Chaohua Cui

Single‐Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency

A single-junction polymer solar cell with an efficiency of 10.1% is demonstrated by using deterministic aperiodic nanostructures for broadband light harvesting with optimum charge extraction. The performance enhancement is ascribed to the self-enhanced absorption due to collective effects, including pattern-induced anti-reflection and light scattering, as well as surface plasmonic resonance, together with a minimized recombination probability.

High efficiency polymer solar cells based on poly(3-hexylthiophene)/indene-C70 bisadduct with solvent additive

The photovoltaic performance of the polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) as donor and indene-C70 bisadduct (IC70BA) as acceptor was optimized by using 3 vol% high boiling point solvent additive of 1-chloronaphthalene (CN), N-methyl pyrrolidone (NMP), 1,8-octanedithiol (OT) or 1,8-diiodooctane (DIO) without solvent annealing. The optimized PSC based on P3HT : IC70BA (1 : 1, w/w) with 3 vol% CN and pre-thermal annealing at 150 °C for 10 min, exhibits a high power conversion efficiency (PCE) of 7.40% with Voc of 0.87 V, Jsc of 11.35 mA cm−2 and FF of 75.0%, under the illumination of AM1.5G, 100 mW cm−2. The PCE of 7.40%, the Voc of 0.87 V, and the FF of 75.0% are all the highest values reported in the literature so far for P3HT-based PSCs. The high efficiency is due to the optimized P3HT/IC70BA interpenetrating network and stronger absorption of the active layer by using the additive treatment. Taking into account the advantages of thickness-insensitivity and good reproducibility of the photovoltaic performance of the P3HT-based PSCs as well as the simple device fabrication processes without the need of solvent annealing, the high-efficiency PSCs based on P3HT : IC70BA using CN additive are very promising for future commercialization of PSC devices.

Enhanced performance and stability of a polymer solar cell by incorporation of vertically aligned, cross-linked fullerene nanorods.

Research on polymer solar cells (PSC) using organic p-type (donor) and n-type (acceptor) semiconductors has attracted tremendous scientific and industrial interest in recent years. The charge generation and charge transport play equally important roles in determining the device efficiency. To dramatically increase the area of the donor–acceptor interface for efficient charge separation, a bulk heterojunction (BHJ) is adopted to form an interpenetrating network of donor and acceptor materials. This configuration decreases the distance that excitons need to travel to reach the heterojunction interface, thus reducing exciton recombination. However, the donor and acceptor are randomly interspersed; pathways for charges to reach the electrodes through the active layer are disordered. Free charges are likely to encounter an opposite charge, resulting in charge recombination and reduced current. 4] Moreover, space charge may be built up if charges are locally trapped on isolated domains. Furthermore, an increase in the thickness of the BHJ layer to enhance absorption is usually accompanied by deteriorated charge collection. Consequently, controlling phase separation toward optimal morphology in BHJ by external treatments, such as thermal or solvent annealing, is an important but challenging task. To provide a direct path for charge transport while maintaining a large interfacial area, the ideal architecture of the donor and acceptor is the periodic, vertically aligned, and interpenetrating ordered bulk heterojunction (OBHJ). The electrons and holes have straight and independent pathways to the electrodes to shorten the carrier transport length and reduce the probability of charge recombination. Several elegant studies have attempted to demonstrate this conceptual architecture, for example by a template-assisted strategy or self-assembly of block copolymer. However, realization of high-performance OBHJ devices has not been successful. We envision that designing a system that combines a BHJ for efficient charge generation with an OBHJ for efficient charge transport and collection would be a more practical strategy. Such a configuration is specifically suitable for solar cells with inverted architecture, because an electron-selective layer is required at the bottom of the active layer for electron extraction and hole blocking. For instance, the upper BHJ active layer of an inverted solar cell is infiltrated into vertically aligned nanorods extending from a bottom layer of an inorganic semiconductor (e.g. ZnO or TiO2). [18–20] However, owing to the poor electrical coherence at the organic/inorganic interface, the improvement in efficiency is moderate (PCE ranges from 2.1 to 2.7%). Recently, we reported a cross-linkable fullerene material, [6,6]-phenyl-C61-butyric styryl dendron ester (PCBSD). The formation of a cross-linked PCBSD (C-PCBSD, Figure 1a) planar layer allows realization of a multilayer inverted device by all-solution processing. By using indene–C60 bisadduct (ICBA, Figure 1a) with a higher-lying lowest unoccupied molecular orbital (LUMO) energy level to serve as the acceptor in the blend, an inverted solar cell device based on the ITO/ZnO/C-PCBSD/ICBA:P3HT/PEDOT:PSS/Ag configuration achieved an enhanced power conversion efficiency

High-performance polymer solar cells based on a 2D-conjugated polymer with an alkylthio side-chain

Two new two-dimension (2D)-conjugated copolymers (PBDTT-S-TT-CF and PBDTT-O-TT-CF) were designed and synthesized for the application as donor materials in polymer solar cells (PSCs) and for further investigation of the effect of alkylthio side chains on the photovoltaic performance of 2D-conjugated polymers. The two copolymers were prepared by the copolymerization of alkylthio- or alkoxy-thienyl-benzodithiophene (BDTT-S or BDTT-O) and thienothiophene with carbonyl and fluorine substituents (TT-CF), and they demonstrated strong and broad absorption spectra in the wavelength region from 450 nm to ca. 800 nm. The HOMO energy level of PBDTT-S-TT-CF was further down-shifted to −5.44 eV by alkylthio substitution on thiophene conjugated side chain of BDT unit and the carbonyl and fluorine substitution on TT unit. The inverted-structured PSCs based on PBDTT-S-TT-CF:PC70BM exhibited a high PCE of 9.58% with a remarkably high Voc of 0.89 V and a high FF of 71.0%. The PCE of the PSCs based on PBDTT-O-TT-CF also reached a high value of 8.68% with a Voc = 0.78 V and a higher Jsc = 16.5 mA cm−2, which is benefited from the broad absorption of PBDTT-O-TT-CF. The results further confirm the unique advantages of the alkylthio side chain in the design of state-of-the-art polymer donor materials for high performance PSCs with high Voc.

Efficient Polymer Solar Cells Based on Poly(3-hexylthiophene) and Indene–C60 Bisadduct Fabricated with Non-halogenated Solvents

The photovoltaic performance of poly(3-hexylthiophene) (P3HT) has been improved greatly by using indene-C60 bisadduct (ICBA) as acceptor instead of phenyl-C61-butyric acid methyl ester (PCBM). However, the solvent of dichlorobenzene (DCB) used in fabricating polymer solar cells (PSCs) limited the application of the PSCs, because of the environmental problem caused by the harmful halogenated solvent. In this work, we fabricated the PSCs based on P3HT/ICBA processed with four low-harmful non-halogenated solvents of toluene, o-xylene, m-xylene, and p-xylene. The PSCs based on P3HT/ICBA (1:1, w/w) with toluene as the solvent exhibit the optimized power conversion efficiency (PCE) of 4.5% with open-circuit voltage (Voc) of 0.84 V, short circuit current density (Jsc) of 7.2 mA/cm(2), and fill factor (FF) of 71%, under the illumination of AM 1.5G at 100 mW/cm(2). Upon using 1% N-methyl pyrrolidone (NMP) as a solvent additive in the toluene solvent, the PCE of the PSCs was greatly improved to 6.6% with a higher Jsc of 10.3 mA/cm(2) and a high FF of 75%, which is even higher than that of the devices fabricated with halogenated DCB solvent. The X-ray diffraction (XRD) measurement shows that the crystallinity of P3HT increased with the NMP additive. The investigations on morphology of the active layers by atomic force microscopy (AFM) and transmission electron microscopy (TEM) indicate that the NMP additive promotes effective phase separation and formation of nanoscaled interpenetrating network structure of the active layer, which is beneficial to the improvement of Jsc and PCE for the PSCs fabricated with toluene as the solvent.

Performance improvement of polymer solar cells by using a solvent-treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) buffer layer

Photovoltaic performance of the polymer solar cell (PSC) based on poly(3-hexylthiophene) (P3HT) as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor was improved by using the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) modification layer treated by ethanol or 2-propanol. Power conversion efficiency (PCE) of the PSC based on P3HT:PCBM (1:1, w/w) with the 2-propanol-treated PEDOT:PSS modification layer reached 4.74%, in comparison with a PCE of 3.39% for the PSC with the PEDOT:PSS layer without the organic solvent treatment. The enhanced performance of the PSCs is attributed to higher conductivity and optimized surface morphology of the PEDOT:PSS layers treated by the organic solvent.

Effects of Alkylthio and Alkoxy Side Chains in Polymer Donor Materials for Organic Solar Cells

Side chains play a considerable role not only in improving the solubility of polymers for solution-processed device fabrication, but also in affecting the molecular packing, electron affinity and thus the device performance. In particular, electron-donating side chains show unique properties when employed to tune the electronic character of conjugated polymers in many cases. Therefore, rational electron-donating side chain engineering can improve the photovoltaic properties of the resulting polymer donors to some extent. Here, a survey of some representative examples which use electron-donating alkylthio and alkoxy side chains in conjugated organic polymers for polymer solar cell applications will be presented. It is envisioned that an analysis of the effect of such electron-donating side chains in polymer donors would contribute to a better understanding of this kind of side chain behavior in solution-processed conjugated organic polymers for polymer solar cells.

Transfer-Printed PEDOT:PSS Electrodes Using Mild Acids for High Conductivity and Improved Stability with Application to Flexible Organic Solar Cells.

UNLABELLED Highly conductive, flexible, and transparent electrodes (FTEs) of PEDOT PSS films on plastic substrates have been achieved using strong acid treatments. However, it is rare to realize a performance attenuation of PEDOT PSS FTEs on plastic substrates and flexible optoelectronic devices because of strong acid residues in the PEDOT PSS matrix. Herein, we develop a feasible transfer-printing technique using mild acids. Because of a mild and weak property of these acids and less acid residues in PEDOT PSS matrix, the transferred PEDOT PSS FTEs exhibited a significant enhancement in stability, conductivity (3500 S cm(-1)), transparency, and mechanical flexibility on plastic substrates. Flexible organic solar cells with the FTEs also showed a remarkable enhancement in power conversion efficiency and stability in the ambient atmosphere. It is expected that the novel transfer-printing technique for making PEDOT PSS FTEs is also useful in many other types of flexible optoelectronic devices.

Unsymmetric platinum(II) bis(aryleneethynylene) complexes as photosensitizers for dye-sensitized solar cells.

Four new unsymmetric platinum(II) bis(aryleneethynylene) derivatives have been designed and synthesized, which showed good light-harvesting capabilities for application as photosensitizers in dye-sensitized solar cells (DSSCs). The absorption, electrochemical, time-dependent density functional theory (TD-DFT), impedance spectroscopic, and photovoltaic properties of these platinum(II)-based sensitizers have been fully characterized. The optical and TD-DFT studies show that the incorporation of a strongly electron-donating group significantly enhances the absorption abilities of the complexes. The maximum absorption wavelength of these four organometallic dyes can be tuned by various structural modifications of the triphenylamine and/or thiophene electron donor, improving the light absorption range up to 650 nm. The photovoltaic performance of these dyes as photosensitizers in mesoporous TiO(2) solar cells was investigated, and a power conversion efficiency as high as 1.57% was achieved, with an open-circuit voltage of 0.59 V, short-circuit current density of 3.63 mA cm(-2), and fill factor of 0.73 under simulated AM 1.5G solar illumination.

Synthesis and photovoltaic properties of D–A copolymers of benzodithiophene and naphtho[2,3-c]thiophene-4,9-dione

Two donor–acceptor (D–A) alternative copolymers of benzodithiophene (BDT) donor unit and an acceptor unit of naphtho[2,3-c]thiophene-4,9-dione (NTDO) with different alkyl side chains, PBDTNTDO-1 and PBDTNTDO-2, were synthesized for application as donor materials in polymer solar cells (PSCs). The copolymers show good solubility in common organic solvents, broad visible absorption from 350 nm to 670 nm, and relatively lower HOMO energy levels at −5.14 eV for PBDTNTDO-1 and −5.19 eV for PBDTNTDO-2. The PSCs based on PBDTNTDO-2 as donor and PC70BM as acceptor demonstrated power conversion efficiency of 1.52% with an open circuit voltage of 0.88 V and a short circuit current of 5.67 mA cm−2, under the illumination of AM1.5, 100 mW cm−2.