arXiv subjects

V. Dyakonov

Publications and source records attributed to V. Dyakonov.

41 records · Page 3Linked to original sources

arXiv2008

Bipolar Charge Transport in Poly(3-hexyl thiophene)/Methanofullerene Blends: A Ratio Dependent Study

We investigated the charge carrier mobility in pristine poly(3-hexyl thiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend devices by applying the time resolved photoconductivity experiment in dependence on the donor:acceptor ratio. We observe a bipolar transport in all studied samples ranging from pure polymer to polymer:fullerene with 90% PCBM content. For the ratios P3HT:PCBM 1:4 and 1:1 we observe two transit times in the electron current transients, as well as hole double transients for P3HT:PCBM 1:2. We find high hole and electron mobilities in the order of 10^(-3) - 10^(-2) cm^2/Vs for a concentration of 90% PCBM in the blend.

cond-mat.mtrl-sci↗

arXiv2008

Polaron Recombination in Pristine and Annealed Bulk Heterojunction Solar Cells

We determined the dominant polaron recombination loss mechanism in pristine and annealed polythiophene:fullerene blend solar cells by applying the photo-induced charge extraction by linearly increasing voltage (photo-CELIV) method in dependence on temperature. In pristine samples, we find a strongly temperature dependent bimolecular polaron recombination rate, which is reduced as compared to the Langevin theory. For the annealed sample, we observe a polaron decay rate which follows a third order of carrier concentration almost temperature independently.

cond-mat.mtrl-sci↗

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2007

Spectroscopy of electronic defect states in Cu(In, Ga)(S, Se)$_2$-based heterojunctions and Schottky diodes under damp-heat exposure

The changes of defect characteristics induced by accelerated lifetime tests on the heterostructure n-ZnO/i-ZnO/CdS/Cu(In, Ga)(S, Se)$_2$/Mo relevant for photovoltaic energy conversion are investigated. We subject heterojunction and Schottky devices to extended damp heat exposure at 85$^{\circ}$C ambient temperature and 85% relative humidity for various time periods. In order to understand the origin of the pronounced changes of the devices, we apply current--voltage and capacitance--voltage measurements, admittance spectroscopy, and deep-level transient spectroscopy. The fill factor and open-circuit voltage of test devices are reduced after prolonged damp heat treatment, leading to a reduced energy conversion efficiency. We observe the presence of defect states in the vicinity of the CdS/chalcopyrite interface. Their activation energy increases due to damp heat exposure, indicating a reduced band bending at the Cu(In, Ga)(S, Se)$_2$ surface. The Fermi-level pinning at the buffer/chalcopyrite interface, maintaining a high band bending in as-grown cells, is lifted due to the damp-heat exposure. We also observe changes in the bulk defect spectra due to the damp-heat treatment.

cond-mat.mtrl-sci↗

arXiv2007

Band Bending Independent of Surface Passivation in ZnO/CdS/Cu(In,Ga)(S,Se)$_2$ Heterojunctions and Cr/Cu(In,Ga)(S,Se)$_2$ Schottky Contacts

We have employed admittance spectroscopy and deep-level transient spectroscopy in order to investigate the electronic properties of ZnO/CdS/Cu(In,Ga)(S,Se)$_2$ heterojunctions and Cr/Cu(In,Ga)(S,Se)$_2$ Schottky contacts. Our work concentrates on the origin of an energy-distributed defect state commonly found in these systems. The activation energy of the defect state addressed continuously shifts upon air annealing or damp-heat treatment and is a valuable measure of the degree of band bending in Cu(In,Ga)(S,Se)$_2$-based junctions. We demonstrate that the band bending within the Cu(In,Ga)(S,Se)$_2$ layer, reported in the literature to become minimal after air exposure, returns after the formation of either a Schottky contact or a heterojunction. The earlier phenomenon turns out to be independent of a surface passivation due to the CdS bath deposition.

cond-mat.mtrl-sci↗