Subcontract Report 

NREL/SR-520-44186 

October 2008 

Novel High Efficiency 

Photovoltaic Devices Based 

on the III-N Material System 

Final Technical Report 

7 December 2005 – 29 August 2008 

C. Honsberg, W.A. Doolittle, and I. Ferguson 

University of Delaware 

Newark, Delaware 
Novel High Efficiency 

Photovoltaic Devices Based 

on the III-N Material System 

Final Technical Report 

7 December 2005 – 29 August 2008 

C. Honsberg, W.A. Doolittle, and I. Ferguson 

University of Delaware 

Newark, Delaware 

NREL Technical Monitor: L. Greene and M. Symko-Davies

Prepared under Subcontract No. XEJ-6-55157-01 

Subcontract Report 

NREL/SR-520-44186 

October 2008 

National Renewable Energy Laboratory

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Contract No. DE-AC36-08-GO28308
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Introduction 

The initial motivation for the project laid in the then-newly discovered band gap of InN, 

re-measured at ~0.7 eV rather than the previous 1.9 eV [1-3]. This makes InGaN a potential 

material for solar cell with the possibility to absorb 99% of the solar irradiance. The advantages 

of the InGaN material system are the wide range of direct band gap, high absorption coefficient, 

a low effective mass (high mobility), and strong polarization effects [4-6]. However, the 

challenges include material quality, defect density, doping, substrates and growth issues. The 

current project led to the understanding of the above challenges; in particular the material quality 

in terms of phase separation was studied and suppressed. The polarization property was modeled 

and solar cell design was developed with these new models. 

The following section describes the issue of phase separation and its suppression. Then a 

brief description of polarization and its modeling is presented. The effect of phase separation and 

polarization on device operation follows. The next section presents the solar cell design and 

results achieved. 

Phase Separation and Its Suppression

There exists a solid phase miscibility gap in the InGaN alloy due to the large difference in 

the lattice constants between GaN and InN, which is also the probable cause of multiple phases 

and consequent multi-peak luminescence observed in the material [7,8]. The equilibrium 

solubility of InN in the bulk GaN is approximately 6% at typical growth temperatures used in 

MOCVD. However, the situation in thin InGaN films epitaxially deposited on GaN virtual 

substrates is significantly different. Theoretical calculations [9] based on a valence-force-field 

(VFF) model [7, 10] predict that phase separation in InGaN strongly depends not only on the 

temperature and In composition, but also on the strain state of the InGaN films. Thus, one or 

more indium-rich phases come into existence in the InGaN alloy layers during growth in an 

attempt to reach thermodynamic equilibrium during growth as shown in Figure 1. 

Phase separation is usually identified as secondary peaks in addition to the primary peak 

corresponding to the bulk material during photoluminescence and, while higher degrees of phase 

separation are also identified via X-ray diffraction (XRD), shown in Figure 2. 

In addition to acting as a recombination channel, it can be correlated from fabricated 

quantum-well solar cells that the lower-band gap phase separated material will also tend to pin 

down the open-circuit voltage (VOC) of the solar cell. 

(a) (b)

Figure 1: Schematic comparison of band structures of (a) an ideal material, and (b) a 

phase separated material. 

1
Figure 2: Summary of (a) Photoluminescence (PL) (b) PL and X-ray diffraction data for InGaN 

grown by MOCVD with indium composition ranging from 0 to 35%.

To minimize the effect of phase separation and increase the material quality of the grown 

layers, the effect of TMIn flow, temperature, growth pressure, growth rate and TEGa flow were 

examined and optimized. The effect of TMIn flow is shown in Figure 3. Increasing the flow rate 

suppresses the secondary peak in PL signals, indicating that phase separation is suppressed. 

The effect of growth temperature is shown in Figure 5. As the growth temperature is 

decreased, the surface morphology becomes rough and the growth mode changes from step growth 

to nucleation of discrete islands. However, the growth temperature, in conjunction with the TEGa 

flow rates determines the growth rate, which also influences phase separation and the indium 

composition. As the TEGa flow rate increases, the FWHM of XRD increases, indicating poorer 

material quality. 

The final critical parameter in controlling phase separation is the thickness of the layer. 

The effect of thickness is shown in Figure 4. Figure 4 shows, the appearance of a secondary 

phase of lower band gap InGaN with In ≈ 10% for thickness of 100nm. Further optimization 

allows minimization of the phase separation for layers as thick as 200 nm. 

2
Figure 3: PL data summary of InGaN growth 

with variable TMIn flow rate.

Figure 4: Photoluminescence data indicating 

increasing emission from phase separated 

InGaN with increasing thickness.

Polarization Modeling 

800 oC 760 oC 720 oC 680 oC 640 800 oC oC 760 oC 720 oC 680 oC 640 oC

Figure 5: Effect of growth temperature on surface morphology, showing change of growth 

mode from step-like growth to nucleation of discrete islands. 

One of the unique characteristics about the wurtzite III-nitrides is its polarization effects. 

The net polarization, Spontaneous polarization ( Psp

r ) + Piezoelectric polarization ( Ppzr ), and 

consequent internal electric fields have been shown to be detrimental to the performance of 

optoelectronic devices [11-13]. Polarization discontinuities lead to potential barriers, band 

bending that can have undesired consequences on the device; electric fields with values as high 

as 1 MV/cm have been reported [14, 15]. Polarization, however, can be utilized constructively 

by accommodating in the solar cell design to improve its performance. Thus, it becomes vital to 

successfully model this effect and incorporate it during device design. 

Polarization is present in the III-nitrides as a consequence of the non-centrosymmetry of 

the wurtzite structure and the large ionicity of the covalent metal-nitrogen bonds. Spontaneous 

polarization ( ) exists due to the non-centrosymmetry of the crystal, where the crystal behaves 

like an electric dipole. In the III-nitrides, the direction of spontaneous polarization is from the NPsp

r

3
atom to the closest metal atom, i.e. along the +c direction, which is also the typical MOCVD or 

MBE growth direction. Hence, in Ga-face GaN, spontaneous polarization is opposite to the 

[0001] direction, i.e. in the downward direction pointing towards the substrate as shown in 

Figure 6(a). 

(a)

(b) 

(c)

Figure 6 (a) Direction of spontaneous polarization jn Ga-face GaN and 

GaN/InGaN p-i-n test structure (b) without and (c) with spontaneous polarization effects

Spontaneous polarization in ternary compounds can be calculated by interpolating the 

binary compounds using a bowing factor ‘b’ as shown in Equation 1. 

xbxxPxPxP )1()1()()( BN

sp

AN

sp

ABN

sp −+−+= [Units: C/m2

] (1) 

The direction of piezoelectric polarization is dependent on the polarity of the material as 

well as on the strain, shown in Figure 6(b). Spontaneous and piezoelectric polarizations are 

parallel and add to each other when planar strain (perpendicular to c-axis) is tensile and are antiparallel when planar strain is compressive. 

The polarization-induced charge density with a gradient of polarization in space given by: 

σ P −∇= P [Units: C/m2] (2) 

The polarization-induced charge density or electric field is used as the starting point for 

the modified PC1D solving routine at a given hetero-interface. If the charge density at a heterointerface is positive, it will tend to accumulate a 2-Dimensional Electron Gas (2DEG); similarly, 

a negative hetero-interface charge density will tend to form a 2-Dimensional Hole Gas (2DHG). 

The modified PC1D, incorporates polarization, is used in the design of InGaN solar cells. 

The design of pn-junction solar cells starts with a p-i-n structure and evolves to the current status 

of utilizing n-GaN capping layer. Alongside the design and fabrication of pn-junction solar cells 

4
5

alternative approaches in designing solar cells are considered. The pn-junctions and alternative 

approaches are presented in the next section. 

Device Design and Results 

 Two approaches have been attempted in the design of InGaN solar cells, pn-junction 

solar cells and Schottky-barrier solar cells. This section presents the evolution of InGaN pnjunction solar cell designs fabricated and tested is summarized in Figure 7, followed by results of 

Schottky-barrier solar cells. 

 The solar cells are designed, grown by MOCVD/MBE, fabricated, characterized and 

analyzed before designing the next generation of devices. The p-i-n device structure (Figure 7(a)) 

is taken as the starting point to design subsequent InGaN solar cells due to a detailed 

understanding obtained from preliminary experiments. The indium composition in the test 

devices is fixed at 12% (Eg – 2.9 eV) to ensure successful p-type and n-type doping. Here, the pand n-GaN regions are fixed at 100 nm and 500 nm thicknesses, respectively and doped at 

maximum concentrations without reaching a level of degeneracy. The thickness of the i-region 

is fixed at 200 nm. 

As the i-InGaN region adds to the series resistance of the solar cell, this region is 

replaced by a p-n InGaN junction in second-generation solar cells as shown in Figure 7(b). In 

this case, the additional electric field generated by the InGaN p-n junction enhances the QE of 

the solar cell. Moreover, the p-GaN layer not only acts as a window layer for the underlying 

InGaN p-n junction, but the GaN junctions also induce front and back surface fields for this 

device. The thicknesses of p- and n-InGaN are set to 50 nm and 150 nm, respectively to 

maintain a net InGaN thickness of 200 nm. 

 The I-V curves for the second-generation InGaN devices show a substantial improvement 

in short-circuit current of the solar cell compared to the first-generation devices as shown in 

Figure 8(a) and (b). The increase in short-circuit current is supported by the increase in internal 

quantum efficiency of the second generation devices up to 50% as seen in Figure 9. However, 

there is a decrease in the open-circuit voltage due to reduction in quasi-fermi level energy 

difference caused by additional doping in the InGaN junctions and/or formation of phaseseparated defects. The third-generation InGaN devices are designed to minimize the voltage loss 

caused by the p-GaN window layer due to its high resistivity. Hence, the thickness of the p-GaN 

window layer is reduced from 100 nm to 10 m as shown in Figure 7(c). However, the 50 nm pInGaN layer is not thick enough to provide charge to the depletion region, which results into an 

incomplete junction formation. Hence, these solar cells measure very low open-circuit voltages 

and quantum efficiencies. 

The fourth-generation devices are designed to enhance the window layer by replacing the 

10 nm p-GaN with n-GaN. The strained window layer forms a 2DEG at the n-GaN/p-InGaN 

interface, as explained in previous section, improving the lateral conduction in the material, 

while the n-GaN window enhances tunneling to reduce the top contact resistance of the device. 

However, these devices also fail to provide substantial open-circuit voltage and quantum 

efficiency due to incomplete charge provision by the thin p-junctions. 
(a) Gen 1: p-i-n structure. (b) Gen 2: i-InGaN replaced by p-n InGaN. 

(c) Gen 3: Thickness of p-GaN reduced. (d) Gen 4: p-GaN cap replaced by n-GaN. 

(e) Gen 5: Thickness of InGaN increased. (f) Gen 6: Thickness of InGaN increased. 

Figure 7: Generations of InGaN solar cell evolution. 

6
(a)

(b)

Figure 8: Sample I-V characteristics of (a) first-generation, and (b) second generation test 

solar cells. 

Figure 9: Internal quantum efficiency comparison of first and second-generation test solar 

cells. 

7
The next generation solar cells are designed to overcome the limitations caused by the 

thin p-type InGaN junctions. Here, the thickness of InGaN p-n junction is increased to 300 nm, 

where the thickness of p- and n- InGaN are designed at 100 nm and 200 nm, respectively. The 

fifth and sixth-generation solar cells employ a 10 nm thick p- and n-GaN window layer, 

respectively. These devices yield a substantially higher performance compared to the fourth and 

fifth- generation devices. Highest open-circuit voltages are obtained from the sixth-generation 

solar cells, which employ a thin n-GaN tunneling window layer as shown in Figure 10.

(a) (b) 

Figure 10: Sixth-generation (a) test solar cells structure (b) IV photo response. 

The pn-junction InGaN solar cells have a good response but the understanding of the 

electrical and optical issues originating in each layer are better studied with Schottky-barrier 

devices. The Schottky device structures play an important role in understanding the electrical 

properties of the material along with being good alternative approaches for making solar cells. 

Two types of Schottky-barrier solar cells can be formed (a) Metal-Semiconductor (b) MetalInsulator-Semiconductor. Each of these is presented below. 

The difference between an Ohmic contact and Schottky contact is the difference in the 

work function between metal and semiconductor. In the present work a study of metal-InGaN is 

being studied. Figure 11 pictorially demonstrates the formation of Ohmic and Schottky junction. 

Initial experiments on p-GaN were conducted with Ti/Au as the Schottky metal contact. 

The metal contacts were annealed in the temperature range of 550 o

C and 700 oC. Below 550 oC 

no Schottky diodes were formed. At annealing temperatures greater than 650o

C the metal 

transformed into small clusters on the surface. Forming gas was used for gas flow, and no 

significant effect of gas flow rate was observed. The initial results are shown in Figure 12(a) for 

a Ti/Au metal contact to p-GaN annealed at 650o

C for 2 min in forming gas, highlighting the 

photovoltaic effect. 

The Schottky-barrier heights are calculated based on Richardson plots Figure 12(b). The 

barrier height of the device is calculated using the following formula [16]: 

φ B = V1

n − k

q

d[ln(I/T2)]

d(1/T) , where V1 is a reference voltage, and n is the ideality factor. 

The Schottky-barrier height is calculated to be 0.48 eV. 

8
(a) (b) 

Figure 11: Metal-Semiconductor junction formation 

-6.00E-06

-4.00E-06

-2.00E-06

0.00E+00

2.00E-06

4.00E-06

-1 -0.5 0 0.5 1 1.5 2 2.5

Voltage (V)

Current (A)

Dark

UV

)UV (With UV pass filter

Solux Lamp

(a) 

1.00E-06

1.00E-05

1.00E-04

1.00E-03

2.5 2.7 2.9 3.1 3.3

Temperature (1000/K)

A/K2

0.499 0.998

1.549 1.999

(b) 

Figure 12: (a) IV results of Ti/Au metal-p-GaN Schottky junction (b) Richardson Plot

Conclusions 

The current work researched into understanding InGaN as the potential material for 

photovoltaics. Phase separation was identified as a key loss mechanism. Phase separated material 

(a lower band gap) controls the open circuit voltage, while the non-phase separated material 

(higher band gap) controls the absorption. Phase separation in InGaN layers was controlled via 

optimization of growth conditions and device thickness. 

InGaN solar cells with Voc ≈ Eg – 0.4 V and using 2.5 eV band gap have been 

demonstrated. This is the first report of (a) a InGaN solar cell with such a low band gap and (b) 

an open circuit voltage with approximately 0.4 V of the band gap, for any material composition 

in the InGaN material system. The realization of an open circuit voltage close to its practical 

limit for a new material system in such a short time frame is a major accomplishment. Internal 

quantum efficiency > 60% for near band gap light in GaN solar cells have been demonstrated. 

The ability to make photovoltaic devices without requiring a pn junction by using Schottky/MIS 

approaches has been demonstrated. These are the first reports of Schottky/MIS solar cells. 

9
Modeling of polarization effects in InGaN solar cells has been implemented in PC1D. This is 

the first photovoltaic modeling to include peizo-electric and polarization effects in a solar cell 

model. Device design rules for InGaN solar cells, including the impact of p-i-n structures, the 

impact of polarization and piezoelectric properties of InGaN have been set forth. 

In addition to material quality and growth, modeling and fundamental device design issues, 

fabrication and processing approaches have also been investigates. Of particular importance is 

the demonstration of ohmic contacts to InGaN solar cells using metal contacts and tunneling 

contacts have been demonstrated. Further, a standard processing sequence has been developed 

for the fabrication of InGaN solar cells. 

Overall, the research shows that InGaN material system can be used to realize high efficiency 

solar cells, making contributions to growth, modeling, understanding of loss mechanisms, and 

process optimization. 

REFERENCES: 

1] V. Yu. Davydov, A. A. Klochikhin, R. P. Seisyan, V. V. Emtsev, S. V. Ivanov, F. Bechstedt, J. 

Furthmuller, H. Harima, A. V. Mudryi, J. Aderhold, O. Semchinova, and J. Graul, “Absorption and 

Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap,” Phys. Stat. Sol. B, vol. 229, 3, 

p. R1-R3, 2002. 

2] T. Matsuoka, H. Okamoto, M. Nakao, H. Harima, and E. Kurimoto, “Optical Bandgap Energy of Wurtzite 

InN”, Appl. Phys. Lett., 81, 7, p. 1246-1248, 2002. 

3] J. Wu et al., “Temperature Dependence of the Fundamental Band Gap of InN”, J. Appl. Phys., vol. 94, 7, 

p. 4457-4460, 2003. 

4] F. Bernardini, and V. Fiorentini, “Nonlinear Macroscopic Polarization in III-V Nitride Alloys”, Phys. Rev. 

B, 64, 8 (2001), p. 085207/1. 

5] V. Fiorentini and F. Bernardini, “Spontaneous versus Piezoelectric Polarization in III-V Nitrides: 

Conceptual Aspects and Practical Consequences”, Phys. Stat. Sol. B, 216 (1999), p. 391. 

6] Y. Nanishi, Y. Saito and T. Yamaguchi, “R-F Molecular Beam Epitaxy Growth and Properties of InN and 

Related Alloys”, Jpn. J. Appl. Phys., vol. 42, 5A, p. 2549-2559, 2003. 

7] I. Ho, and G. B. Stringfellow, “Solid phase Immiscibility in GaInN,” Appl. Phys. Lett., vol. 69, p. 2701, 

1996. 

8] S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, “Luminescence from Localized States in InGaN 

Epilayers,” Appl. Phys. Lett., vol. 70, p. 2822, 1997. 

9] V. A. Elyukhin and S. A. Nikishin, “Internal Strain Energy of AX3B1-X3N Ternary Solid Solutions of 

Cubic Modification,” Semicond. Sci. Technol., vol. 11, p. 917-920, 1996. 

10] I. H. Ho and G. B. Stringfellow, “Incomplete Solubility in Nitride Alloys,” Mater. Res. Soc. Symp. Proc., 

vol. 449, p. 871-880, 1997. 

11] P. Lefebvre, A. Morel, M. Gallart, T. Taliercio, J. Allègre, B. Gil, H. Mathieu, B. Damilano, N. Grandjean, 

and J. Massies, “High internal electric field in a graded-width InGaN/GaN quantum well: Accurate 

determination by time-resolved photoluminescence spectroscopy,” Appl. Phys. Lett., vol. 78, p. 1252, 

2001. 

12] R. Langer, J. Simon, V. Ortiz, N. T. Pelekanos, A. Barski, R. André, and M. Godlewski, “Giant electric 

fields in unstrained GaN single quantum well,” Appl. Phys. Lett., vol. 74, p. 3827, 1999. 

13] F. Bernardini and V. Fiorentini, “Polarization fields in nitride nanostructures: 10 points to think about,” 

App. Surface Science, vol. 166, p. 23, 2000. 

14] J. C. Freeman, “Basic equations for the modeling of Gallium Nitride (GaN) High Electron Mobility 

Transistors (HEMT),” NASA/TM – 2003-211983. 

15] O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy,W. J. Schaff, L. F. Eastman, R. 

Dimitrov, L. Wittmer, M. Stutzmann, W.Rieger, and J. Hilsnbeck, “Two-dimensional electron gases 

induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN 

heterostructures,” J. Appl. Phys. 85 (1999), p. 3222. 

16] Schroder, D. K. (1990). Semiconductor material and device characterization, Wiley 

10
11

PUBLICATIONS: 

[1] Christiana Honsberg, Omkar Jani, Alan Doolittle, Elaissa Trybus, Gon Namkoong Ian Ferguson, David 

Nicole, Adam Payne. “InGaN – A new solar cell material,” 19th European Photovoltaic Science and Energy 

Conference, Paris, (2004). Invited Oral Presentation. 

[2] Elaissa Trybus, Gon Namkoong, Walter Henderson, W. Alan Doolittle, Rong Liu, Jin Mei, Fernando Ponce, 

Maurice Cheung, Fei Chen, Madalina Furis, Alexander Cartwright. “Growth of InN on Ge substrate by 

molecular beam epitaxy”, Journal of Crystal Growth 279, 311–315, 2005. 

[3] Omkar Jani, Christiana Honsberg, Ali Asghar, David Nicol, Ian Ferguson, Alan Doolittle, Sarah Kurtz, 

“Characterization and analysis of InGaN photovoltaic devices, Conference Record of the Thirty-First IEEE 

Photovoltaic Specialist Conference (IEEE Cat. No. 05CH37608), Orlando, 2005, p 37-42. Invited oral

presentation. 

[4] Omkar Jani, Christiana Honsberg, Yong Huang, June-O Song, Ian Ferguson, Gon Namkoong, Elaissa 

Trybus, Alan Doolittle, Sarah Kurtz, “ Design, Growth, fabrication and characterization of high-band gap 

InGaN/GaN solar cells, 4th World Conference of Photovoltaic Energy Conversion, May 2006. Oral 

presentation. 

[5] Elaissa Trybus, Gon Namkoong, Walter Henderson, Shawn Burnham, W. Alan Doolittle, Maurice Cheung, 

Alexander Cartwright, “InN: A material with photovoltaic promise and challenges,” Journal of Crystal 

Growth 288 (2006) 218–224. 

[6] M Mehta, O Jani, C Honsberg, B Jampana, I Ferguson, A Doolittle, “Modifying PC1D to model 

spontaneous and Piezo-electric polarization in III-V nitride solar cells,” Proceedings of the 22nd European 

Photovoltaic Solar Energy Conference, Milan Italy, (2007). 

[7] Omkar Jani, Hongbo Yu, Elaissa Trybus, Balakrishnam Jampana, Ian Ferguson, Alan Doolittle, Christiana 

Honsberg. Effect of phase separation on performance of III-V nitride solar cells, Proceedings of the 22nd 

European Photovoltaic Solar Energy Conference, Milan Italy, (2007). Oral presentation. 

[8] Nikolai Faleev, Christiana Honsberg, Omkar Jani, Ian Ferguson, “Crystalline perfection of GaN and AlN 

epitaxial layers and the main features of structural transformation of crystalline defects”, Journal of Crystal 

Growth, 300, 246–250, 2007. 

[9] Omkar Jani, Ian Ferguson, Christiana Honsberg, Sarah Kurtz, “ Design and characterization of GaN/InGaN 

solar cells,” Applied Physics Letters 91, 132117, 2007. 

[10] Yong Huang, Omkar Jani, Eun Hyun Park, and Ian Ferguson. “Influence of Growth Conditions on Phase 

Separation of InGaN Bulk Material Grown by MOCVD”, Mater. Res. Soc. Symp. Proc. Vol. 955, 2007. 

[11] E. Trybus, O. Jani, S. Burnham, J. Bai, D. S. Citrin, I. Ferguson, W. A. Doolittle, C. Honsberg and M. 

Steiner, "Characteristics of InGaN Designed for Photovoltaic Applications", 7th International Conference of 

Nitride Semiconductors (ICNS-7), Las Vegas, NV, USA, September 16-21, 2007. Oral Presentation

[12] Yong Huang, Omkar Jani, Eun Hyun Park, and Ian Ferguson, “Influence of Growth Conditions on Phase 

Separation of InGaN Bulk Material Grown by MOCVD”, Mater. Res. Soc. Symp. Proc. Vol. 955 2007. 

[13] Balakrishnam R Jampana, Omkar K Jani, Hongbo Yu, Ian Ferguson, Brian E McCandless, Steven S 

Hegedus, Robert L Opila, Christiana B Honsberg, “Nitride based Schottky-barrier photovoltaic devices,” 

Proceedings of the Material Research Society Symposium, 2007. 

[14] Nikolai Faleev, Balakrishnam Jampana, Anup Pancholi, Omkar Jani, Hongbo Yu, Ian Ferguson, Valeria 

Stoleru, Robert Opila, and Christiana Honsberg. “High quality InGaN for photovoltaic applications: type 

and spatial distribution of crystalline defects and “phase” separation,” Proceedings of the 33rd IEEE 

Photovoltaic Specialists Conference, San Diego, May 2008. Oral Presentation. 

[15] Omkar Jani, Balakrishnam Jampana, Mohit Mehta, Hongbo Yu, Ian Ferguson, Robert Opila, Christiana 

Honsberg. “Optimization of GaN window layer for InGaN solar cells using polarization effect,” 

Proceedings of the 33rd IEEE Photovoltaic Specialists Conference, San Diego, May 2008. 

[16] E. Trybus, O. Jani, S. Burnham, I. Ferguson C. Honsberg, M. Steiner, and W. A. Doolittle. “Characteristics 

of InGaN designed for photovoltaic applications,” phys. stat. sol. (c) 5, no. 6, 1843–1845, 2008. 

THESIS: 

Omkar Jani, Development Of Wide-Band Gap InGaN Solar Cells For High-Efficiency Photovoltaics, PhD Thesis, 

Georgia Institute of Technology, 2008.
F1146-E(09/2007) 

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Novel High Efficiency Photovoltaic Devices Based on the III-N 

Material System, Final Technical Report, 7 December 2005 - 

29 August 2008 

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14. ABSTRACT (Maximum 200 Words)

The current work by University of Delaware researchers sought to understand InGaN as the potential material for 

photovoltaics. Phase separation was identified as a key loss mechanism. Phase-separated material (a lower 

bandgap) controls the open-circuit voltage, while the non-phase separated material (higher bandgap) controls the 

absorption. Phase separation in InGaN layers was controlled via optimization of growth conditions and device 

thickness. 

15. SUBJECT TERMS

PV; high efficiency; devices; solar cells; bandgap; open-circuit voltage; ohmic contact; optimization; InGaN; phase 

separation; 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified 19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18