Influence of Charge Carrier Mobility on the Performance of Organic Solar Cells

The power conversion efficiency of organic solar cells based on donor--acceptor blends is governed by an interplay of polaron pair dissociation and bimolecular polaron recombination. Both processes are strongly dependent on the charge carrier mobility, the dissociation increasing with faster charge transport, with raised recombination losses at the same time. Using a macroscopic effective medium simulation, we calculate the optimum charge carrier mobility for the highest power conversion efficiency, for the first time accounting for injection barriers and a reduced Langevin-type recombination. An enhancement of the charge carrier mobility from $10^{-8}$m$^2$/Vs for state of the art polymer:fullerene solar cells to about $10^{-6}$m$^2$/Vs, which yields the maximum efficiency, corresponds to an improvement of only about 20% for the given parameter set.Comment: 3 pages, 4 figure

Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge-Transfer State Electroluminescence

Charge-transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free-carrier bimolecular recombination at the donor-acceptor interface, rather than a charge-trap-mediated process. The fingerprint of the presence of nonradiative trap-assisted recombination, a voltage-dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination-intensity dependence of the open-circuit voltage

Trap-limited electron transport in disordered semiconducting polymers

The electron transport in diodes of poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives is strongly reduced as compared to the hole transport. A recent reexamination [M. M. Mandoc , Phys. Rev. B 73, 155205 (2006)] revealed that the room-temperature electron current shows the fingerprints of trap-limited transport with a distribution of traps in energy. Here, we report on the measured temperature dependence of the electron current in these PPV derivatives. This dependence is weak and seems to be in contradiction with existing trap-limited models. We demonstrate that the presence of a Gaussian density of states (DOS) for the mobile carriers, being characteristic for disordered semiconductors, reduces the temperature dependence of the trap-limited charge transport. The reduction is governed by the width of the Gaussian DOS and originates from the equilibrium concentrations of the mobile and trapped carriers

Effective passivation of Si surfaces by plasma deposited SiOx/a-SiNx:H stacks

Very low surface recombination velocities 800¿°C

Comparison between Al2O3 surface passivation films deposited with thermal ALD, plasma ALD and PECVD

Surface passivation schemes based on Al2O3 have enabled increased efficiencies for silicon solar cells. The key distinguishing factor of Al2O3 is the high fixed negative charge density (Qf = 1012-1013 cm-2), which is especially beneficial for p- and p+ type c-Si, as it leads to a high level of field-effect passivation. Here we discuss the properties of Al2O3 surface passivation films synthesized with plasma atomic layer deposition (ALD), thermal ALD (using H2O as oxidant) and PECVD. We will show that with all three methods a high level of surface passivation can be obtained for Al2O3 deposited at substrate temperatures in the range of 150-250oC. Furthermore, the role of chemical and field-effect passivation will be briefly addressed. It is concluded that the passivation performance of Al2O3 is relatively insensitive to variations in structural properties. Al2O3 is therefore a very robust solution for silicon surface passivation