This is in a series of literature reviews on InGaN solar cells, which supported the comprehensive review by D.V.P. McLaughlin & J.M. Pearce, "Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion"Metallurgical and Materials Transactions A 44(4) pp. 1947-1954 (2013). open access
Others: InGaN solar cells| InGaN PV| InGaN materials| InGan LEDs| Nanocolumns and nanowires| Optical modeling of thin film microstructure| Misc.


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Authors User:Ankitvora
Joseph Rozario
Published 2011
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InGaN photovoltaics.[edit | edit source]

Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion [1][edit | edit source]

  • Comprehensive 2013 review

Abstract: The world requires inexpensive, reliable, and sustainable energy sources. Solar photovoltaic (PV) technology, which converts sunlight directly into electricity, is an enormously promising solution to our energy challenges. This promise increases as the efficiencies are improved. One straightforward method of increasing PV device efficiency is to utilize multi-junction cells, each of which is responsible for absorbing a different range of wavelengths in the solar spectrum. Indium gallium nitride (In x Ga1−x N) has a variable band gap from 0.7 to 3.4 eV that covers nearly the whole solar spectrum. In addition, In x Ga1−x N can be viewed as an ideal candidate PV material for both this potential band gap engineering and microstructural engineering in nanocolumns that offer optical enhancement. It is clear that In x Ga1−x N is an extremely versatile potential PV material that enables several known photovoltaic device configurations and multi-junctions with theoretic efficiencies over 50 pct. This potential is driving immense scientific interest in the material system. This paper reviews the solar PV technology field and the basic properties of In x Ga1−x N materials and PV devices. The challenges that remain in realizing a high-efficiency In x Ga1−x N PV device are summarized along with paths for future work. Finally, conclusions are drawn about the potential for In x Ga1−x N photovoltaic technology in the future.

Fabrication and characterization of InGaN p-i-n homojunction solar cell[2][edit | edit source]

Abstract: InxGa1-xN p-i-n homojunction solar cells with different In content are studied. The measured open circuit voltages (Voc) are 2.24, 1.34, and 0.96 V, for x = 0.02, 0.12, and 0.15, respectively. By comparing the x-ray rocking curves, the I-V characteristics and the external quantum efficiencies, it's demonstrated that the deterioration of InGaN crystal quality for larger In contents causes the decrease of Voc. The result demonstrates that reduction of defect is a key factor in the fabrication of nitride solar cell.

  • High absorption efficiency of InGaN at the band edge makes it beneficial in absorbing light with minimal thickness of layer at the nano level
  • X-ray diffraction shows that with increment of In content the FWHM of the intensity spectrum broadens. It can be attributed to the increment of defects due to the enhanced lattice mismatch and thermal mismatch between the InGaN and GaN
  • IV curve illustrates that the FF for different In concentration ranges from 64%-69%
  • Short circuit current increases with the increase of In content- can be attributed to the enhanced absorption of the incident light due to band gap reduction of InGaN. On the other hand, open circuit voltage decreases significantly with the increment of In content (similar outcome obtained for the heterojunction structure) - due to degradation of crystalline quality. Indium content is dominating factor in determining Voc
  • With higher Indium content the dark current rises which indicates higher level of defects and dislocations. The larger reversed bias leakage current is the evidence of deterioration at crystalline quality
  • Peak EQE decreases with Indium rise. The EQE curve for different composition of InGaN shows that the absorption edge of incident light is shifted to longer wavelengths with higher In incorporation –characterizes that more incident light can be absorbed by the device and indicates high prospect

Nearly lattice-matched n, i, and p layers for InGaN p-i-n photodiodes in the 365–500 nm spectral range[3][edit | edit source]

Abstract: Author(s) report on nearly lattice-matched grown InGaN based p-i-n photodiodes detecting in the 365–500 nm range with tunable peak responsivity tailored by the i-layer properties. The growth of lattice matched i- and n-InGaN layer leads to improvement in the device performance. This approach produced photodiodes with zero-bias responsivities up to 0.037 A/W at 426 nm, corresponding to 15.5% internal quantum efficiency. The peak responsivity wavelength ranged between 416 and 466 nm, the longest reported for III-N photodiodes. The effects of InN content and i-layer thickness on photodiode properties and performance are discussed.

  • A thick i layer increases the probability of absorption of incoming photons in active region; however, increased thickness leads to higher series resistance, surface roughness, and can affect carrier collection efficiency. The p- and n-InGaN layers should be as thin as possible to minimize absorption of photons in these regions. Bandgaps of doped layers were adjusted so that absorption of photons takes place inside or within the diffusion length distance of the active region, i.e., Egi < Eg</sub>n,p.
  • bandgap properties of the i-layer determine the cutoff and peak responsivity wavelengths in a p-i-n photodiode.
  • deterioration of photodiode performance can be attributed to high defect density in the i-layer instigated by higher InN content and thicker films.
  • peak responsivity wavelength ranged between 416 and 466 nm. This is the longest peak responsivity wavelength reported for III-N based photodetectors.
  • Maximum responsivity has been observed for 0.1 um thick i-layer and an InN composition of x=0.07. The relation between photocurrent and incident light intensity/optical power is linear indicates that the responsivity is persistent with changing light intensity
  • Obtained External Quantum Efficiency (EQE) = 11% and Internal Quantum Efficiency (IQE) = 15.5%
  • Wafers had surfaces with metallic appearance after growth, attributable to In metal segregation at low growth temperatures

InGaN quantum dot photodetectors[4][edit | edit source]

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)
  • Nanoscale indium composition fluctuation due to InGaN phase separation could result in the formation of indium-rich clusters, which acts as quantum dots (QDs) (conclusion drawn from references)
  • It is a sophisticated quantum capture system and deeply localizes the charge carriers in quantum dots which eventually hinder the carriers migration toward nonradiative defects (dislocations) (conclusion drawn from references)
  • Author(s) reports the interrupted growth method in MOCVD to fabricate nano-scale InGaN self-assembled QDs.
  • Samples used in this study were grown on (0 0 0 1)- oriented sapphire (Al2O3) substrates in a vertical low-pressure MOCVD reactor with a high-speed rotation disk
  • InGaN QDs were grown by periodically growing and interrupting the deposition process for 12 s untill desired growth size was achieved.
  • The diameter of the circular QDs was in the range of 20–38 nm, with an average height of 4 nm and the density of these circular QDs was estimated to be around 2 X 1010 cm-2
  • To increase photocurrent to dark current contrast ratio of the conventional InGaN MSM photodetector metals with larger Schottky barrier height on InGaN should be used and epitaxial lateral overgrowth should be adopted to reduce the dislocation density in the sample
  • 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.

InGaN/GaN multiple quantum well solar cells with long operating wavelengths[5][edit | edit source]

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]
  • By directly depositing on GaN or AlN epitemplates without buffer layers, single phase InGaN epilayers across the entire alloy range could be produced by MOCVD technique. XRD data for several representative InxGa1−xN epilayers shows that InN peak positions doesn't show multiple peaks- implying that the InxGa1−xN epilayers are not phase separated
  • trend of reduced crystalline quality with increasing x makes the realization of solar cells based on InxGa1-xN with x>0.25 highly challenging.
  • The thickness of p-GaN �(n-GaN�) is �~150nm ��(~0.5u�m)�and the device structure was grown on a GaN epilayer(3u�m)/sapphire template
  • For quantum efficiency versus excitation wavelength characterization, monochromatic illumination was obtained by using white light source in conjunction with a monochromator (with a spectral resolution of about 2.5 nm)
  • 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.
  • Obtained fill factor is about 60%
  • Device delivers a quantum efficiency of 40% at 420 nm and 10% at 450 nm.
  • The response in the shorter wavelength region (<300 nm) is limited by the use of p-GaN window and can be improved if a larger band gap material such as p-AlGaN or p-InAlGaN is incorporated
  • There are three quantum efficiency limiting factors: 1) Absorption in the semitransparent p-contact layer 2) Thin light absorption layer and 3) Low crystalline quality of InGaN alloys with relatively high In contents

High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap[6][edit | edit source]

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.

  • InGaN alloys have been shown to have superior high energy radiation resistance for space based PV applications
  • The III-N alloys also tend to exhibit very strong absorption of approximately 105 cm−1 at the band edge
  • The III-N crystals grown on sapphire contain relatively high densities of threading dislocations, which negatively impact GaN p-n junction device performance by increasing leakage current
  • Large lattice mismatch between InN and GaN, makes it difficult to grow good quality material with high In content due to V-pit formation and InN segregation
  • Strain relaxation can result in defect formation that can increase nonradiative recombination18 and in turn degrade solar cell performance. (from reference)
  • Monochromatic illumination for quantum efficiency measurements was supplied by coupling the Xe lamp to an Oriel 260 monochromator with a spectral linewidth of <5 nm full width at half maximum
  • Measured solar cell parameters for typical devices with 25 and 166 um grid spacing are summarized. The devices with 166 um grid spacing demonstrated high peak external quantum efficiency ne of 63% at 392 nm and a flat (±3%) quantum efficiency response from 370 to 410 nm
  • 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
  • An estimated peak internal quantum efficiency ni of 94% was calculated
  • 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.

Photoelectric characteristics of metal/InGaN/GaN heterojunction structure[7][edit | edit source]

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.

  • A Schottkey Contact is formed between the InGaN and Ni/Au metal determined by the I-V curve measured between Ni/Au and InGaN. Also it is confirmed by Electron Beam induced current measurements. The diode has an apparently rectifying current characteristic
  • The photoresponse of the front-side illuminated PC spectrum consists of two parts apparently. The signal at the wavelength between 250 and 360 nm comes mainly from the GaN layer (and partly from the InGaN layer) and that between 360 and about 422 nm comes from the InGaN layer
  • The slope of the PC curve at about 422 nm (between 420 and 450 nm) is not as steep as at 360 nm, indicating that the alloy composition and then the related band gap are relatively inhomogeneous in the InGaN layer
  • The signal at the wavelength longer than about 475 nm in the front-side illuminated PC spectrum is not caused by the real PC, but comes from the second harmonic effect of monochromator grating and thus will be neglected
  • The photocurrent of the GaN layer (IGaN) is much higher than that of the InGaN layer (IInGaN) in the device, i.e. a higher optical responsivity is contributed by the GaN layer than the InGaN one
  • Two types of junction are formed between layers i.e InGaN/GaN isotype heterojunction and metal/InGaN Schottky junction. Although the current of the two junctions is not well matched in an optimized way, the two junctions are in series and can produce an additive contribution to the photoresponse
  • Although the current of the two junctions is not well matched in an optimized way, the two junctions are in series and can produce an additive contribution to the photoresponse
  • The EQE under front-side illumination presents a peak value of 14% at 360 nm and it is about 0.7% at 420 nm
  • Polarization will induce a fixed sheet charge density of 2DEG or two-dimensional hole gas (2DHG) due to polarization discontinuities at the nitride heterointerfaces
  • As the 2DEG sheet concentration at the InGaN side is rather high, the depletion layer at InGaN is extremely narrow; correspondingly almost no bright region could be resolved in the EBIC image of figure
  • The intensity of EBIC decreases while the CL intensity increases along the direction from the sample surface to the substrate
  • 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.

Growth and Characterization of InGaN for Photovoltaic Devices[8][edit | edit source]

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
  • Phase segregation occurs due to the high surface mobility of the indium atoms compared to the gallium atoms, the large disparity in the vapor pressures of these two metals, and the resulting narrow window of growth conditions in which reasonable properties can be achieved
  • The decay times of the PL and corresponding spectral dispersions are strong evidence for localized emission in this material. The most likely explanation for the localized emission is from small indium content fluctuations
  • Although the x-ray data from the 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
  • Observations- InxGa1-xN devices with x up to 0.54, corresponding to turn-on energies below 2.0 eV
  • Peak internal quantum efficiencies of the order 0.20 were observed from devices spanning an In compositional range from 15 % to nearly 40%
  • Isc's of the tested devices generally tended to increase with indium mole fraction, the Voc and other device characteristics overall tended to degrade with increasing indium content
  • 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

High internal and external quantum efficiency InGaN/GaN solar cells[9][edit | edit source]

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
  • The lattice mismatch between InN and GaN and low temperature growth of InGaN induce impurity incorporation, point, extended, and morphological defects, which generates nonradiative recombination centers (NRCs)
  • These NRCs reduce carrier lifetimes and reduce the solar cell shortcircuit current
  • An efficient conversion of lower energy photons requires high In content InGaN, which limits the maximum thickness of this layer, reducing its light absorption
  • There are significant polarization-related charges at the InGaN/GaN interfaces which result in large electric fields in the InGaN layer. For Ga-polar p-GaN/i- InGaN/n-GaN structures, the polarization-related electric field is in opposite sense to the depletion field of the diode,12,13 and forward biasing the device further increases the electric field in the InGaN layer
  • The 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
  • An improved light incoupling into the solar cells was achieved by rough surface, induced during the p-GaN growth, which reduced the reflection of the incident light at the device surface and increased the path length of the light inside the device active region
  • The active region was formed by a 60 nm thick InGaN grown at 880 °C, with In content ~12%, below a 300 nm thick p-type doped GaN layer. The n- and p-type GaN doping was [Si] ~6X1018 cm−3 and [Mg] ~8X1019 cm−3
  • The p-GaN in the smoother sample was grown at 955 °C (rms roughness of ~7 nm) and at 880 °C in the rougher sample (rms roughness of ~41 nm)
  • The IQE of the solar cells was assessed from the ratio between the EQE and the light absorption, which were determined by independent measurements
  • The light absorption was measured from the combination of the transmission and reflection measurements, performed in the unprocessed wafer, using a Shimadzu UV- 3600 UV-VIS-NIR spectrophotometer
  • In the GaN spectral absorbing region (λ<365 nm), the generated carriers from the strong light absorption do not contribute to current generation (EQE~0); instead, they mostly recombine due to the short carrier diffusion in the neutral p-GaN region
  • The light absorption follows closely the EQE curve, both in shape and magnitude. This means that nearly all absorbed photons in this layer were converted into electrons and holes, and these charges were efficiently separated and transported out of the device
  • The nonzero absorption observed in λ>410 nm region in the smoother sample is an artifact of the measurement. It corresponds to light propagating laterally in this nonabsorbing medium that was converted from the vertically incoming light by scattering at the surface roughness
  • The measurement artifact was verified by changing the size of the integrating sphere aperture, which consequently changes the amount of collected scattered light
  • The IQE is higher than 90% in both samples for λ from 380 to 410 nm. The smoother sample had a slightly higher IQE, reaching values up to 97%, while the rough sample had an IQE up to 93%
  • The reason for the higher IQE observed in the smoother solar cell is not fully understood and is attributed to run to run variations in the MOCVD growth. The high measured IQE revealed an efficient electron-hole generation and carrier transport out of the active region
  • One initial attempt to improve the light absorption is to increase the surface roughness of the solar cells. This reduces the reflection of the incoming light in the top surface and increases the optical path length of the light inside the solar cell due to a change in the incidence angle
  • Light absorption (at 380 nm) increased from 60 to 80 percent due to rougher surface
  • The peak EQE (average EQE curve) increased from 56% to 72%, at 380 nm, even though the IQE at this same wavelength was slightly reduced from 94% to 90%
  • The solar cell with rough surface presented a short-circuit current density of 1.06 mA/cm2, which was 27% higher than the smoother sample (0.83 mA/cm2)
  • The open circuit voltages were 1.89 and 1.83 V for the rough and smoother samples, respectively. These devices presented high fill factors (FF), 78.6% (rough) and 76.6% (smoother), revealing excellent solar cell performances
  • The better light coupling of the solar cell with rough surface to the device active region resulted in an enhancement of 35% in maximum output power
  • An enhanced light coupling scheme is required to improve the InGaN/GaN solar cell performances, which can be achieved by an improved surface texture, by antireflecting and high reflecting coatings in the top and bottom surfaces, respectively, or by coupling the incoming light to guided light using photonic crystals, which can be then fulabsorbed by the active region

Design and characterization of GaN/InGaN solar cells[10][edit | edit source]

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

Detailed balance modeling indicate that in order to achieve practical terrestrial photovoltaic efficiencies of greater than 50%, materials with band gaps greater than 2.4 eV are required

  • In addition to the wide band gap range, the nitrides also demonstrate favorable photovoltaic properties such as low effective mass of carriers, high mobilities, high peak and saturation velocities, high absorption coefficients, and radiation tolerance
  • To maximize absorption in the field bearing i-region of a p-i-n solar cell for maximum collection, the thickness of the top p-region is limited to 100 nm, which is just enough to provide charge for the junction and the top metal contacts
  • Typical rocking curves for the In0.05Ga0.95N/GaN solar cell epilayers measure full widths at half maximum for GaN (peak: 17.28°) at 24.5 arc sec and In0.05Ga0.95N (peak: ~17.19°) at 72.1 arc sec confirming a high crystalline quality
  • Optical characterization using room-temperature (RT) photoluminescence reveals phase separation within the material
  • Each Gaussian curve in the PL spectra indicates a separate InGaN phase with band gap corresponding to the center of that curve
  • A Schottky barrier at the nonoptimal p-GaN–Ni metal contact interface opposes the light generated current so that the current asymptotically approaches zero prior to the device reaching its VOC
  • The solar cells with semitransparent current spreading layers measure internal quantum efficiencies (IQEs) around 60% at the band edge, as shown in Fig. 4 and external quantum efficiencies around 43%. The major loss mechanisms here are absorption of up to 40% of the incident light by the semitransparent current spreading layer and transmittance of at least 10% of the incident light through the solar cell, as confirmed through spectrometry
  • Devices with interdigitated grid contacts measure a lower IQE of around 50% at the band edge due to the Schottky barrier at the p-GaN–Ni nonoptimized contact and high series resistances. Moreover, this Schottky barrier causes an increase in the cutoff voltage by 0.3 V compared to the devices with semitransparent current spreading layers
  • The III-V nitrides are highly pyroelectric materials. The large polarization charge densities present at nitride heterojunction interfaces profoundly influence electric field and mobile carrier distributions, necessitating their incorporation into device design and analysis
  • Short-period superlattice (SPS) structures provide better tunneling contacts to p-type nitrides and increase the lateral conductivity of the device
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