Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (18 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2021)

This dissertation addresses several divides that must be bridged in materials science in order to transition to a renewable energy society. Despite our increasing demand for renewable energy materials over the past decade, as well as recent advances in photovoltaics (PV), solar energy still accounts for less than 1% of global energy generation. Additionally, real-world efficiencies of solar cells still lag behind theoretical limits, despite high-quality solar absorbers. These lags are in part from energetic losses due to suboptimal solar cell contacts; specifically, in contacts that have both p-type conductivity and optical transparency, which are the focus of this dissertation. A high performance p-type transparent conductor (TC) could enable advances in a wide range of PV applications, for example as a junction partner to new thin film absorbers, as part of a hole-selective top contact stack, as a semi-transparent hole-selective back contact, or as a window layer in tandem PV devices.

However, such a material with properties comparable to n-type transparent conducting oxides (TCOs) such as Sn-doped In2O3 (ITO) have not yet been found, and nearly all TCs used in industry are still n-type. The contradictory properties of transparency and conductivity are already difficult to achieve, but p-type TCs suffer from two additional physical challenges: (1) localized valence bands and (2) doping difficulties in wide-gap oxides. Although TCs are conventionally oxides, chalcogenide (S, Se, Te), pnictide (N, P, As), and mixed anion semiconductors show promise of higher hole transport and greater p-type doping propensity. High-throughput computational materials screenings offer a pathway to discover such materials, but so far no computationally predicted p-type TC has high enough performance for solar devices, which can be attributed to a disconnect between theory and experiment. How can we transform computational design principles to yield experimentally-realizable p-type TC materials?

This dissertation research combines high-throughput computation, combinatorial experimental methodologies, and PV device integration to discover and optimize new p-type TCs. My work addresses various aspects of this combined "p-type TC materials discovery pipeline," depicted graphically here and introduced in Chapter 1. Chapter 2 reviews the theoretical background and describes the computational and experimental methods that are used in conjunction throughout my research. In Chapter 3, before applying these methods to search for new p-type TCs, I define my search space in the context of computational material databases, as well as strategies to predict new materials not present in these databases. I also introduce and explore a database of new potential semiconductor alloys.

The first phase of my research focuses on computational approaches to materials discovery. In Chapter 4, I define and evaluate descriptors currently used in computational screenings, which I show to have a remarkably high degree of false negatives and positives. Subsequently, I introduce new experimentally-guided metrics to assess high performance such as widening of a transparency window due to forbidden optical transitions. By performing various tiered computational searches in Chapter 5, I identify a set of promising p-type TC compounds such as BaSnS2, ZnZrN2, and BeSiP2 that were not previously grown as thin films, and CaMg2P2 and spinel MgAl2S4 which are not yet in databases and have not yet been synthesized. Screening for alloy endpoints also reveals a variety of new candidates to examine.

The second phase of this dissertation involves synthesizing these predicted semiconductors as thin films. My approach uses combinatorial sputter deposition, which allows for varying stoichiometry, synthesis temperature, and thickness within a single sample in order to probe a wide array of phase space. I then perform a variety of structural, optical, and electronic characterization, first using combinatorial characterization and then zooming into a region of interest using more targeted analysis techniques. In Chapter 6, I introduce this methodology and focus on a few ternary case studies: CuxZn1-xS, BaSnS2, and ZnZrN2. I discuss some successes, such as an unexpected phase change and a high p-type TC figure of merit in CuxZn1-xS, and I also discuss challenges encountered in the laboratory such as difficulty crystallizing targeted ZnZrN2 and BaSnS2. I also investigate how to assess whether a predicted material is actually synthesizable as a thin film, using a descriptor-based approach I develope based on "disorder tolerance" in Chapter 8.

The next step after synthesis and characterization is incorporating these new p-type TC materials into solar cell devices. I fabricate both silicon heterojunction (SHJ) and cadmium telluride (CdTe) solar cells, each using a novel p-type contact, to explore the challenges of bringing designed materials to actual highly-optimized device stacks; these two case studies are discussed in Chapter 7. This understanding, alongside simulations I perform, then leads to the development of new computational descriptors and screening criteria, discussed in Chapter 7 and Chapter 4. Thus, each phase of this process continues to inform subsequent phases. With this combined theoretical and experimental framework, I have discovered new p-type TC materials and have developed infrastructure and insight to inform future strategies for discovery of new solar cell materials.

I summarize my findings and lessons learned in Chapter 9, and conclude in Chapter 10 by discussing some of the environmental and human impacts of materials research. Renewable materials can lead to positive impacts, such as increased PV efficiency enabled by discovery of a high-performing p-type TC, but also materials can and have caused harm, and have led to climate catastrophe. I first propose a thought experiment to estimate the carbon cost of delaying PV deployment until solar cells are more efficient and show that our research cannot come at the cost of delaying PV installations. I also estimate my CO2 footprint during my PhD, demonstrating a significantly higher footprint than the average American and highlighting a specific cost of doing research. Finally, I reflect upon the ethical responsibility of scientists in the modern world: we have to change the way we do science to truly work towards a more equitable and sustainable future.

    Cover page: Bridging the Computational-Experimental Divide to Design Transparent Contact Materials for Solar Energy
    • Article
    • Peer Reviewed
    LBL Publications (2021)
    With the HTEM, an open online database containing experimental synthesis and characterization data of thin film inorganic materials, Talley et al. (2021) lay a foundation for a new era of high-throughput materials design.
      Cover page: Addressing the critical need for open experimental databases in materials science
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2021)
      Silicon heterojunction (SHJ) solar cell efficiencies are limited by parasitic absorption from the hydrogenated amorphous silicon (a-Si:H) front contact, but this may be mitigated by selecting an alternative carrier selective contact material with a wider band gap. When choosing such a material as the hole-selective contact ('p-layer'), the alignment of the material's valence band edge energy ( boldsymbol{E{text{VB}} ) with that of crystalline silicon (c-Si) is an important criterion, but several other material parameters can also influence the band bending at the contact interface. In this article, we simulate an (n)c-Si/(i)a-Si:H/p-layer interface to explore the influence of six materials parameters in a variable p-layer on the SHJ performance. We find a strong influence on the fill factor (FF) from thickness, doping, and boldsymbol{E{text{VB}} , and on V{text{OC} from the interfacial defect density; notably, optimal boldsymbol{E{text{VB}} is sim 0.1 eV higher than the valence band edge energy of a-Si:H. Multiparameter sensitivity analyses demonstrate how performance is simultaneously influenced by boldsymbol{E{text{VB} and doping; thus, both parameters should be optimized alongside one another. To assess the influence of these parameters experimentally, we grow p-type text{NiO}{x} as a test-case p-layer, which shows that FFs decrease with the oxygen content likely from the increased misalignment of boldsymbol{E{text{VB}} . Although modest efficiencies are achieved experimentally (>7%), what is important is that our model simulates performance trends. With these results, we apply a materials discovery pipeline to suggest new materials (e.g., ZnTe and BeTe) to try as p-layers in the SHJ. This combination of simulations, experiments, and materials discovery informs a better understanding of contact selection in SHJ cells.
        Cover page: Evaluating Materials Design Parameters of Hole-Selective Contacts for Silicon Heterojunction Solar Cells
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2017)
        With the development of new generations of optoelectronic devices that combine high performance and novel functionalities (e.g., flexibility/bendability, adaptability, semi or full transparency), several classes of transparent electrodes have been developed in recent years. These range from optimized transparent conductive oxides (TCOs), which are historically the most commonly used transparent electrodes, to new electrodes made from nano- and 2D materials (e.g., metal nanowire networks and graphene), and to hybrid electrodes that integrate TCOs or dielectrics with nanowires, metal grids, or ultrathin metal films. Here, the most relevant transparent electrodes developed to date are introduced, their fundamental properties are described, and their materials are classified according to specific application requirements in high efficiency solar cells and flexible organic light-emitting diodes (OLEDs). This information serves as a guideline for selecting and developing appropriate transparent electrodes according to intended application requirements and functionality.
          Cover page: Transparent Electrodes for Efficient Optoelectronics
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Wide band gap semiconductors are essential for today's electronic devices and energy applications because of their high optical transparency, controllable carrier concentration, and tunable electrical conductivity. The most intensively investigated wide band gap semiconductors are transparent conductive oxides (TCOs), such as tin-doped indium oxide (ITO) and amorphous In-Ga-Zn-O (IGZO), used in displays and solar cells, carbides (e.g., SiC) and nitrides (e.g., GaN) used in power electronics, and emerging halides (e.g., γ-CuI) and 2D electronic materials (e.g., graphene) used in various optoelectronic devices. Compared to these prominent materials families, chalcogen-based (Ch = S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out because of their propensity for p-type doping, high mobilities, high valence band positions (i.e., low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent semiconductors, as well as the theoretical and experimental underpinnings of the corresponding research methods. We proceed to summarize progress in wide band gap (EG > 2 eV) chalcogenide materials-namely, II-VI MCh binaries, CuMCh2 chalcopyrites, Cu3MCh4 sulvanites, mixed-anion layered CuMCh(O,F), and 2D materials-and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications-for example, photovoltaic and photoelectrochemical solar cells, transistors, and light emitting diodes-that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this Review aims to inspire continued research on this emerging class of transparent semiconductors and thereby enable future innovations for optoelectronic devices.
            Cover page: Wide Band Gap Chalcogenide Semiconductors
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            The original version of the article contained a number of problems related to references that may lead to error propagation in the scientific literature. We would like to correct these issues, and sincerely apologize to the readers and reference authors for any confusion they might have caused. 1. ref 120 should be replaced with ref 146 to indicate ZnSe band gap, in section 2.2.2 2. ref 132 should be replaced with ref 131 to indicate ZnS lattice matching to other materials, in section 3.1.1 3. ref 136 claims electrical conductivity of ∼10−5 S cm−1 for 3% Cu doped ZnS rather than for 0.1% Cu doped ZnS, in section 3.1.1 4. ref 148 claims conductivity of 0.06 S cm−1 and mobility of 86 cm2 V−1 s−1 instead of 0.02 S cm−1 and mobility of 13 cm2 V−1 s−1, respectively, for NSe doped ZnSe in section 3.1.1 (also in Table 1) 5. ref 163 and ref 67 should be cited for ∼3.5 and ∼3.0 eV band gaps of ZB and WZ MgTe respectively, in Table 1 6. ref 163, ref 225, and ref 226 to support data of intrinsic In2S3 in addition to ref 227, in Table 1 7. ref 171 claims p-type conduction due to Mn vacancy in RS instead of WZ phase of MnS, in Table 1 8. ref 188 claims conductivities up to ∼1550 S cm−1 instead of 50 S cm−1 in In-doped CdS (also in Table 1), and ref 187 is to In doping in CdTe, in section 3.1.4 9. ref 249 should read “Bhandari, R. K.; Hashimoto, Y.; Ito, K. CuAlS2 Thin-Films Prepared by Sulfurization of Metallic Precursors and their Properties. Jpn. J. Appl. Phys., 2004, 43 (10), 6890−6893”, in section 3.3.1. 10. ref 268 should be added to GeGa and SnGa defects in its following sentence, and TiGa defect should be referred to “Palacios, P.; Aguilera, I.; Wahnon, P.; Conesa J. C. Thermodynamics of the Formation of Ti- and Cr-doped CuGaS2 Intermediate-band Photovoltaic Materials. J. Phys. Chem. C 2008, 112 (25), 9525−9529”, in section 3.3.1 11. ref 276 claims 2.2 to 2.5 eV instead of 2.5 to 2.7 eV, as experimental gap of AgAlSe2, in section 3.3.2 12. ref 305 and ref 306 should be swapped; ref 305 should refer to ionic conductivity, and ref 306 should refer to the electronic structure of Cu3TaS4, in section 3.4.2 13. ref 308 and ref 309 should be swapped, where ref 309 should refer to synthesis and ref 308 should refer to band gap of Cu3NbSe4, respectively, in section 3.4.2 14. ref 325 claims 2.6 eV instead of 2.2−2.6 eV as the gap for K2Hg3S4, in section 3.4.5 15. ref 327 should be added to ref 326 for the band gap of Cu3SbS3, and the band gaps of Li3SbS3, Na3SbS3, and Ca2Sb2S5 should refer to “Bhattacharya, S.; Chmielowski, R.; Dennler, G.; Madsen, G. K. H. Novel ternary sulfide thermoelectric materials from high throughput transport and defect calculations. J. Mater. Chem. A 2016, 4 (28), 11086−11093”, in section 3.4.6 16. ref 345 claims the field-effect mobility of CuSCN to be 0.01−0.1 cm2 V−1 s−1 instead of 0.001−0.1 cm2 V−1 s−1, in section 3.5.3 17. ref 360 is to band gaps of CsYZnSe3, CsSmZnSe3, and CsErZnSe3, and ref 357 is with respect to band gaps of BaYCuS3, BaNdCuS3, and BaNdAgS3, in section 3.5.6 18. ref 424 refers to the previous sentence about bulk As2S3 material; instead the sentence about As2S3 monolayer, indirect gap, strain, and thickness should refer to “Debbichi, L.; Kim, H.; Björkman, T.; Eriksson, O.; Lebegue S. First-principles investigation of two-dimensional trichalcogenide and sesquichalcogenide monolayers. Phys. Rev. B 2016, 93, 245307” and “Miao, N.; Zhou, J.; Sa, B.; Xu, B.; Sun, Z. Few-layer arsenic trichalcogenides: Emerging two-dimensional semiconductors with tunable indirect−direct band-gaps. J. Alloys Compd. 2017, 699, 554−560”, in section 3.6.2 19. ref 478 and ref 479 should be swapped, where ref 478 refers to theoretical predictions for CdSe, ZnS, ZnSe, and ZnTe, and ref 479 refers to experimental observations of Ga in CuInSe2, in section 4.1.2 20. ref 504 and ref 277 discuss AgAlTe2 and AgAlSe2 instead of AgGaTe2 and AgGaSe2, respectively, and CuAlSe2 instead of CuAlS2 should be referenced to ref 269 in the same sentence, in section 4.1.3.
              Cover page: Correction to Wide Band Gap Chalcogenide Semiconductors
              • Article
              • Peer Reviewed
              LBL Publications (2021)
              The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH3NH3PbI3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment (rs = 0.93) improves significantly on results obtained using a constant relaxation time approximation (rs = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.
                Cover page: Efficient calculation of carrier scattering rates from first principles
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2018)
                The growth of materials databases has yielded significant quantities of data to mine for new energy materials using high-throughput screening methodologies. One application of interest to energy and optoelectronics is the prediction of new high performing p-type transparent conductors (TCs). However, screening methods for such materials have never been validated over the breadth of computed materials properties. In this study, we compile an experimental data set of 74 bulk crystal structures corresponding to known state-of-the-art n-type and p-type TCs and compute a series of corresponding computational descriptor properties. Our goals are to (1) compare computational descriptors to experimentally demonstrated properties of real materials in the data set, (2) determine the ability of ground state, density functional theory (DFT)-based computational screening methodologies to identify these experimentally realized TCs, and (3) use this understanding to estimate reasonable screening cutoffs for four commonly used descriptors. First, stability calculations demonstrate that most materials in the data set have an energy above the convex hull (Ehull) of <100 meV/atom, and we also propose a Pourbaix analysis technique to estimate moisture stability. Second, we find a lenient cutoff for the DFT PBE band gap of 0.5 eV is sufficient to include a majority of the wide gap candidates and exclude narrow gap compounds. Next, the effective mass, m, is found to correlate weakly to conductivity in the p-type materials as compared with n-type materials, which may motivate an increase in the m∗ cutoff as well. Lastly, we perform an uncertainty analysis and literature comparison for the branch point energy (BPE), a qualitative descriptor for dopability. We find the BPEs of most n-type materials to lie near the conduction band and those of most p-type materials to lie at midgap; this is a significant distinction, suggesting BPE to be a more definitive descriptor for n-type TC materials. By assessing the validity of this simple screening methodology via comparing experimental data to computational descriptors, we aim to motivate and strengthen future materials discovery efforts in the field of transparent conductors and beyond.
                  Cover page: Assessing High-Throughput Descriptors for Prediction of Transparent Conductors
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2022)
                  In materials science, it is often assumed that ground-state crystal structures predicted by density functional theory are the easiest polymorphs to synthesize. Ternary nitride materials, with many possible metastable polymorphs, provide a rich materials space to study what influences thermodynamic stability and polymorph synthesizability. For example, ZnZrN2 is theoretically predicted at zero Kelvin to have an unusual layered "wurtsalt"ground-state crystal structure with compelling optoelectronic properties, but it is unknown whether this structure can be realized experimentally under practical synthesis conditions. Here, we use combinatorial sputtering to synthesize hundreds of ZnxZr1-xNy thin-film samples, and find metastable rocksalt-derived or boron-nitride-derived structures rather than the predicted wurtsalt structure. Using a statistical polymorph sampler approach, it is demonstrated that although rocksalt is the least stable polymorph at zero Kelvin, it becomes the most stable polymorph at high effective temperatures similar to those achieved using this sputter deposition method, and thus corroborates experimental results. Additional calculations show that this destabilization of the wurtsalt polymorph is due to configurational entropic and enthalpic effects, and that vibrational contributions are negligible. Specifically, rocksalt- and boron-nitride-derived structures become the most stable polymorphs in the presence of disorder because of higher tolerances to cation cross substitution and off stoichiometry than the wurtsalt structure. This understanding of the role of disorder tolerance in the synthesis of competing polymorphs can enable more accurate predictions of synthesizable crystal structures and their achievable material properties.
                    Cover page: Role of disorder in the synthesis of metastable zinc zirconium nitridesCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2017)
                    P-Type transparent conducting Cu alloyed ZnS thin films from CuZnS targets (, 0.2, 0.3, 0.4, and 0.5) were deposited on glass substrates via radio frequency sputtering. X-ray diffraction and TEM-SAED analysis show that all the films have sphalerite ZnS as the majority crystalline phase. In addition, films with 30% and 40% Cu show the presence of increasing amounts of crystalline Cu2S phase. Conductivity values 400 S were obtained for the films having 30% and 40% Cu, with the maximum conductivity of 752 S obtained for the film with 40% Cu. Temperature dependent electrical transport measurements indicate metallic as well as degenerate hole conductivity in the deposited films. The reflection-corrected transmittance of this Cu alloyed ZnS (40% Cu) film was determined to be 75% at 550 nm. The transparent conductor figure of merit () of the Cu alloyed ZnS (40% Cu), calculated with the average value of transmittance between 1.5 to 2.5 eV, was ≈276 .
                      Cover page: High figure-of-merit p-type transparent conductor, Cu alloyed ZnS via radio frequency magnetron sputtering
                      Top