George F. Burkhard

Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays

Hydrogenated amorphous Si (a-Si:H) is an important solar cell material. Here we demonstrate the fabrication of a-Si:H nanowires (NWs) and nanocones (NCs), using an easily scalable and IC-compatible process. We also investigate the optical properties of these nanostructures. These a-Si:H nanostructures display greatly enhanced absorption over a large range of wavelengths and angles of incidence, due to suppressed reflection. The enhancement effect is particularly strong for a-Si:H NC arrays, which provide nearly perfect impedance matching between a-Si:H and air through a gradual reduction of the effective refractive index. More than 90% of light is absorbed at angles of incidence up to 60 degrees for a-Si:H NC arrays, which is significantly better than NW arrays (70%) and thin films (45%). In addition, the absorption of NC arrays is 88% at the band gap edge of a-Si:H, which is much higher than NW arrays (70%) and thin films (53%). Our experimental data agree very well with simulation. The a-Si:H nanocones function as both absorber and antireflection layers, which offer a promising approach to enhance the solar cell energy conversion efficiency.

Smooth Nanowire/Polymer Composite Transparent Electrodes

IO N Transparent electrodes are critical components of thin-fi lm optoelectronic devices such as displays and thin-fi lm solar cells. Most high-performance transparent conducting fi lms in use today are composed of sputtered metal oxides. [ 1 , 2 ] These fi lms can have sheet resistances under 20 Ω − 1 with 90% transmission when deposited at a high temperature onto glass and resistances increasing to 40–200 Ω − 1 with the same transmission when deposited at lower temperatures onto plastic substrates. [ 2 , 3 ] Recent research has focused on replacing conductive metal oxides with alternative materials that can be deposited from solution and can reproduce the performance of metal oxides on glass on various substrates, including plastics. In addition, metal oxides are brittle, [ 4 , 5 ] and thus alternative transparent conductor technologies are also focusing on fl exibility and robustness to enable lightweight, fl exible solar cells and other thin fi lm devices. Strategies for non-vacuum deposition of transparent electrodes make use of materials other than metal oxides [ 6 ]

Accounting for Interference, Scattering, and Electrode Absorption to Make Accurate Internal Quantum Efficiency Measurements in Organic and Other Thin Solar Cells

In solar cells, internal quantum effi ciency (IQE) is the ratio of the number of charge carriers extracted from the cell to the number of photons absorbed in the active layer. Because IQE measurements normalize the current generation effi ciency by the light absorption effi ciency, they separate electronic properties from optical properties and provide useful information about the electrical properties of cells that external quantum effi ciency measurements alone cannot. The magnitude of the IQE is inversely related to the amount of recombination that is occurring in the cell, while the spectral shape of the curve can provide information about the effi ciency of harvesting excitons in the cell or spatial dependence of charge recombination. [ 1 , 2 ] Effects like multiple exciton generation [ 3‐5 ] and singlet exciton fi ssion [ 6 ] as well as bias-dependent photoconductivity [ 7 ] can lead to interesting spectral shapes and be detected by measuring IQEs greater than 100%. Despite its usefulness as a characterization tool, IQE is rarely reported. When IQE is reported, absorption is frequently not measured in actual devices; this can lead to errors since refl ective electrodes induce strong interference effects that substantially affect absorption. When absorption is measured in actual devices, parasitic absorptions are almost never taken into account. We hope that by demonstrating a straightforward method of measuring IQE, it will become a standard measurement and the community may benefi t from a better understanding of how the best performing cells work. Organic photovoltaics (OPVs) and other ultra-thin solar cells [ 8‐11 ] are made as a stack of materials including an active semiconducting layer, electrodes, and in some cases modifi er layers such as charge blocking layers and optical spacers. [ 12‐15 ] The active layer is responsible for all charge generation in the cell. Typically 5‐10% of the incident light is absorbed in the electrodes. In many solar cells, the IQE should not vary with wavelength. Since parasitic absorption does vary with wavelength, one must account for it to observe the correct spectral shape. [ 1 ] Consequently in the general case, it is critically important to take this parasitic absorption into account when calculating internal quantum effi ciency. Determining the active layer’s contribution to the total absorption can be a challenge, as it generally requires optical modeling to relate the experimentally measurable total absorption to the absorption in each layer. The absorption of each layer cannot independently be measured because, due to interference effects, the optical density of the stack is not simply the sum of the optical densities of each layer. The most accurate commonly used model uses a transfer matrix formalism to calculate the interference of coherent refl ected and transmitted waves at each interface in the stack. [ 16 , 17 ] This calculation requires knowledge of the wavelengthdependent complex index of refraction of each material. The imaginary part, k , is related to the extinction coeffi cient and is responsible for absorption in a medium. The real part, n , determines the wavelength of light of a given energy in a material and is important for calculating where areas of constructive and destructive interference occur. Typically the optical constants are measured using variable angle spectroscopic ellipsometry (VASE). [ 18‐22 ] The data produced by this technique when measuring anisotropic organic materials are diffi cult to interpret and require complicated modeling not available to many research groups. In blended donor-acceptor fi lms, the optical properties depend strongly on morphology and therefore on processing conditions. Thus fi lms of different thicknesses, cast from different solvents, or dried for different amounts of time have different optical constants. [ 23 , 24 ] In such composite materials, morphology is also a function of depth due to vertical phase segregation. [ 24 , 25 ] In these cases the optical constants are spatially dependent and the data gathered by these methods are approximations themselves. It is not always feasible to use VASE to measure n and k for each fi lm, so a simpler method of determining active layer absorption is desirable. In this article we show that for typical OPVs, precise knowledge of the real part of the complex index of refraction of the active layer is not required for making the measurements of the active layer absorption necessary for calculating IQE. We have investigated several methods to calculate the active layer absorption using published values of the optical constants. [ 18‐22 ] We propose a method that minimizes error by using an optical model to calculate the parasitic absorption (the absorption by the layers that do not contribute to photocurrent) and subtracting this from the experimentally measured total absorption.

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells

A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly efficient low cost double-junction solar cells on crystalline Si. Solution-processable organic-inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and though it is possible to tune the bandgap of (CH3NH3)Pb(BrxI1-x)3 between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photoinduced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV.

Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells

We investigate the internal quantum efficiencies (IQEs) of high efficiency poly-3-hexylthiophene:[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) solar cells and find them to be lower at wavelengths where the PCBM absorbs. Because the exciton diffusion length in PCBM is too small, excitons generated in PCBM decay before reaching the donor-acceptor interface. This result has implications for most state of the art organic solar cells, since all of the most efficient devices use fullerenes as electron acceptors.

Efficient energy sensitization of C60 and application to organic photovoltaics.

Fullerenes are currently the most popular electron-acceptor material used in organic photovoltaics (OPVs) due to their superior properties, such as good electron conductivity and efficient charge separation at the donor/acceptor interface. However, low absorptivity in the visible spectral region is a significant drawback of fullerenes. In this study, we have designed a zinc chlorodipyrrin derivative (ZCl) that absorbs strongly in the visible region (450-600 nm) with an optical density 7-fold higher than a C60 film. ZCl efficiently transfers absorbed photoenergy to C60 in mixed films. Application of ZCl as an energy sensitizer in OPV devices leads to an increase in the photocurrent from the acceptor layer, without changing the other device characteristics, i.e., open circuit voltage and fill factor. For example, C60-based OPVs with and without the sensitizer give 4.03 and 3.05 mA/cm(2), respectively, while both have V(OC) = 0.88 V and FF = 0.44. Our ZCl sensitization approach improves the absorbance of the electron-acceptor layer while still utilizing the beneficial characteristics of C60 in OPVs.

Encapsulating perovskite solar cells to withstand damp heat and thermal cycling

Perovskite solar cells (PSCs) are highly promising, but they are mechanically fragile, composed of layers with mismatches in thermal expansion coefficients, and known to decompose in the presence of heat and moisture. Here we show the development of a glass–glass encapsulation methodology for PSCs that enables them to pass the industry standard IEC 61646 damp heat and thermal cycling tests. It is important to select a thermally stable perovskite composition to withstand the encapsulation process at 150 °C and design a cell that minimizes metal diffusion. Moreover, the package needs an edge seal to effectively prevent moisture ingress and an inert encapsulant with an appropriate elastic modulus to hold the package together while allowing for compliance during temperature fluctuations. Our work demonstrates that industrially relevant encapsulation techniques have the potential to enable the commercial viability of PSCs.

Fully inorganic cesium lead halide perovskites with improved stability for tandem solar cells

A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly-efficient low cost double-junction solar cells. Solution-processable organic-inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and while it is possible to tune the bandgap of (CH3NH3)Pb(BrxI1-x)3 between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photo-induced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV.

The roles of bulk and interfacial molecular orientations in determining the performance of organic bilayer solar cells (presentation video)

Molecular orientation plays a significant role in determining the performance of small molecule solar cells. Key photovoltaic processes in these cells are strongly dependent on how the molecules are oriented in the active layer. We isolate contributions arising from the bulk molecular orientations vs. those from interfacial orientations in ZnPc/C60 bilayer systems and we probe these contributions by comparing device pairs in which only the bulk or the interface differ. By controlling the orientation in the bulk the current can be strongly modulated, whereas controlling the interfacial molecular orientation and degree of intermixing mediate the voltage.