Small Molecular Organic Solar Cells Research Articles

Insight the difference of free charge generation in two small molecular accepter organic solar cells

Totally understanding the excitons dissociation in nonfullerene organic solar cells (OSCs) is the first priority for the minimum of the device energy loss and so that it can get the highest power conversion efficiency (PCE). The free charges harvesting is complicated though it has already published some evident examples to investigate this issue by small driving force between donors (D) and accepters (A). Here, a robust minus highest occupied molecular orbital (HOMO) level offset D-A couple unveiled this phenomena in greater depth by a time-resolved photo physical processes. The OSCs D-A heterojunction interfacial doping resonant coupling relaxation effect was observed and found some clues with device open-circuit voltage (Voc) energy loss. It undoubtedly demonstrated the device PCE up to 2.37% by a −0.12 eV HOMO offsets value. It was also found that the donor polymer aggregation behavior was greatly affected by the small molecular acceptor. These results will be beneficial to promote the OSCs comprehensive properties in future.

Precise Control of Phase Separation Enables 12% Efficiency in All Small Molecule Solar Cells

AbstractCompared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.

Burn-In Degradation Mechanism Identified for Small Molecular Acceptor-Based High-Efficiency Nonfullerene Organic Solar Cells.

Organic solar cells (OSCs) have again become a hot research topic in recent years. The record power conversion efficiency (PCE) of OSCs has boosted to over 17% in 2020. Apart from the high PCE, the stability of OSCs is also critical for their future applications and commercialization. Recently, many studies have proposed that burn-in degradation can be considered as an ineluctable barrier to long-term stable OSCs. However, there is still lack of studies to explain the detailed mechanism of this burn-in process. In this work, we first investigated the mechanism of the burn-in process in the high-efficiency PM6:N3-based nonfullerene OSCs. The PM6:N3-based device achieved a profound average PCE of 14.10% but also showed a significant performance loss after the burn-in degradation. Following characterizations such as dark J-V, photoluminescence (PL), time-resolved PL, Urbach energy estimation, and electrochemical impedance spectroscopy reveal that the burn-in degradation observed is closely related to the current extraction, energy transfer, nonradiative recombination, and charge transport process in the PM6:N3-based device. At the same time, it has small effects on the exciton dissociation process and energetic disorder in the PM6:N3-based device. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and grazing incidence X-ray diffraction measurements gratifyingly found that the morphology of the PM6:N3 active layer is relatively stable during the burn-in degradation. Therefore, these observed degradations are suspected results from the instability of interfaces and electrodes. The atoms in carrier transport layers and electrodes may diffuse to the active layer during the degradation, which changes the energy levels of each layer and causes traps at the interface and in the active layer. Conquering the instability of interfaces and electrodes is proposed as the prior task for PM6:N3-based OSCs to achieve long-term stability. Our study provides insights into the mechanism behind the burn-in degradation of the PM6:N3-based OSCs, which takes the first step to conquer this barrier.

An effective heteroatom-substituted strategy on photovoltaic properties of D(A-Ar)2 small molecules for efficient organic solar cells

To research the effects of different kinds of terminals on the solar cell performance, in this paper, carbazole (Cz), 2-octylthiophene (T) and 2-octylhiothiophene (TS) units are picked as end-capping units, with a pyran-bridged indacenodithiophene (IDTP) as central core and two fluorinated benzothiadiazole (DFBT) as acceptors, three π-conjugated small molecules (SMs) with D(A-Ar)2 framework, namely IDTP(DFBT-Cz)2, IDTP(DFBT-T)2 and IDTP(DFBT-TS)2, respectively, were designed and synthesized for application as donor materials in solution-processed small molecular organic solar cells (SM-OSCs). Encouragingly, after the tetrahydrofuran (THF) solvent vapor annealing (SVA) process, the device based on IDTP(DFBT-TS)2 exhibited outstanding efficiencies up to 7.33%, which is 1.71 times equal to the 4.29% PCE of the alkyl terminal substitution IDTP(DFBT-T)2, and 1.45 times equal to the 5.06% PCE of the carbazolyl terminal substitution IDTP(DFBT-Cz)2, which is mainly attributed from its better molecular planarity, higher hole mobility, and better morphologies of blend films in comparison with the other two SMs. To our best knowledge, this work with the IDTP core in SM donor materials is reported for the first time, compared to the IDT core, the IDTP core proved to effectively improve molecular crystallinity and fill factor (FF). The results reported here clearly demonstrate that the alkylthio substitution terminal-group engineering is an effective strategy to further improve the photovoltaic performance of the molecules.

A novel π-D1-A-D2 type low bandgap squaraine dye for efficient small molecular organic solar cells

Squaraine dyes have attracted increasing attention as photovoltaic materials for organic solar cells (OSCs) because of their facile and low-cost synthesis, intense and broad absorption in the visible and near-infrared (Vis-NIR) regions as well as colorful films. Herein, we report the synthesis of a novel π-D1-A-D2 type squaraine dye (DTT-USQ-11) with a low optical bandgap of 1.50 eV. In comparison with the classic D1-A-D2 type squaraine dye (USQ-11), DTT-USQ-11 demonstrates a lower optical bandgap, deeper highest occupied molecular orbital (HOMO) energy level, and higher hole mobility. Consequently, solution-processed bulk heterojunction small molecular OSC fabricated with the DTT-USQ-11/PC71BM blend film exhibits a much higher power conversion efficiency (PCE: 5.69%) compared with the classical USQ-11/PC71BM system (PCE: 4.15%). This enhanced PCE can be attributed to the simultaneous increase in the open-circuit voltage, short-circuit current density and fill factor of the DTT-USQ-11-based devices.

Homo-tandem structures to achieve the ideal external quantum efficiency in small molecular organic solar cells.

In this study, we report a homo-tandem structure of small molecular organic solar cells (SMOSCs) using identical single-junction devices as sub-cells. The trade-off between the absorption and internal quantum efficiency (IQE) of single-junction devices tends to limit the external quantum efficiency (EQE). However, multiple-stacked thin cells with maximized IQE in homo-tandem structures amplify the absorption to achieve the ideal EQE. As a result, a high power conversion efficiency of 7.81% is achieved in tetraphenyldibenzoperiflanthene (DBP):C70-based homo-tandem SMOSCs, which is 21.8% higher than that in a single-junction device.

Improved efficiency of small-molecular (S-M) tandem organic solar cells (TOSCs) by employing low work function alloy nanoparticle intermediate layer

We improved the power conversion efficiency (PCE) of the small molecular (S-M) tandem organic solar cells (TOSCs) by employing different low work function alloy nanoparticle intermediate layers. The enhancement of the PCE was mainly attributed to the gap states formed at the interface between the buffer layer and alloy nanoparticle intermediate layer. The gap states result in the disappearance of the electron injection barrier. Compared with the planar heterojunction (PHJ) TOSCs with single Ag nanoparticle intermediate layer, the PCE of the PHJ TOSC with the Mg-Ag alloy nanoparticle intermediate layer exhibits an enhancement of 7.5%. Moreover, the Mg-Ag alloy nanoparticle intermediate layer was also employed in the bulk-heterojunction (BHJ) TOSCs. Compared with the PHJ TOSCs, the PCE of the BHJ TOSCs with Mg-Ag alloy nanoparticle intermediate layer is doubled and achieves a value of 5.54%.

Efficient chemical structure and device engineering for achieving difluorinated 2,2′-bithiophene-based small molecular organic solar cells with 9.0% efficiency

Three novel narrow band gap small molecules (SMs) with D(A–Ar)2 molecular frameworks, based on fluorinated 2,2′-bithiophene (FBT) as an electron-donating (D) central core coupled with different electron-acceptor (A) bridge units, and 2-octylthiophene (Ar) units as end-capping units, were designed and synthesized for application as donor materials in solution-processed SM-OSCs. Under optimized conditions, a high PCE of 9.00% with a very high Jsc of 16.14 mA cm−2 and a FF of 73.52% for FBT(TDPP-T)2/PC71BM is obtained in single layer OSCs, which is among the highest reported for D(A–Ar)2-type SM based solar cells with a PCE up to 9% so far.

D-A structural protean small molecule donor materials for solution-processed organic solar cells

Under the synergistic effect of molecular design and devices engineering, small molecular organic solar cells have presented an unstoppable tendency for rapid development with putting forward donor-acceptor (D-A) structures. Up to now, the highest power conversion efficiency of small molecules has exceeded 11%, comparable to that of polymers. In this review, we summarize the high performance small molecule donors in various classes of typical donor-acceptor (D-A) structures and discuss their relationships briefly.

Effects of alkyl chains on intermolecular packing and device performance in small molecule based organic solar cells

Two donor molecules named as DR3TDTC-C6 and DR3TDTC-C8 with n-hexyl and n-octyl alkyl chains on the central building block 7H-cyclopenta-[1,2-b:3,4-b’]-dithiophene (DTC) were designed and synthesized. Both of them exhibited PCEs >4%. The photovoltaic properties of these molecules were superior to their analogous donor molecule DR3TDTC, which possess two 2-ethyl hexyl alkyl chains on the same core unit and only demonstrated a PCE of 0.75% after elaborative post-treatment. The influence of the alkyl chains on the optical, electrochemical properties, packing properties and morphology of these three molecules was systematically investigated. The results demonstrated that the difference between their device performances is mainly affected by their intermolecular packing state. This indicates that length and branch structure of alkyl chains on the central unit should be given careful consideration while designing donor molecules for small molecular organic solar cells.

Nonfullerene Small Molecular Acceptors with a Three-Dimensional (3D) Structure for Organic Solar Cells

Two spirobifluorene (SF)-functioned 3D nonfullerene electron acceptors—SF-OR and SF-ORCN—that use rhodanine and 2-(1,1-dicyanomethylene)rhodanine as the terminal units, respectively, were designed and synthesized. These new acceptor materials show reversible electrochemical reduction and a high optical absorption coefficient, which are critical in device operation. Electronic and structural characterizations reveal that the inclusion of cyano group on rhodanine improve the electron accepting ability at the sacrifice of structure order, since the conjugated backbone becomes less planar, which prohibits cofacial stacking. SF-OR and SF-ORCN show good photovoltaic performances when paired with a poly(3-hexylthiophene) (P3HT) donor, and the optimized devices give power conversion efficiencies of 4.66% and 4.48%, respectively. These values are higher than that of fullerene acceptor-based control devices (3.55%), which are also among the best in small molecular nonfullerene acceptor organic solar cells based on ...

The prediction of the morphology and PCE of small molecular organic solar cells

The predicted morphology, domain size, PCE (power conversion efficiency) of Small Molecular Organic Solar Cells.

Stable inverted small molecular organic solar cells using a p-doped optical spacer.

We report inverted small molecular organic solar cells using a doped window layer as an optical spacer. The optical spacer was used to shift the optical field distribution inside the active layers, generating more charge carriers from sunlight. In this report, N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) was doped with 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ), a p-type dopant material. P-doped MeO-TPD was adopted as an optical spacer because it has a large energy band gap, and its conductivity can be increased by several orders of magnitude through a doping process. As a result, a power conversion efficiency of 4.15% was achieved with the doped window layer of optimized thickness. Lastly, we present significantly improved stability of the inverted devices with the MeO-TPD layer.

Cyano-substitution on the end-capping group: facile access toward asymmetrical squaraine showing strong dipole–dipole interactions as a high performance small molecular organic solar cells material

Cyano-substituted asymmetrical squaraines for organic solar cells with a high PCE was synthesized.

Effects of end-capped acceptors subject to subtle structural changes on solution-processable small molecules for organic solar cells.

Self-assembly is crucial for small molecular organic solar cells, which are extremely sensitive to the molecular structure. In this work, three subtle structural changed end-capped acceptors with increased electron withdrawing ability, named octyl 2-cyanoacetate (=CNCOOC8H17), 3-oxoundecanenitrile (=CNCOC8H17), and 2-(octylsulfonyl) acetonitrile (=CNSOOC8H17), were synthesized and introduced into a planar conjugated backbone using ethylhexyl-thiophene substituted benzodithiophene (TBDT) as a core and trithiophene as a π-bridge (labelled M1, M2 and M3, respectively). Their effects on absorption, thermal properties, and morphologies were studied and compared. In particular, the molecular packing of the three materials varied significantly, and the hole mobilities differed by orders of magnitude. As a result, the fill factors of devices varied from 52% to 72%. Combining the effects of electron-withdrawing capability and molecular packing, the power conversion efficiencies of the optimized devices increased from 3.0% for M3 to 6.4% for M1 and M2. The relationship between end-capped acceptors and photovoltaic properties would generate valuable insights into further producing more efficient solution-processable organic solar cells.

MoO3 anode buffer layer for efficient and stable small molecular organic solar cells

The effect of MoO3 anode buffer layer on power conversion efficiency and stability of small molecular photovoltaic devices based on CuPc/PTCDI-C8 heterojunction was investigated. Substantial improvement in device efficiency and stability was achieved with the incorporation of a thin MoO3 anode buffer layer. The improvement in efficiency is attributed to the decrease in contact resistance due to the reduction of barrier height at the CuPc/ITO interface. For the optimized MoO3 thickness the device exhibited a fill factor of 50.32% and efficiency of 1.6%. Stability enhancement is due to the prevention of oxygen and humidity infiltration by MoO3 anode buffer layer.

Degradation in organic solar cells under illumination and electrical stresses in air

The degradation phenomenon in small molecular organic solar cells with an indium tin oxide (anode)/copper phthalocyanine (donor)/fullerene (acceptor)/bathocuproine (buffer)/Ag (cathode) structure is experimentally investigated. The effect of three kinds of stress, that is, air exposure, repetitive illumination, and repetitive electrical stress, on device performance is examined. Both air exposure and repetitive illumination deteriorate the device performance appreciably. The decrease in power conversion efficiency is dominated by the decrease in the fill factor. Repetitive electrical stress does not cause substantial device degradation. The degree of degradation under a combination of stresses from repetitive illumination and air exposure is lower than that under illumination stress alone. Furthermore, there is a possibility that this combination of stresses induces some passivation against further air exposure stress.

Effect of crystallinity in small molecular weight organic heterojunction solar cells

Planar-heterojunction (PHJ; pn) solar cells were fabricated using a thermally convertible well soluble benzoporphycene (BPc) precursor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The conversion temperature was controlled in order to investigate the influence of the crystallinity of the organic semiconducting layer in the PHJ solar cell on the cell performance. The crystallinity of BPc increased with annealing temperature, but the surface roughness did not change. The cell with BPc converted at 250 °C had a power conversion efficiency that was 2.5 times better (1.49%) than that of the cell converted at 150 °C, and 1.5 times better than the cell with a vacuum-deposited BPc layer. The mobility of BPc, measured from the space charge limited current characteristics, supports a clear correlation between crystallinity and cell performance. Thus, the importance of increasing the crystallinity and mobility in organic solar cells was experimentally demonstrated.

Surface Plasmon Enhanced Organic Solar Cells with a MoO3 Buffer Layer

High-efficiency surface plasmon enhanced 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane:C70 small molecular bulk heterojunction organic solar cells with a MoO3 anode buffer layer have been demonstrated. The optimized device based on thermal evaporated Ag nanoparticles (NPs) shows a power conversion efficiency of 5.42%, which is 17% higher than the reference device. The improvement is attributed to both the enhanced conductivity and increased absorption due to the near-field enhancement of the localized surface plasmon resonance of Ag NPs.

Enhancing quantum efficiency of parallel-like bulk heterojunction solar cells

We report enhanced internal quantum efficiency and absorption of parallel-like bulk heterojunction small molecular organic solar cells by inserting multi-functional layers (MFLs). The inserted MFL has an energy level between main donor and acceptor levels, assisting exciton dissociation of the charge transfer state and charge transport from active layers to electrodes. Furthermore, two donors having complementary absorption spectra yield higher and broader absorption efficiency. When a ClAlPc:C60 layer was inserted as a MFL on CuPc:C60, the short circuit current (Jsc) was improved, leading to an increase of the power conversion efficiency from 2.34% to 2.71%.