Miles C. Barr

Direct Monolithic Integration of Organic Photovoltaic Circuits on Unmodified Paper

There has been signifi cant recent interest in integrating electronics into low-cost paper substrates, including transistors, storage devices, displays, and circuitry. [ 1–4 ] Paper-based photovoltaics (PVs) could serve as an “on-chip” power source for these paper electronics, and also create attractive new paradigms for solar power distribution, including seamless integration into ubiquitous formats such as window shades, wall coverings, apparel, and documents. Module installation may be as simple as cutting paper to size with scissors or tearing it by hand and then stapling it to roof structures or gluing it onto walls. Moreover, paper is ∼ 1000 times less expensive ( ∼ 0.01

Designing polymer surfaces via vapor deposition

· m − 2 ) than traditional glass substrates ( ∼ 10

Organic Solar Cells with Graphene Electrodes and Vapor Printed Poly(3,4-ethylenedioxythiophene) as the Hole Transporting Layers

· m − 2 ) [ 5 , 6 ]

Oxidative chemical vapor deposition (oCVD) of patterned and functional grafted conducting polymer nanostructures

Chemical Vapor Deposition (CVD) methods significantly augment the capabilities of traditional surface modification techniques for designing polymeric surfaces. In CVD polymerization, the monomer(s) are delivered to the surface through the vapor phase and then undergo simultaneous polymerization and thin film formation. By eliminating the need to dissolve macromolecules, CVD enables insoluble polymers to be coated and prevents solvent damage to the substrate. Since de-wetting and surface tension effects are absent, CVD coatings conform to the geometry of the underlying substrate. Hence, CVD polymers can be readily applied to virtually any substrate: organic, inorganic, rigid, flexible, planar, three-dimensional, dense, or porous. CVD methods integrate readily with other vacuum processes used to fabricate patterned surfaces and devices. CVD film growth proceeds from the substrate up, allowing for interfacial engineering, real-time monitoring, thickness control, and the synthesis of films with graded composition. This article focuses on two CVD polymerization methods that closely translate solution chemistry to vapor deposition; initiated CVD and oxidative CVD. The basic concepts underlying these methods and the resultant advantages over other thin film coating techniques are described, along with selected applications where CVD polymers are an enabling technology.

Cathode buffer layers based on vacuum and solution deposited poly(3,4-ethylenedioxythiophene) for efficient inverted organic solar cells

For the successful integration of graphene as a transparent conducting electrode in organic solar cells, proper energy level alignment at the interface between the graphene and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus becomes more significant due to the generally lower work function of graphene compared to ITO. A commonly used HTL material with ITO anodes is poly(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrenesulfonate) (PSS) as the solid-state dopant. However, graphenes hydrophobic surface renders uniform coverage of PEDOT:PSS (aqueous solution) by spin-casting very challenging. Here, we introduce a novel, yet simple, vapor printing method for creating patterned HTL PEDOT layers directly onto the graphene surface. Vapor printing represents the implementation of shadow masking in combination with oxidative chemical vapor deposition (oCVD). The oCVD method was developed for the formation of blanket (i.e., unpatterened) layers of pure PEDOT (i.e., no PSS) with systematically variable work function. In the unmasked regions, vapor printing produces complete, uniform, smooth layers of pure PEDOT over graphene. Graphene electrodes were synthesized under low-pressure chemical vapor deposition (LPCVD) using a copper catalyst. The use of another electron donor material, tetraphenyldibenzoperiflanthene, instead of copper phthalocyanine in the organic solar cells also improves the power conversion efficiency. With the vapor printed HTL, the devices using graphene electrodes yield comparable performances to the ITO reference devices (η(p,LPCVD) = 3.01%, and η(p,ITO) = 3.20%).

Multijunction organic photovoltaics with a broad spectral response

We present a simple one-step process to simultaneously create patterned and amine functionalized biocompatible conducting polymer nanostructures, using grafting reactions between oxidative chemical vapor deposition (oCVD) PEDOT conducting polymers and amine functionalized polystyrene (PS) colloidal templates. The functionality of the colloidal template is directly transferred to the surface of the grafted PEDOT, which is patterned as nanobowls, while preserving the advantageous electrical properties of the bulk conducting polymer. This surface functionality affords the ability to couple bioactive molecules or sensing elements for various applications, which we demonstrate by immobilizing fluorescent ligands onto the PEDOT nanopatterns. Nanoscale substructure is introduced into the patterned oCVD layer by replacing the FeCl3 oxidizing agent with CuCl2.

Angle dependence of transparent photovoltaics in conventional and optically inverted configurations

Vacuum and solution processed versions of poly(3,4-ethylenedioxythiophene) (PEDOT) are used as cathode interlayers in inverted organic photovoltaic cells comprising tetraphenyldibenzoperiflanthene as the electron donor and C60 as the electron acceptor. Chemical treatment of the as-deposited PEDOT layers with tetrakis(dimethylamino)ethylene or cesium carbonate reduces the work function by up to 0.8 eV. Inserting these PEDOT layers at the indium tin oxide cathode results in improved electron collection and efficiencies of up to 2.3 ± 0.2%, approaching the 3.2 ± 0.3% of the conventional device. This illustrates the potential for efficient polymer cathode materials and inverted device architectures compatible with either solution or vacuum processing.

Efficient zinc sulfide cathode layers for organic photovoltaic applications via n-type doping

We demonstrate series-integrated multijunction organic photovoltaics fabricated monolithically by vapor-deposition in a transposed subcell order with the near-infrared-absorbing subcell in front of the green-absorbing subcell. This transposed subcell order is enabled by the highly complementary absorption spectra of a near-infrared-absorbing visibly-transparent subcell and a visible-absorbing subcell and motivated by the non-spatially-uniform optical intensity in nanoscale photovoltaics. The subcell order and thicknesses are optimized via transfer-matrix formalism and short-circuit current simulations. An efficient charge recombination zone consisting of layers of BCP/Ag/MoOx leads to negligible voltage and series-resistance losses. Under 1-sun illumination the multijunction solar cells exhibit a power conversion efficiency of 5.5 ± 0.2% with an FF of 0.685 ± 0.002 and a V(OC) of 1.65 ± 0.02 V, corresponding to the sum of the V(OC) of the component subcells. These devices exhibit a broad spectral response (in the wavelength range of 350 nm to 850 nm) but are limited by subcell external quantum efficiencies between 20% and 30% over the photoactive spectrum.

Bilayer heterojunction polymer solar cells using unsubstituted polythiophene via oxidative chemical vapor deposition

Integration of transparent photovoltaics into the building envelope creates unique opportunities to reduce the levelized electricity cost of solar power. However, this integration warrants consideration of the angular dependence of these devices as illumination around the building envelope is rarely at normal incidence. Here we correctly update transfer-matrix and equations to accurately model the quantum efficiency and optical properties under oblique illumination. We use this model to demonstrate the various angular performance characteristics possible for proof-of-concept optimizations of transparent planar-heterojunction solar cells and discuss considerations needed to fully account for optical, electrical, and positional configurations in this optimization.

Top‐illuminated Organic Photovoltaics on a Variety of Opaque Substrates with Vapor‐printed Poly(3,4‐ethylenedioxythiophene) Top Electrodes and MoO3 Buffer Layer

We demonstrate efficient zinc sulfide cathode window layers in thin-film organic photovoltaics enabled by n-type doping zinc sulfide (ZnS) with aluminum sulfide (Al2S3) directly through co-deposition. By optimizing the Al2S3 concentration, the power conversion efficiency is improved from 0.6% ± 0.2% in undoped ZnS window layer devices to 1.8% ± 0.1%, identical to control devices. The mechanism for this performance enhancement is shown to stem from the enhanced conductivity and interface energetics of ZnS upon n-type doping. This work expands the catalog of efficient, inorganic, non-toxic, cathode side window layers that could be effective in a range of thin-film photovoltaic technologies.