**3.2. MQWs solar cells** *p-i-n* **performance**

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

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

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

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

window regions were

nm)/Au (200 nm) metal deposited by electron beam evaporation. 2 mm2

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

deposited over the window area of all devices.

344 Solar Cells - New Approaches and Reviews

voltage is about 0.4 V.

The photocurrent response is shown in Figure 11 for an InP homojunction control device, a *p*-InP/ *i*-In0.91Ga0.09As0.2P0.8 /*n*-InP barrier control device, and a *p*-*i*-*n* multiple quantum wells solar cells device. These epitaxial layer structures for the control device and barrier control device and multiple quantum wells device were grown on 225 nm thicknesses intrinsic layer under identical reactor conditions. The photocurrent response only extends to 950 nm of the InP absorption edge for the InP homojunction control device, which is determined by room temperature photoluminescence measurements, but the photocurrent responses can extend to 1050 nm of the In0.91Ga0.09As0.2P0.8 absorption edges and 1150 nm of In0.81Ga0.19As0.4P0.6 absorption edges for the barrier control device and multiple quantum wells solar cells device, respectively.

**Figure 11.** Photocurrent response spectra for InP homojunction device, barrier device and quantum well solar cell de‐ vice

The maximum power curves are shown in Figure 12 for InP homo-junction control device, *p*-InP/*i*-In0.91Ga0.09As0.2P0.8/*n*-InP barrier control device and *p*-*i*-*n* multiple quantum wells solar cells device which all grown on 250 nm intrinsic layer thicknesses. Despite *V*oc drops to 0.63V for the homo-junction control device and barrier control devices, and drops to 0.53 V for the quantum wells solar cells device, the maximum power output of the quantum wells solar cells device increases 7.4% and 4.6% relative to the homo-junction control device and barrier control device, respectively. [8]

**Figure 12.** Power output curves for InP control device, barrier control device and quantum wells solar cells device

**Figure 13.** Photocurrent response spectra of quantum well solar cells with Au nanoparticles and SiO2 nanoparticles

To illustrate the effect of nanoparticle scattering on the solar cells device in improved photo‐ current response and optoelectronic conversion efficiency, Figure 13 shows photocurrent response spectra for quantum wells solar cells with either 100 nm diameter Au or 150 nm diameter SiO2 nanoparticles deposited on these solar cells surface, which plotted by photo‐ current response ratios relative to the spectrum for the solar cells device without nanoparticles. Au and SiO2 nanoparticles densities were employed with about 2.7×109 cm−2 and 2.1×109 cm−2, respectively. The nanoparticles deposition proceeding and photocurrent measurement apparatus are described in reference [27]. The incident light scattered by Au nanoparticles leads to a reduction of photocurrent response at wavelengths about 560 nm, at the same time, a phase shift is accompanied in the scattered wavelength near the nanoparticle plasmon resonance, which results in partially destructive interference between the scattered waves and the transmitted waves. [28] The scattering of incident light by the nanoparticles arise a broad wavelength range increased from 560 nm to 900 nm. But no surface plasmon polarization resonance is present for the SiO2 nanoparticles, and the transmission and photocurrent response are increased over the range from 400 nm to 1200 nm wavelengths. [29]

The maximum power curves are shown in Figure 12 for InP homo-junction control device, *p*-InP/*i*-In0.91Ga0.09As0.2P0.8/*n*-InP barrier control device and *p*-*i*-*n* multiple quantum wells solar cells device which all grown on 250 nm intrinsic layer thicknesses. Despite *V*oc drops to 0.63V for the homo-junction control device and barrier control devices, and drops to 0.53 V for the quantum wells solar cells device, the maximum power output of the quantum wells solar cells device increases 7.4% and 4.6% relative to the homo-junction control device and barrier control

**Figure 12.** Power output curves for InP control device, barrier control device and quantum wells solar cells device

**Figure 13.** Photocurrent response spectra of quantum well solar cells with Au nanoparticles and SiO2 nanoparticles

Au and SiO2 nanoparticles densities were employed with about 2.7×109

To illustrate the effect of nanoparticle scattering on the solar cells device in improved photo‐ current response and optoelectronic conversion efficiency, Figure 13 shows photocurrent response spectra for quantum wells solar cells with either 100 nm diameter Au or 150 nm diameter SiO2 nanoparticles deposited on these solar cells surface, which plotted by photo‐ current response ratios relative to the spectrum for the solar cells device without nanoparticles.

cm−2 and 2.1×109

cm−2,

device, respectively. [8]

346 Solar Cells - New Approaches and Reviews

The photocurrent response is increased at near 960 nm and cut off at about 1200 nm for the solar cells devices deposited by Au nanoparticles. This phenomenon is attributed to the scattering of incident light into optical propagation path modes, which associated with the slabby waveguide formed by the multiple quantum wells region and surrounding p-layers and n-layers. [8]A standard calculation shows that the slabby waveguide supports two confined modes at 960~1200 nm wavelengths range. [30]Furthermore, the optical waveguide structure mode becomes better confined with increasing wavelength because of the depend‐ ence of the wavelength and semiconductor refractive indices, resulting in the waveguide modes efficiency increased. [26]The photocurrent response is improved in these wavelength ranges because of incident light coming into the waveguide mode, and substrate radiation modes leads to photon propagation path lengths increased dramatically within the multiple quantum wells region that associated with lateral photon propagation path rather than vertical path. Consequently, the efficiency of photon absorption is improved greatly. [8]

**Figure 14.** *J*-*V* and *P-V* curves measured for quantum well solar cell with and without SiO2 nanoparticles

Previously, some groups have reported that the short circuit current density and optoelectronic conversion efficiency are increased due to optical scattering from metal nanoparticles depos‐ ited on Si solar cells [31] and *a*-Si solar cells [32]. The photocurrent response is enhanced on the surface deposited nanoparticle quantum wells solar cells, which leads to the short circuit current density and optoelectronic conversion efficiency improved greatly under normal illumination incidence provided by a solar simulator with a Xenon arc lamp. The short circuit current density and voltage and the power output and voltage characteristics are shown in Figure 14 for the multiple quantum wells solar cells deposited SiO2 nanoparticles on the surface before and after. For a SiO2 nanoparticle surface density about 2.1×109 cm−2, the short circuit current density increased 12.9% and maximum power conversion efficiency increased to 17.0%. For Au nanoparticle surface density about 2.7×109 cm−2, the short circuit current density increased 7.3% and maximum power conversion efficiency increased only 1%.

The conversion efficiency and photocurrent response are improved substantially for the solar cells device structures whose quantum wells region bound with a lower refractive index substrate. A model developed by Soller and Hall [33] shows that when a horizontal electric dipole is located on a silicon insulator substrate, an excess of 80% of the light emitted by the electric dipole is coupled into the waveguide modes of the high refractive index Si insulator layer. [34]The ratio of the power of the electric dipole into waveguide modes fully to the total power of the electric dipole for the solar cells device structure over 600~1200 nm wavelengths occurs with a maximum efficiency no more than 10% in the course of the emission into waveguide, and leaky modes is in the range of 85~90%. This low efficiency is due to the small refractive index contrast to the solar cells device structure and could be improved with greater refractive index. [8]
