Understanding Triplet Formation Pathways in Bulk Heterojunction Polymer:Fullerene Photovoltaic Devices

Abstract

Triplet exciton (TE) formation pathways have systematically been investigated in prototype bulk heterojunction (BHJ) SY-PPV:PCBM, SY-PPV:PC70BM, and SY-PPV:ICBA solar cell devices of varying compositions using complementary optoelectrical and electrically detected magnetic resonance (EDMR) spectroscopies. In this investigation it is shown that EDMR spectroscopy allows the unambiguous demonstration of fullerene triplet production in BHJ polymer:fullerene solar cells. EDMR triplet detection under selective photoexcitation of each blend component and of the interfacial charge transfer (CT) state unravels that low lying fullerene TEs are produced by direct intersystem crossing from singlet excitons (SEs). The direct CT-TE recombination pathway, although energetically feasible, is kinetically suppressed in these devices. However, high energy CT states in the CT manifold can contribute to the population of the fullerene triplet state via a direct CT-SE conversion. This undesirable energetic alignment could be one of the causes for the severe reduction in photocurrent observed when the open circuit voltage of polymer:fullerene solar cells is pushed to or beyond 1.0 V.

Triplet Tellurophene‐Based Acceptors for Organic Solar Cells

“…Thes inglet state can be calculated from the absorption onset, [50] and the energies of the CT states is measured from the D/A blends emission. [51] Thet riplet state has been found to consistently lie about 0.6-0.7 eV below the --…”

The Twisted Benzo[ghi]‐Perylenetriimide Dimer as a 3D Electron Acceptor for Fullerene‐Free Organic Photovoltaics

“…Consequently, charge recombination from charged states to the triplet state of PDI can be thermodynamically favorable and efficient. In contrast to fullerene system, the triplet formation of fullerene is thermodynamically inhibited owing to their higher triplet excited state energies such as PC 61 BM ([6,6]‐phenyl‐C 61 ‐butyric acid methyl ester; E (T 1 )=1.50 eV) . In the rylene imide dye family, particularly, benzo[ ghi ]‐perylenetriimide (BPTI) shows a remarkably high triplet energy level of 1.69 eV .…”

“…In contrastt of ullerene system, the triplet formation of fullerenei st hermodynamically inhibited owing to their highert riplet excited state energies such as PC 61 BM ( [6,6]-phenyl-C 61 -butyric acid methyl ester; E(T 1 ) = 1.50 eV). [22] In the rylene imide dye family,p articularly, benzo[ghi]-perylenetriimide (BPTI) shows ar emarkably high triplete nergy level of 1.69 eV. [23,24] Thus, addressing the issue of triplet state formation through ac harge recombination pathway for the loss mechanism in organic photovoltaic cells, BPTI shows its potential as an ideal buildingb lock for the noveln onfullerene acceptord esign.…”

Higher triplet state of fullerene C70 revealed by electron spin relaxation

“…2 While the triplet formation via direct intersystem crossing is usually strongly suppressed when fullerenes are mixed with donor materials, triplet excitons can still affect the photocurrent generated in polymer:fullerene solar cells. 3,4 In particular, the formation of triplet excitons via back electron transfer is a crucial step that can limit the free charge carrier yield and thus the efficiency of organic solar cells. [5][6][7] Moreover, triplet excitons play a key role in photochemical upconversion 8 and downconversion 9 processes that open the intriguing perspective of increasing the efficiencies of solar cells beyond the Shockley-Queisser limit.…”