Session B16: Focus Session: Organic Electronics and Photonics: Solar Cell Materials and Devices

Photoconductivity in the Poly(2,7-Carbazole) Copolymer PCDTBT, and in Bulk Heterojunction Composites with PC$_{70}$BM

We have studied the carrier generation in an alternating donor-acceptor low bandgap copolymer and in composites of that polymer with a soluble fullerene derivative, using steady-state and transient photoconductivity. The Poly(2,7-Carbazole) copolymer PCDTBT we studied represents a class of donor-acceptor copolymers that hold promise for photovoltaic applications because of the ability to tune the electronic energy levels. Photovoltaic devices fabricated from PCDTBT with the soluble fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC$_{70}$BM) have exhibited a higher solar cell power conversion efficiency than has been achieved in P3HT based devices. In PCDTBT, the absorption extends out to 1.75 eV, with two distinct but broad absorption bands that are centered at $\sim $2 eV and $\sim $3 eV. We have used steady-state and transient photoconductivity to investigate the carrier generation and collection efficiency of PCDTBT and its composite with PC$_{70}$BM after photoexcitation at each of its distinct absorption bands.   

Phthalimide Copolymer Solar Cells

Photovoltaic properties of bulk heterojunction solar cells based on phthalimide donor-acceptor copolymers have been investigated. Due to the strong $\pi -\pi $ stacking of the polymers, the state-of-the-art thermal annealing approach resulted in micro-scale phase separation and thus negligible photocurrent. To achieve ideal bicontinuous morphology, different strategies including quickly film drying and mixed solvent for film processing have been explored. In these films, nano-sale phase separation was achieved and a power conversion efficiency of 3.0{\%} was obtained. Absorption and space-charge limited current mobility measurements reveal similar light harvesting and hole mobilities in all the films, indicating that the morphology is the dominant factor determining the photovoltaic performance. Our results demonstrate that for highly crystalline and/or low-solubility polymers, finding a way to prevent polymer aggregation and large scale phase separation is critical to realizing high performance solar cells.   

A Multiscale Approach to Realistic Simulation of Organic Photovoltaic Devices

We have generated three dimensional bulk heterostructures consistent with neutron reflectivity data from polymer-fullerene systems and have used these nanostructures in fully three dimensional simulations of solar cells fabricated from these materials. The simulations are based on a multiscale model of organic photovoltaic devices, incorporating exciton generation and dis-association, photoinduced charge transport, dark current injection from contacts, and losses due to charge recombination and trapping. Through the use of precalculated charge-pair escape probabilities based upon likely local internal morphologies, interface effects, and associated electric fields, the most computationally intensive parts of many other dynamic Monte Carlo device simulators can be efficiently approximated. Comparison with continuum device models and experimental data will be used to illustrate the aspects that require a fully three dimensional atomistic model.   

Architectures for enhanced exciton collection in organic photovoltaic cells

Organic semiconductors have received considerable attention for application in a variety of optoelectronic systems including light-emitting devices, lasers, and photovoltaic cells. Due to their compatibility with lightweight flexible substrates and high throughput processing techniques, organic photovoltaic cells (OPVs) represent an intriguing renewable energy option. In these materials, photogenerated excitons must be dissociated in order to generate a photocurrent. Exciton dissociation is typically realized using a donor-acceptor (D-A) heterojunction, where the energy level offset exceeds the exciton binding energy. Mobile excitons diffuse to the D-A heterojunction and are dissociated into their component charge carriers. In most organic materials, the exciton diffusion length is much shorter than the optical absorption length. Consequently, not all of the photogenerated excitons reach the D-A interface, limiting cell efficiency. For small molecule active materials, routes around this bottleneck have centered on the use of mixed D-A film morphologies to increase the area of the dissociating interface. This work instead focuses on the use of OPVs with continuously graded film composition and morphology as a means to simultaneously optimize the exciton diffusion and charge collection efficiencies. In these graded heterojunction OPVs, the power conversion efficiency is noted to exceed that of comparable devices containing a planar or mixed heterojunction. Overall, this approach provides the ability to tune the exciton diffusion and charge collection efficiencies based on the composition profile, permitting greater control over device performance.   

Elucidating Vertical Phase Separation of Active Layers in Polymer Solar Cells via NEXAFS

Using synchrotron-based near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, we have quantified the surface compositions of bulk-heterojunction active layers comprising poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester. By delaminating the active layers from the underlying substrates, we have also quantified the compositions at the once-buried film-substrate interface. For active layers on hydrophilic substrates, the surface composition is 97wt\% P3HT. In comparison, the P3HT composition is lower at the film- substrate interface (65wt\%). By increasing the hydrophobicity of the substrates through the adsorption of phosphonic acid derived self-assembled monolayers, we observe an enrichment of P3HT (89wt\%) at the film-substrate interface of the active layers. Accordingly, devices of the conventional architecture fabricated on hydrophobic surfaces show improved performance.   

Block Copolymers for Polymer Photovoltaic Applications

Block copolymers have been recently proposed as a promising candidate for polymer photovoltaic applications, due to their unique properties to form microphase separated domains and ability to self-assemble into nanoscale domains compatible with excitonic solar cells. We recently studied the photovoltaic properties of a series of block copolythiophenes with different block compositions. By blending these block copolymer donors with fullerene acceptors, bulk heterojunction (BHJ) solar cells are fabricated and tested. Our results show that solar cells from block copolymers show preeminent photovoltaic properties compared with their parent homopolymers. We find that in all the block copolymer blend thin films, nanoscale donor/acceptor phase separation is achieved. Enhanced hole transport is achieved in block copolymer blends, as shown in space-charge limited current (SCLC) devices. Factors of one- and two-order of magnitude increase in SCLC hole mobility have been achieved compared with homopolymers. The enhanced power conversion efficiency (PCE) in block copolymers is primarily attributed to the enhanced charge transport. The block composition dependence of both photovoltaic properties and charge transport provides insight into future design of block copolymers for photovoltaic applications.   

Molecular Engineering of Polymer Semiconductors for Solar Cells

We have developed a series of new donor-acceptor copolymers which allow us to tune the electronic structures, charge transport, and thus their photovoltaic properties. The new materials combined high field-effect hole mobilities (0.01-0.1 cm$^{2}$/Vs) with broad absorption spectra and optical band gaps as small as 1.7 eV. Bulk heterojunction solar cells using these copolymers as donor and fullerene derivative as acceptor were fabricated and their comparative performance will be discussed. A power conversion efficiency of 4.54{\%} was achieved in ambient air from one of the polymers with a current density ($J_{sc})$ of 12.19 mA cm$^{-2}$, an open circuit voltage ($V_{oc})$ of 0.60 V, and a fill factor (FF) of 0.62. The variation in the photovoltaic properties in the series of copolymers is explained by the differences in optical properties and electronic structures of these polymers as well as the nanoscale morphology of the polymer-fullerene blend thin films. Our results provide new insights in the design of donor-acceptor copolymers for organic solar cell applications.   

Increased exciton harvesting in organic thin film solar cells

The optimization of organic solar cells involves a fundamental tradeoff between optical absorption length, mobility, and exciton diffusion length ($L_{D})$. Organic semiconductors possess $L_{D}$ that are at least one order of magnitude less than their respective absorption lengths, meaning that many excitons decay before reaching a dissociating interface. The bulk heterojunction concept, whereby one mixes donor and acceptor components into a single layer, is an effective way to avoid this bottleneck. However, because mixed layers tend to have lower mobilities compared with pure films, carrier transport in devices composed of mixed layers thick enough to absorb a significant amount of light is poor, producing an inefficient device. In this talk, we explore two promising approaches to solve these challenges. In one, we investigate the possibility of increasing $L_{D}$ of a given material. By employing a properly chosen phosphorescent dopant, we are able to sensitize a population of long-lived triplet excitons in a normally fluorescent material, increasing the diffusion length by more than a factor of 2. In another approach, we look into the possibility of exploiting surface plasmon resonances of metal nanoparticles. These surface plasmon resonances lead to strongly enhanced near fields, increasing absorption of nearby chromophores. With this approach, therefore, the thickness of organic semiconductor layers containing metal nanoparticles could be reduced without compromising absorption. Here, we investigate exciton-plasmon interactions through photoluminescence and absorption measurements of thin-films consisting of organic semiconductors and metal nanoparticles, as a function of film thickness with and without the presence of spacer layers between the nanoparticles and absorbers. From this knowledge, we assess the prospect of using plasmonic effects in thin film organic solar cells.   

Exploring mechanisms in excitonic photovoltaics from first principles

Organic-based photovoltaics (OPV) are being explored as an alternative to inorganic PVs due to their potential for low cost and more flexible form factors. Single-molecule heterojunctions, containing donor and acceptor moieties linked by covalent bonds, provide an interesting model system for understanding processes fundamental to OPVs, such as light absorption and charge separation. In this work, the necessity of type II heterojunction electronic structure is critically examined for a series of small asymmetric molecules containing covalently-linked aromatic moieties. We present density functional theory and many body perturbation theory calculations of both single electron addition and removal energies using the GW approximation, and the neutral excitations using a Bethe-Salpeter equation approach. Implications for charge separation in these systems in the presence of metal contacts is also discussed. This work supported by DOE via Helios Solar Energy Research Center. Computational support provided by NERSC.   

Electric and Optical Performances of Perylene Diimides (PDI) and Their Applications in Electronic Devices

Perylene diimide (PDI) based compounds were synthesized through a condensation reaction of perylene dianhydride (PDO) with various amines such as, 1-amino octane (PDI-C8), 1-amino hexane (PDI-C6), etc. The chemical structure and their electric and optical properties have being identified and characterized by UV- Vis, FTIR, CV, etc. It was found that the band gaps and charge motilities of these compounds can be adjusted by the monomer selection and condensation reaction controlling. Self-assembled PDI nano fibers can be formed through solvents interaction methods. For example, a PDI-C8 nano fiber was made through mixing the PDI-C8/Chloroform solution with Hexane. The diameters and lengths of those nano fibers can be adjusted through the PDI solution concentration and interaction temperature. The PDI materials can also dissolve in many organic solvents such as Chloroform, and Dichlorobenzene, for spinning coating and solution blending during the fabrication of organic photovoltaic device. In summary, PDI materials have many unique features such as high thermally stable, low cost, broad product offering, excellent electronic properties, and flexible in processing, which are very attractive in making electronic devices such as, OLED, OPV, and organic transistors.   

ZnO Nanoparticles and Nanowire Arrays with Liquid Crystals for Photovoltaic Apprications

Liquid crystals are small monodisperse molecules with high mobilities and are easy and cheap to process. In addition, some of their phases exhibit molecular orientation that can provide a path for the electrons, or holes, to move from one electrode to the other. We have mixed a smectic A liquid crystal (8CB) with varying concentrations of ZnO nanoparticles of $\sim $5 nm in diameter and have observed a photovoltaic effect as a function of the concentration of ZnO. The liquid crystal is believed to enhance the alignment of the nanoparticles and aid in the diffusion of electrons through the particles to the collection electrode. We have also made PV cells of ZnO nanowire arrays grown on Au layers. The nanowire arrays are covered with 8CB liquid crystal for hole conduction. We compare the absorption of the PV cells as a function of wavelength of the light for the ZnO nanoparticle and the ZnO nanowire cells.