Zichun Zhou

Efficient Semitransparent Solar Cells with High NIR Responsiveness Enabled by a Small-Bandgap Electron Acceptor

Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%-6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV-vis-near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an Egopt of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm-2 . Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.

An electron-rich 2-alkylthieno[3,4-b]thiophene building block with excellent electronic and morphological tunability for high-performance small-molecule solar cells

Bulk-heterojunction organic solar cells have been attracting much attention because of the potential for producing low-cost, large-area, and flexible PV panels. Generally, the ideal donor materials should have an appropriate electronic structure to absorb more sunlight and drive charge separation and to form an optimized morphology that can efficiently split excitons and collect charges. 2-Alkylthieno[3,4-b]thiophene (T34bT) as an electron-rich moiety possesses three features that are highly desirable for donor materials: (i) to modulate electronic structure; (ii) to manipulate the blend morphology of the active layer; (iii) to function as the π-bridge to link donor and acceptor moieties. We report herein a new category of small molecules STB-n based on the electron-rich T34bT moiety. STB-n featuring T34bT and rhodanine components showed intense absorption, reduced bandgap, and proper alignment of frontier orbital energy levels. Because of the optimized charge-transport properties and weak charge recombination, STB-3-based solar cells exhibit a power conversion efficiency of 9.26%, which is among the best reported for small-molecule-based solar cells. The relationships among molecular structure, thin-film morphology, and device performance were further investigated systematically.

Design of a New Fused‐Ring Electron Acceptor with Excellent Compatibility to Wide‐Bandgap Polymer Donors for High‐Performance Organic Photovoltaics

Fused-ring electron acceptors (FREAs) have recently received intensive attention. Besides the continuing development of new FREAs, the demand for FREAs featuring good compatibility to donor materials is becoming more and more urgent, which is highly desirable for screening donor materials and achieving new breakthroughs. In this work, a new FREA is developed, ZITI, featuring an octacyclic dithienocyclopentaindenoindene central core. The core is designed by linking 2,7-dithienyl substituents and indenoindene with small methylene groups, in which the indeno[1,2-b]thiophene-2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile part provides a large and unoccupied π-surface. Most notably, ZITI possesses an excellent compatibility with commercially available polymer donors, delivering very high power conversion efficiencies of over 13%.

1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells

A small molecule TBTT-1 with 5-(2-ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. TBTT-1 exhibits a broad absorption with a low optical band gap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm-2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π-π stacking and reduce the scales of phase separation, which led to improved exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance.

Poly(3-hexylthiophene)-based non-fullerene solar cells achieve high photovoltaic performance with small energy loss

Regioregular poly(3-hexylthiophene) (P3HT) as one of the most commonly used and thoroughly studied homopolymers has been demonstrated to be a promising commercial candidate for large-area, roll-to-roll printed solar cells. In this work, we designed a new low-bandgap electron acceptor material ATT-3 by combining rhodanine and octyl thieno[3,4-b]thiophene-2-carboxylate as the electron-deficient part. ATT-3 shows a planar backbone configuration facilitating charge transport and exhibits strong and broad absorption ranging from 300 to 770 nm with an optical bandgap of 1.61 eV. ATT-3 shows a complementary absorption and well-matched HOMO and LUMO energy levels with P3HT. P3HT:ATT-3-based PSCs show a high PCE of 6.26% with a high Voc of 0.927 V and a small energy loss of 0.68 eV, which is comparable to the best value obtained by using an O-IDTBR acceptor.

Applying the heteroatom effect of chalcogen for high-performance small-molecule solar cells

Small molecules with defined chemical structures and low quality variation are important for organic solar cells (OSCs). Three thieno[3,4-b]thiophene small-molecule donor materials, STB-C, STB-O and STB-S, with different side-chain substitutions of alkyl, alkoxy and alkylthio were synthesized and applied to investigate the heteroatom effects on the OSC performance. Optimized devices based on STB-C, STB-O and STB-S delivered power conversion efficiencies (PCEs) of 7.84%, 8.68% and 4.05%, respectively, revealing the distinct influence of heteroatoms. Systematic structure–property relationships were further investigated by using incident X-ray diffraction, transmission electron microscopy, atomic force microscopy, and resonant soft X-ray scattering. Compared with those of STB-C and STB-S, the highest efficiency of STB-O can be attributed to the excellent charge transport properties, originating from the stronger π–π stacking, a multi-length scaled phase separation and a slightly elevated LUMO energy level. It is noteworthy that STB-O delivered the highest PCE among small-molecule donors based on alkoxy-substituted BDTs.

A thieno[3,4-b]thiophene-based small-molecule donor with a π-extended dithienobenzodithiophene core for efficient solution-processed organic solar cells

A small molecule donor STB-4 with dithieno[2,3-d′:2′,3′-d′]benzo[1,2-b:4′,5′-b′]dithiophene (DTBDT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. An optimized power conversion efficiency of 8.17% with an open-circuit voltage of 0.904 V, a short-circuit current of 13.33 mA cm−2, and a fill factor FF of 0.67 was achieved after solvent annealing (SVA). According to the detailed morphology investigations, we found that SVA refined the phase-separated morphologies of the blends, allowing the domains to become well defined with anappropriate size that is beneficial for device performance.

Design and synthesis of medium-bandgap small-molecule electron acceptors for efficient tandem solar cells

Medium-bandgap small-molecule acceptors including YITI-0F, YITI-2F, and YITI-4F with a thiophene π-bridge are designed and synthesized. The PBDB-T:YITI-2F-based device shows the best photovoltaic performance with a PCE of up to 10.05%, a Voc of 0.93 V, a Jsc of 15.54 mA cm−2 and a FF of 69%. Tandem devices are fabricated by applying it as a front cell and the PTB7-Th:ATT-2-based device as a rear cell, giving a high PCE up to 11.86%.