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Photovoltaik


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Thomas Hannappel

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Preparation and characterization of solar cells structures


Preparative and analytical work is devoted to preparing new highly efficient solar cells in the form of thin epitaxial multi-layer systems of silicon, germanium, or  III-V materials. Novel solar cell concepts and new routes of multi-junction solar cells are prepared applying the metal organic chemical vapour deposition (MOCVD) technique. The development of new highly efficient solar cells and fundamental issues of interfacial reactions are guided by insights gained with new powerful analytical tools, i.e. several methods of optical spectroscopy, in situ analysis, and surface science in ultra high vacuum. 

Multi-junction solar cells represent today’s most efficient solar cells. The growth process is monitored via optical in situ signals (such as reflectance anisotropy spectroscopy, RAS) allowing for systematic monitoring and for systematic improvements of the growth procedure and interface formation. Here, critical interfaces like the III-V/SI(100) interfaces are targeted. 


Aspects of High-Efficiency III-V Multijunction Solar Cells

Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today’s most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/ GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.
 

Observation of critical interfaces

The in situ measured optical signals show whether desired bulk properties and a specific surface reconstruction have been realized in the MOCVD reactor. A direct relationship is established between a specific optical signal and the corresponding surface reconstruction. This strategy is successful with a unique experimental tool. The latter enables contamination free sample transfer from the MOCVD reactor to ultra-high-vacuum. Thus, signals like LEED, UPS, XPS, AES, STM images, FTIR, and again RAS are measured in ultra-high-vacuum that characterize the specific surface reconstruction. 

With increasing experience and know-how in this field optical fingerprint signals have been established also for buried interfaces. Easier to handle and more cost effective III-V solar cells can be realized by depositing III-V materials on Ge- or rather Si-wafers. Current estimates indicate a competitive cost level for multi-junction III-V cells with conversion efficiencies towards 50 percent if they are employed at concentrated sunlight. 

Example: III-V on silicon

There is a strong scientific and technological interest in the realization of III-V semiconductor heteroepitaxy on silicon substrates, in particular, by applying the metal-organic vapour phase epitaxy MOVPE(1) technique for industrial scalability. The integration of optoelectronic devices such as high-efficiency solar cells on Si100(1) substrates requires a significant reduction of the defect concentration induced by the III-V on silicon heterointerface.

However, major challenges have to be met at the III–V/Si heterojunction: crucial issues are differences in lattice constants and thermal expansion coefficients as well as the formation of the heterovalent (polar-on-non-polar) interface necessitating a suitable substrate preparation prior to heteroepitaxy. New defect mechanisms – typically not observed in III–V homoepitaxy – arise from the interface with the Si(100) substrate and need to be controlled to achieve defect concentrations suitable for applications in advanced optoelectronic devices.

MOCVD Preparation of III-V materials

Metal Organic Chemical Vapor Deposition (MOCVD) or Metal-Organic Vapor Phase Epitaxy (MOVPE) is a widely used method for preparing epitaxial structures by depositing atoms on a wafer substrate. It is utilized for a broad variety of applications in industry and research. The principle of MOCVD is quite simple. Atoms are deposited by decomposing organic molecules (precursors) while they are passing over the hot substrate. The undesired remnants are removed or deposited on the walls of the reactor. III-V semiconductors of high purity and structural order are prepared via MOCVD. 

Figure Principle of a MOCVD-process: (a) laminar flow of carrier gas (H2 or N2) and of the precursor molecules (metal-organic compounds) over the substrate (wafer) placed on a graphite susceptor inside a reaction vessel (reactor) (b) supply of thermal energy (usually between 400°C and 700°C) for decomposing the molecules, (c) deposition of the material, evapuration of the molecular fragments into the gas phase.

The group operates an AIXTRON-200 MOCVD reactor. 

It holds a German patent for the specific protocol of contamination-free sample transfer into ultra-high vacuum (UHV). The sample can be transported in a mobile UHV chamber and transferred into any other desired UHV chamber, provided the latter is equipped with an appropriate load-lock port. So-called alternative precursor molecules are preferred since they are less poisonous and permit sample growth at lower temperatures than conventional precursors. 

Figure down to the right: MOCVD reactor, arrangement of the in-situ RDS, the MOCVD to UHV transfer system, and the mobile stand-alone UHV shuttle.

 
Characterization of III-V materials, see below methods

  • In-situ RAS/RDS (reflectance anisotropy spectroscopy)
  • Patented UHV transport chamber
  • Surface science methods
  • Optical and electrical characterization

 

In situ reflectance anisotropy spectroscopy

Sample growth is monitored in the MOCVD reactor with Reflectance Difference/Anisotropy Spectroscopy (RDS/RAS). RDS is a normal incidence reflectance technique probing the anisotropy that arises at a reconstructed surface. RDS measures the difference in the normal-incidence reflectance rx and ry of linearly polarized light directed onto the sample's surface. The signal is normalized to the total reflection <r>. The basic principle of RDS is the measurement of the complex reflectance anisotropy rx-ry/<r>, caused e.g. by the reduced symmetry of the reconstructed surface of a cubic crystals.
The group measures RDS signals of MOCVD-prepared surfaces down to 20K, in particular clean and well-ordered III-V (100) and (111) surfaces.

RD spectra of InP(100) and GaP(100) surfaces measured in the MOCVD reactor. The reconstruction is changing from ordered Phosphorus-rich to ordered Indium-rich and Gallium-rich, respectively.


RD spectra measured (red lines) for the In-rich and Ga-rich surfaces of InP(100) and GaP(100), respectively (UHV, 20K). Spectra shown as dashed lines are predicted by theory [Schmidt, Bechstedt et al.] for the mixed-dimer model.
 

Apparatus for investigating metalorganic chemical vapor deposition-grown semiconductors with ultrahigh-vacuum based techniques

An apparatus has been developed for the transfer of a sample from a metalorganic chemical vapor deposition (MOCVD) reactor to an ultrahigh-vacuum (UHV) chamber without introducing any contamination. The surface of the sample does not change during transfer as is borne out by the identical reflectance difference (RD) spectrum measured first in the MOCVD reactor, i.e., in situ, and afterwards again in the UHV chamber. Making use of the earlier apparatus a semiconductor can be grown in the MOCVD reactor and can afterwards be investigated with any desired tool of surface science, in particular also those that require UHV. All the data collected in UHV can be identified with the RD spectrum measured already in the MOCVD reactor. Several examples are presented here for data collection in UHV on III–V semiconductors grown in the MOCVD reactor. They illustrate the ease and reliability of the here described apparatus for contamination-free sample transfer. Signals are presented in particular for the genuine MOCVD-grown P-rich seemingly (231)/(232)InP(100) reconstructed surface that until now can only be investigated in UHV if one makes use of the sample transfer system described in this article.

 

Image of our MOCVD/UHV transfer system and the battery powered UHV shuttle

Methods of Surface Science

  • Low energy electron diffraction (LEED)
  • Ultraviolet photoelectron spectroscopy (UPS)
  • X-ray photoelectron spectroscopy (XPS)
  • Auger electron spectroscopy (AES)
  • 20K Reflection difference/anisotropy spectroscopy (RDS/RAS)
  • Fourier transform infrared spectroscopy (FTIR)
  • 2 photon photoelectron spectroscopy (2PPE)


Research topics

  • Inorganic semiconductors and device structure:
  • III-V- semiconductors, silicon, germanium, 
  • (100)- and (111)-surfaces,
  •  µ-crystalline interfaces and surfaces,
  • Solar cells, opto-electronic  devices:
  • high-efficiency, 3rd generation
  • tandem/multi  junction solar cells
  • nanowire- and quantum well solar cells
  • silicon /thin film solar cells
  • Analytics/characterization:
  • Optical in situ-spectroscopy (u. a. reflectance anisotropy/difference  spectroscopy, RAS/RDS)
  • Benchmarking of the optical in-situ signals employing surface science tools in ultra-high vacuum (UHV), also in collaboration with external partners:  XPS, UPS, LEED, FTIR, STM, LEEM, …


Services offered to companies:

  • ‘surface science’-methods (XPS, UPS, LEED, RAS, FTIR, STM)
  • Optical spectroscopy (photoluminescence, time-resolved and spatially resolved, curvature measurement)
  • Preparation of epitaxial Si-, Ge- and III-V-based layers and solar cells
  • Characterization of solar cells (I-U-curves, efficiency, also of tandem solar cells), quantum efficiency measurements (internal und external)


Special equipment:

  • Metal-organic  chemical vapor deposition/-epitaxy (MOCVD /  MOVPE)
  • MOCVD-to- UHV-transfer system
  • UHV-transport chamber, transfer to various UHV-based measurement opportunities
  • ‘surface science‘, see above