Key Parameters Requirements for Non‐Fullerene‐Based Organic Solar Cells with Power Conversion Efficiency >20%

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10−3 cm2 V−1 s−1 in combination with low nongeminate recombination rate constants of 10−12 cm3 s−1 could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics

Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells

In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs

Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3HT

Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3H

Homo-Tandem Polymer Solar Cells with V<inf>OC</inf>1.8 V for Efficient PV-Driven Water Splitting

High-voltage tandem and triple-junction polymer solar cells (PSCs) were demonstrated by using one of the highest- V OC, high-efficiency polymer donors in bulk heterojunction (BHJs) with fullerenes, namely poly(benzo[1,2- b :4,5- b'] dithiophene?thieno[3,4- c ]pyrrole-4,6-dione) (PBDTTPD). While the efficiency of PBDTTPD-based single-junction PSCs is limited by incomplete optical absorption, we show that homo-tandem PSCs with MoO3 /ultrathin Al/ZnO intermediate recombination layers can achieve higher PCE values. The PV performance of single-junction PBDTTPD:PC 71 BM PSCs were investigated with the standard device structure ITO/PEDOT:PSS/PBDTTPD:PC71 BM/Ca/Al. Next, the potential for further PCE increments were examined via a triple-junction PSC device approach, using the same ZnO/Al/MoO3 interconnection layer between the three subcells. PBDTTPD homo-tandem PSCs yield high operating voltages of 1.54 V at their maximum power point, providing sufficient potential for the dissociation of water and the evolution of hydrogen and oxygen in a standard electrochemical cell

Electropolymerized Star-Shaped Benzotrithiophenes Yield π‑Conjugated Hierarchical Networks with High Areal Capacitance

High-surface-area π-conjugated polymeric networks have the potential to lend outstanding capacitance to supercapacitors because of the pronounced faradaic processes that take place across the dense intimate interface between active material and electrolytes. In this report, we describe how benzo­[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″]­trithiophene (<b>BTT</b>) and tris­(ethylenedioxythiophene)­benzo­[1,2-<i>b</i>:3,4-<i>b</i>′:5,6-<i>b</i>″]­trithiophene (<b>TEBTT</b>) can serve as 2D (trivalent) building blocks in the development of electropolymerized hierarchical π-conjugated frameworks with particularly high areal capacitance. In comparing electropolymerized networks of <b>BTT</b>, <b>TEBTT</b>, and their copolymers with EDOT, we show that <b>TEBTT</b>/EDOT-based copolymers, i.e., P­(<b>TEBTT</b>/EDOT), can achieve higher areal capacitance (e.g., as high as 443.8 mF cm^–2 at 1 mA cm^–2) than those achieved by their respective homopolymers (<b>PTEBTT</b> and PEDOT) in the same experimental conditions of electrodeposition (<b>PTEBTT</b>: 271.1 mF cm^–2 (at 1 mA cm^–2) and PEDOT: 12.1 mF cm^–2 (at 1 mA cm^–2)). For example, P­(<b>TEBTT</b>/EDOT)-based frameworks synthesized in a 1:1 monomer-to-comonomer ratio show a ca. 35× capacitance improvement over PEDOT. The high areal capacitance measured for P­(<b>TEBTT</b>/EDOT)-based frameworks can be explained by the open, highly porous hierarchical morphologies formed during the electropolymerization step. With >70% capacitance retention over 1000 cycles (up to 89% achieved), both <b>PTEBTT</b>- and P­(<b>TEBTT</b>/EDOT)-based frameworks are resilient to repeated electrochemical cycling and can be considered promising systems for high life cycle capacitive electrode applications

Impact of Polymer Side Chain Modification on OPV Morphology and Performance

Efficiencies of organic photovoltaic (OPV) devices have been steadily climbing, but there is still a prominent gap in understanding the relationship between fabrication and performance. Side chain substitution is one processing parameter that can change OPV device efficiency considerably, primarily because of variations in morphology. In this work, we explain the morphological link between side chain selection and device performance in one polymer to aid in the development of design rules more broadly. We study the morphology of an OPV active layer using a PBDTTPD-backbone polymer with four different side chain configurations, which are shown to change device efficiency by up to 4 times. The optimal device has the smallest domain sizes, the highest degree of crystallinity, and the most face-on character. This is achieved with two branched 2-ethylhexyl (2EH) side chains placed symmetrically on the BDT unit and a linear octyl (C8) side chain on the TPD unit. Substituting either side chain (C14 on BDT and/or 2EH on TPD) makes the orientation less face-on, while the TPD side chain primarily affects domain size. For all side chains, the addition of fullerene increases polymer crystallization compared to the neat film, but the degree of mixing between polymer and fullerene varies with side chain. Interestingly, the optimal device has a negligible amount of mixed phase. The domain sizes present in the optimal system are remarkably unchanged with a changing fullerene ratio between 10 and 90%, hinting that the polymer preferentially self-assembles into 10-20 nm crystallites regardless of concentration. The formation of this crystallite may be the key factor inhibiting mixed phase

Electron-Deficient <i>N</i>‑Alkyloyl Derivatives of Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione Yield Efficient Polymer Solar Cells with Open-Circuit Voltages > 1 V

Poly­(benzo­[1,2-b:4,5-b′]­dithiophene–thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione) (PBDTTPD) polymer donors yield some of the highest open-circuit voltages (<i>V</i>_OC, ca. 0.9 V) and fill factors (FF, ca. 70%) in conventional bulk-heterojunction (BHJ) solar cells with PCBM acceptors. Recent work has shown that the incorporation of ring substituents into the side chains of the BDT motifs in PBDTTPD can induce subtle variations in material properties, resulting in an increase of the BHJ device <i>V</i>_OC to ∼1 V. In this contribution, we report on the synthesis of <i>N</i>-alkyloyl-substituted TPD motifs (TPD­(CO)) and show that the electron-deficient motifs can further lower both the polymer LUMO and HOMO levels, yielding device <i>V</i>_OC > 1 V (up to ca. 1.1 V) in BHJ solar cells with PCBM. Despite the high <i>V</i>_OC achieved (i.e., low polymer HOMO), BHJ devices cast from TPD­(CO)-based polymer donors can reach power conversion efficiencies (PCEs) of up to 6.7%, making these promising systems for use in the high-band-gap cell of tandem solar cells