§ 4-1 Introduction
To date, InGaN-based MQW SCs have drawn much research attention due to
their favorable photovoltaic (PV) characteristics, including direct and tunable
bandgaps covering nearly the entire solar spectrum, high absorption coefficient, high
mobility, superior radiation resistance, and additional control of the light absorption
through the quantized energy levels.1,2 Despite these promising characteristics, the
conversion efficiencies of InGaN-based MQW SCs are still unsatisfactory. To
improve the PV performances of InGaN SCs, much research has been reported.3-7 In
order to mitigate the abrupt change in refractive indices from GaN (or ITO) to air,
which blocks a great portion of photons propagating through the air-GaN (or air-ITO)
interface,5a variety of roughened structures have been employed on the top of InGaN
MQW SCs.3,6,7 Periodic subwavelength structures can be fabricated with various
techniques, such as the anodic aluminum oxide templates8 and the nanosphere
lithography.9,10 For the practical applications, the interests in the subwavelength
structures have been extended to disordered structures.3,11,12However, the processing
complexity and the mechanical robustness of the additional roughened structures on
the top of SCs might be a limit to PV applications.
Among various roughened structures, the microdomes (or lens-like
microstructures) have been used for increasing the light escape cone in InGaN
light-emitting diodes (LEDs),13-15resulting in improved light extraction efficiency and
engineered far-field.15 Recently, the use of colloidal microlens arrays had also been
implemented for increasing the light extraction in organic light-emitting diodes,16 as
well as improving the light collection in SCs.17 Additionally, it is found that
roughened p-GaN structures in InGaN LEDs can effectively reduce reflectance loss in
GaN,18 and provides an effective way to make p-metal contact deposition without
using a filling process with the insulating materials in the air gaps caused by
nanostructing/microstructing processes at the device surface, which is required for the
axial p-n nanorod devices to form electrical connection.19 Moreover, the roughened
p-GaN also leads to a reduction of piezoelectric field and improved crystal qualities
due to the relaxation of in-plane strain.20,21Roughening p-GaN is expected to benefit
the PV performances of InGaN MQW SCs since numerical studies have demonstrated
the detrimental effects of strain-induced piezoelectric polarization, forcing the
photocurrent to the opposite direction and thus resulting in low Jsc and open-circuit
voltage (Voc).22 Thus, the pursuit of microdome/lens-like structure via roughening
p-GaN is of great importance for increasing the critical angle and collection efficiency
in InGaN SCs.
In this study, we realize the p-GaN microdome surfaces with the attempt to boost
the conversion efficiencies (η) of InGaN MQW SCs. Compared with flat p-GaN, the
p-GaN microdomes not only generate additional photocurrents (from 0.43 to 0.54
mA/cm2) by suppressing surface reflections considerably but also exhibit an improved
fill factor (from 44 % to 72 %), indicating the strain relaxation and the piezoelectric
field reduction. Accordingly, the p-GaN microdome leads to a 102 % enhancement of
η. The optical enhancement is confirmed using the simulation based on
finite-difference time-domain (FDTD) analysis, reflection measurements and external
quantum efficiency (EQE) measurements. The internal quantum efficiency (IQE)
measurements indicate the possible improvement in photocarrier separation/collection
due to the strain relaxation. The concept of the microdome directly grown during SC
growth preserving mechanical robustness and wafer-scale uniformity without any
additional process represents a viable, promising path toward high-efficiency SCs.
§ 4-2 Experiment
The multiple quantum well (MQW) solar cells (SCs) were grown by
metal–organic chemical vapor deposition on c-plane sapphire substrates. The layer
structures consist of fifteen periods of intentionally undoped In Ga N (2.4
nm)/GaN (14 nm) MQWs, sandwiched by a 2.5-μm n-type (Si-doped) and a 0.2-μm
p-type (Mg-doped) GaN layer. The free carrier concentrations for n-type and p-type
GaN are around 2×1018 cm-3 and 5×1017 cm-3, respectively. Ammonia (NH3),
trimethylgallium (TMGa) for n-GaN and p-GaN, triethylgallium for MQWs, and
trimethylindium were used as the precursors. Surface morphologies of p-GaN were
controlled by TMG flows and substrate temperatures during the p-GaN growth. In
device fabrication, ITO was deposited by electron beam evaporation on p-GaN to
form transparent ohmic contacts. The 1×1 mm2 mesas were then defined by
chlorine-based plasma etching. The contacting scheme consists of fingered
Ti/Al/Ni/Au metal grids with the thicknesses of 20/400/20/2000 nm deposited on the
ITO and the n-GaN.
§ 4-3 Results and Discussion
Surface morphologies of p-GaN were controlled by TMGa flows and substrate
temperatures during the p-GaN growth. For flat p-GaN, TMGa flow rate was 40-50
μmol/min and substrate temperature was in the range of 950-1100 °C. To fabricate the
micro-roughened p-GaN, TMGa flow rate was increased to more than 55 μmol/min
and substrate temperature was decreased to lower than 920 °C. Fig. 4.1 shows
scanning electron microscopy (SEM) images of the MQW SCs with p-GaN
microdome surfaces. The p-GaN microdomes are 530 250 nm in height and the base
of microdomes are 600370 nm in diameter. Fig. 4.2 is the specular reflection
spectra obtained on flat and microdome surfaces at the wavelengths ranging from 340
to 600 nm. One can see that the MQW SC with p-GaN microdome surfaces exhibits a
low reflectance for all studied wavelengths, demonstrating that surface reflectance can
be effectively suppressed by the p-GaN microdomes.
Figure 4.1 45 degree-tilted SEM image of the MQW SCs with p-GaN microdomes.
The inset shows the cross-sectional SEM image.
Figure 4.2 Specular reflection measured on the MQW SCs with flat and microdome surfaces. The p-GaN microdomes are 530 250 nm in height and the base
of microdomes are 600 370 nm in diameter. Fig. 4.2 is the specular reflection
spectra obtained on flat and microdome surfaces at the wavelengths ranging from 340
to 600 nm. One can see that the MQW SC with p-GaN microdome surfaces exhibits a
low reflectance for all studied wavelengths, demonstrating that surface reflectance can
be effectively suppressed by the p-GaN microdomes.
Figure 4.1 45 degree-tilted SEM image of the MQW SCs with p-GaN microdomes.
The inset shows the cross-sectional SEM image.
Figure 4.2 Specular reflection measured on the MQW SCs with flat and microdome surfaces. The p-GaN microdomes are 530 250 nm in height and the base
of microdomes are 600 370 nm in diameter. Fig. 4.2 is the specular reflection
spectra obtained on flat and microdome surfaces at the wavelengths ranging from 340
to 600 nm. One can see that the MQW SC with p-GaN microdome surfaces exhibits a
low reflectance for all studied wavelengths, demonstrating that surface reflectance can
be effectively suppressed by the p-GaN microdomes.
Figure 4.1 45 degree-tilted SEM image of the MQW SCs with p-GaN microdomes.
The inset shows the cross-sectional SEM image.
Figure 4.2 Specular reflection measured on the MQW SCs with flat and
For the incident wavelengths comparable or shorter than the geometric sizes of the
microdomes, the low reflectance can be caused by the light trapping effect due to
severe light scattering. As light impinges on the structured surface, it is diffracted to
several beams with different diffraction angles. The diffracted beams re-bounces
between the p-GaN microdomes, which prevents the light from escaping back to air,
and thus increases the opportunity of optical absorption by the underneath material. It
should be mentioned that the light trapping effect results in the suppressed reflectance
over not only a wide range of wavelengths but also a wide range of incident angles.
23,24
In order to reveal the light propagation across the surface structures, the
distributions of electromagnetic fields within the device structures were simulated by
FDTD analysis. We modeled two kinds of device surfaces: flat p-GaN and p-GaN
microdomes. Fig. 4.3 visualizes the time-averaged TE-polarized electric field
intensity distributions for the MQW SCs with two surface conditions at the incident
wavelength of 400 nm. All of the calculated values in Fig. 4.3 are normalized to those
of the excitation source. It can be seen that the light propagating in the MQW region
for the SC with p-GaN microdomes is strongly and widely scattered, as compared
with the case of flat surface. One can also notice that strong fields are confined within
the p-GaN microdomes. The normalized optical power integrated over the MQW
region as a function of times for the SCs with p-GaN flat and microdome surfaces is
shown in Fig. 4.3(c). The steady-state power values for the two devices are 0.65 and
0.75, respectively. These results indicate that the roughened p-GaN surface not only
helps light propagating across the interfaces but also widens the field distribution
within the device by increasing the light scattering on the surface. Strong light
scattering by the microdome surface is desired for improving the efficiencies of SCs
due to the increase in optical paths, which benefits the light absorption in MQW
regions.
Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN region as a function of times for the SCs with p-GaN flat and microdome surfaces is
shown in Fig. 4.3(c). The steady-state power values for the two devices are 0.65 and
0.75, respectively. These results indicate that the roughened p-GaN surface not only
helps light propagating across the interfaces but also widens the field distribution
within the device by increasing the light scattering on the surface. Strong light
scattering by the microdome surface is desired for improving the efficiencies of SCs
due to the increase in optical paths, which benefits the light absorption in MQW
regions.
Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN region as a function of times for the SCs with p-GaN flat and microdome surfaces is
shown in Fig. 4.3(c). The steady-state power values for the two devices are 0.65 and
0.75, respectively. These results indicate that the roughened p-GaN surface not only
helps light propagating across the interfaces but also widens the field distribution
within the device by increasing the light scattering on the surface. Strong light
scattering by the microdome surface is desired for improving the efficiencies of SCs
due to the increase in optical paths, which benefits the light absorption in MQW
regions.
Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN
microdomes. (c) Normalized optical power, integrated over the MQW region, as a function of times for the two kinds of SCs at 400 nm wavelength.
Fig. 4.4 shows the measured current density–voltage (J–V) curves of the SCs with
two kinds of surfaces. PV characteristics obtained by the J–V curves are listed in the
inset table of Fig. 4.4. One can clearly see that the microdome surfaces lead to
enhanced Jsc, which confirms that the microstructured surface enhances light
absorption. It is also found that the fill factors are enhanced for the SCs with
microdome surfaces suspectedly due to the strain relaxation and the piezoelectric field
reduction caused by the roughened structure.18,20,21 Accordingly, for the SCs with
p-GaN microdomes, η is increased from 0.43 % (with flat surface) to 0.87 %, which is
an improvement of 102 %. The results prove that the microdomes are effective in
boosting the PV performances of the MQW SCs. Note that previous simulation works
focusing on wire SCs reported that increasing the wire length could lead to a constant
decrease in Voc in spite of increases in η.25,26 Consequently, a slight decrease in Voc is
also observed in the MQW SCs with microdome surfaces. We note that the
optimization considerations must be made in future with respect to the geometric
parameters of microdome structures due to the inverse behavior of Jscand Voc.25,26
Figure 4.4 J–V characteristics measured on the MQW SCs with two kinds of surface structures. The inset table shows PV characteristics of InGaN MQW SCs with two kinds of surface structures. FF is the fill factor of SCs, which is defined as the ratio of the actual maximum obtainable power, to the product of Jscand Voc.
To gain insight into the correlation between Jsc enhancement, optical absorption,
and carrier separation/collection efficiency in the active layer, EQE and IQE spectra
were investigated. Fig. 4.5(a) presents the EQE spectra for the two kinds of SCs,
showing the influence of p-GaN microdomes on PV performances. EQEs of the SCs
with microdomes are mostly improved in the region of 360–450 nm, which reveals
the enhancement in light collection efficiency and agrees with the suppressed
reflection on the microdome surface shown in Fig. 4.2. The results also echo with
those revealed by FDTD analysis in Fig. 4.3. Since E. Matioli et al. had found the Figure 4.4 J–V characteristics measured on the MQW SCs with two kinds of surface structures. The inset table shows PV characteristics of InGaN MQW SCs with two kinds of surface structures. FF is the fill factor of SCs, which is defined as the ratio of the actual maximum obtainable power, to the product of Jscand Voc.
To gain insight into the correlation between Jsc enhancement, optical absorption,
and carrier separation/collection efficiency in the active layer, EQE and IQE spectra
were investigated. Fig. 4.5(a) presents the EQE spectra for the two kinds of SCs,
showing the influence of p-GaN microdomes on PV performances. EQEs of the SCs
with microdomes are mostly improved in the region of 360–450 nm, which reveals
the enhancement in light collection efficiency and agrees with the suppressed
reflection on the microdome surface shown in Fig. 4.2. The results also echo with
those revealed by FDTD analysis in Fig. 4.3. Since E. Matioli et al. had found the Figure 4.4 J–V characteristics measured on the MQW SCs with two kinds of surface structures. The inset table shows PV characteristics of InGaN MQW SCs with two kinds of surface structures. FF is the fill factor of SCs, which is defined as the ratio of the actual maximum obtainable power, to the product of Jscand Voc.
To gain insight into the correlation between Jsc enhancement, optical absorption,
and carrier separation/collection efficiency in the active layer, EQE and IQE spectra
were investigated. Fig. 4.5(a) presents the EQE spectra for the two kinds of SCs,
showing the influence of p-GaN microdomes on PV performances. EQEs of the SCs
with microdomes are mostly improved in the region of 360–450 nm, which reveals
the enhancement in light collection efficiency and agrees with the suppressed
reflection on the microdome surface shown in Fig. 4.2. The results also echo with
those revealed by FDTD analysis in Fig. 4.3. Since E. Matioli et al. had found the
possible IQE change in roughened SCs and Wierer et al. had demonstrated that SC
performances including IQE can be further improved by optimizing the barrier
thickness of QWs,4,27 IQE measurements are then taken to evaluate the carrier
separation/collection occurring in the active region. EQE is converted into IQE
through
IQE = EQE/abs(λ) (1)
where abs(λ) the absorption spectra of the SCs, which is shown in Fig. 4.6. For
absorption spectrum measurements, by measuring reflection and transmission in the
spectral range from 360 to 540 nm, the absorption spectra can be obtained by
subtracting reflection and transmission from unity at every wavelength. Accordingly,
the obtained IQE in Fig. 4.5(b) may point out the possibility of IQE changes due to
p-GaN roughening. The variation in IQE after introducing p-GaN microdomes
indicates the possible change in carrier separation/collection efficiency resulted from
relaxation of in-plane strain and consequent reduction of piezoelectric fields during
p-GaN microdome growth.22 One should note that the possible inaccuracy in
determining the absorption spectra due to the difference in the surfaces for both SCs
might not enable one to provide accurate IQE comparison data for both devices. For
correctly determining the IQE for both devices, more detailed experiments in
providing conclusive comparison are required in future. In short, the p-GaN
microdome results in the increases in EQE, which are attributed to the suppressed
interface reflection and possibly efficient separation/collection of photocarriers.
Figure 4.5 (a) EQE curves and (b) IQE curves for the SCs with two kinds of surface structures.
Figure 4.6 The absorption spectra of InGaN MQW SCs with and without microdome structures.
microdome results in the increases in EQE, which are attributed to the suppressed
interface reflection and possibly efficient separation/collection of photocarriers.
Figure 4.5 (a) EQE curves and (b) IQE curves for the SCs with two kinds of surface structures.
Figure 4.6 The absorption spectra of InGaN MQW SCs with and without microdome structures.
microdome results in the increases in EQE, which are attributed to the suppressed
interface reflection and possibly efficient separation/collection of photocarriers.
Figure 4.5 (a) EQE curves and (b) IQE curves for the SCs with two kinds of surface structures.
Figure 4.6 The absorption spectra of InGaN MQW SCs with and without microdome structures.
§ 4-4 Summary
In conclusion, InGaN MQW SCs with microdome surfaces were fabricated. In
comparison with the flat surface, the microdome structures show improved fill factor
and Jsc, leading to the η enhancement by up to 102 %. The p-GaN microdomes exhibit
the enhanced optical absorption due to light trapping effects and possible
improvement of photocarrier separation/collection for the MQW SCs due to the strain
relaxation, resulting in enhanced EQE and IQE. With the advantages of the simple
process and the mechanical robustness, the microdomes grown during SC epitaxial
growth offer a viable way to boost solar efficiency of a variety of SCs.
§ 4-5 References
He, Appl. Phys. Lett. 100, 013105 (2012).
[7] G. J. Lin, K. Y. Lai, C. A. Lin, and J. H. He, Opt. Lett. 37, 61 (2012). and J. A. Smart, Nature Photonic 1, 176 (2007).
[13] Y. K. Ee, P. Kumnorkaew, R. A. Arif, H. Tong, H. Zhao, J. F. Gilchrist, and N.
[14] P. Kumnorkaew, Y. K. Ee, N. Tansu, and J. F. Gilchrist, Langmuir 24, 12150 K. S. Chung, Nano Lett. 4, 1059 (2004).
[20] Y. H. Sun, Y. W. Cheng, S. C. Wang, Y. Y. Huang, C. H. Chang, S. C. Yang, L. Y. and D. Wang, IEEE J. Sel. Top. Quant. 17, 1033 (2011).
[27] J. J. Wierer, Jr., D. D. Koleske, and S. R. Lee, Appl. Phys. Lett. 100, 111119 (2012).
Chapter 5 Efficient light harvesting scheme for InGaN-based quantum well solar cells employing the
hierarchical structure: SiO
2nanorods/p-GaN microdomes
§ 5-1 Introduction
InGaN-based multiple quantum well (MQW) solar cells (SCs) have a direct and
tunable bandgap, which covers nearly the entire solar spectrum. In addition, they also
possess advantages as high absorption coefficients, high mobilities, and superior
radiation resistance, which allow the operation under harsh environments.[1-3]
Employing MQW structures can provide the independent design between the
short-circuit current (Jsc) and open-circuit voltage (Voc) [4], and also avoid the
undesired trade-off between solar response and crystalline quality [4-6]. However, the
conversion efficiencies (η) of InGaN-based SCs are still limited. Methods for either
internal quantum efficiency (IQE) or external quantum efficiency (EQE)
improvements of SCs have been devoted to boost their photovoltaic (PV)
performances.[7-9]
Recently, a variety of subwavelength structures (SWSs) have been demonstrated
to effectively suppress the undesired Fresnel reflections. These roughened structures
can be used as the light trapping layer with many superior antireflective (AR)
characteristics.[10-13] One efficient AR SWS is the subwavelength nanorod arrays
(NRAs) structure, which can be regarded as an intermediate medium with an effective
refractive index (neff) between the air and device surface.[14,15] This leads to the
suppressed interface reflection over a wide range of wavelengths due to the effective
medium effect.[16] To avoid the abrupt refractive index (n) change at air/device
interface, SiO2is an attractive candidate AR material due to its intermediate refractive
index (n=1.56) between air (n=1) and GaN (n=2.5). Since structuring techniques that
require a high-cost lithography process might be a limit to the practical use, a process
with the nature lithography is of great interest due to its simplicity and effectiveness.
Therefore, SiO2 NRAs fabricated by simple and photoresist-free techniques are
particularly attractive to SC applications.[17]
Besides utilizing the added AR SWSs to improve the performances of InGaN SCs,
surface texturing using the structured p-GaN is also under consideration. For
surface texturing using the structured p-GaN is also under consideration. For