Session K56: Organic Electronics and Photonics III: Organic Photovoltaics

Mixed Domains Enhance Charge Generation and Extraction in Small-Molecule Bulk Heterojunction Solar Cells

It is established that the nanomorphology plays an important role in performance of bulk-heterojunction (BHJ) organic solar cells. From intense research in polymer-fullerene systems, some trends are becoming apparent. For example, small ~10 nm domains, high crystallinity, and low miscibility are typically measured in high-performance systems. However, the generality of these concepts for small-molecule (SM) BHJs is unclear. We present a comprehensive study of performance, charge generation and extraction dynamics, and nanomorphology in SM-fullerene BHJ devices to probe these critical structure-property relationships in this class of materials. In the systems investigated, small domains remain important for performance. However, devices composed of highly mixed domains with modest crystallinity outperform those consisting of pure/highly crystalline domains. Such a result points to an alternative ideal morphology for SM-based devices that involves a predominant mixed phase. This stems from SM aggregation in highly mixed domains that both maximize interface for charge generation and establish continuous pathways for efficient charge extraction. Such a morphological paradigm should be considered in future SM systems in pursuit of high-efficiency large-scale solar power production.   

PTB7-Th-b-PNDI fully conjugated donor-acceptor block copolymers for thermal stability of photovoltaic devices

In recent, the long-term stability of organic solar cells, including both photo-stability and thermal stability, has received growing attention. As the sole active layer material and phase compatibilizer in blend systems, photovoltaic devices including fully conjugated donor-acceptor block copolymers are expected to display enhanced thermal stability than the corresponding blend systems. We prepared Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]-block-Poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PTB7-Th-b-PNDI) donor-acceptor block copolymer using a Stille reaction twice. Formation of block copolymers was confirmed by NMR and GPC. Solar cell devices were fabricated using the donor-acceptor block copolymer as the sole active layer material as well as with the block copolymer as an additive in PTB7-Th/PNDI and PTB7-Th/PCBM blend systems. Upon thermal stress at 80 ^oC for up to 168 hours, blend solar cells with addition of block copolymers exhibited enhanced thermal stability. Morphology of blends with or without addition of the block copolymer was investigated by RSoXS and EF-TEM.   

Donor/acceptor interfacial structure from 13C {2H} REDOR NMR

Robust relationships between structure and function are generally lacking in organic photovoltaic (OPV) thin film active layers. To predict performance there exists a need for tools that can measure structure on length scales fine enough to be relatable to inter-molecular energy transfer. Electron microscopy lacks sufficient spatial resolution due to a lack of electron density contrast, and scattering curves can be ambiguous because there typically is not a unique fitting model. In this talk, I will give highlights of recent ¹³C {²H} REDOR measurements to characterize the donor/acceptor interfaces in bulk heterojunction thin films. Heteronuclear couplings are measured between ¹³C nuclei on the acceptor C₆₀ cage and thiophene hydrogens on the donor main chain, which has been isotopically enriched with ²H. I will discuss models of the interface that are used to fit the REDOR dephasing curve, and the constraints that these models have on local composition and packing. We will also show that the REDOR measurements can help solve the mystery of which model to use in fitting small angle neutron scattering curves.
  

Nanoscale Energetic Mapping of Interfaces in Organic Bulk Heterojunction Solar Cells

The bulk heterojunction (BHJ) layer in organic solar cells is a delicate nanoscale blend of electron donating (D) and accepting (A) molecules which assembles to form a three-dimensional distributed D-A heterointerface. Yet, the real-space energetic landscape responsible for charge generation from excitons at the heart of these blends has remained elusive, fueling significant debate about the roles of sharp D-A interfaces versus the mixed (M) phase. In this talk, we discuss recent developments in the direct imaging of the real-space energetic landscape of BHJ layers via scanning tunneling microscopy and spectroscopy (STM/STS) measurements. This approach is shown to work on BHJ films directly usable in OPV devices and reveal several heterointerfaces with different energetics that surround aggregated D, A and and mixed (M) phase domains. We reveal the coexistence of three types of heterointerfaces within the BHJ and, in total, four types of D-A intermolecular interactions exhibiting different energetics. These scenarios are universally present in classical and modern BHJs exhibiting a wide range of power conversion efficiencies (PCE, ca. 3.5 – 10.8%; P3HT:PCBM, P3HT:O-IDTBR and PCE11:PCBM). We provide accurate HOMO/LUMO energy diagrams for D/M, A/M and D/A heterointerfaces in these systems and discuss the most likely pathways for charge generation and recombination. Our study also points to new pathways to engineer BHJ interfaces so as to maximize the Voc and the power conversion efficiency of next generation organic solar cells.
  

Study of Fracture and Tensile Properties of All-Polymer and Fullerene-Polymer Solar Cells Based on Same Polymer Donor

The mechanical properties of the polymer solar cells (PSCs) are important for their applications as flexible and wearable devices. In this work, we compare the mechanical properties including both fracture and tensile properties of all-polymer solar cells (all-PSCs) and PCBM-PSCs based on same polymer donor. First, we measured the fracture energy of two different PSCs using double cantilever beam (DCB) test. Surprisingly, the all-PSCs showed much higher fracture energy of 2.45 J m⁻² than that (0.29 J m⁻²) of PCBM-PSCs and the value of the all-PSCs was not dependent on the acceptor contents. Next, we examine the tensile properties of all-PSCs and PCBM-PSCs using pseudo free-standing test. Interestingly, 20-fold enhancement in elongation at break was observed in all-PSCs compared to that in PCBM-PSCs. This is because a large degree of plastic deformation occurred in all-PSCs due to polymer bridging in film during crack growth, in contrast to very weak interface between polymer and highly crystalline PCBM domains for the PCBM-PSCs. Therefore, our work suggests that replacing PCBM with polymer acceptor can be an effective strategy to obtain mechanically more robust PSCs.   

A General Strategy for Stretchable Organic Photovoltaics through Interpenetrating Thiol-Ene Networks

Thin films of semiconductive polymers and small molecules exhibit a low crack-onset strain and poor mechanical properties under deformation, limiting their viability for flexible organic electronic devices. Molecular engineering of the semiconductive material can enhance mechanical properties but oftentimes is detrimental to electronic properties and overall device performance. Here, we present a general approach to fabricating flexible bulk heterojunction organic photovoltaics (OPVs) through the incorporation of an elastic and mechanically robust interpenetrating network. Reactive small molecule thiol-enes are incorporated in the active layer through solution blending, casting, and post-deposition crosslinking. A thiol-ene network is formed through short, one-minute exposure to UV light or with an amine catalyst. This results in fully functional devices with significantly increased crack-to-onset strain and flexibility. The approach is compatible with a variety of donors and acceptors and results in significant enhancement of mechanical properties and device flexibility.
  

Solution Shearing to Control the Morphology of Continuously Processed P3HT/PCBM Films

We demonstrate the use of solution shearing prior to film deposition as a means to control the morphology of model poly(3-hexylthiophene)/phenyl-C₆₁-butyric acid methyl ester (P3HT/PCBM) films. Specifically, P3HT/PCBM solutions in the gel state were subjected to oscillatory and steady torsional shear forces in the nonlinear regime. The sheared solutions were then doctor bladed onto solar cell device platforms. P3HT/PCBM solar cells with pre-sheared active layers displayed up to double the efficiency of those comprising unsheared active layers. Rheological measurements, 2D x-ray diffraction experiments and confocal fluorescence spectroscopy revealed that solution shearing induced crystallization of and phase separation between P3HT and PCBM, necessary for efficient charge transport during solar energy harvesting. Unlike techniques developed to optimize P3HT/PCBM films deposited via batch spin coating processes, this method of morphology control via solution shearing is compatible with continuous processing methods. The ability to continuously process active layers is critical for advancing organic solar cell technologies towards commercialization.   

Predicting charge transport properties in organic photovoltaic devices with coarse grained models

In this work we examine the structure-function relationships between chemical structure, morphology, and electronic properties in organic photovoltaic (OPV) devices. Understanding these relationships is critical to improving OPV device efficiency. We evaluate the limit to which we can predict charge transport properties of OPV devices using coarse-grained (CG) models, molecular dynamics simulations, and quantum chemical calculations. CG models allow us to access the relevant length and time scales required to study the nanostructure morphology of OPV active layers but they lack the fine-grained detail required to perform electronic structure calculations. We therefore use the CG to atomistic fine-graining capabilities of the MorphCT software package, so that we can conduct quantum chemical calculations to determine the charge transport properties of a variety of morphologies at different state points, to highlight processing conditions that are expected to produce the most efficient devices. In particular, we evaluate the efficacy of our high-throughput OPV device morphology screening for blends of poly(benzodithiophene-thienopyrrolo-dione) (pBDT-TPD) and phenyl C71 butyric acid methyl ester (PC71BM).   

Computational Characterization of Structure and Electronic Properties of Periflanthene:Fullerene Blends

Organic electronic devices are highly desirable in part due to their mechanical flexibility and low costs. The electronic properties of these devices are highly dependent on the self-assembled morphology - the intermolecular structure of constituent n-type and p-type compounds. Recently, the p-type compound Dibenzo-Tetraphenyl-Periflanthene (DBP) has been used successfully in devices such as: transistors, organic light emitting diodes and organic photovoltaics. Although studies have looked at different processing methods to produce devices with varying crystallinity, there has not yet been a study that directly links the morphology of DBP to its electronic properties. In this work, we use molecular dynamics to predict the self-assembled morphology DBP with fullerene derivatives at a variety of densities, temperature and solvents. We then use kinetic Monte Carlo to track the movement of charge-carriers throughout the morphology and calculate zero-field mobilities. As a result of this study, we elucidate the processing methodologies needed to obtain the desired morphologies and identify morphologies that have the most promising electronic properties, which will guide the creation of future devices.   

Elucidating Structure-Property Relationships for Charge Carrier Mobility and Mobility Relaxation in Organic Semiconductor Blends using Kinetic Monte Carlo Simulations Informed by TEM Tomography

A detailed understanding of how materials structure on the nano- to meso-scale affects long-range charge transport behavior is needed to refine design rules for organic electronic devices. While this problem is acutely relevant in donor-acceptor blends for photovoltaic applications where the bulk heterojunction morphology is well-known to play a critical role in device performance, charge transport through crystalline-amorphous microstructures and in semiconductor-insulator blends for flexible transistors share similar physical characteristics. In this presentation, we will introduce a unique approach to developing structure-property relationships for charge transport in these systems using kinetic Monte Carlo simulations with material morphologies derived from spectroscopic transmission electron microscope tomography. We will then quantify how morphological features such as the domain size, domain shape, tortuosity, and structural hierarchy are expected to impact detailed charge transport characteristics such as the carrier density, temperature, and electric field dependence of the mobility and the mobility relaxation dynamics.   

Relating Molecular Morphology to Mesoscale Device Performance Characteristics

Solution-processed organic semiconductors promise to be attractive alternatives to conventional inorganics for use in electronic devices where large-scale, inexpensive production is a priority. The electronic properties of these materials are strongly dependent on the molecular morphology, which is dictated by the material and processing choices. For such materials to ubiquitously replace inorganic semiconductors, it is critical to elucidate the links between molecule conformation on the Angström lengthscales and subsequent device performance over hundreds of nanometers.
We propose an open-source computational pipeline MorphCT that combines molecular dynamics, quantum chemical, and kinetic Monte Carlo simulations to explore the important length- and time-scales for a variety of different molecules. We are able to replicate experimental diffusive carrier mobility trends for ordered and disordered systems. Additionally, we combine donor and acceptor molecular morphologies to construct a thin-film photovoltaic device without the use of a periodically repeating supercell. By incorporating the important kinetic processes into our code, we can obtain spectroscopy data, J-V curves, fill-factors, and device efficiencies for our toy systems, for comparison to experiment.   

Energy Transfer Dynamics in Complexes of Oppositely Charged Conjugated Polyelectrolytes

Conjugated polyelectrolytes (CPEs) are an attractive class of materials for light harvesting in energy applications due to their high absorption cross section, spectral tunability, delocalized electronic states, and tendency toward ionic self-assembly in water. These features make them well suited to mimicking natural electronic energy transfer (EET) relays – using donor-acceptor complexes of oppositely charged CPEs in place of biological pigments. However, the electronic properties of CPEs are highly complex, having a significant interplay between polyelectrolyte backbone microstructure, intra- and inter- chain aggregation, and local chemical environment. Understanding EET in such complexes represents a serious challenge, as the CPE microstructure – and hence electronic properties – are highly perturbed upon complexation. I will discuss recent work characterizing EET dynamics in a series of stable oppositely charged CPE complexes using time-resolved fluorescence anisotropy, transient absorption, as well as steady-state measurements to probe the relationship between CPE complex structure and EET. We find that both the excited-state dynamics and the complex morphology are very sensitive to relative energy donor/acceptor polyion charge ratio, allowing us to control the EET process.   

Electron Donor-Acceptor Nano-Domains in Bulk Heterojunctions of Organic Solar Cell Active Layers - Cross Sectional Scanning Tunneling Microscopy and Spectroscopy

Bulk heterojunctions in organic photovoltaic solar cell (OPVC) active layer are believed to be the ideal quasi-random nano-structures for high solar cell performance. It is also known that the OPVC efficiency strongly depends on the synthesis conditions, such as annealing temperatures, electron donor-acceptor compositions, thickness, electric field etc. In this research, the electron donor-acceptor nano-domain textures and sizes are studied by cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). By varying different synthesis parameters, we visualized drastic evolution of the nano-domain dimensions and textures. Correlations between these nano-domains to the solar cell efficiency will be established as the next step in near future. In this work, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) are used as the model OPVC active layer materials.