Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

Abstract

Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.

Study of the Contributions of Donor and Acceptor Photoexcitations to Open Circuit Voltage in Bulk Heterojunction Organic Solar Cells

“…This implies that both the donor and acceptor photoexcitations can contribute to the achievable open circuit voltage as well as the photovoltaic performance of BHJ OSCs; this agrees with our earlier results [45] [68], 0.34-0.44 V for a range of donor-acceptor blends [69], 0.38 V for OC 1 C 10 -PPV:PCBM solar cells [12], and 0.30-0.60 V for BHJ OSCs [20]. Other low voltage offsets ( E DA/q − V j OC ) that have been reported recently are 0.25 V for an evaporated bilayer OSC [70], 0.23 V and 0.26 V for diketopyrrolopyrrole-thieno[2,3-f]benzofuran (DTD):PC 60 BM and DTD:naphthalene diimide acceptor-polymer (N2200) systems, respectively [71], and 0.34-0.40 V for BHJ OSCs [72], with which our calculated values also somewhat agree in the range.…”

Recent Advances in Nonfullerene Acceptors for Organic Solar Cells

“…In addition, when combined with the new PffBT‐T3(1,2)‐2 donor, which obtained a much higher PCE value over 10.0% with the PCBM acceptor, this acceptor yielded an excellent device performance about 6.90% without any further treatment. Very recently, Bartynski et al developed a novel nonfullerene small molecule acceptor D31 which was similar to the D27 acceptor . The UV–vis spectrum showed that this D31 molecule possessed a narrow absorption region from 450 to 600 nm and the CV result indicated this acceptor owned very low HOMO and LUMO energy levels about −6.00 and −3.88 eV, respectively.…”

Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages

“…1–3 Thus, a substantial amount of work on organic photovoltaics has focused on minimizing the voltage loss required for exciton dissociation and to achieve the best possible compromise between photocurrent and photovoltage. 4–9 Fig. 1 demonstrates the empirical consequences of this compromise showing that the smaller the voltage difference between the band gap E g (divided by the elementary charge q ) and the actual V oc , the smaller will be the efficiency of photocurrent generation represented by the maximum external quantum efficiency (EQE max ) (see Table S1, ESI† for details).…”