Tek Basel

spin-enhanced organic bulk heterojunction photovoltaic solar cells

Recently, much effort has been devoted to improve the efficiency of organic photovoltaic solar cells based on blends of donors and acceptors molecules in bulk heterojunction architecture. One of the major losses in organic photovoltaic devices has been recombination of polaron pairs at the donor-acceptor domain interfaces. Here, we present a novel method to suppress polaron pair recombination at the donor-acceptor domain interfaces and thus improve the organic photovoltaic solar cell efficiency, by doping the device active layer with spin 1/2 radical galvinoxyl. At an optimal doping level of 3 wt%, the efficiency of a standard poly(3-hexylthiophene)/1-(3-(methoxycarbonyl)propyl)-1-1-phenyl)(6,6)C(61) solar cell improves by 18%. A spin-flip mechanism is proposed and supported by magneto-photocurrent measurements, as well as by density functional theory calculations in which polaron pair recombination rate is suppressed by resonant exchange interaction between the spin 1/2 radicals and charged acceptors, which convert the polaron pair spin state from singlet to triplet.

Study of photoexcitations in poly(3-hexylthiophene) for photovoltaic applications

We used a variety of steady state and transient optical techniques for studying the primary and steady state photoexcitations in pristine films of poly(3-hexylthiophene) [P3HT] with different molecular weight used for organic photovoltaic solar cells. The employed techniques include picosecond transient and steady state photoluminescence and photomodulation spectroscopies, laser action, and doping induced absorption. We show that solar cell device qualities based on donor-acceptor blends of the P3HT polymers with substituted fullerene molecules crucially depend on the polymer properties, which can be readily discerned by the steady state photomodulation technique. This technique can, therefore, be applied to specific polymer donors for estimating their solar cell device properties prior to actually fabricating an optimal device.

Admittance spectroscopy study of polymer diodes in small magnetic fields

We performed a systematic study of bipolar and unipolar organic diodes based on the π-conjugated polymer, 2-methoxy-5-(2′-ethylhexyloxy) (MEH-PPV), using electronic and magneto-transport measurements with magnetic field in the range 0–180 mT and admittance spectroscopy in the frequency range 1 Hz < f < 10 MHz. The admittance spectra of bipolar devices reveal two relaxation processes with distinct time scales that are influenced by the magnetic field. The slower process, which dominates the device capacitance at f < 10 Hz, is attributed to the trap-assisted monomolecular recombination. The faster process is attributed to the electron-hole bimolecular recombination kinetics. We found that the time scale of this process, τ2, decreases exponentially with the bias voltage. Application of magnetic field, B = 30 mT decreases τ2 by ∼30%. We also found that the magneto-conductance, ΔG(ω,B)/G(ω,0), has a characteristic cutoff frequency that shifts to higher frequencies with increasing bias voltage. In particular, t...

Magnetic-field dependent differential capacitance of polymer diodes

Using admittance spectroscopy, we found that bipolar organic diodes based on pi-conjugated polymer, 2-methoxy-5-(2′-ethylhexyloxy), MEH-PPV, have strong divergent contribution to the device differential capacitance. It is positive at low bias voltages, turns negative at intermediate biases, and becomes positive again at stronger biases. In addition, we found that at certain biases, a small magnetic field can change the capacitance from divergent negative to divergent positive. Possible physical processes responsible for this anomalous behavior of the capacitance and its relation to the phenomenon of organic magnetoresistance are discussed.

Room-temperature magnetically modulated electroluminescence from hybrid organic/inorganic spintronics devices

We report a hybrid organic/inorganic magnetic-field controlled light-emitting diode (h-OLED), in which an inorganic magnetic tunnel junction (MTJ) with large room temperature magnetoresistance is coupled to an OLED having efficient electroluminescence. In operation, the large resistance mismatch between the MTJ and OLED components is suppressed due to the non-linear I-V characteristic of the OLED. This leads to ∼80% giant magneto-electroluminescence at room temperature with emission in the red, green, and blue spectral ranges.

Ultrafast transient spectroscopy of nano-domains of polymer/fullerene blend for organic photovoltaic applications

We measured the picoseconds (ps) transient photomodulation (PM) dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) [RR-P3HT] (donors-D) and indene-C60 bisadduct (fullerene derivative) [ICBA] (acceptor-A) that phase-separate into D- and A-nano-domains, in a broad spectral range from 0.25 to 2.5 eV; in comparison with steady state PM spectra. We correlate our measurements with organic photovoltaic solar cell performance made from the same D and A materials. In D-A blends of RR-P3HT/ICBA with (1.2:1) weight ratio having solar cell power conversion efficiency of ∼5.1%, we found that although the intrachain excitons in the polymer nano-domains decay within ∼10 ps, no charge polarons are generated on their expense up to ∼1 ns. Instead, there is a built-up of charge-transfer (CT) excitons at the D-A domain interfaces that occurs with the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D- and A-nano-domains at a much later time (≫1 ns). Thi...

Optical properties of low bandgap copolymer PTB7 for organic photovoltaic applications

We used both cw and transient spectroscopies for studying the optical properties and photoexcitations in the low bandgap copolymer PTB7 that has been used in organic photovoltaic applications (OPV). Surprisingly we observed two primary photoexcitations that are generated within ~150 fs (our time resolution); we identify them as singlet exciton (S1) and triplet-pair (1TT). The singlet exciton has been considered to be the only primary photoexcitation in regular π-conjugated polymers and is related with a transient absorption band that peaks at an energy value close to the exciton binding energy (~0.4 eV in PTB7). The TT pair is a novel photoexcitation species in low band-gap π-conjugated copolymers. It has an absorption band close to that of isolated triplet exciton, and may readily dissociate at the donoracceptor interfaces in the PTB7/fullerene blend. This finding may explain the underlying mechanism for the high obtained power conversion efficiency in OPV devices based on the PTB7 copolymer.

Photoexcitations dynamics in pristine low bandgap π-conjugated copolymers

Abstract. We present a comprehensive review on the optical properties in pristine low bandgap copolymers, namely PTB7 and PDTP–DFBT, which are used as electron donors in copolymer/phenyl C71 butyric acid methyl ester blends for high efficiency solar cell devices. The copolymer backbone chain comprises of donor (D) and acceptor (A) moieties, which lower the band gap to the near-IR spectral region. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in D–A copolymers we find at short times coexistence of two primary photoexcitation species, namely SE and triplet–triplet (TT) pairs, which are directly photogenerated upon photon absorption from the ground state within 300 fs, our transient time resolution. Using the transient magnetophotoinduced absorption (t-MPA) spectroscopy, we reveal the spin coupling between the SE and TT spin states from their correlated t-MPA responses. In addition, we show that the TT species dissociates into two individual triplet excitons (TE) in the picosecond time scale; however, the two-geminate TEs are entangled and maintain their spin coherence into the microsecond time domain.

Magnetic field effect in organic light emitting diodes based on donor-acceptor exciplexes showing thermally activated delayed fluorescence

A new type of organic light-emitting diode (OLED) has emerged that shows enhanced operational stability and large internal quantum efficiency approaching 100%, which is based on exciplexes in donor-acceptor (D-A) blends having thermally activated delayed fluorescence (TADF) when doped with fluorescent emitters. We have investigated magnetoelectroluminescence (MEL) and magneto-conductivity in such TADF-based OLEDs, as well as magnetophotoluminescence (MPL) in thin films based on the OLEDs active layers, with various fluorescence emitters. We found that both MEL and MPL responses are thermally activated with substantially lower activation energy compared to that in the pristine undoped D-A exciplex host blend. In addition, both MPL and MEL steeply decrease with the emitters’ concentration. This indicates the existence of a loss mechanism, whereby the triplet charge-transfer state in the D-A exciplex host blend may directly decay to the lowest, non-emissive triplet state of the additive fluorescent emitter molecules.

Magnetic field enhancement of generation-recombination and shot noise in organic light emitting diodes

We have studied the effect of magnetic field on noise in series of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene-based organic light emitting diodes with dominant hole injection, dominant electron injection, and balanced electron and hole injection. The noise spectra of the balanced devices revealed the generation-recombination (g-r) noise term, which we associated with bimolecular electron-hole recombination. The presence of the g-r noise term is correlated with the strong organic magnetoresistance (up to 25%) observed in the balanced devices. The noise spectra also have the shot noise contribution with the Fano factor 0.25–0.4. We found that time constant of the g-r term decreases and the magnitude of shot noise increases when magnetic field is applied. This behavior can be consistently explained within the polaron-polaron model of organic magnetoresistance. We have not found any evidence that the magnetoresistance in studied devices is affected by traps.