Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site.The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization.

PBDTTPD is one of the best conjugated polymers for solar cell applications (up to 8.5% efficiency). We have investigated the dynamics of charge generation in the blend with fullerene (PCBM) and addressed highly relevant topics such as the role of bulk heterojunction structure, fullerene excitation, and excess energy. We show that there are multiple charge separation pathways. These include electron transfer from photoexcited polymer, hole transfer from photoexcited PCBM, prompt (<100 fs) charge generation in intimately mixed polymer:fullerene regions (which can occur from hot states), as well as slower electron and hole transfer from excitons formed in pure PBDTTPD or PCBM domains (diffusion to an interface is necessary). Very interestingly, all the charge separation pathways are highly efficient. For example, the yield of long-lived carriers is not significantly affected by the excitation wavelength, although this changes the fraction of photons absorbed by PCBM and the amount of excess energy brought to the system. Overall, the favorable properties of the PBDTTPD:PCBM blend in terms of morphology and exciton delocalization allow excellent charge generation in all circumstances and strongly contribute to the high photovoltaic performance of the blend. ■ INTRODUCTIONOrganic solar cells based on electron-donating conjugated polymers blended with electron-accepting fullerene derivatives have reached high efficiencies beyond 8%.1−3 In order to further enhance their performance, it is essential to deeply understand the underlying photophysical processes. Highly relevant topics that need to be addressed include the role of delocalization and exciton diffusion for efficient charge generation at the donor:acceptor interface, the role of hot neutral and charge transfer states in the formation of free carriers, and the role of the fullerene as light harvester.4 Both the nanoscale structure of the bulk heterojunction (BHJ) and the photophysical dynamics must be considered to gain answers to those questions. Femtosecond transient absorption (TA) spectroscopy is an undeniably useful tool to study the dynamics of neutral excited states and photoinduced charges in the materials of interest. 5−9Here, we will show that this purely spectroscopic technique additionally yields detailed morphological insights. Our study leads to a comprehensive picture of how different charge generation pathways relate to BHJ structure.We focus the investigation on PBDTTPD (poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione), and its blend with PCBM ([6,6]-phenyl C 60 butyric acid methyl ester). The polymer is among the best performing for photovoltaic applications, with up to 8.5% efficiency.1 Recent reports on morphology control, 1,10−12 device design, 13 charge trapping, 14 long-term stability, 15 quantum calculations, 16,17 and spectroscopy 7,18−20 confirm high interest for the material. PBDTTPD also distinguishes itself from many other conjugated polymers through its very planar structure in both the ground and ex...

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...

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO 2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions.