119 research outputs found
Charge density dependent nongeminate recombination in organic bulk heterojunction solar cells
Apparent recombination orders exceeding the value of two expected for
bimolecular recombination have been reported for organic solar cells in various
publications. Two prominent explanations are bimolecular losses with a carrier
concentration dependent prefactor due to a trapping limited mobility, and
protection of trapped charge carriers from recombination by a donor--acceptor
phase separation until reemission from these deep states. In order to clarify
which mechanism is dominant we performed temperature and illumination dependent
charge extraction measurements under open circuit as well as short circuit
conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-Cbutyric acid
methyl ester (P3HT:PCBM) and PTB7:PCBM
(Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]])
solar cells in combination with current--voltage characteristics. We show that
the charge carrier density dependence of the mobility and the
recombination prefactor are different for PCBM at temperatures below
300K and PTB7:PCBM at room temperature. Therefore, in addition to
a detrapping limited recombination in systems with at least partial
donor--acceptor phase separation is required to explain the high recombination
orders.Comment: 7 pages, 4 figure
Impact of nongeminate recombination on the performance of pristine and annealed P3HT:PCBM solar cells
Transient photovoltage (TPV) and voltage dependent charge extraction (CE)
measurements were applied to poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61
butyric acid methyl ester (PCBM) bulk heterojunction solar cells to analyze the
limitations of solar cell performance in pristine and annealed devices. From
the determined charge carrier decay rate under open circuit conditions and the
voltage dependent charge carrier densities n(V) the nongeminate loss current
jloss of the device is accessible. We found that jloss alone is sufficient to
describe the j-V characteristics across the whole operational range, for
annealed and, not yet shown before, also for the lower performing pristine
solar cells. Even in a temperature range from 300 K to 200 K nongeminate
recombination is found to be the dominant and, therefore, performance limiting
loss process. Consequently, charge photogeneration is voltage independent in
the voltage range studied.Comment: 3 pages, 3 figures; Rapid Research Letter 201
Influence of Phase Segregation on Recombination Dynamics in Organic Bulk-Heterojunction Solar Cells
We studied the recombination dynamics of charge carriers in organic bulk
heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl
thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric
acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The
techniques of charge carrier extraction by linearly increasing voltage
(photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC)
were used. We observed a difference in the initially extracted charge carrier
concentration in the photo-CELIV experiment by one order of magnitude, which we
assigned to an enhanced geminate recombination due to a fine interpenetrating
network with isolated phase regions in the 1:1 pBTCT-C12:PC61BM bulk
heterojunction solar cells. In contrast, extensive phase segregation in 1:4
blend devices leads to an efficient polaron generation resulting in an
increased short circuit current density of the solar cell. For both studied
ratios a bimolecular recombination of polarons was found using the
complementary experiments. The charge carrier decay order of above two for
temperatures below 300 K can be explained by a release of trapped charges. This
mechanism leads to a delayed bimolecular recombination processes. The
experimental findings can be generalized to all polymer:fullerene blend systems
allowing for phase segregation.Comment: 14 pages, 5 figure
Monomolecular and Bimolecular Recombination of Electron-Hole Pairs at the Interface of a Bilayer Organic Solar Cell
While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBTtype polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh2)(2) layer as donor material and C-60 as acceptor. The authors measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (L-p) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor-acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor-acceptor interface. This effect is absent in the p-DTS(FBTTh2)(2) diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness.The authors acknowledge financial support by the Bavarian State Ministry
of Science, Research, and the Arts through the Collaborative Research
Network âSolar Technologies go Hybridâ, by the Volkswagen foundation
and by the German Science Foundation DFG through the doctoral
training center âGRK 1640.â This project further received funding from
the Universidad Carlos III de Madrid, the European Unionâs Seventh
Framework Programme for research, technological development and
demonstration under grant agreement no. 600371, el Ministerio de EconomĂa y Competitividad (COFUND2014-51509), el Ministerio de
EducaciĂłn, cultura y Deporte (CEI-15-17), and Banco Santander. M.R.
additionally acknowledges support from the Hanns Seidel Foundation
for a stipend through funds from the German Ministry of Education
and Research (BMBF). T.-Q.N. thanks the Office of Naval Research
(#N000141410076) for the support. Furthermore, the authors would like
to thank the anonymous referees for helpful suggestions
Exciton diffusion length and charge extraction yield in organic bilayer solar cells
The authors thank the German BMBF for funding within the scope of the projects InnoProfile 2.2 (03IPT602X) and MEDOS (03EK3503A) as well as the European Commission within the scope of the Career Integration Grant (FP7, MSCA, 630864). I.D.W.S. and M.T.S. acknowledge support from the European Research Council (grant number 321305) and from EPSRC (grant number EP/L017008/1). I.D.W.S. also acknowledges a Royal Society Wolfson Research Merit Award. K.L. is a fellow of the Canadian Institute for Advanced Research (CIFAR). The research data supporting this publication can be accessed at https://doi.org/10.17630/af263bfb-620c-40a8-9929-86658e5187d3A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.PostprintPostprintPeer reviewe
High performance ternary solar cells based on P3HT:PCBM and ZnPc-hybrids
Single walled carbon nanotubes (SWCNTs) and reduced graphene oxide (rGO) covalently and non-covalently functionalised by zinc phthalocyanine (ZnPc) were added to P3HT:PCBM blend in order to investigate the effects of these hybrid materials on P3HT:PCBM organic solar cell performance. Adding a small amount of these hybrids to P3HT:PCBM blend does not significantly alter the absorption spectra of the latter nor its structure. ZnPcârGO and ZnPcâSWCNT hybrid features have appeared on the P3HT:PCBM surface morphology as verified by SEM and AFM images. However these hybrid materials have caused significant effects on the electrical properties of the studied blends. An increase of about two orders of magnitudes has been observed in the electrical conductivity. Space charge limited conduction theory was employed to investigate the charge carriers' mobility whereas a thermionic emission model was used to evaluate the recombination rate based on an estimated diode ideality factor. Solar cell devices based on P3HT:PCBM:ZnPcâSWCNTs-co bonded have demonstrated best device performance with PCE of 5.3%, Jsc of 12.6 mA cmâ2, Voc of 0.62 V and FF of 68%. A reference device based on bare P3HT:PCBM blend has exhibited PCE of just under 3.5%, Jsc of 9.3 mA cmâ2, Voc of 0.62 V and FF of 60%
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