Free carrier generation in organic photovoltaic bulk heterojunctions of conjugated polymers with molecular acceptors: planar versus spherical acceptors.

A comparative study of the photophysical performance of the prototypical fullerene derivative PC61BM with a planar small-molecule acceptor in an organic photovoltaic device is presented. The small-molecule planar acceptor is 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile, termed K12. We discuss photoinduced free charge-carrier generation and transport in blends of PC61BM or K12 with poly(3-n-hexylthiophene) (P3HT), surveying literature results for P3HT:PC61BM and presenting new results on P3HT:K12. For both systems we also review previous work on film structure and correlate the structural and photophysical results. In both cases, a disordered mixed phase is formed between P3HT and the acceptor, although the photophysical properties of this mixed phase differ markedly for PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region that can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC61BM blends yield free carriers detected by the contactless time-resolved microwave conductivity (TRMC) method. In contrast, approximately 85% of the excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron-transport properties in the mixed P3HT:K12 phase. We propose that the observed differences can be traced to the respective shapes of PC61BM and K12: the three-dimensional nature of the fullerene cage facilitates coupling between PC61BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their π systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC61BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub-percolating volume fraction of the planar acceptor. Furthermore, the planar K12 co-assembles with P3HT into a disordered, glassy phase that partly accounts for the poor electron-transport properties, and may also enhance recombination due to the strong intermolecular interactions between the donor and the acceptor. The implication for the performance of organic photovoltaic devices with the two acceptors is also discussed.

Photovoltaic devices with a low band gap polymer and CdSe nanostructures exceeding 3% efficiency.

We report the fabrication and measurement of solar cells approaching a power conversion efficiency of 3.2% using a low band gap conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] and CdSe nanoparticles. These devices exhibit an external quantum efficiency (EQE) of >30% in a broad range of 350-800 nm with a maximum EQE of 55% in a range of 630-720 nm. We also present certified device efficiencies of 3.13% under AM 1.5 illumination.

Prolonging charge separation in P3HT-SWNT composites using highly enriched semiconducting nanotubes.

Single-walled carbon nanotubes (SWNTs) have potential as electron acceptors in organic photovoltaics (OPVs), but the currently low-power conversion efficiencies of devices remain largely unexplained. We demonstrate effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into poly(3-hexylthiophene) (P3HT). We use these enriched blends to provide the first experimental evidence of the negative impact of metallic nanotubes. Time-resolved microwave conductivity reveals that the long-lived carrier population can be significantly increased by incorporating highly enriched semiconducting SWNTs into semiconducting polymer composites.

Endohedral fullerenes for organic photovoltaic devices.

So far, one of the fundamental limitations of organic photovoltaic (OPV) device power conversion efficiencies (PCEs) has been the low voltage output caused by a molecular orbital mismatch between the donor polymer and acceptor molecules. Here, we present a means of addressing the low voltage output by introducing novel trimetallic nitride endohedral fullerenes (TNEFs) as acceptor materials for use in photovoltaic devices. TNEFs were discovered in 1999 by Stevenson et al. ; for the first time derivatives of the TNEF acceptor, Lu(3)N@C(80), are synthesized and integrated into OPV devices. The reduced energy offset of the molecular orbitals of Lu(3)N@C(80) to the donor, poly(3-hexyl)thiophene (P3HT), reduces energy losses in the charge transfer process and increases the open circuit voltage (Voc) to 260 mV above reference devices made with [6,6]-phenyl-C(61)-butyric methyl ester (C(60)-PCBM) acceptor. PCEs >4% have been observed using P3HT as the donor material. This work clears a path towards higher PCEs in OPV devices by demonstrating that high-yield charge separation can occur with OPV systems that have a reduced donor/acceptor lowest unoccupied molecular orbital energy offset.

Direct synthesis of CdSe nanoparticles in poly(3-hexylthiophene).

A new approach was developed for the synthesis of nearly monodisperse CdSe nanoparticles directly in a polymer-containing solution in the absence of any other surface capping molecules. The comparatively high synthesis temperature reaction produces good quality crystalline CdSe nanoparticles. Time-resolved microwave conductivity measurements show that photoinduced charge separation occurs at the interface between the CdSe quantum dots and the polymer. This method can be extended to the synthesis of other II-VI semiconductor nanomaterials directly in a polymer-containing solution.

Exciton migration in conjugated dendrimers: a joint experimental and theoretical study.

We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials.

Application of an A-A'-A-Containing Acceptor Polymer in Sequentially Deposited All-Polymer Solar Cells.

PNNT has been prepared as a polymeric electron acceptor for organic solar cells. The polymer has an A-A'-A acceptor motif linked alternatively with thiophene and vinyl moieties. The A'-unit is a naphthalene diimide, while the A groups are thiazoles. PNNT films were found to have an estimated electron affinity of ≈4.3 eV and an electron mobility of the order of 10-4 cm2 V-1 s-1. Its relatively low solubility in common chlorinated solvents at ambient temperature allowed the manufacture of sequentially deposited (SD) devices, which were found to have significantly higher efficiency than that of bulk heterojunction (BHJ) solar cells containing the same materials. Grazing-incidence wide-angle X-ray scattering measurements indicated that the SD films retained the ordering of the individual polymers to a greater extent compared to the BHJ films. The best SD devices were found to have a power conversion efficiency of up to 4.5%, with stable performance under thermal stress.

Effect of an adsorbent on recombination and band-edge movement in dye-sensitized TiO2 solar cells: evidence for surface passivation.

The mechanism by which the adsorbent guanidinium affects the open-circuit photovoltage of dye-sensitized TiO2 nanocrystalline solar cells was investigated. The influence of the guanidinium cation on the rate of recombination and band-edge movement was measured by transient photovoltage. When guanidinium is present in the electrolyte recombination becomes slower by a factor of about 20. At the same time, the adsorbent causes the band edges to move downward, toward positive electrochemical potentials, by 100 mV. The collective effect of both a downward shift of the band edges and slower recombination, owing to the presence of guanidinium, results in an overall improvement in the open-circuit photovoltage.

Influence of surface area on charge transport and recombination in dye-sensitized TiO2 solar cells.

The dependence of the electron transport and recombination dynamics on the internal surface area of mesoporous nanocrystalline TiO2 films in dye-sensitized solar cells was investigated. The internal surface area was varied by altering the average particle size in the films. The scaling of the photoelectron density and the electron diffusion coefficient at short circuit with internal surface area confirms the results of a recent study (Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Appl. Phys. Lett. 2005, 87, 202106) that transport-limiting traps are located predominately on the surfaces of the particles. The recombination current density was found to increase superlinearly (with an exponent of 1.40 +/- 0.12) with the internal surface area. This result is at odds with the expected linear dependence of the recombination current density on the surface area when only the film thickness is increased. The observed scaling of the recombination current density with surface area is consistent with recombination being transport-limited. Evidence is also presented confirming that photoinjected electrons recombine with redox species in the electrolyte via surface states rather than from the TiO2 conduction band.

Structure and Conductivity of Semiconducting Polymer Hydrogels.

Poly(fluorene-alt-thiophene) (PFT) is a conjugated polyelectrolyte that self-assembles into rod-like micelles in water, with the conjugated polymer backbone running along the length of the micelle. At modest concentrations (∼10 mg/mL in aqueous solutions), PFT forms hydrogels, and this work focuses on understanding the structure and intermolecular interactions in those gel networks. The network structure can be directly visualized using cryo electron microscopy. Oscillatory rheology studies further tell us about connectivity within the gel network, and the data are consistent with a picture where polymer chains bridge between micelles to hold the network together. Addition of tetrahydrofuran (THF) to the gels breaks those connections, but once the THF is removed, the gel becomes stronger than it was before, presumably due to the creation of a more interconnected nanoscale architecture. Small polymer oligomers can also passivate the bridging polymer chains, breaking connections between micelles and dramatically weakening the hydrogel network. Fits to solution-phase small-angle X-ray scattering data using a Dammin bead model support the hypothesis of a bridging connection between PFT micelles, even in dilute aqueous solutions. Finally, time-resolved microwave conductivity measurements on dried samples show an increase in carrier mobility after THF annealing of the PFT gel, likely due to increased connectivity within the polymer network.

Effect of a coadsorbent on the performance of dye-sensitized TiO2 solar cells: shielding versus band-edge movement.

The mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, alters the open-circuit photovoltage and short-circuit current of dye-sensitized solar cells was investigated. The influence of tetrabutylammonium chenodeoxycholate on dye loading was studied under a variety of conditions in which the TiO2 films were exposed to the sensitizing dye and coadsorbent. Photocurrent--voltage measurements combined with desorption studies revealed that adding chenodeoxycholate reduces the dye loading by as much as 60% while having a relatively small effect on the short-circuit photocurrent. Calculations along with measurements showed that even at low loading, enough dye is present to absorb a significant fraction of the incident light in the visible spectrum. In concurrence with the observations of others, we find evidence for weakly and strongly adsorbed forms of the dye resulting from either different binding conformations or aggregates. The most strongly adsorbed dyes are less susceptible to displacement by chenodeoxycholate than those that are weakly adsorbed. While having no observable effect on dye coverage, multiple exposures of a TiO2 film to a dye solution substantially increased the fraction of strongly adsorbed dye as judged by the resistance of the adsorbed dye to displacement by chenodeoxycholate. Measurements of the open-circuit voltage as a function of the photocharge density, determined by infrared transmittance, showed that chenodeoxycholate not only shifts the conduction band edge to negative potentials, but also significantly increases the rate of recombination. The net effect of adding chenodeoxycholate is, however, to improve the photovoltage.

Narrowband light detection via internal quantum efficiency manipulation of organic photodiodes.

Spectrally selective light detection is vital for full-colour and near-infrared (NIR) imaging and machine vision. This is not possible with traditional broadband-absorbing inorganic semiconductors without input filtering, and is yet to be achieved for narrowband absorbing organic semiconductors. We demonstrate the first sub-100 nm full-width-at-half-maximum visible-blind red and NIR photodetectors with state-of-the-art performance across critical response metrics. These devices are based on organic photodiodes with optically thick junctions. Paradoxically, we use broadband-absorbing organic semiconductors and utilize the electro-optical properties of the junction to create the narrowest NIR-band photoresponses yet demonstrated. In this context, these photodiodes outperform the encumbent technology (input filtered inorganic semiconductor diodes) and emerging technologies such as narrow absorber organic semiconductors or quantum nanocrystals. The design concept allows for response tuning and is generic for other spectral windows. Furthermore, it is material-agnostic and applicable to other disordered and polycrystalline semiconductors.

Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te(2-) Ligands Lead To Charge Carrier Diffusion Lengths Over 2 µm.

We report on photoconductivity of films of CdTe nanocrystals (NCs) using time-resolved microwave photoconductivity (TRMC). Spherical and tetrapodal CdTe NCs with tunable size-dependent properties are studied as a function of surface ligand (including inorganic molecular chalcogenide species) and annealing temperature. Relatively high carrier mobility is measured for films of sintered tetrapod NCs (4 cm(2)/(V s)). Our TRMC findings show that Te(2-) capped CdTe NCs show a marked improvement in carrier mobility (11 cm(2)/(V s)), indicating that NC surface termination can be altered to play a crucial role in charge-carrier mobility even after the NC solids are sintered into bulk films.

Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor-Acceptor Conjugated Polymers.

Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

A faux hawk fullerene with PCBM-like properties.

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)- can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F-. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C-F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/-) values of 2 and C60, -0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/-) of PCBM is -0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene-fullerene interactions with centroid···centroid (⊙···âŠ™) distances of ≤10.34 Å is significantly greater, and the average ⊙···âŠ™ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙···âŠ™ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙···âŠ™ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

Semi-random vs Well-Defined Alternating Donor-Acceptor Copolymers.

The influence of backbone composition on the physical properties of donor-acceptor (D-A) copolymers composed of varying amounts of benzodithiophene (BDT) donor with the thienoisoindoledione (TID) acceptor is investigated. First, the synthesis of bis- and tris-BDT monomers is reported; these monomers are subsequently used in Stille copolymerizations to create well-defined alternating polymer structures with repeating (D-A), (D-D-A), and (D-D-D-A) units. For comparison, five semi-random D-A copolymers with a D:A ratio of 1.5, 2, 3, 4, and 7 were synthesized by reacting trimethyltin-functionalized BDT with various ratios of iodinated BDT and brominated TID. While the HOMO levels of all the resultant polymers are very similar, a systematic red shift in the absorbance spectra onset of the D-A copolymer films from 687 to 883 nm is observed with increasing acceptor content, suggesting the LUMO can be fine-tuned over a range of 0.4 eV. When the solid-state absorbance spectra of well-defined alternating copolymers are compared to those of semi-random copolymers with analogous D:A ratios, the spectra of the alternating copolymers are significantly more red-shifted. Organic photovoltaic device efficiencies show that the semi-random materials all outperform the well-defined alternating copolymers, and an optimal D:A ratio of 2 produces the highest efficiency. Additional considerations concerning fine-tuning the lifetimes of the photoconductance transients of copolymer:fullerene films measured by time-resolved microwave conductivity are discussed. Overall, the results of this work indicate that the semi-random approach is a powerful synthetic strategy for fine-tuning the optoelectronic and photophysical properties of D-A materials for a number of systematic studies, especially given the ease with which the D:A ratios in the semi-random copolymers can be tuned.

Heterojunction modification for highly efficient organic-inorganic perovskite solar cells.

Organic-inorganic perovskites, such as CH3NH3PbX3 (X=I, Br, Cl), have emerged as attractive absorber materials for the fabrication of low cost high efficiency solar cells. Over the last 3 years, there has been an exceptional rise in power conversion efficiencies (PCEs), demonstrating the outstanding potential of these perovskite materials. However, in most device architectures, including the simplest thin-film planar structure, a current-voltage response displays an "anomalous hysteresis", whereby the power output of the cell varies with measurement time, direction and light exposure or bias history. Here we provide insight into the physical processes occurring at the interface between the n-type charge collection layer and the perovskite absorber. Through spectroscopic measurements, we find that electron transfer from the perovskite to the TiO2 in the standard planar junction cells is very slow. By modifying the n-type contact with a self-assembled fullerene monolayer, electron transfer is "switched on", and both the n-type and p-type heterojunctions with the perovskite are active in driving the photovoltaic operation. The fullerene-modified devices achieve up to 17.3% power conversion efficiency with significantly reduced hysteresis, and stabilized power output reaching 15.7% in the planar p-i-n heterojunction solar cells measured under simulated AM 1.5 sunlight.

Photoinduced electron transfer in composites of conjugated polymers and dendrimers with branched colloidal nanoparticles.

Charge generation and separation dynamics in donor:acceptor systems based on composites of branched CdSe nanoparticles with a phenyl-cored thiophene-containing dendrimer (4G1-3S), or a low-bandgap conjugated polymer (PCPDTBT) are reported upon exclusive excitation of the donor or the acceptor. Time-resolved microwave conductivity is used to study the dynamics of either transfer of holes from the nanoparticle to dendrimer, or conversely the transfer of electrons from the polymer to the nanoparticle. Higher photoconductance signals and longer decay-times are correlated with device efficiencies, where composites with higher nanoparticle concentration exhibit higher solar photovoltaic power conversion efficiencies and an increase in external quantum efficiencies. This work evaluates the contribution of both components to device performance, but specifically the role of photoexcited nanoparticles.

Photoinduced carrier generation and decay dynamics in intercalated and non-intercalated polymer:fullerene bulk heterojunctions.

The dependence of photoinduced carrier generation and decay on donor-acceptor nanomorphology is reported as a function of composition for blends of the polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C(14)) with two electron-accepting fullerenes: phenyl-C(71)-butyric acid methyl ester (PC(71)BM) or the bisadduct of phenyl-C(61)-butyric acid methyl ester (bis-PC(61)BM). The formation of partially or fully intercalated bimolecular crystals at weight ratios up to 1:1 for pBTTT-C(14):PC(71)BM blends leads to efficient exciton quenching due to a combination of static and dynamic mechanisms. At higher fullerene loadings, pure PC(71)BM domains are formed that result in an enhanced free carrier lifetime, as a consequence of spatial separation of the electron and hole into different phases, and the dominant contribution to the photoconductance comes from the high-frequency electron mobility in the fullerene clusters. In the pBTTT-C(14):bis-PC(61)BM system, phase separation results in a non-intercalated structure, independent of composition, which is characterized by exciton quenching that is dominated by a dynamic process, an enhanced carrier lifetime and a hole-dominated photoconductance signal. The results indicate that intercalation of fullerene into crystalline polymer domains is not detrimental to the density of long-lived carriers, suggesting that efficient organic photovoltaic devices could be fabricated that incorporate intercalated structures, provided that an additional pure fullerene phase is present for charge extraction.

Photovoltaic charge generation in organic semiconductors based on long-range energy transfer.

For efficient charge generation in organic solar cells, photogenerated excitons must migrate to a donor/acceptor interface where they can be dissociated. This migration is traditionally presumed to be based on diffusion through the absorber material. Herein we study an alternative migration route--two-step exciton dissociation--whereby the exciton jumps from the donor to acceptor before charge creation takes place. We study this process in a series of multilayer donor/barrier/acceptor samples, where either poly(3-hexylthiophene) (P3HT) or copper phthalocyanine (CuPc) is the donor, fullerene (C60) is the acceptor, and N,N-diphenyl-N,N-bis(3-methylphenyl)-[1,1-bisphenyl]-4,4-diamine (TPD) acts as a barrier to energy transfer. By varying the thickness of the barrier layer, we find that energy transfer from P3HT to C60 proceeds over large distances (∼50% probability of transfer across a 11 nm barrier), and that this process is consistent with long-range Förster resonance energy transfer (FRET). Finally, we demonstrate a fundamentally different architecture concept that utilizes the two-step mechanism to enhance performance in a series of P3HT/CuPc/C60 devices.