Small Molecule Organic Solar Cells Research Articles

The importance of enhancing D-Dand A-Astacking simultaneously for the balance of hole and electron mobility of small molecule single-component organic solar cells.

Single-component organic solar cells (SCOSCs) have attracted extensive attention due to their simplified device manufacturing and excellent stability. However, the relationship between morphology and charge carrier mobility in the active layers of SCOSCs is not well understood. In this work, we present a comprehensive investigation on this issue by studying four dyads (fullerenes as acceptor units) used as materials of active layers in small-molecule single-component organic solar cells (SM-SCOSCs), in which dyad 4 created the record of power conversion efficiency (PCE) of SM-SCOSC until now. Utilizing a multiscale theoretical approach, the results identify that the acceptor-acceptor stacking is dominant in amorphous films, significantly improving electron mobility and lowering hole mobility. We also find the importance of achieving a balance between electron and hole mobility to further improve PCE of SM-SCOSC because dyad 4 exhibits a more balanced electron/hole mobility than the other three molecules. These findings indicate the importance of tuning and enhancing donor-donor and acceptor-acceptor stacking simultaneously, offering insights for the design and optimization of future SM-SCOSC.

Photoinduced charge transfer in small-molecule organic solar cells dependent on external electric field

In this paper, MTDATA is selected as the electron donor and BPhen as the acceptor. The trianiline structure of the donor molecule provides a variety of charge transfer pathways for charge separation. Based on Marcus theory, the photoinduced charge transfer properties of non-fullerene acceptor organic solar cells (NFA-OSCs) in external electric field (Fext) were studied. By analyzing the changes of reorganization energy (λ), Gibbs free energy (ΔG), electron transfer integral (Vda) and charge transfer rate under the influence of Fext, it was found that Fext can effectively regulate the charge transfer. Moreover, the change of charge transfer rate under Fext is similar to Vda, indicating that Vda is the main factor affecting charge transfer rate. To a certain extent, this study provides theoretical support for improving the photoelectric performance of NFA-OSCs.

Design of Isoindigo-Based Small-Molecule Donors for Bulk Heterojunction Organic Solar Cell Applications in Combination with Nonfullerene Acceptors.

The development of small-molecule organic solar cells with the required efficiency depends on the information obtained from molecular-level studies. In this context, 39 small-molecule donors featuring isoindigo as an acceptor moiety have been meticulously crafted for potential applications in bulk heterojunction organic solar cells. These molecules follow the D2-A-D1-A-D2 and D2-A-π-D1-π-A-D2 framework. Similar molecules considered in the previous experimental study (molecules R1 ((3E,3″E)-6,6″-(benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione)) and R2 ((3E,3″E)-6,6″-(4,8-dimethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione))) have been chosen as reference molecules. Molecules with and without π-spacers have been considered to understand the impact of the length of the π-spacer on intramolecular charge-transfer transitions and absorption properties. A detailed investigation is carried out to establish the relationship between the structure and photovoltaic parameters using density functional theory and time-dependent density functional theory methods. The newly developed molecules exhibit better electronic, excited-state, and charge transport properties than the reference molecules. Additionally, model donor-acceptor interfaces are constructed by integrating the designed donor molecules with fullerene/nonfullerene acceptors. The electronic and excited-state properties of these interfaces are rigorously evaluated. Results elucidate that the donor comprising of isoindigo-bithiophene-pyrroloindacenodithiophene (IIG-T2-PIDT) emerges as a promising candidate for bulk heterojunction solar cells based on nonfullerene acceptors. This research provides systematic design strategies for the development of small-molecule donors for organic solar cells.

Increasing terminal alkyl chain length for a better small molecule organic solar cell donor

Optimizing the alkyl chains within a terminal group is considered to be a highly effective approach for boosting the power conversion efficiency (PCE) of all small-molecule organic solar cells (ASM-OSCs). In this study, we focus on designing and synthesizing small molecular (SM) donor compounds with different end-group alkyl chain lengths to study their impact on OSC performance. Under optimized conditions, OSC devices based on BTR-Cl-C8:Y6 demonstrate higher PCE of 14.43 % compared to those based on BTR-Cl:Y6. Moreover, BTR-Cl-C8 exhibits reduced levels of bimolecular recombination and trap-assisted recombination, enhanced capability for exciton dissociation and charge collection, and extended carrier lifetimes. These results highlight the significance of molecular design strategies in optimizing OSC performance and offer valuable insights for the development of efficient and stable ASM-OSCs.

Enhancing predictive modeling of photovoltaic materials’ solar power conversion efficiency using explainable AI

We present a study on Explainable AI-based prediction of power conversion efficiency (PCE) of organic solar cells, conducted on a dataset of 566 small-molecule organic solar cell materials samples with varying donor and acceptor species combinations. This research uncovers an interesting phenomenon, the first of its kind to be reported, of PCE quantization, where the PCE values increase in steps with the increase in feature values. Our findings have significant implications for the development of efficient organic solar cells, as they provide a better understanding of the factors that influence PCE, and highlight the feature value ranges for which more efficient PCE would be achieved. Our study demonstrates the power of XAI techniques in uncovering hidden patterns in scientific datasets and highlights the importance of interdisciplinary research in the field of materials science.

A highly crystalline donor enables over 17% efficiency for small-molecule organic solar cells

A highly crystalline donor B2 was synthesized, and an outstanding PCE of 17.1% was achieved with a low energy loss of 0.579 eV for small-molecule organic solar cells.

A computational investigation of the effects of changes of donor–acceptor linker length and acceptor on electronic spectra of small-molecule single-component organic solar cells

Small-molecule single-component organic solar cells (SM-SCOSCs) have great potential for commercial application. However, the effects of changes of donor–acceptor linker length and acceptor unit of small molecules of active layers on their photoelectric properties are still not well understood. In this work, four state-of-the-art dyads 1–4 were selected to investigate these effects since dyad 4 created the highest power conversion efficiency (PCE) of 5.34% of SM-SCOSC; and their electronic structures were investigated. The results indicate that increasing the length of the donor–acceptor connecting bridge leads to a decrease in exciton binding energy, and so does replacing the acceptor component PC61BM with PC71BM. Furthermore, the simulated electronic absorption spectrum of dyad 4 is enhanced with respect to others. We also find that the introduction of PC71BM creates more channels for transitions from local excited states to charge transfer states. This work gains a deep insight into the relationship between molecular structure and performance of SM-SCOSC.

Benzotriazole‐Based Donor–Acceptor–Acceptor Electron Donors for Vacuum‐Deposited Small Molecule Organic Solar Cells

Three novel donor–acceptor–acceptor configured small molecules with benzotriazole as the central A building block are synthesized as donor materials for vacuum‐deposited small‐molecule organic solar cells (SMOSCs). The effects of different lengths of the side chains attached to the benzotriazole block on the molecular structure, electrochemical behavior, and optical properties of these donors are investigated systematically. Vacuum‐deposited SMOSCs fabricated with these small molecule donors and C70 as the acceptor exhibit power conversion efficiencies (PCEs) in the range of 6.42–7.43% under air mass 1.5 global (AM 1.5 G) 100 mW cm−2 simulated solar illumination. Furthermore, the Me‐DTDCPT‐based devices deliver a promising PCE of 14.84% under 600 lux illumination by a fluorescent lamp, demonstrating its potential in indoor photovoltaic applications.

Miscibility Regulation and Thermal Annealing Induced Hierarchical Morphology Enables High‐Efficiency All‐Small‐Molecule Organic Solar Cells Over 17%

AbstractAchieving an ideal morphology to realize efficient charge generation and transport is an imperative avenue to improve the photovoltaic performance of all small‐molecule organic solar cells (SM‐OSCs). Here, ternary SM‐OSCs are fabricated based on a new small molecule donor, SM‐mB, and an alloyed blend acceptor of Y6 and its derivative, L8‐BO, and desirable hierarchical morphology with appropriate nanoscale phase separation is successfully realized through adjusting the thermal annealing treatment conditions and compositions of mixed acceptors in the active layer. Then the ternary SM‐OSCs achieve an excellent PCE of 17.06 %, which is one of the best results for the SM‐OSCs so far. The desirable morphology can be ascribed to the optimization of the miscibility‐driven donor and acceptor blend morphology that takes full advantage of the individual advantages of both acceptors, which facilitate efficient charge generation and extraction with more balanced charge carrier mobilities. More importantly, the photovoltaic performance of the ternary SM‐OSCs possesses a high tolerance to the device fabrication conditions, including thermal annealing treatment, and is insensitive to film thickness, which is beneficial for large‐area manufacture and future practical applications.

New Benzotrithiophene-Based Molecules as Organic P-Type Semiconductor for Small-Molecule Organic Solar Cells.

A new benzotrithiophene-based small molecule, namely 2,5,8-Tris[5-(2,2-dicyanovinyl)-2-thienyl]-benzo[1,2-b:3,4-b':6,5-b″]-trithiophene (DCVT-BTT), was successfully synthesized and subsequently characterized. This compound was found to present an intense absorption band at a wavelength position of ∼544 nm and displayed potentially relevant optoelectronic properties for photovoltaic devices. Theoretical studies demonstrated an interesting behavior of charge transport as electron donor (hole-transporting) active material for heterojunction cells. A preliminary study of small-molecule organic solar cells based on DCVT-BTT (as the P-type organic semiconductor) and phenyl-C61-butyric acid methyl ester (as the N-type organic semiconductor) exhibited a power conversion efficiency of 2.04% at a donor: acceptor weight ratio of 1:1.

Understanding Improved Performance of Vacuum-Deposited All Small-Molecule Organic Solar Cells Upon Postprocessing Thermal Treatment

An all-inclusive investigation of the effect of postprocessing thermal treatment on all vacuum-deposited small-molecule organic solar cells (SM-OSC) is presented. Herein, DTDCTB is chosen as the donor (D) molecule, and three fullerene-derivative acceptor (A) molecules, namely, ICBA, C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> , and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> , are chosen for the study, and the devices were optimized for PV. As the first step, OSC cells were fabricated and characterized for photophysical, morphology, as well as various photovoltaic parameters, such as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${V}_{\text{OC}}$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${J}_{\text{SC}}$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$FF,{\rm{and\ }}\eta $</tex-math></inline-formula> , and external quantum efficiency for different processing parameters, such as active layer concentration ratios and annealing temperatures. The devices based on ICBA were found to have outperformed the devices using C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> . OSC devices using ICBA as an acceptor are chosen for further characterization to establish the role of thermal treatment on their device performance. To this, 1-diode Shockley equation modeling is employed, and a qualitative relationship between diode saturation current and thermal annealing is obtained. Additionally, the charge recombination dynamics of the binary bulk heterojunction systems were investigated using the light intensity-dependent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J–V</i> characteristics, and the role of annealing in the reduction of trap-assistance recombination was established that corroborates well with the obtained annealing-dependent morphology information from AFM measurement.

High-Performance Small Molecule Organic Solar Cells Enabled by a Symmetric-Asymmetric Alloy Acceptor with a Broad Composition Tolerance.

Using a combinatory blending strategy is demonstrated as a promising path for designing efficient organic solar cells (OSCs) by boosting the short-circuit current density and fill factor. Herein, a high-performance ternary all-small molecule OSC (all-SMOSCs) using a narrow-bandgap alloy acceptor containing symmetric and asymmetric molecules (BTP-eC9 and SSe-NIC) and a wide-bandgap small molecule donor MPhS-C2 is reported. Introducing the synthesized SSe-NIC into the MPhS-C2:BTP-eC9 host system can broaden the absorption spectrum, modulate energy offsets, and optimize the molecular packing of the host materials. After systematically optimizing the weight ratio of MPhS-C2:BTP-eC9:SSe-NIC, a champion efficiency of 18.02% is achieved. Impressively, the ternary system not only delivered a broad composition tolerance with device efficiencies over 17% throughout the whole blend ratios, but also exhibited less non-geminate recombination and energy loss, and better-light-soaking stability than the corresponding binary systems. This work promotes the development of high-performance ternary all-SMOSCs and heralds their brighter application prospects.

High‐Efficiency All‐Small‐Molecule Organic Solar Cells Based on New Molecule Donors with Conjugated Symmetric/Asymmetric Hybrid Cyclopentyl‐Hexyl Side Chains

AbstractAll small molecule organic solar cells (ASM‐OSCs) have numerous advantages but lower power conversion efficiencies (PCEs) than their polymer equivalents, which is largely due to the suboptimal nanoscale network structure in a bulk heterojunction (BHJ). Herein, new small molecule donors with symmetric/asymmetric hybrid cyclopentyl‐hexyl side chains are designed, accounting for manipulated intermolecular interactions and BHJ morphology. Theoretical and experimental results reveal that the asymmetric cyclopentyl‐hexyl side chains modification has a significant influence on potential energy surface and intermolecular interaction that can ensure preferable molecular assembly and regulate the D/A interfacial energetics, thus boosting the exciton dissociation and charge transport when pairing with a wide‐used acceptor L8‐BO. Concurrently, a nanoscale bicontinuous interpenetrating network with optimal domain size can be fully evolved in the BHJ layer. As a consequence, the As‐TCp‐based binary device achieves a superior PCE of 16.46% in comparison to that of the controlled symmetric counterparts S‐BF (14.92%) and A‐TCp (15.77%), and ranks one of best performance among ASM‐OSCs. This study demonstrates that precise manipulation of the cyclo‐alkyl chain in combination with the asymmetric 2D side chain strategy is an effective synergistic approach to control intermolecular interaction and nanoscale bicontinuous phase separation for achieving high‐performance ASM‐OSCs.

Linear-Shaped Low-Bandgap Asymmetric Conjugated Donor Molecule for Fabrication of Bulk Heterojunction Small-Molecule Organic Solar Cells.

A linear-shaped small organic molecule (E)-4-(5-(3,5-dimethoxy-styryl)thiophen-2-yl)-7-(5″-hexyl-[2,2':5',2″-terthiophen]-5-yl)benzo[c][1,2,5]thiadiazole (MBTR) comprising a benzothiadiazole (BTD) acceptor linked with the terminal donors bithiophene and dimethoxy vinylbenzene through a π-bridge thiophene was synthesized and analyzed. The MBTR efficiently tuned the thermal, absorption, and emission characteristics to enhance the molecular packing and aggregation behaviors in the solid state. The obtained optical bandgap of 1.86 eV and low-lying highest occupied molecular orbital (HOMO) level of -5.42 eV efficiently lowered the energy losses in the fabricated devices, thereby achieving enhanced photovoltaic performances. The optimized MBTR:PC71BM (1:2.5 w/w%) fullerene-based devices showed a maximum power conversion efficiency (PCE) of 7.05%, with an open-circuit voltage (VOC) of 0.943 V, short-circuit current density (JSC) of 12.63 mA/cm2, and fill factor (FF) of 59.2%. With the addition of 3% 1,8-diiodooctane (DIO), the PCE improved to 8.76% with a high VOC of 1.02 V, JSC of 13.78 mA/cm2, and FF of 62.3%, which are associated with improved charge transport at the donor/acceptor interfaces owing to the fibrous active layer morphology and favorable phase separation. These results demonstrate that the introduction of suitable donor/acceptor groups in molecular design and device engineering is an effective approach to enhancing the photovoltaic performances of organic solar cells.

Oligothiophene electron donor and electron acceptor for all small molecule organic solar cells with efficiency over 9%

By using easily accessible oligothiophene donors and acceptor, all-oligothiophene organic solar cells (AOT-OSCs) were realized. The devices fabricated with thiazole-centered donor Tz6T and quaterthiophene-based acceptor A4T-16 exhibited a satisfactory power conversion efficiencies (PCEs) of 9.30 %. Moreover, the devices based on Tz6T/A4T-16 binary demonstrated an excellent industrial figure of merit (i-FoM) of up to 0.168 which represented one of the best cost-effectiveness in all small molecule organic solar cells, suggesting the strong potential of all-oligothiophene combinations in developing OSCs with both low cost and high efficiency.

Panchromatic small-molecule organic solar cells based on a pyrrolopyrrole aza-BODIPY with a small energy loss

A small photon energy loss (Eloss) is one of the requisites for achieving near-infrared (NIR) small-molecule organic solar cells (SM-OSCs). Herein, we present two novel pyrrolopyrrole aza-BODIPY (PPAB)-based panchromatic chromophores, TT-2PPAB and DFBT-2PPAB, with an acceptor-donor-acceptor configuration for photovoltaic studies. Among them, in combination with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) acceptor, TT-2PPAB exhibits a moderate power conversion efficiency (PCE) of 3.11% due to an Eloss of 0.48 eV, which is smaller than the empirical limit of 0.6 eV. Based on the electrochemistry and theoretical calculations, we revealed that the efficient photovoltaic performance of the TT-2PPAB-based device can be ascribed to the sufficiently deep LUMO level of TT-2PPAB for charge transfer to PC71BM.

Anti-reflection metallic anode-enhanced performance of organic solar cells via cross coupling between Fabry-Perot cavity modes and microcavity modes.

An effective anti-reflection metallic anode with the structure of glass/dielectric2 /Ag (D1D2M) is demonstrated both in small-molecule (SM) and conjugated polymer (CP) organic solar cells (OSCs). The anti-reflection mechanism is investigated by the finite-difference time-domain numerical calculation method and the experimental method. By tuning the refractive index and the thickness of the D2 layer, the reflection light is confined in the Fabry-Perot (F-P) cavity modes, which effectively enhances the transmittance of the D1D2M anode in the wavelength range of 420 nm-800 nm. Compared with the conventional glass/Ag (D1M) anode, the experimental transmittance of the D1D2M anode is enhanced by 33.24% at a wavelength of 550 nm. By replacing the D1M anode with the D1D2M anode in the OSCs, the F-P cavity modes cross couple with the microcavity modes in the active layers. As a result, the absorption intensity is obviously increasing in a wide angle range (0≤θ≤85∘) in the wavelength ranges of 475 nm-650 nm and 540 nm-720 nm for the SM and CP OSCs, respectively. The short circuit current density and power conversion efficiency of the SM OSC is increased by 25.07% and 27.23%, respectively.

Nanomorphology dependence of the environmental stability of organic solar cells

Previous studies have reported contradictory effects of small-molecule acceptors on the environmental stability of polymer:small-molecule blends, with one showing that a small-molecule acceptor stabilizes and another showing that it destabilizes the polymer donor. In this work, to investigate the origin of these contradictory results, the effects of the nanomorphologies of small-molecule acceptors on the environmental stability of polymer:small-molecule blends are demonstrated. Investigations on the environmental stabilities of polymer:fullerene blends of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):phenyl-C61-butyric acid methyl ester (PCBM) with contrasting nanomorphologies of PCBM reveal that dispersed PCBM in a mixed phase is the critical factor that causes triplet-mediated singlet oxygen generation and, hence, the severe photooxidation of PTB7, whereas an aggregated PCBM phase stabilizes PTB7 by reducing the formation of PTB7 triplet excitons. In addition, the photooxidation of PTB7 substantially degrades hole transport in the PTB7:PCBM blends by destroying the crystalline PTB7 phases within the films; this effect is strongly correlated with the efficiency losses of the PTB7:PCBM organic solar cells. These conclusions are also extended to polymer:nonfullerene blends of PTB7:ITIC and PTB7:Y6, thereby confirming the generality of this phenomenon for polymer:small-molecule organic solar cells.

Physicochemical insights into the rational designing of new acceptor molecules by donor bridge modifications for efficient solar cells: In silico chemistry

AbstractIn small‐molecule organic solar cells (SM‐OSCs), it remains a big challenge to obtain favorable bulk heterojunction morphology by acceptor material design. In this report, efforts are being made to design efficient acceptor molecules for the active layer of the organic solar cell. Donor bridge modifications have been made by using effective donor units, and thus, four new molecules (KSD1–KSD4) have been designed and examined theoretically. Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) have been employed for the calculation of various geometric and structural parameters. Transition density matrix, open‐circuit voltage, reorganizational energy of hole and electron, and photovoltaic characteristics of newly designed molecules have been examined through quantum chemical techniques. Additionally, the frontier molecular orbital analysis along with excitation and binding energies has been performed for these molecules. A narrow bandgap with a redshift in absorption spectrum is observed for KSD1–KSD4 molecules. Results of different analyses have recommended that the designed molecules are effective contributors for organic solar cell applications.

Investigation of charge transfer between donor and acceptor for small-molecule organic solar cells by scanning tunneling microscopy and ultrafast transient absorption spectroscopy

Investigation of charge transfer between donor and acceptor for small-molecule organic solar cells by scanning tunneling microscopy and ultrafast transient absorption spectroscopy