Article pubs.acs.org/JPCC
Temperature- and Energy-Dependent Separation of Charge-Transfer States in PTB7-Based Organic Solar Cells Marina Gerhard,† Andreas P. Arndt,‡ Ian A. Howard,§ Arash Rahimi-Iman,† Uli Lemmer,*,‡ and Martin Koch† †
Faculty of Physics and Materials Science Center, Philipps-Universität Marburg, Renthof 5, D-35032 Marburg, Germany Light Technology Institute, Karlsruhe Institute of Technology, Kaiserstrasse 12, D-76131 Karlsruhe, Germany § Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡
S Supporting Information *
ABSTRACT: A decisive factor for the performance of organic bulk heterojunction solar cells is the competition between charge separation and geminate electron−hole recombination through a charge-transfer (CT) state after exciton separation across the heterointerface. By time-resolving the near-infrared emission of the high-performance low-bandgap PTB7:PC71BM material system, we selectively study the properties of CT states formed after optical excitation with a femtosecond laser pulse. We observe that the CT emission yield is higher after 705 nm excitation of the polymer rather than after 400 nm excitation of the fullerene, which can be attributed to better charge separation for excitons generated in fullerene aggregates. Additionally, the CT states are weakly bound with their emission not far red-shifted from that of the exciton. Examining the time-resolved CT emission from room temperature to 10 K, we observe changes in CT state lifetime with energy and temperature, indicative of CT state separation effectively competing with recombination, especially for the higher-energy CT states. Our findings suggest that this weak binding of CT states in the polymer−fullerene mixed phase is a key factor for the highly efficient charge separation in this material system.
1. INTRODUCTION
One critical factor that is not so well understood and cannot be rationally designed yet in organic solar cells is the separation of charges from the heterointerface following exciton quenching. How interfacial charge-transfer (CT) states play a role in charge separation and how morphology changes the yield of charge generation for a given material system has been widely investigated, but clear-cut predictive understanding is still absent. Transient absorption (TA) measurements on homopolymers10−13 and donor−acceptor type copolymers14−17 suggest that relaxed CT states formed at interfaces are subject to geminate recombination on a nanosecond-to-subnanosecond time scale and are therefore unlikely to contribute to photocurrent. This is also indicated by pump-push photocurrent studies18 and electronic structure calculations on phenyl-C61-butyric acid methyl ester (PCBM) crystallites,19 which outline the important difference between thermally relaxed CT states, acting as traps, and excited CT states, promoting exciton separation according to their more delocalized nature. Indeed, this view of the relaxed CT as a
Power-conversion efficiencies of single-junction organic solar cells approaching 10% have recently been achieved, demonstrating the steady progress being made in the chemical design of organic semiconductors and the engineering of organic photovoltaic devices.1,2 On the basis of the polymer/fullerene blend bulk heterojunction concept,3 these devices allow for both efficient exciton dissociation and charge extraction from the device. Copolymers designed with either alternating electron-rich and electron-poor monomer units or aromatic quinoid resonance character exhibit a lower bandgap, improving the overlap with the solar spectrum.4−7 Yu and coworkers designed the impressively performing PTB7 polymer by studying how side-chains affect π−π overlap and thereby device fill factor, as well as how side-chains and electronwithdrawing units can lower the highest occupied molecular orbital (HOMO) levels to increase the open-circuit voltage in the poly(thieno[3,4-b]-thiophene−benzodithiophene) (PTB) family of quinoid-type low-bandgap polymers.7,8 Since then, PTB7 has become a benchmark high-performance polymer with single-junction power-conversion efficiencies exceeding 9%.1,9 © XXXX American Chemical Society
Received: October 8, 2015 Revised: November 16, 2015
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DOI: 10.1021/acs.jpcc.5b09842 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
work by Collins et al.34 based on X-ray scattering experiments indicates that the inclusion of DIO reduces the size of pure PC71BM agglomerates in a 70/30 PTB7/PC71BM mixed phase. They conclude that proximity to a pure fullerene phase is essential for efficient charge separation, and the drastically increased number of sites with such proximity in the DIOprocessed sample plays a large role in its increased photovoltaic performance. Hedley et al.35 recently used the time-resolved emission of the PC71BM to study the diffusion of PC71BM excitons to the polymer interface, thereby directly learning about the PC71BM agglomerate size. Combining this optical technique with photoconductive atomic force microscopy (AFM), they found the morphology after processing with DIO suggested that the phases formed fibrils roughly 10 nm in width and 100 nm in length. Similar conclusions concerning the nanomorphology were recently obtained in a study regarding blends based on the polymer PBDTTT-C and PC71BM with varying portions of DIO.36 For further progress in this field, it is important to understand the interrelation between nanomorphology, photophysics, and carrier dynamics. For example, the impact of geminate recombination has also been successfully investigated in time-delayed collection field (TDCF) experiments, where a dependence of the extracted photocurrent on the prebias indicated that geminate recombination of CT states contributes to the bias dependence of the photocurrent in PTB7 devices processed without DIO.37,38 However, the high interfacial area created in the mixed polymer−fullerene phase in the PTB7 blends could be expected to favor the formation of CT states, even in blends processed with DIO. This motivates us to selectively and directly study the subset of emissive CT states in this high-performance benchmark system by time-resolving their near-infrared (NIR) emission after selective excitation of PTB7 and PC71BM, over a wide temperature range. A better understanding of the CT state and its dynamics is essential for developing the rational design of organic heterojunctions. The presented work is structured as follows. First, we compare time-resolved photoluminescence (TRPL) data of pristine films and a blend, which allows us to identify the signature of an interfacial emissive CT state. By performing selective excitation at 705 and 400 nm, we are able to address the polymer-rich mixed phase and the fullerene agglomerate domains of the blend separately. In the second and third part, we compare two blends with different PCBM loadings concerning their energy- and temperature-dependent dynamics. We observe that emissive CT states with a variety of energies are generated in PTB7 blends. They are formed preferentially after excitation of the polymer and, therefore, are likely to reside in the mixed phase and are not as likely to be formed when excitations are created within or near a fullerene aggregate. The kinetics of the CT states depends on their energy and the temperature. Higher-energy CT states are able to separate independently of temperature, whereas lowerenergy CT states can only separate at higher temperatures, indicative of charge separation with an activation energy on the order of 30 meV.
trap state is supported by our recent observations of the radiative recombination of such CT states in the homopolymer polythiophene/PC61CM system.20 On the other hand, efficient charge generation from CT states has been demonstrated after sub-bandgap excitation of several material systems.21,22 In a recent study, Vandewal et al.23 directly investigated the influence of charge generation from thermally relaxed CT states by exploiting the weakly allowed radiative transition to the ground state, which can be probed via electroluminescence (EL). They found that directly excited CT states yield separated charges with the same quantum efficiency irrespective of their absorption energy. This demonstrated that charge separation can occur efficiently through CT states irrespective of their energy. Even the lowest-energy CT states can contribute to charge generation. This is compelling evidence and, in our opinion, not inconsistent with our observations of CT-state photoluminescence (PL). In the experimental literature on CT states at organic interfaces, one must be aware of how different measurement techniques may probe different subsets of the CT density of states (DOS). Photothermal deflection spectroscopy probes the entire CT DOS (where absorption at a given energy is proportional to the DOS at this energy and the absorption cross section of the CT state with this energy). External quantum efficiency (EQE) measurements will weigh more heavily the subset of CT states in which charge generation is more efficient. Transient CT emission measurements, on the other hand, will be most influenced by the subset of CT states most likely to recombine before the charges separate, if such a subset exists. This implies that, if a subset of optically created CT states exists whose separation is slow or not possible, our transient PL measurements will be most influenced by these. These CT states will be a part of quantum efficiency loss mechanisms in the solar cell that reduce the internal quantum efficiency (IQE) from unity. We feel that the selectivity of time-resolved CT emission to study CT states, albeit perhaps a subset of the total population, plus the unique ability it gives to directly and selectively study CT states and their dynamics as a function of temperature make such studies a useful contribution to consider within the context of the broader experimental constellation. This will help to develop a comprehensive theoretical understanding of charge separation and organic interfaces encompassing all experimentally observed facets. Beside ongoing experimental work, also the theoretical understanding of charge generation and separation at organic heterojunctions is an active topic with a number of significant contributions in recent years.19,24−29 In general for organic semiconductors, device characteristics and optoelectronic properties depend on the morphology of the blend. An effective method of tuning the nanomorphology was found with the introduction of processing additives such as octanedithiol (ODT)30,31 and diiodooctane (DIO),7,32,33 which are high-boiling-point selective solvents for PCBM. For some donor−acceptor polymers the solvent additive increases the polymer and fullerene domain sizes, similar to annealing of poly(3-hexylthiophene) (P3HT) blends, leading to enhanced absorption of red photons in the polymer, better charge separation, and better charge extraction. For PTB7 blends, the additive also has a strongly beneficial effect on device performance, but here it reduces the domain size of fullerene clusters that are originally too large for efficient device function. There have been various detailed studies of the morphology change induced by cosolvents in PTB7 blends.34−36 Recent
2. EXPERIMENTAL METHODS 2.1. Device Fabrication and Standard Characterization. Samples were fabricated from the polymer thieno[3,4-b]thiophene-alt-benzoditiophene (PTB7, purchased from 1-Material Inc.) and the fullerene derivative [6,6]-phenyl-C71butyric acid methyl ester (PC71BM, purchased from Solenne B
DOI: 10.1021/acs.jpcc.5b09842 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a) Two-dimensional false color plots of the recorded PL data at a sample temperature of 10 K. The PTB7 film was excited at 705 nm, whereas for PCBM a wavelength of 400 nm was chosen due to the weak absorptivity around 700 nm. The PTB7/PCBM blend investigated here revealed a PCBM content of 60%. The PL data is normalized and presented with a logarithmic scale to allow comparison of the faint CT emission. Panel (b) shows PL spectra of the pristine films and the blend for different time intervals after optical excitation. The time ranges were chosen for demonstration of (i) the initial PL signatures and (ii) the emissive CT state. In (c) the transients for pristine films and the blend are plotted for different excitation conditions. The transients were integrated over the whole spectral window provided by the instrument, corresponding to an energy range of approximately 1.10−1.85 eV.
BV), which were dissolved in dichlorobenzene solution with 4% of the solvent additive diiodooctane (DIO). Devices of pristine films and blends were fabricated in the same manner. Initially, a layer of zinc acetate dehydrate (ZAD) was spin-cast and afterward annealed for 40 min at 200 °C. Subsequently, the active layer was deposited. Beside pristine films of PTB7 and PCBM, two blends were fabricated. One of them comprised a PCBM loading of 60%, which has been reported as the optimized stoichiometric ratio.7 The other blend revealed a higher PCBM fraction of 75%. The films were annealed for 10 min at 60 °C. Finally, metal electrodes were evaporated, consisting of molybdenium oxide and a 150 nm layer of aluminum. Current−voltage curves of the solar cells were measured in an Oriel solar simulator and recorded with a Keithley 2400 source meter, giving photoconversion efficiencies of 5% (60% PCBM) and 4% (75% PCBM), respectively (see Figure S1). 2.2. Time-Resolved Photoluminescence. The TRPL studies presented in this work were carried out with a streak camera (Hamamatsu, C5680), which was operated in synchroscan mode and triggered to the 80 MHz repetition rate of a titanium-sapphire oscillator (Spectra Physics, Tsunami). The sensitivity of the streak camera, employing a S1 cathode, extended from the visible to the near-infrared (300−1300 nm), allowing us to study the faint emission of CT states. The time resolution of the experiments was limited to the streak camera response time, which was 3 ps in the mode with the best time resolution. In the experiments carried out here, we estimate the time resolution to 35 ps, according to the
rise time of the recorded transients in a time window of 2 ns. The laser was tunable over a wide range of wavelengths from 700 to 1000 nm, allowing for selective excitation of PTB7 at a wavelength of 705 nm. To achieve an excitation wavelength of 400 nm, we employed a lithium triborate crystal for second harmonic generation. Sample degradation under blue excitation was identified by a nonreversible decrease in PL lifetimes of the PCBM emission. Under red excitation no change of the PL signatures was observed, even after long exposure times and for high fluences. However, to exclude sample degradation, the excitation fluences were kept at moderate values around 0.5 μJ/ cm2 for the experiments shown here. Additionally, to avoid oxygen exposure, the samples were mounted in an evacuated microscope cryostat, which also allowed us to vary the sample temperature between room temperature and 10 K.
3. RESULTS 3.1. Selective Excitation of PTB7 and PC71BM. To study the influence of intermixing on the emission properties, we first compare the PL signatures of pristine PTB7 and PC71BM films to that of a blend with a fullerene content of 60%, as summarized in Figure 1. Figure 1a shows streak camera images of the PL emission with a logarithmic intensity scale, recorded at a sample temperature of 10 K. After preferential excitation of the fullerene at 400 nm (3.1 eV), the blend luminescence spectrum is dominated by the PL signature of the PCBM; the prominent peak of PC71BM at 800 nm (1.55 eV) is present in the blend at early times (
