The
donor/acceptor weight ratio is crucial for photovoltaic performance
of organic solar cells (OSCs). Here, we systematically investigate
the photovoltaic behaviors of PM6:Y6 solar cells with different stoichiometries.
It is found that the photovoltaic performance is tolerant to PM6 contents
ranging from 10 to 60 wt %. Especially an impressive efficiency over
10% has been achieved in dilute donor solar cells with 10 wt % PM6
enabled by efficient charge generation, electron/hole transport, slow
charge recombination, and field-insensitive extraction. This raises
the question about the origin of efficient hole transport in such
dilute donor structure. By investigating hole mobilities of PM6 diluted
in Y6 and insulators, we find that effective hole transport pathway
is mainly through PM6 phase in PM6:Y6 blends despite with low PM6
content. The results indicate that a low fraction of polymer donors
combines with near-infrared nonfullerene acceptors could achieve high
photovoltaic performance, which might be a candidate for semitransparent
windows.
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.
Recently,
benefiting from the merits of small-molecule acceptors
(NFAs), polymer solar cells (PSCs) have achieved tremendous advances.
From the perspective of the structural characteristics of the π-conjugated
acceptor–donor–acceptor (A–D–A) type of
organic molecules, the backbone’s planarity and the terminal
groups and their substituents have strong influences on the performances
of the constructed NFAs. Through enlarging the dihedral angle of the
conjugated main chain of NFAs, a certain degree of enhancement of
photovoltaic parameters has been achieved. To further probe the influences
of ending groups on the performances of nonplanar NFAs, we synthesized
two new NFAs i-cc23 and i-cc34 with isomerized
thiophene-fused ending groups and a twisted π-conjugated main
chain. Compared to i-cc23 containing the 2-(6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene)malononitrile
ending group, the acceptor i-cc34 containing 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile
has a relatively higher molar extinction coefficient, bathochromic-shifted
absorption spectrum, and deepened energy levels. When mixed with PBDB-T
in solar cells, the i-cc23-based device achieved an excellent
open-circuit voltage (V
OC) of 1.10 V and
a moderate power conversion efficiency of 7.34%. Although the V
OC of the i-cc34-related device
was decreased to 0.96 V, the short-circuit current density and fill
factor were improved, giving rise to an enhanced efficiency of 9.51%.
Apart from the distinct photovoltaic performances, the two isomer-based
devices exhibit a high radiative efficiency of 8 × 10–4, leading to a very small nonradiative loss of 0.19 V. Our results
emphasize the importance of the isomerized thiophene-fused ending
groups on the performances of nonplanar NFA-based PSCs.
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