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
(poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-<i>b</i>:20,30-<i>d</i>]silole)-2,6-diyl-<i>alt</i>-(4,7-bis(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