Daniel A. M. Egbe

Photovoltaic action of conjugated polymer/fullerene bulk heterojunction solar cells using novel PPE-PPV copolymers

The design of novel conjugated polymers suitable for use in plastic solar cells is one of todays challenges aiming towards improved key properties like the increase of photocurrent and open circuit voltage of such devices. In this work we present first results on arylene-ethynylene/arylene-vinylene hybrid polymers 3 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and 5 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), demonstrating photovoltaic action in combination with the soluble C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 (PCBM). Devices with an active layer thickness of about 100 nm yielded power conversion efficiencies of up to 2% under 100 mW cm−2 AM 1.5 white light illumination. The coarse grained morphology of the active layers was identified as the main limitation for the photocurrent, revealed by AFM measurements. The photovoltaic devices were characterized by current–voltage and spectral photocurrent measurements. The results show that the open circuit voltage is weakly dependent on the HOMO (highest occupied molecular orbital) level of the conjugated polymer used as donor.

Film thickness dependency of the emission colors of PPE–PPVs in inkjet printed libraries

The influence on the emission color of π-conjugated polymers varying side chains, film thicknesses and thermal treatment were investigated in a parallel fashion. The respective thickness libraries of six alkoxy-substituted poly(p-phenyleneethynylene)-alt-poly(p-phenylenevinylene)s (PPE–PPVs) were prepared by inkjet printing in a controlled way. The optical properties of the printed libraries were screened utilizing high-throughput methods. It was found that the emission colors of the investigated polymers strongly depend on the inter-chain interactions which are increased with increasing film thickness and influenced by the side chains. Moreover, upon annealing at 70 °C, white emission was observed from the printed films.

Synthesis and Properties of Novel Well-Defined Alternating PPE/PPV Copolymers

The polycondensation reactions of luminophoric dialdehydes 5 and bisphosphonates 3 provide a new type of π-conjugated polymers 7 with well-defined structure (—Ar—C≡C—Ar—C≡C—Ar—CH=CH—Ar— CH=CH—)n , which was confirmed by NMR, infrared and UV-Vis spectroscopy. High molecular weight (Mw up to 500 000 g/mol), thermostable, soluble and transparent film-forming materials were obtained. The grafting of large alkoxy side chains enhances the solubility and processability of the new compounds. The incorporation of triple bonds into the PPV backbone increases the oxidation and reduction potentials, thus making these polymers potentially good electron-transporting materials if used in light-emissive-diode devices. The polymers are photoconductive and show very good photoluminescent properties in solution as well as in the solid state. Identical absorption (λmax,abs = 445 nm) and emission (λmax,em = 490 nm) behaviors were observed for all polymers in solution (CHCl3), resulting in a fluorescence quantum yield of 70%, but the photophysical behavior in solid state was side group dependent. Polymer 7ac shows a very large Stokes shift (137 nm) and lower fluorescence quantum yield (19%), whereas 7aa, 7bb and 7ab, consisting of side groups equal to or longer than O(CH2)11CH3 are characterized by smaller Stokes shifts (around 30 nm) and comparatively higher fluorescence quantum yields (34 to 44%).

Conjugated polymers with 2,2′-bipyridine and diethinylenebenzene units: absorption and luminescence properties

Alternating oligomers and polymers consisting of 2,2′-bipyridine and diethinylenebenzene units and corresponding model compounds were synthesized and investigated in dilute solutions by absorption spectroscopy and by stationary and time-resolved emission spectroscopy. The strictly linear (rod-like) π-chain oligomers/polymers were compared with the angularly linked oligomers/polymers and with related model compounds. The model compounds which already show the essential spectroscopic properties of the oligomers/polymers consist of three (hetero)aromatics linearly connected by two diethenylene groups. These models exhibit fluorescence quantum yields close to unity and short fluorescence decay times around 1 ns. Fluorescence anisotropy and rotational relaxation times are consistent with the Stokes–Einstein equation and the Perrin equation. The absorption and emission spectra of the polymers and their radiative rate constants determined by fluorescence quantum yield and lifetime and according to the Strickler/Berg equation show a conjugation length of one to two repetition units. The conjugation along the chain is stronger in linear than in angular polymers and stronger in alkoxy-substituted than in unsubstituted polymers. In angular polymers at least two different emitting segments were found. The shortened mean lifetimes and the reduced fluorescence quantum yields and anisotropies of the oligomers/polymers indicate an additional radiationless deactivation channel which is opened by energy migration along the chain. Rates of energy transfer calculated for linear and angular polymers correlate with rates of radiationless deactivation. Copyright

Mobility and photovoltaic performance studies on polymer blends: effects of side chains volume fraction

A 1 : 1 mixture of two thiophene based poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene)s denoted DO-PThE1-PPV2 (D1) and MEH-PThE1-PPV2 (D2), consisting of the same conjugated backbone but different types and volume fraction of alkoxy side chains on the phenylene ethynylene unit, has led to enhanced charge carrier mobility (measured using CELIV technique) as compared to the individual polymers. The resulting ternary blend with PC60BM showed better photovoltaic performance as compared to binary blends from the single polymers mixed with PCBM. This is due to the improved active layer nanomorphology in the ternary system as revealed by AFM studies.

Improvement of photovoltaic performance by ternary blending of amorphous and semi-crystalline polymer analogues with PCBM

Ternary blending of amorphous and semi-crystalline anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) copolymers (AnE-PVs) with PCBM was investigated in bulk heterojunction solar cells. In general, a strong impact on all photovoltaic parameters was observed by increasing the amount of amorphous AnE-PVba-derivative in relation to its semi-crystalline counterpart AnE-PVab. Interestingly, small additions of the amorphous copolymer were beneficial for overall solar cell performance. The observed performance increase of the ternary blend could be related to an improved open-circuit voltage, despite the fact that the binary blend of the amorphous copolymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) did not exhibit a larger photovoltage than the binary blend based on the semi-crystalline copolymer. These results indicate that a certain amorphous fraction of the donor polymer may be required for obtaining optimal bulk heterojunction morphologies, yielding maximum photovoltaic performance.

Improvement in carrier mobility and photovoltaic performance through random distribution of segments of linear and branched side chains

The random distribution of segments of linear octyloxy side chains and of branched 2-ethylhexyloxy side chains, on the backbone of anthracene containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) has resulted in a side chain based statistical copolymer, denoted AnE-PVstat, showing optimized features as compared to the well defined homologues whose constitutional units are incorporated into its backbone. Electric field independent charge carrier mobility (µhole) for AnE-PVstat was demonstrated by CELIV and OFET measurements, both methods resulting in similar µhole values of up to 5.43 × 10−4 cm2 V−1 s−1. Upon comparison, our results show that charge carrier mobility as measured by CELIV technique is predominantly an intrachain process and less an interchain one, which is in line with past photoconductivity results from PPE-PPV based materials. The present side chain distribution favors efficient solar cell active layer phase separation. As a result, a smaller amount of PC60BM is needed to achieve relatively high energy conversion efficiencies above 3%. The efficiency of ηAM1.5 ≈ 3.8% obtained for AnE-PVstat:PC60BM blend is presently the state-of-art value for PPV-based materials.

Characterization of potential donor acceptor pairs for polymer solar cells by ESR, optical, and electrochemical investigations

In this report new PPV-PPE copolymers DE 69, DE 11 were compared with the state of the art materials MDMO-PPV and poly(3-alkylthiophenes) (P3DDT, P3OT). The optical band gap energy of the two copolymers DE 69, DE11 is somewhat higher than that one of MDMO-PPV. The electrochemical band gap was found to be lower for DE 69, DE11 related to for MDMO-PPV. The absorption coefficient of the new PPV-PPE copolymers is higher than for MDMO-PPV but in the same order of magnitude. Films from composites of MDMO-PPV/PCBM and DE69 or DE11 with PCBM show a clear photoluminescence quenching effect. At 77 K two kinds of LESR signals were identified, one of polarons (P+.) and one of radical anions of fullerenes. The LESR results show strong differences in kinetics between the separation and recombination processes of photoexcited charge carriers. The relaxation rates of paramagnetic centers were estimated by microwave power saturation experiments. Photovoltaic devices were prepared under ambient conditions on flexible PET-ITO foils with MDMO-PPV/ PCBM (1:3 wt. %) with ηAM1.5 = 2.4% and DE 69/ PCBM (1:3 wt. %) with ηAM1.5 = 1.75% (A=0.25 cm2, Pin = 100 mW/cm2).

PHOTOVOLTAIC PERFORMANCE OF PPE-PPV COPOLYMERS: EFFECT OF THE FULLERENE COMPONENT

Two conjugated PPE-PPV copolymers were studied as electron donor materials in bulk heterojunction organic solar cells in combination with a library of electron acceptor fullerene derivatives. It was shown that molecular structure and solubility of the fullerene counterpart significantly affect the photovoltaic performance of both polymers. Use of [60]PCBM as an electron acceptor material yielded quite moderate power conversion efficiencies. The best results were achieved when fullerene derivatives with suitable molecular structures and solubilities were applied. The obtained results suggest that every newly designed conjugated polymer should be evaluated in solar cells using a library of fullerene derivatives instead of just conventional PCBMs. We believe that only this combinatorial approach might bring the best performing donor/acceptor combinations for future generations of efficient organic solar cells.

Alkoxy-substituted poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s: synthesis, electroluminescence and photovoltaic applications

Poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s (PAE-PAVs) combine the intrinsic features of both poly(arylene-ethynylene) (PAE) and poly(arylene-vinylene) (PAV) in a single polymeric backbone. They exhibit enhanced electron affinity, as compared to parent poly(p-phenylene-vinylene) (PPV), making electron injection easier, placing them as potential candidates for low turn-on voltage organic light emitting diodes (OLEDs). Depending on the chemical structures, PAE-PAVs have been efficiently used either as donor materials in polymer-PCBM (phenyl-C61-butyric acid methylester) or polymer-Vinazene (2-vinyl-4,5-dicyanoimidazole) bulk heterojunction solar cells or as acceptor materials in polymer-polymer bilayer and blend solar cells. This article reviews the synthesis and properties (electroluminescence and photovoltaic) of π-conjugated alkoxy-substituted PAE-PAVs, designed by us, and which were obtained either by Horner–Wadsworth–Emmons olefination reaction of luminophoric dialdehydes with bisphosphonate esters, or Knoevenagel reaction of the same dialdehydes with dinitriles.