Tracey M. Clarke

Charge Photogeneration in Organic Solar Cells

In recent years, organic solar cells utilizing π-conjugated polymers have attracted widespread interest in both the academic and, increasingly, the commercial communities. These polymers are promising in terms of their electronic properties, low cost, versatility of functionalization, thin film flexibility, and ease of processing. These factors indicate that organic solar cells, although currently producing relatively low power conversion efficiencies (∼5-7%),1–3 compared to inorganic solar cells, have the potential to compete effectively with alternative solar cell technologies. However, in order for this to be feasible, the efficiencies of organic solar cells need further improvement. This is the focus of extensive studies worldwide. The backbone of a π-conjugated polymer is comprised of a linear series of overlapping pz orbitals that have formed via sp2 hybridization, thereby creating a conjugated chain of delocalized electron density. It is the interaction of these π electrons that dictates the electronic characteristics of the polymer. The energy levels become closely spaced as the delocalization length increases, resulting in a ‘band’ structure somewhat similar to that observed in inorganic solid-state semiconductors. In contrast to the latter, however, the primary photoexcitations in conjugated polymers are bound electron-hole pairs (excitons) rather than free charge carriers; this is largely due to their low dielectric constant and the presence of significant electron-lattice interactions and electron correlation effects.4 In the absence of a mechanism to dissociate the excitons into free charge carriers, the exciton will undergo radiative and nonradiative decay, with a typical exciton lifetime in the range from 100 ps to 1 ns. Achieving efficient charge photogeneration has long been recognized as a vital challenge for molecular-based solar cells. For example, the first organic solar cells were simple single-layer devices based on the pristine polymer and two electrodes of different work function. These devices, based on a Schottky diode structure, resulted in poor photocurrent efficiency.5–7 Relatively efficient photocurrent generation in an organic device was first reported by Tang in 1986,8 employing a vacuum-deposited CuPc/ perylene derivative donor/acceptor bilayer device. The differing electron affinities (and/or ionization potentials) between these two materials created an energy offset at their interface, thereby driving exciton dissociation. However, the efficiency of such bilayer devices is limited by the requirement of exciton diffusion to the donor/acceptor interface, typically requiring film thicknesses less than the optical absorption depth. Organic materials usually exhibit exciton diffusion lengths of ∼10 nm and optical absorption depths of 100 nm, although we note significant progress is now being made with organic materials with exciton diffusion lengths comparable to or exceeding their optical absorption depth.9–12 The observation of ultrafast photoinduced electron transfer13,14 from a conjugated polymer to C60 and the * To whom correspondence should be addressed. E-mail: j.durrant@ imperial.ac.uk. Chem. Rev. 2010, 110, 6736–6767 6736

Analysis of charge photogeneration as a key determinant of photocurrent density in polymer: fullerene solar cells.

Signifi cant progress has been made in relating the voltage output of organic solar cells to materials’ properties, specifi cally to the energy difference between the donor ionisation potential and acceptor electron affi nity. [ 1–3 ] However, progress in predicting device photocurrent densities on the basis of materials or fi lm properties has proved much more problematic. Signifi cant attention has focused upon enhancing light-harvesting effi ciency by reducing the optical bandgap of the photoactive layer, as discussed in recent reviews. [ 4–6 ] Most models of device effi ciency have typically assumed a unity yield for exciton dissociation into separated charges, requiring only that the donor/acceptor LUMO level offset is greater than 0.3 eV (corresponding to the assumed exciton binding energy). In practice, these models have proved rather poor in predicting the photocurrent densities of real devices, even after processing optimization. [ 7 ] Whilst some materials (e.g. P3HT:PCBM) have indeed achieved photocurrent densities consistent with near unity internal quantum effi ciencies for photocurrent generation, most new materials (with some notable exceptions) evaluated for their performance in organic photovoltaic devices have yielded much lower photocurrent densities, and consequently poor device performance. [ 6 , 7 ] In this paper, we consider the extent to which such variations in photocurrent density can be largely understood in terms of the effi ciency of charge photogeneration. The key processes involved in charge photogeneration in organic bulk heterojunciton solar cells are illustrated in Figure 1 . By ‘charge photogeneration’ we refer to the overall process by which photon absorption leads to the generation of dissociated

Non-Langevin bimolecular recombination in a silole-based polymer:PCBM solar cell measured by time-resolved charge extraction and resistance-dependent time-of-flight techniques

The silole-based non-Langevin conjugated polymer KP115 has been used to demonstrate that circuit resistance is a crucial parameter in time-of-flight measurements of organic photovoltaic cells, providing a resistance-dependent bimolecular recombination coefficient. The origin of this behaviour is the biphasic decay dynamics present in KP115:PCBM devices (observed using a novel time-dependent charge extraction technique), which time-of-flight cannot accurately characterise.

Energy versus electron transfer in organic solar cells: a comparison of the photophysics of two indenofluorene: fullerene blend films

In this paper, we compare the photophysics and photovoltaic device performance of two indenofluorene based polymers: poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-4,7-(2,1,3-benzothiodiazole] (IF8BT) and poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiodiazole] (IF8TBTT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Photovoltaic devices made with IF8TBTT exhibit greatly superior photocurrent generation and photovoltaic efficiency compared to those made with IF8BT. The poor device efficiency of IF8BT/PCBM devices is shown to result from efficient, ultrafast singlet Forster energy transfer from IF8BT to PCBM, with the resultant PCBM singlet exciton lacking sufficient energy to drive charge photogeneration. The higher photocurrent generation observed for IF8TBTT/PCBM devices is shown to result from IF8TBTTs relatively weak, red-shifted photoluminescence characteristics, which switches off the polymer to fullerene singlet energy transfer pathway. As a consequence, IF8TBTT singlet excitons are able to drive charge separation at the polymer/fullerene interface, resulting in efficient photocurrent generation. These results are discussed in terms of the impact of donor/acceptor energy transfer upon photophysics and energetics of charge photogeneration in organic photovoltaic devices. The relevance of these results to the design of polymers for organic photovoltaic applications is also discussed, particularly with regard to explaining why highly luminescent polymers developed for organic light emitting diode applications often give relatively poor performance in organic photovoltaic devices.

Enhanced performance of dye-sensitized solar cells using carbazole-substituted di-chromophoric porphyrin dyes

The purpose of this work is to investigate the origin of improved photovoltaic performance of a series of di-chromophoric carbazole-substituted porphyrin dyes employed as sensitizers in dye-sensitized solar cells. Five di-chromophoric zinc porphyrin dyes with the same porphyrin core, a carbazole unit attached in the meso-position through a phenylethenyl linkage, and substituents spanning a range of electron affinities, in an attempt to tune the electronic level of the carbazole unit, have been synthesized (CZPs). Density functional theory (DFT) calculations predicted the nature of the electronic transitions observed in the CZP systems, showing a large degree of orbital mixing. In contrast, UV-vis absorption, resonance Raman spectroscopy and differential pulse voltammetry investigations suggested negligible interaction between the porphyrin and carbazole chromophores. Carbazole substitution led to a moderate increase in photon absorption intensity within the ∼300 nm to 400 nm wavelength region, a smaller but broader Soret band absorption and slightly increased photon absorption intensity in the 550 nm to 650 nm Q band region. Despite the rather small changes in light harvesting and negligible changes in the HOMO/LUMO electronic levels, the photovoltaic performance of the new dyes is increased by as much as 30% compared to the single chromophore Zn porphyrin dye 5-(4-(2-cyano-2-carboxyethenyl)phenyl-15-phenyl-10,20-bis(2,4,6-trimethylphenyl)porphyrinato zinc(II) (ZP1), leading to over 6% power conversion efficiencies (PCEs). Both open circuit voltage (VOC) and short circuit current (JSC) have increased. The increased VOC is attributed to increased electron lifetimes due to a steric blocking effect. Analysis of the increased short circuit current (ΔJSC) showed that only less than 10% of ΔJSC originates from increased light absorption under simulated air mass 1.5 illumination, while the rest of the improvements are attributed to a steric effect enhancing the electron injection efficiency. These results suggest that developing non-conjugated multichromophoric dyes can lead to simultaneous increases in both the photocurrent and the photovoltage of dye-sensitized solar cells.

An intermediate band dye-sensitised solar cell using triplet–triplet annihilation

A new mechanism of charge photogeneration is demonstrated for the first time, based on organic molecular structures. This intermediate band approach, integrated into a dye-sensitised solar cell configuration is shown to generate charges upon illumination with low energy photons. Specifically 610 nm photoexcitation of Pt porphyrins, through a series of energy transfer steps and triplet-triplet annihilation, excites a higher energy absorption onset molecule, which is then capable of charge injection into TiO2. Transient absorption measurements reveal further detail of the processes involved.

The role of emissive charge transfer states in two polymer–fullerene organic photovoltaic blends: tuning charge photogeneration through the use of processing additives

The role of charge transfer (CT) states in organic photovoltaic systems has been debated in the recent literature. In this paper the device performances of two structurally analogous polymers PDTSiTTz (also known as KP115) and PCPDTTTz blended with PCBM are investigated, focusing on the effect the processing additive diiodooctane (DIO) has on morphology, charge photogeneration, and, in particular, the CT state characteristics. While DIO has a considerable beneficial effect for PCPDTTTz:PCBM photovoltaic devices, negligible effects are observed for PDTSiTTz:PCBM devices. An emissive CT state able to be quenched by DIO was observed for PCPDTTTz:PCBM, despite relatively small morphological changes. This is only the second instance of CT state quenching by a processing additive to be reported. Formation of an emissive CT state is therefore a loss pathway for PCPDTTTz:PCBM, which can be alleviated through the use of DIO to increase the proportion of CT states that dissociate into free charges. Conversely, the CT state of PDTSiTTZ:PCBM is weak and short-lived, with the DIO having little effect. The CT state dissociates more efficiently for this higher crystallinity system, leading to less evidence of emissive CT state recombination, and high charge photogeneration yields and device efficiencies.

Synthesis and photo-induced charge separation of confined conjugation length phenylene vinylene-based polymers

We report the synthesis and photophysics of four confined chromophore phenylene vinylene (PPV)-based polymers where PPV-type trimer chromophores are linked by alkyl ether chains of increasing length (PT3–PT6). These confined chromophore polymers were blended in a 1 : 1 w/w ratio with the fullerene derivative PCBM and the photoinduced charge-separation properties compared to that of a fully conjugated polymer analogue (alt-co-MEH-PPV). Fluorescence from all polymers is quenched in the presence of PCBM and transient absorption measurements confirm the presence of polymer polarons arising from charge transfer from the polymer to the PCBM acceptor. The polaron decay kinetics of all the blend films were similar but the yields of polarons for the confined conjugation length polymers were lower than for alt-co-MEH-PPV:PCBM. These observations suggest that many of the initially formed radical ion pairs for the confined length polymers are unable to fully charge dissociate and undergo fast geminate recombination.

Charge photogeneration in donor/acceptor organic solar cells

We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.

Enhancement of dye regeneration kinetics in dichromophoric porphyrin–carbazole triphenylamine dyes influenced by more exposed radical cation orbitals

Reduction kinetics of oxidized dyes absorbed on semiconductor surfaces and immersed in redox active electrolytes has been mainly modeled based on the free energy difference between the oxidation potential of the dye and the redox potential of the electrolyte. Only a few mechanisms have been demonstrated to enhance the kinetics by other means. In this work, the rate constant of the reduction of oxidized porphyrin dye is enhanced by attaching non-conjugated carbazole triphenylamine moiety using iodine/triiodide and tris(2,2′-bispyridinium)cobalt II/III electrolytes. These results are obtained using transient absorption spectroscopy by selectively probing the regeneration kinetics at the porphyrin radical cation and the carbazole triphenylamine radical cation absorption wavelengths. The enhancement in the reduction kinetics is not attributed to changes in the driving force, but to the more exposed dye cation radical orbitals of the dichromophoric dye. The results are important for the development of high efficiency photo-electrochemical devices with minimalized energy loss at electron transfer interfaces.