Yingzhi Jin

9.2%-efficient core-shell structured antimony selenide nanorod array solar cells

9.2%-efficient core-shell structured antimony selenide nanorod array solar cells, Published online: 10 January 2019; doi:10.1038/s41467-018-07903-6

Small‐molecule‐based ternary BHJ solar cells with the SM donor DR3, the SM acceptor ICC6, and the fullerene PC₇₁BM yield high power conversion efficiencies nearing 11% for active layer thicknesses >200 nm. With low geminate and nongeminate recombination, long carrier lifetimes, and high/balanced carrier mobilities, the ternary system maintains PCEs >8% over a wide range of active layer thicknesses within 200–500 nm.

Abstract

Solution‐processed small molecule (SM) solar cells have the prospect to outperform their polymer‐fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk‐heterojunction (BHJ) charge carrier mobility (<10^‐4 cm² V^‐1 s^‐1) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large‐scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC₇₁BM can be used to achieve SM‐based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10^‐4 cm² V^‐1 s^‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC₇₁BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC₇₁BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment.

Replacement of alkyl chains with alkoxyl chains of the backbone of nonfullerene acceptors successfully increases the energy levels, enhances the light absorption, and improves charge transport mobility, which simultaneously enhances three parameters (short circuit current density (J _SC), open circuit voltage (V _OC), and fill factor (FF)), resulting in a highest efficiency for poly(3‐hexylthiophene) (P3HT)‐based organic solar cells (OSCs) (6.6%).

Abstract

Poly(3‐hexylthiophene) (P3HT)‐based organic solar cells (OSCs) have attracted much attention due to their advantages of low‐cost production and matured roll‐to‐roll manufacture. However, the efficiency of P3HT‐based OSCs lag much behind the non‐P3HT ones due to their negligible absorption of long wavelengths of light over 650 nm, high‐lying highest occupied molecular orbitals (HOMO), and difficulty of controlling morphology. In this study, the alkyl chains of the nonfullerene acceptors are replaced with alkoxy chains to achieve synergistic enhancement of all three parameters ( short circuit current density (J _SC), open circuit voltage (V _OC), and fill factor (FF)) and thus significant increase of power conversion efficiency for P3HT‐based OSCs. As a result, the OSCs exhibit a maxima efficiency of 6.6%. The P3HT‐based systems are systematically studied with optical spectroscopy, photoluminescence, cyclic voltametry, space charge limit current, grazing incident wide‐angle X‐ray scattering, transient absorption spectroscopy, transmission electron microscope, and atomic force microscopy to probe the mechanism, which reveal that introducing alkoxy chains simultaneously increases the energy levels of the HOMO and the lowest unoccupied molecular orbitals, enhances the light absorption, improves the rigidity of the backbone and charge transport mobility, and tunes the molecular orientation and film morphology, thus improving the photovoltaic performance. This contribution provides an important guidance in the design of novel nonfullerene acceptors for high‐performance P3HT‐based OSCs.

Quaternary organic solar cells based on PBDB‐T:PTB7‐Th:ITIC:FOIC are reported to deliver a parallel‐alloy morphology mode in non‐fullerene‐based devices. In the quaternary system, the parallel‐like donors and alloy‐like acceptors together facilitate transfer kinetics, optimize blend morphology, and drive the device efficiency toward over 12.5%, which indicates the great potential of quaternary organic solar cells.

Abstract

Two compatible donors (PBDB‐T and PTB7‐Th) and two miscible acceptors (ITIC and FOIC) are employed to deliver a parallel‐alloy morphology model in non‐fullerene‐based quaternary organic solar cells. PBDB‐T and PTB7‐Th form a parallel link with a slight adjustment of molecular packing into enhanced face‐on crystallites while ITIC disperses into discontinuous FOIC microcrystal regions to form continuous and ordered alloy‐like acceptor phases. Characterization of blend morphology highlights the parallel‐alloy model—enabled by the introduction of PBDB‐T and ITIC, which contributes to improved molecular packing and reduced domain size resulting in efficient charge generation and consistent transport channels. This successful parallel‐alloy quaternary blend morphology demonstrates an enhanced optical absorption, optimized domain size, and nanostructures toward simultaneous improvement in charge transfer and transport. Therefore, a power conversion efficiency of 12.52% is realized for a quaternary device which is 6% higher than the ternary device (PBDB‐T:PTB7‐Th:FOIC) and 12% higher than the binary device (PTB7‐Th:FOIC). Domination of quaternary devices over ternary and binary blends, which is another feasible way to realize highly efficient devices through further investigation of quaternary OSCs, is presented.

Multijunction organic solar cells provide higher power conversion efficiencies than the corresponding single junction solar cells by reducing thermalization and transmission losses and are fabricated by sequential layer deposition from solution. In recent years, important progress has been made in terms of novel materials and device design and the most salient advances are discussed.

Abstract

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution‐processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.

Strategies to enhance the stretchability of conductive poly(3,4‐ethylenedioxythiophene) (PEDOT) and PEDOT: poly(styrene sulfonate) for applications in energy, electronics, and biology are highlighted. The benefits and drawbacks of each method on the mechanical properties and conductivity are discussed along with some of the challenges that remain to be addressed.

Abstract

The conductive polymer poly(3,4‐ethylenedioxythiophene) (PEDOT), and especially its complex with poly(styrene sulfonate) (PEDOT:PSS), is perhaps the most well‐known example of an organic conductor. It is highly conductive, largely transmissive to light, processible in water, and highly flexible. Much recent work on this ubiquitous material has been devoted to increasing its deformability beyond flexibility—a characteristic possessed by any material that is sufficiently thin—toward stretchability, a characteristic that requires engineering of the structure at the molecular‐ or nanoscale. Stretchability is the enabling characteristic of a range of applications envisioned for PEDOT in energy and healthcare, such as wearable, implantable, and large‐area electronic devices. High degrees of mechanical deformability allow intimate contact with biological tissues and solution‐processable printing techniques (e.g., roll‐to‐roll printing). PEDOT:PSS, however, is only stretchable up to around 10%. Here, the strategies that have been reported to enhance the stretchability of conductive polymers and composites based on PEDOT and PEDOT:PSS are highlighted. These strategies include blending with plasticizers or polymers, deposition on elastomers, formation of fibers and gels, and the use of intrinsically stretchable scaffolds for the polymerization of PEDOT.

The combination of a liquid‐crystalline small‐molecule donor and a nonfullerene acceptor with good crystallinity provides appropriate phase separation and molecular stacking orientation for all‐small‐molecule solar cells. The fill factors and power conversion efficiencies (PCEs) are obviously improved in the proposed systems, and the maximum PCE reaches 10.7%. The results are significant for the construction and development of all‐small‐molecule solar cells.

Abstract

Compared with nonfullerene‐based polymer solar cells, all‐small‐molecule solar cells normally show low power conversion efficiencies (PCEs) due to their low fill factors (FFs). Molecular stacking orientation and phase separation are the main factors affecting the performance of all‐small‐molecule solar cells. In this work, two liquid‐crystalline small‐molecule donors are designed and synthesized and a novel nonfullerene acceptor with good crystallinity developed. Owing to the face‐on orientation of the component molecules and appropriate phase separation in the active layer, a high FF of over 70% and a PCE of 10.7% are obtained from the resulting solar cells; these values are among the best obtained thus far for all‐small‐molecule solar cells. The high FF reported here is significant for a further design of high‐performance all‐small‐molecule solar cells.

Ternary organic solar cells with improved power conversion efficiency (PCE) and ambient stability are developed by combining a nonfullerene acceptor and a fullerene acceptor. Such a ternary system is insensitive to the content of the third component, and PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of the binary reference device.

Abstract

The ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC₇₁BM) are reported. It is found that the addition of PC₇₁BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC₇₁BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.

High performance fullerene‐free organic photovoltaic devices are fabricated via temperature controlled slot die coating. The additive‐free approach results in not only high performance with a 10% power conversion efficiency (PCE) but also exceptional stability under continuous illumination. The scalable method is then used in a roll‐to‐roll process to achieve over 7% PCE on a flexible substrate.

Abstract

Solution‐processed organic photovoltaics (OPVs) have continued to show their potential as a low‐cost power generation technology; however, there has been a significant gap between device efficiencies fabricated with lab‐scale techniques—i.e., spin coating—and scalable deposition methods. Herein, temperature‐controlled slot die deposition is developed for the photoactive layer of OPVs. The influence of solution and substrate temperatures on photoactive films and their effects on power conversion efficiency (PCE) in slot die coated OPVs using a 3D printer‐based slot die coater are studied on the basis of device performance, molecular structure, film morphology, and carrier transport behavior. These studies clearly demonstrate that both substrate and solution temperatures during slot die coating can influence device performance, and the combination of hot substrate (120 °C) and hot solution (90 °C) conditions result in mechanically robust films with PCE values up to 10.0% using this scalable deposition method in air. The efficiency is close to that of state‐of‐the‐art devices fabricated by spin coating. The deposition condition is translated to roll‐to‐roll processing without further modification and results in flexible OPVs with PCE values above 7%. The results underscore the promising potential of temperature‐controlled slot die coating for roll‐to‐roll manufacturing of high performance OPVs.

The molecular order of nonfullerene electron acceptor INPIC‐4F is manipulated by varying the self‐organization time during solution casting. With the presence of solvent vapor, INPIC‐4F grows into spherulites with poor efficiency. On the contrary, casting on hot substrates promotes face‐on π−π stacking, which improves absorption as well as efficient exciton dissociation and balanced charge mobility for a maximum efficiency of 13.1%.

Abstract

Developing a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB‐T:INPIC‐4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC‐4F in achieving high performance is demonstrated. When PBDB‐T:INPIC‐4F is cast with the presence of solvent vapor to prolong the organization time, INPIC‐4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face‐on π−π stacking of INPIC‐4F. This molecular transformation of INPIC‐4F significantly enhances the absorption ability of INPIC‐4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.

Ultraflexible organic photovoltaics are developed by employing a novel metal‐oxide‐free cathode that consists of a printed ultrathin metallic transparent electrode and an organic electron transport layer. The proposed ultraflexible organic photovoltaics achieve a power conversion efficiency of 9.7% and durability with 74% efficiency retention after 500 cycles of deformation at 37% compression through buckling.

Abstract

Flexible and stretchable organic photovoltaics (OPVs) are promising as a power source for wearable devices with multifunctions ranging from sensing to locomotion. Achieving mechanical robustness and high power conversion efficiency for ultraflexible OPVs is essential for their successful application. However, it is challenging to simultaneously achieve these features by the difficulty to maintain stable performance under a microscale bending radius. Ultraflexible OPVs are proposed by employing a novel metal‐oxide‐free cathode that consists of a printed ultrathin metallic transparent electrode and an organic electron transport layer to achieve high electron‐collecting capabilities and mechanical robustness. In fact, the proposed ultraflexible OPV achieves a power conversion efficiency of 9.7% and durability with 74% efficiency retention after 500 cycles of deformation at 37% compression through buckling. The proposed approach can be applied to active layers with different morphologies, thus suggesting its universality and potential for high‐performance ultraflexible OPV devices.