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.

This page describes selected literature available on InGaN photovoltaics.

InGaN quantum dot photodetectors[1]

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[2]

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[3]

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[4]

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[5]

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[6]

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[7]

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 cellsCite error: Invalid <ref> tag; invalid names, e.g. too many

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.

Photovoltaic Effects of InGaN/GaN Double Heterojunctions With p-GaN Nanorod Arrays[8]

Abstract: The p-GaN/In0.06Ga0.94N/n-GaN double heterojunctional solar cells with solely formed nanorod arrays of p-GaN have been fabricated on sapphire (0001). The p-GaN nanorod arrays are demonstrated to significantly reduce the reflectance loss of light incidence. A stress relief of the intrinsic InGaN region is observed from high-resolution X-ray diffraction analyses. The electroluminescence emission peak is blue shifted compared with the conventional solar cells. These results are reflected by the spectral dependences of the external quantum efficiency (EQE) that show a shorter cutoff wavelength response. The maximum EQE value is 55.5%, which is an enhancement of 10% as compared with the conventional devices.

  • Group-III nitrides have a reasonably high refractive index (~2.5), which leads to a reflectance of 18% according to Fresnel’s law from reference.
  • till date, due to problems such as material selection and thermal expansion, the devices (InGaN) with antireflective coatings are not reported.
  • solely formed p-GaN nanostructures are proposed and explored to reduce the reflectance loss.
  • vertically aligned p-GaN nanorod arrays may be useful in improving the conversion efficiency and reducing material consumption.
  • p-GaN nanorod arrays can effectively reduce optical loss in PV applications
  • p-GaN nanorod arrays applied in InGaN/GaN heterojunctional solar cells (author(s) prototype) yielded up to 55.5% peak EQE.
  • low reflectance of p-GaN nanorod arrays originated from high surface area and subwavelength scale of the nanorods, can be used effectively for their enhanced antireflection ability.
  • p-nanorod type solarcell exhibited considerably small reflectance of up to less than 1% within entire wavelength range whereas for p-GaN (planar type) showed reflectivity of 18% as determined by Fresnel’s law for air/GaN interface
  • reduction in the piezoelectric field caused by partial strain release induces a blue-shift in p-nanorod solar cell

Improved Conversion Efficiency of GaN/InGaN Thin-Film Solar Cells[9]

Abstract: In this letter, Author(s) report on the fabrication and photovoltaic characteristics of p-i-n GaN/InGaN thin-film solar cells. The thin-film solar cells were fabricated by removing sapphire using a laser lift-off technique and, then, transferring the remaining p-i-n structure onto a Ti/Ag mirror-coated Si substrate via wafer bonding. The mirror structure is helpful to enhance light absorption for a solar cell with a thin absorption layer. After the thin-film process for a conventional sapphire-based p-i-n solar cell, the device exhibits an enhancement factor of 57.6% in current density and an increment in conversion efficiency from 0.55% to 0.80%. The physical origin for the photocurrent enhancement in the thin-film solar cell is related to multireflection of light by the mirror structure.

  • crystalline defects commonly observed include v-shaped pits, phase separation, and dislocations, which have been shown to deteriorate device performance by increasing leakage current
  • conventional solar cell with a thin InGaN absorption layer exhibits smaller photocurrent than that with a thick InGaN layer does, the thin-InGaN solar cell shows better performance in open-circuit voltage, fill factor, and shunt resistance
  • For enhancing light absorption, a highly reflective mirror is employed in thin-InGaN solar cell, which improved current density from 0.33 to 0.52 mA/cm2 and an increment in conversion efficiency from 0.55% to 0.80%
  • mirrorcoated Si substrate has another advantage of good heat dissipation as compared with conventional sapphire-based devices

High-quality InGaN/GaN heterojunctions and their photovoltaic effects[10]

Abstract: High-quality p-GaN/i-In0.1Ga0.9N/n-GaN heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1 eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.

  • crystalline defects commonly observed include v-shaped pits, phase separation, and dislocations, which have been shown to deteriorate device performance by increasing leakage current.
  • fabricated high quality p-i-n type solar cell using MOCVD by adjusting the layer thickness using critical thickness calculations
  • leakage current density increases with area/periphery ratio of diode, reavealing leakage current as one of main components degrading photovolatic performance

Growth, fabrication, and characterization of InGaN solar cells[11]

Abstract: The InGaN alloy system offers a unique opportunity to develop high efficiency multi-junction solar cells. In this study, single junction solar cells made of Inx Ga1–xN are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si-doped InGaN layer, an intrinsic layer and an Mg-doped InGaN layer on the top. The I –V curves indicate that the cell made of all-GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo-responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films.

  • good control of p-type doping in InGaN is one of most critical issues. Due to unusual low position of the conduction band edge at 0.9 eV below Fermi level stabilization energy (EFs), p-type doping of InGaN has proved extremely difficult. evaluation of p-type doping with Magnesium (Mg) as an acceptor still remains a challenge because strong surface accumulation of electrons exists throughout most of the Indium (In) composition range.
  • surface accumulation represents a possible significant parasitic conductivity path between p and n contacts on solar cell structures.
  • suggested that a thicker buffer layer or GaN template can be applied to reduce defect density of InGaN
  • devices with high In compositions have much higher leakage current, which cause difficulty to determine turn-on voltage and output power of cells
  • high In mole fraction alloy has a strong surface electron accumulation which can also contribute to the leakage in InGaN cells
  • leakage current density goes up as area/periphery ratio of the diode increases, which tell that bulk leakage is one of main component in leakage current in devices.
  • high defect density in InGaN materials may cause the abnormal bulk leakage current in devices

Characteristics of InGaN designed for photovoltaic applications[12]

Abstract: This work addresses the required properties and device structures for an InGaN solar cell. Homojunction InGaN solar cells with a bandgap greater than 2.0 eV are specifically targeted due to material limitations. These devices are attractive because over half the available power in the solar spectrum is above 2.0 eV. Using high growth rates, InGaN films with indium compositions ranging from 1 to 32% have been grown by Molecular Beam Epitaxy with negligible phase separation according to X-ray diffraction analysis, and better than 190 arc-sec ω-2θ FWHM for ∼0.6 μm thick In0.32Ga0.68N film. Using measured transmission data, the adsorption coefficient of InGaN at 2.4 eV was calculated as α ≅ 2×105 cm–1 near the band edge. This results in the optimal solar cell thickness of less than a micron and may lead to high open circuit voltage while reducing the constraints on limited minority carrier lifetimes.

  • suggested that InN films must be grown at low temperatures, such as 360–550 °C because of low dissociation temperature of InN
  • at low substrate temperatures, it is difficult to achieve p-type InGaN while maintaining good crystalline structures
  • InGaN has phase separation for high In composition due to the immiscibility of InN in GaN
  • phase separation also affects the bandgap of material, creating localized regions of different composition materials, seriously limiting efficiency of solar cell
  • phase separation can be suppressed by increasing growth rate, author(s) successfully grew InGaN films with various In compositions by Molecular Beam Epitaxy (MBE) at rates in excess of 0.6–1.3 μm/hr.
  • absorption coefficient of InGaN at 2.4 eV was calculated to be α ~ 2×105 cm–1, near the band edge

Growth of InGaN self-assembled quantum dots and their application to photodiodes[13]

Abstract: Nanometer-scale InGaN self-assembled quantum dots (QDs) have been prepared by growth interruption during 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 QD density was about 2×1010 cm−2. In contrast, much larger InGaN QDs were obtained without growth interruption. InGaN metal-semiconductor-metal photodiodes with and without QDs were also fabricated. It was found that the QD photodiode with lower dark current could operate in the normal incidence mode, and exhibit a stronger photoresponse.

  • Author(s) could achieve a much larger photocurrent to dark current contrast ratio from MSM photodiodes with nanoscale InGaN SAQDs

InGaN/GaN multiple quantum well concentrator solar cells[14]

Abstract: Author(s) present the growth, fabrication, and photovoltaic characteristics of Inx Ga1−xN/GaN(x ~ 0.35) multiple quantum well solar cells for concentrator applications. The open circuit voltage, short circuit current density, and solar-energy-to-electricity conversion efficiency were found to increase under concentrated sunlight. The overall efficiency increases from 2.95% to 3.03% when solar concentration increases from 1 to 30 suns and could be enhanced by further improving the material quality.

  • requirements of an active material system for obtaining solar cells with a conversion efficiency greater than 50% can be fulfilled by InGaN alloys with In-content of about 40%.
  • large lattice mismatch between InN and GaN, results in phase separation and as a consequence reported values of open circuit voltages (Voc) for different In contents in general are significantly lower than theoretical values (thermodynamic limit)
  • strain could suppress phase separation in InGaN
  • used strain to suppress phase separation for In(x)Ga(1-x)N (x~0.3) based multiple quantum well
  • advantages of low dimensional InGaN MQW solar cells include (i) crystalline quality of thin light absorption layers (InGaN wells) embedded between GaN barriers is higher than that of InGaN epilayers with thickness exceeding the critical thickness, (ii) with incorporation of MQW structure in the i-region, Voc and Jsc can be independently optimized. (iii) MQW solar cells are expected to outperform bulk i-layer solar cells under concentrated sunlight
  • described Voc as a function of concentrated sun light by mathematical expression

Optimization of GaN window layer for InGaN solar cells using polarization effect[15]

Abstract: The III-nitride material system offers substantial potential to develop high-efficiency solar cells. Theoretical modeling of InGaN solar cells indicate strong band bending at the top surface of p-InGaN junction caused due to piezoelectric polarization-induced charge at the strained p-GaN window interface. A counterintuitive strained n-GaN window layer is proposed, modeled and experimentally verified to improve performance of InGaN solar cells. InGaN solar cells with band gap of 2.9 eV are grown using MOCVD with p-type and n-type strained GaN window layers, and fabricated using variable metallization schemes. Fabricated solar cells using n-GaN window layers yield superior VOC and FF compared to those using p-GaN window layers. The VOC's of InGaN solar cells with n-GaN window layers are further enhanced from 1.5 V to 2 V by replacing the conventional NiOX top contact metal with Ti/Al, which also verifies the tunneling principle.

  • net polarization and consequent internal electric fields have detrimental effect on performance of optoelectronic devices due to polarization-induced potential barriers and band bending.
  • demonstrated counterintuitive design, fabrication and optimization of the GaN window layer for InGaN solar cells mediated by polarization effects
  • thin GaN window layers, designed in a 2 – 10 nm thickness range, are typically strained due to lattice mismatch with the underlying InGaN layer and hence, generate substantial piezoelectric polarization.
  • simulations indicated that tunneling contacts using n-type material can potentially provide superior Ohmic characteristics to p-type GaN or InGaN contacts.
  • due to lower resistivity and ease of forming Ohmic contacts to n-GaN compared to p-GaN, tunneling contact using n-type material indicates a viable alternative
  • Strained n-GaN window layers enhance tunneling of holes from the p-InGaN junction due to piezoelectrically induced sheet charge and strong band bending at heterointerface
  • Fabricated InGaN solar cells with band gap of 2.9 eV and n-GaN window layer demonstrate a superior performance compared to those with p-GaN window layers.

Simulation of In0.65Ga0.35 N single-junction solar cell[16]

Abstract: The performances of In0.65Ga0.35N single-junction solar cells with different structures, including various doping densities and thicknesses of each layer, have been simulated. It is found that the optimum efficiency of a In0.65Ga0.35N solar cell is 20.284% with 5 × 1017 cm−3 carrier concentration of the front and basic regions, a 130 nm thick p-layer and a 270 nm thick n-layer.

  • When carrier concentrations of front and basic regions are 5×1017 cm−3, thickness of p-layer and n-layer are 130 nm and 270 nm, respectively, optimum efficiency calculated is 20.284% (AM1.5G, 100mWcm−2, 0.32–1.32μm)
  • summarized expression for efficiency, minority carrier, diffusion length, open circuit voltage short-circuit current and other experimentally measured properties of GaN and InGaN.

Photoconductivity in nanocrystalline GaN and amorphous GaON[17]

Abstract: In this work Author(s) present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a‐GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a‐GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, Author(s) build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, Author(s) are able to characterize the density of states in the gap for the two materials.

  • nc-GaN film exhibits a weak photoconductivity (approximately 50 fA at 9 V, corresponding to 10−7 A/W) which is fast, switching on and off
  • GaON film has a response several orders of magnitude greater (several nanoamperes at 9 V, corresponding to 10−2 A/W) which is very slow, decaying nonexponentially over the course of hours, i.e., showing PPC (persistent photoconductivity)
  • By examining intensity dependence of photoconductivity under 325 nm illumination, photoresponse was found to be linear in generation rate in nc-GaN films and had a squareroot dependence in GaON films.
  • Spectral dependence of photoconductivity showed an exponential drop-off in combined densities of states of conduction- and valence-band tails with decreasing energy for both types of film.
  • Author(s) developed a model which considers the effect of a large density of localized states in the gap to describe the photoconductive behavior observed in these two types of films

Theoretical possibilities of InxGa1−xN tandem PV structures[18]

Abstract: Author(s) designed a model of InxGa1−xN tandem structure made of N successive p–n junctions going from two junctions for the less sophisticated structure to six junctions for the most sophisticated. Author(s) simulated the photocurrent density and the open-circuit voltage of each structure under AM 1.5 illumination in goal to optimize the number of successive junctions forming one structure.

For each value of N, Author(s) assumed that each junction absorbs the photons that are not absorbed by the preceding one. From the repartition of photons in the solar spectrum and starting from the energy gap of GaN, Author(s) fixed the gap of each junction that gives the same amount of photocurrent density in the structure. Then Author(s) calculated the current density accurately and optimized the thicknesses of p and n layers of each junction to make it give the same output current density. The evaluation of ni: the intrinsic concentration permitted to calculate the saturation current density and the open-circuit voltage of each junction. Assuming an overall fill factor of 80%, Author(s) divided the output peak power by the incident solar power and obtained the efficiency of each structure.

The numerical values for InxGa1−xN were taken from the relevant literature. The calculated efficiency goes from 27.49% for the two-junction tandem structure to 40.35% for a six-junction structure. The six-junction InxGa1−xN tandem structure has an open-circuit voltage of about 5.34 V and a short circuit current density of 9.1 mA/cm2.

The impact of piezoelectric polarization and nonradiative recombination on the performance of (0001) face GaN/InGaN photovoltaic devices[19]

Abstract: The impact of piezoelectric polarization and nonradiative recombination on the short-circuit current densities (Jsc) of (0001) face GaN/InGaN photovoltaic devices is demonstrated. P-i-n diodes consisting of 170 nm thick intrinsic In0.09Ga0.91N layers sandwiched by GaN layers exhibit low Jsc ~ 40 μA/cm2. The piezoelectric polarization at the GaN/InGaN heterointerfaces creates drift currents opposite in direction needed for efficient carrier collection. Also, nonradiative recombination centers produce short carrier lifetimes, limiting Jsc. Alternative structures with intrinsic InGaN layers sandwiched by n-type InGaN or graded InyGa1−yN (y = 0–0.09) layer and a p-type In0.015Ga0.985N layer have favorable potentials, longer carrier lifetimes, and improve Jsc to ~ 0.40 mA/cm2.


Superior radiation resistance of In1−xGaxN alloys: Full-solar-spectrum photovoltaic material system[20]

Abstract: High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently author(s) have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edges relative to the average dangling bond defect energy represented by the Fermi level stabilization energy in In1−xGaxN alloys.

Plasmonic nanoparticle enhanced photocurrent in GaN/InGaN/GaN quantum well solar cells[21]

Abstract: Author(s) demonstrate enhanced external quantum efficiency and current-voltage characteristics due to scattering by 100 nm silver nanoparticles in a single 2.5 nm thick InGaN quantum well photovoltaic device. Nanoparticle arrays were fabricated on the surface of the device using an anodic alumina template masking process. The Ag nanoparticles increase light scattering, light trapping, and carrier collection in the III-N semiconductor layers leading to enhancement of the external quantum efficiency by up to 54%. Additionally, the short-circuit current in cells with 200 nm p-GaN emitter regions is increased by 6% under AM 1.5 illumination. AFORS-Het simulation software results were used to predict cell performance and optimize emitter layer thickness.

Nearly lattice-matched n, i, and p layers for InGaN p-i-n photodiodes in the 365–500 nm spectral range[22]

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.

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells[23]

Abstract: Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.

Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates[24]

Abstract: Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, Author(s) report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, Author(s) demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, Author(s) demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

Nanowire Solar Cells[25]

Abstract: The nanowire geometry provides potential advantages over planar waferbased or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the maximum efficiency above standard limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost reductions. Additionally, nanowires provide opportunities to fabricate complex single-crystalline semiconductor devices directly on low-cost substrates andelectrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technology. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.

40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells

Abstract: An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP/GaInAs/Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm2, AM1.5D, low aerosol optical depth, 25 °C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

Silicon nanowire solar cells

Abstract: Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼ 1.6 mA/cm2 for 1.8 cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼ 12% at 690 nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.

Effects of concentrated sunlight on organic photovoltaics

Abstract: Author(s) report the effects of concentrated sunlight on key photovoltaic parameters and stability of organic photovoltaics (OPV). Sunlight collected and concentrated outdoors was focused into an optical fiber and delivered onto a 1 cm2 bulk-heterojunction cell. Sunlight concentration C was varied gradually from 0.2 to 27 suns. Power conversion efficiency exhibited slow increase with C that was followed by saturation around 2% at C = 0.5–2.5 suns and subsequent strong reduction. Possible OPV applications in stationary solar concentrators (C ≤ 2 suns) are discussed. Finally, experiments at C = 55–58 suns demonstrated potential of our approach for accelerated studies of light induced mechanisms in the OPV degradation.

The interdigitated back contact solar cell: A silicon solar cell for use in concentrated sunlight

Abstract: The theoretical and experimental performance of an interdigitated back contact solar cell is described. This type of cell is shown to have significant advantages over a conventional solar cell design when used at high concentration levels, namely, reduced internal series resistance, nonsaturating open-circuit voltage, and an absence of shadowing by front surface contacting fingers. The results of a computer study are presented showing the effects of bulk lifetime, surface recombination velocity, device thickness, contact dimensions, and illumination intensity on the conversion efficiency and general device operation. Experimental results are presented for solar illumination intensities up to 28 W/cm2.

Temperature dependences of InxGa1−xN multiple quantum well solar cells

Abstract: In this work, high open circuit voltages (Voc) of In0.2Ga0.8N and In0.28Ga0.72N multiple quantum well solar cells (MQWSCs) are experimentally obtained (2.2 V and 1.8 V, respectively). The Voc of In0.28Ga0.72N MQWSCs is lower than the expected value due to serious indium segregation problems causing more defects in In0.28Ga0.72N films, which is consistent with the observation of a high ideality factor in dark current measurement. The temperature dependence of the Voc and the short circuit current (Jsc) in In0.2Ga0.8N MQWSCs is found to be larger than the corresponding values in In0.28Ga0.72N MQWSCs. It is also observed that higher quantum well energy barrier exhibits a low fill factor of 0.52 due possibly to the loss of electric field and the higher energy barrier. This obtained efficiency increases with temperatures up to 100 °C and then decreases due to competing results between the reduction in Voc and an increase in Jsc.

Substantial photo-response of InGaN p–i–n homojunction solar cells

Abstract: InGaN p–i–n homojunction structures were grown by metal-organic chemical vapor deposition, and solar cells with different p-contact schemes were fabricated. X-ray diffraction measurements demonstrated that the epitaxial layers have a high crystalline quality. Solar cells with semitransparent p-contact exhibited a fill factor (FF) of 69.4%, an open-circuit voltage (Voc) of 2.24 V and an external quantum efficiency (EQE) of 41.0%. On the other hand, devices with grid p-contact showed the corresponding values of 57.6%, 2.36 V, 47.9% and a higher power density. These results indicate that significant photo-responses can be achieved in InGaN p–i–n solar cells.

Low-resistance ohmic contacts to p-type GaN achieved by the oxidation of Ni/Au films

Abstract: A contact has been developed to achieve a low specific contact resistance to p-type GaN. The contact consisted of a bi-layer Ni/Au film deposited on p-type GaN followed by heat treatment in air to transform the metallic Ni into NiO along with an amorphous Ni–Ga–O phase and large Au grains. A specific contact resistance as low as 4.0×10−6 Ω cm2 was obtained at 500 °C. This low value was obtained by the optimization of Ni/Au film thickness and heat treatment temperatures. Below about 400 °C, Ni was not completely oxidized. On the other hand, at temperatures higher than about 600 °C, the specific contact resistance increased because the NiO detached from p-GaN and the amount of amorphous Ni–Ga–O phase formed was more than that of the sample annealed at 500 °C. The mechanism of obtaining low-resistance ohmic contacts for the oxidized Ni/Au films was explained with a model using energy band diagrams of the Au/p-NiO/p-GaN structure.

Engineering light absorption in semiconductor nanowire devices

Abstract: The use of quantum and photon confinement has enabled a true revolution in the development of high-performance semiconductor materials and devices. Harnessing these powerful physical effects relies on an ability to design and fashion structures at length scales comparable to the wavelength of electrons (1 nm) or photons (1 m). Unfortunately, many practical optoelectronic devices exhibit intermediate sizes where resonant enhancement effects seem to be insignificant. Here, Author(s) show that leaky-mode resonances, which can gently confine light within subwavelength, high-refractive-index semiconductor nanostructures, are ideally suited to enhance and spectrally engineer light absorption in this important size regime. This is illustrated with a series of individual germanium nanowire photodetectors. This notion, together with the ever-increasing control over nanostructure synthesis opens up tremendous opportunities for the realization of a wide range of high-performance, nanowire-based optoelectronic devices, including solar cells, photodetectors, optical modulators14 and light sources.

Contact effects of solution-processed polymer electrodes: Limited conductivity and interfacial doping

Abstract: Contact effects between solution processed conducting polymer electrodes with semiconducting polymers in field effect transistors are investigated. Limited conductivity of polymer electrodes and interfacial doping of the active semiconducting polymer by the conducting polymer electrode are found to be two important factors in determining the performance of polymer field effect transistors with printed conducting polymer electrodes.

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