TY  - JOUR
A1  - Salert, Beatrice Ch. D.
A1  - Krueger, Hartmut
A1  - Bagnich, Sergey A.
A1  - Unger, Thomas
A1  - Jaiser, Frank
A1  - Al-Sa'di, Mahmoud
A1  - Neher, Dieter
A1  - Hayer, Anna
A1  - Eberle, Thomas
T1  - New polymer matrix system for phosphorescent organic light-emitting diodes and the role of the small molecular co-host
JF  - Journal of polymer science : A, Polymer chemistry
N2  - A new matrix system for phosphorescent organic light-emitting diodes (OLEDs) based on an electron transporting component attached to an inert polymer backbone, an electronically neutral co-host, and a phosphorescent dye that serves as both emitter and hole conductor are presented. The inert co-host is used either as small molecules or covalently connected to the same chain as the electron-transporting host. The use of a small molecular inert co-host in the active layer is shown to be highly advantageous in comparison to a purely polymeric matrix bearing the same functionalities. Analysis of the dye phosphorescence decay in pure polymer, small molecular co-host film, and their blend lets to conclude that dye molecules distribute mostly in the small molecular co-host phase, where the co-host prevents agglomeration and self-quenching of the phosphorescence as well as energy transfer to the electron transporting units. In addition, the co-host accumulates at the anode interface where it acts as electron blocking layer and improves hole injection. This favorable phase separation between polymeric and small molecular components results in devices with efficiencies of about 47 cd/A at a luminance of 1000 cd/m(2). Investigation of OLED degradation demonstrates the presence of two time regimes: one fast component that leads to a strong decrease at short times followed by a slower decrease at longer times. Unlike the long time degradation, the efficiency loss that occurs at short times is reversible and can be recovered by annealing of the device at 180 degrees C. We also show that the long-time degradation must be related to a change of the optical and electrical bulk properties.
KW  - charge transport
KW  - conducting polymer
KW  - degradation
KW  - host-guest systems
KW  - light-emitting diodes
KW  - random copolymer
KW  - synthesis
KW  - UV-vis spectroscopy
Y1  - 2013
U6  - https://doi.org/10.1002/pola.26409
SN  - 0887-624X
VL  - 51
IS  - 3
SP  - 601
EP  - 613
PB  - Wiley-Blackwell
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Proctor, Christopher M.
A1  - Kim, Chunki
A1  - Neher, Dieter
A1  - Thuc-Quyen Nguyen, 
T1  - Nongeminate recombination and charge transport limitations in diketopyrrolopyrrole-based solution-processed small molecule solar cells
JF  - Advanced functional materials
N2  - Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fill factor (62%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono-DPP system (2 x 10(-5) cm(2) V-1 s(-1) versus 34 x 10(-5) cm(2) V-1 s(-1)). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.
KW  - charge transport
KW  - solar cells
KW  - photovoltaic devices
KW  - organic electronics
KW  - characterization tools
Y1  - 2013
U6  - https://doi.org/10.1002/adfm.201202643
SN  - 1616-301X
SN  - 1616-3028
VL  - 23
IS  - 28
SP  - 3584
EP  - 3594
PB  - Wiley-VCH
CY  - Weinheim
ER  -