[Paper Review] Two-dimensional coherent photocurrent excitation spectroscopy in a polymer solar cell

Eleonora Vella, Pascal Grégoire+7 more|arXiv (Cornell University)|Jun 25, 2015

Spectroscopy and Quantum Chemical Studies3 citations

TL;DR

This study introduces two-dimensional coherent photocurrent excitation spectroscopy (2DPC) to probe ultrafast charge generation dynamics in polymer solar cells. By measuring off-diagonal spectral correlations in a 2D photocurrent spectrum with femtosecond resolution, the technique reveals coherent coupling between polymer excitons and delocalized charge-transfer states, with experimental data showing a 100 meV off-diagonal signal consistent with noise-induced resonant tunnelling, as predicted by quantum mechanical models.

ABSTRACT

In high-performance solar cells based on polymeric semiconductors, the mechanism of photocarrier generation on $<100$-fs timescales is yet to be unravelled. In particular the dynamics of early-time electronic coupling between excitons on polymer chains and charge-transfer states need to be investigated in order to develop a detailed picture of ultrafast processes involved in photocurrent production. In this proceeding, we report preliminary measurements using a novel spectroscopy that can measure such correlations: two-dimensional coherent photocurrent excitation spectroscopy. This nonlinear technique measures off-diagonal spectral correlations in a two-dimensional photocurrent excitation spectrum. We interpret these sectroscopic measurements in light of recent theoretical predictions.

Motivation & Objective

• To investigate the ultrafast mechanisms of photocarrier generation in polymeric semiconductors on sub-100 fs timescales.
• To address the unresolved question of how tightly bound excitons on polymer chains rapidly evolve into free charges despite high binding energies (~0.5 eV).
• To develop and apply a novel nonlinear spectroscopic technique capable of probing off-diagonal couplings between localized excitonic states and delocalized charge-transfer states.
• To directly measure photocurrent excitation spectra in the operating regime of organic solar cells, enabling observation of correlations relevant to primary charge generation.

Proposed method

• The study employs a four-pulse, collinear femtosecond laser setup based on acousto-optic phase modulation to generate ultrafast excitation sequences.
• Two-dimensional photocurrent excitation spectroscopy (2DPC) is used to record rephasing and non-rephasing 2D spectra, with signal detection via a two-channel lock-in amplifier.
• The total correlation function maps are constructed by summing rephasing and non-rephasing spectra, enhancing signal-to-noise ratio and revealing off-diagonal cross peaks.
• The experimental 2DPC spectra are compared with theoretical simulations based on a finite-temperature, quantum mechanical model of polymer heterojunctions, incorporating noise-induced transitions between excitonic and charge-separated states.
• Population waiting time is set to 50 fs to probe early-time dynamics in the photocurrent response.
• The technique is applied to a working polymer solar cell under conditions matching actual device operation, ensuring relevance to real photocurrent generation.

Experimental results

Research questions

• RQ1 What is the nature of the electronic coupling between intra-chain excitons and delocalized charge-transfer states during ultrafast charge separation in polymer solar cells?
• RQ2 Can off-diagonal spectral correlations in a 2D photocurrent spectrum reveal coherent dynamics involved in early-stage charge generation?
• RQ3 How do environmental fluctuations (noise) influence the transition from localized excitons to delocalized charge carriers on sub-100 fs timescales?
• RQ4 To what extent do experimental 2DPC spectra match theoretical predictions of noise-induced resonant tunnelling between excitonic and charge-separated states?
• RQ5 Can 2DPC spectroscopy resolve the mechanism of rapid electron-hole separation, as suggested by previous transient absorption and Raman studies?

Key findings

• The 2DPC experiment successfully recorded high-signal-to-noise 2D photocurrent spectra on a working polymer solar cell, demonstrating the feasibility of the technique under operational conditions.
• A distinct off-diagonal signal at approximately 100 meV above the diagonal was observed in the total correlation spectrum, indicating spectral coupling between excitonic and delocalized charge-transfer states.
• The experimental 2DPC spectra show a pronounced high-energy tail and asymmetric line shape, consistent with the presence of coherent, delocalized states involved in early charge separation.
• The observed off-diagonal signal at 100 meV matches theoretical predictions from a finite-temperature quantum mechanical model that includes noise-induced resonant tunnelling between excitons and charge-separated states.
• The data support the hypothesis that ultrafast charge generation in polymer solar cells proceeds via coherent, delocalized pathways rather than sequential, localized charge-transfer states.
• The results provide direct experimental evidence for the role of environmental fluctuations in enabling rapid, coherent population transfer from localized excitons to delocalized charge carriers, explaining high internal quantum efficiency despite high exciton binding energy.