PacSurf2014 Session EH-TuM: Solar Cells

Tuesday, December 9, 2014 8:40 AM in Room Lehua

Tuesday Morning

Time Period TuM Sessions | Abstract Timeline | Topic EH Sessions | Time Periods | Topics | PacSurf2014 Schedule

Start Invited? Item
8:40 AM EH-TuM-3 Understanding and Improving Solar Energy Conversion through Interface Engineering
Stacey Bent (Stanford University, USA)
Meeting the world’s growing energy needs in a sustainable fashion is one of the most pressing problems of our time. The most abundant source of renewable energy is the sun, which can be converted directly to useful forms of energy such as electricity and fuels by photovoltaics and photoelectrocatalysts, respectively. In this talk, we will discuss the use of nanoscale materials in solar energy conversion, and in particular the role of interface engineering in improving conversion efficiencies. Quantum dots (QDs), which are nanoscale materials typically based on low-band-gap metal chalcogenides, have been widely explored for next generation solar absorbers due to their tunable band gap and high absorption coefficient. Colloidal quantum dot (CQD) solar cells made from lead sulfide (PbS) QDs have achieved a power efficiency of ~ 8%. In these devices, it is important to control the band gap as well the band position of the QDs to efficiently inject electrons and holes into their respective electrodes. We will describe experimental and theoretical studies of the effects of interface engineering through surface ligand modification on the band gap and relative band positions in lead chalcogenide (PbSexS1-x) QDs. Multilayer CQD solar cells were fabricated to investigate the effect on carrier collection of QD layers with different relative band positions. We will show that interface engineering can be applied to lead chalcogenide QDs in order to create a favorable band diagram and achieve enhanced photogenerated carrier collection in multilayer CQD devices.
9:20 AM EH-TuM-5 Understanding Carrier Dynamics in Cu2ZnSn(S,Se)4 Using Time-Resolved Terahertz Spectroscopy
Glenn Guglietta (Drexel University, USA); Kaushik Roy Choudhury, Jonathan Caspar (DuPont Central Research and Development, USA); Jason Baxter (Drexel University, USA)

We have used time-resolved terahertz spectroscopy (TRTS) to measure lifetimes and determine recombination mechanisms in Cu2ZnSn(S,Se)4 (CZTSSe) thin films fabricated from nanocrystal inks. TRTS probes photoconductivity on femtosecond to nanosecond time scales that are relevant for recombination in thin film photovoltaics. Terahertz frequencies (0.2-2.5 THz) correspond to typical scattering rates in semiconductors, enabling determination of carrier density and mobility. Ultrafast time resolution permits tracking the evolution of carrier density to determine recombination mechanisms. By manipulating the photoexcitation wavelength and fluence, we can tailor the generation profile of photoexcited carriers to distinguish between surface, Shockley-Read-Hall (SRH), and Auger recombination mechanisms and determine rate constants.

TRTS experiments and modeling were used to understand the role of recombination mechanisms and their contribution to CZTSSe photovoltaic performance. TRTS photoconductivity shows an instrument-limited onset within 1 ps of an ultrafast pump pulse, followed by a slow decay over nanoseconds. Photoconductivity decay kinetics were fit with a bi-exponential model with two time constants and a weight fraction. The short time constant is typically ~200 ps and roughly corresponds to diffusion to and recombination at the surface. The long time constant is typically ~2 ns and is attributed to SRH recombination. Assignment of these mechanisms is supported by the dependence of kinetics upon excitation fluence and wavelength. Normalized kinetics are independent of fluence over a range of 40x, indicating that no Auger recombination is occurring. Without Auger recombination, we can distinguish between surface and SRH rates by tuning the pump wavelength. As the excitation wavelength is shifted towards the blue, carriers are generated nearer to the front surface and the photoconductivity kinetics are sensitive to the surface recombination velocity. With blue excitation, we see that a larger fraction, ~0.5, of carriers recombine with a short time constant. With redder excitation wavelengths, the carriers are generated more evenly throughout the film and the kinetics are dominated by SRH recombination with the long time constant having a majority of the weight fraction, ~0.8. TRTS provides a pathway to determine performance-limiting recombination mechanisms and measure key parameters like SRH lifetime and surface recombination velocity, helping to direct the design of efficient thin film photovoltaics.

9:40 AM EH-TuM-6 The CdS/CdTe Solar Cell with the Back Contact Protected by ITO/Mo
Juan Peña, Víctor Rejón (Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Mexico); Ines Riech (Universidad Autónoma de Yucatán, Mexico); Eric Hernández-Rodríguez (Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Mexico)

Today CdS/CdTe heterojunction is an important photovoltaic system for conversion of solar energy. Due to the simple techniques used for thin films deposition, it has been brought from laboratory to industrial scale. Although the thin films are deposited by using techniques such as sputtering and CSS, the back contact layers are a challenge in both: the formation of an ohmic contact and that endures a long time. There are several prospects for back contacts using Mo as can be seen in [1]. The solar cell with Cu/Mo as a back contact deposited by using rf-sputtering is reported in [2]. There are few researches that studied how Mo film can be protected against oxidation without adding series resistance. The bilayer Mo/ITO has been used in microelectronics devices [3].

In this work, the CdS/CdTe solar cells that uses Cu/Mo as back contact is investigated. It is shown that the formation of MoOx at surface of Mo thin film influences the series resistance. The oxide is formed when the solar cell is used at outdoor conditions without any encapsulation. Here some indoor experiments were made. First, we show how the Mo/Glass film is oxidized when it is annealed at 400 °C in air. Second, how the Mo is degraded at same conditions when it is used on CdTe/CdS cell and annealed at same conditions. Third, how ITO works when it is deposited over of Mo on the solar cell. Preliminary results indicates that the ITO avoids the formation of MoOx and mechanical scratching , preserves its electrical properties and the solar cell shows good stability after an annealing at 400 °C.

The films Cu, Mo and ITO were deposited by rf-sputtering. We show evidences that the p-n junction remains working properly and the oxidation of Mo is the main cause of the cell efficiency degradation. By using the thin film of ITO on Mo layer all solar cell characteristics are preserved. The CdTe film was grown by conventional CSS technique. The cells were activated by using CHClF2-argon-oxygen gases. DRX spectra and HR-SEM were made for Mo/Glass and ITO/Mo/CdTe structure before and after annealing at 400 °C in air.


This work has been supported by CONACYT-México under contract FORDECYT-116157, FOMIX-169739 and FOMIX-172298, CeMIE-Sol PY-207450/25. Measurements were performed at LANNBIO CINVESTAV-Mérida, under support from projects FOMIX-Yucatán 2008-108160, CONACYT LAB-2009-01 No. 123913 and CB2012/ 178947.


[1] D. L. Batzner, et al. Thin Solid Films, vol. 451-452, pp. 536-543, 2004.

[2] V. Rejón, et al. Solar Energy, vol. 95, pp. 319-324, 2013.

[3] H. Piao, et al. Surface and Interface Analysis, vol. 39, pp. 493-500, 2007.

10:00 AM BREAK
10:20 AM EH-TuM-8 Study of Energy Level Alignment at Electrode Interfaces in Organic Solar Cells
Takeaki Sakurai, Shenghao Wang, Tetsuya Miyazawa, Hao Xia, Wei Fu, Katsuhiro Akimoto (University of Tsukuba, Japan)

Control of electrical properties at organic semiconductor/metal electrode interfaces is one of the significant issues for improving performances of the organic solar cells since it has an influence on carrier extraction and exciton quenching. In small molecule based organic solar cells, bathocuproine (BCP) is used well as a buffer layer between C60 and metal cathode to improve the device efficiency. Lifetime and the stability of the device with BCP buffer layer, however, are not good for the practical application. To obtain the strategies for the development of the effective buffer layer, we have investigated the electronic structures at the interfaces between C60 and a large variety of organic semiconductors by means of synchrotron based in-situ ultraviolet photoelectron spectroscopy. The C60/buffer/Ag heterostructures were formed by depositing buffer materials on Ag and subsequently depositing C60 onto buffer/Ag stack layer in a step-by-step way in a vacuum deposition chamber. A series of perylene derivatives (PTCDA, PTCDI and PTCBI), TCNQ derivatives (TCNQ and F4TCNQ) and pyridine based acceptor molecules (BCP, TPBi and TAZ) were applied as buffer layers. For all buffer/Ag stack structures, the LUMO level of buffer layers almost accords with Fermi level of Ag, that is, electron is easily transferred from the Ag electrode to the buffer layers due to the disappearance of the electron injection barrier. In contrast, the electron injection barrier height between LUMO of C60 and LUMO of buffer materials correlates with the work function of the buffer/Ag stack structures (Φbuff). We conclude that to select a buffer/Ag heterostructure with low work function is important in order to maintain good electric contact near Ag cathode. The detailed physical mechanism about the energy level alignment will be discussed using charge transfer model.

11:00 AM EH-TuM-10 n-type Thin Film WSe2 for use in Homojunction WSe2 Solar Cells
Amithaba Sarkar, Natale Ianno, Rodney Soukup (University of Nebraska-Lincoln)

Previously we have shown that as-grown p-type thin film WSe2 is an excellent candidate for an earth abundant photovoltaic absorber.1 The vast majority of thin solar cells are heterojunction devices with a wide band gap n-type window layer. Many of the device issues are centered on the heterojunction interface, making a homojunction more desirable. The problem here is growing n-type material in thin film systems. In this work we present the structural, optical, and electrical properties of n-type thin film WSe2 grown via the selenization of sputter deposited copper doped tungsten films. We will show that highly textured films with an optical band gap in range of 1.45 eV, and absorption coefficients greater than 105/cm across the visible spectrum can be easily achieved. In addition we will present Hall Effect and carrier density measurements as a measure of film quality. We employ these results to numerically simulate homojunction solar cells based on this material, where we will show efficiencies greater than 20% are possible.

1. Q. Ma, H. Kyureghian, J. D. Banninga and N. J. Ianno, MRS Proceedings , Volume 1670 , 2014. DOI:

11:20 AM EH-TuM-11 Samarium Sulfide-Cerium Sulfide, A Potential Band Gap Variable Alloy System for Use in Spectrum Splitting Photovoltaic Systems
Rodney Soukup, Natale Ianno, Fernando Urias-Cordero, Casey Berger, Amithaba Sarkar, Matthew Hilfiker (University of Nebraska-Lincoln)

A new research thrust to obtaining high efficiency photovoltaic systems is spectrum splitting, where the input solar radiation is physically separated via input optics and channeled to a planar array of individual cells, each with a band gap tuned to absorb a specific band width of radiation. This approach allows for simpler device fabrication and a wider breadth of band gaps than the common multi-junction design, in exchange for a complex optics structure to separate the incoming light. Both these approaches require high efficiency cells over a broad range of band gaps. Currently this is met by III-V alloy singe crystal cells and while significant advances have been made in reducing the fabrication cost, recent best estimates place their cost at 1-5x single crystal silicon cells. In addition many of these materials contain indium, whose supply may be limited.

In order for the multi-junction and split spectrum designs to fully realize their potential, it is necessary to explore other classes of semiconducting materials with tunable band gap, better crustal abundance and lower production costs. The Sm2-xCexS3 alloy system with its potentially broad band gap range (0.8-1.9 eV), as grown p-type majority carrier, potentially excellent optoelectronic properties in polycrystalline form and abundance is an excellent candidate. We will present preliminary data on the growth, optical and electrical properties of thin film Sm2S3, and Ce2S3 as an initial study of the alloy formation.

11:40 AM EH-TuM-12 Nano-patterned Pt Counter electrode Enhancing Light harvesting for Dye-Sensitized Solar Cells
Won Seok Chi, Dong Jun Kim, Jung Pyo Jung, Jong Hak Kim (Yonsei University, Korea)
Nano-patterning is one of the most simple and effective techniques for efficiently light trapping to boost the energy conversion efficiency. By imprinting with poly(dimethyl siloxane) (PDMS) nanostamp cross-rotate two steps onto thin layered commercial TiO2 paste, a mesh-shaped (200 nm × 200nm) TiO2 scaffold electrode with a large area was fabricated. Platinum (Pt) nanoparticles deposited onto TiO2 pattern by diverse method such as sputtering, thermal deposition and UV-radiation deposition. The Pt nanoparticles distribution as mesh-shaped structure onto counter electrode was characterized by a field emission scanning electron microscope (FE-SEM) and an atomic force microscope (AFM). Moreover, the light trapping ability was measured reflectance by comparison of mesh-shaped patterned and non-patterned counter electrode as a function of angle. For the four cases : 1) patterned Pt counter electrode by sputtering, 2) non-patterned Pt counter electrode by sputtering, 3) non-patterned Pt counter electrode by thermal deposition and 4) patterned counter electrode by UV-radiation deposition were fabricated for all-solid-state dye-sensitized solar cells (ssDSSCs). Among them, patterned Pt counter electrode by sputtering assembled solar cell showed highest solar energy conversion efficiency up to 7.0%. This remarkable result, observed for N719 dye based DSSC, was due to enhanced light harvesting and superior surface area confirmed by incident photon-to current efficiency (IPCE), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) measurements. More importantly, we believe this approach is universally applicable to a variety of electrochemical cells requiring Pt catalysts such as fuel cells and catalytic chemical reactors. Furthermore, this process could include nano- and micro-sized patterns and can be extended to metal nanoparticles other than Pt.
Time Period TuM Sessions | Abstract Timeline | Topic EH Sessions | Time Periods | Topics | PacSurf2014 Schedule