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.

We studied the spin response of various magnetic field effects and magneto-transport in both protonated and deuterated aluminum tris(8-hydroxyquinoline) [Alq 3 ]-based organic light emitting diodes and spin-valve devices. Both conductivity-detected magnetic resonance in diodes and magneto-resistance in spin valves show substantial isotope dependence pointing to the importance of the hyperfine interaction (HFI) in the spin response of spin ½ charge polarons in Alq 3 . In addition the low field (B<20 mT) magneto-electroluminescence (MEL) response is also isotope sensitive, showing that HFI-induced spin mixing of polaron-pairs spin sublevels dominates this response too. However, the magneto-conductance (MC) response was found to be much less sensitive to isotope exchange at low fields, in agreement with previous studies. The disparity between the isotope sensitivity of MC and MEL responses in Alq 3 indicates that the HFI in the MC response is overwhelmed by an isotope independent spin mixing mechanism. We propose that collisions of spin ½ carriers -with triplet species such as polaron pairs may be the main spin mixing mechanism in the low field MC response in Alq 3 diodes.

Several methods of harvesting singlet excitons via delayed fl uorescence have been introduced in OLED so far. These methods include up-conversion to singlet excitons by triplet-triplet annihilation (TTA) [ 3,4 ] or triplet fusion in materials that show a strong singlet fi ssion. [ 5 ] A different approach for enhancing the singlet emission that involves triplet excitons was introduced recently, whereby the triplet excitons may undergo reverse intersystem crossing (RISC) to singlet excitons and consequently give rise to thermally activated delayed fl uorescence (TADF). [6][7][8][9][10][11][12] This occurs in compounds with small electron exchange energy, and thus small singlet-triplet energy splitting, Δ E ST that enables triplet excitons to undergo thermally activated RISC to the singlet manifold. [ 13 ] A fi rm indication for TADFrelated emission in compounds that possess RISC is that the EL in these OLED is thermally activated, with activation energy E act ≈ Δ E ST Ͻ Ͻ 0.7 eV (which is Δ E ST in traditional organic semiconductors). During the last few years there has been a large interest in magnetic fi eld effect (MFE) in conjugated organic compounds, mainly because of the possibility to enhance the electroluminescence effi ciency, which was dubbed magneto-EL (MEL). [ 11,[14][15][16][17][18][19][20][21] In this effect, the magnetic fi eld changes the exchange rate between PP singlet (PP S ) and triplet (PP T ), which can be detected through the induced change in the EL emission intensity (MEL) or the current density (MC) in the device. This occurs if the PP S and PP T recombination rates ( R S , R T ) and/or dissociation rates ( d S , d T ) differ from each other. [ 17,22 ] So far the MEL maximum value, MEL max at room temperature (RT) has been less than ≈20% in OLEDs.In conventional OLEDs, spin mixing occurs within the PP states rather than at the exciton levels because the electron-hole orbitals strongly overlap in the latter species leading to large exchange energy, J , that consequently causes large energy gap, Δ E ST (=2 J ) between singlet and triplet states. In contrast, materials showing RISC may allow spin-mixing among the PP spin levels and in the exciton levels because Δ E ST is small. [ 23 ] In this case, possible spin-mixing mechanism may be the hyperfi ne interaction [ 24,25 ] and/or the Δ g mechanism, [ 26 ] where the difference, Δ g in the g -values of positive and negative carrier in the pair may promote intersystem crossing. The obtained full width Reverse intersystem crossing (RISC) from triplet to singlet states has been recently introduced to photophysics of organic chromophores. One type of RISC occurs in donor (D)-acceptor (A) composites that form an exciplex manifold in which the energy difference, Δ E ST between the lowest singlet (S 1 ) and triplet (T 1 ) levels of the exciplex is small (<100 meV) thus allowing RISC at room temperature. This adds a delayed component to the photoluminescence emission that is widely known as thermally activated delayed fl uorescence. Here, it is found t...

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of poly (3-hexyl-thiophene) [P3HT] (donors-D) and fullerene [PCBM] (acceptor-A) using the transient pump/probe photomodulation technique in an unprecedented broad spectral range from 0.25 to 2.5 eV; and compared the results with organic solar cell performance based on the same blends.In D-A blends with maximum domain separation such as regio-regular P3HT/PCBM with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the photogenerated intrachain excitons in the polymer nano-domains decay within ~10 ps, no charge polarons are generated on their expense up to ~ 2 ns. Instead, there is a build-up of chargetransfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay, which dissociate into separate polarons in the D and A domains at a much later time (>>1 ns). This 'two-step' charge photogeneration process may be typical in organic bulk heterojunction cells.Although the CT excitons are photogenerated on the exciton expense much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM, their dissociation is less efficient because of larger binding energy. This explains the poor solar cell power conversion efficiency (<0.1%) based on this blend. Our results support the 'two-step' charge photogeneration mechanism in polymer/fullerene blends, and emphasize the important role of the CT binding energy in generating free charge polarons in organic solar cells. For the polarized transient photomodulation spectroscopy we used the femtoseconds (fs) twocolor pump/probe correlation technique with two different pulsed laser systems based on a Ti:Sapphire oscillator [26]. These are: a low-power (energy/pulse ~0.1 nJ) high repetition rate (~80 MHz) pulsed laser system for the mid-IR spectral range; and a high power (energy/pulse ~10 µJ) low repetition rate (~1 kHz) pulsed laser system for the near-IR/visible spectral range. The pump excitation photon energy, ω(pump) was set at 3.1 eV for above-gap and 1.55 eV for below-gap excitation, respectively. The pulse energy flux on the film was adjusted so that the initial was modulated at frequency, f=40 kHz and the changes, ΔT in the probe transmission, T were measured using a phase-sensitive technique. For the transient near-IR/visible spectroscopy measurements we used white light super-continuum as probe, spanning the spectral range from 1.15 to 2.5 eV; the pump modulation frequency here was synchronized with the laser rep. rate.The transient photomodulation signal, ΔT/T is positive for photoinduced absorption (PA) and negative for photo-bleaching and stimulated emission. ΔT(t)/T was measured using a computer controlled translation stage up to 2 ns, with time resolution of ~150 fs set by the pump/probe cross-correlation. ΔT(0)/T spectra from the two laser systems were normalized to each other at several wavelengths in the near-IR/visible spectral range [26].Since some photoexcitations may live longer than...

The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fl uorinated poly-thienothiophene-benzodithiophene) with C60-PCBM using optical, electrical, and magnetic measurements in thin fi lms and devices is studied. A variety of steady state optical and magneto-optical techniques were employed, such as photoinduced absorption (PA), magneto-PA, dopinginduced absorption, and PA-detected magnetic resonance (PADMR); as well as picosecond time-resolved PA. The charge polarons and triplet exciton dynamics in fi lms of pristine PTB7, PTB7/fullerene donor-acceptor (D-A) blend is followed. It is found that a major loss mechanism that limits the power conversion effi ciency (PCE) of PTB7-based solar cell devices is the "back reaction" that leads to triplet exciton formation in the polymer donor from the photogenerated charge-transfer excitons at the D-A interfaces. A method of suppressing this "back reaction" by adding spin½ radicals Galvinoxyl to the D-A blend is presented; this enhances the cell PCE by ≈30%. The same method is not effective for cells based on PTB7/C70-PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.