Eric T. Hoke

Hysteresis and transient behavior in current–voltage measurements of hybrid-perovskite absorber solar cells

Hybrid organo-metal halide perovskites are an exciting new class of solar absorber materials and have exhibited a rapid increase in solar cell efficiencies throughout the past two years to over 17% in both meso-structured and thin-film device architectures. We observe slow transient effects causing hysteresis in the current–voltage characterization of these devices that can lead to an over- or underestimation of the solar cell device efficiency. We find that the current–voltage (IV) measurement scan direction, measurement delay time, and light and voltage bias conditions prior to measurement can all have a significant impact upon the shape of the measured IV light curves and the apparent device efficiency. We observe that hysteresis-free light IV curves can be obtained at both extremely fast and slow voltage scan rates but only in the latter case are quasi-steady-state conditions achieved for a valid power conversion efficiency measurement. Hysteretic effects are also observed in devices utilizing alternative selective contacts but differ in magnitude and time scale, suggesting that the contact interfaces have a big effect on transients in perovskite-absorber devices. The transient processes giving rise to hysteresis are consistent with a polarization response of the perovskite absorber that results in changes in the photocurrent extraction efficiency of the device. The strong dependence of the hysteresis on light and voltage biasing conditions in thin film devices for a period of time prior to the measurement suggests that photo-induced ion migration may additionally play an important role in device hysteresis. Based on these observations, we provide recommendations for correct measurement and reporting of IV curves for perovskite solar cell devices.

A Layered Hybrid Perovskite Solar‐Cell Absorber with Enhanced Moisture Stability

Two-dimensional hybrid perovskites are used as absorbers in solar cells. Our first-generation devices containing (PEA)2(MA)2[Pb3I10] (1; PEA=C6H5(CH2)2NH3(+), MA=CH3NH3(+)) show an open-circuit voltage of 1.18 V and a power conversion efficiency of 4.73%. The layered structure allows for high-quality films to be deposited through spin coating and high-temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI3] (2) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual-absorber tandem device. Compared to 2, the layered perovskite structure may offer greater tunability at the molecular level for material optimization.

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.

A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.

The mechanism of burn-in loss in a high efficiency polymer solar cell.

Degradation in a high efficiency polymer solar cell is caused by the formation of states in the bandgap. These states increase the energetic disorder in the system. The power conversion efficiency loss does not occur when current is run through the device in the dark but occurs when the active layer is photo-excited.

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.

Molecular Packing and Solar Cell Performance in Blends of Polymers with a Bisadduct Fullerene

We compare the solar cell performance of several polymers with the conventional electron acceptor phenyl-C61-butyric acid methyl ester (PCBM) to fullerenes with one to three indene adducts. We find that the multiadduct fullerenes with lower electron affinity improve the efficiency of the solar cells only when they do not intercalate between the polymer side chains. When they intercalate between the side chains, the multiadduct fullerenes substantially reduce solar cell photocurrent. We use X-ray diffraction to determine how the fullerenes are arranged within crystals of poly-(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and suggest that poor electron transport in the molecularly mixed domains may account for the reduced solar cell performance of blends with fullerene intercalation.

High Excitation Transfer Efficiency from Energy Relay Dyes in Dye-Sensitized Solar Cells

The energy relay dye, 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), was used with a near-infrared sensitizing dye, TT1, to increase the overall power conversion efficiency of a dye-sensitized solar cell (DSC) from 3.5% to 4.5%. The unattached DCM dyes exhibit an average excitation transfer efficiency (ETE) of 96% inside TT1-covered, mesostructured TiO(2) films. Further performance increases were limited by the solubility of DCM in an acetonitrile based electrolyte. This demonstration shows that energy relay dyes can be efficiently implemented in optimized dye-sensitized solar cells, but also highlights the need to design highly soluble energy relay dyes with high molar extinction coefficients.