High-efficiency Organic Solar Cells Research Articles

Enhancing Open-Circuit Voltage of High-Efficiency Nonfullerene Ternary Solar Cells with a Star-Shaped Acceptor.

The ternary strategy has been widely used in high-efficiency organic solar cells (OSCs). Herein, we successfully incorporated a mid-band-gap star-shaped acceptor, FBTIC, as the third component into the PM6/Y6 binary blend film, which not only achieved a panchromatic absorption but also significantly improved the open-circuit voltage (VOC) of the devices due to the high-lying lowest unoccupied molecular orbital (LUMO) of the FBTIC. Morphology characterizations show that star-shaped FBTIC molecules are amorphously distributed in the ternary system, and the finely tuned ternary film morphology facilitates the exciton dissociation and charge collection in ternary devices. As a result, the best PM6/Y6/FBTIC-based ternary OSCs achieved a power conversion efficiency (PCE) of 16.7% at a weight ratio of 1.0:1.0:0.2.

Enhanced organic photovoltaic performance through promoting crystallinity of photoactive layer and conductivity of hole-transporting layer by V2O5 doped PEDOT:PSS hole-transporting layers

The fill factor (FF) as an important photovoltaic parameter is related to the property of transport layers and active layers for organic solar cells (OSCs). Herein, we propose a simple method to improve FF by using inorganic-organic composite hole transport layer (HTL) in OSCs. Through incorporating inorganic metal oxide, vanadium pentoxide (V2O5) nanoparticles into PEDOT:PSS, the morphology and conductivity of the composite HTL are improved, due to V2O5 nanoparticles can fill the pinholes in PEDOT:PSS and expose more PEDOT chains to the surface core–shell structure. The PTB7-Th:PC71BM-based OSCs with different HTLs were prepared. The power conversion efficiency and FF of the OSCs for the composite HTL can be greatly improved compared with the traditional HTL, which can reach up to 9.44% and 70.14%, respectively. The results of water contact angle and grazing incidence wide-angle X-ray scattering show that the hydrophobicity of the composite HTL is improved, which can not only improve the physical contact but also greatly affect the subsequent active layer formation, thus increasing the crystallinity of PTB7-Th:PC71BM. These results demonstrate that V2O5:PEDOT:PSS has good application prospects as a HTL to realize high-efficient OSCs.

Correlation of Nanomorphology with Structural and Spectroscopic Studies in Organic Solar Cells

The nanomorphology of bulk heterojunction (BHJ) blends based on 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) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is systematically varied by using the volume fraction of the solvent additive 1,8-diiodooctane (DIO) from 0 to 20 vol % in chlorobenzene to prepare organic solar cells (OSCs). Blends prepared without DIO forms large phase-separated domains of PC71BM which are suppressed on addition of 3 vol % DIO, resulting in improved nanoscale features. Addition of 20 vol % DIO results in a finer interconnected morphology along with increased roughness due to polymer aggregation, which contrasts with previous reports. With increase in addition of DIO the photoluminescence (PL) from the blend is reduced; however, a relative increase in PL from 750 nm onward is observed for blends with 20 vol % DIO. As quenching of the blend PL is related to the donor/acceptor interface, structural characterizations in real-space (microscopy) and k-space (scattering) are performed to unravel the nanomorphology and correlate it with photophysical and charge transport processes. Electron-transport length scales measured by scanning photocurrent microscopy are found to increase with the addition of up to 3 vol % DIO associated with the breakup of the large PC71BM agglomerates, while the hole-transport length is found to increase on adding DIO up to 20 vol % due to aggregation of polymer chains. Hence, this work represents a unique set of results systematically examining the effect of nanomorphology on structural and solar cells properties of BHJ blends, which can have a direct implication on better understanding of the emerging high-efficiency OSC systems.

Thermally assisted charge transfer and charge separation in organic donor–acceptor solar cells

We propose a mechanism based on thermally assisted charge transfer (CT) to study the thermal effect on the formation of CT states and subsequent charge separation in an organic donor–acceptor solar cell. We reveal that the difference between phonons in the donor and acceptor caused by elastic energy acts as a thermally assisted driving force for charge transfer. It is found that the system exhibits a quite different CT process in the high and low temperature regions. Remarkably, combined with the entropy driving mechanics, the thermally assisted CT yields charge separation probability as high as 70% at room temperature. Our model and results provide a microscopic quantum understanding of the relevant recent experiments and open up a route to realize high-efficiency organic solar cells by effectively taking advantage of the thermal effect.

Efficient tuning of triphenylamine-based donor materials for high-efficiency organic solar cells

Small molecular donors (SMDs) manifest promising photovoltaic performance and rapid progress in solar cells community. Herein, we investigate high-performance triphenylamine (TPA) small molecular donors for solar cells applications. We specifically designed four donor molecules (D1–D4) with a strong donor moiety of triphenylamine (TPA) linked to four different end-capped units. The structure-property relationship and effects of end-capped units on D1-D4 are theoretically calculated and compared with reference molecule R. DFT and TD-DFT approach have been employed and assessments of photophysical characteristics, frontier molecular orbitals (FMOs), reorganization energies, density of state (DOS) and overlap population density of state (OPDOS), open-circuit voltages (Voc), transition density matrix (TDMs), surfaces and charge transfer analysis were conducted. In comparison to synthesized reference molecule R, designed D1-D4 molecules show significant and comparable optoelectronic properties. The D3 designed molecule is proven as the best candidate for solar cells applications than all other due to its promising photovoltaic properties involving the lowest band gap (3.329 eV) and excitation energy (3.094 eV), small reorganization energies for electron (λe = 0.110 eV) and hole (λh = 0.055 eV), low binding energy (Eb = 0.221 eV), highest λmax values 396.69 nm, 400.63 nm both in gas and solvent form, respectively. This theoretical designed model confirms that the end-capped unit modifications proves an efficient alternative solution in achieving the desired optoelectronic properties. Therefore, D1-D4 designed molecules are outstanding and highly recommended to experimentalists for developments of highly efficient solar cells devices in future.

Designing of near-infrared sensitive asymmetric small molecular donors for high-efficiency organic solar cells

Herein, we have designed four small molecular donors (SMDs) with Donor–Acceptor–Acceptor (D–Á–A) backbone having different acceptor units for highly efficient organic solar cells (OSCs). The specific molecular modeling has been made by replacing the additional acceptor unit (A) of recently synthesized TPA-DAA-MDN molecule (R) by employing different highly efficient acceptor units in order to improve the photovoltaic performances of the molecules. A theoretical approach (DFT and TD-DFT) has been applied to investigate the photophysical, opto-electronic and photovoltaic parameters of the designed molecules (DAA1–DAA4) and compared with the reference molecule (R). The red-shifting absorption of SMDs is the most important factor for highly efficient OSCs. Our all formulated molecules showed a red shifted absorption spectrum and also exhibit near IR sensitivity. Acceptor unit modification of R molecule causes reduction in HOMO-LUMO energy gap; therefore, all designed molecules offer better opto-electronic properties as compared to R molecule. A variety of certain critical factors essential for efficient SMDs like frontier molecular orbitals (FMOs), absorption maxima, dipole moment, exciton binding energy along with transition density matrix, excitation energy, open circuit voltages and charge mobilities of (DAA1–DAA4) and R have also been investigated. Generally, low values of reorganizational energy (hole and electron) offer high charge mobility and our all designed molecules are enriched in this aspect. High open circuit voltage values, low excitation energies, large dipole moment values indicate that our designed SMDs are suitable candidates for high-efficiency OSCs. Furthermore, conceptualized molecules are superior and thus are suggested to experimentalist for out-looking future progresses of highly efficient OSCs devices.

Random terpolymer based on thiophene-thiazolothiazole unit enabling efficient non-fullerene organic solar cells

Developing a high-performance donor polymer is critical for achieving efficient non-fullerene organic solar cells (OSCs). Currently, most high-efficiency OSCs are based on a donor polymer named PM6, unfortunately, whose performance is highly sensitive to its molecular weight and thus has significant batch-to-batch variations. Here we report a donor polymer (named PM1) based on a random ternary polymerization strategy that enables highly efficient non-fullerene OSCs with efficiencies reaching 17.6%. Importantly, the PM1 polymer exhibits excellent batch-to-batch reproducibility. By including 20% of a weak electron-withdrawing thiophene-thiazolothiazole (TTz) into the PM6 polymer backbone, the resulting polymer (PM1) can maintain the positive effects (such as downshifted energy level and reduced miscibility) while minimize the negative ones (including reduced temperature-dependent aggregation property). With higher performance and greater synthesis reproducibility, the PM1 polymer has the promise to become the work-horse material for the non-fullerene OSC community.

Molecular engineering of A–D–C–D–A configured small molecular acceptors (SMAs) with promising photovoltaic properties for high-efficiency fullerene-free organic solar cells

Non fullerene small acceptor molecules in organic photovoltaics are proven beneficial than the traditional fullerene based acceptors for their fine contribution in organic solar cells. Researchers are constantly doing efforts for designing novel acceptor materials with promising photovoltaic properties. Designing of novel molecules by end-capped modifications is a convenient strategy to obtain high efficiency acceptor molecules for OSCs. Herein, we studied optoelectronic characteristics of five novel acceptor–donor–core–donor–acceptor configured small acceptor molecules (S1–S5) after end-capped modifications of recently synthesized DF-PCIC molecule. Designed molecules S1–S5 consist of 1,4-difluorobenzene (as central core), 4,4-bis(2-ethylhexyl)-2,6-dimethyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene as donor which directly attached with different end-capped acceptors. The electronic and optical properties of newly designed (S1–S5) molecules are examined and compared with reference molecule with the aid of DFT and TD-DFT. Certain key parameters like frontier molecular orbitals analysis, density of states, dipole moment, binding energy along with transition density matrix, excitation energy, charge mobility, absorption maxima and charge transfer analysis have been performed in order to explore the photo-physical, optoelectronic and photovoltaic properties of designed and reference molecule. Out of newly designed structures, S4 displayed lowest energy band-gap (2.25 eV) with red-shifting in absorption spectrum (λmax = 712.43 nm) in chloroform which disclosed the perfect relationship between end-capped acceptor with large electron withdrawing character through extended conjugation. Similarly, S1 exhibited highest value (1.53 V) of open circuit voltage (Voc) with respect to PTB7-Th donor material owing to lower values of λe. Designed molecules exhibit better electron and hole mobility as compared to reference molecule R. All molecules express better absorption maximum, open circuit voltage, low excitation energies, comparable binding energies, large dipole moment and efficient electron and hole transport as compared to reference molecules. So, results these parameters suggest that end-capped modification is a convenient strategy in order to enhance the efficiency of OSCs. Therefore, conceptualized molecules are recommended to experimentalist for out-looking future developments of highly efficient solar cells.

In Operando GISAXS and GIWAXS Stability Study of Organic Solar Cells Based on PffBT4T‐2OD:PC71BM with and without Solvent Additive

Solvent additives are known to modify the morphology of bulk heterojunction active layers to achieve high efficiency organic solar cells. However, the knowledge about the influence of solvent additives on the morphology degradation is limited. Hence, in operando grazing‐incidence small and wide angle X‐ray scattering (GISAXS and GIWAXS) measurements are applied on a series of PffBT4T‐2OD:PC71BM‐based solar cells prepared without and with solvent additives. The solar cells fabricated without a solvent additive, with 1,8‐diiodoctane (DIO), and with o‐chlorobenzaldehyde (CBA) additive show differences in the device degradation and changes in the morphology and crystallinity of the active layers. The mesoscale morphology changes are correlated with the decay of the short‐circuit current J sc and the evolution of crystalline grain sizes is codependent with the decay of open‐circuit voltage V oc. Without additive, the loss in J sc dominates the degradation, whereas with solvent additive (DIO and CBA) the loss in V oc rules the degradation. CBA addition increases the overall device stability as compared to DIO or absence of additive.

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads: New insights for high efficiency organic solar cells

In the active layer of organic solar cells (OSCs), the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices. Herein, we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides (PDI) dyads (2PDI-Th and fused-2PDI-Th), in order to provide a unique explanation in depth on their different performances in OSC devices. From the transient absorption (TA) spectra, the singlet excitons of 2PDI-Th form excimers in the time scale of 1.5 ps. Then the excimers go into the triplet state via intersystem crossing (ISC). In fused-2PDI-Th, triplet excitons are generated directly from the singlet excited excitons via the efficient ISC. Density functional theory (DFT) calculations further support the formation of excimers. DFT results indicate that 2PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers, while fused-2PDI-Th gives a twisted X-shaped configuration in the optimized ground and excited state. In steady-state emission spectra, 2PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps, which is much shorter than those of PDI (5.5 ns) and fused-2PDI-Th (3.3 ns). The triplet lifetime (67 μs) of fused-2PDI-Th is factor of 3 longer than that of 2PDI-Th (22 μs). These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons, which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.

Optimized active layer morphology toward efficient and polymer batch insensitive organic solar cells

Morphology control in laboratory and industry setting remains as a major challenge for organic solar cells (OSCs) due to the difference in film-drying kinetics between spin coating and the printing process. A two-step sequential deposition method is developed to control the active layer morphology. A conjugated polymer that self-assembles into a well-defined fibril structure is used as the first layer, and then a non-fullerene acceptor is introduced into the fibril mesh as the second layer to form an optimal morphology. A benefit of the combined fibril network morphology and non-fullerene acceptor properties was that a high efficiency of 16.5% (certified as 16.1%) was achieved. The preformed fibril network layer and the sequentially deposited non-fullerene acceptor form a robust morphology that is insensitive to the polymer batches, solving a notorious issue in OSCs. Such progress demonstrates that the utilization of polymer fibril networks in a sequential deposition process is a promising approach towards the fabrication of high-efficiency OSCs.

High performance organic solar cells based on ZnO: POT2T as an effective cathode interfacial layer

Interface engineering in organic solar cells (OSCs) plays an important role in improving electron extraction and suppressing carrier recombination. In this article, the improvement of device was achieved by doping POT2T into ZnO as a new electron transport layer (ETL). For PTB7-Th: PC71BM system, the device fabricated by using ZnO as ETL achieved a PCE of 9.03% with a JSC of 17.33 mA/cm2, a VOC of 0.783V and a FF of 64.17%. After doping 5wt% POT2T in ZnO, the JSC increased to 18.01 mA/cm2 and the FF increased slightly to 69.87%, as a result, the PCE increased to 9.84% with about 10% enhancement. The enhancement is attributed to the improved electron transport and conductivity, optimized ZnO surface morphology after adding POT2T, resulting in higher JSC and FF. Therefore, doping POT2T into ZnO as an ETL is a simple and effective method for obtaining high efficient OSCs.

Asymmetrically noncovalently fused-ring acceptor for high-efficiency organic solar cells with reduced voltage loss and excellent thermal stability

Abstract Simultaneously broadening the spectral response and reducing the energy loss are challenging tasks in the material design of organic solar cells (OSCs). Herein, a novel asymmetrically noncovalently fused-ring electron acceptor (NFEA) with unilateral alkylthio-substituted thiophene π-bridge, namely IDST-4F, is synthesized. IDST-4F exhibits a broader absorption, higher-lying energy levels, larger dipole moments and suppressed crystallinity than its symmetric counterpart (ID-4F) without the π-bridge. Compared to the devices of PM6:ID-4F, the optimized PM6:IDST-4F-based devices display simultaneously enhanced current density and photovoltage, resulting in an excellent power conversion efficiency (PCE) of 14.3%, which is the highest value among the OSCs based on NFEAs reported in the literature to date. More importantly, the PM6:IDST-4F-based OSCs possess excellent thermal stability with 82% of the initial PCE after thermal treatment at 150 °C for 1200 min. In summary, this study indicates that asymmetrically NFEAs are promising to achieve high efficiency with excellent thermal stability.

Diflurobenzothiadiazole core-based noncovalently fused small molecule acceptor exhibiting over 12% efficiency and high fill factor

The versatility and flexibility of organic photoelectric materials endow organic photovoltaic cells fine function modulation and huge commercial potential. In this work, a new noncovalent fused-ring small molecule acceptor (SMA) BID-4F has been synthesized for high-efficient organic solar cells (OSCs). BID-4F consists of a diflurobenzothiadiazole (DFBT) core, ladder-like indacenodithiophene (IDT) spacers, and dicyanoindanone electron-withdrawing end groups, which are supposed to be conformationally interlocked by noncovalent interactions, leading to good molecular planarity. In addition, compact solid state stacking was revealed by UV–vis–NIR absorption spectrum. The optimized PM6:BID-4F based device delivers an eminent power conversion efficiency (PCE) of 12.30% with a high open-circuit voltage (Voc) of 0.92 V and a high fill factor (FF) of 74.3%. Most importantly, the PCE and FF are among one of the highest values reported for the OSCs based on the unfused-ring SMAs. Overall, our work demonstrates that the unfused ring central framework with high molecular planarity through noncovalent interactions provides a good strategy to construct highly efficient SMAs.

Ultrafast Hole Transfer and Carrier Transport Controlled by Nanoscale-Phase Morphology in Nonfullerene Organic Solar Cells.

Nonfullerene acceptors (NFAs) have attracted great attention in high-efficiency organic solar cells (OSCs). While the effect of molecular properties including structures and energetics on charge transfer has been extensively investigated, the effect of macroscopic-phase properties is yet to be revealed. Here we have performed a correlation study of the nanoscale-phase morphology on the photoexcited hole transfer (HT) process and photovoltaic performance by combining ultrafast spectroscopy with high temporal resolution and photo-induced force microscopy (PiFM) with high spatial and chemical resolution. In PM6/IT-4F, we observe biphasic HT behavior with a minor ultrafast (<100 fs) interfacial process and a major diffusion-mediated HT process until ∼100 ps, which depends strongly on phase segregation. Because of the interplay between charge transfer and transport, a compromised domain size of 20-30 nm for NFAs shows the best performance. This study highlights the critical role of phase morphology in high-efficiency OSCs.

High-Efficiency Nonfullerene Organic Solar Cells Enabled by 1000 nm Thick Active Layers with a Low Trap-State Density.

The high-efficiency organic solar cells (OSCs) with thicker active layers are potential candidates for the fabrication of large-area solar panels. The low charge carrier mobility of the photoactive materials has been identified as the major problem hindering the photovoltaic performance of the thick-film OSCs. In this study, high performance of ultra-thick-film OSCs employing a nonfullerene acceptor BTP-4Cl and a polymer donor PBDB-TF is demonstrated. Two blends (PBDB-TF:BTP-4Cl and PBDB-TF:IT-4F) show comparable mobilities and excellent photovoltaic characteristics in thin-film devices, while in the 1000 nm thick devices, although they both exhibit desirable and balanced mobilities, the PBDB-TF:BTP-4Cl-based blend possesses lower trap-state density than the IT-4F-based counterpart, leading to lower trap-assist recombination, longer carrier lifetime, and thus a much higher short-circuit current density in the device. As a result, the BTP-4Cl-based 1000 nm thick OSC achieves a remarkable power conversion efficiency of 12.1%, which greatly outperforms the IT-4F-based devices (4.72%). Furthermore, for a 1000 nm thick device with an active area of 4 cm2, a promising efficiency of 10.1% was obtained, showing its great potential in future large-scale production.

Exploiting Ternary Blends for Improved Photostability in High-Efficiency Organic Solar Cells

Ternary organic solar cells based on polymer donor and nonfullerene acceptors (NFAs) are delivering high power conversion efficiencies (PCE). Now, further improvement needs to be directed to enhanc...

Introducing Trifluoromethyl to Strengthen Hydrogen Bond for High Efficiency Organic Solar Cells.

Nowadays, the ternary strategy has become a common way to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The intermolecular interaction between the third component and donor or acceptor plays a key role in achieving a high performance. However, hydrogen bond as a strong intermolecular interaction is rarely considered in ternary OSCs. In this work, we introduce trifluoromethyl on a newly synthesized small molecular DTBO to strength hydrogen bonds between DTBO and IEICO-4F. Due to the existence of hydrogen bonds has a strong impact on electrostatic potential (ESP) and benefits π-π stacking in the active layer, the ternary OSCs show superior charge extraction and low charge recombination. In DTBO, PTB7-Th and IEICO-4F based ternary devices, the PCE increases from 11.02 to 12.48%, and short-circuit current density (JSC) increases from 24.94 to 26.43 mA/cm2 compared with typical binary devices. Moreover, the addition of DTBO can realize an energy transfer from DTBO to PTB7-Th and broaden the absorption spectrum of blend films. Grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns show that the π-π stacking distance of IEICO-4F decreased after adding 10 wt% DTBO. The effect of the hydrogen bond is also achieved in the PM6: Y6 system, showing 16.64% efficiency by comparison to the 15.49% efficiency of binary system. This work demonstrates that introduce trifluoromethyl to enhance hydrogen bond which improve π-π stacking can achieve higher performance in OSCs.

High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency1–4. In particular, non-radiative recombination loss3 and energetic disorder5, which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. An organic solar cell designed with minimal energetic disorder exhibits very low energy loss due to non-radiative recombination and highly efficient operation.

Stability of Lead and Tin Halide Perovskites: The Link between Defects and Degradation.

The field of photovoltaic research has been lately dominated by the rapid evolution of low-cost and high-efficiency hybrid organic lead halide perovskite solar cells. Despite the considerable progress made in the efficiency of such devices, the achievement of long-term material and device stability remains a challenge. In this Perspective, insights into the role structural defects play in the stability of these perovskite absorbers are examined, highlighting the critical importance of vacancy type defects as the initiation sites for moisture-, oxygen-, and light-induced degradation and the approaches that are emerging to help overcome these issues. In the second part of the Perspective we consider the stability of tin-based perovskites. Here, the Sn4+ defects that arise upon material degradation are described along with the strategies being developed to enhance stability and decrease their formation. Finally, the discussion is extended to innately more stable layered tin-based perovskites, identifying them as a route to the development of efficient lead-free perovskite solar cells.