerogel morphology are presented. Almost any substrate including polymers, metals, or ceramics can be coated using a method that is suitable for mass production. The thin, porous films can be prepared from the wet gels via conventional drying, supercritical drying is not necessary. The filigree ZnO network is thermally very stable and exhibits sufficient electrical conductivity for advanced electronic applications. The latter was tested by realizing a highly desired architecture of organic-inorganic hybrid solar cells. After sensitizing of the ZnO with a purely organic squarine dye (SQ2), a nanostructured, interpenetrating 3D network of the inorganic semiconductor (ZnO) and organic semiconductor (P3HT) was prepared. The solar cell device was tested under illumination with AM 1.5G solar light (100 mW/cm(2)) and exhibited an energy conversion efficiency (η(eff)) of 0.69%.

Fu ll y organic sola r cell s (OSCs) based o n polymers and fullerenes have attracted remarkable interest during the last decade and hi g h power conversion efficie ncies (PCEs) beyond 8% have been realized. However. air stabi li ty of these ce ll s remain s poor. The conventional geometry of OSCs utili zes strongly oxidizing metal top contacts lik e AI or Ca . These metals are easily oxid ized in air resu lting in rapid decrease of PCE if cell s are not perfectly encapsulated. Usin g a thin electron-selective hole- bl ocki ng bottom layer lik e TiO, enables fabricat ion of solar ce ll s in a so-ca ll ed inverted geometry. In this geometry. noble metal s lik e Ag or Au can be used as top contacts. which are less sensitive to ambient oxygen. Thus. air-stabi li ty of these inverted solar ce ll s is s ig nifi ca ntly improved. In this study we investi ga te inverted polythiophene-methanofull erene solar ce ll s. W e find signifi ca nt influence of the TiO, layer thickness on li ght absorption and illumination stability of the solar cells . as w e ll as the trap filling by photoinduced carrie rs. Even though TiO, layers as thick as 500 nm seem not to be detrimental for cha rge transport, li ght intensity lo sses limit the dev ice pe rfo rmance. In turn. illumination stability is better for thicker TiO, layers. which can serv e as UV filt ers and protect the photoactive material s from degradation. when compared to thi n TiO, layers. Considering these different effects we state that a t hi ckn ess of 100 nm is the optimization of the TiO, layer.

The method of spray-depositing PEDOT:PSS allows the fabrication of thin films with controlled thickness on polymer layers. PEDOT:PSS is used in inverted ITO/TiO2/P3HT:PCBM/PEDOT:PSS/Ag solar cells to optimize the work function of the hole-collecting electrode. The interlayer is also found to protect the organic layer during metal top deposition and improve the contact between P3HT–PCBM and the Ag electrode, which is confirmed using two different metal-deposition techniques; thermal evaporation and sputtering. Cells with PEDOT:PSS show full VOC and efficiency immediately after fabrication, whereas devices without PEDOT:PSS exhibit low performance in the beginning and improve significantly during the first 10 days after production. Devices are long-term stable if stored in the dark and in ambient air and show no significant performance decrease after 80 days. No inert nitrogen atmosphere is needed for any fabrication step, thus reducing the potential production costs since no glove box has to be used.

Discotic materials have attracted remarkable interest for application in organic solar cells. We analyze a series of phenyl-substituted hexa-peri-hexabenzocoronenes (HBCs) with residue modifications blended with perylenediimide (PDI) as donor material. The effect of differing alkyl chain lengths of 6, 8, 12 and 16 carbon atoms, introduction of a triple bond linker between HBC core and residual phenyl group and a swallow tailed dialkylphenyl chain on the device performance is investigated. Detailed insight to device physics and morphology is gained by analysis of photoluminescence quenching, transient photovoltage and photocurrent decay experiments and atomic force microscopy. Complementary the investigations explain why using short alkyl side chains higher currents and consequently increased device performance can be achieved. We report an external quantum efficiency of over 27% for devices based on discotic molecules. In this work the molecular assembly and its impact on performance in photovoltaic devices is studied. The study also reveals pathways to further increase the quantum yield of small molecule organic solar cells.

An electrode structured with a TiO2/Ag/TiO2 (TAT) multilayer as indium tin oxide (ITO) replacement with a superior thermal stability has been successfully fabricated. This electrode allows to directly tune the optical cavity mode towards maximized photocurrent generation by varying the thickness of the layers in the sandwich structure. This enables tailored optimization of the transparent electrode for different organic thin film photovoltaics without alteration of their electro-optical properties. Organic photovoltaic featuring our TAT multilayer shows an improvement of ∼12% over the ITO reference and allows power conversion efficiencies (PCEs) up to 8.7% in PTB7:PC71BM devices.

Periodically structured electrodes are typically introduced to thin-film photovoltaics for the purpose of light management. Highly effective light-trapping and optimal in-coupling of light is crucial to enhance the overall device performance in such thin-film systems. Here, wavelength-scale structures are transferred via direct laser interference patterning to electron-selective TiO2 electrodes. Two representative thin-film solar cell architectures are deposited on top: an organic solar cell featuring blended P3HT:PCBM as active material, and a hybrid solar cell with Sb2S3 as inorganic active material. A direct correlation in the asymmetry in total absorption enhancement and in structure-induced light in-coupling is spectroscopically observed for the two systems. The structuring is shown to be beneficial for the total absorption enhancement if a high n active material is deposited on TiO2, but detrimental for a low n material. The refractive indices of the employed materials are determined via spectroscopic ellipsometry. The study outlines that the macroscopic Fresnel equations can be used to investigate the spectroscopically observed asymmetry in light in-coupling at the nanostructured TiO2 active material interfaces by visualizing the difference in reflectivity caused by the asymmetry in refractive indices. published

Low‐cost hybrid solar cells have made tremendous steps forward during the past decade owing to the implementation of extremely thin inorganic coatings as absorber layers, typically in combination with organic hole transporters. Using only extremely thin films of these absorbers reduces the requirement of single crystalline high‐quality materials and paves the way for low‐cost solution processing compatible with roll‐to‐roll fabrication processes. To date, the most efficient absorber material, except for the recently introduced organic–inorganic lead halide perovskites, has been Sb2S3, which can be implemented in hybrid photovoltaics using a simple chemical bath deposition. Current high‐efficiency Sb2S3 devices utilize absorber coatings on nanostructured TiO2 electrodes in combination with polymeric hole transporters. This geometry has so far been the state of the art, even though flat junction devices would be conceptually simpler with the additional potential of higher open circuit voltages due to reduced charge carrier recombination. Besides, the role of the hole transporter is not completely clarified yet. In particular, additional photocurrent contribution from the polymers has not been directly shown, which points toward detrimental parasitic light absorption in the polymers. This study presents a fine‐tuned chemical bath deposition method that allows fabricating solution‐processed low‐cost flat junction Sb2S3 solar cells with the highest open circuit voltage reported so far for chemical bath devices and efficiencies exceeding 4%. Characterization of back‐illuminated solar cells in combination with transfer matrix‐based simulations further allows to address the issue of absorption losses in the hole transport material and outline a pathway toward more efficient future devices.