TY  - JOUR
A1  - Tokmoldin, Nurlan
A1  - Vollbrecht, Joachim
A1  - Hosseini, Seyed Mehrdad
A1  - Sun, Bowen
A1  - Perdigón-Toro, Lorena
A1  - Woo, Han Young
A1  - Zou, Yingping
A1  - Neher, Dieter
A1  - Shoaee, Safa
T1  - Explaining the fill-factor and photocurrent losses of nonfullerene acceptor-based solar cells by probing the long-range charge carrier diffusion and drift lengths
JF  - Advanced energy materials
N2  - Organic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open-circuit voltage loss and fill-factor, with state-of-the-art OSCs rarely displaying fill-factor of 80% and above. The fill-factor of transport-limited solar cells, including organic photovoltaic devices, is affected by material and device-specific parameters, whose combination is represented in terms of the established figures of merit, such as theta and alpha. Herein, it is demonstrated that these figures of merit are closely related to the long-range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady-state photoconductance measurements. This yields a straightforward way of determining theta and alpha in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices-including the high efficiency nonfullerene acceptor blends-and show that the diffusion length of the free carriers provides a good correlation with the fill-factor. It is, finally, concluded that most state-of-the-art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.
KW  - diffusion length
KW  - drift length
KW  - figure of merit
KW  - lifetime‐ mobility product
KW  - steady‐ state photoconductance
Y1  - 2021
U6  - https://doi.org/10.1002/aenm.202100804
SN  - 1614-6840
VL  - 11
IS  - 22
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Ryll, Britta
A1  - Schmitz, Andreas
A1  - de Boor, Johannes
A1  - Franz, Alexandra
A1  - Whitfield, Pamela S.
A1  - Reehuis, Manfred
A1  - Hoser, Andreas
A1  - Müller, Eckhard
A1  - Habicht, Klaus
A1  - Fritscht, Katharina
T1  - Structure, phase composition, and thermoelectric properties of YbxCo4Sb12 and their dependence on synthesis method
JF  - ACS applied energy materials
N2  - We present a combined microscopic and macroscopic study of YbxCo4Sb12 skutterudites for a range of nominal filling fractions, 0.15 < x < 0.75. The samples were synthesized using two different methods — a melt–quench–annealing route in evacuated quartz ampoules and a non-equilibrium ball-mill route — for which we directly compare the crystal structure and phase composition as well as the thermoelectric properties. Rietveld refinements of high-quality neutron powder diffraction data reveal about a 30–40% smaller Yb occupancy on the crystallographic 2a site than nominally expected for both synthesis routes. We observe a maximum filling fraction of at least 0.439(7) for a sample synthesized by the ball-mill routine, exceeding theoretical predictions of the filling fraction limit of 0.2–0.3. A single secondary phase of CoSb2 is observed in ball-mill-synthesized samples, while two secondary phases, CoSb2 and YbSb2, are detected for samples prepared by the ampoule route. A detrimental influence of the secondary phases on the thermoelectric properties is observed for secondary-phase fractions larger than 8 wt % regardless of the kind of secondary phase. The largest figure of merit of all samples with a ZT ∼ 1.0 at 723 K is observed for the sample with a refined Yb content of x2a = 0.159(3), synthesized by the ampoule route.
KW  - thermoelectric materials
KW  - skutterudite
KW  - melt-quench-anneal
KW  - ball mill
KW  - neutron powder diffraction
KW  - thermoelectric properties
KW  - figure of merit
Y1  - 2017
U6  - https://doi.org/10.1021/acsaem.7b00015
SN  - 2574-0962
VL  - 1
IS  - 1
SP  - 113
EP  - 122
PB  - American Chemical Society
CY  - Washington
ER  -