Small Bandgap Polymers for Organic Solar Cells(Polymer Material Development in the Last 5 Years)

Sub-bandgap absorption in organic solar cells: experiment and theory

Most high-performance organic solar cells involve bulk-heterojunctions in order to increase the active donor-acceptor interface area. The power conversion efficiency depends critically on the nano-morphology of the blend and the interface. Spectroscopy of the sub-bandgap region, i.e., below the bulk absorption of the individual components, provides unique opportunities to study interface-related properties. We present absorption measurements in the sub-bandgap region of bulk heterojunctions made of poly(3-hexylthiophene-2,5-diyl) as an electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as an electron acceptor and compare them with quantum-chemical calculations and recently published data on the external quantum efficiency (EQE). The very weak absorption of the deep sub-bandgap region measured by the ultra-sensitive Photothermal Deflection Spectroscopy (PDS) features Urbach tails, polaronic transitions, conventional excitons, and possibly charge-transfer states. The quantum-chemical calculations allow characterizing some of the unsettled spectral features.

Carbon nanotube electrodes in organic transistors

The scope of this Minireview is to provide an overview of the recent progress on carbon nanotube electrodes applied to organic thin film transistors. After an introduction on the general aspects of the charge injection processes at various electrode-semiconductor interfaces, we discuss the great potential of carbon nanotube electrodes for organic thin film transistors and the recent achievements in the field.

Synthesis and characterization of poly(3-hexylthiophene)-functionalized siloxane nanoparticles

Poly(3-hexylthiophene)-functionalized siloxane nanoparticles were prepared by a modified Stöber method. The photovoltaic performance of P3HT-nanohybrids with C60 derivative PCBM was evaluated. The device made from 1 : 1 blends of P3HT-NPs:PCBM showed reasonably good photovoltaic performance with a power conversion efficiency of 2.5% under standard test conditions (AM 1.5G, 100 mW cm(-2)).

Low-lying electronic states and their nonradiative deactivation of thieno[3,4-b]pyrazine: an ab initio study

State-averaged complete active space self-consistent field (SA-CASSCF) calculations have been used to locate the four low-lying electronic states of thieno[3,4-b]pyrazine (TP), and their vertical excitation energies and emission energies have been determined by means of the multistate complete active space with second-order perturbation theory (MS-CASPT2) calculations. The present results indicate that the first weak (1)nπ∗ excited state has a C(s)-symmetry structure, unlike two bright (1)ππ∗ excited states in C(2v) symmetry. The predicted vertical excitation energies of the three low-lying excited states in the gas phase are 3.41, 3.92, and 4.13 eV at the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] optimized geometry, respectively. On the basis of calculations, a new assignment to the observed spectra of TP was proposed, in which the (1)nπ∗ state should be responsible for the weak absorption centred at 3.54 eV and the two closely spaced (1)ππ∗ states account for the two adjacent absorption bands observed at 3.99 and 4.15 eV. The predicted vertical emission energies lend further support to our assignments. Surface hopping dynamics simulations performed at the SA-CASSCF level suggest that the plausible deactivation mechanism comprises an ultrafast relaxation of the (1)ππ∗ excited states to (1)nπ∗ excited state, followed by a slow conversion to the S(0) ground state via a conical intersection. This internal conversion is accessible, since the MS-CASPT2 predicted energy barrier is ∼0.55 eV, much lower than the Franck-Condon point populated initially under excitation. The dynamical simulations on the low-lying states for 500 fs reveal that the relatively high (1)ππ∗ excited states can be easily trapped in the (1)nπ∗ excited state, which will increase the lifetime of the excited thieno[3,4-b]pyrazine.

Organic solar cells: understanding the role of Förster resonance energy transfer

Organic solar cells have the potential to become a low-cost sustainable energy source. Understanding the photoconversion mechanism is key to the design of efficient organic solar cells. In this review, we discuss the processes involved in the photo-electron conversion mechanism, which may be subdivided into exciton harvesting, exciton transport, exciton dissociation, charge transport and extraction stages. In particular, we focus on the role of energy transfer as described by Förster resonance energy transfer (FRET) theory in the photoconversion mechanism. FRET plays a major role in exciton transport, harvesting and dissociation. The spectral absorption range of organic solar cells may be extended using sensitizers that efficiently transfer absorbed energy to the photoactive materials. The limitations of Förster theory to accurately calculate energy transfer rates are discussed. Energy transfer is the first step of an efficient two-step exciton dissociation process and may also be used to preferentially transport excitons to the heterointerface, where efficient exciton dissociation may occur. However, FRET also competes with charge transfer at the heterointerface turning it in a potential loss mechanism. An energy cascade comprising both energy transfer and charge transfer may aid in separating charges and is briefly discussed. Considering the extent to which the photo-electron conversion efficiency is governed by energy transfer, optimisation of this process offers the prospect of improved organic photovoltaic performance and thus aids in realising the potential of organic solar cells.

Effect of thickness-dependent microstructure on the out-of-plane hole mobility in poly(3-hexylthiophene) films

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, μ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100-200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10(-4) cm(2)/V·s, for a 80 nm thick film, to a value of 10(-3) cm(2)/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.

Guidelines for the Bandgap Combinations and Absorption Windows for Organic Tandem and Triple-Junction Solar Cells

Organic solar cells have narrow absorption windows, compared to the absorption band of inorganic semiconductors. A possible way to capture a wider band of the solar spectrum—and thus increasing the power conversion efficiency—is using more solar cells with different bandgaps in a row, i.e., a multi-junction solar cell. We calculate the ideal material characteristics (bandgap combinations and absorption windows) for an organic tandem and triple-junction solar cell, as well as their acceptable range. In this way, we give guidelines to organic material designers.

On the importance of morphology control in polymer solar cells

Nanostructured polymer-based solar cells (PSCs) have emerged as a promising low-cost alternative to conventional inorganic photovoltaic devices and are now a subject of intensive research both in academia and industry. For PSCs to become practical efficient devices, several issues should still be addressed, including further understanding of their operation and stability, which in turn are largely determined by the morphological organisation in the photoactive layer. The latter is typically a few hundred nanometres thick film and is a blend composed of two materials: the bulk heterojunction consisting of the electron donor and the electron acceptor. The main requirements for the morphology of efficient photoactive layers are nanoscale phase segregation for a high donor/acceptor interface area and hence efficient exciton dissociation, short and continuous percolation pathways of both components leading through the layer thickness to the corresponding electrodes for efficient charge transport and collection, and high crystallinity of both donor and acceptor materials for high charge mobility. In this paper, we review recent progress of our understanding on how the efficiency of a bulk heterojunction PSC largely depends on the local nanoscale volume organisation of the photoactive layer.

Plasmonic nanograting design for inverted polymer solar cells

Plasmonic nanostructures for effective light trapping in a variety of photovoltaics have been actively studied. Metallic nanograting structures are one of promising architectures. In this study, we investigated numerically absorption enhancement mechanisms in inverted polymer photovoltaics with one dimensional Ag nanograting in backcontact. An optical spacer layer of TiO₂, which also may act as an electron transport layer, was introduced between nanograting pillars. Using a finite-difference-time domain method and performing a modal analysis, we explored correlations between absorption enhancements and dimensional parameters of nanograting such as period as well as height and width. The optimal design of nanograting for effective light trapping especially near optical band gap of an active layer was discussed, and 23% of absorption enhancement in a random polarization was demonstrated numerically with the optimally designed nanograting. In addition, the beneficial role of the optical spacer in plasmonic light trapping was also discussed.

Photo-Fries-based photosensitive polymeric interlayers for patterned organic devices

This work reports on the investigation of the photosensitive polymer poly(diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PPNB), which undergoes the photo-Fries rearrangement upon illumination with UV-light, used as interfacial layers in organic electronic devices. Two cases were investigated: the use of a blend of PPNB with poly-vinylcarbazole (PVK) as an interlayer in para-sexiphenyl (PSP) based organic light emitting diodes (OLEDs) and the use of PPNB as gate dielectric layer in organic field effect transistors (OFETs). The photo-Fries rearrangement reaction causes a change of the polymer chemical structure resulting in a change of its physical and chemical properties. The electroluminescence spectra and emission of the PSP OLEDs are not affected when fabricated with a non-UV-illuminated PPNB:PVK blend. However, the electroluminescence is totally quenched in those OLEDs fabricated with UV-illuminated PPNB:PVK blend. Although the dielectric constant of PPNB increases upon UV-treatment, it is demonstrated that those OFETs built with UV-treated PPNB as gate dielectric have lower performance than those OFETs built with non-UV-treated PPNB. Furthermore, the effect of the UV-illumination of PPNB and PPNB:PVK blend on the growth of the small molecules C60 and PSP has been studied by atomic force microscopy. Using photolithography, this kind of photochemistry can be performed to spatially control and tune the optical and electrical performance of organic electronic devices.