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Recent evidence indicates that foamy viruses (FVs) are the oldest retroviruses (RVs) that we know and coevolved with their hosts for several hundred million years. This coevolution may have contributed to the non-pathogenicity of FVs, an important factor in development of foamy viral vectors in gene therapy. However, various questions on the molecular evolution of FVs remain still unanswered. The analysis of the spectrum of animal species infected by exogenous FVs or harboring endogenous FV elements in their genome is pivotal. Furthermore, animal studies might reveal important issues, such as the identification of the FV in vivo target cells, which than require a detailed characterization, to resolve the molecular basis of the accuracy with which FVs copy their genome. The issues of the extent of FV viremia and of the nature of the virion genome (RNA vs. DNA) also need to be experimentally addressed.
Besides established, conventional inorganic photovoltaics—mainly based on silicon—organic photovoltaics (OPV) are well on the way to represent a lowcost, environment friendly, complementary technology in near future. Production costs, solar cell lifetime and performance are the relevant factors which need to be optimized to enable a market launch of OPV. In this work, the efficiency of organic solar cells and their limitation due to charge carrier recombination are investigated. To analyze solar cells under operating conditions, time-resolved techniques such as transient photovoltage (TPV), transient photocurrent (TPC) and charge extraction (CE) are applied in combination with time delayed collection field (TDCF) measurements. Solution processed and evaporated samples of different material composition and varying device architectures are studied. The standard OPV reference system, P3HT:PC61BM, is analyzed for various temperatures in terms of charge carrier lifetime and charge carrier density for a range of illumination intensities. The applicability of the Shockley Equation for organic solar cells is validated in case of field-independent charge photogeneration. In addition, a consistent model is presented, directly relating the ideality factor to the recombination of free with trapped charge carriers in an exponential density of states. An approach known as j=V reconstruction enables to identify the performance limiting loss mechanism of as-prepared and thermally treated P3HT:PC61BM solar cells. This procedure, involving TPV, CE and TDCF measurements, is extended to samples based on the rather new, low-band gap polymer PTB7 in combination with PC71BM. While in the devices processed from pure chlorobenzene solution considerable geminate and nongeminate losses are observed, the use of a solvent additive facilitates efficient polaron pair dissociation minimizing geminate recombination. Finally, in collaboration with the IMEC institute in Leuven, the two main organic solar cell device architectures, planar and bulk heterojunction—both based on CuPc and C60—are directly compared in terms of nongeminate recombination and charge carrier distribution. Two experimental techniques, TPV and CE, as well as a macroscopic device simulation are applied to reveal the origin of different Voc vs. light intensity dependence.
The focus of this work is studying recombination mechanisms occurring in organic solar cells, as well as their impact on one of their most important parameters — the open circuit voltage Voc.
Firstly, the relationship between Voc and the respective charge carrier density n in the active layer under open circuit conditions is analyzed. Therefor, a model after Shockley for the open circuit voltage is used, whose validity is proven with the aid of fits to the measured data. Thereby, it is emphasized that the equation is only valid under special conditions. In the used reference system P3HT:PC61BM the fits are in agreement with the measurement data only in the range of high temperatures (150 - 300 K), where Voc increases linearly with decreasing temperature. At lower temperatures (50 – 150 K), the experiment shows a saturation of Voc. This saturation cannot be explained with the model by the measured falling charge carrier density with decreasing temperatures. In this temperature range Voc is not directly related to the intrinsic properties of the active layer. Voc saturation is due to injection energy barriers at the contacts, which is ascertained by macroscopic simulations. Furthermore, it is observed that Voc in the case of saturation is equivalent to the so-called built-in potential. The difference between the built-in potential and the energy gap corresponds thereby to the sum of the energy barriers at both contacts.
With the knowledge of the Voc(n) dependency for not contact limited solar cells, it is possible to investigate the recombination mechanisms of charge carriers in the active layer. For Langevin recombination the recombination rate is Rn2 (recombination order RO = 2), for Shockley-Read-Hall (SRH) Rn1 (RO=1); in various publications RO higher than two is reported with two main explanations.
1: Trap states for charge carriers exist in the respective separated phases, i.e. electrons in the acceptor phase and holes in the donor phase, which leads to a delayed recombination of the charge carriers at the interface of both phases and finally to an apparent recombination order higher than 2.
2: The enhanced R(n) dependency is attributed to the so called recombination prefactor, which again is dependent from n dependent mobility µ.
It is shown that for the system P3HT:PC61BM at room temperature the µ(n) dependency does nearly completely explain the higher RO but not at lower temperatures which in this case supports the first explanation. In the material system PTB7:PC71BM the increased RO cannot be explained by the µ(n) dependency even at room temperature.
To support the importance of trap states in combination with a phase separation for the explanation of the enhanced RO, additional trap states were incorporated in the solar cells to investigate their influence on the recombination mechanisms. To achieve this, P3HT:PC61BM solar cells were exposed to synthetic air (in the dark and under illumination) or TCNQ was added in small concentrations to the active layer which act as electron traps. For the oxygen degraded solar cell the recombination order is determined by a combination of open Voc-transients and Voc(n) measurements. Thereby, a continuous increase of the recombination order from 2.4 to more than 5 is observed with higher degradation times. By the evaluation of the ideality factor it can be shown that the impact of SRH recombination is increasing with higher trap concentration in relation to Langevin recombination. A similar picture is revealed for solar cells with TCNQ as extrinsic trap states.
Finally, a phenomenon called s-shaped IV-curves is investigated, which can sometimes occur for solar cells under illumination. As course of this a reduced surface recombination velocity can be found. Experimentally, the solar cells were fabricated using a special plasma treatment of the ITO contact. The measured IV-curves of such solar cells are reproduced by macroscopic simulations, where the surface recombination velocity is reduced. Hereby, it has to be distinguished between the surface recombination of majority and minority charge carriers at the respective contacts. The theory can be experimentally confirmed by illumination level dependent IV-curves as well as short circuit current density and open circuit voltage transients.