Session H36: Photovoltaics: Compound Semiconductors and Organics

Effects of doping on the band gap of iron pyrite

Iron pyrite (FeS$_{2})$, a highly abundant materials in Earth's upper continental crust, is of great interests in photovoltaic and photo-electrochemical applications, due to its wide band gap of 0.9 - 0.95 eV and large absorption coefficient of $\alpha >$10$^{5}$ cm-1 for $\lambda <$10$^{3}$ nm. However, its electron/hole mobility is typically very low, caused by the presence of sulfur deficiency in crystalline FeS$_{2}$ bulks or nanostructures. In principle, doping electrons or holes by exotic elements may not only compensate sulfur deficiency but also create mobile carriers. In this work, we systematically studied the dopabilities of N, P, F and Cl in bulk FeS$_{2}$, by using the first-principles calculations. First of all, we found that these elements substitute S under sulfur poor condition in the FeS$_{2}$ bulk and nanostructures. While N, P and F dopants merely induce deep localized defect levels, doping 1.6{\%} Cl not only generates a 0.996 $\mu _{B}$ local magnetic moment per Cl and but also increases the carrier density by 3x10$^{18}$/cm$^{-3}$. The defect states are delocalized and hence doping of Cl also improve the carrier mobility in FeS$_{2}$. We found that incorporation of Cl also leads to significant structural distortions around Cl atom.   

The reason FeS$_2$ is not a good PV absorber

FeS$_2$ is representative of an ideal earth-abundant candidate absorber for thin film PV, because of its appropriate band gap, high absorption coefficient and good electron/hole mobility. Yet, despite $\sim$15 years of research, the promise of FeS$_2$ as an absorber layer has been unrealized, manifesting as a low open circuit voltage which has been attributed to E$_f$ pinning arising from bulk sulfur vacancies. Our first-principles calculations and experimental thermogravimetirc analyses, however, show that S vacancies and other point defects have rather high formation energies. Hence, they are unlikely to form and pin E$_f$. We find that the widely observed S deficiency in FeS$_2$ is accommodated by phase-coexistence of a few Fe$_{1-x}$S compounds, rather than S vacancies. These minority phases are metallic and detrimental for PV. We find select ternary Fe sulfides do not have thermodynamically-mandated phase-coexistence like FeS$_2$, yet they retain optimal band gaps and high absorption strengths comparable to FeS$_2$. These properties and associated surface-defect calculations will be discussed.   

Direct measurement of the built-in potential in a nanoscale heterostructure

Recently synthesized heterostructured nanorods are a promising material for applications such as photovoltaics. Understanding the electronic structure of these materials is both an interesting scientific question and vitally important for applications. We present the measurement of the built-in potential across individual Cu2S-CdS heterostructured nanorods by combining transmission electron microscopy with electrostatic force microscopy. This represents the first experimental determination of the electrostatic potential across an isolated nanostructure. We observe a variation of built-in potentials, ranging from 100 to 920 mV, with an average of 250 mV. Nanorods of a uniform composition with no heterojunction do not show built-in potential, as expected.   

First Principles Calculation of Optical Properties of Ternary Semiconductors Cu$_3$PSe$_4$ and Cu$_3$PS$_4$

The ternary semiconducting compounds Cu$_3$PSe$_4$ and Cu$_3$PS$_4$ are of interest as potential optoelectronic materials. Of particular interest for solar photovoltaic devices is Cu$_3$PSe$_4$, as its band gap lies in the desired 1.0 to 1.6 eV range for an absorber. We have theoretically calculated the optical properties of these materials using density functional theory with GGA and hybrid exchange-correlation functionals, as well as with the GW$_0$ approximation from many-particle theory. We find that Cu$_3$PSe$_4$ has a direct band gap with relatively strong optical absorption above 2 eV, indicating that this compound is a candidate photovoltaic absorber. Cu$_3$PS$_4$ has a larger band gap of approximately 2.4 eV, placing it outside consideration as a solar absorber.   

Crystalline ordered states and local surface potential variations of photovoltaic Cu(In,Ga)Se$_{2}$ thin-films

Structural and electrical properties of CuInSe$_{2 }$(CIS), Cu(In,Ga)Se$_{2}$ (CIGS) and CuGaSe$_{2}$ (CGS) grown by co-evaporation were studied. Intriguing morphology and grain growth behaviors were found in the surface of the films. X-ray diffraction of the films exhibited phase formation of the stoichiometric chalcopyrite while Cu$_{2}$Se and CuSe$_{2}$ were observed. Using Raman scattering spectroscopy, shift of A$_{1}$ mode was observed from 177 cm$^{-1}$ for CIS to 189 cm$^{-1}$ for CGS as Ga content increased. It is very interesting that two different crystalline ordered states with chalcopyrite (CH) and CuAu structure (CA) were found. Effects of the grain boundaries on local electrical properties of the films with different chemical contents were examined. Local current mapping and surface potential distribution were obtained in the film by conductive atomic force microscopy and Kelvin probe microscopy. Minority carrier transport behaviors and local variations of potential values on and near the grain boundaries were characterized. These results suggested that a local built-in potential is possibly formed on positively charged grain boundaries.   

Minority-Carrier Lifetimes in GaInP

Minority-carrier lifetimes are very important to the performance of photovoltaic materials and are quite sensitive to the structure of the material. The impact of lifetimes can be readily illustrated using computer modeling of cell performance, and a brief discussion of the results of our modeling will be given. AlInP/GaInP double heterojunctions of varying thickness and doping concentration were grown on GaAs substrates by metallorganic chemical vapor deposition (MOCVD). Lifetimes were measured using time-resolved photo-luminescence. Carrier concentrations were determined using capacitance-voltage measurements. Here we report on the minority carrier lifetime as a function of active layer thickness and doping concentration for n-type and p-type GaInP that is lattice-matched to the substrate.   

Photvoltaic effects in ferroelectrics due to nonlinear optical processes

The physical mechanism for the bulk photovoltaic effect that appears in noncentrosymmetric materials, especially in ferroelectric devices, is not well understood. A promising candidate for a truly bulk photovoltaic effect is non-linear optical processes -- most notably ``shift current,'' which describes the net motion of coherently excited electrons in the absence of inversion symmetry, and has been described analytically several times in the literature. Shift current is also of interest due to the appearance of a gauge invariant phase describing the carrier mobility. We have developed an expression for shift current suitable for efficient computation and analysis utilizing wavefunctions of arbitrary origin, and calculated the response for several prominent ferroelectrics -- including LiNbO$_3$, BaTiO$_3$, and PbTiO$_3$ -- using KS eigenstates. The calculated short-circuit currents appear to be in rough agreement with available experimental results where they exist. Furthermore, they indicate a more subtle relationship between polarization and band gap than has heretofore been presumed, with strong implications for the materials design process, as well as shift current's overall viability as a mechanism for efficiently harvesting solar energy.   

Nanostructured multiferroic materials for optoelectronics and energy-related nanodevices

Combining properties into multifunctional materials is one of innovative ways explored by the modern technology to achieve high miniaturization of integrated devices. In this context, besides their exciting physics, multiferroic materials hich combine two or more ferroic order offer opportunities for potential applications in emerging fields of spintronics, optoelectronics and data storage. For such applications, successful integration of these multifunctional materials needs to develop adequate fabrication processes and to the coexistence in singe phase of robust properties at room temperature (RT). Furthermore, a synergistic interaction between magnetic and electric orders leads to additional freedom for designing related devices. Here we review recent progress of our group in growth and nanopatterning of multiferroic thin films developed to overcome those drawbacks. We will present the fabrication of RT-multiferroic Bi$_{2}$FeCrO$_{6 }$thin films. Successful nanopatterning of these complex oxides by a versatile and generic approach and their photovoltaic properties will be also discussed.   

Energy Transfer in Organic Photovoltaic Cells and its Impact on Measurements of the Exciton Diffusion Length

In order to generate photocurrent from an organic photovoltaic cell (OPV), the optically generated exciton must be dissociated into its constituent charge carriers. This process is carried out at the interface between electron donating and accepting materials. Consequently, photocurrent is generated only at the donor-acceptor (D-A) interface, and exciton diffusion to the interface is a critical step in the photoconversion process. The focus of this work is on the development of methods that permit the accurate measurement of the exciton diffusion length, and realizing architectures that demonstrate enhanced exciton harvesting. In measuring the exciton diffusion length, emphasis is placed on quantifying the role of excitonic energy transfer in the dissociation process by explicitly measuring the F\"{o}rster radius between donor and acceptor materials. Many of the techniques currently used to estimate the exciton diffusion length incorrectly ignore these effects, potentially leading to overestimates. Efforts to overcome the short diffusion length are focused on small molecule OPVs that contain a continuously graded D-A film composition as a means to simultaneously optimize both exciton diffusion and charge collection. In a properly optimized graded heterojunction OPV, power conversion efficiencies $>$4{\%} can be realized, exceeding the performance of conventional planar and uniformly mixed structures.   

Effect of Thin Polymer Layers on the Performance of ZnO/Cu$_2$O Solar Cells

Transition metal oxides are a class of stable, non-toxic, and inexpensive semiconductors with great potential in low-cost photovoltaics (PV) applications. Cu$_2$O is a versatile p-type oxide that absorbs visible light and can be solution-processed at low temperatures. ZnO is a wide-E$_g$ n-type material with good electronic properties and has already been widely incorporated into other low-cost PV technologies such as organic and dye-sensitized solar cells. While ZnO/Cu$_2$O devices have large theoretical efficiencies (as high as 20\%) [1], practical devices do not reach their full potential due to poor charge collection and recombination. ZnO/Cu$_2$O PV's can be improved by optimizing deposition conditions, such as solution pH and temperature, and device geometry, such as layer thickness [2]. This talk, however, will discuss how semiconducting polymer layers can further enhance performance for scalable device fabrication. In particular, polymer type and the Cu$_2$O/polymer interface will be discussed as routes to better performance. \\[4pt] [1] J. Nelson. \emph{The Physics of Solar Cells}. Imperial College Press, 2003 \newline [2] Musselman et al., unpublished   

Nanoscale Morphology and Charge Transport in Hybrid Solar Cells by Conducting Probe Atomic Force Microscopy

Measurements of the dependence of photoactive response on nanoscale morphology provide essential insights to further improve processing and achieve morphologies with enhanced device performance. To study the correlation between local morphology and photoactive response, we have fabricated hybrid polymer/zinc oxide thin films and have characterized their electrical properties at nanoscale resolution with conducting probe atomic force microscopy (c-AFM). The charge carrier mobilities were extracted based on local IV characteristics. The surface morphology and current mapping were recorded simultaneously under various illumination and biasing conditions, enabling direct study of morphology dependent transport processes in these photoactive devices.   

Development of Phase Stable Organic Photon Upconverters

Recently, high efficiency photon upconversions (UCs) utilizing triplet-triplet annihilation (TTA) in aromatic hydrocarbon molecules, applicable to sunlight intensity, have been actively studied for the purpose of improving external quantum efficiencies of photovoltaics. These studies have been using volatile organic solvents as media in order for TTA to occur, which are currently hindering its applications. I have discovered that those aromatic molecules can be stably dispersed by a simple method within certain class of ionic liquids (ILs), which are non-volatile and thermally stable up to several hundred degree C, to form unprecedented organic photon upconverters with improved phase stability [1,2]. The proposed mechanism for the molecular stabilization in ILs as well as the UC quantum efficiencies is presented.\\[4pt] [1] Y. Murakami and I. Sato, Patent 2010-230938JP (pending)\\[0pt] [2] Y. Murakami et al., submitted.   

Charge transport and absorption study of metal nanoparticle plasmonics for organic photovoltaics

A hybrid plasmonic nanostructure of an optically sensitive heterojunction organic film incorporating metal nanoparticles is fabricated. From the Charge Extraction in Linearly Increasing Voltage (CELIV) measurements, the mobility of this hybrid plasmonic nanostructure has been experimentally extracted to be at least one order of the magnitude higher than that of the organic film without metal nanoparticles. The measured absorption spectrum also shows the increasing of the intensity by around 28{\%} as well as the broadening of the spectrum. The theoretical calculation confirms this broadband optical absorption enhancement results from localized surface plasmon resonance. The optimization of the density of the metal nanoparticles has been done to achieve the best performance for the photovoltaic devices.   

The Effect of a Self Assembled Monolayer in Small Molecule Organic Solar Cells

We have previously found that a Self Assembled Monolayer (SAM) of Flouroalkyl TrichloroSilane (FTS) molecules on Single-Walled and Multi-Walled Carbon Nanotubes (SWCNT {\&} MWCNT) can greatly improve the conductivity [1]. In present work we have studied the effect of SAM modified carbon nanotubes in Small molecule organic photovoltaic cells. (OPV) We have fabricated and characterized OPV of the general structure: CNT(FTS)/CuPC/C60/BCP/Al. We observed improvement of the performance of the OPV with CNT anodes with FTS SAM both for SW and MW CNT. The major effect is an improvement of the open circuit voltage and also small improvements in both short circuit current and filling factor. The increase in open circuit voltage is likely due to modifications of the carbon nanotube work function by the strong dipole moments of the FTS molecules. The improvements in short circuit current and filling factor is probably due to improved active layer morphology and removal of absorbed water from the substrate. \\[4pt] [1] Cook, Alexander; Lee, Bumsu; Kuznetsov, Alexander; Podzorov, Vitaly; Zakhidov, Anvar. Self Assembled Dipole Monolayers on CNTs: Effect on Transport and Charge Collectio. Oral Presentation APS March Meeting 2010   

Inverted Polymeric Photovoltaic Cells and Parallel Tandems with Transparent Single Wall Carbon Nanotubes Interlayer

We demonstrate an organic photovoltaic (OPV) monolithic multi junction cell in a parallel electrical configuration utilizing polymers with complementary absorption spectra and transparent single wall CNT (SWCNT) as an interlayer electrode (IE). Parallel tandem cells are of importance because they can append to the limited spectral coverage of available polymers and because there is no need balance current as is the case with in-series configurations. Devices comprise of polymeric sub cells where one is inverted using ZnO nanoparticles and a MoO$_{3}$ buffer layers, this inverted structure allows for the SWCNT IE to function as a cathode. Each sub cell is characterized independently and the short circuit current of the tandem device is shown to increase. Overall increase in efficiency is observed and attributed to enhanced spectral coverage by spectrally complimentary polymers and the effective use of parallel tandem architecture. We also demonstrate a semi transparent inverted OPV structure with a SWCNT electrode and a efficiency of over 3{\%}.