Steven K. Hau

Air-stable inverted flexible polymer solar cells using zinc oxide nanoparticles as an electron selective layer

The performance and stability of unencapsulated inverted bulk-heterojunction solar cells with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO nanoparticles on indium tin oxide plastic substrates showed high power conversion efficiency of ∼3.3%. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after 40days while the conventional one showed negligible photovoltaic activity after 4days. This is due to the improved stability at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Ag interface.

Development of New Conjugated Polymers with Donor−π-Bridge−Acceptor Side Chains for High Performance Solar Cells

Two new conjugated polymers have been designed and synthesized for polymer solar cells. Both of them exhibit excellent photovoltaic properties with a power conversion efficiency as high as 4.74%. Different from the traditional linear donor-acceptor (D-A) type conjugated polymers, these newly designed polymers have a two-dimensional conjugated structure with their tunable acceptors located at the end of D-A side chains and connected with the donors on the main chain through an efficient pi-bridge. This approach provides great flexibility in fine-tuning the absorption spectra and energy levels of the resultant polymers for achieving high device performance.

High performance ambient processed inverted polymer solar cells through interfacial modification with a fullerene self-assembled monolayer

The performance of inverted bulk-heterojunction solar cells with zinc oxide nanoparticles as the electron selective contact is compared to those modified with a fullerene self-assembled monolayer (C60-SAM). The devices modified with a C60-SAM show very significant improvement in conversion efficiencies compared to unmodified devices leading to efficiencies as high as 4.9%. This is due to enhanced electronic coupling of the inorganic/organic interface from the C60-SAM leading to improved fill factor and photocurrent. Furthermore, devices fabricated in an inert environment were compared to those fabricated in ambient showing comparable device performance.

Interfacial modification to improve inverted polymer solar cells

We report improved device performance of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl C61butyric acid methyl ester (PCBM)-based inverted bulk-heterojunction (BHJ) solar cells through the modified interface of the TiO2/BHJ with a series of carboxylic acid functionalized self-assembled monolayers (SAMs). The SAMs reduce the series resistance and improve the shunt resistance of the cell leading to increased fill factor and photocurrent density. Different aspects of device improvement can be affected depending on the nature of the SAMs. Modification with a C60-SAM shows the largest enhancement leading to a 35% improvement (η = 3.78%) over unmodified inverted devices (η = 2.80%). This SAM serves multiple functions to affect the photoinduced charge transfer at the interface to reduce the recombination of charges, passivation of inorganic surface trap states, improve the exciton dissociation efficiency at the polymer/TiO2 interface as well as a template to influence the overlayer BHJ distribution of phases, morphology and crystallinity leading to better charge selectivity and improved solar cell performance.

A Review on the Development of the Inverted Polymer Solar Cell Architecture

The increase in energy production costs for fossil fuels has led to a search for an economically viable alternative energy source. One alternative energy source of particular interest is solar energy. A promising alternative to inorganic materials is organic semiconductor polymer solar cells due to their advantages of being cheaper, light weight, flexible and made into large areas by roll-to-roll processing. However, the conventional architecture that is typically used for fabricating solar cells requires high vacuum to deposit the top metal electrode which is not suitable for roll-to-roll processing. Recently an inverted device architecture has been investigated as a suitable architecture for developing the ideal roll-to-roll type processing of polymer-based solar cells. This review will go over the recent advances and approaches in the development of this type of inverted device architecture. We will highlight some of the work that we have done to integrate materials, device, interface, and processing of the inverted device architecture platform to produce more idealized polymer-based solar cells.

Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cells

A simple method was developed using metal grid/conducting polymer hybrid transparent electrode to replace indium tin oxide (ITO) for the fabrication of inverted structure polymer solar cells. The performance of the devices could be tuned easily by varying the width and separation of the metal grids. By combining the appropriate metal grid geometry with a thin conductive polymer layer, substrates with comparable transparency and sheet resistance to those of ITO could be achieved. Polymer solar cells fabricated using this hybrid electrode show efficiencies as high as ∼3.2%. This method provides a feasible way for fabricating low-cost, large-area organic solar cells.

Self-assembled monolayer modified ZnO/metal bilayer cathodes for polymer/fullerene bulk-heterojunction solar cells

A simple method was developed to tune the interface of cathode in polymer solar cells by inserting a layer of ZnO/self-assembled monolayer (SAM) between a poly(3-hexylthiophene): [6,6]-phenyl-C61 butyric acid methyl ester bulk-heterojunction film and a metal cathode. We found that the device performance could be significantly altered depending on the dipole direction and chemical bonding between the SAM and metals. With appropriate choice of SAMs, devices show dramatically improved efficiencies and even high work-function metals such as Ag and Au could be used as cathodes. This finding provides an efficient method for interface engineering in organic-based optoelectronic devices.

Anode modification of inverted polymer solar cells using graphene oxide

A simple method has been developed to modify the anode interface of inverted bulk-heterojunction (BHJ) polymer solar cells by spin-coating a thin layer of graphene oxide (GO) on top of the organic active layer. The device with GO exhibited a remarkable improvement in power conversion efficiency compared to devices without any interfacial layer, indicating that GO can effectively modify the BHJ/metal anode interface to facilitate efficient hole collection. The dependence of the device performance on the GO layer thickness was also investigated showing an optimum performance from a GO thickness of ∼2–3 nm.

Highly efficient electro-optic polymers through improved poling using a thin TiO2-modified transparent electrode

A sol-gel derived thin titanium dioxide (TiO2) layer was spin-coated onto indium-tin-oxide substrate to improve poling efficiency of recently developed electro-optic (E-O) polymers. The thin TiO2 layer significantly blocks excessive charge injection and reduces the leakage current during high field poling. Ultralarge E-O coefficients, up to 160–350 pm/V at 1310 nm, have been achieved. These results show higher poling efficiency (enhancement of 26%–40%) compared to the results of poled films without the TiO2 layer. This enhancement can be explained by field distribution flattening effect at high injection barrier with the insertion of TiO2 barrier layer.

n-Doping of thermally polymerizable fullerenes as an electron transporting layer for inverted polymer solar cells

A novel [6,6]-phenyl-C61-butyric acid methyl styryl ester (PCBM-S) was synthesized and employed as an electron transporting interfacial layer for bulk heterojunction polymer solar cells with an inverted device configuration. After the deposition of PCBM-S film from solution, the styryl groups of PCBM-S were polymerized by post-thermal treatment to form a robust film which is resistive to common organic solvents. This allows the solution processing of upper bulk heterojunction film without eroding the PCBM-S layer. Additionally, the PCBM-S was n-doped with decamethylcobaltocene (DMC) to increase the conductivity of the film, which resulted in a significantly improved power conversion efficiency from 1.24% to 2.33%. The improved device performance is due to the decrease of series resistance and improved electron extraction property of the n-doped PCBM-S film.