Comment on "Coherence and uncertainty in nanostructured organic photovoltaics".

Comment pubs.acs.org/JPCA Comment on “Coherence and Uncertainty in Nanostructured Organic Photovoltaics” Shaul Mukamel Department of Chemistry, University of California, Irvine, California 92697-2025, United States J. Phys. Chem. A 2013, 117. DOI: 10.1021/jp408700m I strong classical ﬁelds are used, the response and the signals depend on the spatial, temporal, and spectral proﬁle of the electric f ield, which is a collective representation of many photons. 2 Diﬀraction limits the beam size to λ/2, which happens to agree with “Heisenberg’s” estimate. 1 What a single photon does is irrelevant to these measurements. Charge transport must depend on properties of matter (energy levels, density of states, eﬀective size of quasiparticles, couplings) and cannot be universal. After all, real electrons have to move. Thus the desire to demonstrate universal “coherence eﬀects” in photovoltaics is misguided. There are two types of coherences in many-chromophore systems. The ﬁrst is related to exciton delocalization (i.e., the eigenstates can spread over few or many chromophores). 3−14 The second is coherence between eigenstates (i.e., when the incoming light prepares a superposition of eigenstates with well-deﬁned phases). Coherence between eigenstates is expected to decay on the time scale of the exciton bandwidth, but coherence within eigenstates, known as exciton delocaliza- tion, may persist for longer times. Exciton delocalization and, more generally, carrier delocalization are well understood phenomena. The exciton size can be attributed to the interplay of coupling and disorder. Delocalized excitons can span many chromophores and can thus give rise to fast transients that do not require exciton hopping. These eﬀects depend on details of material properties and are not universal. The Marcus Theory 15 when formulated in the delocalized basis 16 can give rise to fast electron transfer. There is nothing particularly novel or surprising about this. With classical light, elementary quantum mechanics tells us that, immediately following photoexcitation, molecules are prepared in a superposition of eigenstates with a broad bandwidth. Subsequent dephasing can create eigenstates. All of these depend on the material level structure, the eigenstates of the system, and the dipole matrix elements. The size of the light beam is a factor, for example, for ﬁelds shaped on the nanoscale, 17 but this only involves classical optics and has nothing to do with quantum uncertainty. The cited quantum eﬀects in photosynthesis 18 is another example where the quest for universal principles has triggered extensive activity centered on how biology uses quantum coherence to optimize the charge-separation yield. Phase coherence was found to persist for 0.5 ps in photon echo experiments of the light harvesting antenna complex FMO and other systems. However, despite the original claims, that ﬁnding n a recent paper 1 the authors make the following claims. (1) The spatially extended excited-state wave functions can be understood as originating from fundamental quantum uncertainty. During the light-absorption process, the existence of the photon is uncertain, implying a momentum uncertainty equal to the momentum of the photon. As a result, its position, as well as the position of the photo- excitation it creates, is uncertain, as required by Heisenberg’s famous equation: ΔxΔp ≥ ℏ/2. The length scale imposed by the uncertainty principle is λ/4π, which is greater than 20 nm for visible radiation. Thus, the photoexcitation process generates a delocalized coherent superposition of the eigenfunctions of the Schrodinger equation that describes the nanostructured organic photovoltaic blend. (2) A general means of understanding excited-state delocaliza- tion on short time scales is through fundamental quantum uncertainty; the initial photoexcitation is a coherent superposition of eigenfunctions of the Schrodinger equation, having a spatial distribution of λ/4π. (3) We observed a generality in the charge-generation dynamics of organic bulk heterojunction materials that is unexpected from the perspective of a highly localized initial photoexcitation. The creation of long-range coherent superposition states is a natural consequence of the Heisenberg uncertainty principle, as applied to the photon absorption process. We expect that phenomena of this type are important not only for organic bulk heterojunction solar cells but also for nanostructured materials in general. The authors have performed pump−probe measurements in various electron donor/acceptor blends and found similar fast transients , ℏ/2 for the absorbed photon. They argued that thanks to Heisenberg, the absorbed photon is delocalized and can cover the entire sample instantaneously, hence the ultrafast charge transfer time scale. They claim that exciton migration is not necessary for charge separation because a delocalized photon creates delocalized excitations. Because photon delocalization is involved, it is a universal mechanism. This argument is false on many fronts. The pump−probe signal does not depend on any property of individual photons or on the corpuscular nature of light. When © 2013 American Chemical Society Received: July 17, 2013 Published: September 13, 2013 dx.doi.org/10.1021/jp4071086 | J. Phys. Chem. A 2013, 117, 10563−10564