Effect of temperature on carrier formation efficiency in organic photovoltaic cells (original) (raw)
Related papers
Fast Carrier Formation from Acceptor Exciton in Low-Gap Organic Photovotalic
Applied Physics Express, 2012
Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic. Here, we investigated the femtosecond charge dynamics after acceptor excitation in poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b 0 ] dithiophene-2,6diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7)/[6,6]-phenyl C 71-butyric acid methyl ester (PC 70 BM), which is a lowgap (E gap % 1:6 eV) organic photovoltaic. We observed fast conversion from the acceptor exciton (A Ã) to the donor polaron (D þ) within rise ¼ 0:45 ps. The fast carrier formation is ascribed to the prompt charge transfer of an electron from the D highest occupied molecular orbital (HOMO) to the A HOMO.
Carrier Formation Dynamics of Organic Photovoltaics as Investigated by Time-Resolved Spectroscopy
Advances in Optical Technologies, 2012
Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaics (OPV). In order to improve the incident photon-to-current efficiency (IPCE) of the BHJ solar cell, a comprehensive understanding of the ultrafast dynamics of excited species, such as singlet exciton (D *), interfacial charge-transfer (CT) state, and carrier (D +), is indispensable. Here, we performed femtosecond time-resolved spectroscopy of two prototypical BHJ blend films: poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C 61-butyric acid methyl ester (PCBM) blend film and poly(9,9-dioctylfluorene-cobithiophene) (F8T2)/[6,6]-phenyl C 71-butyric acid methyl ester (PC 70 BM) blend film. We decomposed differential absorption spectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition procedure clearly distinguished photoinduced absorptions (PIAs) due to D * , CT, and D +. Based on these assignments, we will compare the charge dynamics between the F8T2/PC 70 BM and P3HT/PCBM blend films.
2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009
In order to understand the dependence of photoinduced initial processes on thermal annealing, the femtosecond time-resolved fluorescence dynamics of regioregular poly(3-hexylthiophene) (P3HT) in (thermally) annealed P3HT/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films has been studied by using the fluorescence up-conversion technique. For comparison, a P3HT solution, pristine P3HT, and unannealed P3HT/ PCBM blend films have been investigated as well. The fluorescence dynamics of the P3HT solution showed wavelength dependence. Excitation energy transfer between the segments and torsional relaxation possibly occurred in a time scale of several ps in the solution. Observed rise times at longer wavelength emission suggested the formation of these relatively lower emission states (at 650 and 700 nm). Charge transfer (or excitonic quenching) was the dominant process in the fs time scale with emission at 650 nm in the unannealed blend film. In the annealed blend film, the charge transfer (334 fs) and downhill relaxation (942 fs) of selftrapped (dynamic localized) excitons were competitive processes due to the well aligned nanodomains in the P3HT/PCBM blend films. There were different charge transfer rates at different excited states (650 and 700 nm) in the annealed film. The charge transfer process occurred faster at a lower excited state, and a stronger electronic and vibrational coupling in the annealed P3HT/PCBM films was revealed within these measurements as well. The ultrafast anisotropy decays suggested that a strong and ultrafast reorientation of the molecular dipole moments occurred at excited states. The anisotropy decay was mainly determined by the ultrafast process, whereas the energy could continuously migrate along or between P3HT chains in a time scale of ∼100 ps. The ultrafast process suggested that there was an excitation delocalization associated with vibrational modes, as was consistent with the observation from steady-state measurements. On the basis of the understanding of the mechanisms above, the optimized cell performance has been established.
Advanced Materials, 2014
be bound by Coulombic interaction. Direct evidence for the formation of strongly correlated interfacial charge pairs came from electron spin resonance studies on polymer-fullerene blends. As most donor-acceptor blends exhibit additional sub-bandgap absorption and emission features, it was proposed that these interfacial excitations are charge transfer states (CT states) coupled to the ground state via radiative transitions. The reasons for highly effi cient charge generation despite the existence of Coulombically bound geminate pairs is subject of ongoing debate. Correlations have been found between the free carrier generation yield and the energy difference
Ultrafast Transient Spectroscopy of Polymer/Fullerene Blends for Organic Photovoltaic Applications
Materials, 2013
We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (>1 ns). This "two-step" charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM.
The Journal of Physical Chemistry C, 2008
The results of time-resolved spectroscopic studies of the structurally ordered liquid-crystalline polymer, poly(2,5bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT, and of composites of PBTTT with the soluble fullerene derivative, [6,6]-phenyl-C 61 butyric acid methyl ester, PCBM are reported. These ultrafast studies focus on the photogeneration of mobile carriers in PBTTT/PCBM composites following sub-picosecond electron transfer from PBTTT to PCBM. The recombination and loss of mobile carriers at early times (approximately 200 ps following photoexcitation) were measured with different ratios of the PBTTT and PCBM components (1:1 and 1:3) by monitoring the time decay of the photoinduced absorption features associated with the chargetransferred state. The results are well-correlated with the solar-cell efficiency obtained using PBTTT/PCBM bulk heterojunction materials.
Advanced Functional Materials, 2008
Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer:[6,6]-phenyl C 61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak groundstate charge-transfer complexes in the studied polymer:fullerene blends. The effective band gap (E g ) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all Submitted to 2 2 devices, under various preparation conditions, the open-circuit voltage (V oc ) scales linearly with E g . The redshift of the CT band upon thermal annealing of regioregular poly(3hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open-circuit voltage of hightemperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of E g , proportional with the observed changes in V oc for different PPV:PCBM ratios. As E g corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells. Submitted to 3 3