Polymer solar cells are a promising technology for future power generation. In particular the polymer poly (3-hexylthiophene) (P3HT) has attracted widespread interest with power conversion efficiencies close to 5%. [1][2][3][4] Such devices employ the bulk heterojunction device structure, where the polymer is blended with a strong charge acceptor such as a fullerene. The processing of the blend affects its morphology and the formation of domains of P3HT-rich and fullerene-rich regions. It is at the interface between these two domains that excitons are dissociated into their constituent charges, a critical step in the operation of a solar cell. Excitons are only able to diffuse a short distance during their lifetime and therefore the size of the domains should ideally be on the order of the diffusion length, maximising the number of excitons reaching the interface and undergoing dissociation. The development of accurate measurements of the exciton diffusion length is therefore important for organic photovoltaics and the optimisation of materials, processing and device structure. To date there has been a wide range of reported values for different materials obtained by techniques such as surface quenching, [5][6][7][8] volume quenching, [9,10] microwave conductivity, [11] exciton-exciton annihilation [12,13] and photocurrent modelling of solar cells. [14] Of these the surface quenching technique is probably the most used, where the organic material is deposited onto a suitable quencher, resulting in a loss of luminescence. This loss can be quantified by comparing the emission from the quenched film with that of an identical film on a non-quenching substrate and will depend on the diffusion coefficient and the thickness of the organic film. This experiment can be performed via steady-state or time-resolved with most employing the former. However, a problem in steady-state measurements is that interference effects in the layer structures used can strongly modify the amount of light absorbed. [7] Time-resolved techniques do not require absolute measurements of the luminescence as it is only the decay of the emission from the material that is actually needed, though the initial excitation profile is influenced by optical interference. Exciton diffusion in polymers occurs on a time range from 1 ps to $1 ns, [15] thus any measurements should aim to cover this range. In this communication we will describe how time-resolved measurements of fluorescence, coupled with an appropriate quencher, enable robust measurements of the diffusion coefficient. We have applied this technique to the polymer P3HT, which despite being the most used polymer in organic photovoltaic research, has had little published on its exciton diffusion. Kroeze et al.[11] reported a value for the diffusion length of 2.6-5.3 nm from time-resolved microwave conductivity measurements, depending on whether or not excitons were reflected at the polymer/air interface. Using oxygen-induced fluorescence quenching in P3HT, Lü er et al. [9] obtained a minimum value...

Organic photovoltaic (OPV) cells have attracted substantial commercial and scientifi c interest as they provide a pathway for a renewable energy source that is portable and potentially inexpensive. [ 1,2 ] Current state-of-the-art solution-processed OPV cells are based on the so-called bulk heterojunction (BHJ) architecture, consisting of an active layer that is a "blend" between an electron donor and acceptor forming an interpenetrating network structure due to self-organized phase separation. [3][4][5] Compared to a simpler planar heterojunction (i.e., a bilayer architecture), the BHJ geometry provides a greater total surface area for charge separation and reduced recombination, and these advantages should theoretically translate to dramatically enhanced power conversion effi ciency. [ 6,7 ] BHJ cells based on a donoracceptor pair of poly(3-n -hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) have shown effi ciencies approaching 5%, [ 1 ] while effi ciencies of 6.77% have been reported with "low-bandgap" polymer donors. [ 8 ] Given that blending and morphology are critical for device performance, signifi cant research effort has focused on studying the phase separation mechanism and optimum conditions to permit effi cient splitting of excitons into charge carriers. [9][10][11][12] This is widely acknowledged as a delicate balance between forming the interpenetrating network and excessive phase segregation causing carrier traps. [ 13 ] Additionally, due to the short exciton diffusion length in organic semiconductors (approximately 10 nm), [ 14 ] the nanoscale morphology presents characterization and processing challenges. [15][16][17] In contrast, sequential processing to form the active bilayer is conceptually more straightforward. Additionally, as discussed in Ayzner et al. [ 18 ] sequential processing allows the use of organic electron acceptors that may not survive thermal evaporation and in principle allows each layer to be controlled and optimized independently. Such a strategy also lends itself more readily to multiple junction devices which are widely believed to be the pathway for OPV cells to be competitive with inorganic systems (conversion effi ciencies > 10%). However, until very recently, no solution processed bilayer device has performed anywhere near as well as its BHJ counterpart. [ 19 ] In this regard, Ayzner et al. [ 18 ] have reignited interest (and much debate) by reporting all-solution-processed P3HT/PCBM bilayer cells with performance approaching that of a BHJ. This performance was somewhat surprising given the individual active layer thicknesses were larger than the expected exciton diffusion lengths in either component and the planar heterojunction yields a reduced interfacial area for exciton separation. Motivated by these potentially very signifi cant, yet somewhat controversial observations, we have undertaken a detailed study of the morphology of solution processed P3HT/ PCBM devices formed by sequential layer deposition using a very similar approa...

Photocurrent generation in organic bulk heterojunction (BHJ) solar cells is most commonly understood as a process which predominantly involves photoexcitation of the lower ionization potential species (donor) followed by electron transfer to the higher electron affinity material (acceptor) [i.e., photoinduced electron transfer (PET), which we term Channel I]. A mirror process also occurs in which photocurrent is generated through photoexcitation of the acceptor followed by hole transfer to the nonexcited donor or photoinduced hole transfer (PHT), which we term Channel II. The role of Channel II photocurrent generation has often been neglected due to overlap of the individual absorption spectra of the donor and acceptor materials that are commonly used. More recently Channel II charge generation has been explored for several reasons. First, many of the new high-efficiency polymeric donors are used as the minority component in bulk heterojunction blends, and therefore, the acceptor absorption is a significant fraction of the total; second, nonfullerene acceptors have been prepared, which through careful design, allow for spectral separation from the donor material, facilitating fundamental studies on charge generation. In this article, we review the methodologies for investigating the two charge generation channels. We also discuss the factors that affect charge generation via Channel I and II pathways, including energy levels of the materials involved, exciton diffusion, and other considerations. Finally, we take a comprehensive look at the nonfullerene acceptor literature and discuss what information about Channel I and Channel II can be obtained from the experiments conducted and what other experiments could be undertaken to provide further information about the operational efficiencies of Channels I and II.

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.

Singlet–singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of β‐phase chains using time‐resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time‐independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time‐dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the β‐phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low‐energy sites. The annihilation rate in the mixed‐phase films increases with increasing fraction of β‐phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersedβ‐phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line‐dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy andβ‐phase chromophores are intimately mixed rather than clustered or phase‐separated.

The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes.

A blue-emitting distributed feedback laser based on a star-shaped oligofluorene truxene molecule is presented. The gain, loss, refractive index, and (lack of) anisotropy are measured by amplified spontaneous emission and variable-angle ellipsometry. The waveguide losses are very low for an organic semiconductor gain medium, particularly for a neat film. The results suggest that truxenes are promising for reducing loss, a key parameter in the operation of organic semiconductor lasers. Distributed feedback lasers fabricated from solution by spin-coating show a low lasing threshold of 270 W/cm(2) and broad tunability across 25 nm in the blue part of the spectrum

We have investigated a series of branched fluorescent sensing compounds with thiophene units in the arms and triphenylamine centers for the detection of nitrated model compounds for 2,4,6-trinitrotoluene (TNT) and the plastic explosives taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB). Stern−Volmer measurements in solution show that the fluorescence is more efficiently quenched by nitroaromatic compounds when compared to a non-nitrated quencher, benzophenone. Simple modification of the structure of the sensing compound was found to result in significant changes to the sensitivity and selectivity toward the nitrated analytes. A key result from time-resolved fluorescent measurements showed that the chromophore−analyte interaction was primarily a collisional process. This process is in contrast to conjugated polymers where static quenching dominates, a difference that could offer a potentially more powerful detection mechanism.