Dan Chi

Perylene diimides: a thickness-insensitive cathode interlayer for high performance polymer solar cells

With the power conversion efficiency of polymer solar cells (PSCs) approaching the milestone value of 10%, their instability associated with a low work function metal cathode, particularly in the presence of oxygen and moisture, becomes a critical issue for real applications. To alleviate the air-sensitive problem, two easy-accessible solution-processed, environmentally friendly organic small-molecule cathode interlayers, with perylene diimides (PDI) as the core and amino (PDIN) or amino N-oxide (PDINO) as the terminal substituent, are explored. Benefitting from the extended planar structure of the PDI units, the two interlayer materials show high conductivities of ∼10−5 S cm−1, which make them capable of functioning efficiently in a wide thickness range of 6 to 25 nm. This is the first time that thickness-insensitive small-molecule-based cathode interlayers are reported. It is also found that the work function tuning effect of the two PDI-based interlayers allows high work function metals (such as Au and Ag) to act as the cathode. With the conventional device structure with PTB7 as a donor and PC70BM as an acceptor, the PDINO-based devices exhibit an efficiency of 8.24% with Al as the top electrode and 8.16% with Ag as the top electrode, much higher than that of the corresponding Ca/Al-based device (6.98%). The high efficiency of 8.35% is also achieved in the device with PTB7-Th as the donor. The success of the two PDI-interlayers indicates that π-delocalized planar structures with high electron affinities could be particularly useful in developing high-performance organic interlayer materials.

High efficiency P3HT:PCBM solar cells with an inserted PCBM layer

By inserting a PCBM (phenyl-C61-butyric acid methyl ester) layer between a P3HT (poly(3-hexylthiophene)):PCBM blend and a Ca/Al cathode, the performance of P3HT:PCBM bulk-heterojunction polymer solar cells is greatly improved. The maximum power conversion efficiency reached 4.24%, which is much higher than that of the traditional standard P3HT:PCBM based device (3.57%). By exploring various experimental techniques including absorption spectroscopy, X-ray photoelectron spectroscopy and impedance spectroscopy, we have found that the enhancement of the device performance can mainly be attributed to two reasons: (1) the inserted-PCBM layer can enhance the overall light absorption of the whole device and hence improve the photocurrent. (2) The inserted-PCBM layer can increase the amount of PCBM at the interface between the active layer and the cathode electrode, and hence suppress carrier recombination and facilitate electron extraction.

High-performance hybrid organic-inorganic solar cell based on planar n-type silicon

Hybrid organic-inorganic solar cells were fabricated by spin coating the hole transporting conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film on n-type crystalline silicon (n-Si). By incorporating different additives into the PEDOT:PSS, the conductivity and wettability of PEDOT:PSS film are markedly improved, and the device performance is greatly enhanced accordingly. To further optimize the device performance, poly(3-hexylthiophene) (P3HT) layer was inserted between the n-Si and PEDOT:PSS layer. The P3HT layer blocks electrons from diffusing to the PEDOT:PSS, and hence reduces recombination at the anode side. The device eventually exhibits a high power conversion efficiency of 11.52%.

Efficient hybrid plasmonic polymer solar cells with Ag nanoparticle decorated TiO2 nanorods embedded in the active layer

A hybrid plasmonic polymer solar cell, in which plasmonic metallic nanostructures (such as Ag, Au, and Pt nanoparticles) are embedded in the active layer, has been under intense scrutiny recently because it provides a promising new approach to enhance the efficiency of the device. We propose a brand new hybrid plasmonic nanostructure, which combines a plasmonic metallic nanostructure and one-dimensional semiconductor nanocrystals, to enhance the photocurrent of the device through a strong localized electric field and an enhanced charge transport channel. We demonstrate that when Ag nanoparticle decorated TiO2 nanorods were introduced into the active layer of polymer-fullerene based bulk heterojunction solar cells, the photocurrent significantly increased to 14.15 mA cm(-2) from 6.51 mA cm(-2) without a decrease in the open voltage; thus, the energy conversion efficiency was dramatically enhanced to 4.87% from 2.57%.

Synthesis of novel acceptor molecules of mono- and multiadduct fullerene derivatives for improving photovoltaic performance.

We have successfully synthesized and separated a series of tert-butyl 4-C(61)-benzoate (t-BCB) organofullerenes, including monoadduct, diadduct, and triadduct compounds, and investigated their photophysics, electrochemistry, thermal properties, and high-performance liquid chromatography analysis. The photovoltaic devices were fabricated based on monoadduct, diadduct, and triadduct products, and the devices based on them exhibited power conversion efficiencies of 2.43%, 0.48%, and 1.68%, respectively. This was the first time to study the dependent relationship on the device performance and the different isomer numbers.

Fully understanding the positive roles of plasmonic nanoparticles in ameliorating the efficiency of organic solar cells

Herein, we constructed inverted PBDTTT-CF:PC70BM bulk-heterojunction organic solar cells by introducing Au nanoparticles to a ZnO buffer layer and a great improvement in energy conversion efficiency has been realized. To discover the positive roles of such plasmonic nanoparticles in the process of solar energy conversion, photovoltaic devices with the same architecture but different sized Au nanoparticles were purposely fabricated and it has been observed that the overall efficiency can be remarkably improved from 6.67% to 7.86% by embedding 41 nm Au nanoparticles in the buffer layer. The devices with other sizes of Au nanoparticles show a relatively low performance. Subsequent investigations including finite difference time domain simulation and transient photoluminescence studies reveal that the existence of the plasmonic particles could not only improve the optical absorption and facilitate the exciton separation, but can also benefit the collection of charge carriers. Thus, this paper provides a comprehensive perspective on the roles of plasmonic particles in organic solar cells and insights into the photo energy conversion process in the plasmonic surroundings.

Hybrid silicon nanocone–polymer solar cells based on a transparent top electrode

Instead of using a metal grid line as the top electrode for Si nanocone/PEDOT:PSS hybrid solar cells, herein we utilize a fully covered thin layer of transparent MoO3/Ag/ZnS to efficiently collect photo-generated charge carriers without sacrificing photon absorption. Vertical silicon nanocone (SiNC) arrays with short cone lengths are prepared by a metal-assisted chemical etching technique. Such a unique structure favors the subsequent infiltration of PEDOT:PSS into the nanocone array. In addition to decent antireflection and light trapping properties, which can be attributed to the nanostructure array, the power conversion efficiency (PCE) of the hybrid solar cells reaches 5.12% upon optimizing the length of the nanocone and the thickness of the PEDOT:PSS layer. This paper not only provides an elaborate investigation on Si-based hybrid solar cells, but also gives a universal methodology for the preparation of transparent electrodes for other categories of photoelectric devices.

A Facile Way for Synthesis of High Performance Electron Receptor MCB: A Promising Replacer of PCBM

Methyl 4-C61-benzoate (MCB) series fullerene materials with low cost and high yield, including monoadduct (MCBM) and bisadduct (MCBB) compounds, were synthesized and their photophysical and electrochemical properties were investigated. Fabricated photovoltaic devices based on both two materials showed power conversion efficiency of 3.48% and 0.16%, respectively. The MCBM exhibited higher PCE relative to PCBMs 3.40%. The LUMO energy level of MCBM was 0.03 eV lower than that of PCBM, and it was facile to be synthesized by two steps with high yield of 55% from low-cost industrial commercials, whose molecular weight was 868.0 g/mol. This work supplied new route to design fullerene materials as PCBMs alternative.

Ultra-thin ZnO film as an electron transport layer for realizing the high efficiency of organic solar cells

To overcome the limits of low charge transport efficiency and high absorption in the UV region of conventional thick ZnO layers in organic solar cells, herein we introduce an ultra-thin ZnO film (4 nm) into PBDTTT-CF:PC70BM bulk heterojunction organic solar cells, as the electron transport layer, and realize a power conversion efficiency of 7.51%, which is dramatically higher than that of a device using general ZnO film (28.1 nm). Various techniques from both steady-state and ultra-fast views reveal that the devices with an ultra-thin ZnO film (less than 10 nm) show a higher built-in potential compared to the device with a 28.1 nm ZnO film. Such an enhancement of the built-in potential could facilitate the photo-generated excitons dissociating into free charge carriers and benefit the transport of charge carriers to the electrode. Thus, we have supplied an efficient electron conducting layer not only for the photovoltaic community but also for other photoelectronic devices.