Electrical Initialization and Readout of Triplet Exciton Spins

Molecular electronics offer several important advantages for spintronic, quantum sensing, and quantum computing fields. The ease of synthesis and fabrication combined with tunable spin physics makes some of these organic systems promising initial candidates for use in quantum information, however a key challenge is the ability to address individual molecular spins. Promising gate operations and coherence times have already been demonstrated with triplet exciton spin transfer using optical techniques, but electrical readout of these states would move these systems closer towards functional microelectronic circuits. Here we present a pathway towards electrical initialization and readout of triplet spins using the phenomenon of organic magnetoresistance (OMAR) with polycrystalline pentacene.<br/><br/>Pentacene and many of its derivatives and dimers, including 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), are studied for their ability to undergo a process known as singlet fission. Under illumination, these molecules readily excite to the first excited singlet state and efficiently and rapidly fission into two first excited triplet states onto neighboring molecules. This phenomenon has been fairly well explored for use in photovoltaics, and because of this, the process is well known to generate triplets of relative polarized spin due to their initial entanglement upon formation. Interactions of these polarized triplet spins with charge carriers creates a mixed state that holds information of both triplet spins. This triplet-charge spin-spin interaction is electrically detectable and forms the basis for singlet fission molecules to be explored for quantum information science. Polycrystalline pentacene is therefore a great model system for manipulation of these spin-spin interactions due to its large carrier mobility and ability to generate a high density of long-lived triplet excitations.<br/><br/>As the detection mechanism of these spin states, we look to theory of OMAR, where it has been shown that the interaction between triplets and mobile charges fundamentally changes the resistance through a thin film organic layer based on the applied magnetic field. We have fabricated polycrystalline pentacene devices that minimize the other commonly observed OMAR effects to isolate the response seen from these triplet-polaron encounters. Development of a quantum kinetic model through the application of the Stochastic Liouville equation of the triplet-charge mixed states reveals predictions that agree well with experiment. The model also provides predictions of the magnetoresistance when spin selective contacts are introduced, which would allow only anti-aligned triplet spins to quench, eventually purifying the triplet spins aligned with the applied field.<br/><br/>This triplet spin purification acts as an initialization process and can be achieved through a secondary thin film using a phenomenon known as chirality induced spin selectivity (CISS). This relatively newly discovered effect uses spiral-like chiral constituents to polarize either light or polaron spin of a specific orientation under an applied magnetic field. We show the effect of using the (S) and (R) chiral versions of (PEA)₂PbI₄perovskite as a polarizing hole injection layer into polycrystalline pentacene. These results combined with detection via magnetoresistance provide promising first steps in the room temperature initialization and readout of molecular triplet spins.