Kamil Mielczarek

Nanoimprinted Polymer Solar Cell

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. A vertically bicontinuous and interdigitized heterojunction between donor and acceptor has been regarded as one of the ideal structures to enable both efficient charge separation and transport. Synergistic control of polymer orientation in the nanostructured heterojunction is also critical to improve the performance of polymer solar cells. Nanoimprint lithography has emerged as a new approach to simultaneously control both the heterojunction morphology and polymer chains in organic photovoltaics. Currently, in the area of nanoimprinted polymer solar cells, much progress has been achieved in the fabrication of nanostructured morphology, control of molecular orientation/crystallinity, deposition of acceptor materials, patterned electrodes, understanding of structure-property correlations, and device performance. This review article summarizes the recent studies on nanoimprinted polymer solar cells and discusses the outstanding challenges and opportunities for future work.

Imprinted large-scale high density polymer nanopillars for organic solar cells

Nanoimprint with a large-scale nanoporous Si mold is developed to fabricate high density periodic nanopillars (∼1010∕cm2) in various functional polymers. A anodic alumina membrane is first obtained using electrochemical anodization. The membrane is used as a mask for a two-step plasma etching process to obtain a Si mold of 50–80nm wide and 100–900nm deep pores. The mold is used in nanoimprint lithography to fabricate ordered and high density polymer nanopillars and nanopores in SU-8, hydrogen silsesquixane, polymethylmethacrylate, poly(3-hexylthiophane) (P3HT), and phenyl-C61-butyric acid methyl ester (PCBM). Then, the imprinted P3HT nanopillars were used to make bulk heterojunction solar cells by depositing PCBM on top of the pillars. Imprinting provides a way to precisely control the interdigitized heterojunction morphology, leading to improved solar cell performance.

Monolithic parallel tandem organic photovoltaic cell with transparent carbon nanotube interlayer

We demonstrate an organic photovoltaic cell with a monolithic tandem structure in parallel connection. Transparent multiwalled carbon nanotube sheets are used as an interlayer anode electrode for this parallel tandem. The characteristics of front and back cells are measured independently. The short circuit current density of the parallel tandem cell is larger than the currents of each individual cell. The wavelength dependence of photocurrent for the parallel tandem cell shows the superposition spectrum of the two spectral sensitivities of the front and back cells. The monolithic three-electrode photovoltaic cell indeed operates as a parallel tandem with improved efficiency.

Hole mobility enhancement by chain alignment in nanoimprinted poly(3-hexylthiophene) nanogratings for organic electronics

The authors report that the poly(3-hexylthiophene-2,5-diyl) (P3HT) nanogratings shaped by nanoimprint lithography show enhanced hole mobility and strong anisotropy of conductance due to nanoimprint-induced three-dimensional polymer chain alignment. Field effect transistors were fabricated using these nanogratings and device measurements show a hole mobility of 0.03 cm2/V s along the grating direction, which is about 60 times higher than that of nonoptimized thin film transistors. Organic photovoltaic devices (OPV) were made using the P3HT nanograting with infiltration of [6,6]-phenyl-C61-butyric acid methyl ester. Compared to similar bilayer and bulk heterojunction devices, the nanoimprinted OPV shows improved device performance.The authors report that the poly(3-hexylthiophene-2,5-diyl) (P3HT) nanogratings shaped by nanoimprint lithography show enhanced hole mobility and strong anisotropy of conductance due to nanoimprint-induced three-dimensional polymer chain alignment. Field effect transistors were fabricated using these nanogratings and device measurements show a hole mobility of 0.03 cm2/V s along the grating direction, which is about 60 times higher than that of nonoptimized thin film transistors. Organic photovoltaic devices (OPV) were made using the P3HT nanograting with infiltration of [6,6]-phenyl-C61-butyric acid methyl ester. Compared to similar bilayer and bulk heterojunction devices, the nanoimprinted OPV shows improved device performance.

Nanoimprinted P3HT/C60 solar cells optimized by oblique deposition of C60

Poly(3-hexylthiophene) (P3HT)-C60 organic photovoltaic devices with interpenetrating donor-acceptor interfaces were fabricated by oblique thermal deposition of C60 into the P3HT nanogratings. The uniformity and step coverage of C60 infiltration into the P3HT nanostructures, which can determine the device performance, were dependent on the C60 evaporation angle. It was also observed that the C60 deposition rate and thickness determine the efficiency. A 50% improvement in power conversion efficiency is observed due to the increased exciton dissociation rate at the larger area P3HT-C60 interface at optimal C60 deposition filling. With the proposed technique, a highly efficient organic solar cell using an insoluble acceptor has been fabricated.

Effects of nanostructure geometry on nanoimprinted polymer photovoltaics.

We demonstrate the effects of nanostructure geometry on the nanoimprint induced poly(3-hexylthiophene-2,5-diyl) (P3HT) chain alignment and the performance of nanoimprinted photovoltaic devices. Out-of-plane and in-plane grazing incident X-ray diffraction techniques are employed to characterize the nanoimprint induced chain alignment in P3HT nanogratings with different widths, spacings and heights. We observe the dependence of the crystallite orientation on nanostructure geometry such that a larger width of P3HT nanogratings leads to more edge-on chain alignment while the increase in height gives more vertical alignment. Consequently, P3HT/[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) solar cells with the highest density and aspect ratio P3HT nanostructures show the highest power conversion efficiency among others, which is attributed to the efficient charge separation, transport and light absorption.

Large Molecular Weight Polymer Solar Cells with Strong Chain Alignment Created by Nanoimprint Lithography

In this work, strong chain alignment in large molecular weight polymer solar cells is for the first time demonstrated by nanoimprint lithography (NIL). The polymer crystallizations in nonimprinted thin films and imprinted nanogratings with different molecular weight poly(3-hexylthiophene-2,5-diyl) (P3HT) are compared. We first observe that the chain alignment is favored by medium molecular weight (Mn = 25 kDa) P3HT for nonimprinted thin films. However, NIL allows large molecular weight P3HT (>40 kDa) to organize more strongly, which has been desired for efficient charge transport but is difficult to achieve through any other technique. Consequently P3HT/[6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM) solar cells with large molecular weight P3HT nanogratings show a high power conversion efficiency of 4.4%, which is among the best reported P3HT/PCBM photovoltaics devices.

Nanoimprinted solar cells with large molecular weight P3HT nanogratings with enhanced molecular alignment

We demonstrate efficient polymer solar cells made from large molecular weight (MW) poly(3-hexylthiophene-2,5-diyl) (P3HT) nanostructures made by nanoimprint lithography (NIL). We found NIL allows the polymer with higher MW (> 40 kDa) to organize more strongly than lower MW, which is desired for efficient charge transport but difficult to achieve previously. Solar cells with large MW P3HT nanogratings show a high power conversion efficiency of 4.4%, which is among the best reported for the same materials.

Effects of nanostructure geometry on polymer chain alignment and device performance in nanoimprinted polymer solar cell

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. This article demonstrates our recent studies on the nanostructure geometry effects on the nanoimprint induced poly(3 hexylthiophene-2,5-diyl) (P3HT) chain alignment and photovoltaic performance. Out-of-plane and in-plane grazing incident X-ray diffractions are employed to characterize the chain orientations in P3HT nanogratings with different widths and heights. It is found that nanoimprint procedure changes the initial edge-on alignment in non-imprinted P3HT thin film to a vertical orientation which favors the hole transport, with an organization height H≥ 170 nm and width in the range of 60 nm≤ W< 210 nm. Samples with better aligned molecules lead to a larger crystallite sizes as well. Imprinted P3HT/[6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM) solar cells show an increase in power conversion efficiency (PCE) with the decrease of nanostructure width, and with the increase of height and junction area. Devices with the highest PCE are made by the fully aligned and highest P3HT nanostructures (width w= 60 nm, height h= 170 nm), allowing for the most efficient charge separation, transport and light absorption. We believe this work will contribute to the optimal geometry design of nanoimprinted polymer solar cells.