4 research outputs found
Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge
We investigate the reasons for the dependence of photovoltaic
performance
on the absorber thickness of organic solar cells using experiments
and drift-diffusion simulations. The main trend in photocurrent and
fill factor versus thickness is determined by mobility and lifetime
of the charge carriers. In addition, space charge becomes more and
more important the thicker the device is because it creates field
free regions with low collection efficiency. The two main sources
of space-charge effects are doping and asymmetric mobilities. We show
that for our experimental results on Si-PCPDTBT:PC<sub>71</sub>BM
(polyÂ[(4,40-bisÂ(2-ethylhexyl)ÂdithienoÂ[3,2-<i>b</i>:20,30-<i>d</i>]Âsilole)-2,6-diyl-<i>alt</i>-(4,7-bisÂ(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl
C71-butyric acid methyl ester) solar cells, the influence of doping
is most likely the dominant influence on the space charge and has
an important effect on the thickness dependence of performance
Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge
We investigate the reasons for the dependence of photovoltaic
performance
on the absorber thickness of organic solar cells using experiments
and drift-diffusion simulations. The main trend in photocurrent and
fill factor versus thickness is determined by mobility and lifetime
of the charge carriers. In addition, space charge becomes more and
more important the thicker the device is because it creates field
free regions with low collection efficiency. The two main sources
of space-charge effects are doping and asymmetric mobilities. We show
that for our experimental results on Si-PCPDTBT:PC<sub>71</sub>BM
(polyÂ[(4,40-bisÂ(2-ethylhexyl)ÂdithienoÂ[3,2-<i>b</i>:20,30-<i>d</i>]Âsilole)-2,6-diyl-<i>alt</i>-(4,7-bisÂ(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl
C71-butyric acid methyl ester) solar cells, the influence of doping
is most likely the dominant influence on the space charge and has
an important effect on the thickness dependence of performance
Sensitivity of the Mott–Schottky Analysis in Organic Solar Cells
The application of Mott–Schottky analysis to capacitance–voltage
measurements of polymer:fullerene
solar cells is a frequently used method to determine doping densities
and built-in voltages, which have important implications for understanding
the device physics of these cells. Here we compare drift-diffusion
simulations with experiments to explore the influence and the detection
limit of doping in situations where device thickness and doping density
are too low for the depletion approximation to be valid. The results
of our simulations suggest that the typically measured values on the
order of 5 × 10<sup>16</sup> cm<sup>–3</sup> for doping
density in thin films of 100 nm or lower may not be reliably determined
from capacitance measurements and could originate from a completely
intrinsic active layer. In addition, we explain how the violation
of the depletion approximation leads to a strong underestimation of
the actual built-in voltage by the built-in voltage <i>V</i><sub>MS</sub> determined by Mott–Schottky analysis
Dependence of Charge Separation Efficiency on Film Microstructure in Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C<sub>61</sub> Butyric Acid Methyl Ester Blend Films
Herein we address the factors controlling photocurrent generation in P3HT:PCBM blend films as a function of blend composition and annealing treatment. Absorption, photoluminescence, and transient absorption spectroscopy are used to distinguish the role of exciton dissociation, charge pair separation, and charge collection. Variations in blend film microstructure with composition and annealing treatment are studied using X-ray diffraction. While the trend in photocurrent generation with composition and annealing [Muller, et al., <i>Adv. Mater.</i> <b>2008</b>, <i>20</i>, 3510] does not follow the trend in exciton dissociation, it closely follows the trend in charge pair generation. Moreover, charge pair generation efficiency is positively correlated to the degree of polymer crystallization and the appearance of large domains of both polymer and fullerene phases. We argue that larger domains assist charge pair separation by increasing the probability of escape from the P3HT:PCBM interface, thus reducing geminate charge recombination