This PDF file contains the front matter associated with SPIE Proceedings Volume 8477, including the Title Page, Copyright Information, Table of Contents, the Conference Committee Listing, and the Introduction.

Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more solar cells with different absorption ranges. However, for polymer solar cells (PSCs), the performance of tandem devices lags behind single-layer cells mainly due to the lack of a high-performance low-bandgap polymer with appropriate spectral response range. Here, we demonstrate a novel low bandgap conjugated polymer (~1.44 eV) specifically suitable for tandem structure. In the single-layer device, power conversion efficiency (PCE) of 6.5% was achieved. When the polymer was applied to tandem solar cells, we demonstrated a NREL certified PCE of 8.62%^[1] . Further optimization on materials and devices of this system has lead to record breaking efficiency of 10.6%. Furthermore, the tandem devices show excellent stability due both to the intrinsic stability of the polymer and the advanced device structure.

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.

We report on the latest progress in the field of organic p-i-n tandem solar cells. The results of tandem solar cells with an efficiency of 9.8% are shown (certified by Fraunhofer ISE) with an active area of about 1.1 cm². These solar cells show a promising intrinsic stability: a relative reduction of 5.7% of its initial power conversion efficiency was measured when stored at 85°C for 2400 hours. Additionally, we present a small OPV module with an active area of 122cm² showing an efficiency of with 9%, and an excellent low light behavior. Furthermore, we present the latest results on optimized tandem solar cells showing a power conversion efficiency of 10.7 % (measured by SGS, accredited and independent testing facility, active area of 1.1cm²).

Poly(3-alkylthiophene)-based diblock copolymers with controllable block lengths were synthesized by combining Grignard metathesis (GRIM) method, Ni-catalyzed quasi-living polymerization and a subsequent azide-alkyne click reaction to introduce a fullerene functionality into the side chains of one of the blocks. The fullerene-attached copolymers had good solubility (> 30 g L^-1 in chlorobenzene) with high molecular weights (Mn > 20000). The diblock copolymer films showed the formation of clear nanostructures with the size of 20 nm in AFM phase image driven by the crystallization of poly(3-hexylthiophene) block and aggregation of the fullerene groups. The photovoltaic device based on the copolymers showed a power conversion efficiency of 2.5% with a much higher fill factor of 0.63 compared with the single component devices previously reported. These results indicate that the rational material designs enable to construct the donor-acceptor nanostructure suitable for the photovoltaic application without relying on the mixing of the materials.

Herein we present an extension of our work on indacenodithiophenes (IDT) by replacing the central benzene ring with a thieno(3,2-b) thiophene unit. This newly developed thieno[3,2-b]thieno bisthiophene (4T) donor moiety was synthesized from commercially available reagents and incorporated into a series of donor-acceptor polymers. We will discuss the pronounced donating character of 4T compared to IDT and the choice of bridging atom in those new polymers with an emphasis on field effect transistor and photovoltaic device performance.

We demonstrate a new organic solar cell fabrication and characterization technique that allows for a quick screening of new materials and material combinations (i.e. blends) as active layers for solution processed organic solar cells with respect to the optimization of the active layer thicknesses, thereby saving precious material resources. Therefore we bar coat wedge-shaped layers by “horizontal dipping”. The photocurrent under short circuit conditions, the external quantum efficiency and the absorption of those wedge-shaped layers were then recorded spatially resolved. From the results the device photocurrent vs. the layer thickness can be extracted allowing for conclusions about the optimum absorber layer thickness.

The most promising active material for organic photovoltaic (OPV) cells is the polymer/fullerene bulk heterojunction (BHJ) system, but charge transport and recombination mechanisms in these materials have yet to be completely understood. We report the use of lateral bulk heterojunction devices to perform novel material diagnostics on the BHJ system. Using electron beam lithography, we fabricate devices with up to 24 voltage probes embedded in the channel in order to perform in situ potentiometry. From current vs. voltage measurements performed at a variety of light intensities, we are able to describe the charge transport properties in three distinct regions of a polymer/fullerene BHJ device and determine the dominant recombination mechanism of the OPV material. We note that these are the first such measurements performed on OPV materials. Such measurements will be very useful for materials diagnostics.

Organic solar cell (OSC)-organic light emitting Diode (OLED) stacked structures are investigated for their use in imaging the morphology of the bulk heterojunction layer used in OSCs. In lieu of the top cathode, bilayer OLED devices can be deposited directly on top of the otherwise traditional OSC device. Design considerations and device optimization are detailed. Results show that the bulk heterojunction layer composed of a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is sufficiently hole-conductive to allow for reasonably strong emission, peaking at a brightness of 175 cd/m² at a driving voltage of 8.5V and a current density of 12.5mA/cm². Since the OLED is in intimate contact with the P3HT:PCBM layer, it provides enhanced images of the bulk heterojunction with information on macro-scale defects and morphological variations. By tuning the emission intensity, detailed images can be obtained without the need for overly sensitive or costly cameras. Further applications of this technique, such as its potential in testing new donor polymer formulations as well as its capacity to monitor degradation due to residual solvents, are briefly discussed.

Recent progress on solution processable polymeric photovoltaic (PV) cells has drawn a lot of attention in both industry and academia. Over 8% power conversion efficiencies (PCE) have been demonstrated. In order to realize the application of organic PV, high efficiency (~10%) is not the only criteria, but also the low material and processing costs and device stability. For mostly demonstrated laboratory high efficiency cells, the devices consists of high work-function bottom anode and low work-function top cathode, e.g. Al, which is well known that the oxidation of the cathode accelerates the device degradation. In order to accommodate the issue, recent effort has been focusing on developing inverted structure. In such case, the low work-function metal can be eliminated by using a composite electrode with a work-function modifying interlayer. Solution derived TiOx and ZnO nano-particles are widely used as the interlayer. It has been shown such interlayer can efficiently reduce the work-function of bottom ITO electrode and significantly improve the device stability. However, it is often found that the inverted cells processed a lower performance than their counterpart with conventional structure. Such low efficiency is caused by the surface trap states of the nanoparticles which introduce charge recombination.

This paper is a review of our previous work on the field of low temperature, solution processed metal oxide buffer layers published in various journals. Our work focuses on zinc oxide (ZnO) and aluminum-doped zinc oxide (AZO) as n-type and molybdenum oxide (MoO₃) as p-type solution processed buffer layer. In addition to that, we investigate the surface modification of AZO using phosphonic acid-anchored aliphatic and fullerene self assembled monolayers (SAMs).

Organic solar cell (OSC) materials have recently gained rich attention due to capable of efficient power conversion, cost-effective, mechanically flexible and light weight solar cells. At the same time further materials developments for high performance will be necessary for commercial production of organic photovoltaics. The increase of efficiency has resulted from the low band gab materials, combination of polymer: fullerene and presence of blend micro structure. In this regard, the authors have achieved an efficient polymer solar cells based on Poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl](PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as donor and acceptor respectively. The photocurrent active layers were fabricated by spray coating technique which enables large area, low cost solar cells. A systematic analysis of PTB7:PC71BM devices carried out with TiOx electron transport layer, Chlorobenzene (CB) and 1,8 Diiodooctane (DIO) solvents. Optical and surface characterization analysis carried out by UV-visible and AFM techniques respectively. From the J-V characteristics, the device prepared with CB+DIO mixture solvents and TiO_x layer exhibits the best power conversion efficiency of 4.90%. It shows the device efficiency is one order of magnitude higher compared to that achieved with a TiO_x electron transport layer and without DIO solvent. The obtained results shows that DIO cosolvent induced changes in active layer morphology down to nano scale range and the TiO_x layer decrease the resistance between the active layer and electrode material.

Inkjet printing is considered a promising technique for industrial production of Organic Photovoltaic (OPV) devices, especially due to its minimal consumption of materials, the easy modification of the numerical design and because this is a non-contact process. The objective of this study is to make efficient modules at a semi-industrial scale using 128 nozzle heads. In order to maximize the performance and lifetime, an inverted device structure was used consisting of: transparent plastic conductive substrate / N-layer / active layer / P-layer / silver electrode. Formulations and processes were therefore developed for substituting each spin-coated layer with an ink-jet printed layer. For 3.7 cm² cells power conversion efficiency (PCE) reaches 3.3 % with only N-layer printed, 2.4 % with only active layer printed, 3.0 % with only P-layer printed and 2.9 % with only the silver electrode printed. Three-cell modules of 11 cm² on 5x5 cm² substrates were also made. Most PCE reach >2 % for each inkjet printed layer.

The key process in organic photovoltaics cells is the separation of an exciton, close to the donor/acceptor interface into a free hole (in the donor) and a free electron (in the acceptor). In an efficient solar cell, the majority of absorbed photons generate such hole-electron pairs but it is not clear why such a charge separation process is so efficient in some blends (for example in the blend formed by poly(3- hexylthiophene) (P3HT) and a C60 derivative (PCBM)) and how can one design better OPV materials. The electronic and geometric structure of the prototypical polymer:fullerene interface (P3HT:PCBM) is investigated theoretically using a combination of classical and quantum simulation methods. It is shown that the electronic structure of P3HT in contact with PCBM is significantly altered compared to bulk P3HT. Due to the additional free volume of the interface, P3HT chains close to PCBM are more disordered and, consequently, they are characterized by an increased band gap. Excitons and holes are therefore repelled by the interface. This provides a possible explanation of the low recombination efficiency and supports the direct formation of “quasi-free” charge separated species at the interface. This idea is further explored here by using a more general system-independent model Hamiltonian. The long range exciton dissociation rate is computed as a function of the exciton distance from the interface and the average dissociation distance is evaluated by comparing this rate with the exciton migration rate with a kinetic model. The phenomenological model shows that also in a generic interface the direct formation if quasi-free charges is extremely likely.

Extensive interface between donors and acceptors and respective connected networks of both created in bulk heterojunction (BHJ) solar cells have been proposed to effectively boost the photovoltaic efficiency via facilitating exciton dissociation and charge transport. The multi-scale nature of intermolecular interaction involved however renders the fabrication of such nano-morphology to try-and-error. Our recently proposed freeze-dry method to fabricate the BHJ polymer solar cells has demonstrated comparable efficiencies, regardless the intermolecular interaction strengths of polymers. A fibrous polymer scaffold, being first concocted with the simultaneously grouted PCBM in solution, sustains while the solid-phase solvent sublimates at a low temperature. The formation of such polymer structure can only be unraveled with in-situ monitoring means. Here, we report in-situ characterization of such structure during the initial cooling process with Raman spectroscopy. Raman spectroscopy – revealing molecular vibrational signatures – scrutinizes short-range structural regularity. In comparison with the Raman spectrum of the thermally annealed films, the sequential Raman spectra, acquired during cooling drop-cast o-dichlorobenzene solution of pristine P3HT and its blend with PCBM, show promptly emergent Raman signatures below -5°C – significantly narrowed peaks and new prominent peaks, signifying homogeneously packed P3HT agglomerates. These distinct Raman characteristics accompanied by real-time photoluminescence and absorption measurements suggest extended conjugation and high homogeneity of the P3HT network formed under the dynamic cooling process. This in-situ study thus opens a new utility of Raman spectroscopy to investigate intricate molecular packing that is relevant to the efficient transport of excitons and charges in polymer solar cells.

Photovoltaics (PV) offer a solution for the development of sustainable energy sources, relying on the sheer abundance of sunlight: More sunlight falls on the Earth’s surface in one hour than is required by its inhabitants in a year. However, it is imperative to manage the wide distribution of photon energies available in order to generate more cost efficient PV devices because single threshold PV devices are fundamentally limited to a maximum conversion efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the production of c-Si cells with efficiencies as high as 25%,¹ close to the limiting efficiency of ∼30%. But these cells are rather expensive, and ultimately the cost of energy is determined by the ratio of system cost and efficiency of the PV device. A strategy to radically decrease this ratio is to circumvent the SQ limit in cheaper, second generation PV devices. One promising approach is the use of hydrogenated amorphous silicon (a-Si:H), where film thicknesses on the order of several 100nm are sufficient. Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and the material suffers from light-induced degradation. Thinner absorber layers in a-Si:H devices are generally more stable than thicker films due to the better charge carrier extraction, but at the expense of reduced conversion efficiencies, especially in the red part of the solar spectrum (absorption losses). Hence for higher bandgap materials, which includes a-Si as well as organic and dye-sensitized cells, the major loss mechanism is the inability to harvest low energy photons.

In this work we discuss an elegant, alternative strategy to extend the spectral sensitivity of wide bandgap polymers in the near IR region. We discuss the microstructure of different functional ternary systems and compare them from different perspectives.

Luminescent solar concentrators (LSCs) aim to deliver high concentration ratio for photovoltaic cells without tracking the Sun, however, experimental realizations to date underperform their limiting theoretical potential by more than an order of magnitude. Here, we pursue a new path to improve LSC performance by combining highly directional spontaneous emission with integrated nonimaging optical elements. By minimizing the etendúe of emitted light through use of simple and scalable photonic structure, we employ the nonimaging optical design to transform limited angular extent into high spatial compression. We discuss the conceptual basis and theoretical potential of this approach and show through a combination of experiment and ray-tracing simulation that dramatic increases in LSC concentration ratio can be realized.

Typically, most low bandgap materials have low absorption with wavelength at around 500 nm. Besides, the restrictions of active layer thickness of thin film organic solar cells (OSCs) make the devices reduce to absorb light in long wavelength region (around 700 nm). As absorption would be a joint effect of material band properties and optical structures, well-designed light-trapping strategies for these low-bandgap PSCs will be more useful to further enhance efficiencies. We investigate the change of optical properties and device performances of organic solar cells based on our newly synthesized low-bandgap material with embedded poly-(3,4-ethylenedioxythiophene): poly(styrenesulfonate) PEDOT:PSS grating in the photoactive bulk heterojunction layer.

Solar cells based on the combination of conjugated polymers and fullerenes are among the most promising devices for low-cost solar energy conversion. Significant improvements in the efficiency have been accomplished, but some bottlenecks still persist. The substitution of fullerenes by inorganic semiconductor nanoparticles, especially CdSe and CdS, has been investigated as a promising alternative. In this work, we highlight two aspects to be considered in the pursuit of more efficient devices. By comparing different polymer/CdSe systems, we show how the polymer structure can be used to tune the charge transfer from the polymer to CdSe. Even if this process is efficient, the charges will be trapped in the inorganic phase if the charge carrier transport of the nanoparticles is poor. An elegant way to improve the electron hopping is to form an electrically integrated network of nanoparticles. The use of chalcogenide aerogels is a new alternative which may be interesting for applications requiring maximal transport of charge and is also discussed here.

Certified efficiency of dye-sensitized solar cells (DSC) with a cell area larger than 1 cm² reached 11.0%, which is almost same as that of amorphous silicon type solar cells. However, the efficiency is not as high as 20-25 % of crystal silicon type solar cells. Therefore, researches to find photo-conversion systems in the area of near infrared and infrared regions are being done to increase the efficiency. It has been reported that the efficiency of DSCs is affected by dye-adsorption behaviors on titania surfaces. However, there was no report on how dyes are adsorbed on titania and the relationship between dye adsorption and solar cell efficiency. We now report the adsorption behavior of dye molecules, which are monitored by Quartz Crystal Microbalance (QCM), and discuss the role of dye aggregation inhibitors which affect seriously the solar cell efficiency.

To enhance the light trapping of organic solar cells (OSCs), metallic (e.g. Au, Ag) nanoparticles (NPs) have been incorporated into the polymer layers conveniently in solution process. Although power conversion efficiency (PCE) of OSCs has been shown to improve by incorporating metallic NPs in either the buffer layer such as poly-(3,4-ethylenedioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS)[1] or the active layer[2], the understanding on the changes is still not quite clear. Moreover, there are very limited studies on incorporating metallic NPs in more than one organic layer and investigating their effects on the optical and electrical properties as well as the performances of OSCs. In this work, monofunctional poly(ethylene glycol) (PEG)-capped Au NPs of sizes 18 nm and 35 nm are doped in the PEDOT:PSS and poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) layers respectively, leading to an improvement of PCE by ~22% compared to the optimized control device. We will firstly identify the impact of NPs in each polymer layer on OSC characteristics by doping Au NPs in either the PEDOT:PSS or P3HT:PCBM layer. Then, we will investigate Au NPs incorporated in all polymer layers. We demonstrate that the accumulated benefits of incorporating Au NPs in all organic layers of OSCs can achieve larger improvements in OSC performances.

Although various optical designs and physical mechanisms have been studied both experimentally and theoretically to improve the optical absorption of organic solar cells (OSCs) by incorporating metallic nanostructures, the effects of plasmonic nanostructures on the electrical properties of OSCs is still not fully understood. Hence, it is highly desirable to study the changes of electrical properties induced by plasmonic structures and the corresponding physics for OSCs. In this work, we develop a multiphysics model for plasmonic OSCs by solving the Maxwell’s equations and semiconductor equations (Poisson, continuity, and drift-diffusion equations) with unified finite-difference method. Both the optical and electrical properties of OSCs incorporating a 2D metallic grating anode are investigated. For typical active polymer materials, low hole mobility, which is about one magnitude smaller than electron mobility, dominates the electrical property of OSCs. Since surface plasmon resonances excited by the metallic grating will produce concentrated near-field penetrated into the active polymer layer and decayed exponentially away from the metal-polymer interface, a significantly nonuniform and extremely high exciton generation rate is obtained near the grating. Interestingly, the reduced recombination loss and the increased open-circuit voltage can be achieved in plasmonic OSCs. The physical origin of the phenomena lies at direct hole collections to the metallic grating anode with a short transport path. In comparison with the plasmonic OSC, the hole transport in a multilayer planar OSC experiences a long transport path and time because the standard planar OSC has a high exciton generation rate at the transparent front cathode. The unveiled multiphysics is particularly helpful for designing high-performance plasmonic OSCs.

In this letter, poly(methyl methacrylate) (PMMA), in which ZnCdS/ZnS core/shell QDs or ZnCdSe/ZnS core/shell QDs were embedded, was used to enhance the absorbance of light and, thereby, deliver power at greater efficiencies, as shown in Fig. 1. In order to attach QDs on the opposite side of an indium tin oxide (ITO)/glass substrate, a PMMA layer was used as a supporting material. The effects of the QDs + PMMA on the performance of the OPV cells are discussed. Furthermore, the dependence of the efficiency of the OPV cell on the photoluminescence wavelength of the core/shell QDs is discussed by comparing the two types of QDs.

Bulk heterojunction (BHJ) solar cells based on blends comprising conjugated polymers and fullerene acceptors are the subject of considerable investigation because of their potential to enable the fabrication of low-cost devices that convert sunlight into electricity. Recently, poly(2,7-carbazole) derivatives have gained momentum as a class of promising alternative materials to poly(3-hexylthiophene) (P3HT) in organic solar cell applications. Among them, poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzo thiadiazole)] (PCDTBT) has a relatively deeper highest occupied molecular orbital (HOMO) of 5.45 eV compared to the HOMO of 5.1 eV of the P3HT. In this work we systematically study the effect of donor and acceptor ratio on the device performance of bulk heterojunction solar cells made with blends of PCDTBT and PC₇₁BM. We used PEDOT: PSS as a hole transport layer, and TiOX as a hole-blocking layer in order to improve the power conversion efficiency. The current density-voltage (JV) characteristics of photovoltaic cells were measured under the illumination of simulated solar light with 100 mW/cm² (AM 1.5G) by an Oriel 1000 W solar simulator. The power conversion efficiency of the solar cell is more than 5%.

Solar energy is the most abundant and reliable source of energy and we have to provide for the multi-terawatt challenge we are facing. In recent years organic photovoltaic's have become one of the most interesting research areas due to their potential towards a cheap and broad applicability. We report the optical and electrical properties of PBDTTT-CF: PC₇₁BM bulk hetero-junction (BHJ) solar cell. The devices were prepared by spin coating technique with the device structure of Glass/ITO/PEDOT: PSS/Active layer/Al. The ratio of polymer donor and fullerene acceptor varied between 1:1 to 1:4. Optical absorption spectroscopy measurements of the films indicated absorption peaks in the range from 500-800 nm which were attributed to PBDTTT-CF. The surface morphology of the active layers deposited was examined using Atomic Force Microscopy. The current density (J)-voltage (V) characteristics of the PBDTTT-CF: PC₇₁BM bulk hetero-junction solar cells were studied. The devices fabricated using the selective active layer show overall power conversion efficiency of 3%.

ZnPc and CuPc molecules stacked similar way in the film, but showed different growth modes in thermal evaporation. The distribution of CuPc crystals did not change by the film thickness, whereas the distribution of ZnPc became random as the increase of the film thickness. The disc type nanograins of CuPc were quite regularly distributed at the initial growth regime and the regular distribution of nanograins was kept during the film growth. On the other hand, ZnPc consisted in ellipsoid shaped nanograins and the distribution of nanograins was not regular in the initial growth regime. The irregular distribution of nanograins changed to the regular mode at the later growth regime by showing structure factor in GISAXS measurement. The different initial nanograin distribution in ZnPc and CuPc was related to the different nanostructure in the mixed layer with C60 to form the bulk heterojunction.

In this work, we focus on introducing an alternative approach to realize transparent graphene anodes. We report the use of very thin thermally evaporated gold (Au) nanoclusters with proper UVO treatments to facilitate efficient hole collection at graphene electrodes, which significantly benefits device performance while avoiding issues arising from PEDOT:PSS. We will investigate the effects of Au thickness and UVO treatments for optimizing device performance. Ultraviolet photoemission spectroscopy (UPS) is conducted to further analyze the WF shift at the graphene/polymer interface modified by UVO-treated Au.

In this report, we demonstrate an efficient planar-mixed heterojunction organic photovoltaic (OPV) device employing a mixed structure of subphthalocyanine (SubPc) donor and C₇₀ acceptor. Compared to a SubPc:C₆₀ cell, the SubPc:C₇₀ cell exhibits high performance with a fill factor (FF) of 52%, short-circuit current density (J_SC) of 8.8 mA/cm², open-circuit voltage (V_OC) of 1 V, and power conversion efficiency of 4.6% under air-mass 1.5 G illumination (1 sun). The high V_OC is mainly due to the wide interface gap between SubPc and C₇₀ as in the SubPc:C₆₀ device. On comparing the absorption spectra of C₇₀ and C₆₀, C₇₀ shows the higher absorption coefficient and wider absorption band, leading to the significantly improved J_SC. Moreover, the FF of the device using C₇₀ is enhanced to 52%; by comparison it is 45% when using C₆₀. In order to understand this phenomenon, the space-charge limited current is measured for estimating the carrier mobility of SubPc, C₇₀, and C₆₀. It reveals that the electron mobility of C₇₀ is lower than that of C₆₀ by orders of magnitude but is close to the hole mobility of SubPc. As a result, a better charge balance condition is achieved when SubPc is blended with C₇₀ and therefore a higher FF is obtained. Consequently, C₇₀ seems to be a more suitable acceptor for the mixed structure to develop a highly efficient OPV device due to the improvements in both electrical and optical properties.

Due to low manufacturing costs, printed organic solar cells are on the short-list of renewable and environmentally- friendly energy production technologies of the future. However, electrode materials and each photoactive layer require different techniques and approaches. Printing technologies have attracted considerable attention for organic electronics due to their potentially high volume and low cost processing. A case in point is the interface between the substrate and solution (ink) drop, which is a particularly critical issue for printing quality. In addition, methods such as UV, oxygen and argon plasma treatments have proven suitable to increasing the hydrophilicity of treated surfaces. Among several methods of measuring the ink-substrate interface, the simplest and most reliable is the contact angle method. In terms of nanoscale device applications, zinc oxide (ZnO) has gained popularity, owing to its physical and chemical properties. In particular, there is a growing interest in exploiting the unique properties that the so-called nanorod structure exhibits for future 1-dimensional opto-electronic devices. Applications, such as photodiodes, thin-film transistors, sensors and photo anodes in photovoltaic cells have already been demonstrated. This paper presents the wettability properties of ZnO nanorods treated with UV illumination, oxygen and argon plasma for various periods of time. Since this work concentrates on solar cell applications, four of the most common solutions used in organic solar cell manufacture were tested: P3HT:PCBM DCB, P3HT:PCBM CHB, PEDOT:PSS and water. The achieved results prove that different treatments change the contact angle differently. Moreover, solvent behaviour varied uniquely with the applied treatment.

The synthesis and analysis of solution processable polymers for organic solar cells is crucial for innovative solar cell technologies such as printing processes. In the field of donor materials for photovoltaic applications, polymers based on tetraphenylamine (TPA) are well known hole conducting materials. Here, we synthesized two conjugated TPA containing copolymers via Suzuki polycondensation. We investigated the tuning of the energy levels of the TPA based polymers by two different concepts. Firstly, we introduced an acceptor unit in the side chain. The main-chain of this copolymer was built from TPA units. The resulting copolymer 2-(4-((4'-((4-(2-ethylhexyloxy)phenyl)(paratolyl) amino)biphenyl-4-yl)(para-tolyl)amino)benzylidene) malononitrile P1 showed a broader absorption up to 550 nm. Secondly, we used a donor-acceptor concept by synthesizing a copolymer with alternating electron donating TPA and electron withdrawing Thieno[3,4-b]thiophene ester units. Consequently, the absorption maximum in the copolymer octyl-6-(4-((4-(2-ethylhexyloxy)phenyl)(p-tolyl)amino)phenyl)-4-methylthieno[3,4-b]thiophene-2-carboxylate P2 was red shifted to 580 nm. All three polymers showed high thermal stability. By UV-vis and Cyclic voltammetry measurements the optical and electrochemical properties of the polymers were analyzed.

Achieving efficient charge generation has been a challenge in the field of organic photovoltaics (OPVs). Therefore, it is important to understand the photophysical behaviour of a system in relation to the material properties. In this study, photophysical properties of a range of conjugated donor-acceptor polymers with varying crystallinity are investigated. Preliminary study shows a correlation between photophysics and polymer crystallinity.

Currently, the field of organic photovoltaics experiences tremendous growth because this technology offers competitive efficiency of light – to – energy conversion and compliance with requirements for high-throughput manufacturing (roll – to – roll, screen printing, etc.). However, several challenges exist, such as relatively short device lifetime and optimization of device structure to achieve a commercial viability threshold of 10% power conversion efficiency for this technology, exist. For research purposes quick, simple and inexpensive approaches for device encapsulation are desired for high-throughput screening of samples. In this paper we show that encapsulation of organic photovoltaic devices using silicone adhesive and Kapton or glass is a viable approach for preserving devices in ambient conditions at ~25 °C in the dark for at least 24 hours. Also, PET, Kapton and glass encapsulation materials can be used to limit oxygen and water access to the device and to determine prevalent degradation pathways in organic solar cells.

For advanced organic thin film photovoltaic cell, stacked structure of single cells, tandem structure, would be a key issue. Many kinds of tandem structure have been already reported. When an appropriate intermediate layer was inserted between the single cells, open circuit voltage (Voc) can be doubled compared with the single cell. For small molecules, vacuum evaporation can be applicable for fabrication. Systematic investigations have been made to reveal the requirement for the intermediate materials. Quite thin, a few nm thick, metal layer can act as intermediate layer[1]. The metal cannot form continuous layer but island lake structure in such small amount. On the other hand, the combination of the metal oxide (such as ZnO, TiO2 and ITO) and PEDOT:PSS are used for the intermediate layer. We need to reveal minimum requirement for the intermediate materials for polymer based bulkhererojunction cells for low-cost high performance organic photovoltaic cells. We have developed a polymer thin film preparation technique, Evaporative Spray Deposition using Ultradilute Solution (ESDUS)[2]. This method has enabled fabricating organic thin films applicable to polymer light-emitting diodes, organic photovoltaics and organic field-effect transistors13 from highly diluted solutions of 1-10 ppm. Moreover, it has been exhibited that a successive polymer layer can be deposited without damaging the preceding polymer layer by use of a same solvent for each layer deposition. We conduct the systematic investigation of the intermediate materials. Onto the bottom cell/intermediate layer, top cell can be deposited by use of ESDUS.

High efficiency near-infrared (NIR) absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the J_SC compared to the case in the absence of the CuI layer. This is due to the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an x-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.

Real time grazing incidence small angle x-ray scattering (GI-SAXS) and x-ray reflectivity measurements were conducted in order to investigate the thermal evolution of the nano-grain structure and surface of 5nm thick Copper(II) Phthalocyanine (CuPc) films. The evolution was strongly influenced by the surface energy of silicon substrate. On the low surface energy (hydrophobic) Si substrate, CuPc nano-grains are randomly distributed and the crystal size did not increase in size upon thermal annealing. Thermal annealing induced a more random distribution of nano-grains with an increase in roughness, and large islands formed by the coalescence of small grains. On the high surface energy (hydrophilic) Si substrate, CuPc film consisted of disk shaped nano-grains of two different sizes. The larger grains showed lateral crystal growth and planarization by thermal annealing, while the smaller grains did not increase in size. Large clusters were observed at high temperature, which were derived by large grains. The different thermal evolution models of CuPc films based on GI-SAXS analysis are consistent with the different temperature behavior of the hole mobilities of organic field-effect transistor (OFET) devices fabricated on both surfaces.

A novel hyperbranched zinc phthalocyanine dye, i.e. HBZnPc-COOH, was synthesized, characterized, and applied into dye-sensitized solar cells (DSSCs) as TiO₂ sensitizer. UV-visible absorption, steady-state fluorescence, femtosecond time-resolved fluorescence, cyclic voltammetry, current–voltage characteristics, and photoelectrical properties of the active material/device were investigated. The utilization of hyperbranched structure was proved to be able to solve the aggregation issue of phthalocyanine dyes on TiO₂ surface which has been widely considered as one of the key limiting issues that severely lower the efficiencies of phthalocyanine dye sensitized solar cells. With appropriate highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels, HBZnPc-COOH exhibited efficient and ultrafast multi-phasic electron injection from both the Soret band and Q band to the conduction band of TiO₂, leading to a solar cell power conversion efficiency of 1.15% and a high incident photon to current conversion efficiency of 66.7% at 670 nm.