Delayed Luminescence Spectroscopy of Organic Photovoltaic Binary Blend Films: Probing the Emissive Non-geminate Charge Recombination (original) (raw)
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ACS Applied Materials & Interfaces, 2019
Disentangling temporally-overlapping charge carrier recombination events in organic bulk heterojunctions by optical spectroscopy is challenging. Here, a new methodology for employing delayed luminescence spectroscopy is presented. The proposed method is capable of distinguishing between recombination of spatially-separated charge carriers and trap-assisted charge recombination simply by monitoring the delayed luminescence (afterglow) of bulk heterojunctions with a quasi timeintegrated detection scheme. Applied on the model composite of the donor poly(6,12-dihydro-6,6,12,12-tetraoctyl-indeno[1,2-b]fluorene-alt-benzothiadiazole) (PIF8BT) polymer and the acceptor ethyl-propyl perylene diimide (PDI) derivative, i.e. PIF8BT:PDI, the luminescence of charge-transfer (CT) states created by non-geminate charge recombination on the ns-μs time scale is observed. Fluence-dependent, quasi time-integrated detection of the CT luminescence monitors exclusively emissive charge recombination events, while rejecting the contribution of other early-time emissive processes. Trap-assisted and bimolecular charge recombination channels are identified based on their distinct dependence on fluence. The importance of the two recombination channels is correlated with the layer's order and electrical properties of the corresponding devices. Four different microstructures of the PIF8BT:PDI composite obtained by thermal annealing are investigated. Thermal annealing of PIF8BT:PDI shrinks the PDI domains in parallel with the growth of the PIF8BT domains in the blend. Common to all states studied, the delayed CT luminescence signal is dominated by trap-assisted recombination. Yet, the minor fraction of fully-separated charge recombination in the overall CT emission increases as the difference in the size of the donor and acceptor domains in the PIF8BT:PDI blend becomes larger. Electric field-induced quenching measurements on complete PIF8BT:PDI devices confirm quantitatively the dominance of emissive trap-limited charge recombination and demonstrate that only 40% of the PIF8BT/PDI CT luminescence comes from the recombination of fully-separated charges, taking place within 200 ns after photoexcitation. The method is applicable to other non-fullerene acceptor blends beyond the system discussed here, if their CT state luminescence can be monitored.
2005
The electronic structure of the widely-used light emitting materials, 2,5-bis͑5-tert-butyl-2-benzoxazolyl͒ thiophene ͑BBOT͒, poly͑N-vinylcarbazole͒ ͑PVK͒ thin films have been characterized using surface photovoltage spectroscopy. The photo-induced charge separation and transfer processes in both blend films of PVK:BBOT and PVK:TPD:BBOT, where TPD is N , NЈ-diphenyl-N , NЈ-bis͑3-methylphenyl͒-͓1,1Ј-biphenyl͔-4,4Ј-diamine have also been investigated. The results of the photo-induced contact potential difference ͑CPD͒ change show that BBOT film is an electron-transporting material while PVK film is a hole-transporting one. The photoluminescence and electroluminescence results of the blend films suggest an exciplex interaction between BBOT and PVK or TPD. A positive CPD change due to photo-excitation of the BBOT in PVK:BBOT blend film is attributed to electron trapping at the localized state induced by dispersed BBOT species. In the PVK:TPD:BBOT blend films, a positive CPD change, which starts at the same transition energy as in the former blend film but is significantly enhanced, is observed and explained in terms of charge transfer between the involved energy structures of the blend components. The dependence of the observed effects on the blend composition and ensuing electronic structure is discussed.
Binary Organic Photovoltaic Blends: A Simple Rationale for Optimum Compositions
Advanced Materials, 2008
The most promising device structure for organic photovoltaic devices presented to date is the ''bulk-heterojunction'' whereby a hole-conducting (electron-donating) conjugated polymer, such as poly(3-hexylthiophene) (P3HT), is blended with an electron-conducting (electron-accepting) smallmolecular compound, such as a fullerene derivative. The reported strong composition-and thermal-treatment dependence of the power conversion efficiency of such binaries suggests that phase behavior, processing conditions and the resulting microstructure play a dominant role in the performance of devices based on these systems. Here, we propose a simple rationale for selecting the optimum composition of such crystalline/crystalline polymer/small molecule blends. We find that these binary systems feature simple eutectic phase behavior, and that the optimum composition for device performance is slightly hypoeutectic when expressed in terms of the polymer component. In accord with classical understanding of eutectic solidification, these blends feature a finely phase-separated matrix surrounding primary crystals of the small-molecular species. The combination of large interfacial area and component connectivity yield a desired microstructure for use in bulk-heterojunctions.
The Journal of Physical Chemistry B, 2013
Photoinduced charge separation in a mixture of Perylene Red (N,N′-bis(2,6-di-isopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxylic acid bis-imide) and pyrene, organized in thin solid film on quartz, was studied by means of steady-state absorption and emission spectroscopy and by femtosecond transient absorption spectroscopy. Steady state spectroscopy shows substantial interaction between the two chromophores in the ground and excited states. Luminescence quenching indicates charge transfer interaction. Global and target analysis of the transient absorption data indicates a complex photophysical behavior with the formation of long-lived charges (free charge carriers) and of a triplet excited state species (with rates of ∼10.4 × 10 9 and 72.1 × 10 6 s −1 ) via charge recombination pathways from charged states. A generally applicable target model for the analysis of photophysical data of photovoltaic blends is developed.
Journal of Physical Chemistry C, 2013
Low-bandgap diketopyrrolopyrrole-and carbazolebased polymer bulk-heterojunction solar cells exhibit much faster charge carrier recombination kinetics than that encountered for less-recombining poly(3-hexylthiophene). Solar cells comprising these polymers exhibit energy losses caused by carrier recombination of approximately 100 mV, expressed as reduction in open-circuit voltage, and consequently photovoltaic conversion efficiency lowers in more than 20%. The analysis presented here unravels the origin of that energy loss by connecting the limiting mechanism governing recombination dynamics to the electronic coupling occurring at the donor polymer and acceptor fullerene interfaces. Previous approaches correlate carrier transport properties and recombination kinetics by means of Langevin-like mechanisms. However, neither carrier mobility nor polymer ionization energy helps understanding the variation of the recombination coefficient among the studied polymers. In the framework of the charge transfer Marcus theory, it is proposed that recombination time scale is linked with charge transfer molecular mechanisms at the polymer/fullerene interfaces. As expected for efficient organic solar cells, small electronic coupling existing between donor polymers and acceptor fullerene (V if < 1 meV) and large reorganization energy (λ ≈ 0.7 eV) are encountered. Differences in the electronic coupling among polymer/fullerene blends suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based solar cells. Our approach reveals how to directly connect photovoltaic parameters as open-circuit voltage to molecular properties of blended materials.
Acceptor Energy Level Control of Charge Photogeneration in Organic Donor/Acceptor Blends
Journal of the American Chemical Society, 2010
In this paper we focus upon the role of interfacial energetics in influencing the separation of charge transfer states into dissociated charge carriers in organic donor/acceptor blend films. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) electron acceptors. For this film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron acceptor 3′-phenyl-3′H-cyclopropa [1,9](C 60 -I h )[5,6]fullerene-3′-butanoic acid methyl ester (PCBM).
Solution-processed bulk heterojunction (BHJ) based on electron-donor (D) polymer and acceptor (A) fullerene is a promising technology for organic photovoltaics. Geminate charge recombination is regarded as one of the main loss mechanisms limiting device performances. This stems from the dynamics of the initial charge transfer state (CTS), which depend on the blend morphology, the molecular conformation, and the energetics of the D:A interface. Here we study the photophysics of a crystalline phase-separated blend of regioregular poly(3-hexylthiophene) (P3HT) with [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) with a coarsened morphology, by mapping the transient absorption signal with submicrometer space and subpicosecond time resolution. At the P3HT:PCBM interface, we detect a long-lived photoinduced dynamic that we assign to a peculiar coherent CTS forming in ∼10 ps, not affected by geminate recombination and characterized by a different polarization with respect to the one in the usual polydispersed blend. Quantum chemical calculations on supramolecular P3HT:PCBM complexes confirm the presence of low-lying and highly polarized CTS, validating the experimental findings.
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.