TY - GEN
T1 - Tailored heterojunctions for efficient thin-film organic solar cells
T2 - Organic Photovoltaics VIII
AU - Schueppel, R.
AU - Schmidt, K.
AU - Uhrich, C.
AU - Schulze, K.
AU - Wynands, D.
AU - Brédas, J. L.
AU - Maennig, B.
AU - Pfeiffer, M.
AU - Leo, K.
AU - Brier, E.
AU - Reinold, E.
AU - Bu, H. B.
AU - Baeuerle, P.
PY - 2007
Y1 - 2007
N2 - Recently, we have demonstrated an open circuit voltage of 1.0V and a power conversion efficiency of 3.4% in thin film solar cells, utilizing a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor and C60 as acceptor [1]. Stimulated by this result, we systematically study the energy and electron transfer processes taking place at the oligothiophene:fullerene heterojunction along a homologous series of these oligothiophenes, The heterojunction is modified by tuning the HOMO level using different oligothiophene chain lengths, while the LUMO level is essentially fixed by the choice of the acceptor-type end-groups (dicyanovinyl) attached to the oligothiophene. We study electron transfer at the heterojunction to C 60 using photoinduced absorption. The observed transitions are unambiguously identified by TD-DFT calculations. With increasing the effective energy gap of the donor-acceptor pair, charge carrier dissociation following the photoinduced electron transfer is eventually replaced by recombination into the triplet: state, which alters the photovoltaic operation conditions. Therefore, the optimum open-circuit voltage of a solar cell is a trade-off between an efficient charge separation at the interface and a maximized effective gap. We conclude that values between 1.0 and 1.1 V for the open-circuit voltage in our solar cell devices present an optimum, as higher voltages were only achieved with concomitant losses in charge separation efficiency.
AB - Recently, we have demonstrated an open circuit voltage of 1.0V and a power conversion efficiency of 3.4% in thin film solar cells, utilizing a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor and C60 as acceptor [1]. Stimulated by this result, we systematically study the energy and electron transfer processes taking place at the oligothiophene:fullerene heterojunction along a homologous series of these oligothiophenes, The heterojunction is modified by tuning the HOMO level using different oligothiophene chain lengths, while the LUMO level is essentially fixed by the choice of the acceptor-type end-groups (dicyanovinyl) attached to the oligothiophene. We study electron transfer at the heterojunction to C 60 using photoinduced absorption. The observed transitions are unambiguously identified by TD-DFT calculations. With increasing the effective energy gap of the donor-acceptor pair, charge carrier dissociation following the photoinduced electron transfer is eventually replaced by recombination into the triplet: state, which alters the photovoltaic operation conditions. Therefore, the optimum open-circuit voltage of a solar cell is a trade-off between an efficient charge separation at the interface and a maximized effective gap. We conclude that values between 1.0 and 1.1 V for the open-circuit voltage in our solar cell devices present an optimum, as higher voltages were only achieved with concomitant losses in charge separation efficiency.
KW - Donor-acceptor heterojunction
KW - Open circuit voltage
KW - Organic solar cells
KW - Triplet recombination
UR - http://www.scopus.com/inward/record.url?scp=42149137339&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=42149137339&partnerID=8YFLogxK
U2 - 10.1117/12.732295
DO - 10.1117/12.732295
M3 - Conference contribution
AN - SCOPUS:42149137339
SN - 9780819468048
T3 - Proceedings of SPIE - The International Society for Optical Engineering
BT - Organic Photovoltaics VIII
Y2 - 28 August 2007 through 30 August 2007
ER -