Patrick Boland

Design of organic tandem solar cells using PCPDTBT: PC61BM and P3HT: PC71BM

We conducted optical and electrical simulations with the goal of determining the optimal design for conjugated polymer-fullerene tandem solar cells using poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT): [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as a bottom cell and poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as a top cell. The effects of photon density, absorption, balanced and unbalanced charge carrier transport, and bimolecular recombination in the two subcells were incorporated into the simulations. We found that the maximum energy conversion efficiency (η) is 9% when charge carrier mobilities in both top and bottom cells are balanced. However, the efficiency drops significantly if the carrier mobilities are unbalanced in either the top or bottom cell. In addition, we found that unbalanced carrier mobilities in the top cell require a reduction in the thickness of the bottom cell wherea...

Optimization of Active Layer Thickness in Planar Organic Solar Cells via Optical Simulation Methods

Optimizing the efficiency of polymer-based, planar organic solar cells requires nanoscale control of the thicknesses of constituent layer materials composing the device. A thin film optical simulation modeling tool has been used to determine ideal active layer thicknesses for regioregular poly(3-hexylthiophene) and phenyl-C61/C71-butyric acid methyl ester (P3HT-PC61BM and P3HT-PC71BM) organic blends used as photoactive components in polymer solar cells. Using the well understood transfer matrix formalism, solar cells are simulated layer-by-layer after varying such factors as active layer thickness and electron and hole mobilities. Spectral absorption, fill factor (FF), power conversion efficiency (PCE), and current-voltage (I–V) characteristics are obtained and the most effective active layer thicknesses determined that will enable maximum efficiency to be achieved. Figure 1 shows the simulated current densities of P3HT:PC61BM and P3HT:PC71BM as a function of device thickness and carrier mobility. This device simulation indicates the potential of higher current density achievable using P3HT:PC71BM. We will discuss the systematic study of the effect of the charge carrier mobilities on the current density, open circuit voltage, and fill factor as a function of device thickness.

Toward ultra thin CIGS solar cells

In this paper, we present our results on the fabrication of solar cells down to thicknesses of 0.5 μm, and how real time and in situ analysis by spectroscopic ellipsometry (SE) can help in (i) understanding the results of the devices; and (ii) modeling the growth and properties of the CIGS solar cell. These in situ and real time measurements are correlated with ex situ structural measurements of the films such as XRD and AFM; broad spectral range optical measurements of the films and devices such as T&R, variable angle SE; and device specific measurements such as I-V and QE measurements.

Self-Organized Crystal Growth of Nanostructured ZnO Morphologies by Hydrothermal Synthesis

A template-free method was developed to synthesize zinc oxide (ZnO) nanoflowers, nanorods, microdandelions, and microspheres using hydrothermal synthesis. The effect of pH value on ZnO morphology was investigated using zinc acetate dehydrate and urea as an additive. We found that ZnO microspheres were developed at pH=12.0 while nanoflowers and nanorods were developed at pH less than 11.0. Furthermore, we also found that urea plays a crucial role in the formation of ZnO microspheres.

Real time analysis of ultra-thin CIGS thin film deposition

Thin films of Cu(In,Ga)Se2 with various copper contents as functions of the copper and gallium contents were deposited by co-evaporation onto thermally oxidized silicon wafer (100). In-situ Real Time Spectroscopic Ellipsometry (RTSE) is used to understand the effect of the Ga/(In+Ga) ratio and the Cu atomic % on the growth and optical properties of ultra -thin CIGS films. We have demonstrated that RTSE can be used effectively to identify the growth process and to distinguish the effects of copper from those of gallium on the surface roughness evolution and dielectric functions.

Fabrication methodology for nanostructured hybrid organic/inorganic photovoltaic devices formed with alternating sacrificial spacers in a nested nanotube configuration

An active area of research in the field of photovoltaics that strives to significantly improve energy conversion efficiencies involves the exploration and manipulation of device physical structures. With knowledge gleaned from the fabrication of standard planar organic solar cells, our group has developed a technique for creating nanoscale nested structures capable of accommodating organic photoactive material that serves as a light-absorbing layer. The technique employs highly-ordered macroporous silicon templates that are used as the supporting structure and bottom contact for concentrically-nested metal oxide cylinders.

Optimization of active layer thickness in planar organic solar cells via optical simulation methods

Optimizing the efficiency of polymer-based, planar organic solar cells requires nanoscale control of the thicknesses of constituent layer materials composing the device. A thin film optical simulation modeling tool has been used to determine ideal active layer thicknesses for regioregular poly(3-hexylthiophene) and phenyl-C61/C71-butyric acid methyl ester (P3HTPC61BM and P3HT-PC71BM) organic blends used as photoactive components in polymer solar cells. Using the well understood transfer matrix formalism, solar cells are simulated layer-bylayer after varying such factors as active layer thickness and electron and hole mobilities. Spectral absorption, fill factor (FF), power conversion efficiency (PCE), and current-voltage (I-V) characteristics are obtained and the most effective active layer thicknesses determined that will enable maximum efficiency to be achieved. Figure 1 shows the simulated current densities of P3HT:PC61BM and P3HT:PC71BM as a function of device thickness and carrier mobility. This device simulation indicates the potential of higher current density achievable using P3HT:PC71BM. We will discuss the systematic study of the effect of the charge carrier mobilities on the current density, open circuit voltage, and fill factor as a function of device thickness. A key parameter impacting photovoltaic power conversion efficiency (PCE) is the fill factor. Fill factor represents the overall quality of the device and is mainly influenced by the shunt (RSH) and serial (RS) resistances [1-2]. Therefore, device optimization requires continuous effort to lower RS and increase RSH by varying such factors as thermal annealing, solvent properties, active layer thickness, and the organic-electrode interfaces [2-3]. FF influences PCE according to the relationship

Characterization of TCO deposition for CIGS solar cells

Transparent conducting ZnO:Al (AZO) thin films were deposited by using radio frequency (RF) magnetron sputtering system and were analyzed with regards to their potential application as a window layer for CIGS solar cells. Their properties were measured in-situ and ex-situ by real time spectroscopic ellipsometry (RTSE) and were correlated with ex-situ characterization, such as AFM, XRD, and T&R measurements. As the deposition power was increased, an increase in the conductivity (via an increase by the contribution of the Drude oscillator), a reduction of the transmittance in the IR and an increase in the grain size was observed by RTSE, while the growth mechanism remained the same for all powers.

Characterization of ZnS films deposited by ALD for CIGS solar cells

Alternative deposition methods and materials are of interest for the fabrication of thin film solar cells since they offer potential enhancements for either low cost, high speed or high efficiency but also because they can help in better understanding the underlying physical and chemical processes that could lead to the next generation of solar cells. In this study, we will present new results on the deposition of ZnS by atomic layer deposition (ALD) as an alternate to CdS deposited by chemical bath deposition.

Estimation of Organic Tandem Solar Cell Power Conversion Efficiency via Optical Simulation Methods

Research and development of organic photovoltaics continues to progress as efforts to harness limitless and pollution-free solar energy remains an intense focus of scientific interest. Organic solar cells that are inexpensive, easy to fabricate, and highly reliable are the goal of numerous research groups both in industry and academia and the understanding of device physics and organic materials has increased markedly over the past couple of decades. Still, energy conversion efficiency for these types of cells remains far too low for commercial viability . Improvements in performance have been made with bulk heterojunction (BHJ) devices 3 where electron-donating organic polymers and electronaccepting fullerene components are mixed to form nanoscale donor/acceptor interfaces. Device efficiencies exceeding 4%are easily achievable with this fabrication method, however, the limited absorption profiles of currently available organic materials prevent the attainment of higher efficiencies. An approach being explored presently to overcome these limitations involves the design of multijunction organic solar cells. This structural configuration employs individual cells arranged in a “tandem” configuration that offers a number of advantages including increased open circuit voltage (VOC) and short circuit current density (JSC). The most common configuration comprises a series connection of two or three sub-cells where the photogenerated current extracted from the tandem structure is determined by the subcell producing the lowest value of photocurrent. The VOC of the tandem cell is approximately the sum of each subcell’s open circuit voltage. Therefore, ideal tandem solar cells in a series configuration require that each subcell be engineered such that light absorption is accurately controlled to balance photocurrent. Several researchers have used optical transfer matrix methods (TMM) 15 to predict organic thin film thicknesses for matching photocurrents between heterojunction organic blends. Commonly used materials include poly (3-hexylthiophene) (P3HT): 1-(3methoxycarbonyl) propyl-1-phenyl [6,6] C61/71 (PC61/71BM), and poly [2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta [2,1-b;3,4-b’]dithiophene)-alt-4,7-(2,1,3benzothiadiazole)] (PCPDTBT): 1-(3methoxycarbonyl) propyl-1-phenyl [6,6] C61/71 (PC61/71BM). Our simulation model employs TMM in addition to a number of electrical models to realistically calculate several charge transport properties 16,17,18 such as exciton generation and dissociation, bimolecular recombination, and unbalanced charge transport inside the active layers. We conducted optical and electrical simulations with the goal of determining the optimal design for conjugated polymer-fullerene tandem solar cells using varying combinations of low-, and high-bandgap polymers as top and bottom subcells in a series tandem configuration. The effects of photon density, absorption, balanced and unbalanced charge carrier transport, and bimolecular recombination in the two subcells were incorporated into the simulations. Also, the mutual effects of both subcells in tandem were analyzed to determine such parameters as current density as a function of active layer thickness, open circuit voltage, fill factor, and power conversion efficiency. Our results indicate that appropriate selection of polymer:fullerene composition and spatial ordering of the subcells can allow achievement of device efficiencies exceeding 9%.