Dalei Yang

New benzotrithiophene derivative with a broad band gap for high performance polymer solar cells

A new planar conjugated polymer, poly{benzo(1,2-b:3,4-b′:5,6-d′′)trithiophene-alt-4,4′-dihexyl-2,2′-bithiazole} (BTT-BTz), with a broad band gap and higher Voc based on benzotrithiophene and bithiazole units, was designed and synthesized. This copolymer possesses a deeper HOMO (−5.65 eV), and after 1,8-diiodoctane treatment, the power conversion efficiency of the photovoltaic device based on the BTT-BTz/PC71BM photoactive layer reaches 5.06% with 0.81 V of open-circuit voltage, 10.9 mA cm−2 of short-circuit current and 0.57 of fill factor, which is the highest one among the conjugated polymers based on BTT or BTz derivatives. This new conjugated copolymer seems a promising candidate for the application in tandem solar cells.

Side‐Chain Engineering for Enhancing the Thermal Stability of Polymer Solar Cells

An effective strategy of engineering side chains is proposed for enhancing solar-cell-device thermal stability. As the conjugated length of the side chains increases, the morphological stability of the blend film is enhanced. The thermal stability of corresponding devices is consequently improved.

Fluorinated low band gap copolymer based on dithienosilole-benzothiadiazole for high-performance photovoltaic device

A new fluorinated low band gap copolymer, poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-4,7-(5-fluoro-2,1,3-benzothiadiazole)] (PDTSBT-F), was designed and synthesized. The introduction of fluorine atom to a classical low band gap copolymer (PDTSBT) has a little influence on the polymer absorption spectrum and band gap, which was 1.48 eV for PDTSBT-F. However, the HOMO level was lowered to −5.17 eV for PDTSBT-F, the film crystallinity was improved, and PDTSBT-F showed higher charge carrier mobility than its non-fluorinated analogue (PDTSBT). For the PDTSBT-F/PC71BM device, a Jsc of 15.96 mA cm−2, a Voc of 0.70 V, and a FF of 0.60 were attained, resulting in a PCE of 6.70%. To the best of our knowledge, this is the highest value to date in devices based on copolymers with C-, Si- and Ge-bridged dithiophene as the electron-rich unit and benzothiadiazole derivatives as electron-deficient unit. A high PCE in combination with a wide absorption spectrum in the visible range could induce PDTSBT-F to be a potentially promising low band gap polymer for polymer solar cells.

Simultaneous enhancement of performance and insensitivity to active layer thickness for OPVs by functionalizing π-spacer's side chain

In the design of high performance conjugated copolymers, π-spacer plays one of the most important roles in finely manipulating photovoltaic properties of the polymers. However, up to now, little attention has been paid to functionalize the π-spacer. In this work, a novel D–A conjugated copolymer PBTI3T-S, with 3-(decylthio)thiophene as the π-spacer, is designed and synthesized. The introduction of a modified π-spacer has little influence on the absorption spectrum and band gap of the polymer. However, it is found that the modified π-spacer could create a noncovalent attractive interaction between neighboring moieties, resulting in good planarity and decreased π–π stacking distance in comparison to its analogue PBTI3T with decyl substitution on the π-spacers. More interestingly, the face-on population of crystallite orientation is significantly enhanced. Thus, the hole mobility of the PBTI3T-S/PC71BM blend is 1.29 × 10−2 cm2 V−1 s−1, which is one order of magnitude higher than that of the PBTI3T/PC71BM blend (1.15 × 10−3 cm2 V−1 s−1). The PBTI3T-S device provides a higher PCE of 7.14%, while the optimized PBTI3T device provides 6.51%. Impressively, the PCEs of the PBTI3T-S device can remain above 7% without substantial loss as the active layer thickness increases up to ∼270 nm, while the PCE reduces to 5.70% for the PBTI3T. These results demonstrate that the introduction of an alkylthio chain modified π-spacer would be an effective way to further improve the device performance and provide a guideline for molecular engineering towards the application of the roll to roll printing technique.

Synergistic effect of fluorination and regio-regularity on the long-term thermal stability of polymer solar cells

Three structurally identical polymers, except for the number of fluorine substitutions, namely PF0, PF1 and PF2, are designed and synthesized, in order to investigate the impact of fluorination on the thermal stability of polymer solar cells. Devices based on the three polymers can retain ∼90% of their initial efficiency, after annealing for 30 days at 100 °C. To the best of our knowledge, this is the first time that a series of novel materials achieving such long-term device thermal stability at high temperature have been reported. Accelerated aging tests show that device thermal stability ranks in the following order: PF2 > PF0 > PF1. Our findings demonstrate that PF2, with two fluorine substitutions and featuring regio-regularity, has the strongest ability to preserve the morphology, which endows the PF2 blend with a relative slow rate of morphological deterioration. In contrast, the morphology of mono-fluorinated PF1 shows the poorest thermal stability, which is ascribed to the regio-random characteristic induced by mono-fluorination. This work discloses the synergistic effect of fluorination and regio-regularity on device thermal stability for the first time, and systematically elucidates the “structure-thermal stability” relationship, which provides a guideline for designing materials with high device thermal stability.

Novel wide band gap copolymers featuring excellent comprehensive performance towards the practical application for organic solar cells

In the past few decades, extensive efforts have been devoted to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs), while their comprehensive performance including high efficiency, fulfilling the prerequisites of solution printing technology (active layer thickness >200 nm) and long-term thermal stability, which are essential for the practical application of OSCs, have not been paid adequate attention yet. In this communication, two novel wide band gap (WBG) polymers of PBTIBDTT and PBTIBDTT-S were designed and synthesized. PBTIBDTT devices exhibit a high PCE of 9.42%, which is one of the highest values reported for OSCs fabricated from WBG materials and the PCEs could remain above 8.6% as the active layer thickness further increased to 280 nm. More importantly, the device displays remarkable thermal stability for 120 hours at 100 °C. To the best of our knowledge, this is the first time to report a WBG material that features excellent comprehensive performance, which would open a new avenue to design meaningful conjugated copolymers towards future practical applications.

High‐Performance Additive‐/Post‐Treatment‐Free Nonfullerene Polymer Solar Cells via Tuning Molecular Weight of Conjugated Polymers

In recent years, rapid advances are achieved in polymer solar cells (PSCs) using nonfullerene small molecular acceptors. However, no research disclosing the influence of molecular weight (Mn ) of conjugated polymer on the nonfullerene device performance is reported. In this work, a series of polymers with different Mn s are synthesized to systematically investigate the connection between Mn and performance of nonfullerene devices for the first time. It is found that the device performance improves substantially as the Mn increases from 12 to 38 kDa and a power conversion efficiency (PCE) as high as 10.5% is realized. It has to be noted this PCE is achieved without using any additives and post-treatments, which is among the top efficiencies of additive- and post-treatment-free PSCs. Most importantly, the variation trend of the optimal active layer thickness and morphology is significantly different from the device with fullerene as acceptor. The findings clarify the effect of Mn on the performance of nonfullerene PSCs, which would benefit further efficiency improvement of nonfullerene PSCs.

New PDI-based small-molecule cathode interlayer material with strong electron extracting ability for polymer solar cells

A water-soluble small-molecule perylene diimide derivative functionalized with a propylenetrimethylammonium end group, namely N,N-bis(propylenetrimethyl ammonium)-3,4,9,10-perylenediimide (PDI-N3I), was synthesized and successfully applied to conventional polymer solar cells as the cathode interlayer firstly. The small-molecule PDI-N3I showed excellent solubility in water and a desirable LUMO energy level of −4.09 eV which matches well with that of PC71BM. With incorporation of the PDI-N3I cathode interlayer, a PDI-N3I/Al device exhibited a higher PCE of 7.00% than that (6.54%) of a LiF/Al device resulting from the improved short current density from 10.41 mA cm−2 to 11.44 mA cm−2, which benefitted from the more efficient electron extracting ability of PDI-N3I compared with LiF proved by photoluminescence quenching experiment. The high vertical conductivity of PDI-N3I endows it with the outstanding property of thickness insensitivity, which is compatible with roll-to-roll production. Additionally, device performance was further improved by the use of a PDI-N3I/LiF bilayered cathode interlayer and the highest PCE of 7.18% was achieved. This work provides a novel solution-processed cathode interlayer material which could effectively improve the efficiency of polymer solar cells.