Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes

Qingzhen Bian,Qi Wei,Khadga J Karki,Xiaojun Su,Fei Ma,Weimin M Chen,Olle Inganäs,Carlito S Ponseca,Shula Chen,Mathieu Linares,Irina A Buyanova,Arkady Yartsev

doi:10.1038/s41467-020-14476-w

Abstract

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial ~200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.

Highlights

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices
In organic photovoltaics (OPVs), where the active layer consists of a nanostructured blend of donor (D) and acceptor (A) semiconductors, the key photophysical process is the generation of free charge carriers
Despite extensive studies indicating that the charge transfer (CT) state determines the photovoltage of OPVs, there are reports on the absence of this state in some efficient OPV material systems, claiming that long-range charge separation may occur directly from excitons generated within the polymer domain[1,2]

Summary

Introduction

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. Insights on the timescale and processes leading to the formation of mobile charge carriers can be obtained using ultrafast transient absorption spectroscopy (TA) with sub-50 femtosecond (fs) temporal resolution, complemented by transient photoluminescence, transient photoconductivity and transient two-dimensional photocurrent spectroscopy (2DPS). Both films and working devices have been investigated to confirm the consistency of the transient behavior. Our 2D photocurrent results clearly demonstrates a distinctive coherent photocurrent generation that persists for the initial ~200 fs in a high performance device, indicating that the vibronic (electronic-vibrational) coherences directly contributes to the formation of charge carriers and enhancement in the photocurrent in the device

Methods

Results

Conclusion