**2.1. MQWs solar cells layers structure**

The MQWs solar cells layers structure fabricated by InGaN alloys and GaN materials is shown in Figure 1, the light absorption region consists of eight periods of InGaN (3 nm)/GaN (8 nm) MQWs. The MQWs were grown under the established MOCVD growth conditions; InGaN alloys materials with In content about 0.3 or 0.4 work in longer wavelengths in order to obtain better photovoltaic responses. The thickness of *p*-GaN or *n*-GaN is about 150 nm. The device structure was grown on a GaN/Al2O3 template. As demonstration, the device fabrication steps are adopted to commercial group III-nitride LEDs and implemented a thin Ni/Au semitrans‐ parent current spreading layer to minimize the *p*-contact resistance on the *p*-GaN window layer.

The growth of high quality crystalline structure InGaN alloys materials is highly challenging in all of In composition range. One of the main problems is that the lattice is seriously mis‐ matched between InN and GaN atoms, resulting in low solubility and phase separation between InN and GaN materials. [17]Recently, single crystalline phase InGaN alloys materials with all of In composition range could be grown by metal organic chemical vapor deposition (MOCVD) by directly depositing on GaN templates without using buffer layers. [18] These InGaN alloys characteristics are illustrated in Figure 2, in which X-ray diffraction data of the (002) plane for several InGaN alloys materials for *θ*-2*θ* scans are shown. All scanned curves

**Figure 1.** Schematic of layers structure based on InGaN/GaN MQWs solar cells.

**2. InGaN/GaN MQWs solar cells**

338 Solar Cells - New Approaches and Reviews

velocity and low radiation resistance. [14]

**2.1. MQWs solar cells layers structure**

layer.

incorporated InGaN alloys with higher In contents. [13]

performance of solar cells operating in wavelengths longer than 420 nm.

InGaN alloys semiconductor materials as active layers of light emitting diodes have been widely investigated, which emission wavelengths cover from red to near ultraviolet spectral regions. [13] Recently, InGaN alloys as a new solar cells materials have been interested by their tunable energy band gaps which vary from 0.7 eV to 3.4 eV, covering almost the whole sunlight spectrum, and also their superior photovoltaic characteristics, such as high carrier mobility, direct energy band gap, high optical absorption coefficient near the band edge, high drift

Although InGaN alloys material solar cells offer tremendous potential for photovoltaic applications, there are only a few references on InGaN alloys solar cells. Furthermore, most of reported InGaN alloys solar cells usually have In contents no more than 15% and band gaps about 3.0 eV or larger, therefore, external quantum efficiency is nearly not exist at wavelengths longer than 420 nm. [15] An earlier theoretical calculation has been indicated that the use of a special active material in solar cells can obtain conversion efficiency greater than 50%, that material is InGaN alloys whose In content exceed 40%. [16]Additionally, group III-nitride multi-junction solar cells with ideal band gaps for maximum conversion efficiency must be

In this section, the optoelectronic properties of InGaN alloys solar cells are researched by merging into InGaN/GaN multiple quantum wells (MQWs), attempt to alleviate the phase separation issue of InN and GaN in a certain extent, and demonstrate the photovoltaic

The MQWs solar cells layers structure fabricated by InGaN alloys and GaN materials is shown in Figure 1, the light absorption region consists of eight periods of InGaN (3 nm)/GaN (8 nm) MQWs. The MQWs were grown under the established MOCVD growth conditions; InGaN alloys materials with In content about 0.3 or 0.4 work in longer wavelengths in order to obtain better photovoltaic responses. The thickness of *p*-GaN or *n*-GaN is about 150 nm. The device structure was grown on a GaN/Al2O3 template. As demonstration, the device fabrication steps are adopted to commercial group III-nitride LEDs and implemented a thin Ni/Au semitrans‐ parent current spreading layer to minimize the *p*-contact resistance on the *p*-GaN window

The growth of high quality crystalline structure InGaN alloys materials is highly challenging in all of In composition range. One of the main problems is that the lattice is seriously mis‐ matched between InN and GaN atoms, resulting in low solubility and phase separation between InN and GaN materials. [17]Recently, single crystalline phase InGaN alloys materials with all of In composition range could be grown by metal organic chemical vapor deposition (MOCVD) by directly depositing on GaN templates without using buffer layers. [18] These InGaN alloys characteristics are illustrated in Figure 2, in which X-ray diffraction data of the (002) plane for several InGaN alloys materials for *θ*-2*θ* scans are shown. All scanned curves

exhibit no multiple peaks except for closing the InN peak position, showing that InGaN alloys materials have not been phase separation. The results show a significant growth improvement of InGaN alloys materials by MOCVD. [13]

separation can be grown when the alloys is embedded by an InGaN/GaN double heterostructure.

Figure 2. X-ray diffraction data for *θ*-2*θ* scan curves of the (002) plane **Figure 2.** X-ray diffraction data for *θ*-2*θ* scan curves of the (002) plane

**2.2. MQWs solar cells performance** 

Intensity of PL emission s

pectru

m

Intensity of PL emission spectrum However, when In composition range exceed to the 0.5, the homogeneity of InGaN alloys materials is pretty poorer. The full width at half maximum of the *θ*-2*θ*-scans rocking curves of the (002) plane increases from about 1000 arcsec when In composition is 0.2 to about 3000 arcsec when In content is 0.5 for InGaN alloys materials of 200 nm thickness. The photolumi‐ nescence (PL) emission spectrum of InGaN alloys materials also is deteriorated with an increase of In composition, as shown in Figure 3. The intensity of PL emission spectrum of In0.4Ga0.6N alloys is about 100 times lower than that of In0.2Ga0.8N. This trend of crystalline

Figure 3. PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates.

Photon energy (eV)

Figure 4. Emission spectrum of the white light source and the inset is the microscopy image of MQWs solar cells.

Wavelength of incident light(nm)

An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown

in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a

quality reduced with In composition increasing makes the realization of solar cells based on InGaN alloys materials with In content greater than 0.3 highly challenging. Evidence that strain could suppress phase separation in InGaN alloys materials has been reported. [19] It was shown that InGaN alloys materials with In content lower than 0.5 without phase separation can be grown when the alloys is embedded by an InGaN/GaN double heterostructure. Intensity of X-ray diffraction Degree of X-ray diffraction Intensity of X-ray diffraction

Figure 2. X-ray diffraction data for *θ*-2*θ* scan curves of the (002) plane

separation can be grown when the alloys is embedded by an InGaN/GaN double heterostructure.

Figure 3. PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates.

Figure 3. PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates.

**Figure 3.** PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates. Photon energy (eV)

m

**2.2. MQWs solar cells performance 2.2. MQWs solar cells performance Figure 4.** Emission spectrum of the white light source and the inset is the microscopy image of MQWs solar cells.

An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown

Figure 4. Emission spectrum of the white light source and the inset is the microscopy image of MQWs solar cells.

in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a

#### in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown **2.2. MQWs solar cells performance**

An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a Keithley 2400 source meter. The solar cells were illuminated by a white light source with no optical filters to measure the current versus voltage characteristics, whose PL emission spectrum is shown in Figure 4. The solar cells were illuminated monochromatically

8

by using the same white light source to achieve the characterization of quantum efficiency versus excitation wavelength.[13]The PL emission spectrum for MQWs solar cells structure fabricated by InGaN alloys materials with In content about 0.3 and GaN is shown in Figure 5, and exhibits an PL emission peak about 472 nm. Figure 4. The solar cells were illuminated monochromatically by using the same white light source to achieve the characterization of quantum efficiency versus excitation wavelength.[13]The PL emission spectrum for MQWs solar cells structure fabricated by InGaN alloys materials with In content about Keithley 2400 source meter. The solar cells were illuminated by a white light source with no optical filters to measure the current versus voltage characteristics, whose PL emission spectrum is shown in Figure 4. The solar cells were illuminated monochromatically by using the same white light source to

achieve the characterization of quantum efficiency versus excitation wavelength.[13]The PL emission

0.3 and GaN is shown in Figure 5, and exhibits an PL emission peak about 472 nm.

Figure 5. PL emission spectrum of an InGaN/GaN MQWs solar cells structure.

Figure 5. PL emission spectrum of an InGaN/GaN MQWs solar cells structure.

**Figure 5.** PL emission spectrum of an InGaN/GaN MQWs solar cells structure. Wavelength of incident light(nm)

)

quality reduced with In composition increasing makes the realization of solar cells based on InGaN alloys materials with In content greater than 0.3 highly challenging. Evidence that strain could suppress phase separation in InGaN alloys materials has been reported. [19] It was shown that InGaN alloys materials with In content lower than 0.5 without phase separation can be grown when the alloys is embedded by an InGaN/GaN double heterostructure.

Intensity of X-ray diffraction

340 Solar Cells - New Approaches and Reviews

Intensity of X-ray diffraction

Intensity of PL emission spectrum

Intensity of PL emission spectrum

Intensity of PL emission spectru

Intensity of PL emission spectru

m

m

**Figure 3.** PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates.

Figure 2. X-ray diffraction data for *θ*-2*θ* scan curves of the (002) plane

Figure 2. X-ray diffraction data for *θ*-2*θ* scan curves of the (002) plane

Degree of X-ray diffraction

Degree of X-ray diffraction

Figure 3. PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates.

Figure 3. PL emission spectrum of InGaN alloys materials grown on GaN/Al2O3 templates.

Photon energy (eV)

Photon energy (eV)

Figure 4. Emission spectrum of the white light source and the inset is the microscopy image of MQWs solar cells.

Wavelength of incident light(nm)

Figure 4. Emission spectrum of the white light source and the inset is the microscopy image of MQWs solar cells.

**Figure 4.** Emission spectrum of the white light source and the inset is the microscopy image of MQWs solar cells.

Wavelength of incident light(nm)

An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a

An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a

An optical microscopy image of a MQWs solar cells fabricated by InGaN alloys materials is shown in the inset of Figure 4. The MQWs solar cells were characterized by a microprobe station with a Keithley 2400 source meter. The solar cells were illuminated by a white light source with no optical filters to measure the current versus voltage characteristics, whose PL emission spectrum is shown in Figure 4. The solar cells were illuminated monochromatically

**2.2. MQWs solar cells performance** 

**2.2. MQWs solar cells performance** 

**2.2. MQWs solar cells performance**

separation can be grown when the alloys is embedded by an InGaN/GaN double heterostructure.

separation can be grown when the alloys is embedded by an InGaN/GaN double heterostructure.

Current versus voltage (*I*-*V*) characteristics of two MQWs solar cells fabricated by InGaN alloys Current versus voltage (*I*-*V*) characteristics of two MQWs solar cells fabricated by InGaN alloys **Figure 6.** Curves of *I*-*V* characteristics for In*x*Ga1−*<sup>x</sup>*N/GaN MQWs solar cells.

8

8

materials with In composition about 0.3 or 0.4 in the quantum well region and GaN are shown in Figure 6. The open-circuit voltage (*V*oc) is about 2.0 V or 1.8 V for two MQWs solar cells with In composition about 0.3 or 0.4, respectively. These values are in good agreement with the band gaps of materials with In composition about 0.3 or 0.4 in the quantum well region and GaN are shown in Figure 6. The open-circuit voltage (*V*oc) is about 2.0 V or 1.8 V for two MQWs solar cells with In composition about 0.3 or 0.4, respectively. These values are in good agreement with the band gaps of Current versus voltage (*I*-*V*) characteristics of two MQWs solar cells fabricated by InGaN alloys materials with In composition about 0.3 or 0.4 in the quantum well region and GaN are shown in Figure 6. The open-circuit voltage (*V*oc) is about 2.0 V or 1.8 V for two MQWs solar cells with In composition about 0.3 or 0.4, respectively. These values are in good agreement with the

Figure 6. Curves of *I*-*V* characteristics for In*x*Ga1−*<sup>x</sup>*N/GaN MQWs solar cells.

9

band gaps of In0.3Ga0.7N and In0.4Ga0.6N. However, the performance of the solar cells with In0.4Ga0.6N/GaN MQWs as active region is no more than that of the solar cells with In0.3Ga0.7N/ GaN MQWs, despite the In0.4Ga0.6N/GaN MQWs solar cells active layers are shown to have a much better spectral overlap with the excitation light source. [3]This degradation of these solar cells performance and the X-ray diffraction results are shown in Figure 2 and it is a direct reason of the InGaN alloys materials quality degradation with In composition increasing, which further leads to much loss of the photogenerated carriers. The observed photovoltaic charac‐ teristics of these solar cells are consistent with the quantum efficiencies of group III-nitride green LEDs which are much lower than those of blue LEDs. [13] In0.3Ga0.7N and In0.4Ga0.6N. However, the performance of the solar cells with In0.4Ga0.6N/GaN MQWs as active region is no more than that of the solar cells with In0.3Ga0.7N/GaN MQWs, despite the In0.4Ga0.6N/GaN MQWs solar cells active layers are shown to have a much better spectral overlap with the excitation light source. [3]This degradation of these solar cells performance and the X-ray diffraction results are shown in Figure 2 and it is a direct reason of the InGaN alloys materials quality degradation with In composition increasing, which further leads to much loss of the photogenerated carriers. The observed photovoltaic characteristics of these solar cells are consistent with the quantum

efficiencies of group III-nitride green LEDs which are much lower than those of blue LEDs. [13]

Figure 7. Curves of current-density vs voltage and power-density vs voltage.

Current density versus voltage and power density versus voltage curves of the solar cells with

**Figure 7.** Curves of current-density vs voltage and power-density vs voltage.

In0.3Ga0.7N/GaN MQWs as active layer are shown in Figure 7, a fill factor of over 60% is obtained from the solar cells. The external quantum efficiency as a function of excitation wavelength for the In0.3Ga0.7N/GaN MQWs solar cells is shown in Figure 8, from which we can see that the solar cells have an external quantum efficiency of 40% at 420 nm. The spectrum response is limited by using *p*-GaN window in the shorter wavelength region, and it would be improved if the *p*-AlGaN or *p*-InAlGaN material is incorporated. [13]Three major factors limited the external quantum efficiency are the following: (1) Light absorption in the semi-transparent *p*-contact layer. Current spreading in *p*-contact layer was only adopted from LED structure and the *p*-contact layer need to be optimized for solar cells devices. (2) The thickness of light absorption layer is too thin in the In0.3Ga0.7N/GaN MQWs structure. The well thickness and period of the In0.3Ga0.7N/GaN MQWs active region need to be optimized to maximize light absorption and minimize other detrimental effects, which is incorporated by relatively high In composition InGaN alloys materials in the multiple quantum well region. (3) InGaN alloys materials with relatively high In composition are very low crystalline quality. Current density versus voltage and power density versus voltage curves of the solar cells with In0.3Ga0.7N/GaN MQWs as active layer are shown in Figure 7, a fill factor of over 60% is obtained from the solar cells. The external quantum efficiency as a function of excita‐ tion wavelength for the In0.3Ga0.7N/GaN MQWs solar cells is shown in Figure 8, from which we can see that the solar cells have an external quantum efficiency of 40% at 420 nm. The spectrum response is limited by using *p*-GaN window in the shorter wavelength region, and it would be improved if the *p*-AlGaN or *p*-InAlGaN material is incorporated. [13]Three major factors limited the external quantum efficiency are the following: (1) Light absorp‐ tion in the semi-transparent *p*-contact layer. Current spreading in *p*-contact layer was only adopted from LED structure and the *p*-contact layer need to be optimized for solar cells devices. (2) The thickness of light absorption layer is too thin in the In0.3Ga0.7N/GaN MQWs structure. The well thickness and period of the In0.3Ga0.7N/GaN MQWs active region need to be optimized to maximize light absorption and minimize other detrimental effects, which is incorporated by relatively high In composition InGaN alloys materials in the multiple quantum well region. (3) InGaN alloys materials with relatively high In composition are very low crystalline quality. Nevertheless, the InGaN alloys materials solar cells have good external quantum efficiency working at such long wavelengths, so the MQWs is a effec‐

tive method to design MQWs solar cells by InGaN alloys materials with relatively high In compostion for high photoelectric conversion efficiency. [13] Nevertheless, the InGaN alloys materials solar cells have good external quantum efficiency working at such long wavelengths, so the MQWs is a effective method to design MQWs solar cells by InGaN

alloys materials with relatively high In compostion for high photoelectric conversion efficiency. [13]

Figure 8. Curves of external quantum efficiency vs excitation wavelength.

Currently, there are extensively interests in the application of multiple quantum wells structure in

**3. InP/InGaAsP MQWs solar cells Figure 8.** Curves of external quantum efficiency vs excitation wavelength.

#### solar cells devices; their optoelectronic conversion efficiency can exceed the single-junction solar cells theoretical efficiency limit of 31%. [20]Theoretically, maximum optoelectronic conversion **3. InP/InGaAsP MQWs solar cells**

band gaps of In0.3Ga0.7N and In0.4Ga0.6N. However, the performance of the solar cells with In0.4Ga0.6N/GaN MQWs as active region is no more than that of the solar cells with In0.3Ga0.7N/ GaN MQWs, despite the In0.4Ga0.6N/GaN MQWs solar cells active layers are shown to have a much better spectral overlap with the excitation light source. [3]This degradation of these solar cells performance and the X-ray diffraction results are shown in Figure 2 and it is a direct reason of the InGaN alloys materials quality degradation with In composition increasing, which further leads to much loss of the photogenerated carriers. The observed photovoltaic charac‐ teristics of these solar cells are consistent with the quantum efficiencies of group III-nitride

In0.3Ga0.7N and In0.4Ga0.6N. However, the performance of the solar cells with In0.4Ga0.6N/GaN MQWs as active region is no more than that of the solar cells with In0.3Ga0.7N/GaN MQWs, despite the In0.4Ga0.6N/GaN MQWs solar cells active layers are shown to have a much better spectral overlap with the excitation light source. [3]This degradation of these solar cells performance and the X-ray diffraction results are shown in Figure 2 and it is a direct reason of the InGaN alloys materials quality degradation with In composition increasing, which further leads to much loss of the photogenerated carriers. The observed photovoltaic characteristics of these solar cells are consistent with the quantum efficiencies of group III-nitride green LEDs which are much lower than those of blue LEDs. [13]

Figure 7. Curves of current-density vs voltage and power-density vs voltage.

Voltage(V)

Current density versus voltage and power density versus voltage curves of the solar cells with In0.3Ga0.7N/GaN MQWs as active layer are shown in Figure 7, a fill factor of over 60% is obtained from the solar cells. The external quantum efficiency as a function of excita‐ tion wavelength for the In0.3Ga0.7N/GaN MQWs solar cells is shown in Figure 8, from which we can see that the solar cells have an external quantum efficiency of 40% at 420 nm. The spectrum response is limited by using *p*-GaN window in the shorter wavelength region, and it would be improved if the *p*-AlGaN or *p*-InAlGaN material is incorporated. [13]Three major factors limited the external quantum efficiency are the following: (1) Light absorp‐ tion in the semi-transparent *p*-contact layer. Current spreading in *p*-contact layer was only adopted from LED structure and the *p*-contact layer need to be optimized for solar cells devices. (2) The thickness of light absorption layer is too thin in the In0.3Ga0.7N/GaN MQWs structure. The well thickness and period of the In0.3Ga0.7N/GaN MQWs active region need to be optimized to maximize light absorption and minimize other detrimental effects, which is incorporated by relatively high In composition InGaN alloys materials in the multiple quantum well region. (3) InGaN alloys materials with relatively high In composition are very low crystalline quality. Nevertheless, the InGaN alloys materials solar cells have good external quantum efficiency working at such long wavelengths, so the MQWs is a effec‐

Current density versus voltage and power density versus voltage curves of the solar cells with In0.3Ga0.7N/GaN MQWs as active layer are shown in Figure 7, a fill factor of over 60% is obtained from the solar cells. The external quantum efficiency as a function of excitation wavelength for the In0.3Ga0.7N/GaN MQWs solar cells is shown in Figure 8, from which we can see that the solar cells have an external quantum efficiency of 40% at 420 nm. The spectrum response is limited by using *p*-GaN window in the shorter wavelength region, and it would be improved if the *p*-AlGaN or *p*-InAlGaN material is incorporated. [13]Three major factors limited the external quantum efficiency are the following: (1) Light absorption in the semi-transparent *p*-contact layer. Current spreading in *p*-contact layer was only adopted from LED structure and the *p*-contact layer need to be optimized for solar cells devices. (2) The thickness of light absorption layer is too thin in the In0.3Ga0.7N/GaN MQWs structure. The well thickness and period of the In0.3Ga0.7N/GaN MQWs active region need to be optimized to maximize light absorption and minimize other detrimental effects, which is incorporated by relatively high In composition InGaN alloys materials in the multiple quantum well region. (3) InGaN alloys materials with relatively high In composition are very low crystalline quality.

power-density(mW/cm2

)

green LEDs which are much lower than those of blue LEDs. [13]

342 Solar Cells - New Approaches and Reviews

Current-density(mA/cm2

**Figure 7.** Curves of current-density vs voltage and power-density vs voltage.

)

efficiency range in multiple quantum wells solar cells could be predicted from 50% to 65%. The incorporation of multiple quantum wells structure can ensure high energy photon absorption efficiency and improved short-circuit current density (*J*sc) and reduced in open-circuit voltage (*V*oc), because no enough collection efficiency of photogenerated carriers is especially obvious. [21] Regardless of these problems, the maximum power of multiple quantum wells solar cells devices can exceed that of the similar homo-junction solar cells devices by extending the absorption spectrum to longer wavelengths. [22]We have got a conclusion that incident light can be normally got into lateral optical propagation paths in the multiple quantum wells solar cells devices by scattering from metal or dielectric nanoparticles, whose optical confinement is provided by the refractive index contrast between the quantum wells layer and surrounding materials. Substantially, the photogenerated current generation and collection over a large range of incident light wavelengths has been improved, particularly at longer wavelengths. [8] **3.1.** MQWs **solar cells** *p-i-n* **structure**  Currently, there are extensively interests in the application of multiple quantum wells structure in solar cells devices; their optoelectronic conversion efficiency can exceed the singlejunction solar cells theoretical efficiency limit of 31%. [20]Theoretically, maximum optoelec‐ tronic conversion efficiency range in multiple quantum wells solar cells could be predicted from 50% to 65%. The incorporation of multiple quantum wells structure can ensure high energy photon absorption efficiency and improved short-circuit current density (*J*sc) and reduced in open-circuit voltage (*V*oc), because no enough collection efficiency of photogener‐ ated carriers is especially obvious. [21] Regardless of these problems, the maximum power of multiple quantum wells solar cells devices can exceed that of the similar homo-junction solar cells devices by extending the absorption spectrum to longer wavelengths. [22]We have got a conclusion that incident light can be normally got into lateral optical propagation paths in the multiple quantum wells solar cells devices by scattering from metal or dielectric nanoparticles, whose optical confinement is provided by the refractive index contrast between the quantum wells layer and surrounding materials. Substantially, the photogenerated current generation and collection over a large range of incident light wavelengths has been improved, particularly at longer wavelengths. [8]

#### The lattice matched InP/InGaAsP multiple quantum wells *p*-*i*-*n* structure solar cells is nominally **3.1. MQWs solar cells** *p-i-n* **structure**

10

shown in Figure 9. The *n*-type electrode of all *p*-*i*-*n* structures solar cells consist of a S doped InP The lattice matched InP/InGaAsP multiple quantum wells *p*-*i*-*n* structure solar cells is nomi‐ nally shown in Figure 9. The *n*-type electrode of all *p*-*i*-*n* structures solar cells consist of a S

doped InP substrate with doping concentration about 5×1018 cm−3, while the intrinsic region consist of 10 nm In0.91Ga0.09As0.2P0.8 barriers alternating with 10 nm In0.81Ga0.19As0.4P0.6 quantum wells for ten periods with an additional 50 nm or 25 nm In0.91Ga0.09As0.2P0.8 barrier on the top quantum wells layer. The *p*-type electrode of all *p*-*i*-*n* structures solar cells consist of a Zn doped 50 nm *p*-type InP layer or 25 nm *p*-type InP and 10 nm *p*-type In0.47Ga0.53As with doping concentration about 3×1018 cm−3. The *n*-type Ohmic contacts were fabricated by using Ti (40 nm)/Au (200 nm) metal deposited by electron beam evaporation. 2 mm2 window regions were formed by conventional photolithography, and *p*-type contacts were formed by using Ti (20 nm)/Pd (20 nm)/Au (200 nm) metal deposited by electron beam evaporation. [8]The top In0.47Ga0.53As contact layer was removed from the window region by a selective wet etch (H2SO4:H2O2:H2O, 1:10:220) for 15 s, and about 15 nm SiO2 surface passivation layer was sputter deposited over the window area of all devices.

**Figure 9.** InP-based multiple quantum well solar cells with nanoparticles on the surface

To optimize collection efficiency of photogenerated carriers in the multiple quantum wells and to minimize the reduction of *V*oc, a sufficiently large electric field across the intrinsic region is definitely required, in which the electric field intensity is 30 kV/cm or so, [23] and the barriers must be thermally or optically excited usually at 200~450 meV or less. [24] The electric field condition requires that the intrinsic region in the *p*-*i*-*n* structure should be especially thin, where the intrinsic region is required by choosing appropriate materials for the multiple quantum wells and barrier. [8]For the solar cells device structure with an intrinsic layer thickness of 250 nm, shown in Figure 9, the quantum wells electric field intensity is about 48 kV/cm at equilibrium condition, and 32 kV/cm at a maximum power state when operating voltage is about 0.4 V.

Incorporation of the multiple quantum wells region in solar cells device, not only improves photon absorption efficiency at longer wavelengths, but also increases the refractive index in the intrinsic region relative to the surrounding electrode contact layers, as also shown in Figure 9, which produces a slabby waveguide structure. [25] Waveguide mode accompanied by light scattered from metallic nanoparticles has been demonstrated by metal nanoparticles on siliconon-insulator photodetectors. [26] The scattering effect is achieved by depositing metal nanoparticles or dielectric nanoparticles at top of the solar cells device, as shown in Figure 10. The incident light scattered by the nanoparticles not only can improve transmission of photons into the solar cells active layers, but also make normally incident photons into lateral confined paths in the multiple quantum wells waveguide layer, result in photon absorption efficiency increasing, more photocurrent generating and optoelectronic conversion efficiency improving. [8]

**Figure 10.** SEM images of 100 nm diameter Au nanoparticles (left) and 150 nm diameter SiO2 nanoparticles (right).
