Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F24: Photovoltaics, including Cu, Sulfides, and Si Alloys |
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Sponsoring Units: GERA Chair: Gergely Zimanyi, University of California, Davis Room: 504 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F24.00001: Scaleable photovoltaic absorber materials within the Cu-Sb-S system Adam Welch, William Tumas, David Ginley, Colin Wolden, Andriy Zakutayev The Cu-Sb-S system contains four ternary compounds which may hold promise for scalable, non-toxic, and efficient solar photoconversion. Like similar compounds CuInSe$_2$, CIGS, and CZTS, the Cu-Sb-S compounds are predicted to offer high absorption coefficients, and electrically benign grain boundaries. Antimony, instead of indium or gallium, has the advantage of lower cost and greater availability, as well as theoretically predicted better photon absorption. It also has a potential advantage over CZTS, as the Cu-V-S compound avoids the deep traps associated with antisite defects. Here, we synthesize two compounds within the Cu-Sb-S system, Cu$_{12}$Sb$_4$S$_{14}$ (tetrahedrite) and CuSbS$_2$ (chalcostibite), by combinatorial RF magnetron co-sputtering from Cu$_2$S and Sb$_2$S$_3$ targets. Chalcostibite films were found to have good optical and electrical properties, with a steep absorption onset at 1.5eV, high absorption coefficient ($>10^{5}$cm$^{-1}$), good carrier concentration ($p=10^{17}$cm$^{-3}$) and mobility (0.2 cm$^2$/V-s). Chalcostibite growth conditions were therefore further optimized and it was found that an overflux of vapor phase Sb$_2$S$_3$ allowed strict control of stoichiometry for better device integration. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F24.00002: Theoretical study on the growth conditions for single-phase stability of kesterite-Cu$_{2}$ZnSnS$_{4}$ Pranab Sarker, Tyler J. Harrison, Mowafak M. Al-Jassim, Muhammad N. Huda Nowadays, kesterite-Cu$_{2}$ZnSnS$_{4}$ (CZTS) is being pursued as an efficient solar absorber materials for PV cells. By chemical potential landscape analysis of CZTS we will show that the formation of stoichiometric CZTS is practically impossible at thermodynamic equilibrium. This analysis verifies the experimental fact that non-stoichiometry is evident for high efficiency CZTS. In addition, the co-existence of ZnS is found to be highly probable if high efficiency growth condition (Zn rich, Cu-poor) is pursued. Moreover, it is found that Zn-richer growth condition is necessary to minimize the number of competitive secondary phases. Cu-poor condition should be chosen in such a way so that the occurrence of Cu$_{2}$S can be prevented irrespective of the value available chemical potential for S. In addition, defect calculation shows that the suitable Cu-poor condition can prevent anionic defects as well. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F24.00003: Growth of Cu$_{2}$ZnSnS$_{4}$(CZTS) by Pulsed Laser Deposition for Thin film Photovoltaic Absorber Material Abhishek Nandur, Bruce White CZTS (Cu$_{2}$ZnSnS$_{4}$) has become the subject of intense interest because it is an ideal candidate absorber material for thin-film solar cells with an optimal band gap (1.5 eV), high absorption coefficient (10$^{4}$ cm$^{-1}$) and abundant elemental components. Pulsed Laser Deposition (PLD) provides excellent control over film composition since thin films are deposited under high vacuum with excellent stoichiometry transfer from the target. CZTS thin films were deposited using PLD from a stoichiometrically close CZTS target (Cu$_{2.6}$Zn$_{1.1}$Sn$_{0.7}$S$_{3.44}$). The effects of laser energy fluence and substrate temperature and post-deposition sulfur annealing on the surface morphology, composition and optical absorption have been investigated. Optimal CZTS thin films exhibited a band gap of 1.54 eV with an absorption coefficient of 4x10$^{4}$cm$^{-1}$. A solar cell utilizing PLD grown CZTS with the structure SLG/Mo/CZTS/CdS/ZnO/ITO showed a conversion efficiency of 5.85{\%} with V$_{\mathrm{oc}} = $ 376 mV, J$_{\mathrm{sc}}=$ 38.9 mA/cm$^{2}$ and Fill Factor, FF $=$ 0.40. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F24.00004: Non-equilibrium phase map, optical and electrical properties of Cu-Zn-O alloys Archana Subramaniyan, John Perkins, Ryan O'Hayre, David Ginley, Stephan Lany, Andriy Zakutayev Cuprous oxide (Cu$_{2}$O) is a candidate p-type solar cell absorber material that has been spotlighted recently due to its low cost, earth abundant and non-toxic nature. The maximum reported efficiency of Cu$_{2}$O based solar cells is rather low (5. 38{\%}) and it can in part be attributed its forbidden direct band gap (2.1 eV) and higher absorption threshold (2.6 eV). Here, we alloy Cu$_{2}$O with ZnO via combinatorial RF magnetron sputtering as a function of temperature (T) and composition at fixed 20 mTorr Ar pressure to modify the electronic band structure and reduce its absorption threshold, which can potentially enhance the solar cell performance. A non-equilibrium Cu-Zn-O phase map was generated in the T range 100 -- 400 $^{\circ}$C and Zn composition 0 -- 37 at{\%}. Highly crystalline Cu$_{2}$O structured Cu-Zn-O alloys with Zn content of 0 to 17 at{\%} were synthesized in the T range 200 -- 270 $^{\circ}$C. With increasing Zn at{\%}, the preferential orientation in Cu-Zn-O alloy changes from (200) to (111) direction. At lower T (\textless 200 $^{\circ}$C), either amorphous or poor crystalline Cu$_{2}$O structured alloys were observed, whereas at higher T (\textgreater 270 $^{\circ}$ C) and higher Zn composition (\textgreater 25 at{\%}), CuO or ZnO second phases were observed. The absorption coefficient of all Cu-Zn-O alloys was higher than that of phase pure Cu$_{2}$O. The absorption threshold () was also reduced significantly, for example, at $=$ 2*10$^{4}$ cm$^{-1}$ the absorption threshold of Cu-Zn-O alloy with 10 at{\%} Zn reduced from 2.4 eV to 2.1 eV. The electrical conductivity of all Cu-Zn-O alloys was measured to be within 2 -- 5 mS/cm. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F24.00005: First principles simulations of Cu$_2$ZnSnS$_x$O$_{4-x}$ alloys Chaochao Dun, N.A.W. Holzwarth, Yuan Li, Wenxiao Huang, David Carroll Crystalline Cu$_2$ZnSnS$_4$ (CZTS) has been well studied for its photo-voltaic properties. This paper reports a systematic computational study of CZTS alloys with oxygen substituting for S in the form Cu$_2$ZnSnS$_x$O$_{4-x}$ in order to understand their stability and structural forms. The calculations find the heat formation of Cu$_2$ZnSnO$_{4}$ (CZTO) to be 4.7 eV lower than that of CZTS, a result which is consistent with the general observation that CZTS is very reactive when exposed to air. Interestingly, the results find that CZTS is stable with respect to its decomposition products; the calculated enthalpy for CZTS $\rightarrow$ Cu$_2$S + ZnS + SnS$_2$ is $\Delta H_{cal}$= +0.6 eV. However, for CZTO the corresponding decomposition is predicted to be exothermic; the calculated enthalpy for CZTO $\rightarrow$ Cu$_2$O + ZnO + SnO$_2$ is $\Delta H_{cal}$= -1.7 eV. The simulations of S/O alloys show that there are preferred structures for the O configurations. For example, for alloys with $x=2$, the energy difference between the lowest and highest energy O arrangements is 0.25 eV/formula unit. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F24.00006: An EXAFS Analysis of Cu$_2$SnS$_3$ for Extremely Thin Absorber Layer Leila Jewell, Andrew Short, Frank Bridges, Glenn Alers, John Norman, Sue A. Carter We present local structure studies of Cu$_2$S and Cu$_2$SnS$_3$ composite films prepared with CVD, using extended x-ray absorption fine structure (EXAFS) technique. The EXAFS technique has the ability to probe the local environment of specific atoms, and can also give very precise ratios of elements using their fluorescence peaks. Chemical vapor deposition (CVD) deposits highly conformal films and hence is an important tool for developing nanostructured solar cells with scalability. Cu$_2$SnS$_3$ is an earth-abundant absorber that is even more cost-effective when used in an extremely thin absorber solar cell. Composite films of Cu$_2$SnS$_3$ were made using CVD layers of Cu$_2$S and Tin (IV) Sulfide (SnS$_2$) with an anneal step. Cu$_2$SnS$_3$ also has the same structure as ZnS, which allows for the formation of the quaternary Cu$_2$ZnSnS$_4$ by depositing ZnS on top of the Cu$_2$S and SnS$_2$ layers determined for Cu$_2$SnS$_3$. Stoichiometric control was established by varying the deposition times of the binary compounds and was measured using energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), and EXAFS techniques. Optical absorption results are promising for forming a photovoltaic device with copper-based ternary and quaternary materials as the absorber. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F24.00007: Comparative nm-Resolution Electrical Potential and Resistance Mapping of Cu(In,Ga)Se$_{2}$, Cu$_{2}$ZnSnSe$_{4}$, and CdTe Thin Films Chunsheng Jiang, Ingrid Repins, Lorelle Mansfield, Miguel Contreras, Helio Moutinho, Kannan Ramanathan, Mowafak Al-Jassim We report on a comparative study of three leading thin-film PV materials of Cu(In,Ga)Se$_{2}$ (CIGS), Cu$_{2}$ZnSnSe$_{4}$ (CZTS), and CdTe, by mapping the local electrical potential and resistance using atomic force microscopy (AFM)-based electrical techniques of scanning Kelvin probe force microscopy (SKPFM) and scanning spreading resistance microscopy (SSRM). The SKPFM potential mapping shows consistent results among the three films. The energy bands around the grain boundaries (GBs) bent downward and the GBs are positively charged. However, whether the carriers around the GBs are depleted or inverted could not be determined solely by the potential contrast between the GB and grain surface because surface band bending decreases this contrast. The SSRM resistance mapping shows different results between the films. A higher conduction channel was imaged along the GBs of CIGS and CZTS, indicating an inversion of carriers around the GBs. However, no characteristic resistance was imaged on the GBs of CdTe. This difference of local resistance on the GBs suggests a depletion of carriers in CdTe, in contrast to CIGS and CZTS. These nm-electrical mapping proposes an active GB of CdTe for minority carrier recombination, but inactive GBs of CIGS and CZTS. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F24.00008: Structure prediction and electronic structure study of pristine and doped cuprous sulfide (Cu$_{2}$S) Prashant Khatri, Mowafak M. Al-Jassim, Muhammad N. Huda Cuprous sulfide (Cu$_{2}$S) is among the materials that have high potential of being used in solar cells, but it is highly unstable mainly due to the formation of Cu vacancies. Due to this instability of Cu$_{2}$S and mobile nature of Cu in Cu$_{2}$S, it is hard to study Cu$_{2}$S, and as a result not much is known about its structural details. A systematic theoretically understanding is necessary to utilize its potential fully in photovoltaic devices. The goal of this study is to predict the most probable structure for stoichiometric Cu$_{2}$S which is energetically favorable, and to find a mechanism to stabilize it against the formation of Cu vacancy. DFT, DFT$+$U and DFT-Hybrid functional theory has been used in predicting the structure and studying the properties. Many different structures have been considered while performing the calculations. Acanthite like Cu$_{2}$S structure has been found to be the most favorable structure energetically. We have also studied the structures with Cu-vacancy. A detail theoretical analysis of these aspects will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F24.00009: Structural Order and Thermodynamic Stability of Disordered Cu$_{2}$ZnSnS$_{4}$ Alloys Sin Cheng Siah, Rafael Jaramillo, Pete Erslev, Glenn Teeter, Tonio Buonassisi Crystalline kesterite Cu$_{2}$ZnSnS$_{4}$ (c-CZTS) thin films, of interest for photovoltaics, has a narrow window of thermodynamic stability and complex point defect chemistry. Hence, c-CZTS solar cells are thought to suffer from the effects of secondary phase segregation and further improvements in device efficiency may hinge on using kinetic stabilization to inhibit decomposition. By growing films at room temperature (T), we achieve a disordered (CuZnSn)S$_{4}$ alloy with an expanded solid solution window in the pseudo-ternary CuS--ZnS--SnS phase diagram that allows independent tuning of bandgap and carrier concentration. We use extended x-ray absorption fine structure to quantify short range order, and x-ray absorption near edge structure to quantify phase segregation of this new alloy. X-ray diffraction is used to elucidate the long range structural order. We study the structural evolution of the alloy as a function of annealing temperature and see continuous evolution towards c-CZTS phase that is nearly complete at 450$^{\circ}$C. Our results inform the fabrication of conventional c-CZTS solar cells by establishing the temperature range over which thin films transform from a kinetically stabilized, metastable phase to a thermodynamically stabilized, crystalline phase. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F24.00010: Defects in buffer layers: electrical and optical properties of point defects in CdS and ZnS Joel Varley, Vincenzo Lordi The rapid development of thin-film solar cells has largely focused on alternative absorber materials, while the choices for buffer layers remain somewhat limited. The most common buffer layer material is cadmium sulfide (CdS), which exhibits good electrical properties leads to a loss of solar photocurrent due to its band gap of 2.4 eV. Wider band gap alternatives with good electrical properties are desired, but the precise material characteristics dictating the electrical properties are not fully understood. Here we present a first principles study to benchmark the electrical and optical characteristics of intrinsic and common extrinsic point defects in CdS and ZnS, a larger band gap alternative buffer layer. We discuss the role of defects in carrier compensation and recombination events that strongly impact the buffer layer electrical properties in a thin-film solar cell and overall device performance. Correlation of defect properties with growth conditions is made in terms of film stoichiometry and presence of impurities. We also calculate the band alignments in a conventional Cu(In,Ga)Se$_2$ solar cell, showing why CdS performs well and why Zn(O,S) is a promising alternative buffer layer for high-efficiency devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F24.00011: Evidence of p- to n-type inversion at CIGS grain boundaries: A depth-dependent surface electron microscopy study Calvin Chan, Taisuke Ohta, Gary Kellogg, Lorelle Mansfield, Rommel Noufi Chalcopyrite Cu(In$_{1-x}$Ga$_x$)Se$_2$ (CIGS) is an interesting photovoltaic material because it holds the laboratory record for thin-film solar power conversion efficiency ($\eta > 20$\%) despite its disordered microcrystalline structure. However, commercialization of this technology has been limited by structural and chemical variations in CIGS films. Many microscopic and spectroscopic studies have shown built-in electric potentials ($\Phi_{\textrm{bi}}$) at CIGS grain boundaries. This may assist with electron-hole separation, but the reported magnitude and statistical distribution of $\Phi_{\textrm{bi}}$ remains inconsistent between studies. In this work, photoemission and low-energy electron microscopies (PEEM and LEEM) were used to reconcile these reported differences. Highly surface sensitive PEEM measurements showed $\Phi_{\textrm{bi}} \sim 0.5$ V, which was consistent with most other reports. However, more bulk sensitive LEEM measurements showed $\Phi_{\textrm{bi}} \sim 1.5$ V, which strongly suggests p- to n-type inversion at CIGS grain boundaries. This formation of pn junctions at CIGS grain boundaries is likely responsible for the high performance of CIGS photovoltaics. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F24.00012: Plasma and Thermal Assisted Selenization for Preparation of CuInGaSeS absorber film Zehra Cevher, Zhi Huang, Ren Yuhang Cu(In,Ga)Se semiconductor alloys have been the center of attention over the past decades to potentially replace high efficient silicon based~photovoltaic devices.~ In order to improve the conversion efficiency of CuInGaSe photovoltaic devices, the CuInGa precursor film along with a suitable selenization technique must be enhanced.~ We demonstrate the plasma assisted selenium cracking technique and thermal assisted selenium cracking by radio frequency plasma or increased temperature and the deposition of a selenium cap layer above CuInGa metallic precursors.~ The two stage plasma enhanced selenization process includes the modification of the ionization state of the Se species by radio frequency plasma and the deposition of a selenium cap layer above CuInGa precursors and thermal assited selenization includes cracking large selenium molecules without further modification during selenization.~ Improved homogeneity and crystallization is realized in both techniques~as opposed to conventional selenization procedure. The result is explained by the enhancement of reaction kinetics between the reduced Se phase and metallic precursor layers.~ We further demonstrate improvement in CIGS film morphology using sulfurization technique.~ [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F24.00013: Possible reasons for low open circuit voltage in pyrite (FeS$_2$) Predrag Lazic, Rickard Armiento, William Herbert, Rupak Chakraborty, Ruoshi Sun, Maria Chan, Katherine Hartman, Tonio Buonassisi, Bilge Yildiz, Gerbrand Ceder Pyrite (FeS$_2$), being a promising material for future solar technologies, has so far exhibited in experiments an open-circuit voltage (OCV) of around 0.2 V, which is much lower than the frequently quoted ``accepted'' value for the fundamental bandgap of 0.95 eV. Absorption experiments show large subgap absorption, commonly attributed to defects or structural disorder. However, computations using density functional theory with a semi-local functional predict that the bottom of the conduction band consists of a very low intensity sulfur p-band that may be easily overlooked in experiments. The intensity of absorption into the sulfur p-band is found to be of the same magnitude as contributions from defects and disorder. Our findings suggest the need to re-examine the value of the fundamental bandgap of pyrite presently in use in the literature. If the contribution from the p-band has so far been overlooked, the substantially lowered bandgap would partly explain the discrepancy with the OCV. Also, we show that more states appear on the surface within the low energy sulfur p-band, which suggests a mechanism of thermalization into those states that would further reducing the OCV. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F24.00014: Si3AlP: A New Promising Material for Solar Cell Absorber Jihui Yang, Yingteng Zhai, Hengrui Liu, Hongjun Xiang, Xingao Gong, Suhuai Wei First-principles calculations are performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si2)0.6(AlP)0.4 alloy [T. Watkins et al., J. Am. Chem. Soc. 2011, 133, 16212.] The most stable structure of Si3AlP is a superlattice along the \textless 111\textgreater direction with separated AlP and Si layers, which has a similar optical absorption spectrum to silicon. The ordered C1c1-Si3AlP is found to be the most stable one among all the structures with --AlPSi3- motifs, in agreement with the experimental suggestions. We predict that C1c1-Si3AlP has good optical properties, i.e., it has a larger fundamental band gap and a smaller direct band gap than Si, thus it has much higher absorption in the visible light region, making it a promising candidate for improving the performance of the existing Si-based solar cells. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F24.00015: Si-Based Earth Abundant Clathrates for Solar Energy Conversion Yuping He, Giulia Galli We show that recently synthesized Si-based clathrates[1], composed entirely of Earth abundant elements are promising materials for solar energy conversion. Using ab initio calculations we found that the type I clathrate K$_{8}$Al$_{8}$Si$_{38}$ exhibits a quasi-direct band gap of $\simeq$ 1 eV, which may be tuned to span the IR and visible range by strain engineering. We also found that electron and hole states generated by photon absorption are spatially separated on different cages in the material, with low probability of charge recombination. Finally, we computed electron and hole mobilities and obtained values much superior to those of amorphous silicon and approximately six and ten time smaller than those of crystalline silicon.\\[4pt] [1] Y. He, F. Sui, S. Kauzlarich and G. Galli (submitted) [Preview Abstract] |
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