Bipolaron Formation in Organic Solar Cells Observed by Pulsed Electrically Detected Magnetic Resonance

Abstract

We report the observation of a spin-dependent dark transport current, exhibiting spin coherence at room temperature, in a -conjugated polymer-fullerene blend using pulsed electrically detected magnetic resonance. The resonance at g ¼ 2:0028ð3Þ is due to polarons in the polymer, and exhibits spin locking at high microwave fields. The presence of an excess of fullerene, and the operating voltage (1 V) used, suppresses negative polaron formation in the polymer. It is concluded that spin-dependent transport is due to the formation of positive bipolarons. DOI: 10.1103/PhysRevLett.105.176601 PACS numbers: 72.80. Le, 71.38.Mx, 72.25.Rb, 76.30.Mi Organic semiconductors provide a range of commercial optoelectronic display devices, and show great promise in the field of photovoltaics (PV) [1][2][3][4] if improved efficiency, combined with low cost and ease of production, can be achieved. In addition, the weak spin-orbit coupling of organic semiconductors is attractive for carrier spin transport and manipulation, and is driving efforts to develop spintronic devices [5]. All these applications depend upon detailed knowledge of the relevant transport processes, in particular, those influenced by spin selection rules. The observation of large magnetoresistance (MR) has, for example, attracted a range of explanations [6][7][8][9], but aspects of the proposed mechanisms remain controversial [10].Conduction in disordered organic semiconductors is dominated by hopping of charge carriers between localized states. Because of strong electron-phonon coupling the carriers are polarons. Oppositely charged polarons can form excitons and eventually recombine; the process normally depends on the spin state of the coupled pair immediately prior to exciton formation. In addition, the strong coupling between carriers and the environment can markedly reduce the energy cost of doubly occupying states. Two like-charge polarons can form a bipolaron, the correlation energy between the pair and the lattice deformation lowering the formation energy [11]. However, the on-site exchange requires that the final state is a spin singlet, and bipolaron formation will be ''spin-blocked'' if two polarons have the same spin component along the common axis of quantization [8].Organic PV devices have advanced dramatically with the development of bulk heterojunction materials [2][3][4], which comprise a -conjugated polymer blended with an electron acceptor such as a fullerene derivative. The two phase-separated components give interpenetrating networks with vastly increased interfacial regions [3]. The PVeffect is due to photoexcitation of the polymer, followed by highly efficient electron transfer to the fullerene phase. Positive polarons (P þ ) are transported through the polymer matrix, negative polarons through the fullerene phase, efficiently suppressing carrier loss by P þ P À recombination. However, unipolar transport to the electrodes may be influenced by bipolaron formation. Gaining insight into this process, which affects charge carrier collection...

Tuning organic magnetoresistance in polymer-fullerene blends by controlling spin reaction pathways

“…As such, this provides an alternative route towards unravelling underlying mechanisms. To do so, we performed finite element drift-diffusion simulations 19,31,38,41 for realistic parameters (please refer to the Supplementary Note 3 for more details). Here we will briefly discuss the MC(V) in all three regimes, starting with regime (2).…”

Slow Hopping and Spin Dephasing of Coulombically Bound Polaron Pairs in an Organic Semiconductor at Room Temperature

“…The correspondence between electrically and optically detected pulsed electron spin resonance experiments [7] on similar structures indicates that the polaron pairs seen in this work are oppositely charged. However, this method may also be of use for materials which contain bipolaron pairs [8] (pairs of likecharge polarons) or other more complex spin pairings [4].…”

Polaron spin echo envelope modulations in an organic semiconducting polymer

“…(13) shows that, for all orientations of the polymer chains, the maximum modulation depth of the contact hyperfine protons is 0.05 and the maximum depth of the distant protons is 0.007 (recall that, for MEH-PPV, in Eqs. (10) and (11) the contact protons are labelled by the subscript, 1 ≤ j ≤ N c , where N c = 22, and the distant protons are labelled by N c < j ≤ N ).…”

“…[7][8][9] Substantial progress in this direction was made by the pulsed EDMR (pEDMR) experiments. [10][11][12][13][14][15][16] Unlike the continuous wave measurements, these experiments are capable of probing the coherent spin dynamics, and thus provide a closer view on the spin-dependent processes. Importantly, pEDMR (and pODMR) offer the implementation of various spin-echo based spectroscopic techniques in the study of organic semiconductors.…”