Marina Gerhard

THz generation at 1.55 µm excitation: six-fold increase in THz conversion efficiency by separated photoconductive and trapping regions

We present first results on photoconductive THz emitters for 1.55µm excitation. The emitters are based on MBE grown In0.53Ga0.47As/In0.52Al0.48As multilayer heterostructures (MLHS) with high carrier mobility. The high mobility is achieved by spatial separation of photoconductive and trapping regions. Photoconductive antennas made of these MLHS are evaluated as THz emitters in a THz time domain spectrometer (THz TDS). The high carrier mobility and effective absorption significantly increases the optical-to-THz conversion efficiency with THz bandwidth in excess of 3 THz.

64 μW pulsed terahertz emission from growth optimized InGaAs/InAlAs heterostructures with separated photoconductive and trapping regions

We present results on optimized growth temperatures and layer structure design of high mobility photoconductive Terahertz (THz) emitters based on molecular beam epitaxy grown In0.53Ga0.47As/In0.52Al0.48As multilayer heterostructures (MLHS). The photoconductive antennas made of these MLHS are evaluated as THz emitters in a THz time domain spectrometer and with a Golay cell. We measured a THz bandwidth in excess of 4 THz and average THz powers of up to 64 μW corresponding to an optical power-to-THz power conversion efficiency of up to 2 × 10−3.

Room temperature excitonic recombination in GaInN/GaN quantum wells

The dependence of radiative and nonradiative lifetimes on the excess carrier density in GaInN/GaN quantum wells is studied via time-resolved photoluminescence spectroscopy over a wide range of excitation densities. Our results differ from the predictions of simple free-carrier models: density independent radiative lifetimes clearly evidence the excitonic nature even at room temperature. At high densities, nonradiative lifetimes are weakly temperature dependent and proportional to the inverse of the density, implying an excitonic, threshold-less Auger process. Furthermore, in the intermediate density regime between low and high injection, an increase of the nonradiative lifetimes is observed, which is typical for Shockley-Read-Hall-type recombination.

Hybrid continuous wave terahertz spectroscopy

We propose a hybrid architecture for continuous wave terahertz spectroscopy employing a conventional two color photomixing system combined with a quasi time domain spectrometer, driven by a multimode laser diode. This approach fuses high spectral intensity with broadband frequency information and overcomes the ambiguity of standard continuous wave thickness measurements.

Time-resolved spectroscopy of charge transfer phenomena in organic solar cells

Geminate recombination of photo-generated excitons represents a considerable loss mechanism in polymer solar cells. We apply time-resolved photoluminescence (TRPL) to study the radiative recombination which accompanies the process of charge generation. A streak camera is used, which is sensitive for both the photoluminescence (PL) from the initially excited singlet excitons and the weaker emission from charge transfer (CT) states. The latter are formed at internal interfaces when the polymer is blended with a fullerene acceptor. We draw a comparison between our results for two polymers, P3HT and PTB7, respectively, which were studied in blends with the fullerene derivative PCBM. In addition, pristine films were investigated, allowing for the identification of interfacial features in the blends. For both polymers, the PL of the singlet states was rapidly quenched in blends with PCBM. In P3HT, time constants of about 40 ps were recorded for the singlet exciton decay and related to exciton diffusion, whereas the PL of PTB7 was almost completely quenched within the first 3 ps. The decay rates of the emissive CT excitons were 2-3 orders of magnitude smaller than those of the singlet state. Yet, due to their slower dynamics (~ 500 ps), they could be separated from the superimposed singlet emission. The CT decay times in blends with P3HT exhibited no significant temperature dependence, indicating that thermally driven dissociation of emissive excitons is unlikely. For blends with PTB7, however, a faster decay of the CT emission was obtained at room temperature.

Field-induced exciton and CT-state dissociation probed by time-resolved luminescence quenching (Conference Presentation)

The microscopic mechanisms of exciton and charge-transfer-state dissociation in organic semiconductors play a major role for the efficiencies of organic solar cells [1]. One of the most direct experiments for probing the dynamics of these processes is luminescence quenching. Here, we present a comprehensive experimental and simulative study of the field and temperature dependence of the dissociation of singlet excitons in PTB7 and PC71BM, and from charge-transfer states generated across interfaces in PTB7/PC71BM bulk heterojunction solar cells. We deduce the relevant data from time-resolving the near infrared emission of the states from 10K to room temperature and for electric fields ranging from 0 to 2.5 MV/cm. To draw qualitative conclusions from our data, we use an analytical field-assisted hopping model in the presence of disorder [2]. We conclude that singlet excitons can be split by high fields, and that disorder plays a large role in broadening the critical threshold field for which singlet excitons are separated. Charge-transfer (CT) state dissociation can be induced by both field and temperature, and the data imply that a strong reduction of the Coulomb binding potential at the interface facilitates their separation. The observations provided herein of the field dependent separation of CT states as a function of temperature offer a rich dataset against which theoretical models of charge separation can be rigorously tested. References: [1] H. Bassler and A. Kohler, Phys. Chem. Chem. Phys. 17, 28451 (2015) [2] O. Rubel, S. D. Baranovskii, W. Stolz, and F. Gebhard, Phys. Rev. Lett. 100, 196602 (2008).

Loss mechanisms in organic solar cells based on perylene diimide acceptors studied by time-resolved photoluminescence

In organic photovoltaics (OPV), perylene diimide (PDI) acceptor materials are promising candidates to replace the commonly used, but more expensive fullerene derivatives. The use of alternative acceptor materials however implies new design guidelines for OPV devices. It is therefore important to understand the underlying photophysical processes, which either lead to charge generation or geminate recombination. In this contribution, we investigate radiative losses in a series of OPV materials based on two polymers, P3HT and PTB7, respectively, which were blended with different PDI derivatives. Our time-resolved photoluminescence measurements (TRPL) allow us to identify different loss mechanisms by the decay characteristics of several excitonic species. In particular, we find evidence for unfavorable morphologies in terms of large-scale pure domains, inhibited exciton transport and incomplete charge transfer. Furthermore, in one of the P3HT-blends, an interfacial emissive charge transfer (CT) state with strong trapping character is identified.

Identifying interfacial charge transfer states in organic heterostructures(Conference Presentation)

Charge transfer (CT) states play evidently an important role at the interface of organic heterostructures but their identification and characterization is often experimentally less obvious and challenging. We studied two exemplary material systems which both represented a benchmark within the research of organic photovoltaics at their time: the homopolymer P3HT blended with PC61BM and the copolymer PTB7 blended with PC71BM. In both heterostructures, we could identify a distinct CT state emission by the use of NIR time-resolved photoluminescence (PL) [1], [2]. The selectivity of this technique enables us to clearly probe the energetics and dynamics of weak emitting interfacial states and therefore to prove differences in the CT state characteristics between the two systems. We went beyond this previous work and investigated the time and temperature dependent emission anisotropy as well as the electric field dependence of the time-resolved PL for both blends and the pristine polymers, respectively. In both cases the CT state emission clearly deviates from the one of the primarily excited singlet excitons: the emission anisotropy reveals an additional relaxation pathway for the exciton which is connected with a change of the transition dipole moment of the emission, and under applied bias different quenching thresholds can give access to varying binding energies of the emissive excitons involved. Finally, we think that our findings demonstrate how interfacial CT state emission can be clearly identified as such and how it can be unambiguously distinguished from singlet exciton emission.

Charge transfer states as traps in organic solar cells (Presentation Recording)

We investigate the NIR time-resolved photoluminescence of a series of P3HT:PC61BM solar cells with varying blend ratios after preferential excitation of the PC61BM and P3HT components respectively. Besides the rapid and diffusion-limited quenching of singlet excitons we resolve a weak emission feature in the near-infrared that our measurements confirm comes from interfacial charge-transfer (CT) states. This CT state emission becomes stronger for samples with an excess of PC61BM, and also after selective excitation of the PC61BM component. In this way, we show that these NIR time-resolved photoluminescence measurements provide an accurate method of observing subtle changes in the formation and dynamics of CT states at organic heterojunctions due to its high selectivity, and suggest that PC61BM excitons are more likely to lead to geminately recombining CT states than are the excitons created on P3HT. We also measure the temperature dependence of the transient NIR photoluminescence and find that while the intensity of the NIR emission is temperature dependent, its lifetime is not. This interesting observation suggests that the CT states we observe are formed through a precursor state which can either form separated charges or CT states, and that the relative yield of these two pools is temperature dependent. Furthermore, it indicates that charges within these relaxed CT states are trapped at the donor-acceptor interface and cannot contribute to free-charge generation via thermal activation anymore.

Efficiency droop in nitride LEDs revisited: impact of excitonic recombination processes

The efficiency droop in nitride LEDs is currently attributed to either carrier-density-dependent nonradiative recombination or to carrier leakage, both being discussed in terms of a single-particle picture. Our time-resolved photoluminescence results show that the radiative lifetime is independent of carrier density, while the nonradiative lifetime scales with the inverse of the carrier density. This can not be understood in a single-particle model. By means of a many-particle theory approach we obtain a consistent picture with both radiative and Auger recombination enhanced by excitonic electron-hole correlation. In the high carrier density limit single-particle radiative and Auger recombination are recovered.