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* analysis of polarization effect and band diagram of device supports the explanation of physical process of photocurrent generation in the device.
* analysis of polarization effect and band diagram of device supports the explanation of physical process of photocurrent generation in the device.
* It is beneficial to realize the multi-junction solar cell with less tunneling junctions.
* It is beneficial to realize the multi-junction solar cell with less tunneling junctions.
====[http://jap.aip.org/resource/1/japiau/v104/i2/p024507_s1 Modeling of InGaN/Si tandem solar cells]====
'''Abstract:''' Author(s) investigate theoretically the characteristics of monolithic InGaN/Si two-junction series-connected solar cells using the air mass 1.5 global irradiance spectrum. The addition of an InGaN junction is found to produce significant increases in the energy conversion efficiency of the solar cell over that of one-junction Si cells. Even when Si is not of high quality, such two-junction cells could achieve efficiencies high enough to be practically feasible. Author(s) also show that further, though smaller, improvements of the efficiency can be achieved by adding another junction to form an InGaN/InGaN/Si three-junction cell.
* derived expression for achieving maximum efficiency by optimizing thickness for InGaN/Si tandem solar cells by using electron and hole current expression.
* matching operating current rather than short-circuit current in multi-junctions solar cell improves the efficiency and fill factor (but operating current is usually within a few percent of short circuit current)
* most efficient solar cells are designed such that internal reflections can increase the cell’s effective thickness by as much as a factor of 40.
* the efficiency rapidly declines with temperature, careful heat sinking of such cells is critical.
* gain achievable with multicrystalline or otherwise low quality Si by adding an InGaN junction on top of Si can result in increases in energy conversion efficiencies of more than 50% compared to Si alone (27% vs. 17%). Such an increase in efficiency could justify the economic cost associated with increased complexity of growing such cells.





Revision as of 19:07, 12 November 2011

This page describes selected literature available on InGaN photovoltaics.

InGaN quantum dot photodetectors

Abstract: Nanometer-scale InGaN self-assembled quantum dots (QDs) have been prepared by growth stop during the metalorganic chemical vapor deposition growth. With a 12 s growth interruption, author(s) successfully formed InGaN QDs with a typical lateral size of 25 nm and an average height of 4.1 nm. The QDs density was about 2x1010 cm-2. Nitride-based QD metal–semiconductor–metal (MSM) photodetectors were also fabricated. It was found that author(s) could significantly enhance the photocurrent to dark current contrast ratio of the MSM photodetectors by the use of QD structure.

  • The MSM photodetector has an ultra low intrinsic capacitance and its fabrication process is also compatible with field-effect transistor (FET)-based electronics. Thus, one can easily integrate GaN MSM photodetectors with GaN FET based electronics to realize a nitride-based optoelectronic integrated circuit (OEIC).
  • InGaN alloy inhomogeneity plays a key role in the high efficiency of nitride based LEDs grown on sapphire substrates. (conclusion drawn from references)
  • Author(s) reports the interrupted growth method in MOCVD to fabricate nano-scale InGaN self-assembled QDs.
  • InGaN QDs were grown by periodically growing and interrupting the deposition process for 12 s untill desired growth size was achieved.
  • Although dark current of InGaN QD MSM photodetector was about the same as that of conventional InGaN MSM photodetector, its photocurrent was much larger. author(s) reported a photocurrent to dark current contrast ratio larger than 220 by using InGaN QD MSM photodetector. Result seems to suggest that InGaN QDs can actually enhance photo response of nitride-based photodetectors.

Growth and Characterization of InGaN for Photovoltaic Devices

Abstract: In this work author(s) present the growth and characterization of InxGa1-xN-based materials and solar cells with x up to 0.54. Growth of single phase InxGa1-xN was achieved using Plasma Assisted Molecular Beam Epitaxy (PAMBE) with flux modulation for active species. The material was characterized by x-ray diffraction, electrochemical capacitance-voltage, time-resolved photo-luminescence, and contactless electroreflectance. Fabricated devices are then studied for photo-response under simulated AM0 spectral conditions to evaluate solar cell characteristics. The dark and illuminated J-V results indicate the existence of significant shunt and series resistances arising from material defects and non-optimized device design.

  • To combat the miscibility and indium phase segregation issues, author(s) developed InGaN deposition using modulated growth conditions in order to maintain slightly metal rich growth front while avoiding the formation of indium droplets.
  • Electrochemical capacitance-voltage (ECV) measurements were used to evaluate the carrier levels in the layers. This method was chosen over Hall Effect and standard C-V measurements due to the strong band bending that is present at the InGaN surfaces, especially those with indium compositions above 35 - 40%.
  • The carrier concentration for low indium fraction n-InGaN can be calculated through typical Mott-Schottky C-V analysis, but higher fraction n-InGaN and essentially all p-InGaN require non-standard analysis.
  • Contactless electroreflectance (CER) was used to investigate the optical transitions and built-in electric fields. CER spectroscopy is similar to photoreflectance (PR) spectroscopy as it is also insensitive to localized states in the film, and therefore comparison with PL spectra can give insight into carrier localization phenomena.
  • On the Mott-Schottky representation of ECV data taken from various n-InGaN samples at 1 kHz, The lower composition samples have a linear 1/C2 vs V relationship which lends itself to standard depletion capacitance analysis regardless of doping. On the other hand, higher indium fraction layers do not behave in a linear fashion. Through theoretical modeling a strong correlation between the value of (1/C2) peak and the bulk free electron concentration can be determined. Thus the value of (1/C2) peak becomes the metric by which the n-type behavior of InGaN can be calculated.
  • Evaluation of p-InGaN also relies on the magnitude of (1/C2) peak. Due to the large acceptor energy of Mg in GaN and InGaN, low frequency C-V techniques are sensitive to the acceptor concentration as opposed to the free hole density. Therefore, authors have correlated the ECV behavior of our p-InGaN against SIMS measurements of the Mg concentration. From Mott-Schottky representation of ECV data from p-InGaN and p-GaN taken at 300 Hz, the value of (1/C2) peak decreases with increasing Mg concentration for both p-GaN and p-InGaN. The value of (1/C2) peak for a given Mg concentration is also affected by the indium composition.
  • The room temperature bandgap of the InGaN layers has been determined from CER measurements. Because of suspected carrier localization phenomena in the layers, the corresponding PL peaks were observed below the energy gap determined from CER measurements. The Stokes shift at room temperature has been found to increase from 175 to 250 meV with the decrease of the energy gap from 2.5 to 2.2 eV (i.e. an increase in indium content)
  • Room temperature time resolved photoluminescence measurements did confirmed the carrier localization phenomenon in InGaN layers. PL decay times between 5 ns and 25 ns were observed from layers with indium compositions between 31% and 41%. Author(s) observed that the decay time of PL increases from ~ 5 ns to ~ 15 - 25 ns with the increase in emission wavelength. The decay times of the PL and corresponding spectral dispersions are strong evidence for localized emission in this material. Most likely explanation for the localized emission is from small indium content fluctuations. Although the x-ray data from author(s) samples exhibited only a single peak - thus indicating essentially a single phase material - the breadths of the PL emission and the XRD FWHM’s (~600 to 900 arcsec) support the assumption of small compositional variations in the layers.
  • Photoresponse was observed for InxGa1-xN devices with x up to 0.54, corresponding to turn-on energies below 2.0 eV.
  • Photoresponse turn-on becomes more gradual with increasing indium composition. Most likely causes of this behavior are increased localized indium content fluctuations and reduction in the magnitude of the absorption coefficient. Peak internal quantum efficiencies of the order 0.20 were observed from devices spanning an In compositional range from 15 % to nearly 40%.
  • p-n device structures with indium fractions of up to 54% were fabricated and tested. Isc’s of up to 2.2 mA/cm2 were achieved for a In0.39Ga0.61N cell, but Voc was extremely small. In general, the cell's Isc typically increased with indium mole fraction, but conversely the overall device performance tended to degrade. Coupled with the reduction of Voc and IQE with increase of the cell area, the results indicate material quality issues arising from the lattice mismatch as the indium content is increased.

InGaN/GaN multiple quantum well solar cells with long operating wavelengths

Abstract: Author(s) report the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments (420 nm). The fabricated solar cells based on In0.3Ga0.7N/GaN MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).

  • Theoretical calculations has indicated that the requirements of an active material system to obtain solar cells having a solar energy conversion efficiency greater than 50% can be fulfilled by InGaN alloys with In content of about 40% (from reference [1])
  • The realization of high crystalline quality InGaN films in the entire composition range is highly challenging. One of the biggest problems is attributed to the large lattice mismatch between InN and GaN, resulting in low solubility and phase separation. [2],[3]
  • trend of reduced crystalline quality with increasing x makes the realization of solar cells based on InxGa1-xN with x>0.25 highly challenging.
  • strain suppress phase separation in InGaN
  • The open-circuit voltages (Voc) for devices with x ~ 0.3 and 0.4 are about 2.0 and 1.8 V, respectively.
  • performance of In0.4Ga0.6N/GaN MQWs as active region was much poorer than that of In0.3Ga0.7N/GaN MQWs as active region, despite the fact that In0.4Ga0.6N/GaN MQWs are expected to have a much better spectral overlap with the excitation source. Reason is reduced material quality with increasing x, this leads to a higher loss of photogenerated charge carriers.
  • Device delivers a quantum efficiency of 40% at 420 nm and 10% at 450 nm.

High internal and external quantum efficiency InGaN/GaN solar cells

Abstract: High internal and external quantum efficiency GaN/InGaN solar cells are demonstrated. The internal quantum efficiency was assessed through the combination of absorption and external quantum efficiency measurements. The measured internal quantum efficiency, as high as 97%, revealed an efficient conversion of absorbed photons into electrons and holes and an efficient transport of these carriers outside the device. Improved light incoupling into the solar cells was achieved by texturing the surface. A peak external quantum efficiency of 72%, a fill factor of 79%, a short-circuit current density of 1.06 mA/cm2, and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions.

  • high measured IQE reflects efficient conversion of absorbed photons into e- and h+ and also reveals efficient transport of these carriers outside device.
  • degrading effect of the polarization fields was avoided with a high doping of the n- and p-GaN, which helps to screen the polarization-related charges at the heterointerfaces
  • improved light incoupling into the solar cells achieved by rough surface, induced during the p-GaN growth, reduces reflection of incident light at device surface and increases path length of light inside device active region.
  • surface roughness assist in achieving higher efficiency.
  • In InGaN spectral region (370<λ<410 nm), where spectral behavior of light absorption followed closely EQE curve, both in shape and magnitude. implying that nearly all absorbed photons in this layer were converted into electrons and holes.
  • generated carriers from strong light absorption in GaN region do not contribute to current generation (EQE ∼ 0); instead, they mostly recombined due to the short carrier diffusion in neutral p-GaN region
  • IQE for both smooth and rough surface solar cells determined from ratio between the EQE and light absorption curves, IQE found higher than 90% in both samples for λ from 380 to 410 nm.
  • smoother sample had slightly higher IQE, reaching values up to 97%, the rough sample had an IQE up to 93%.
  • high measured IQE revealed efficient electron-hole generation and carrier transport out of the active region.
  • absorption can be increased by making surface more rough, roughness can boost absorption upto 80 %
  • solar cell with rough surface presented a short-circuit current density of 1.06 mA/cm2, which was 27% higher than smoother sample (0.83 mA/cm2).
  • open circuit voltages were 1.89 and 1.83 V for rough and smoother samples, respectively. These devices presented high fill factors (FF), 78.6% (rough) and 76.6% (smoother), revealing excellent solar cell performances.
  • better light coupling of solar cell with rough surface to device active region resulted in an enhancement of 35% in maximum output power

High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap

Abstract: Author(s) report on III-nitride photovoltaic cells with external quantum efficiency as high as 63%. InxGa1−xN/GaN p-i-n double heterojunction solar cells were grown by metal-organic chemical vapor deposition on (0001) sapphire substrates with xIn = 12%. A reciprocal space map of the epitaxial structure showed that the InGaN was coherently strained to the GaN buffer. The solar cells have a fill factor of 75%, short circuit current density of 4.2 mA/cm2, and open circuit voltage of 1.81 V under concentrated AM0 illumination. It was observed that the external quantum efficiency can be improved by optimizing the top contact grid.

  • Strain relaxation can result in defect formation that can increase nonradiative recombination18 and in turn degrade solar cell performance. (from reference)
  • External quantum efficiency of these devices is limited by several factors: reflection at surface, absorption in semitransparent Ni/Au current spreading layer, and incomplete absorption in InGaN layer.
  • By optimizing p-contact grid spacing, peak external quantum efficiency greater than 60% was achieved.
  • Quantum efficiency spectrum showed a flat spectral response from 370 to 410 nm, efficiency of III-N solar cells can be further improved by optimizing p-GaN contact such as optimizing Ni and Au thicknesses and exploring alternate contact schemes such as ITO and ZnO.
  • Short wavelength response could be enhanced by using p-AlGaN as a window layer instead of p-GaN, this could have added benefit of potentially reducing recombination of electrons at surface.
  • Additionally, an antireflection coating on top surface should also increase performance of these devices.

Design and characterization of GaN/InGaN solar cells

Abstract: Author(s) experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4 V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells were designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material was primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.

  • polarization tends to substantially influence the performance of III-V nitride devices
  • Schottky barrier at nonoptimal p-GaN–Ni metal contact interface opposes light-generated current so that current asymptotically approaches zero prior to device reaching its VOC; this Schottky effect was also confirmed through PC1D simulation.
  • major loss mechanisms are (in author(s) prototype) absorption of up to 40% of incident light by semitransparent current spreading layer and transmittance of at least 10% of incident light through the solar cell, and it was also confirmed through spectrometry.
  • quantum efficiency can be further enhanced by optimizing grid contacts for low Ohmic resistance and be brought close to unity as confirmed through simulations.
  • calculated theoretically and measured experimentally that the III-V nitrides are highly pyroelectric materials

Photoelectric characteristics of metal/InGaN/GaN heterojunction structure

Abstract: A heterojunction structure photodetector was fabricated by evaporating a semitransparent Ni/Au metal film on the InGaN/GaN structure. The photocurrent (PC) spectra show that both the Schottky junction (NiAu/InGaN) and the InGaN/GaN isotype heterojunction contribute to the PC signal which suggests that two junctions are connected in series and result in a broader spectral response of the device. Secondary electron, cathodoluminescence and electron-beam-induced current images measured from the same area of the edge surface clearly reveal the profile of the layer structure and distribution of the built-in electric field around the two junctions. A band diagram of the device is drawn based on the consideration of the polarization effect at the InGaN/GaN interface. The analysis is consistent with the physical mechanism of a tandem structure of two junctions connected in series.

  • discussed about photocurrent with respect to different excitation wavelength and overall net photocurrent.
  • discussed about polarization at the InGaN/GaN hetero-junction and band diagram of the metal/InGaN/GaN structure.
  • in wurtzite structure, III-nitride semiconductors, there are often large spontaneous and piezoelectric polarization effects. As a consequence, polarization induce a fixed sheet charge density of two-dimensional hole gas (2DHG) due to polarization discontinuities at the nitride heterointerfaces.
  • Author(s) used combined measurements of SE, CL and EBIC micro-imaging on cleaved edge face of the device to check allocation of two junctions and distribution of related built-in electric field.
  • analysis of polarization effect and band diagram of device supports the explanation of physical process of photocurrent generation in the device.
  • It is beneficial to realize the multi-junction solar cell with less tunneling junctions.

Modeling of InGaN/Si tandem solar cells

Abstract: Author(s) investigate theoretically the characteristics of monolithic InGaN/Si two-junction series-connected solar cells using the air mass 1.5 global irradiance spectrum. The addition of an InGaN junction is found to produce significant increases in the energy conversion efficiency of the solar cell over that of one-junction Si cells. Even when Si is not of high quality, such two-junction cells could achieve efficiencies high enough to be practically feasible. Author(s) also show that further, though smaller, improvements of the efficiency can be achieved by adding another junction to form an InGaN/InGaN/Si three-junction cell.

  • derived expression for achieving maximum efficiency by optimizing thickness for InGaN/Si tandem solar cells by using electron and hole current expression.
  • matching operating current rather than short-circuit current in multi-junctions solar cell improves the efficiency and fill factor (but operating current is usually within a few percent of short circuit current)
  • most efficient solar cells are designed such that internal reflections can increase the cell’s effective thickness by as much as a factor of 40.
  • the efficiency rapidly declines with temperature, careful heat sinking of such cells is critical.
  • gain achievable with multicrystalline or otherwise low quality Si by adding an InGaN junction on top of Si can result in increases in energy conversion efficiencies of more than 50% compared to Si alone (27% vs. 17%). Such an increase in efficiency could justify the economic cost associated with increased complexity of growing such cells.
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