AVS2010 Session EN+TF-TuA: Thin Films for Photovoltaics

Tuesday, October 19, 2010 2:00 PM in Room Pecos
Tuesday Afternoon

Time Period TuA Sessions | Abstract Timeline | Topic EN Sessions | Time Periods | Topics | AVS2010 Schedule

Start Invited? Item
2:00 PM EN+TF-TuA-1 Ar/H2 Plasma Treatment of a-Si:H Thin Films: On the Role of Atomic Hydrogen in µc-Si:H Thin-Film Deposition
Aafke Bronneberg, Adriana Creatore, M.C.M. van de Sanden (Eindhoven University of Technology, the Netherlands)

The transition from amorphous (a-Si:H) to microcrystalline (µc-Si:H) silicon film growth has been ascribed to the interaction of atomic hydrogen with the (sub)surface of the growing film [1]. To gain more insight into the formation of microcrystalline silicon and to the role of atomic hydrogen, several studies have been dedicated to hydrogen treatment of a-Si:H films [2,3,4,5]. However, the observed film crystallization is often wrongly ascribed to the impinging hydrogen atoms. What tended to be overlooked, is that the counter electrode is covered with an a-Si:H film during the deposition step. Interaction of the H2 plasma with the coated electrode will result in formation of molecules and radicals originating from silane [4]. Hence, the resulting plasma conditions are similar to those used for µc-Si:H, which explains the observed crystal formation.

Here, we show that this is not only true for direct plasmas, but that also in remote plasmas the reactor wall has big influences on the growth process. By using in situ spectroscopic ellipsometry and ex situ Raman spectroscopy, we show that H2 plasma treatment of a-Si:H only results in the formation of crystals when there is a source of silicon-based molecules and radicals. Together with a plasma study, comprising mass spectrometry and ion probe measurements, we address the origin and kinetics of the crystal formation and discuss the implications for µc-Si:H growth.

1. A. Matsuda, Journal of Non-Crystalline Solids 338, 1-12 (2004).

2. A. F. I. Morral, J. Bertomeu, and P. R. I. Cabarrocas, Materials Science and Engineering B-Solid State Materials for Advanced Technology 69, 559-563 (2000).

3. A. F. I. Morral and P. R. I. Cabarrocas, Journal of Non-Crystalline Solids 299, 196-200 (2002).

4. K. Saitoh, M. Kondo, M. Fukawa, T. Nishimiya, A. Matsuda, W. Futako, and I. Shimizu, Applied Physics Letters 71, 3403-3405 (1997).

5. G. Dingemans, M. N. van den Donker, A. Gordijn, W. M. M. Kessels, and M. C. M. van de Sanden, Applied Physics Letters 91, (2007).
2:20 PM EN+TF-TuA-2 Spectroscopic Analysis of the Role of Hydrogen in Amorphous Silicon
Philipp Schäfer, Frank Nobis, Ovidiu Gordan, Hartmut Kupfer, Frank Richter, Dietrich Zahn (Chemnitz University of Technology, Germany)

Amorphous hydrogenated silicon (a-Si:H) is widely used in photovoltaic applications. The high absorption renders a-Si:H technically relevant especially for thin film solar cells. Despite lower efficiency, an amorphous silicon solar panel possesses the advantage of higher absorption rate and easier processing at lower production cost.

Here the focus lies on highly (p and n) doped amorphous silicon films. The samples are prepared using d.c.-pulsed magnetron sputtering of crystalline silicon targets. A controlled hydrogen flow is added to the sputtering plasma. Hydrogen in amorphous silicon is known to saturate dangling bonds and improves the short range atomic order [1]. To probe the influence of hydrogen in the sputtering process various spectroscopic techniques were applied for sample characterisation.

Raman spectroscopy is a technique sensitive to the morphological aspects of the film. The relaxation of quasi-momentum conservation in amorphous films results in drastically different spectra of amorphous and crystalline silicon. A broad band at ~485 cm-1 appears instead of the sharp crystalline phonon feature at 520 cm-1. Its shape and asymmetry unveils further information on the short range order like the average dispersion angle from tetrahedral conformation. With the help of Fourier transformed transmission infrared spectroscopy the concentration of hydrogen in the sample is studied. Vibrational hydrogen-silicon stretching modes in the region around 2000 cm-1 are therefore assessed by a modified [2] Brodsky-Cardona-Cuomo approach [3]. Access to the optical constants n and k and therefore the complex dielectric function ε of the sputtered material is granted by variable angle spectroscopic ellipsometry. Thereby important parameters like the Tauc-Lorentz band gap which is mainly determined by interband gap defects are revealed. The combination of these spectroscopic techniques provides a detailed picture of morphological, electrical, and optical parameters of the system. An in depth discussion of the degree of structural improvement, the decrease of interband gap defects, the saturation of hydrogen content, and evolution of optical properties in correlation with the hydrogen flow will be presented.


[1] R. A. Street, “Hydrogenated Amorphous Silicon”, chapter 2.3, Cambridge University Press.

[2] A. A. Langford, M. L. Fleet, B. P. Nelson, W. A. Lanford, N. Maley: Phys. Rev. B 45 (1992) 13367.

[3] M. H. Brodsky, M. Cardona, J. J. Cuomo: Phys. Rev. B 16 (1977) 3556.

2:40 PM Invited EN+TF-TuA-3 Improved Efficiency and Air Stability of Hybrid Thin Film Solar Cells with a ZnO Nanoparticle Layer
Paul H. Holloway, Lei Qian, Jihua Yang, Renjia Zhou, Aiwei Tang, Ying Zheng, Jiangeng Xue (University of Florida)

Hybrid solar cells with active and transport layers based on conjugated polymers and/or inorganic semiconductor nanoparticles are an alternative to all-organic or all-inorganic solar cells. In hybrid cells, inorganic nanoparticles complement the absorption of the organic phase and provide better charge transport properties due to higher carrier mobility, while still maintaining the ability to solution-process. These properties will be illustrated first in hybrid solar cells with a mixed active layer based on poly(3-hexyl thiophene) (P3HT) and colloidal CdSe nanospheres, and with a ZnO nanoparticle buffer layer. The CdSe and ZnO nanoparticles were synthesized using a micelle and a sol-gel method, respectively. Both the active and buffer layers were spin-coated from solution onto a poly(3,4-ethylene dioxythiophene) doped with polystyrenesulfonic acid (PEDOT:PSS) layer on an ITO/glass substrate, and finished by deposition of the Al cathode. Compared to control devices without the ZnO layer, devices with the layer showed only slight changes in open-circuit voltage and fill factor, but showed 40-70% higher short-circuit current density, depending on the size of the CdSe nanospheres. ZnO-containing devices showed a maximum power conversion efficiency of 2.5-2.8%, compared to approximately 1.6-1.9% for the best P3HT/CdSe nanosphere devices without the ZnO layer. Using a ZnO layer and a low-gap poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) to better harvest near-infrared photons, we have achieved a maximum power conversion efficiency of 3.3-3.5%. In addition to the efficiency enhancement, the ZnO layer also drastically improved the air stability of both types of hybrid solar cells. While devices without the ZnO layer degraded completely after one to three days of air exposure, devices with the ZnO layer exhibited only a modest 35% efficiency decrease after >70 days of storage in laboratory air. The mechanisms leading to higher efficiencies and reduced degradation will be discussed.

3:20 PM BREAK
4:00 PM EN+TF-TuA-7 Energy Band Alignments and Influence of Doping on Ga-doped ZnO, CuO and Si
SingYang Chiam, Shijie Wang, JiSheng Pan, Nancy Wong (Institute of Materials Research & Engineering, Singapore); WaiKin Chim (National University of Singapore)

It is important to investigate new materials for thin film solar cells for 2nd generation devices. Materials extraction cost and annual electricity production considerations highlighted several potential new materials including cuprous oxide (Cu2O). In this work, we report on the growth of Cu2O and highlighted the importance of the oxygen partial pressure during growth. Namely, the partial pressure of oxygen determines the transition of Cu2O to CuO with increasing partial pressure. This is accomplished at a fixed total pressure as this may influence Cu2O formation. We then discuss about the interface energy alignments, first between between Cu2O/ZnO and then ZnO/Si. The former is of importance as inorganic thin film p-n junction that is suitable for 2nd generation solar cell devices. For the latter case, we fabricated device structure on differently doped Si, to investigate influence on doping on the transport characteristics of the hetero- pn junction. It is found that forward bias characteristics for a heterojunction, is not critically dependant on the band offsets, but rather the build-in-field at the heterojunction. If the physics is considered from the point of view of quais-fermi level separation during light illumination, this build-in-field will also determine the Voc. In this sense, band offset measurements can only give an indication of the maximum limit of the Voc for differently doped semiconductors (non-degenerate) heterojunction solar cells. In addition, we show that under illumination, the current conduction for the ZnO/Si at zero-bias is a “forward bias” current, unlike all homojunction devices. This can be understood with a detail examination of the energy band diagrams. This work shows the importance of using measured band offsets to aid in understanding the relative Fermi-level alignment instead of using bulk electron affinity values. The work also demonstrates a whole array of playground possible for thin film heterojunction of different materials to engineer an ideal junction for solar cell devices.

4:20 PM EN+TF-TuA-8 Growing Low-Dislocation-Density Ge on Si through Nanometer Sized Voids in Chemical Oxide and Subsequent Integration of III-V Films for Multijunction Solar Cells
Darin Leonhardt, Josephine J. Sheng (University of New Mexico); Jeffrey G. Cederberg, Malcolm S. Carroll (Sandia National Laboratories); Manuel J. Romero (National Renewable Energy Laboratory); Sang M. Han (University of New Mexico)
In an effort to reduce the manufacturing cost of multijunction solar cells, which currently utilize Ge substrates, we have scaled up a process to produce low-defect-density Ge films on 2-inch-diameter Si substrates. This process begins with the nucleation of over 1x1011/cm2 Ge islands on Si through 3-to-7 nanometer diameter voids in chemical SiO2. It is shown that upon Ge island coalescence, the Ge film primarily contains stacking faults as opposed to threading dislocations. We have found that annealing the Ge islands at an early stage of island growth removes the stacking faults, but results in the formation of 5x107/cm2 threading dislocations upon further growth. Herein, we report on a detailed investigation using transmission electron microscopy (TEM) to clarify the mechanism of the stacking fault formation in the Ge. We will also discuss the effect of annealing both on the Ge island morphology and in eliminating the stacking faults. Additionally, the origin of the threading dislocations obtained after annealing is revealed through the TEM study. Lastly, we report characterization results of GaAs-based double heterostructures integrated on the annealed Ge films, whose photoluminescence intensity over the entire 2-inch wafer is comparable to the same structures grown on commercially available GaAs and Ge substrates.
4:40 PM EN+TF-TuA-9 Optimizing Heterojunctions of ZnTe/ZnSe Solar Cells: Effect of Surface Treatment and Growth Conditions
Fang Fang, Brian McCandless, Robert Opila (University of Delaware)

II-VI direct band gap semiconductors are attractive for thin film solar cell (TFSC) applications owing to their potential flexibility in tunable opto-electronic properties and possible application in tandem cells for being band gap materials (EG > 2 eV). For the n-ZnSe/p-ZnTe heterojunction solar cell, the defect states and electronic band alignment at the ZnSe/ZnTe interface are crucial for device performance. We have employed Al-Kα X-ray photoelectron spectroscopy as well as synchrotron source ultra-violet photoelectron spectroscopy to study the surface chemical composition and electronic structures at heterojunction interface. Scanning electron microscopy (SEM) was used to study observe the film microstructure morphology of the interface.

We used two different deposition techniques: Close Space Sublimation (CCS), a low-cost deposition method already demonstrated for high efficiency and commercial CdTe TFSC, and conventional thermal evaporation. Our preliminary results indicated that surface oxides on CCS-grown ZnSe film formed once open to air, and a significant valence band offset induced by this oxide is observed which acts like additional energy barrier for carrier transport, resulting in low open circuit voltage. Also, during sequential CSS deposition of the two stacking films, the covering ZnTe thin film layer growth damage the microstructure of the underlying ZnSe film, i.e., enlarged pores are observed in ZnSe films in the locations where partially covering ZnTe film was deposited. A degraded device performance is expected and low short circuit currents and fill factors of the cells are detected. By analogy to CdS/CdTe TFSC, we are aiming for close-packed column polycrystalline of ZnSe/ZnTe film growth. Therefore, we are exploring etching processes, annealing temperatures and ambient settings to optimize the growth conditions. Evaporation is under investigation, since we have the option of dual-sources in the self-designed chamber, sequential growth of ZnSe and ZnTe films without vacuum break is feasible. Film morphology as well as energy band alignment at the heterojunctions using evaporation growth is being studied.
5:00 PM EN+TF-TuA-10 Investigation of NbSe2 as Potential High Work Function Back Contact for CdTe Solar Cells
Matthaus A. Wolak, Sebastian Gutmann, Martin M. Beerbom, Chris S. Ferekides, Rudy Schlaf (University of South Florida)
The layered semi-metal NbSe2 combines a chemically inert van der Waals surface with a high work function of about 5.8 eV. This motivated an investigation of NbSe2 as Ohmic hole injection contact for CdTe solar cells. Current back contacts made from Cu suffer from interdiffusion issues leading to cell degradation. In the discussed experiments, the interface between NbSe2 and CdTe was investigated using x-ray and ultraviolet photoemission spectroscopy (XPS, UPS). In these experiments CdTe and NbSe2 thin films were grown in-situ in a vacuum chamber attached to the photoemission system. This enabled the investigation of the CdTe/NbSe2 interface without interference by ambient contamination. After growth of a CdTe thin film, the NbSe2 film was prepared in several steps. Photoemission spectroscopic characterization between each of the deposition steps allowed the observation of the formation of the band line-up at the interface. The results of the experiments indicate that an intermixed layer forms at the interface. This layer causes the formation of an interface dipole, preventing the formation of an Ohmic contact. A Schottky-type band line-up formed instead.
5:20 PM EN+TF-TuA-11 In-Rich InGaN Films for Efficient Photovoltaic Devices Grown by ENABLE
Todd L. Williamson, Mark A. Hoffbauer (Los Alamos National Laboratory); Kin M. Yu, Lothar A. Reichertz, Wladek Walukiewicz (Lawrence Berkeley National Laboratory)

The wide band gap tunability of InxGa1-xN thin films (0.7 eV to 3.4 eV, 1>x>0) makes them ideal for efficient photovoltaic (PV) devices. However, growing high-quality In-rich InxGa1-xN films with strong photoluminescence in the green-to-red portions of the visible spectrum has faced considerable challenges due to indium phase segregation and other materials issues. These challenges have precluded the growth of both In-rich InGaN and compositionally graded InGaN materials, and make it difficult to grow higher bandgap Ga-rich materials on top of lower bandgap In-rich materials. Overcoming these difficulties using conventional epitaxial techniques is challenging due to the low decomposition temperatures of In-rich materials (e.g. InN~550°C) and the relatively high growth temperatures for Ga-rich materials (e.g. GaN >800°C).

Energetic neutral atom beam lithography & epitaxy (ENABLE) is a low-temperature thin film growth technology recently developed at LANL that utilizes a collimated beam of energetic neutral N atoms (kinetic energies 0.5 to 5.0 eV) to react with evaporated Ga and In metals to grow InGaN. ENABLE is similar to MBE, but provides a much larger N atom flux and correspondingly high film growth rate. The high kinetic energy of the reactive N atoms substantially reduces the need for high substrate temperatures, making isothermal growth over the entire InGaN alloy composition range possible at rates of >3 microns/hr with no toxic precursors or waste products.

Data on film photoluminescence, crystallinity, electrical properties, doping, and electro-luminescence of InxGa1-xN, graded InxGa1-xN, and GaN films grown using ENABLE over the full composition range will be presented. ENABLE-grown InxGa1-xN films show strong photo- and electro-luminescence spanning the entire visible region of the spectrum, with reasonable carrier mobilities background carrier concentrations typically in the low 1017 range. Evidence for p-type doping of In-rich InGaN films and characterization of p/n junctions will be discussed along with the prospects for using ENABLE to fabricate efficient PV devices.

5:40 PM EN+TF-TuA-12 Copper Oxide Thin Films: Preparation and Modulation of Semiconducting Properties by Electrochemical Methods
Felipe Caballero-Briones (CICATA-IPN/Universitat de Barcelona, Spain); Anna Palacios-Padrós (Instituto de Bioingenieria de Catalunya, Spain); Fausto Sanz (Instituto de Biongenieria de Catalunya/CIBER-BBN, Spain)
Copper oxide is a p-type semiconductor with a direct band gap of 2 eV, suitable for photovoltaic applications. In this work we present an electrochemical method to prepare p-type semiconducting Cu2O films around 100 nm thick with noticeable photocurrent response. The film properties were modulated by varying different conditions such as the time at a dissolution potential and the film doping with alkaline ions. The modification of the time of exposure to the dissolution potential allows the tailoring of the crystallinity, the band gap energy and the disorder parameter E0 and also provided elements to outline the growth mechanism of the Cu2O films that involve surface reaction, diffusion of oxygen species that react in the solid state accordingly to the point defect model, and heterogeneous deposition of Cu2O from the Cu+ ions dissolved in a chemical bath-like fashion. On the other side, the study of the behavior of different alkaline metal ions (A: Li, Na, K, Cs) present in the electrolyte used to prepare the Cu2O films lead to important results. It was observed that important amounts of the alkaline ion (around 1%) can be incorporated to the film and that are indeed electrically active impurities that modify the band gap energy probably by introducing states within the band gap in the case of Cs or by getting incorporated to the crystalline lattice for Na or Li. Changes in the optical absorption, thickness, density of carriers and in defects are related with the size of the employed ion. To complete the study, an electronic diagram of the Cu|Cu2O|Electrolyte interface was prepared by using a combination of techniques including Electrochemical Impedance and Electrochemical Tunneling Spectroscopy/Microscopy.
Time Period TuA Sessions | Abstract Timeline | Topic EN Sessions | Time Periods | Topics | AVS2010 Schedule