Xiwen Chen

Organic Solar Cells Using a High‐Molecular‐Weight Benzodithiophene–Benzothiadiazole Copolymer with an Efficiency of 9.4%

A high molecular weight donor-acceptor conjugated polymer is synthesized using the Suzuki polycondensation method. Using this polymer, a single-junction bulk-heterojunction solar cell is fabricated giving a power conversion efficiency of 9.4% using a fullerene-modified ZnO interlayer at the cathode contact.

A Hyperbranched Conjugated Polymer as the Cathode Interlayer for High‐Performance Polymer Solar Cells

An alcohol-soluble hyperbranched conjugated polymer HBPFN with a dimethylamino moiety is synthesized and used as a cathode interlayer. A PCE of 7.7% is obtained for PBDTTT-C-T/PC71 BM based solar cells. No obvious interfacial dipole is found at the interface between the active layer and HBPFN however, an interfacial dipole with the cathode could be one of the reasons for the enhanced performance.

Efficiency enhancement for bulk heterojunction photovoltaic cells via incorporation of alcohol soluble conjugated polymer interlayer

Three dimensional conjugated polymers with pendant ionic ammoniums or polar amines and their linear analogues as cathode interfacial layers were used for organic photovoltaic cells based on blends of poly [(9,9-di-n-octyl-2,7-fluorene)-alt-(5,5-(4′,7′-di-2-thienyl)-2′,1′,3′-benzothiadiazole)] (PFOTBT) or poly(3-hexylthiophene) as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor. The alcohol soluble conjugated polymer interlayers can improve the device performance significantly by simultaneous enhancements of the open-circuit voltage, short-circuit current density, and fill factor. An increase of the power conversion efficiency from 2.62% to 4.67% by 78% was observed with poly[(2,7,2′,7′-spirobifluorene-co-(9,9-bis(6′-((N,N,N-trimethyl) ammonium) hexyl)-2,7-fluorene) dibromide)] based on PFOTBT-PC61BM blend.

Self n-doped [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium) ethyl ester diiodides as a cathode interlayer for inverted polymer solar cells

A series of self n-doped fullerene ammonium derivatives have been synthesized and confirmed with electron paramagnetic resonance and conductivity measurements. The existence of stable C60O2˙− anion radical in these materials resulted in intrinsically high conductivities between 1.05 × 10−2 and 1.98 × 10−2 S cm−1. Among fullerenes with different numbers of ammonium and counter anions, [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium)-ethyl ester diiodides (PCBDANI) showed the best solvent resistance, which was confirmed by the measurement of film thickness and corresponding UV-vis absorption before and after rinsing with dichlorobenzene. Most importantly, the inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO3/Ag retained reasonably high power conversion efficiency even at a thickness of 82 nm of the PCBDANI film as the cathode interlayer. Thus large-area devices via printing this interlayer or printing on this interlayer could become feasible.

[6,6]-Phenyl-C61-butyric Acid 2-((2-(Dimethylamino)ethyl)(methyl)amino)-ethyl Ester as an Acceptor and Cathode Interfacial Material in Polymer Solar Cells

An amine-based, alcohol-soluble fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino)-ethyl ester (PCBDAN) with 4-fold electron mobility of 6,6-phenyl-C61-butyric acid methyl ester (PCBM) is applied successfully as an acceptor and cathode interfacial material in polymer solar cells ITO/P3HT:PCBDAN/MoO3/Ag, where indium tin oxide (ITO) alone is used as the cathode and poly(3-hexylthiophene) (P3HT) is used as a donor. The X-ray photoelectron spectroscopy (XPS) depth profile confirming a favorable vertical phase separation is formed where P3HT is rich at the air/active blend interface and PCBDAN is rich at the buried interface with ITO and, thus, reduces the work function of ITO for use as the cathode. A moderate power conversion efficiency (PCE) of 3.1% is achieved. The slightly low PCE could be due to unoptimized morphology and low structure ordering of P3HT in the blends. However, this result demonstrates that the amine-based fullerene could be used as the acceptor and cathode interfacial material, which eliminated the multilayer device fabrication process. Because PCBDAN has high electron mobility, it would have potential applications in nano-structured organic solar cells. In the near future, alcohol-processable, high-efficient organic/polymer solar cells can be anticipated.

A hybrid organic-inorganic three-dimensional cathode interfacial material for organic solar cells

An alcohol soluble hybrid organic–inorganic three-dimensional material 1,3,5,7,9,11,13,15-(9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-octavinylpentacyclo-octasiloxane (POSS-FN) has been synthesized and assessed as a cathode interlayer within organic solar cells consisting of a PBDT-BT:PC61BM bulk heterojunction. For comparison, we also studied another two linear interfacial materials: a typical conjugated polymer poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) and an insulating polymer poly(4-N,N-dimethylamino-styene) (PStN) in the same system. The hybrid interlayer caused a significant improvement to the device power conversion efficiency by 32%, comparable to the other two interlayers. We found that there are two kinds of interfacial dipoles formation: one weak but unfavourable between the interlayer and the active layer, and the other larger, favourable and significant between the interlayer and the cathode. This latter factor maximized the built-in electric field across the interlayer-modified devices, which provides one of the major reasons for the improved performance. The thermodynamics study revealed that the driving force for the dipole formation could be ascribed to the amino groups.