Small Molecule Donor Research Articles

Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study

To investigate photocurrent generation mechanisms in these organic solar cells (OSCs), we design and synthesize four thienothiophene (TT)-based small-molecule donors with the highest occupied molecular orbital (HOMO) levels varying from −6.4 eV to −5.1 eV, which span across the HOMO value of the [6,6]-phenyl-C70-butyric acid methyl ester (PC 71 BM) acceptor. We measure TT-based donor:PC 71 BM films’ electronic and optical properties, OSC current density-voltage characteristic, and external quantum efficiency, and perform density functional theory (DFT) calculations. Our results show that photocurrent generation depends strongly on the substitutions of the center TT groups, cyano (-CN) versus hexyloxy (-OHex). With 1 wt% donor, TTOHex:PC 71 BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC 71 BM devices. In contrast, TTCN:PC 71 BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC 71 BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC 71 BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC 71 BM devices are comparable to neat PC 71 BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC 71 BM devices are low due to the low hole mobility, ~10 −8 cm 2 /V. Following exciton dissociation, hole transport is analyzed according to three possible mechanisms: tunneling, percolation pathways, and hole back transfer. We find that the hole back transfer mechanism can explain all experimental results and therefore is the primary hole transport mechanism for photocurrent generation in TT-based donor:PC 71 BM dilute-donor OSCs. • Design and synthesis of thienothiophene (TT)-based small molecule donors with varied HOMO levels. • Functional group substitutions on center TT groups have stronger effect on energy levels and photocurrent generation. • Favorable HOMO offset between donor and acceptor is required for exciton dissociation. • Hole back transfer is the primary hole transport mechanism for photocurrent generation in the TT:PCBM dilute-donor devices.

Synergistically minimized nonradiative energy loss and optimized morphology achieved via the incorporation of small molecule donor in 17.7% efficiency ternary polymer solar cells

To further improve the photovoltaic performance of the polymer solar cells (PSCs), it is an essential and significantly challenging issue to synergistically minimize non-radiative energy loss and optimize morphology. In this work, ternary PSCs are fabricated by incorporating a small molecule donor, namely BTTzR, into the state-of-the-art PM6:Y6 binary system. Detailed investigations suggest that BTTzR can form energy cascade with binary components to facilitate charge transfer. Furthermore, the integration of BTTzR can enhance crystallization and domain purity of the binary blend, and effectively suppress the aggregation of Y6, which ultimately leads to more efficient charge transport and lower non-radiative energy loss (ΔEnr). As a result, compared to the binary devices with PCE of 15.8% and ΔEnr of 0.23 eV, the ternary PSCs with 20% BTTzR contents achieved a significantly enhanced performance with PCE of 17.7% and lower ΔEnr of 0.19 eV, which are among the best results reported in the literatures to date. Therefore, this work not only offers a high-efficiency ternary blend system but also provides an efficient strategy to synergistically minimize non-radiative energy loss and optimize the morphology for the development of PSCs.

Efficient Electron Transport Layer Free Small-Molecule Organic Solar Cells with Superior Device Stability.

Electron transport layers (ETLs) placed between the electrodes and a photoactive layer can enhance the performance of organic solar cells but also impose limitations. Most ETLs are ultrathin films, and their deposition can disturb the morphology of the photoactive layers, complicate device fabrication, raise cost, and also affect device stability. To fully overcome such drawbacks, efficient organic solar cells that operate without an ETL are preferred. In this study, a new small-molecule electron donor (H31) based on a thiophene-substituted benzodithiophene core unit with trialkylsilyl side chains is designed and synthesized. Blending H31 with the electron acceptor Y6 gives solar cells with power conversion efficiencies exceeding 13% with and without 2,9-bis[3-(dimethyloxidoamino)propyl]anthra[2,1,9-def:6,5,10-d'e'f ']diisoquinoline-1,3,8,10(2H,9H)-tetrone (PDINO) as the ETL. The ETL-free cells deliver a superior shelf life compared to devices with an ETL. Small-molecule donor-acceptor blends thus provide interesting perspectives for achieving efficient, reproducible, and stable device architectures without electrode interlayers.

Suppressing Kinetic Aggregation of Non‐Fullerene Acceptor via Versatile Alloy States Enables High‐Efficiency and Stable Ternary Polymer Solar Cells

AbstractDespite considerable advances devoted to improving the operational stability of organic solar cells (OSCs), the metastable morphology degradation remains a challenging obstacle for their practical application. Herein, the stabilizing function of the alloy states in the photoactive layer from the perspective of controlling the aggregation characteristics of non‐fullerene acceptors (NFAs), is revealed. The alloy‐like model is adopted separately into host donor and acceptor materials of the state‐of‐the‐art binary PM6:BTP‐4Cl blend with the self‐stable polymer acceptor PDI‐2T and small molecule donor DRCN5T as the third components, delivering the simultaneously enhanced photovoltaic efficiency and storage stability. In such ternary systems, two separate arguments can rationalize their operating principles: (1) the acceptor alloys strengthen the conformational rigidity of BTP‐4Cl molecules to restrain the intramolecular vibrations for rapid relaxation of high‐energy excited states to stabilize BTP‐4Cl acceptor. (2) The donor alloys optimize the fibril network microstructure of PM6 polymer to restrict the kinetic diffusion and aggregation of BTP‐4Cl molecules. According to the superior morphological stability, non‐radiative defect trapping coefficients can be drastically reduced without forming the long‐lived, trapped charge species in ternary blends. The results highlight the novel protective mechanisms of engineering the alloy‐like composites for reinforcing the long‐term stability of NFA‐based ternary OSCs.

Backbone Engineering with Asymmetric Core to Finely Tune Phase Separation for High-Performance All-Small-Molecule Organic Solar Cells.

In order to obtain high-performance all-small-molecule organic solar cells (ASM-OSCs), it is crucial to exploit the available strategy for molecular design and to further understand key structure-property relationship that can rationally control the blend nanomorphology and influence the physical process. In this work, we design two small molecule donors FBD-S1 and TBD-S2 with identical electron-withdrawing units but various asymmetric central cores, which exhibit differing phase separation in Y6-based blend films. It is found that TBD-S2 with increased phase separation between donor and acceptor can lead to more favorable interpenetrating networks, effective exciton dissociation, and enhanced and more balanced charge transport. Importantly, a remarkable PCE of 13.1% is obtained for TBD-S2:Y6 based ASM-OSCs, which is an attractive photovoltaic performance for ASM-OSCs. This result demonstrates that the central core modification at the atomic level for small molecule donors can delicately control the phase separation and optimize photophysical processes, and refines device performance, which facilitate development in the ASM-OSC research field.

Illuminating and alleviating cellular oxidative stress with an ROS-activated, H2S-donating theranostic

This paper is dedicated to Prof. Dale L. Boger on the occasion of his receipt of the 2020 Tetrahedron Prize for Creativity. Hydrogen sulfide (H2S) is an endogenous signaling molecule that has been shown to provide impressive antioxidative and cytoprotective effects. To further gauge its physiological and pathophysiological properties, small molecule donors with precise control over their H2S delivery within a biological setting are needed. Herein, we describe a novel, self-reporting H2S donor that is selectively activated by reactive oxygen species (ROS). Within our design, we append an ROS-responsive aryl boronate ester to 7-hydroxy-4-methylcoumarin via an S-alkyl thiocarbonate linkage—a key feature which provides the efficient release of both H2S (via COS hydrolysis) and a fluorescent reporter in response to elevated levels of ROS. Thus, this design was found to provide a useful bifunctional tool given its rare ability to simultaneously image and neutralize hydrogen peroxide within live cells.

15.3% Efficiency All-Small-Molecule Organic Solar Cells Achieved by a Locally Asymmetric F, Cl Disubstitution Strategy.

Single junction binary all‐small‐molecule (ASM) organic solar cells (OSCs) with power conversion efficiency (PCE) beyond 14% are achieved by using non‐fullerene acceptor Y6 as the electron acceptor, but still lag behind that of polymer OSCs. Herein, an asymmetric Y6‐like acceptor, BTP‐FCl‐FCl, is designed and synthesized to match the recently reported high performance small molecule donor BTR‐Cl, and a record efficiency of 15.3% for single‐junction binary ASM OSCs is achieved. BTP‐FCl‐FCl features a F,Cl disubstitution on the same end group affording locally asymmetric structures, and so has a lower total dipole moment, larger average electronic static potential, and lower distribution disorder than those of the globally asymmetric isomer BTP‐2F‐2Cl, resulting in improved charge generation and extraction. In addition, BTP‐FCl‐FCl based active layer presents more favorable domain size and finer phase separation contributing to the faster charge extraction, longer charge carrier lifetime, and much lower recombination rate. Therefore, compared with BTP‐2F‐2Cl, BTP‐FCl‐FCl based devices provide better performance with FF enhanced from 71.41% to 75.36% and J sc increased from 22.35 to 24.58 mA cm−2, leading to a higher PCE of 15.3%. The locally asymmetric F, Cl disubstitution on the same end group is a new strategy to achieve high performance ASM OSCs.

Star-shaped small molecular donors based on a Zn-Porphyrin core and DPP arms via different linkers for organic solar cells

Star-shaped porphyrin-based small-molecule donors (SMDs) with all four-directional substitution have been rarely reported. Herein, we designed and synthesized two SMDs, namely ZnP-4PhDPP and ZnP-4ThDPP, by employing zinc porphyrin as the core, thiophene-capped diketopyrrolopyrrole (DPP) as the four conjugated arms via a phenyl or thienyl linking bridge. The varied linker resulted in differences in absorption, energy level and molecular geometry. Blended with Y6 as acceptor, all-small-molecule devices achieved power conversion efficiency (PCE) of 6.86% and 6.93% for ZnP-4PhDPP and ZnP-4ThDPP, upon different device post-treatment conditions, respectively. The more orthogonal spatial configuration of ZnP-4ThDPP was favorable for constructing additional charge pathways, which was in accord with the results from photoluminescence quenching efficiency, charge transport ability and thickness sensitivity evaluation. The results in this work enriched the category of star-shaped porphyrin-derivatives and might afford guidelines into the structure-property correlation of various linkers-based small-molecule photovoltaic materials.

Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells

Dilute donor organic solar cells (OSCs) are a promising technology to circumvent the trade‐off between open‐circuit voltage (Voc) and short‐circuit current density (Jsc). The origin of hole transport in OSCs with donor concentrations below the percolation threshold is diversely discussed in the community. Herein, both hole back transfer and long‐range hopping (tunneling) are analyzed as possible mechanisms of photocurrent in small molecule dilute donor OSCs using kinetic Monte Carlo (kMC) simulations. In contrast to previous kMC studies, the driving force for exciton dissociation is accounted for. As a study system, nitrogen‐bridged terthiophene (NBTT) molecules in a [6,6]‐phenyl‐C70‐butyric acid methyl ester (PC71BM) matrix are investigated. The simulations show that hole back transfer from the small molecule donor to the fullerene matrix explains the measured concentration dependences of the photocurrents as well as the Jsc dependence on the light intensity for donor concentrations below 5 wt%. For 5 wt%, distances between NBTT molecules decrease to reasonable ranges that long‐range hopping or tunneling cannot be discounted. Compared with polymer donors, larger hole localization is observed. The results emphasize that the barrier for hole back transfer is not only due to the highest occupied molecular orbital (HOMO) offset, but also by hole localization.

Theoretical investigations about the effect of electron-withdrawing groups on proprieties of A-π-D-π-A type small molecules donor for organic solar cells.

Taking the reporting donor SM0 as reference, three new A-π-D-π-A type small molecules donors (SM1, SM2, and SM3) were designed via replacing acceptor moieties. The molecular proprieties affecting the cell performance, such as, frontier molecular orbital, open circuit voltage (Voc), absorption spectrum, vertical and adiabatic ionization potential, vertical and adiabatic electronic affinity, and energy driving force ΔEL-L were investigated through the density functional theory (DFT) and the time dependent density functional theory (TD-DFT). All new designed molecules exhibit reduced energy gap, show a red shift absorption band, small energy binding Eb, moderate Voc, and improve the creation of hole and electron as compared to SM0, which demonstrate that these new designed molecules could be used as a promising donors materials for organic solar cell. Additionally, the three new designed molecules present a larger amount charge transfer qCT, a longer length charge transfer DCT than SM0 and have a smaller reorganization energy of hole λ(h) which improve the charge carrier mobilities.

Influence of alkyne spacers on the performance of thiophene-based donors in bulk-heterojunction organic photovoltaic cells

The ever-increasing demand for renewable energy has led to the intensification of research focused upon harnessing the abundant energy from sunlight. Solution processed organic solar cells fabricated using conjugated polymer donor materials dominated the early development of organic photovoltaics (OPV). More recently, organic solar cells fabricated using small molecule donors have become increasingly attractive due to the advantages these systems have over their polymeric counterparts. These include reproducible synthesis and the ability to more accurately develop structure-activity relationships. Here, we report the synthesis of thiophene based small molecule donors that have been used to investigate the influence of alkyne π-spacers on the optical, electronic and photovoltaic properties of solution-processable bulk-heterojunction (BHJ) organic OPV cells. Additionally, we compare the addition of 3,4-ethylenedioxythiophene (EDOT) units in place of the end-thiophene of these derivatives. We found that the compounds featuring alkyne moieties displayed poor charge transfer and blue-shifted UV–vis spectra, whereas the incorporation of the EDOT units increased charge transfer for the non-alkyne derivative resulting in red-shifted absorbance. DFT calculations predicted higher charge separation/mobility and lower charge generation for the compounds without alkyne units. Indeed, greater Jsc values and higher PCEs were found for the compounds lacking alkyne spacers. EDOT containing compounds showed poor performance which is likely due to an unfavourable morphology and higher HOMO level, leading to lower Voc. Hence, the simplest 5T-1 molecule showed the highest performance of 3.24%, which is c.a. 1.6x that of the alkyne-containing 5TA-1 and c.a. 2.9x that of the EDOT-containing derivative. This study demonstrates that alkyne spacers can be detrimental to orbital interactions, charge transfer and OPV performance, and that the EDOT unit can be added to provide greater charge transfer due to its auxiliary donating ability. However, careful design is required to ensure higher Voc values and a favourable morphology in thiophene-based solution-processable OPVs.

Novel thieno[3,2-b]thiophene-embedded small-molecule donors for highly efficient and photostable vacuum-processed organic photovoltaics

Four thieno[3,2-b]thiophene (TT)-based donor-π-acceptor–type small-molecule donors DTCPTT, DTCPTT-2CN, DTDCPTT, and DTDCPTT-2CN were synthesized and characterized. A clear structure-property relationship correlating the effects of introducing cyano groups onto the TT and the end-capping groups has been successfully established. DTDCPTT and DTDCPTT-2CN were adopted as electron donors for organic photovoltaics (OPVs) mainly due to the strong intramolecular charge transfer absorption in the visible light region, sufficiently high thermal stability, and appropriate energy level alignment with C70. Moreover, the analyses of crystal structures reveal that DTDCPTT and DTDCPTT-2CN form antiparallel dimer pairs through strong dipolar and non-covalent intermolecular interactions. X-ray structures also indicate that introducing cyano groups onto the TT moiety does not affect the backbone planarity significantly. Compared with DTDCPTT-2CN, the more compact and uniform crystal packing renders DTDCPTT advantageous for exciton dissociation and charge carrier transport. DTDCPTT-based device achieves power conversion efficiency as high as 7.81% and 16.89%, respectively, under simulated AM 1.5G illumination and 500 lux TLD-840 fluorescent lamp illumination, which could be attributed to stronger absorption and better photocurrent extraction. In addition, both devices exhibit high stability under continuous simulated 1 sun illumination for more than 300 h. As far as we know, this is the best performance for vacuum-processed small-molecule indoor OPVs to date.

Research Progress in Organic Solar Cells Based on Small Molecule Donors and Polymer Acceptors

Research Progress in Organic Solar Cells Based on Small Molecule Donors and Polymer Acceptors

A small-molecule donor with a thieno[3,2-c]isochromene unit to synchronously improve the efficiency and stability of ternary fullerene organic solar cells

A small-molecule donor of TiC8 was primarily used as a third component to construct ternary fullerene OSCs. A high PCE of 11.12% and record device stability were simultaneously obtained in the ternary fullerene OSCs by adding TiC8 into the PTB7-Th:PC71BM host matrix, which maintains 80% of the initial PCE after 4752 h at room temperature in a N2-filled glovebox.

Assessing the effects of increasing conjugation length on exciton diffusion: from small molecules to the polymeric limit.

Organic solar cells (OSC) generally contain long-chain π-conjugated polymers as donor materials, but, more recently, small-molecule donors have also attracted considerable attention. The nature of these compounds is of crucial importance concerning the various processes that determine device performance, among which singlet exciton diffusion is one of the most relevant. The efficiency of the diffusion mechanism depends on several aspects, from system morphology to electronic structure properties, which vary importantly with molecular size. In this work, we investigated the effects of conjugation length on the exciton diffusion length through electronic structure calculations and an exciton diffusion model. By applying extrapolation procedures to thiophene and phenylene vinylene oligomer series, we investigate their electronic and optical properties from the small-molecule point of view to the polymeric limit. Several properties are calculated as a function of oligomer size, including transition energies, absorption and emission spectra, reorganization energies, exciton coupling and Förster radii. Finally, an exciton diffusion model is used to estimate diffusion lengths as a function of oligomer size and for the polymeric limit showing agreement with experimental data. Results also show that longer conjugation lengths correlate with longer exciton diffusion lengths in spite of also being associated with shorter exciton lifetimes.

Effects of lateral-chain thiophene fluorination on morphology and charge transport of BDT-T based small molecule donors: a study with multiscale simulations

The number and position of fluorinations on BDT-Ts affect morphology by changing molecule stacking patterns, and the trend of hole mobility is consistent with that of the proportion of face-on configurations.

Impact of fluorine substituted π-bridges on the photovoltaic performance of organic small-molecule donor materials

Two analogue small molecules, BDTP-2F with a difluorobenzene π-bridge and BDTP-4F with a tetrafluorobenzene π-bridge, are designed to investigate the impact of a fluorine substituted π-bridge on photovoltaic performance.

Adjusting the photovoltaic performance of big fused ring-based small molecules by tailoring with different modifications.

Three novel A–D–A type small-molecule donor materials, namely AAN-DPP2, AAN(T-DPP)2 and AANT(T-DPP)2, with anthanthrene (AAN) as the electron-donating core, diketopyrrolopyrrole (DPP) as the electron-accepting moiety, and thiophene as π-bridge units, have been designed and synthesized for application in bulk-heterojunction (BHJ) organic solar cells (OSCs). Compared to AAN-DPP2, devices based on AAN(T-DPP)2 and AANT(T-DPP)2 show better photovoltaic performance due to broader absorption and better planarity of the molecular backbone. A maximum power conversion efficiency (PCE) of 2.33% with a short-circuit current density (Jsc) of 6.82 mA cm−2 and a fill factor (FF) of 39.80 was obtained in the AAN(T-DPP)2/PC71BM-based solar cells. This is resulting from the suitable thickness of the active layer, improving the ability of catching light and decreasing the twist angle of the backbone by inserting a thiophene spacer. The results indicate that strategic substitution of π-bridges and side-chains in A–D–A type SMs is an efficient strategy to improve photovoltaic performance.

Indacenodithiophene-based small-molecule donor with strong crystallinity for efficient organic solar cells

An indacenodithiophene (IDT)-based small molecule is designed as a donor material for efficient all-small-molecule organic solar cells, which recorded a notable power conversion efficiency of 11.04%.

Optimization of active layer morphology by small-molecule donor design enables over 15% efficiency in small-molecule organic solar cells

Two conjugated small molecules with different conjugated side chains were synthesized to study their photovoltaic performances.