Correlating Molecular Structures with Transport Dynamics in High-Efficiency Small-Molecule Organic Photovoltaics

Efficient charge transport is a key step toward high efficiency in small-molecule organic photovoltaics. Here we applied time-of-flight and organic field-effect transistor to complementarily study the influences of molecular structure, trap states, and molecular orientation on charge transport of small-molecule DRCN7T (D1) and its analogue DERHD7T (D2). It is revealed that, despite the subtle difference of the chemical structures, D1 exhibits higher charge mobility, the absence of shallow traps, and better photosensitivity than D2. Moreover, charge transport is favored in the out-of-plane structure within D1-based organic solar cells, while D2 prefers in-plane charge transport

Dithienosilole-Based Small-Molecule Organic Solar Cells with an Efficiency over 8%: Investigation of the Relationship between the Molecular Structure and Photovoltaic Performance

Two new acceptor–donor–acceptor (A-D-A) small molecules with 2,6-(4,4-bis­(2-ethylhexyl)-4H-cyclopenta­[2,1-b;3,4-b′]-dithiophene (DTC) and (4,4′-bis­(2-ethylhexyl) dithieno­[3,2-b:2′,3′-d]­silole)–2,6-diyl (DTS) as the central building block unit and 3-ethyl-rhodanine as the end-capping groups have been designed and synthesized. The influence of the bridging atoms on the optical, electrochemical properties, packing properties, morphology, and device performance of these two molecules was systematically investigated. Although with only the difference of one atom on the central core units, the two molecules showed great different properties such as film absorption, molecular packing, and charge transport properties. The optimized device based on molecule DR3TDTS exhibited a power conversion efficiency (PCE) of >8%

Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%

A small molecule named DR3TSBDT with dialkylthiol-substituted benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithio­phene (BDT) as the central unit was designed and synthesized for solution-processed bulk-heterojunction solar cells. A notable power conversion efficiency of 9.95% (certified 9.938%) has been achieved under AM 1.5G irradiation (100 mW cm^–2), with an average PCE of 9.60% based on 50 devices

Multiarmed Aromatic Ammonium Salts Boost the Efficiency and Stability of Inverted Organic Solar Cells

Inverted organic solar cells (OSCs) have attracted much attention because of their outstanding stability, with zinc oxide (ZnO) being commonly used as the electron transport layer (ETL). However, both surface defects and the photocatalytic effect of ZnO could lead to serious photodegradation of acceptor materials. This, in turn, hampers the improvement of the efficiency and stability in OSCs. Herein, we developed a multiarmed aromatic ammonium salt, namely, benzene-1,3,5-triyltrimethanaminium bromide (PhTMABr), for modifying ZnO. This compound possesses mild weak acidity aimed at removing the residual amines present within ZnO film. In addition, the PhTMABr could also passivate surface defects of ZnO through multiple hydrogen-bonding interactions between its terminal amino groups and the oxygen anion of ZnO, leading to a better interface contact, which effectively enhances charge transport. As a result, an efficiency of 18.75% was achieved based on the modified ETL compared to the bare ZnO (PCE = 17.34%). The devices utilizing the modified ZnO retained 87% and 90% of their initial PCE after thermal stress aging at 65 °C for 1500 h and continuous 1-sun illumination with maximum power point (MPP) tracking for 1780 h, respectively. Importantly, the extrapolated T80 lifetime with MPP tracking exceeds 10 000 h. The new class of materials employed in this work to modify the ZnO ETL should pave the way for enhancing the efficiency and stability of OSCs, potentially advancing their commercialization process

Small-Molecule Acceptor Based on the Heptacyclic Benzodi(cyclopentadithiophene) Unit for Highly Efficient Nonfullerene Organic Solar Cells

A new nonfullerene small molecule with acceptor–donor–acceptor (A–D–A) structure, namely, NFBDT, based on a heptacyclic benzodi­(cyclopentadithiophene) (FBDT) unit using benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene as the core unit, was designed and synthesized. Its absorption ability, energy levels, thermal stability, as well as photovoltaic performances were fully investigated. NFBDT exhibits a low optical bandgap of 1.56 eV resulting in wide and efficient absorption that covered the range from 600 to 800 nm, and suitable energy levels as an electron acceptor. With the widely used and successful wide bandgap polymer PBDB-T selected as the electron donor material, an optimized PCE of 10.42% was obtained for the PBDB-T:NFBDT-based device with an outstanding short-circuit current density of 17.85 mA cm^–2 under AM 1.5G irradiation (100 mW cm^–2), which is so far among the highest performance of NF-OSC devices. These results demonstrate that the BDT unit could also be applied for designing NF-acceptors, and the fused-ring benzodi­(cyclopentadithiophene) unit is a prospective block for designing new NF-acceptors with excellent performance

Small Molecules Based on Alkyl/Alkylthio-thieno[3,2‑<i>b</i>]thiophene-Substituted Benzo[1,2‑<i>b</i>:4,5-b′]dithiophene for Solution-Processed Solar Cells with High Performance

Two acceptor–donor–acceptor small molecules based on thieno­[3,2-<i>b</i>]­thiophene-substituted benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene, DRBDT-TT with alkyl side chain and DRBDT-STT with alkylthio side chain, were designed and synthesized. Both molecules exhibit good thermal stability, suitable energy levels, and ordered molecular packing. Replacing the alkyl chain with alkylthio increases the dihedral angle between the thieno­[3,2-<i>b</i>]­thiophene (TT) and benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene (BDT) unit, and thus slightly decreases its intermolecular interactions leading to its blue-shift absorption in the solid state. The best devices based on DRBDT-TT and DRBDT-STT both exhibited power conversion efficiencies (PCEs) over 8% with high fill factors (FFs) over 0.70 under AM 1.5G irradiation (100 mW cm^–2), which are attributed to their optimized morphologies with feature size of 20–30 nm and well-balanced charge transport properties. The devices based on DRBDT-STT exhibited relatively lower short-circuit current density (<i>J</i>_sc) and thus slightly lower PCE as compared to the devices of DRBDT-TT, mainly due to its relatively poorer absorption. These results demonstrate that thieno­[3,2-<i>b</i>]­thiophene-substituted benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene derivatives could be promising donor materials for obtaining high efficiencies and fill factors

An A‑D‑A Type Small-Molecule Electron Acceptor with End-Extended Conjugation for High Performance Organic Solar Cells

A new non-fullerene small molecule with an acceptor-donor-acceptor (A-D-A) structure, FDNCTF, incorporating fluorenedicyclopentathiophene as core and naphthyl-fused indanone as end groups, was designed and synthesized. Compared with the previous molecule FDICTF with the phenyl-fused indanone as the end groups, the extended π-conjugation at the end group has only little impact on its molecular orbital energy levels, and thus, the open-circuit voltage (<i>V</i>_oc) of its solar cell devices has been kept high. However, its light absorption and mobility, together with the short-current density (<i>J</i>_sc) and the fill factor (FF), of its devices have been all improved simultaneously. Through morphology, transient absorption, and theoretical studies, it is believed that these favorable changes are caused by (1) the appropriately enhanced molecular interaction between donor/acceptor which makes the charge separation at the interface more efficient, and (2) enhanced light absorption and more ordered packing at solid state, all due to the extended end-group conjugation of this molecule. With these, the solar cells with FDNCTF as the acceptor and a wide band gap polymer PBDB-T as the donor demonstrated a high power conversion efficiency (PCE) of 11.2% with an enhanced <i>J</i>_sc and a maintained high <i>V</i>_oc, and significantly improved FF of 72.7% compared with that of the devices of FDICTF with the phenyl-fused indanone as the end groups. These results indicate that the unexplored conjugation size of the end group plays a critical role for the performance of their solar cell devices

Enhancement of Performance and Mechanism Studies of All-Solution Processed Small-Molecule based Solar Cells with an Inverted Structure

Both solution-processed polymers and small molecule based solar cells have achieved PCEs over 9% with the conventional device structure. However, for the practical applications of photovoltaic technology, further enhancement of both device performance and stability are urgently required, particularly for the inverted structure devices, since this architecture will probably be most promising for the possible coming commercialization. In this work, we have fabricated both conventional and inverted structure devices using the same small molecular donor/acceptor materials and compared the performance of both device structures, and found that the inverted structure based device gave significantly improved performance, the highest PCE so far for inverted structure based device using small molecules as the donor. Furthermore, the inverted device shows a remarkable stability with almost no obvious degradation after three months. Systematic device physics and charge generation dynamics studies, including optical simulation, light-intensity-dependent current–voltage experiments, photocurrent density-effective voltage analyses, transient absorption measurements, and electrical simulations, indicate that the significantly enhanced performance using inverted device is ascribed to the increasing of <i>J</i>_sc compared to the conventional device, which in turn is mainly attributed to the increased absorption of photons in the active layers, rather than the reduced nongeminate recombination

Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules with a Benzodithiophene Unit

Three small molecules named DR3TBDTT, DR3TBDTT-HD, and DR3TBD2T with a benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) unit as the central building block have been designed and synthesized for solution-processed bulk-heterojunction solar cells. Power conversion efficiencies (PCEs) of 8.12% (certified 7.61%) and 8.02% under AM 1.5G irradiation (100 mW cm^–2) have been achieved for DR3TBDTT- and DR3TBDT2T-based organic photovoltaic devices (OPVs) with PC₇₁BM as the acceptor, respectively. The better PCEs were achieved by improving the short-circuit current density without sacrificing the high open-circuit voltage and fill factor through the strategy of incorporating the advantages of both conventional small molecules and polymers for OPVs

Medium-Bandgap Small-Molecule Donors Compatible with Both Fullerene and Nonfullerene Acceptors

Much effort has been devoted to the development of new donor materials for small-molecule organic solar cells due to their inherent advantages of well-defined molecular weight, easy purification, and good reproducibility in photovoltaic performance. Herein, we report two small-molecule donors that are compatible with both fullerene and nonfullerene acceptors. Both molecules consist of an (E)-1,2-di­(thiophen-2-yl)­ethane-substituted (TVT-substituted) benzo­[1,2-b:4,5-b′]­dithiophene (BDT) as the central unit, and two rhodanine units as the terminal electron-withdrawing groups. The central units are modified with either alkyl side chains (DRBDT-TVT) or alkylthio side chains (DRBDT-STVT). Both molecules exhibit a medium bandgap with complementary absorption and proper energy level offset with typical acceptors like PC₇₁BM and IDIC. The optimized devices show a decent power conversion efficiency (PCE) of 6.87% for small-molecule organic solar cells and 6.63% for nonfullerene all small-molecule organic solar cells. Our results reveal that rationally designed medium-bandgap small-molecule donors can be applied in high-performance small-molecule organic solar cells with different types of acceptors