M-LPPP

Figure 8-2. (a) Normalized PL spectra of m-LPPP films al 7=77 K for exeila ion ai 3.2 eV (390 am) with a circular spot al Iwo different flu-ences 0.3 mJ/eni2 (solid line) and 2.4 mJ/cm2 (dashed line). The inset shows ihe room lempcralure absorption spectrum of nt-LPPP. (b) Normalized PL spectrum for excitation with a rectangular spot at a fluence of 0.055 mJ/cm2 (from Ref. 125J with permission). 

Figure 8-5. Transmission difference spectra of m-LPPP films at 7=77 K excited at 3.2 eV for various pump-probe delays. The inset zooms out the low energy region for 0 ps (solid line) and 400 ps (dashed line) delay. Doping induced absorption (D1A) data are also shown for comparison (from Ref. (251 with permission).

Figure 8-7. SE decay al 2.53 cV in m-LPPP films il T=77 K versus pump-probe delay at lhrce different excitation fluences (from Ref. 125] with permission).

We have studied the temporal dynamics of CPG in m-LPPP by performing field-assisted pump-probe experiments on LED structures, as described in Section 8.3.2. The narrow line-width PA assigned to polarons (see Section 8.5.2) is a fingerprint of charge generation in m-LPPP. Monitoring the dynamics of these PA band enables us, for the first time, to directly observe the CPG dynamics in a conjugated polymer with sub-picosecond time resolution [40],... 

Figure 8-16. A picture of the pholoexcilalion scenario in m-LPPP, see text for a discussion. Pv is a positively charged chain (polaron), while X- can be either a negatively charged chain or an electron acceptor, such as oxygen.

Figure 9-1. Materials overview a few sclcclcd conjugated polymers and Ihcir properties have been compiled and ihe following abbreviations arc used DO-PPP...Poly(2-decyloxy-l,4-phcnylcnc), EHO-PPP...Poly(2-(2 -elhylliexyloxy)-l,4-phcnylenc), CN-PPP... Poly(2-(6 -cyano-6 -incthyl-licplyloxy)-l,4-phcnylene), m-LPPP... methyl-substituted ladder-type Poly( 1,4-phenylcne), and PLQY=phololuinincs-ecncc quanluni yield.

In summary, all the transitions expected for the neutral states of a model system for conjugated polymers, the m-LPPP, were observed and described and all of these transitions also show clearly resolved vibronic replicas due to coupling to vibronic modes of the backbone. 

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

In Figure 8-1 we show the chemical structure of m-LPPP. The increase in conjugation and the reduction of geometrical defects was the main motivation to incorporate a poly(/ -phenylene)(PPP) backbone into a ladder polymer structure [21]. Due to the side groups attached to the PPP main chain excellent solubility in nonpolar solvents is achieved. This is the prerequisite for producing polymer films of high optical quality. A detailed presentation of the synthesis, sample preparation,... 

In this section experimental results are described, which are obtained by applying the conventional pump-probe technique to m-LPPP films kept in vacuum at the temperature of liquid nitrogen [25], These results allow the identification of the primary excitations of m-LPPP and the main relaxation channels. In particular, the low and high excitation density regimes are investigated in order to get an insight into the physical processes associated with the emission line-narrowing phenomenon. 

We assume that standard Coulomb-correlated models for luminescent polymers [11] properly described the intrachain electronic structure of m-LPPP. In this case intrachain photoexcitation generate singlet excitons with odd parity wavefunctions (Bu), which are responsible for the spontaneous and stimulated emission. Since the pump energy in our experiments is about 0.5 eV larger than the optical ran... 

It has been demonstrated that the whole photoexcitation dynamics in m-LPPP can be described considering the role of ASE in the population depletion process [33], Due to the collective stimulated emission associated with the propagation of spontaneous PL through the excited material, the exciton population decays faster than the natural lifetime, while the electronic structure of the photoexcited material remains unchanged. Based on the observation that time-integrated PL indicates the presence of ASE while SE decay corresponds to population dynamics, a numerical simulation was used to obtain a correlation of SE and PL at different excitation densities and to support the ASE model [33]. The excited state population N(R.i) at position R and time / within the photoexcited material is worked out based on the following equation ... 

---