Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

Abstract

PBDTTPD is a conjugated polymer with high power conversion efficiency if used in organic solar cells together with fullerene derivatives. We have investigated for the first time the excited state dynamics of pristine PBDTTPD thin film as well as the ultrafast evolution of charge carriers in PBDTTPD:PCBM bulk heterojunction blend using femtosecond transient absorption spectroscopy. In the latter, charges appear within the time resolution of the experiment (<100 fs), but clean spectral signatures allowed to directly follow slower ∼1 ps charge separation. Only the slower quenching component competes with exciton−exciton and exciton−charge annihilation, leading to a reduced yield of charge carriers at high laser fluence. Our excellent measuring sensitivity made it possible to reduce pump power to a point where annihilation is quasi suppressed. In this case >80% of charges survive after 1 ns; the rest recombines (most probably geminately) on the 200 ps time scale. The photophysics of PBDTTPD has only been little explored. 7,8 We report here, to our knowledge, the first transient absorption (TA) study of the ultrafast processes occurring in thin films of pristine PBDTTPD and PBDTTPD:PCBM blend. TA spectroscopy has been used on numerous occasions to understand the charge carrier dynamics in polymer:fullerene blends. 9−17 It is, however, becoming clear that the important energy delivered by pulsed laser excitation in such experiments is not necessarily representative of solar irradiation: The high density of excited species and charges generated at high pump fluence leads to bimolecular loss mechanisms that are not observed under solar cell operating conditions. 10−12,17−20 Excellent measuring sensitivity of our apparatus has allowed us to reduce the excitation power to a point where those loss mechanisms become negligible on the subnanosecond time scale. Moreover, we have taken advantage of the annihilation processes that occur with increasing fluence to gain valuable insight about the evolution of excited and charged species in pristine and blended PBDTTPD. Figure 1A depicts the steady-state absorption and fluorescence spectra of the investigated thin films. Pristine PBDTTPD has a broad and structured absorption band ranging from ∼360−700 nm. The presence of defined vibronic features, uncommon for conjugated donor−acceptor copolymers, has been attributed to a planar backbone conformation of the polymer in the ground state (density functional thoery calculations), 21 and to insignificant torsional relaxation in the excited state (two-dimensional electronic spectroscopy). 8 The quantum simulations also suggest delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and that several electronic bands might be present under the broad envelope of the absorption spectrum. 21 The emission spectrum of pristine PBDTTPD film is weakly Stokes shifted with a maximum at 662 nm and a vibronic shoulder around 720 nm. For the 1:2 PBDTTPD:PCBM blend cast from o-dichlorobenze...

Charge Separation Pathways in a Highly Efficient Polymer: Fullerene Solar Cell Material

“…19 We have determined that positively charged PBDTTPD has a flat absorption in the 660−1000 nm region with a very characteristic peak at 880 nm. 7 The signature has been independently verified by PIA spectroscopy and absorption of the oxidized polymer. 19 In Figure 3B, the TA spectra at selected time delays are compared for excitation at 390 nm (strong PCBM absorption) and at 530 nm (predominant PBDTTPD absorption).…”

“…We have previously found that extremely low excitation fluence (<4 μJ/ cm 2 ) is necessary to avoid laser-induced exciton annihilation and charge recombination artifacts for PBDTTPD-based samples. 7 With an even improved experimental sensitivity, we have now carefully recorded all data below this threshold and verified that the dynamics for each measurement are fluence-independent in this regime. The flux of absorbed photons at the various excitation wavelengths was kept constant at 5.9 × 10 12 photons/ cm 2 for the blend and at 4.9 × 10 12 photons/cm 2 for the neat polymer, taking into account the absorbance at λ ex , the fluence and the photon energy (see explanation in the Supporting Information [SI] and Table S1).…”

“…7 (2) For λ ex = 530 nm, the spectral features of the neutral polymer exciton (SE and ESA, similar as in the neat PBDTTPD film) are present at 0.5 ps. They disappear almost completely within 10 ps, with a concerted rise of the 880 nm charge peak, pointing to fast ET.…”

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

“…4B), the signatures of the neutral PBDTTPD exciton are also visible at early time delays. They include the negative SE band on the red side of the GSB (640-750 nm, coincident with the steady-state emission), 54 and the broad excited state absorption (ESA, 740-1000 nm, rising at long wavelengths), as identied by comparison to the neat PBDTTPD spectrum shown in the inset of Fig. 4B (in grey).…”

“…Conveniently, the signature of positively charged PBDTTPD is highly characteristic, with a at absorption in the 660-1000 nm region and a pronounced peak at 880 nm (shown by TA and PIA spectroscopy as well as absorption of the oxidized polymer). 12,54 At the longest timedelay (1000 ps, blue curve), the TA spectra of PBDTTPD:PCBM are similar for both excitation wavelengths. They are dominated by the polymer's GSB (460-660 nm) and the characteristic charge absorption (880 nm peak), indicating that charges have been efficiently generated by ET or HT.…”