Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Transport and Spectroscopic Studies of the Effects of Fullerene
  Structure on the Efficiency and Lifetime of Polythiophene-based Solar Cells   Time-dependent measurements of both power conversion efficiency and
ultraviolet-visible absorption spectroscopy have been observed for solar cell
blends containing the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) with two
different functionalized C60 electron acceptor molecules: commercially
available [6,6]-phenyl C61 butyric acid methyl ester (PCBM) or [6,6]-phenyl C61
butyric acid octadecyl ester (PCBOD) produced in this laboratory. Efficiency
was found to decay with an exponential time dependence, while spectroscopic
features show saturating exponential behavior. Time constants extracted from
both types of measurements showed reasonable agreement for samples produced
from the same blend. In comparison to the PCBM samples, the stability of the
PCBOD blends was significantly enhanced, while both absorption and power
conversion efficiency were decreased.

2
The categories of this paper are: math
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
36
The categories of this paper are: cmp-lg
26
The categories of this paper are: bayes-an
20
The categories of this paper are: acc-phys
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
3
The categories of this paper are: physics
15
The categories of this paper are: nucl-ex
11
The categories of this paper are: hep-lat
29
The categories of this paper are: funct-an