Perylene Sensitization of Fullerenes for Improved Performance in Organic Photovoltaics

Abstract

We present the addition of an energy relay dye to fullerenes resulting in increased light harvesting and significantly improved power conversion efficiency for organic photovoltaic (OPV) devices. Although exhibiting excellent properties as electron acceptors, visible light absorption of fullerenes is limited. Strongly light absorbing donor materials are needed for efficient light harvesting in the thin active layer of OPV devices. Therefore, photocurrent generation and thus power conversion efficiency of this type of solar cell is confined by the overlap of the relatively narrow absorption band of commonly used donor molecules with the solar spectrum. Herein the concept of fullerene dye sensitization is presented, which allows increased light harvesting on the electron acceptor side of the heterojunction. The concept is exemplarily shown for an UV absorbing small molecule and a near infrared absorbing polymer, namely hexa‐peri‐hexabenzocoronene (HBC) and Poly[2,1,3‐benzothiadiazole‐4,7‐diyl[4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b']dithiophene‐2,6‐diyl]] (PCPDTBT), respectively. In both systems remarkably higher power conversion efficiency is achieved via perylene sensitization of the fullerene acceptor. Steady state photoluminescence, transient absorption and transient photocurrent decay studies reveal pathways of the additionally generated excited states at the sensitizer molecule. The findings suggest fluorescence resonance energy transfer from the photo‐excited dye to the fullerene enabling decoupling of light absorption and charge transport. The presented sensitization method is proposed as a viable new concept for performance enhancement in organic photovoltaic devices.

Sensitization of fullerenes by covalent attachment of a diketopyrrolopyrrole chromophore

“…14,15 There are a few examples of dye molecules used as sensitizers in BHJ solar cells (the active spectral range is increased due to dye absorption, followed by various energy and charge transfer steps depending on the system). [16][17][18][19][20][21][22] However, this typically involves adding the dye as a third component to the donor-acceptor blend. A second challenge to improve BHJ device efficiency is the control of the nanoscale morphology for optimal charge formation and transport.…”

The influence of additive property on performance of organic bulk heterojunction solar cells

“…In the case of sensitization of PC 61 BM, using the PDI as sensitizer can improve its photon absorption, thus significantly enhanced photovoltaic performance [77]. Adding small amounts of PDI to PC 61 BM yields an increase emission from the fullerene, which is attributed to an energy transfer from the strongly absorbing perylene dye to the PC 61 BM.…”

“…One possible reason for this might be provided by the energy-level distribution of the P3HT, SM, and PC 61 BM as mentioned in ''Change the energy-level distribution and enhance the ordering of P3HT'' section. Using the sensitizers as additive, such as PDI and SiPc, could improve the optical property of some component [77,78], leading an enhanced J sc of device. Adding SiPc as additive, the P3HT/PC 61 BM device exhibits a distinguished increase in J sc from 6.5 to 7.9 mA/cm 2 after the thermal annealing.…”

“…While the sensitizer for PC 61 BM, such as 2,9-di(pent-3-yl)-anthra [2,1,9-def:6,5,10-d 0 e 0 f 0 ] diisoquinoline-1,3,8,10-tetrone (PDI), could be used to make up for the weak photon absorption of PC 61 BM. There is energy transfer from photoexcited PDI molecules to PC 61 BM, which results in strongly increased light harvesting of PC 61 BM and leads to improved light harvesting and current generation [77]. In addition to adding sensitizer for electron acceptor, introduction of sensitizer for electron donor is also an effective method to increase the optical absorption.…”

Bridging donor–acceptor energy offset using organic dopants as energy ladders to improve open-circuit voltages in bulk-heterojunction solar cells

“…Among many elements that control V oc , the energy band alignments of the donor-acceptor in the BHJ devices have been reported as the major factor that limit the value of V oc [3][4][5]. Since V oc in donor-acceptor systems correlates to the energetic difference between the highest occupied molecular orbital (HOMO) of the electron donor and lowest unoccupied molecular orbital (LUMO) of the electron acceptor [6][7][8], one of the route toward a larger V oc is to incorporate low-band gap semiconducting polymers with deeper HOMOs as donors [9,10] and electron-accepting fullerene derivatives with shallower LUMOs as acceptors [11,12]. Although many efforts have been devoted to modify the HOMO-LUMO offset of the donor-acceptor by introducing additional substituents or replacing the function group in molecules to enhance the V oc of BHJ solar cells, there is normally more than 0.3 V difference between V oc and the HOMO-LUMO offset.…”