Zhicai He

Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells

Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in highly efficient polymer solar cells by incorporating an alcohol/water-soluble conjugated polymer as cathode interlayer is domonstrated. When combined with a low-bandgap polymer PTB7 as the electron donor material, the power efficiency of the devices is improved to a certified 8.370%. Due to the drastic improvement in efficiency and easy utilization, this method opens new opportunities for PSCs from various material systems to improve towards 10% efficiency.

Largely Enhanced Efficiency with a PFN/Al Bilayer Cathode in High Efficiency Bulk Heterojunction Photovoltaic Cells with a Low Bandgap Polycarbazole Donor

Polymeric photovoltaic cells (PVCs) have attracted considerable attention over the past several years because of their unique advantages of low cost, light weight, and great potential for the realization of fl exible and large-area devices. [ 1 , 2 ] Typically, bulk heterojunction (BHJ) PVCs, a promising device confi guration for high power conversion effi ciency (PCE), involve the use of a phase-separated blend of an electron-donating conjugated poly mer and an electron-accepting fullerene derivative as the active layer. [ 3 − 10 ] Judicious design of a polymer donor and selection of a fullerene acceptor have realized high effi ciency BHJ PVCs with PCE values over 5%. [ 11 − 16 ] Tremendous efforts have also been made to optimize the active layer formation and the device confi guration. Many optimization methods, such as using different solvents to fabricate the active layer, [ 4 ] thermal annealing of the active layer or the device, [ 17 ] fi lm forming speed, [ 18 ] the addition of additives to the active layer, [ 19 ] the use of an optical spacer, [ 15 ]

Recent Advances in Polymer Solar Cells: Realization of High Device Performance by Incorporating Water/Alcohol‐Soluble Conjugated Polymers as Electrode Buffer Layer

This Progress Report highlights recent advances in polymer solar cells with special attention focused on the recent rapid-growing progress in methods that use a thin layer of alcohol/water-soluble conjugated polymers as key component to obtain optimized device performance, but also discusses novel materials and device architectures made by major prestigious institutions in this field. We anticipate that due to drastic improvements in efficiency and easy utilization, this method opens up new opportunities for PSCs from various material systems to improve towards 10% efficiency, and many novel device structures will emerge as suitable architectures for developing the ideal roll-to-roll type processing of polymer-based solar cells.

High-efficiency ITO-free polymer solar cells using highly conductive PEDOT:PSS/surfactant bilayer transparent anodes

By spin-coating a surfactant layer, glycerol monostearate (GMS) atop poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film, a PEDOT:PSS/surfactant bilayer film was prepared facilely for the first time and applied as the transparent anode for high-efficiency ITO-free bulk heterojunction polymer solar cell (BHJ-PSC) devices. A significant improvement of the conductivity of PEDOT:PSS films (from ∼1 S cm−1 to more than 1000 S cm−1) was achieved by GMS modification and the highest conductivity reaches 1019 S cm−1 for Clevios PH 1000 under an optimized spin-coating speed of GMS layer. The Clevios PH 1000/GMS bilayer film exhibits a sheet resistance of 98 Ω sq−1 and a transparency of around 80% in the visible range, which are comparable to those of ITO, fulfilling its function as the transparent anode. The conductivity improvement by GMS modification is proposed to result from the GMS-induced segregation of PSS chains and the conformational change of the conductive PEDOT chains within PEDOT:PSS. While the highly hydrophobic –(CH2)16CH3 groups of GMS preferentially interact with the hydrophobic PEDOT of PEDOT:PSS, the highly hydrophilic –COOCH2–CHOH–CH2OH groups preferentially interact with the hydrophilic PSS chains with the hydroxyl groups playing an important role on the consequent phase separation between PEDOT and PSS chains. Using Clevios PH 1000/GMS bilayer films as the transparent anodes replacing ITO, high-efficiency ITO-free BHJ-PSC devices based on poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) blended with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) and thieno[3,4-b]-thiophene/benzodithiophene (PTB7):PC71BM systems exhibit power conversion efficiencies (PCE) of 5.90% and 7.06%, respectively, which are comparable to the corresponding devices based on the traditional ITO anode.

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.

Flexible polymer solar cells with power conversion efficiency of 8.7

Here we demonstrate flexible polymer solar cells with a record high power conversion efficiency of 8.7% and a very high specific power of 400 W kg−1, by depositing a physical blend of a conjugated semiconducting polymer and a fullerene derivative on a highly flexible polyethylene terephthalate (PET) substrate. The flexible device reported here performs basically as well as those based on rigid glass/ITO substrates with regard to most of the device parameters, however has a relatively lower fill factor, which can be further improved via interface engineering between the photoactive layer and electrodes or by the development of novel flexible substrates. The processing method for the flexible polymer solar cells is solution-based and all of the film deposition processes are completed at room temperature, thus this simplicity of preparing flexible polymer solar cells can offer easy-processability over a large area with fast deposition on an industrial scale at room temperature.

Recent Advances in Organic Photovoltaics: Device Structure and Optical Engineering Optimization on the Nanoscale.

Organic photovoltaic (OPV) devices, which can directly convert absorbed sunlight to electricity, are stacked thin films of tens to hundreds of nanometers. They have emerged as a promising candidate for affordable, clean, and renewable energy. In the past few years, a rapid increase has been seen in the power conversion efficiency of OPV devices toward 10% and above, through comprehensive optimizations via novel photoactive donor and acceptor materials, control of thin-film morphology on the nanoscale, device structure developments, and interfacial and optical engineering. The intrinsic problems of short exciton diffusion length and low carrier mobility in organic semiconductors creates a challenge for OPV designs for achieving optically thick and electrically thin device structures to achieve sufficient light absorption and efficient electron/hole extraction. Recent advances in the field of OPV devices are reviewed, with a focus on the progress in device architecture and optical engineering approaches that lead to improved electrical and optical characteristics in OPV devices. Successful strategies are highlighted for light wave distribution, modulation, and absorption promotion inside the active layer of OPV devices by incorporating periodic nanopatterns/nanostructures or incorporating metallic nanomaterials and nanostructures.

Platinum–Acetylide Polymers with Higher Dimensionality for Organic Solar Cells

A new series of platinum(II)-acetylide polymers P1-P3 containing thiophene-triarylamine chromophores of different dimensions were synthesized and their electronic band structures, field-effect charge transport, and application in bulk heterojunction solar cells were evaluated. These materials are soluble in polar organic solvents and show strong absorptions in the solar spectra (with the highest absorption coefficient of 1.59×10(5)  cm(-1) from thin films), thus rendering them excellent candidates for bulk heterojunction polymer solar cells. The spin-coated polymer thin films showed p-channel field-effect charge transport with hole mobilities of 1.90×10(-5) to 7.86×10(-5)  cm(2)  V(-1)  s(-1) for P1-P3 and an improved charge carrier transport was found for P2 with higher molecular dimensionality than P1. The dependence of their photovoltaic properties and dimensionality was also investigated. Even if the polymers possess relatively high bandgaps and narrow absorption bandwidths, the highest power conversion efficiency of 2.24 % can be obtained based on blends of P3 with [6,6]phenyl-C(61)-butyric acid methyl ester (PCBM) (1:5, w/w) under AM1.5 simulated solar illumination. The present work indicates that multidimensional polymers exhibit a better photovoltaic performance over the linear polymers under the same measurement conditions and can provide an attractive approach to developing highly efficient conjugated metallopolymers for efficient power generation.

Efficiency enhancement in solution-processed organic small molecule: Fullerene solar cells via solvent vapor annealing

We report highly efficient small molecule solar cells (SMSCs) by using dichloromethane solvent vapor annealing method. The resulted devices delivered a power conversion efficiency (PCE) of 8.3%, which is among the highest in SMSCs. Comparing to the control devices, the short circuit current (Jsc), fill factor, and PCE of solvent vapor annealed devices are significantly improved. Summarizing the results of optical absorption, film morphology, and charge carrier transporting properties, we see that the enhanced structure order and reduced size of phase separation are major reasons for the improved device performances, establishing a solid structure-property relationship. The solvent vapor annealing method can thus be a useful method in device fabrication to enhance performances of SMSCs.

Solution-Processed Bulk-Heterojunction Photovoltaic Cells Based on Dendritic and Star-Shaped D-π-A Organic Dyes

A series of D-pi-A organic dendritic and star-shaped molecules based on three various chromophores (i.e., the truxene nodes, triphenylamine moieties as the donor, and benzothiadiazole chromophore as the acceptor) and their corresponding model compounds are facilely developed. Their photophysical and electrochemical properties are investigated in detail by UV/Vis absorption and photoluminescent spectroscopy, and cyclic voltammetry. By changing the various conjugated spacers (i.e., single bond, double bond, and triple bond) among the three chromophores of dendritic series, their photophysical properties (that is, the one-photon absorption range and two-photon absorption cross-section values) are effectively modulated. All D-pi-A conjugated oligomers show a broad and strong absorption band from 250 to 700 nm in thin films. Solution-processed bulk-heterojunction photovoltaic devices using our oligomer as donor and PCBM as acceptor are fabricated and measured. The power conversion efficiency of the devices based on our oligomers continuously increases from DBTTr to TrTD2A as a result of an increasing relative absorption intensity in longer wavelength region by changing the donor-acceptor ratio and conjugated spacers between the donor and acceptor. The power conversion efficiency of the devices based on TrTD2A was 0.54% under the illumination of AM 1.5 and 100 mW cm(-2), which is the highest value recorded based on D-pi-A conjugated oligomers containing triphenylamine moieties and benzothiadiazole chromophores with truxene to date.