Design of Nonfullerene Acceptors with Near‐Infrared Light Absorption Capabilities

Abstract

A series of narrow bandgap electron acceptors was designed and synthesized for efficient nearinfrared (NIR) organic solar cells. Extending π-conjugation of donor frameworks led to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7-Th, IOTIC-2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm −2 with small energy losses (~0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7-Th:IOTIC-2F-based cells achieve a power conversion efficiency (PCE) of 12.1% with good visible transparency (52% transmittance from 370 -740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC-2F, into the PTB7-Th:IOTIC-2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy-level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize non-fullerene acceptor properties for efficient NIR organic photovoltaics.

Chlorine Effects of Heterocyclic Ring‐Based Donor Polymer for Low‐Cost and High‐Performance Nonfullerene Polymer Solar Cells

“…As shown in Figure b, we surveyed and calculated a figure of merit (FOM‐ n = SC/PCE nonfullerene ) and a PCE based on NFAs (PCE nonfullerene ) for the commercial polymers and P(Cl). We obtained PCE nonfullerene values for P3HT, PTB7‐Th, and PBDB‐T, in each case, as the highest recorded performance based on an NFA in a binary‐component single cell, as found in the most recently published literature (Table S10, Supporting Information). Each polymer is ranked according to its FOM‐ n based on its SC and PCE, and some issues relating to the design of the materials, along with some insights about the syntheses, are now discussed.…”

Quantifying the Nongeminate Recombination Dynamics in Nonfullerene Bulk Heterojunction Organic Solar Cells

“…Another difference between bulk-and surface-trap assisted recombination is that the hole mobility μ p was used for the latter. [42] Conversely, the existence of surface recombination mainly decreases the V OC , especially at higher illumination intensities (Figure 6b). Commonly, annealing at higher temperatures can help to remove residual compounds that may act as trapping centers for charge carriers, although each ZnO/BHJ interface has to be specifically investigated under different conditions, which is beyond the scope of this study.…”

“…The value of β depends on the ratio of recombination mechanisms, ideally with β = 1 for trap-assisted recombination (R t = k t n), [55] β = 2 for bimolecular recombination (R bm = k bm n 2 ), and β = 3 for Auger recombination, although the latter case is usually not relevant for organic solar cells. [42,58] It should be noted that in the analysis shown in ref. [56] Similar to alternative methods [57] used to determine mobility other than the well-known Mott-Gurney relationship, in this case, a field and charge carrier dependent, effective mobility μ eff (n,V) can be considered, which for the studied, optimized blend system yields a value that falls within the hole and electron mobilities reported in the literature ( Figure S5, Supporting Information).…”

“…[1][2][3][4][5][6] Most high-performing OSCs use fullerene-based acceptors in a bulk-heterojunction (BHJ) configuration, and integral parameters and the density of charge carriers under different operating conditions. [34,[39][40][41][42] Thus, it is of great importance to develop a general quantitative model that describes total nongeminate recombination losses which originate from different recombination processes. In the case of inverted NFA-OSCs in particular, V OC versus light intensity dependence routinely exhibits additional contributions that are not related to bimolecular recombination, which manifests itself with slopes smaller than kT/q ( Figure S1, Supporting Information).…”

“…As a first step, the light intensity dependence of the V OC is measured to qualitatively determine the type of recombination dominating the studied solar cells, where the slope obtained from the linear fit of the V OC versus the natural logarithm of the light intensity indicates whether the charge recombination is bimolecular recombination (slope ≈kT/q, where k is the Boltzmann-constant, T the absolute temperature, and q the elementary charge) and/or trap-assisted recombination (slope >kT/q). This detailed quantitative analysis of the recombination dynamics was applied to OSCs (V OC = 0.75 V; J SC = 16.8 mA cm -2 ; FF = 0.69; PCE = 8.6%; EQE max = 74.9 %) which considerably deviate from pure bimolecular recombination (slope = 0.8kT/q) using an active layer with poly[[2,6′-4,8-di(5-ethylhexylthienyl)-benzo [1,2-b;3,3-b] dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno- [3,4-b]thiophenediyl]] (PTB7-Th) as the donor and the NFA ITIC-2F [42] as the acceptor (Figure 1) via a quantitative combination of light-intensity dependent current densityvoltage characteristics, capacitance spectroscopy, and V OC decay. [34,[39][40][41][42] Thus, it is of great importance to develop a general quantitative model that describes total nongeminate recombination losses which originate from different recombination processes.…”