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...

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.

The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10–50 nm wide and 200–400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

Six techniques are used to measure the exciton diffusion length as a function of systematic chemical modifications.

The effect of the extent of electron conjugation on the primary photophysics in semiconducting polymers is reported. A rapid depolarization of photoluminescence and transient absorption, which indicates a reorientation of the transition dipole moment by ϳ30°on a sub-100 fs time scale, is observed in the fully conjugated polymer poly͓2-͑2'-ethylhexyloxy͒-5-methoxy-1,4-phenylenevinylene͔ ͑MEH-PPV͒. In contrast, partially conjugated polymers exhibit a much slower depolarization. The results reveal rapid changes of exciton delocalization in the fully conjugated MEH-PPV driven by structural relaxation.

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Articles you may be interested inCorrelation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes J. Chem. Phys. 139, 204306 (2013) In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo-simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV.

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