Warwick J. Belcher

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.

Cooperative anion binding and electrochemical sensing by modular podands

A series of podands based on two or three hydrogen bonding “arms” situated in mutually ortho, meta, or para relationships about an aryl core have been prepared, and their affinities for simple inorganic anions were measured. Of the two-arm hosts the meta compound and to a lesser extent the ortho host exhibit a cooperative anion binding effect. The two arms function essentially independently in the para derivative. The mutually meta three-arm host shows dramatically enhanced cooperative binding. Conformational changes within the meta two-arm host result in significantly enhanced electrochemical anion sensing compared with the more conformationally rigid three-arm host.

Oxo-anion binding by metal containing molecular 'clefts'

Abstract A simple but effective route has been developed to produce a series of molecular clefts, [(η6-p-cymene)RuCl(1)2]PF6 (4a), [(dppe)Pd(1)2](BF4)2 (5a), [(dppe)Pd(2)2](BF4)2 (5b), [(dppe)Pt(1)2](BF4)2 (6a) and [(dppe)Pt(2)2](BF4)2 (6b), that contain either a redox active ferrocenyl or a photoactive anthracenyl side arm, attached to a ruthenium(II), palladium(II) or platinum(II) backbone. Compounds 4a, 5a, 5b and 6a act as hosts for oxo-anions. Anion recognition is achieved via convergent hydrogen bond interactions from secondary amine functionality on the side arms. The binding is also enhanced by the positive charge of the metal centres. The X-ray crystal structure of the related [PdCl2(1)2] (7) shows it to possess a trans geometry. The X-ray crystal structures of the monoadducts [(η6-p-cymene)RuCl2(1)] (3a) and [(η6-p-cymene)RuCl2(2)] (3b), which show contrasting behaviour in their hydrogen bonding to coordinated chloride, are also reported.

Intramolecular binding site competition as a means of tuning the response of a colourimetric anion sensor

Two new viologen-based anion hosts, 2 and 3, are reported, incorporating two tripodal binding domains. Binding of carboxylate anions, particularly acetate, malonate and succinate by pyridinium derivative 2 results in an intense purple colouration. DFT calculations on representative system 2·malonate reveal that this absorption arises from charge-transfer from the anion to the bipyridinium unit. Anion binding by 3 does not initially result in a colour change because anions are bound at the periphery of the receptor; however, addition of more that two equivalents of acetate or more than one equivalent of dicarboxylate turn the colourimetric response on.

Channel-containing 1D coordination polymers based on a linear dimetallic spacer

A new channel-containing 1D coordination polymer has been prepared by the combination of a linear dimetallic spacer, Cu2(O2CMe)4, and a linear didentate ligand, 1,3-di-4-pyridylpropane; the new material includes methanol, acetic acid and diethylene glycol.

Colourimetric Carboxylate Anion Sensors Derived from Viologen‐Based Receptors

A series of tri- and tetrapodal viologen-based anion receptors showing a colourimetric response to carboxylates, such as acetate, have been synthesised. Alteration of the anion binding sites allows for binding site competition within a receptor. This results in a delayed colourimetric response for urea derivatives compared with pyridinium systems because the anions are initially bound to the periphery of the receptor, away from the viologen unit. DFT calculations and experimental measurements allow the colour change to be assigned to an anion-receptor charge-transfer process, facilitated by the exceptionally low reduction potential of the cationic host compounds. Evidence for electron transfer to give the viologen radical cation is also seen in some cases.

Porphyrin complexes containing coordinated BOB groups: synthesis, chemical reactivity and the structure of [BOB(tpClpp)]2+

The reactions of boron halides with free base porphyrins under conditions where partial hydrolysis of the boron halides can occur give diboron porphyrin complexes containing BOB moieties in which each boron is bonded to two porphyrin nitrogen atoms. BF(3).OEt(2) with H(2)(por) gives B(2)OF(2)(por) (por = tpp, ttp, tpClpp, oep) which has an asymmetric structure in which one boron lies in the porphyrin plane (B(ip)) while the other lies above it (B(oop)). BCl(3).MeCN with H(2)(por) gives B(2)O(2)(BCl(3))(2)(por) which contains a four-membered B(2)O(2) ring and is stable only in the presence of excess BCl(3). BBr(3) with Li(2)(tpClpp) gives the dicationic complex [B(2)O(tpClpp)](2+) as its [BBr(4)](-) salt, and is the first example of a boron porphyrin containing three-coordinate boron to be structurally characterised. B(2)O(2)(BCl(3))(2)(por) can be chromatographed on basic alumina to give the hydroxyboron complex B(2)O(OH)(2)(por), which is deduced from its NMR spectra and DFT calculations to have a structure analogous to B(2)OF(2)(por). The OH protons are shifted upfield to near delta -4 (B(oop)-OH) and -10 (B(ip)-OH) by the diamagnetic porphyrin ring current. The reaction of either B(2)O(2)(BCl(3))(2)(por) or B(2)O(OH)(2)(por) (por = ttp, tpClpp) with alcohols (ROH, R = Et, 4-C(6)H(4)CH(3)) gives B(2)O(OR)(2)(por), which can in turn be converted to B(2)O(OR)(OH)(por) by repeated chromatography. The reaction of PhBCl(2) with H(2)(por) (por = ttp, tpClpp) gives B(2)O(Ph)(OH)(por) which has been characterised by spectroscopy in concert with DFT calculations. It is a further example of the B(2)OF(2)(por) structural type, in which the phenyl group is coordinated to the out-of-plane boron and the OH group to the in-plane boron, as are its derivatives B(2)O(Ph)(X)(tpClpp) (X = F, OEt). Steric drivers for the facile hydrolysis of haloboron porphyrins relative to their dipyrromethene and expanded porphyrin counterparts are discussed.

Exciton transport in organic semiconductors: Förster resonance energy transfer compared with a simple random walk

Forster resonance energy transfer theory (FRET) and a simple random walk (RW) are both implemented in a dynamic Monte Carlo simulation with the aim of determining the exciton diffusion length from photoluminescence (PL) measurements. The calculated diffusion lengths obtained from both models are shown to be the same. As such, given that the computational time of a random walk is typically 2–3 orders of magnitude smaller than the FRET approach, this work shows that the RW methodology can be a preferable model for the determination of diffusion lengths. We also show that the RW approach may also be implemented in Monte Carlo simulations that describe organic solar cells. Despite the fact that (compared with FRET) RW does not account for non-nearest neighbor hopping or energy relaxation, we show that the resulting overestimation of the simulated current will not exceed 2% for typical OPV parameters. In addition, by taking advantage of the gain in speed we are able to investigate the impact of the exciton dif...

Towards the development of a virtual organic solar cell: An experimental and dynamic Monte Carlo study of the role of charge blocking layers and active layer thickness

Monte Carlo (MC) simulations have been used to fully model organic solar cells. The quantum efficiency and short-circuit current of these virtual devices are in excellent agreement with experimental measurements. Simulations show that, contrary to expectation, indium tin oxide/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methylester (PCBM)/aluminium devices lack effective charge blocking layers at the electrode interfaces. X-ray photoelectron spectroscopy depth profiling shows that despite a PCBM-rich region near the cathode, interface intermixing at the electrodes combined with incomplete PCBM coverage leads to significant interface recombination. This work highlights the effectiveness of MC simulations as a predictive tool and emphasizes the need to control electrode interface processes.

Comparative degradation and regeneration of polymer solar cells with different cathodes.

A comparative degradation study of solar cells based on a bulk-heterojunction (BHJ) blend of poly(3-hexylethiophene) (P3HT) and phenyl [6,6] C61 butyric acid methyl ester (PCBM) with two different cathodes is reported. Poly(ethylene-dioxythiphene):poly(styrene sulfonate) (PEDOT:PSS) coated ITO electrodes were used as the anode, whereas Ca/Al and Ca/Ag electrodes were used as cathodes. Fully degraded devices were subjected to thermal annealing under inert atmosphere. The performance of degraded solar cells with a Ca/Al cathode exhibited no improvement after treatment. However the solar cells with a Ca/Ag cathode exhibited a considerable recovery in their performance following annealing under a nitrogen atmosphere. Indeed, these solar cells could be subjected to many degradation and regeneration cycles. Current density-voltage (J-V) characteristics and X-ray photoelectron spectroscopy (XPS) studies show that this behavior arises from the complex chemical thermodynamics of the reactions that can occur at the cathode/active layer interface. In particular, the recovery of device performance for solar cells with a Ca/Ag cathode is due to the reversible oxidation of Ag upon thermal annealing.