Safa Shoaee

Charge generation and transport in efficient organic bulk heterojunction solar cells with a perylene acceptor

The origin of high current density in efficient non-fullerene based bulk heterojunction (BHJ) organic solar cells employing a non-planar perylene dimer (TP) as an electron acceptor and a thiophene based donor polymer PBDTTT-CT is investigated using electrical and optical techniques. Photoluminescence measurements reveal almost complete quenching of both the donor and acceptor excitons, indicating efficient electron and hole transfer processes. The nanomorphology of the films shows fine mixing of the donor polymer and TP at 50 : 50% weight ratio with a photon to current conversion efficiency (IPCE) of 45% in the visible regime. At the donor–acceptor interface, both polymer and TP excitons undergo fast dissociation with similar time scales of a few picoseconds. The magnitude of the polaron yield of PBDTTT-CT:TP blends is observed to be comparable to that of PBDTTT-CT:PC70BM blends and exhibits similar μs-decay dynamics. A power conversion efficiency of 3.2% is achieved for devices with 50 : 50% by weight compositional ratio of polymer and TP.

On the role of intermixed phases in organic photovoltaic blends

Recently, an intermixed phase has been identified within organic photovoltaic (OPV) bulk heterojunction (BHJ) systems that can exist in addition to relatively phase-pure regions, highlighting the need for a refined picture of the solid-state microstructure of donor–acceptor blends and for gaining further understanding of the exact nature and role such intermixed phases play in such devices. Here we manipulate the microstructure of polymer–fullerene systems via processing means and the selection of the molecular weight of the donor polymer. This manipulation is used as a tool to vary the fraction of intermixed phase present and its effects on the structure and subsequently the opto-electronic processes. We find clear relationships between the state of mixing and amount of exciton quenching and number of polarons generated per absorbed photon. Furthermore, we observe that blend systems incorporating higher molecular weight polymer result in a greater yield of dissociated polarons, likely due to the increase of the intermixed fraction.

Acoustic Enhancement of Polymer/ZnO Nanorod Photovoltaic Device Performance

Acoustic vibrations are shown to enhance the photovoltaic efficiency of a P3HT/ZnO nanorod solar cell by up to 45%, correlated to a three-fold increase in charge carrier lifetime. This is assigned to the generation of piezoelectric dipoles in the ZnO nanorods, indicating that the efficiency of solar cells may be enhanced in the presence of ambient vibrations by the use of piezoelectric materials.

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

Influence of nanoscale phase separation on geminate versus bimolecular recombination in P3HT:fullerene blend films

In this paper, we compare the charge recombination dynamics observed in films comprising poly(3-hexylthiophene) 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 bimolecular 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 which 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.

Additive-assisted supramolecular manipulation of polymer:fullerene blend phase morphologies and its influence on photophysical processes

It is well known that even small variations in the solid-state microstructure of polymer:fullerene bulk heterojunctions can drastically change their organic solar cell device performance. We employ pBTTT:PC61BM as a model system and manipulate co-crystal formation of 1 : 1 (by weight) blends with the assistance of fatty acid methyl esters as additives. This allows us to evaluate the role of the intermixed phase in such binary blends through manipulation of their phase morphology—from fully intercalated to partially and predominantly non-intercalated systems—and its effect on the exciton- and carrier- dynamics and the efficiency of charge collection, with relevance for future device design and manufacturing.

Slower carriers limit charge generation in organic semiconductor light-harvesting systems

Blends of electron-donating and -accepting organic semiconductors are widely used as photoactive materials in next-generation solar cells and photodetectors. The yield of free charges in these systems is often determined by the separation of interfacial electron–hole pairs, which is expected to depend on the ability of the faster carrier to escape the Coulomb potential. Here we show, by measuring geminate and non-geminate losses and key transport parameters in a series of bulk-heterojunction solar cells, that the charge-generation yield increases with increasing slower carrier mobility. This is in direct contrast with the well-established Braun model where the dissociation rate is proportional to the mobility sum, and recent models that underscore the importance of fullerene aggregation for coherent electron propagation. The behaviour is attributed to the restriction of opposite charges to different phases, and to an entropic contribution that favours the joint separation of both charge carriers.

A comparison of charge separation dynamics in organic blend films employing fullerene and perylene diimide electron acceptors

We report a comparison of charge carrier dynamics and device performance for low band gap polymer PBDTTT-CT in blends with the fullerene acceptor PC71BM and a PDI derivative with similar electron affinities. Charge separation and recombination dynamics are found to be remarkably similar for these two acceptors, with both blends exhibiting efficient, ultrafast charge separation (time constants of 1.6 and 1.4 ps, respectively). The lower device performance for the PDI acceptor (1.75% compared to 3.5% for the equivalent PC71BM device) is shown to result from slower charge transport, increasing nongeminate recombination losses during charge collection.

Impact of concentration self-quenching on the charge generation yield of fullerene based donor-bridge-acceptor compounds in the solid state.

A fullerene based Donor-Bridge-Acceptor (DBA) compound, incorporating a π-extended tetrathiafulvalene electron donor, is investigated with respect to its photophysics in solution versus solid state. Solid films of neat DBA are compared with blend films where the DBA compound is diluted in the inert, low dielectric, polymer poly(styrene). It is found that the moderate intermolecular electronic coupling and donor-acceptor separation (22 Å) in this case leads to the generation of more dissociated, intermolecular charges than a mixture of the donor and acceptor reference compounds. However, the increased intermolecular interactions in the solid state lead to the excited state of the fullerene suffering from concentration self-quenching. This is found to severely affect the charge generation yield in solid films. The impact of competing intra and intermolecular interactions in the solid state upon the film photophysics is analysed in terms of a kinetic model which includes both the effects of concentration self-quenching and the impact of film composition upon the dielectric stabilisation of charge separated states. We conclude that both concentration self-quenching and dielectric stabilisation are critical in determining the photophysics of the blend films, and discuss strategies based upon our observations to enhance the charge photogeneration properties of organic films and photovoltaic devices based upon DBA compounds.

Oxygen diffusion dynamics in organic semiconductor films

Transient absorption spectroscopy is commonly used to probe the yield and kinetics of excited states of materials. We present a transient absorption spectroscopic assay of oxygen diffusion in a series of solution-processed polymer films. The films were partially encapsulated with an epoxy/glass top barrier as a simple model system for organic photovoltaic and light emitting devices with metal top contacts. The results presented herein show that this spectroscopic approach can be a versatile and quantitative in situ assay of local oxygen concentrations in such organic semiconductor films. With our current apparatus, the approach has a time resolution of 5 seconds, thereby enabling direct measurement of oxygen diffusion kinetics into a semiconductor film. The versatility of this approach suggests it could be widely applicable to measurement of oxygen diffusion into organic optoelectronic devices, including for example oxygen diffusion through encapsulation and barrier layers. Employing this approach, we demonstrate significant differences in oxygen diffusion kinetics between different semiconducting polymers. We furthermore demonstrate the impact of an additional getter (ZnO) and light exposure upon the local oxygen concentration, providing new insights into the role of oxygen diffusion kinetics in determining the environmental stability of organic semiconductors.