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We study the top surface composition of blends of the conjugated polymer regioregular poly-3-hexylthiophene (P3HT) with the fullerene (6,6)-phenyl-C(61)-butyric acid methyl ester (PCBM), an important model system for organic photovoltaics (OPVs), using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). We compare the ratio of P3HT to PCBM near the air/film interface that results from preparing blend films on two sets of substrates: (1) poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) coated indium tin oxide (ITO) as is commonly used in conventional OPV structures and (2) ZnO substrates that are either unmodified or modified with a C(60)-like self-assembled monolayer, similar to those that have been recently reported in inverted OPV structures. We find that the top surface (the film/air interface) is enriched in P3HT compared to the bulk, regardless of substrate or annealing conditions, indicating that changes in device performance due to substrate modification treatments should be attributed to the buried substrate/film interface and the bulk of the film rather than the exposed film/air interface.

Inverted ZnO-NPs/C60-self-assembled monolayer (SAM)/poly(3-hexyl-thiophene):[6,6]-phenyl C61 butyric acid methyl ester solar cell devices were systemically optimized by varying the weight blend ratio of donor and acceptor from 1:0 to 1:1, the active layer thickness, the annealing temperature, the annealing time, and the top anode electrode. These inverted cells using C60-SAM modification show a transition to a more bulkheterojunction device at blend ratios of 1:0.3 to 1:0.4, thus leading to large increase in device efficiencies from 1.6% to 3.5%. Further increase in the ratio shows eventual saturation in device efficiency to 4.5%, thus indicating an optimum blend composition. A strong dependence of the annealing temperature and time on the fill factor of the device is observed, which is correlated to changes in the morphology of the active layer. The anode metal electrode work function was varied using Ca/Al, Al, Ag, Cu, Au, and Pd as the contacts. Devices incorporating a layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) between the active layer and anode electrode exhibit similar Voc regardless of electrode work function, whereas devices without the PEDOT:PSS layer exhibited behavior more similar to the MIM model. Optimized devices show efficiencies of 4.5% using a blend ratio of 1:0.7, an active layer thickness of ~200 nm, and a thermal annealing condition of 160°C for 10 min using Ag as the top-metal-anode contact. Inverted polymer-based solar cells using cheaper metals like Cu showed similar device efficiency.

The increase in energy production costs for fossil fuels has led to a search for an economically viable alternative energy source. One alternative energy source of particular interest is solar energy. A promising alternative to inorganic materials is organic semiconductor polymer solar cells due to their advantages of being cheaper, light weight, flexible and made into large areas by roll-to-roll processing. However, the conventional architecture that is typically used for fabricating solar cells requires high vacuum to deposit the top metal electrode which is not suitable for roll-to-roll processing. Recently an inverted device architecture has been investigated as a suitable architecture for developing the ideal roll-to-roll type processing of polymer-based solar cells. This review will go over the recent advances and approaches in the development of this type of inverted device architecture. We will highlight some of the work that we have done to integrate materials, device, interface, and processing o...

The interface of electron-selective ZnO in inverted polymer bulk-heterojunction (BHJ) solar cells was modified with a series of fullerene-based self-assembled monolayers (C60-SAM) containing different anchoring groups (catechol, carboxylic acid, and phosphonic acid), linkage location, and functionalization. The formation of the C60-SAM to the surface of ZnO was investigated by processing the SAM through either a solution immersion technique or a solution spin-coating method. It is found that the C60-SAMs with the carboxylic acid and catechol termination can be formed onto the surface of ZnO by simple solution spin-coating process, whereas all three anchoring groups can be formed by solution immersion technique. Heterojunction devices were fabricated under different processing conditions to form SAM leading to 2-fold, 75%, and 30% efficiency improvement with the carboxylic acid, catechol, and phosphonic acid C60-SAMs, respectively. The main contribution to the variation of efficiency from different SAMs is...

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is investigated as a transparent cathode to replace indium tin oxide (ITO) in inverted polymer solar cells. Increasing the thickness of the PEDOT:PSS electrode leads to a reduction in transparency and sheet resistance which lowers the photocurrent but increases the fill factor of the solar cells. The offset of photocurrent and fill factor as the thickness is increased leads to a saturation of the power conversion efficiency to ∼3%. These electrodes were applied to flexible substrates showing similar device performance to glass based devices. Cyclic bending test of these flexible polymer electrodes show improved conversion efficiency retention (∼92%) when compared to flexible ITO based electrodes (∼50%) after 300 bend cycles. In addition to using PEDOT:PSS as a cathode replacement for ITO in inverted solar cells, its use as a semi-transparent anode replacement to Ag is also examined. Semi-transparent inverted solar cells fabricated with ITO as the cathode and PEDOT:PSS as the top anode electrode were demonstrated showing efficiencies of ∼2.51% while replacement of both ITO and Ag with PEDOT:PSS as both the cathode and anode show efficiencies of ∼0.47%.

Inverted polymer solar cells using non-vacuum processed spraycoated silver nanoparticles (Ag-NPs) as the anode electrode were compared to vacuum deposited Ag electrodes. The number of spray coating layers can affect its final device performance showing with a higher number of coating layers, a better nanoparticle interconnectivity and morphology is achieved which reduces the sheet resistance and transparency of the Ag electrode leading to improved fill factor and device performances (∼3.0%). These devices can be fabricated onto flexible ITO substrates devices showing a performance of ∼1.40%. Using a non-vacuum technique to deposit the electrode is important for low-cost polymer solar cells.

valuesoriginatefromthelossofchargecarriersthroughleakagepathsincluding pinholes in the films and the recombination andtrapping of the carriers during their transit through the cell,leading to a decrease in device performance.Modification of electrodes has been commonly employed toimprove the contact between the active organic layer andelectrodes. At the indium tin oxide (ITO) anode side, a thinbuffer layer of a conductive polymer, poly(3,4-ethylene-dioxylene thiophene):poly(styrene sulfonic acid) (PED-OT:PSS), is often used to increase the work-function of ITOfor effective hole collection.

Recent developments of molecular architectural control and solid-state engineering have led to exceptionally large electro-optic (EO) activities in organic and polymeric nonlinear optical (NLO) materials. A new generation of NLO dendrimers has been developed to generate well-defined nano-objects, minimize strong intermolecular electrostatic interactions, and improve poling efficiency and stability. A facile and reliable Diels−Alder “click” reaction was applied for lattice hardening to improve physical properties of cross-linkable EO polymers. The “click” chemistry also provides means to study the relationship between EO activity, chromophore shape, and number density of the chromophore, systematically. The NLO dendrimers or polymers were used as hosts for guest chromophores to increase chromophore concentration and improve poling efficiency. A variety of nanostructured organic and polymeric materials with ultrahigh r33 values (>350 pm/V at the wavelength of 1310 nm, more than 10 times that of LiNbO3) and ...

One of the most challenging tasks encountered in developing highly efficient electro-optic (EO) devices is to find a material system that possesses all desirable properties such as large EO coefficients, good thermal and mechanical stability, and low optical loss. In order to meet this stringent requirement, we have developed a series of crosslinkable EO dendrimers using the standardized AJL8-type chromophore as the center core and the furyl- and anthryl-containing dendrons as the periphery. Upon adding a trismaleimide (TMI) dienophile, these dendrimers could be in-situ crosslinked via the Diels-Alder cycloaddition and efficiently poled under a high electric field. Through this dynamic process, the spatially voided and π-electron-rich surrounding of the chromophore core changes into a dense and more aliphatic network, with the dipolar chromophore embedded and aligned inside. The resultant materials exhibit large EO coefficients (63-99 pm V -1 at 1.31 μm), excellent temporal stability (the original r 33 values remain unchanged at 100 °C for more than 500 h), and blue-shifted near-IR absorption. With these combined desirable properties, a poled EOD2/TMI film could be processed through multiple lithographic and etching steps to fabricate a racetrack-shaped micro-ring resonator. By coupling this ring resonator with a side-polished optical fiber, a novel broadband electric-field sensor with high sensitivity of 100 mV m -1 at 550 MHz was successfully demonstrated.

Two series of highly efficient and thermally stable nonlinear optical chromophores based on the (4-diarylamino)phenyl electron donors have been synthesized and systematically investigated. A modular approach has been employed to synthesize these electron donors with tunable size, shape, and electron-donating abilities. Efficient conjugated bridges were extended from these donors and coupled with very strong CF3−TCF electron acceptors to afford chromophores with very high β values (up to 7077 × 10-30 esu at 1.907 μm). These chromophores possess much higher thermal stability (with their onset decomposition temperatures all above 220 °C) than those substituted with (4-dialkylamino)phenyl donors. Most importantly, the high molecular hyperpolarizability of these chromophores can be effectively translated into very large electro-optic (E-O) coefficients (r33) in poled polymers through suitable shape engineering. Exemplified by the chromophore B4, which has a fluorinated aromatic substituent anchored at its dono...

Poled electro-optic (EO) polymers have enabled many advances in the exploration of high-speed and broadband information technologies. Polymer based EO devices have been demonstrated to have large bandwidths (over 110 GHz), low driving voltages, and sustain their performance in a flexible form or under extreme environmental conditions. For optical circuits, EO polymers can be easily integrated with striplineor ring-structured waveguides made of sol–gels, low-loss fluorinated polymers, silicon slots, conducting oxides, and photonic crystals. Recently, EO polymers have also been utilized for the generation/detection of a gap-free pulsed THz system with a bandwidth up to ca. 12 THz. Large numbers of discrete photonic components need to be inserted into integrated systems of telecommunication and silicon microphotonics, especially where an extreme amount of data is required to travel in a very small space. Therein lies the great challenge for polymer-based EO technologies: to have thermally stable EO coefficients (r33) of around 500 pm V at wavelengths of 1.31 or 1.55 lm. Currently, the most commonly used materials for polymeric EO devices are based on poled polymers with r33 values around 50–80 pm V at wavelengths of 1.31 or 1.55 lm. In these materials, dipolar nonlinear optical (NLO) chromophores have been doped or incorporated at a level of ca. 20– 25 wt % to reach their maximum r33 values. [5–10] Beyond such a moderate loading of chromophores, strong intermolecular electrostatic interactions severely limit the poling-induced polar order and cause phase-separation problems between chromophores and polymers. To further improve EO activity, research efforts have focused on developing shape-engineered chromophores with high molecular optical nonlinearities (lb), where lb is a product of first hyperpolarizability and the dipole moment of the NLO chromophore, and increasing order within the matrices by controlling the nanoscale architecture of macromolecules. In our recent study we have demonstrated, using Diels–Alder (DA) “click chemistry” to post-functionalize NLO chromophores onto polymers, that high chromophore loading levels (up to 35 wt %) and large r33 values (up to 110 pm V) could be achieved in in situ generated side-chain dendronized NLO polymers with non-reacted chromophores as guest dopants. This opens a new avenue to explore optimal host–guest combinations and to develop an efficient way to control lattice hardening in these hybrid polymers. The ultimate goal is to simultaneously achieve very large EO activity, good thermal stability, high optical transparency, and excellent mechanical properties within the same material via molecular design and facile processing. In this paper, we report a novel method to disperse a highly efficient secondary chromophore into in situ crosslinked NLO polymer networks, leading to both enhanced EO activity (> 260 pm V at 1.31 lm) and alignment stability at 85 °C. In photorefractive (PR) polymers and liquid-crystal (LC) systems, binary chromophore mixtures have been shown to elevate the loading density of small dipolar dyes without causing phase separation. We have extended this further by incorporating highly polarizable NLO chromophores into these binary systems. In addition, DA click chemistry has also been employed to improve the physical properties of these materials. This combined effort demonstrates that binary mixtures of large-bl chromophores can be loaded into side-chain NLO matrices and efficiently poled to give EO activities higher than the summed value of two added chromophores. These systems can also be mildly cured to ensure a thermally stable EO response. The EO polymers studied in this work exist as a three-component host–guest system (Fig. 1). The host polymer is a copolymer, poly[(methyl methacrylate)-co-(9-anthracenyl methyl methacrylate)] (PMMA-AMA), with around 10 mol % of the anthracenyl moiety. To this host, AJC146 was added together with compound 1a, 1b, or 1c as binary guest chromoC O M M U N IC A TI O N S

A macrocyclic trichromophore bundle 1 with parallel-aligned dipole moments has been synthesized to study the influence of aggregation and orientation of a nonlinear optical (NLO) chromophore on its optical properties. The linear and nonlinear optical properties of 1 and a single chromophore standard 2 have been studied by UV-vis absorption, fluorescence, solvatochromic spectrometry, and hyper-Rayleigh scattering (HRS). Reduced first-order hyperpolarizability beta, hypsochromic shift, enhanced solvatochromic shifts, and fluorescence quenching for individual chromophores were observed when 1 was compared with 2. Analysis of the data showed that the transition dipole moment changes only slightly when the chromophores are parallel aligned in the bundle architecture. However, the apparent hyperpolarizability of the individual chromophores decreased significantly by about 20%. The reduction in beta for the individual chromophores in 1 is largely due to the hypsochromic shift, i.e., excitation energy increase of the interband (charge-transfer) energy gap and the reduced difference between the ground-state and excited-state dipole moments. The hypsochromic shift and fluorescence quenching are consistent with exciton theory. Possible reasons for the enhanced solvatochromic shift are discussed.

A new type of benzobis(silolothiophene)-based low band gap copolymers PBSTBT and PBSTDTBT were developed via a facile synthetic method in high yield. The introduction of silicon atoms in the large fused rings provides new polymers with low HOMO levels and high hole mobilities up to 0.01 cm2 V−1 s−1. The results from preliminary characterization of bulk heterojuction solar cells showed PCE above 3.5% for both PBSTBT and PBSTDTBT.

A low band gap polymer based on the copolymerization between benzodithiophene and thieno-pyrroledione units has been investigated in both conventional and inverted polymer bulk-heterojunction photovoltaic cells. High power conversion efficiencies of more than 4% in both device structures were demonstrated.

Two amorphous fullerenes, TPA-PCBM and MF-PCBM, have been developed as efficient electron acceptors to induce a highly stable morphology of active layer in polymer photovoltaic devices. The power conversion efficiency (PCE) of devices using both materials show no degradation, even after 10 h annealing at 150 °C.

Both high charge carrier mobility in OFETs and high power conversion efficiency in solar cells were demonstrated in a series of amorphous metallated π-conjugated polymers. The best solar cell performance is based on a high field-effect mobility (μh = 1.0 × 10−2 cm2 V−1 s−1) polymer blended with PC71BM leading to a power conversion efficiency of PCE = 4.13%.

Photobleaching was used for the fabrication of electro-optic polymer microrings resonators. All the device parameters were theoretically optimized. The photobleaching mask was made using e-beam lithography. UV exposure through the mask defined the pattern in the polymer by an irreversible change of the material properties and decrease of the refractive index. Waveguide coupled microring resonators have shown 15 dB resonance contrast and low total insertion loss of 9 dB, most of which was mainly due to fiber coupling in and out of the chip.

[reaction: see text] A facile synthetic route has been developed to convert an electron-rich, sterically hindered dialkylaminodienone into a conjugated dialkylaminotrienal with good yield. The derived dialkylaminotetraene-type nonlinear optical chromophores possess an all-trans conformation and can be functionalized with fluoro-dendron to provide proper shape modification for poling. Polymers doped with two examples of these chromophores in high concentrations have been poled to afford ultra-large electrooptic coefficients (r(33)) of 208 and 262 pm/V, respectively, at the measuring wavelength of 1.31 mum.

A novel broadband E-field sensor based on electro-optic polymer micro-ring resonator directly coupled to the core of optical fiber is proposed and demonstrated. A flat is made on the side of the optical fiber by polishing and an electro-optic polymer waveguide in the shape of a ring is placed on the polished flat. One side of the ring is directly above the core of the fiber and light is evanescently coupled between the fiber and the micro-ring. External electric fields change the index of refraction of the ring resonator and therefore its resonant wavelengths. The sensor is all dielectric without metal layers to distort the measured E-field. The resonance structure allows the sensor to potentially have much higher sensitivity than other electro-optic sensors based on interferometry or polarization modulation. Since electro-optic polymers have higher electro-optic coefficients, lower dielectric constants and faster electro-optic responses than inorganic crystals, higher sensitivity, lower invasiveness and higher bandwidth of E-field sensing can be expected. The sensor with EO polymer micro-ring directly coupled to side-polished fiber eliminates unreliable and possibly lossy fiber to waveguide butt coupling as well as the high propagation loss which comes from the long straight EO polymer waveguides. Unlike devices based on waveguide technology, a supporting substrate is not necessary in this device. This leads to sensors of small size and low disturbance to the measured electric field. In the proof-of-concept experiment, a sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system) using AJLS103 electro-optic polymer.

Recent breakthroughs in developing exceptional organic electro-optic (EO) materials are reviewed. Whole series of guest-host polymers furnished with high μβ chromophores have shown large electro-optic coefficients around 100~160 pm/V @ 1.31μm. Moreover, new generation of NLO chromophores based on pyrroline and pyrrolizine acceptors have been designed and synthesized. To go beyond the typical oriented gas model limit for poled polymers, new approach of using nanoscale architecture control and supramoleaular self-assembly has been proved as a very effective method to create a new paradigm for materials with very exciting properties. The approaches of employing Diels-Alder reactions for postfunctionalization and lattice hardening also provide a facile and reliable way to generate high-performance EO polymers and dendrimers. This type of "click" chemistry paves the way to systematically study the relationships between chromophore shape and number density, controlled self-assembly, in addition to provide the material properties needed for multi-layer device fabrication. Finally, a new generation of binary monolithic glasses has been developed that exhibit unprecedented high EO activities through careful manipulation of intricate supramolecular interactive forces for self-assembly. The results obtained from these poled binary organic glass materials (r 33 as high as 310 pm/V at 1.31μm) are the highest values ever reported which are >10 times of the commercial lithium niobate crystals. The success of these material developments has recently inspired the exploration of new device concepts trying to take full advantage of the organic EO materials with ultrahigh r 33 values.

A series of side-chain electrooptic (E-O) polymers have been prepared by Diels-Alder reaction in a solid state and characterized for their nonlinear optical properties. A synthesized chromophore were easily attached to a pendent anthracenyl moiety functionalized on the poly(methylmethacrylate-co-anthrylmethylmethacrylate) thermally in the bulk films during the poling process without compromising E-O performances. We have also controlled a chromophore concentration to determine its critical loading density at which chromophore-chromophore electrostatic interaction occurs in the polymer matrix. The highest E-O coefficient was 110 pm/V for the 34 wt% of the doped chromophore in the polymer at the wavelength of 1.3 μm. A high loading density of chromophore was obtained without observing a severe phase separation in the polymer matrix by an AFM morphology study. This novel approach provides to demonstrate a strategy for developing highly efficient E-O materials with the full potential of a chromophore.

Spin-on processing is used in many industries to deposit very thin coatings on flat substrates, including silicon wafers, flat-panel displays, and precision optical components. A liquid precursor solution is first dispensed onto the surface of the substrate; this fluid then spreads out very evenly over the surface due to large rotational forces caused by spinning of the substrate. When looking for an optimum coating procedure process engineers can adjust many variables including the peak spin speed, the ramping rate to reach that speed, the spinning time, as well as allowing for dynamic solution dispense before ramping up, though most protocols focus on the peak spin speed as the primary controlling variable. Engineers often construct spin-speed versus thickness correlations that enable predictable adjustment of spin-speed to achieve a desired thickness. Yet, rather little attention has been paid to the importance of the acceleration rate used to reach the desired peak speed. We show here that ramping rate is also important in helping establish the final coating thickness. We present a numerical model of the fluid flow on a spinning wafer when the spin-speed is ramping linearly up to a desired peak speed and then held constant. It is shown that the coating may ���set��� into its final thickness before the spin-speed reaches its peak value. In these cases then the peak spin-speed parameter is no longer the primary variable that defines the final coating thickness. This also impacts the interpretation of critical exponents found when fitting spin-speed vs. thickness data. We perform parallel experimental measurements for different ramping-up times and confirm the results from the numerical model. Both experimental and theoretical results support use of the simplified model put forth by Meyerhofer over 25 years ago (J. Appl. Phys. 49 (1978) 3993-3997)., Support from the National Science Foundation under grant DMR 98-02334: Dye Probe Studies of Spin Coating of Sol-Gel Solutions.

A novel [6,6]-phenyl-C61-butyric acid methyl styryl ester (PCBM-S) was synthesized and employed as an electron transporting interfacial layer for bulk heterojunction polymer solar cells with an inverted device configuration. After the deposition of PCBM-S film from solution, the styryl groups of PCBM-S were polymerized by post-thermal treatment to form a robust film which is resistive to common organic solvents. This allows the solution processing of upper bulk heterojunction film without eroding the PCBM-S layer. Additionally, the PCBM-S was n-doped with decamethylcobaltocene (DMC) to increase the conductivity of the film, which resulted in a significantly improved power conversion efficiency from 1.24% to 2.33%. The improved device performance is due to the decrease of series resistance and improved electron extraction property of the n-doped PCBM-S film.

Inverted tandem bulk-heterojunction solar cells with comparable efficiency to single layer devices have been demonstrated by utilizing two thin layers (∼50 nm) of poly-(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester as the active material and a fullerene self-assembled monolayer (C60-SAM) to modify the interfaces between the ZnO buffer layer and the active layer. Single and tandem solar cells without the SAM modification have much lower efficiencies than the ones with modification. The successful demonstrations of inverted tandem devices with SAM modification give promise of further device improvements if active materials with complementary absorption can be used.

A simple method has been developed to modify the anode interface of inverted bulk-heterojunction (BHJ) polymer solar cells by spin-coating a thin layer of graphene oxide (GO) on top of the organic active layer. The device with GO exhibited a remarkable improvement in power conversion efficiency compared to devices without any interfacial layer, indicating that GO can effectively modify the BHJ/metal anode interface to facilitate efficient hole collection. The dependence of the device performance on the GO layer thickness was also investigated showing an optimum performance from a GO thickness of ∼2–3 nm.

A sol-gel derived thin titanium dioxide (TiO2) layer was spin-coated onto indium-tin-oxide substrate to improve poling efficiency of recently developed electro-optic (E-O) polymers. The thin TiO2 layer significantly blocks excessive charge injection and reduces the leakage current during high field poling. Ultralarge E-O coefficients, up to 160–350 pm/V at 1310 nm, have been achieved. These results show higher poling efficiency (enhancement of 26%–40%) compared to the results of poled films without the TiO2 layer. This enhancement can be explained by field distribution flattening effect at high injection barrier with the insertion of TiO2 barrier layer.

A simple method was developed using metal grid/conducting polymer hybrid transparent electrode to replace indium tin oxide (ITO) for the fabrication of inverted structure polymer solar cells. The performance of the devices could be tuned easily by varying the width and separation of the metal grids. By combining the appropriate metal grid geometry with a thin conductive polymer layer, substrates with comparable transparency and sheet resistance to those of ITO could be achieved. Polymer solar cells fabricated using this hybrid electrode show efficiencies as high as ∼3.2%. This method provides a feasible way for fabricating low-cost, large-area organic solar cells.

The performance of inverted bulk-heterojunction solar cells with zinc oxide nanoparticles as the electron selective contact is compared to those modified with a fullerene self-assembled monolayer (C60-SAM). The devices modified with a C60-SAM show very significant improvement in conversion efficiencies compared to unmodified devices leading to efficiencies as high as 4.9%. This is due to enhanced electronic coupling of the inorganic/organic interface from the C60-SAM leading to improved fill factor and photocurrent. Furthermore, devices fabricated in an inert environment were compared to those fabricated in ambient showing comparable device performance.

The performance and stability of unencapsulated inverted bulk-heterojunction solar cells with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO nanoparticles on indium tin oxide plastic substrates showed high power conversion efficiency of ∼3.3%. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after 40days while the conventional one showed negligible photovoltaic activity after 4days. This is due to the improved stability at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Ag interface.

A simple method was developed to tune the interface of cathode in polymer solar cells by inserting a layer of ZnO/self-assembled monolayer (SAM) between a poly(3-hexylthiophene): [6,6]-phenyl-C61 butyric acid methyl ester bulk-heterojunction film and a metal cathode. We found that the device performance could be significantly altered depending on the dipole direction and chemical bonding between the SAM and metals. With appropriate choice of SAMs, devices show dramatically improved efficiencies and even high work-function metals such as Ag and Au could be used as cathodes. This finding provides an efficient method for interface engineering in organic-based optoelectronic devices.

We report improved device performance of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl C61butyric acid methyl ester (PCBM)-based inverted bulk-heterojunction (BHJ) solar cells through the modified interface of the TiO2/BHJ with a series of carboxylic acid functionalized self-assembled monolayers (SAMs). The SAMs reduce the series resistance and improve the shunt resistance of the cell leading to increased fill factor and photocurrent density. Different aspects of device improvement can be affected depending on the nature of the SAMs. Modification with a C60-SAM shows the largest enhancement leading to a 35% improvement (η = 3.78%) over unmodified inverted devices (η = 2.80%). This SAM serves multiple functions to affect the photoinduced charge transfer at the interface to reduce the recombination of charges, passivation of inorganic surface trap states, improve the exciton dissociation efficiency at the polymer/TiO2 interface as well as a template to influence the overlayer BHJ distribution of phases, morphology and crystallinity leading to better charge selectivity and improved solar cell performance.

By incorporating efficient NLO chromophore into supramolecular self-assembly, we have recently achieved unprecedented E-O activity (> 300 pm/V @1310nm), which is one order higher than the state-of-the-art inorganic material, LiNbO3.