Zaiyu Wang

Conjugated Polymer–Small Molecule Alloy Leads to High Efficient Ternary Organic Solar Cells

Ternary organic solar cells are promising candidates for bulk heterojunction solar cells; however, improving the power conversion efficiency (PCE) is quite challenging because the ternary system is complicated on phase separation behavior. In this study, a ternary organic solar cell (OSC) with two donors, including one polymer (PTB7-Th), one small molecule (p-DTS(FBTTH2)2), and one acceptor (PC71BM), is fabricated. We propose the two donors in the ternary blend forms an alloy. A notable averaged PCE of 10.5% for ternary OSC is obtained due to the improvement of the fill factor (FF) and the short-circuit current density (J(sc)), and the open-circuit voltage (V(oc)) does not pin to the smaller V(oc) of the corresponding binary blends. A highly ordered face-on orientation of polymer molecules is obtained due to the formation of an alloy structure, which facilitates the enhancement of charge separation and transport and the reduction of charge recombination. This work indicates that a high crystallinity and the face-on orientation of polymers could be obtained by forming alloy with two miscible donors, thus paving a way to largely enhance the PCE of OSCs by using the ternary blend strategy.

A planar electron acceptor for efficient polymer solar cells

A novel planar acceptor IDT-2BR was designed and synthesized. Polymer solar cells (PSCs) based on P3HT:IDT-2BR blended films gave power conversion efficiencies of up to 5.12%, which are much higher than that of PC61BM-based control devices (3.71%) and the highest values reported for P3HT-based fullerene-free PSCs.

High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π-π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices.

Acceptor End‐Capped Oligomeric Conjugated Molecules with Broadened Absorption and Enhanced Extinction Coefficients for High‐Efficiency Organic Solar Cells

Acceptor end-capping of oligomeric conjugated molecules is found to be an effective strategy for simultaneous spectral broadening, extinction coefficient enhancement, and energy level optimization, resulting in profoundly enhanced power conversion efficiencies (of 9.25% and 8.91%) compared to the original oligomers. This strategy is effective in overcoming the absorption disadvantage of oligomers and small molecules due to conjugation limitation.

Enhancing Performance of Large-Area Organic Solar Cells with Thick Film via Ternary Strategy

Large-scale fabrication of organic solar cells requires an active layer with high thickness tolerability and the use of environment-friendly solvents. Thick films with high-performance can be achieved via a ternary strategy studied herein. The ternary system consists of one polymer donor, one small molecule donor, and one fullerene acceptor. The small molecule enhances the crystallinity and face-on orientation of the active layer, leading to improved thickness tolerability compared with that of a polymer-fullerene binary system. An active layer with 270 nm thickness exhibits an average power conversion efficiency (PCE) of 10.78%, while the PCE is less than 8% with such thick film for binary system. Furthermore, large-area devices are successfully fabricated using polyethylene terephthalate (PET)/Silver gride or indium tin oxide (ITO)-based transparent flexible substrates. The product shows a high PCE of 8.28% with an area of 1.25 cm2 for a single cell and 5.18% for a 20 cm2 module. This study demonstrates that ternary organic solar cells exhibit great potential for large-scale fabrication and future applications.

Understanding the Impact of Hierarchical Nanostructure in Ternary Organic Solar Cells

Ternary organic solar cells (OSCs), which blend two donors and fullerene derivatives with different absorption ranges, are a promising potential strategy for high‐power conversion efficiencies (PCEs). In this study, inverted ternary OSCs are fabricated by blending a highly crystalline small molecule BDT‐3T‐CNCOO in a low band gap polymer PBDTTT‐C‐T:PC71BM. As the small molecule is introduced, the overall PCEs increase from 7.60% to 8.58%. The morphologies of ternary blends are studied by combining transmission electron microscopy and X‐ray scattering techniques at different length scales. Hierarchical phase separation is revealed in the ternary blend, which is composed of domains with sizes of ≈88, ≈50, and ≈20 nm, respectively. The hierarchical phase separation balances the charge separation and transport in ternary OSCs. As a result, the fill factors of the devices significantly improve from 58.4% to 71.6%. Thus, ternary blends show higher hole mobility and higher fill factor than binary blends, which demonstrates a facile strategy to increase the performance of OSCs.

From Alloy-Like to Cascade Blended Structure: Designing High-Performance All-Small-Molecule Ternary Solar Cells

Ternary blending strategy has been used to design and fabricate efficient organic solar cells by enhancing the short-circuit current density and the fill factor. In this manuscript, we report all-small-molecule ternary solar cells consisting of two compatible small molecules DR3TBDTT (M1) and DR3TBDTT-E (M2) as donors and PC71BM as acceptor. A transformation from an alloy-like model to a cascade model are first realized by designing a novel molecule M2. It is observed that after thermal and solvent vapor annealing M2 shifts from the mixed region to donor-acceptor (D-A) interfaces which ameliorates the charge transfer and recombination processes. The optimal ternary solar cells with 10% M2 exhibited a power conversion efficiency of 8.48% in the alloy-like model and 10.26% in the cascade model. The proposed working mechanisms are fully characterized and further supported by the density functional theory and atomistic molecular dynamics simulations. This provides an important strategy to design high-performance ternary solar cells which contains one molecule not only is compatible with the main donor molecule but also performs a preference to appear at the D-A interfaces hence builds cascade energy levels.

Evolution of morphology and open-circuit voltage in alloy-energy transfer coexisting ternary organic solar cells

The oligomer-type small molecule PDT2FBT-ID is applied in a polymer/small molecule/fullerene ternary system. The PTB7-Th/PDT2FBT-ID/PC71BM ternary active layer shows complementary absorption spectra, enhanced face-on orientation and energy transfer properties. Compared to the binary system, the FF of the ternary system is improved from 66.72% to 76.14% and the Jsc is improved from 17.90 mA cm−2 to 18.92 mA cm−2. The maximum PCE of 11.1% is obtained in the ternary system with a common inverted device structure. Additionally, an alloy-like domain structure and energy transfer between the two donor materials are found to coexist in the ternary system. Based on the combined model, the variation in morphology and Voc is discussed in detail and compared with previous publications. An in-depth interpretation for the selection of a third compound in high performance ternary organic solar cells is provided. Finally, this facile design strategy could also be widely used in other systems.

Tuning molecule diffusion to control the phase separation of the p-DTS(FBTTh2)2/EP-PDI blend system via thermal annealing

An interpenetrating bulk-heterojunction structure with a domain size of 10–20 nm is the ideal morphology for carrier generation, separation and transportation in organic solar cells. However, depending on the blend composition, the phase-separation behavior of the crystalline small molecule blend system of donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and acceptor N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) is quite different. When the weight ratio of p-DTS(FBTTh2)2/EP-PDI is greater than 8 : 2 or smaller than 4 : 6, a large phase separation structure is observed induced by p-DTS(FBTTh2)2 or EP-PDI crystallization. When the ratio of p-DTS(FBTTh2)2 : EP-PDI changes from 7 : 3 to 5 : 5, no obvious phase separation can be detected due to the interaction between p-DTS(FBTTh2)2 and EP-PDI. In order to obtain the interpenetrating bulk-heterojunction structure with a domain size of 10–20 nm, we proposed to tune the molecule diffusion of p-DTS(FBTTh2)2 and EP-PDI by different thermal annealing temperature to control the phase separation domain size and phase purity. When the annealing temperature is T Tc p-DTS(FBTTh2)2 − Tb (T is the thermal annealing temperature, Ta and Tb are constants), the molecule diffusion rate of both p-DTS(FBTTh2)2 and EP-PDI is too slow or too fast, resulting in no phase separation or large phase separation morphology. When T is between Tm EP-PDI + Ta and Tc p-DTS(FBTTh2)2 − Tb, EP-PDI is in a melting state, but p-DTS(FBTTh2)2 self-assembles into crystals. As a result, p-DTS(FBTTh2)2 crystallized and formed a framework, which inhibited the massive crystallization of EP-PDI due to the spatial confinement of the p-DTS(FBTTh2)2 crystallization framework, leading to the formation of a bi-continuous phase separation structure with suitable domain size and phase purity. Based on the above phase separation structure we got, a power conversion efficiency of 4.25% was obtained, which is relatively high in this system without any additives.

Aromatic end-capped acceptor effects on molecular stacking and the photovoltaic performance of solution-processable small molecules

Aromatic end-capped acceptors are important in constructing donor materials and non-fullerene acceptors in organic solar cells. However, their features (such as electron-withdrawing ability and subtle change in planarity) and effects on molecular stacking and photovoltaic performance, lack a systematic study. This manuscript reports four molecules, namely, BT-RCN, BT-BA, BT-RA, and BT-ID, which are terminated by different acceptors in the same backbone. We quantify the molecular planarity through their dipole moment in the Z direction and investigate the effect of the degree of planarity on molecular properties and device performances. The four acceptors are classified into two groups based on acceptor strength: medium-strong and strong acceptors. Molecules based on medium-strong acceptors exhibit excellent efficiencies as follows: 10.1% for BT-ID, 9.6% for BT-RA, and 8.5% for BT-BA. Their decreased efficiency is quite consistent with their lowered hole mobility. Grazing incidence X-ray diffraction results demonstrate the positive relationship between the non-planarity of the acceptors and d-spacing distance in the π–π stacking direction, which are detrimental to mobility. BT-RCN, with a strong acceptor, obtains the lowest efficiency of 6.1%. These findings indicate the importance of matching the electron-donating ability of donor units and electron-withdrawing ability of acceptor units, and the subtle planarity change in molecular properties and aggregation.