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:PC71BM (poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(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.
†Department of Physics and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.
Thomas
T
Kirchartz
†Department of Physics and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.
Tiziano
T
Agostinelli
‡Institut de Ciència de Materials de Barcelona, UAB Campus, 08193 Bellaterra, Spain.
Mariano
M
Campoy-Quiles
†Department of Physics and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.|§Key Laboratory of Luminescence and Optical Information, Ministry of Education and Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, People's Republic of China.
Wei
W
Gong
†Department of Physics and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.
Jenny
J
Nelson
Si-PCPDTBT; charged defects; doping; drift-diffusion simulations; interferences; matrix transfer algorithm; photovoltaics; polymer:fullerene