Transient photoinduced absorption spectra of organic photovoltaic materials
4.1.1. Spectral signatures of photoinduced processes in polymer fullerene blends A simplified scheme of energy levels in organic materials for solar cells is presented in
fig. 4.1. A more appropriate representation should draw energy levels with respect to the vacuum level as ionization potentials and electron affinities27 but was bypassed for the sake of simplicity. The diagrams in fig. 4.1 show only the dominating transitions, which are present in the materials studied in this Chapter. There is a huge difference between situations with only a single material, and cases with a combination of electron donor and acceptor materials. The two scenarios must be discussed separately.
In a pristine polymer, the lowest energy of a photon that can be absorbed is equal to the energy of the bandgap (Eg in fig. 4.1a). This transition is presented in fig. 4.1 as from S0 (ground state) to S1 (lowest excited state, a singlet exciton in Chapter 1 fig. 1.6a). Transitions from the ground state to higher energy levels are also available, however, any excess energy is quickly dissipated by vibrational relaxation (as shown in Chapter 1 fig. 1.2) to the lowest excited state S128. Singlet (Frenkel) excitons can convert into triplet excitons via intersystem crossing, where the excited electrons flip the spin (T0 in fig. 4.1).
It was reported that pristine polymers can produce photocurrent12; therefore, charge transfer is definitely feasible between two polymer molecules or within different segments of the same polymer chain. When charge transfer occurs, a CT exciton is formed (fig. 1.6b in Chapter 1). Moreover, some polymers, called push-pull polymers, are composed of molecules
that have alternating electron donating and electron accepting units, hence allowing for the formation of CT excitons already within a repeating unit of the polymer (fig. 1.6b in Chapter 1). CT excitons subsequently can form CT states if energy barrier appears between the wavefunction spatial distributions of electron and hole such as typically found at the donor-acceptor interfaces. Direct light absorption leading to the formation of a CT exciton or a CT state is also a known phenomenon29-33 (ECT transition in fig. 4.1 to the CT0 level). When the polymer is photoexcited to the S1 state, transitions from the S1 to Sn states become available;
this process is called excited-state absorption, or exciton absorption. The energy required for the transition to the lowest Sn level typically lies in the IR region with energies ranging from
~0.7 eV to > 1 eV, depending on the particular material (fig. 4.2 Exciton).
Fig. 4.1 Energy level diagram of donor:acceptor systems: when the donor or the donor-acceptor CT state is photoexcited (a) and when the acceptor is photoexcited (b). The vertical scale represents the energy of electronic levels. The blue horizontal lines (on the left side of the diagrams) represent energy levels of the electron donor material; the red horizontal lines (on the right side of the diagrams) represent energy levels of the electron acceptor material. Solid lines represent unrelaxed energy levels while dotted lines represent renormalization of the levels after the charge induced structural relaxation (i.e. the states S"0, S"1 and CT'0). Note that S'0 is equivalent to S0 but with the major difference – it is a state available only when excitation and structural relaxation are present. Green arrows Eg and ECT represent photoexcitation in the bandgap and directly to the charge transfer state, respectively. Magenta arrows HE (high energy), LE (low energy) and CTA (CT absorption of a vibrationally relaxed CT state) represent photoinduced transitions (absorption) due to structural relaxation around positive charges (for HE and LE) and charge transfer states (for CTA), respectively. The magenta arrow S1 =>Sn represents the first singlet to the n-th singlet transition. Arrows ET, HT, and CS represent electron transfer, hole transfer and charge separation, respectively. T0 stands for a triplet exciton state.
When a polymer is mixed with an acceptor molecule, absorption from the ground state also leads to singlet exciton formation. There is a probability of electron transfer from a donor to an acceptor (ET in fig. 4.1) at the donor-acceptor interface. When such charge transfer occurs the wavefunctions of electron and hole reside on different molecules, thereby
105 forming a CT exciton (fig. 1.6b in Chapter 1). However, a CT exciton is not easily detected experimentally. There are two reasons for this: 1) the CT exciton is extremely short-lived (in general this does not apply for a pristine material because the energy barrier created by the LUMO-LUMO and HOMO-HOMO offset present at the donor-acceptor interface, shown in fig. 1.4 and 1.5, is usually missing in the pristine material) and forms a CT state (fig. 1.6c in Chapter 1) within the measured electron (or hole) transfer time of 25-50 fs34-39 and, 2) the spectral response of both CT exciton and CT state is broad and often have overlap. The donor-acceptor materials put together form a CT state to which absorption may occur from the ground state. Therefore, CT state (or CT exciton followed by ultrafast formation of CT state) can be directly photoexcited29-33 (see the ECT transition in fig. 4.1 to the CT0 level). As was already mentioned, the CT state can separate into charges, called CS states (CS in fig.
1.6d in Chapter 1 and fig. 4.1).
A presence of any uncompensated charges at the polymer, be it CS states or CT states, forces the surrounding nuclei of the molecules to reorganize, or in other words to geometrically relax40. The charge surrounded by the geometric reorganization within the molecules, is called a polaron11. Formation of the polaron renormalizes the molecular energy levels31, 45 causing appearance of new transitions not present in the pristine material (dotted lines in fig. 4.1). Typical energy of CT exciton should be close to that of a Frenkel exciton (fig. 4.2). CT states and CS states have low energy (LE) and high energy (HE) transitions due to the renormalized energy levels31, 45 (fig. 4.2). Typical energies of CT states and CS states are at ~0.3-0.5 eV (LE) and around 1 eV (HE).
The acceptor molecules can be photoexcited as well41. The process of appearance of CT or CS states has a certain symmetry to the electron-donor excitation, except that instead of the electron transfer, hole transfer occurs39. The final CS (charges) states should be the same as in the scenario when the donor molecule is photoexcited. The only difference lies in the probability of formation of this state, for instance, either via hot state or by thermal activation, depending on the particular HOMO-HOMO and LUMO-LUMO offsets between donor-acceptor molecules42. Therefore, the rate of generation for CS states, in general, is not the same when comparing donor and acceptor excitation43. However, molecular systems that are optimized for OPV usually have ultrafast charge generation rate34, 35, 39 for both the donor and acceptor excitation (see also Chapter 2 for the hole transfer).
4.1.2. Time signatures of photoinduced processes in polymer fullerene blends