Henini M. Handbook of Self Assembled Semiconductor Nanostructures for Novel devices in Photonics and Electronics

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Nanostructured Solar Cells 559

Typically, very high loading levels of semiconductor nanoparticles are required to overcome the

poor electron transport properties of the polymer. Inclusion of SWCNTs to the polymer/quantum

dot mixture can improve the transport problem.

The most commonly used colloidal quantum dot system for organic solar cell applications has

been CdSe. However, new reports on other II–VIs, III–Vs, and ternary compounds such as CuInS

and CuInSe

are increasing in frequency. CdSe quantum dots are conventionally synthesized

from CdO as the precursor, following the established protocol. In short, CdO, stearic acid, and

1-octadecene are heated to 200 ºC under Ar

(g)

and then cooled to room temperature. Following

the addition of trioctylphosphine oxide (TOPO) and octadeclyamine (ODA), the reaction mix-

ture is heated and a selenium–trioctylphosphine solution is injected. A combination of time and

temperature is then used to control the QD size. Extraction of the CdSe–TOPO quantum dots

requires sequential washes using a methanol:hexanes mixture until a distinct interfacial separa-

tion is achieved. The CdSe–TOPO quantum dots are normally precipitated with acetone and can

be resuspended in chloroform for additional characterization. Figure 18.7 shows two samples of

colloidal CdSe QDs of different sizes under UV illumination. The different colors are a result of

the varying degrees of quantum conﬁ nement related to the difference in particle size. The green

sample have the higher effective band gap and thus the smaller nanocrystallites.

(a) (b)

Figure 18.7 Digital photographs of colloidal InAs quantum dot suspensions showing the changes in the

photoluminescence, and thus effective band gap, with a variation in the size (green to red, smaller to larger) of these

semiconducting nanocrystals under (a) ambient white light and (b) UV illumination.

18.5 Quantum dot solar cell behaviour

18.5.1 III–V quantum dot solar cell behaviour

The key operating principle of the quantum dot enhanced solar cell of the type shown in Fig.

18.1 is absorption of long wavelength (infrared) photons by the QDs. This allows collection

of some portion of the light that would normally be lost in non-QD enhanced solar cells. This

enhanced absorption of infrared light leads to increased photocurrent production and thus,

increased solar cell short circuit current ( I

). In the case of ultra-high-efﬁ ciency InGaP/GaAs/Ge

triple-junction solar cells (TJSC), the GaAs junctions are typically the current limiting junction,

thus increasing the I

of the stack using QD enhancement could potentially lead to improved

global conversion efﬁ ciency for what currently is the highest efﬁ ciency solar cell design being

produced today. It has been predicted that quantum dot enhanced TJSCs have an efﬁ ciency ceil-

ing of 47% under one-sun 6000 K black body illumination spectrum [35] . Additionally, quan-

tum dot array enhanced GaAs cells have the added beneﬁ t of possible intermediate band effects

[36] , anisotropic absorption [37] and enhanced radiation tolerance [38] .

Figure 18.8 shows a band diagram for the quantum dot enhanced GaAs solar cell. As can be

seen, photons below the GaAs band gap are able to excite carriers in the InAs quantum dots.

These carriers may escape by either of two means: photon-assisted extraction from deep bound

QD levels or thermal-assisted extraction from higher energy bound QD levels. Carriers are then

swept across the junctions due to the strong built-in electric ﬁ eld of the p-i-n device.

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560 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

Photon assisted

extraction

Thermal assisted

extraction

⫺

Primary excitation

Secondary excitation

Figure 18.8 Idealized band diagram for a p-i-n solar cell with an embedded quantum dot array in the intrinsic

region showing potential photoconversion mechanisms.

400

1E-3

0.01

0.1

Quantum yield

0.5

500 600 700

Wavelength (nm)

800 900 1000 1100

Baseline

1 layer QD

5 layers QD

QD w/ strain comp

Figure 18.9 Quantum efﬁ ciency of the p-i-n cells with and without QD and strain compensation.

Measurements of the cell quantum efﬁ ciency show the QD absorption effects seen in the above

Figure. As seen in Fig. 18.9 , the quantum efﬁ ciency (QE) of the QD enhanced solar cells directly

show the effects of QD absorption [39] . The cell quantum efﬁ ciency measures the photocurrent

produced by the cell per incident photon as a function of the photon wavelength. For a standard

GaAs cell, the quantum efﬁ ciency will be more or less constant over the visible region of the spec-

trum and rapidly fall off at the GaAs band edge near 870 nm (below band gap photons produce

no photocurrent in the cell). As can be seen in the baseline p-i-n cell (without QDs), the quantum

efﬁ ciency is relatively high over the 500–870 nm region.

However, with ﬁ ve layers of QDs, the cells show an extended QD spectrum at wavelengths

below 870 nm.This indicates that a portion of the short circuit current is being generated from

below band gap absorption in the QD region, one of the requirements of QD cell enhancement.

The ﬁ ve-layer QD cell shows increased below-band gap QE, due to the increased absorption cross-

section of the ﬁ ve-layer QD cell. The QD cells show 1–2% QE in the 870–950 nm range. The QE

would be expected to increase even more with additional layers of QDs. The quantum conﬁ ne-

ment in the QDs should lead to absorption peaks corresponding to transitions from the valence

band to the discrete QD energy levels. These discrete transitions are evident in the ﬁ ve-layer QE

spectrum, as evidenced in peaks appearing in the below-gap tail region.

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Nanostructured Solar Cells 561

18.5.2 Organic bulk heterojunction solar cells

The efﬁ ciencies of bulk heterojunction (BHJ) solar cells based on conventional sandwich struc-

ture ITO/PEDOT:PSS/active layer/Al, where the active layer is conjugated polymer, such as

MEH-PPV or P3HT and a quantum dot, fullerene, or derivatized fullerene (i.e. PCBM) blend

have reached approximately the 5% level under a 1 sun AM 1.5 illumination [www.nrel.gov/pv/

thin_ﬁ lm/does/kaz_best_research_cells.ppt]. At this point, these cells are starting to exhibit what

amounts to a much more conventional current versus voltage behaviour, which is reminiscent

of their crystalline counterparts of a decade or so ago. A thermal treatment of active layer in

these types of solar cells, both before and after contacting, has been shown to be critical to their

improvement as shown in Fig. 18.10 .

875

⫺3

⫺2

⫺1

900 925 950 975 1000 1025 1050 1075 1100

400

600

800

1000

⫺4

⫺3

⫺2

⫺1

5x QD, no SC

Wavelength (nm)

5x SC QD

Baseline

Spectral responsibility

(mA/mW)

Spectral responsibility (mA/mW)

Figure 18.10 The spectral response for a p-i-n GaAs solar cell without quantum dots and with both non-

strain-compensated and strain-compensated quantum dot arrays (QDAs) in the intrinsic region demonstrating the

sub-GaAs band gap photoconversion that is achievable with the QDA introduction. Inset: Spectral response in the

above GaAs band gap region showing how the use of strain compensation can be used to eliminate the loss of above

GaAs band gap photoconversion associated with ordinary QDA inclusion.

No annealing

Pre-annealing

Pre-annealing ⫹ Post-annealing

0.0

⫺6.0

⫺5.0

⫺4.0

⫺3.0

⫺2.0

⫺1.0

0.0

0.1 0.2 0.3 0.4

Voltage (V)

0.5 0.6 0.7 0.8

Current density (mA/cm

)

Figure 18.11 Current versus voltage characteristics under 1 sun AM 1.5 illumination for a P3HT/PCBM bulk

heterojunction solar cell with and without thermal annealing.

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562 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

The band offsets between a given conjugate polymer host and the various types of quantum

dots or nanomaterials used are critical to the performance of BHJ devices. The QD composition

and size, and thus their effective band gap, electron afﬁ nity, and ionization potentials, need to be

appropriately matched to the polymer host or they will act as either electron traps, hole traps,

or both. Even when band offsets are favourably aligned for electron and hole transport, small

changes in the effective optoelectronic structure will affect the photovoltaic performance of the

BHJ devices. Figure 18.12 shows the effect of changing the fullerene type in a typical BHJ device.

Unfortunately, the effect band offsets for the materials used based upon optical spectroscopy or

electrochemical measurements made on the polymers or nanomaterials separately can be mis-

leading (see Fig. 18.13 ). The actual energy levels in the blends are known to shift due to the basic

electronic interaction of materials once placed in intimate contact.

⫺0.8

⫺6

⫺4

⫺2

⫺0.4 0.0 0.4 0.8

C60

C70

Current density (mA/cm

)

Volta

e (v)

Figure 18.12 Current versus voltage behaviour under a 1 sun AM 1.5 illumination of an MEH-PPV bulk

heterojunction solar cell as a function of the fullerene used in the active region [40].

2.44

Energy (ev)

MEH-PPV C60 PCBM C70 PCBM

3.5

3.33

5.77

5.9

4.73

⫽ 4.3

ITO

⫽ 4.7

Figure 18.13 Experimentally determined values for the work functions, ionization potentials, and electron

afﬁ nities associated with the materials used in the devices measured in Fig. 18.11 [40].

The advantageous affects of thermal annealing of polymer solar cells, as demonstrated above,

are well known in the ﬁ eld. However, it has also been recently shown that these types of solar

cells exhibit a positive temperature coefﬁ cient as well (see Fig. 18.14 ). The short circuit current

density is shown to improve dramatically with increasing temperature with only a slight drop

in the open circuit voltage. The net result is almost a factor of 3 improvement in 1 sun AM 1.5

photovoltaic efﬁ ciency over a temperature range of 77 K to 330 K for a conventional bulk hetero-

junction solar cell.

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Nanostructured Solar Cells 563

18.6 Conclusions

Nanostructured solar cells will play an important role in enhancing the efﬁ ciency of future gen-

erations of solar cells, whether they are III–V, II–VI, or hybrid organic–inorganic cells. There is a

great deal of potential in multiple approaches for these nanostructures. It is also clear that fur-

ther work will be necessary in many material areas; however, substantial progress has been made

in the last ﬁ ve years. The future looks bright for nanostructured photovoltaics.

References

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⫺6.0

⫺5.0

⫺4.0

⫺3.0

⫺2.0

⫺1.0

0.0

76 K

95 K

115 K

135 K

155 K

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330 K

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)

Voltage (V)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Figure 18.14 Temperature dependent current versus voltage characteristics for a conventional P3HT/PCBM bulk

heterojunction solar cell showing a positive photovoltaic efﬁ ciency temperature coefﬁ cient [40].

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Quantum Dot Superluminescent Diodes

M. Rossetti ,

L.H. Li

A. Fiore

L. Occhi ,

and C. Vélez

Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 3, Ch-1015 Lausanne (Switzerland)

EXALOS AG, Wagistrasse 21, CH-8952 Schlieren (Switzerland)

19.1 Introduction

19.1.1 Superluminescent diodes and their applications

Superluminescent light-emitting diodes (SLEDs) are semiconductor light sources that show a

broadband output optical spectrum (like LEDs) at high power levels (like semiconductor lasers).

Sometimes they are referred as “ high-power LEDs ” or as “ laser diodes operating below thresh-

old ” . SLEDs have features typical of LEDs like low coherence, as well as features of semicon-

ductor lasers like high optical power. This combination is extremely useful for a wide range of

applications.

SLEDs are based on a pn-junction embedded in an optical waveguide. When electrically biased

in the forward direction they show optical gain and generate ampliﬁ ed spontaneous emission

over a wide range of wavelengths. SLEDs are designed to have high single-pass ampliﬁ cation for

the spontaneous emission generated along the waveguide but unlike laser diodes, insufﬁ cient

feedback to achieve lasing action. The suppression of the cavity modes is typically achieved by

tilting the waveguide with respect to the end facets and by providing the facets with an anti-

reﬂ ection coating. With the suppression of the cavity modes a low ripple or smooth spectra is

obtained. The main technological challenge is to achieve high optical output powers with a

smooth spectrum.

In other words, the SLED is a light source, which combines the spatial coherence of a laser

diode with the temporal incoherence of an LED. The spatial coherence translates into small beam

divergence, which greatly facilitates the light coupling into a single mode ﬁ bre with similar efﬁ -

ciency as for laser diodes. Typically, more than 50% of the chip facet power is coupled to the sin-

gle mode ﬁ bre. The large optical bandwidth or the low temporal coherence is of great advantage

for applications where interferences cause problems like speckle or ghost signals. SLEDs are much

more powerful than standard LEDs and are particularly advantageous for applications requiring

high power density. SLEDs typically have single-mode output powers of the same order of magni-

tude as single-mode laser diodes of several tens of milliwatts when biased with several hundreds

of milliamperes.

Although SLEDs have been known in the literature for many years, it has been only recently

that they have become more popular or even standard devices for various applications. The main

reason is that only recently devices showing high output power, smooth spectra and large band-

width have been demonstrated and made available in larger quantities commercially [1] .

CHAPTER 19

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566 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

The availability of large optical spectral bandwidth at high power levels makes SLEDs the most

cost-effective light source solution for a wide range of applications, as summarized in Table 19.1 .

19.1.2 Application requirements and markets

Optical coherence tomography (OCT) is an emerging technology for producing high-resolution

cross-sectional imaging. Tissue structure can be imaged on the micron scale in real time. The prin-

ciple of OCT is analogous to that of ultrasound imaging, except that it uses light instead of sound

(see Fig. 19.1 ). The light is provided by a broadband source like the SLED. Although sound waves

are able to pass through tissue, light, which has a much shorter wavelength, is unable to penetrate

beyond 2 mm in most non-transparent tissue. Thus, the use of OCT is limited to optically trans-

parent tissues, such as retina, or endoscopic examinations. The large optical bandwidth together

with the good suppression of the cavity modes allows short coherence lengths from some tens of

micrometres down to few micrometres. Moreover, the high power level of the SLEDs allows fast

scan times required in ophthalmologic, cardiovascular, and gastrointestinal imaging applications.

Quantum dot-based SLEDs are leading the development of new SLEDs with increased spectral

bandwidth and large optical output power, in order to achieve higher image resolution to perform

tissue characterization on a scale never before possible within the human body. This technology

has the potential to dramatically change the way that physicians, researchers and scientists see and

understand the human body in order to better diagnose and treat diseases.

Table 19.1 SLED are versatile optical sources. Overview of the main SLED application ﬁ elds.

Application ﬁ eld SLED advantage Examples

Optical coherence tomography

(OCT)

high resolution, fast scan time endoscopes, dermatology,

cornea and retina diagnostics

cardiovascular and gastrointestinal

imaging

Fibre optic gyroscopes (FOG) reliability, stability, cost effective navigation for satellites, airplanes,

ships, land vehicle

Fibre optic test equipment high power density over a large

spectral range, telecom reliability

standards, cost effective

chromatic and polarization

mode dispersion, optical channel

monitoring, passive components

characterization

Fibre optic sensors cost effective, robust temperature and strain

measurements in bridges or oil

pipelines

SLED

Detector

Tissue

Scanning reference

mirror

Amplifier

Computer

Bandpass

filter

Fibre-optic

beamsplitter

Figure 19.1 Basic block diagram of an optical coherence tomography set-up.

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Quantum Dot Superluminescent Diodes 567

Another main SLED application is in navigation systems employing ﬁ bre optic gyroscopes

(FOGs). Fibre optic gyroscopes are mainly used in avionics and aerospace. They are at the heart

of very precise rotation measurement systems. FOGs are based on the measurement of the

Sagnac phase shift occurring to the radiation propagating along a ﬁ bre optic coil when it is sub-

ject to a rotation around the winding axis. When an FOG is mounted within a navigation sys-

tem it allows the tracking of change of orientation. The basic elements within an FOG are a light

source, a single-mode polarization maintaining ﬁ bre coil, a coupler and a detector (see Fig. 19.2 ).

By means of the optical coupler, the light emitted by the source is injected into the ﬁ bre coil in

counter-propagating directions. When the ﬁ bre coil is at rest the two light waves interfere con-

structively at the detector and a maximum signal is detected. During coil rotation the two optical

waves experience different optical paths that depend on the rotation rate; the phase difference

between the two ﬁ elds causes variations of the intensity detected by the photodiode and provides

information on the rotation rate. SLED-based gyroscopes rely on the large bandwidth of the

source to reduce the undesired effect of scattering along the ﬁ bre and of reﬂ ections at the facets

of the internal optical components that could decrease the sensitivity for very low rotation rates.

SLED

Detector

SM PM fibre

coil

50% 2  2

coupler

Figure 19.2 Basic block diagram of a ﬁ bre optic gyroscope.

The requirements on FOGs and on the embedded optical sources are low cost, reduced form

factor size, low power consumption, stability and reliability. Quantum dots (QDs) have the

potential to successfully combine all these properties since they are potentially less sensitive to

temperature.

Broadband optical sources are also used in ﬁ bre optics for component test or network manage-

ment purposes. An example of the ﬁ rst is the characterization of optical components suitable for

coarse wavelength division multiplexing (CWDM) application, which requires multiple wavelength

sources. Thanks to their large optical bandwidth, QD SLEDs allow the integration of fewer sources,

with wavelengths spanning the whole CWDM band (i.e. 1200–1700 nm), in single test equipment.

In optical networks SLEDs are used to measure the polarization-mode dispersion of optical ﬁ bres.

SLEDs are preferred sources for these purposes, because of many aspects. First, SLEDs are fully

compatible with the wavelengths used in the optical communication systems. Second, the SLED’s

optical waveguide allows an efﬁ cient coupling of the light into single-mode optical ﬁ bres. Third,

SLEDs answer well to the pressures in today’s telecommunications carrier market, which requires

test solutions that are cost effective, reliable, provide better productivity and reduce the number

of testers in the ﬁ eld.

The ﬁ nal main SLED application ﬁ eld is in ﬁ bre optic sensors for strain and temperature meas-

urements in civil engineering, structural analysis and composite material manufacturing applica-

tions. Fibre optic sensors have a number of advantages over conventional sensors. Their immunity

to electromagnetic ﬁ elds, their ability to measure at many points along a single ﬁ bre, and their abil-

ity to be embedded within, or bonded to, structures makes them a highly ﬂ exible solution. Over the

past 25 years, ﬁ bre optic sensors have seen a continuous improvement in quality and performance.

SLEDs are used for these applications because of their large optical bandwidth and robustness.

19.1.3 Quantum dots for superluminescent diodes

SLEDs are based on ampliﬁ ed spontaneous emission in an active semiconductor waveguide,

which can simultaneously provide large bandwidth and high output power. In the most simpliﬁ ed

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568 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

picture, assuming a uniform gain proﬁ le along the propagation direction, the spatial dependence

of light power P ( z ) along the waveguide is described by the equation:

PgP

β

(19.1)

where P

is the power spontaneously emitted per unit length, β the spontaneous emission cou-

pling factor, and g the net modal gain. The solution, assuming a perfect antireﬂ ection coating at

z  0, is:

() .

()

(19.2)

The exponential growth of the power with distance is at the basis of the high efﬁ ciencies achiev-

able in SLEDs. However, the exponential factor in Eq. 19.2 also implies a strong wavelength

dependence, as g  g ( λ ). Neglecting the other spectral dependences, the output spectral density

(at z  L ) drops to half of the peak value at the wavelength λ

where the gain drops by Δ g :

Δg

ggL







()

(19.3)

At typical working conditions, for output powers  1 mW a gL  6–10 is needed, which gives

Δ g / g  10%. In order to achieve wide output spectra, an extremely ﬂ at gain spectrum must thus

be provided.

Quantum wells (QWs) are in principle well suited for wide gain spectrum, as their density of

states is ﬂ at – at least ideally – within each subband. However, in practice very large bandwidths

(   50 nm) are hard to achieve with QWs. This is due to the large density of states correspond-

ing to the ﬁ rst QW subband – in order to completely ﬁ ll one subband, an extremely large current

density (  10 kA/cm

) would be needed. Additionally, the modal gain corresponding to the ﬁ rst

subband is already very high, which typically drives the device into lasing unless special (techno-

logically demanding) antireﬂ ection structures are used.

Semiconductor quantum dots (QDs) – semiconductor nanostructures where electrons and

holes are conﬁ ned in three dimensions, resulting in quantization of the energy levels – have sev-

eral advantages over bulk and QW active regions for SLEDs:

1. Inhomogeneously broadened gain spectrum: In self-assembled QDs grown in the Straski–

Krastanov regime (see section 19.2 and Fig. 19.3 ), the natural size dispersion of the

QDs in the ensemble results in a dispersion of the energy states and thus of the inter-

band transition energies – a prototypical example of inhomogeneous broadening. More

precisely, at low temperature (10 K) and low excitation level, each QD contributes few

1180

0.8

0.6

0.4

0.2

1200 1220

40 K, 4 W

1 m

m

1240

Wavelength (nm)

PL (a.u.)

Figure 19.3 Left : Atomic force microscopy image of self-assembled QDs grown by molecular beam epitaxy of InAs

on GaAs. Centre: Low-temperature photoluminescence from an ensemble of about 100 QDs, showing how single QD

lines merge in a wide Gaussian emission. Right : Simpliﬁ ed schematics of energy levels and transitions in a QD (GS:

ground state. ES: excited state. WL: wetting layer).

CH019-I046325.indd 568CH019-I046325.indd 568 6/27/2008 5:21:28 PM6/27/2008 5:21:28 PM