Photocurrent-Detected 2D Electronic Spectroscopy Reveals Ultrafast Hole Transfer in Operating PM6/Y6 Organic Solar Cells

Abstract

The performance of nonfullerene-acceptor-(NFA)-based organic solar cells is rapidly approaching the efficiency of inorganic cells. The chemical versatility of NFAs extends the light-harvesting range to the infrared, while preserving a considerably high open-circuit-voltage, crucial to achieve power-conversion efficiencies >17%. Such low voltage losses in the charge separation process have been attributed to a low-driving-force and efficient exciton dissociation. Here, we address the nature of the subpicosecond dynamics of electron/hole transfer in PM6/Y6 solar cells. While previous reports focused on active layers only, we developed a photocurrent-detected two-dimensional spectroscopy to follow the charge transfer in fully operating devices. Our measurements reveal an efficient hole-transfer from the Y6-acceptor to the PM6-donor on the subpicosecond time scale. On the contrary, at the same time scale, no electron-transfer is seen from the donor to the acceptor. These findings, putting ultrafast spectroscopy in action on operating optoelectronic devices, provide insight for further enhancing NFA solar cell performance.

Figure 1

(a) Pulse-by-pulse phase modulation scheme: the pulses of a 3 kHz amplified Ti:Sa laser, after going through a NOPA and prism compressor, are shaped by Dazzler. The shaping is made such that from one laser pulse 4 delay- and phase-controlled pulses are generated. At every following laser pulse a phase delay is introduced in 3 of the 4 generated pulses such that their phases are modulated with different frequencies. Laser pulses illuminate the photovoltaic cell that generates a current/voltage signal, read-out by a National Instruments card. This allows to retrieve a modulated signal (b) of which the FFT spectrum (c) gives the components of the 4th order population at different frequencies. (d) The 2DES maps are rebuilt by extracting the components at all time-delays. After data processing and Fourier transform along t₁ and t₃, (e) the standard rephasing and non rephasing maps are obtained and (f) the sum of these give the absorptive maps.

Figure 2

(a) Molecular structures of PM6 and Y6. (b) EQE spectrum of the cell, with value close to 80% in visible and NIR ranges. (c) Normalized absorption spectra of single components compared with the laser spectrum. (d) Pictorial representation of the BHJ layer made of PM6 and Y6. The various processes that occur within ultrafast range associated excitation-detection transitions are illustrated. (e) 2DES coordinates of these combination. Numbers 1 and 5 represent GSB for the same transition, for example, when the excitation happens within a moiety. Numbers 2, 3, and 4 represent the possible mechanism at interface, electron, and exciton transfer from PM6 to Y6 and the hole transfer from Y6 to PM6, respectively. If after excitation (bold lines), a transfer occurs between the materials, the GSB will be detected by the transition in the other material (dashed lines) and visualized at cross-peaks in the 2DES map.

Figure 3

(a) Evolution of absorptive 2DES maps between 50 and 1500 fs. (b) Decay associated map associated with 1500 fs decay. Positive and negative amplitude are associated with decreasing and increasing signal, respectively. (c) Population time evolutions of diagonal- and cross-peak plotted with their global fitting curves. (d) A schematic view of the carriers’ distributions just after the excitation and after hole transfer happening.