Influence of the local absorber layer thickness on the performance of ZnO nanorod solar cells

The local absorber layer thickness (d_local) of solar cells with extremely thin absorber was changed between 10 nm and 70 nm. As a model system, ZnO nanorod arrays (electron conductor) with fixed internal surface area coated with In₂S₃ (absorber) and impregnated with CuSCN (transparent hole conductor) were applied. The performance of the small area solar cells depended critically on d_local. The highest short circuit current density was reached for the lowest d_local. In contrast, the highest open circuit voltage was obtained for the highest d_local. A maximum energy conversion efficiency of 3.4% at AM1.5 was achieved. Limiting factors are discussed.(© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microcrystalline silicon‐carbon alloys as anti‐reflection window layers in high efficiency thin film silicon solar cells

Microcrystalline silicon‐carbide (μc‐SiC:H) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc‐SiC:H films were employed as window layers in microcrystalline silicon (μc‐Si:H) solar cells. The short‐circuit current density (J_SC) in these n‐side illuminated n–i–p cells increases with increasing the deposition time t_W of the μc‐SiC:H window layer from 5 min to 60 min. The enhanced J_SC is attributed to both the high transparency and an anti‐reflection effect of the μc‐SiC:H window layer. Using these favourable optical properties of the μc‐SiC:H window layer in μc‐Si:H solar cells, a J_SC value of 23.8 mA/cm² and cell efficiencies above 8.0% were achieved with an absorber layer thickness of 1 μm and a Ag back reflector. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the electrical and optical properties of InAs/InGaAs dot in a well solar cell

The electroreflectance (ER) and current–voltage (J–V) of InAs/InGaAs dots in a well (DWELL) solar cell (SC) were measured to examine the optical and electrical properties. To investigate the carrier capturing and escaping effects in the quantum dot (QD) states the above and below optical biases of the GaAs band gap were used. In the reverse bias region of the J–V curve, the tunneling effect in the QD states was observed at low temperature. The ideality factors (n) were calculated from the J–V curves taken from various optical bias intensities (I_ex). The changes in the ideality factor (n) and short circuit current (J_SC) were attributed mainly to carrier capture at low temperature, whereas the carrier escaping effect was dominant at room temperature. ER measurements revealed a decrease in the junction electric field (F_J) due to the photovoltaic effect, which was independent of the optical bias source at the same temperature. At low temperature, the reduction of photovoltaic effect could be explained by the enhancement carrier capturing effect due to the strong carrier confinement in QDs.     

Comparison of D.C. and A.C. electro-analytical methods for measuring diode ideality factors and series resistances of silicon solar cells

The diode ideality factor (m) and the series resistance (R_s) of a Si solar cell represent two critical performance-indicator parameters of the device. Since both m and R_s are functions of voltage (V) and temperature (T), simultaneous electrical measurements of these parameters under variable conditions of V and T can often be difficult with traditional direct current (D.C.) techniques. Using the electro-analytical method of linear sweep voltammetry (LSV) and a commonly available Si solar cell, we explore these specific confines of such D.C. measurements. The results are compared with those obtained from a parallel set of alternating current (A.C.) measurements using impedance spectroscopy (IS). LSV provides the main D.C. parameters (open circuit voltage, short circuit current, fill factor, and efficiency) of the cell, but is limited in terms of independently measuring m and R_s beyond strong forward biased conditions. The IS approach is free of the latter experimental constraints, and at the same time can provide several other important electrical parameters of the solar cell. Specifically, IS detects the presence of a low-high (p–p⁺) junction at the back surface of the cell, and serves as an efficient probe of certain electrical characteristics of this junction.     

Analysis of the current linearity at low illumination of high‐efficiency back‐junction back‐contact silicon solar cells

The relation between current and illumination intensity of three structures of high‐efficiency back‐junction back‐contact silicon solar cells was analyzed. Both, n‐type cells with non‐diffused front surface and p‐type cell with floating n‐emitter show a pronounced non‐linearity due to strong illumination dependence of the passivation quality of the non‐diffused surface and the floating junction respectively. Quantum efficiency (QE) of this cell type drops significantly for the illumination lower than 0.5 suns. In contrast the QE of n‐type cells with n⁺‐front surface field (FSF) is linear. Low illumination current characteristics of all three of the analyzed structures could be well described by physical models. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Decoupling crystalline volume fraction and VOC in microcrystalline silicon pin solar cells by using a µc‐Si:F:H intrinsic layer

Microcrystalline silicon thin film pin solar cells with a highly crystallized intrinsic μc‐Si:F:H absorber were prepared by RF‐plasma enhanced chemical vapour deposition using SiF₄ as the gas precursor. The cells were produced with a vacuum break between the doped layer and intrinsic layer depositions, and the effect of different subsequent interface treatment processes was studied. The use of an intrinsic μc‐Si:H p/i buffer layer before the first air break increased the short circuit current density from 22.3 mA/cm² to 24.7 mA/cm². However, the use of a hydrogen‐plasma treatment after both air breaks without an interface buffer layer improved both the open circuit voltage and the fill factor. Although the material used for the absorber layer showed a very high crystalline fraction and thus an increased spectral response at long wavelengths, an open‐circuit voltage (V_OC) of 0.523 V was nevertheless observed. Such a value of V_OC is higher than is typically obtained in devices that employ a highly crystallized absorber as reported in the literature (see abstract figure). Using a hydrogen‐plasma treatment, a single junction μc‐Si:F:H pin solar cell with an efficiency of 8.3% was achieved.

     

Influence of shunt conduction on determining the dominant recombination processes in CIGS thin-film solar cells

We investigated the transport and photovoltaic properties of Cu(In_1-xGa_x)Se₂ (CIGS) thin-film solar cells. The shunt-current-eliminated diode current could be obtained from the current–voltage characteristics by subtracting the parasitic shunt leakage current from the total current. The temperature dependence of the open-circuit voltage, extracted from the shunt-eliminated (total) current, suggested that the recombination activation energy is comparable to (much less than) the CIGS bandgap. The low-temperature characteristics of the diode ideality factor supported bulk-dominated recombination in the same cell. This suggests that shunt-current subtraction can provide the proper diode parameters of CIGS solar cells.     

Zinc oxide nanostructures for applications as ethanol sensors and dye-sensitized solar cells

ZnO nanostructures were prepared by thermal oxidation technique for applying as ethanol sensors and dye-sensitized solar cells. To improve sensitivity of the sensor based on ZnO nanostructures, gold doping was performed in ZnO nanostructures. Gold-doped with 0%, 5%, and 10% by weight were investigated. The improvement of sensor sensitivity toward ethanol due to gold doping was observed at entire operating temperature and ethanol concentration. The sensitivity up to 145 was obtained for 10% Au-doped ZnO sensor. This can be explained by an increase of the quantity of oxygen ion due to catalytic effect of gold. Also, it was found that oxygen ion species at the surface of the Au-doped ZnO sensor remained O²⁻ as pure ZnO sensor. For dye-sensitized solar cell application, the dye-sensitized solar cell structure based on ZnO as a photoelectrode was FTO/ZnO/Eosin-Y/electrolyte/Pt counter electrode. ZnO with different morphologies of nanobelt, nano-tetrapod, and powder were investigated. It was found that DSSCs with ZnO powder showed higher photocurrent, photovoltage and overall energy conversion efficiencies than that of ZnO nanobelt and ZnO nano-tetrapod. The best results of DSSCs were the short circuit current (J_sc) of 1.25 mA/cm², the open circuit voltage (V_oc) of 0.45 V, the fill factor (FF) of 0.65 and the overall energy conversion efficiency (η) of 0.68%.     

Photovoltaic and impedance properties of dye-sensitized solar cell based on nature dye from beetroot

The present study involves fabrication and photovoltaic characterization including impedance properties of dye-sensitized solar cells based on natural dye from beetroot. The electrode of the cell was prepared with commercial Fluorine-doped Tin Oxide glass with 100 μm layer of nanostructured TiO₂ whereas, the counter electrode consisted of platinum-coated glass. Fresh juice was extracted from beetroot to use as dye. The dye exhibited high absorption in visible range. Photovoltaic measurements of the solar cell gave a short circuit current density (Jsc) of 130 μA/cm² and an open-circuit voltage (VOC) of 0.38 V under AM 1.5 illumination intensity. The VOC and Jsc showed linear behavior at higher values of illumination intensities. The conductance-voltage, the capacitance-voltage and the series resistance voltage characteristics of the dye solar cell was measured at frequency range from 5 kHz to 5 MHz to study performance of the dye-sensitized solar cells with natural dyes.     

Surface passivation schemes for high‐efficiency n‐type Si solar cells

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J_0e) of 49 fA/cm² have been reached with this approach on SiO₂ passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (V_oc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-resolution mapping of the energy conversion efficiency of solar cells and silicon photodiodes in photovoltaic mode

We demonstrate an optical technique to derive the two-dimensional energy conversion efficiency (η_CE), fill factor (FF) and external quantum efficiency (η_QE) distributions across the surface of photovoltaic devices. A compact, inexpensive optical-feedback laser diode microscope is constructed to acquire the confocal reflectance and efficiency maps enabling the observation of the local parametric behavior in silicon photodiodes in photovoltaic mode and single-junction solar cells. The η_CE and η_QE distributions are greatly influenced by local parasitic resistances that depend on laser irradiance. These parasitic resistances decrease the η_CE and η_QE values with distance from the contact electrode at high laser irradiance. The optical technique enables microscopic comparison of η_CE and η_QE within the pn-overlay region of the photodiode sample, revealing its optimization for photodetection rather than power generation. The technique also elucidates the decreasing local η_CE of the solar cell under intense irradiation.     

Calibrated numerical model of a GaInP–GaAs dual‐junction solar cell

In this letter a calibrated numerical model of a III–V dual‐junction solar cell including tunnel diode and Bragg reflector is presented. The quantum efficiencies of the subcells are computed by using the principle of current‐limitation in monolithic multi‐junction solar cells. A special procedure with bias‐illumination and bias‐voltage was implemented. Numerical simulations are used to study the influence of the top cell thickness on the cells' quantum efficiency and on the current‐matching condition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XPS surface analysis of monocrystalline silicon solar cells for manufacturing control

X-ray photoelectron spectroscopy (XPS) has been applied to surfaces of silicon wafers in the different stages of the assembly line for large-scale monocrystalline silicon solar cell manufacturing (ISOFOTON, Malaga, Spain). XPS results have shown that a considerable amount of carbon is present on the pyramidal-textured monocrystalline silicon surface. This amount decreases slightly but is still present after the process of phosphor diffusion (p-n junction), as well as after subsequent calcination in humid air for SiO₂ film formation (passivation). This amount of carbon may be buried during the process of CVD coating an anti-reflection TiO₂ film. After calcination of the film in order to obtain the TiO₂ rutile phase, an even higher amount of carbon is detected on the TiO₂ anti-reflection coating surface. This indicates that not all organics from the tetra-isopropile ortho-titanate (TPT) precursor were released from the film. Furthermore, in this case phosphor is found in excess on the SiO₂ wafer surface (dead layer) and also on the rutile TiO₂ surface, indicating that an extra phosphor diffusion from the bulk silicon through the TiO₂ film has taken place during calcination. These results demonstrate how thermal treatments applied in the solar cell manufacturing assembly line can influence and may change the intended compositional distribution. These treatments may also introduce defects that act as recombination centres for charge carriers in the solar cell device. Received: 13 September 2000 / Accepted: 10 January 2001 / Published online: 3 May 2001     

Black multi‐crystalline silicon solar cells

A simple method for nano‐scale texturing of silicon surfaces based on local metal‐catalyzed wet chemical etching, which results in an almost complete suppression of reflectivity in a broad spectral range, has been successfully applied to produce black multi‐crystalline silicon solar cells. The performance of the cells is compared to that of reference cells without surface nano‐texturing. A considerable increase of the short circuit current (by 36–42% with respect to the reference cells) without deterioration of other performance parameters is observed under natural sun illumination. Means of further optimization of such black solar cells are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A luminescent solar concentrator with 7.1% power conversion efficiency

The Luminescent Solar Concentrator (LSC) consists of a transparent polymer plate, containing luminescent particles. Solar cells are connected to one or more edges of the polymer plate. Incident light is absorbed by the luminescent particles and re‐emitted. Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection. Since the edge area is smaller than the receiving one, this allows for concentration of sunlight without the need for solar tracking. External Quantum Efficiency (EQE) and current–voltage (I –V) measurements were performed on LSC devices with multicrystalline silicon (mc‐Si) or GaAs cells attached to the sides. The best result was obtained for an LSC with four GaAs cells. The power conversion efficiency of this device, as measured at European Solar Test Installation laboratories, was 7.1% (geometrical concentration of a factor 2.5). With one GaAs cell attached to one edge only, the power efficiency was still as high as 4.6% (geometrical concentration of a factor 10). To our knowledge these efficiencies are among the highest reported for the LSC. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photovoltaic characterization of n-CdTe/p-CdMnTe/GaAs diluted magnetic diode

A CdTe/CdMnTe heterojunction magnetic diode for photovoltaic applications was fabricated by using molecular beam epitaxy (MBE). The ideality factor and the potential barrier height of the diode were determined to be 1.25 and 0.836 eV, respectively. Photovoltaic parameters of the studied device were determined at various illumination intensities. The highest open circuit voltage of the CdTe/CdMnTe heterostructure was equal to 0.56 V at the illumination intensity of 130 mW/cm². The reverse current of the n-CdTe/p-CdMnTe/GaAs diode increases with the increasing illumination intensities. The obtained results suggest that n-CdTe/p-CdMnTe/GaAs diode can be used as a photodiode in photovoltaic and photodetector applications.     

Application of the chemical vapor-etching in polycrystalline silicon solar cells

This paper reports a study of the application of chemical vapor-etching (CVE) for the rear surface and in the emitter of polycrystalline silicon (pc-Si) solar cells. The CVE technique consists of exposing pc-Si wafers to a mixture of HF/HNO₃. This technique is used to groove the rear surface of the pc-Si wafers for acid vapors rich in HNO₃ (HNO₃/HF > 1/4), in order to realize rear-buried metallic contacts (RBMC) and the formation of a porous silicon (PS) layer on the frontal surface of the cell for volume ratio of HNO₃/HF = 1/7. A significant increase of the spectral response in the long wavelength range was observed when a RBMC is formed. This increase was attributed to the reduction of the effective thickness of the base of the cells and grain boundary Al gettering. The achievement of a PS layer on the emitter of the pc-Si cells passivates the surface and reduces the reflectivity. The dark I-V characteristics of pc-Si cells with emitter-based PS show an important reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows an enhancement of both short circuit current density and fill factor. The internal quantum efficiency is improved, particularly in the short wavelengths region.     

Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction

In this article, we report the growth of zinc‐tin nitride (ZnSnN₂) thin films as a potential absorber for photovoltaic applications by fabricating a heterojunction of n‐ZnSnN₂/p‐SnO. The performance of the heterojunction has been monitored through selective deposition of top electrode with different materials (Ni/Au or Al). The electron‐transfer process from the ZnSnN₂ layer to the cathode is facilitated by selecting metal electrode with relatively low work function, which also boosts up the electron injection or/and extraction. The diode exhibits a good J–V response in the dark with a rectification ratio of 3 × 10³ at 1.0 V and an ideality factor of 4.2 in particular with Al as the top electrode. Under illumination, the heterostructure solar cell demonstrates a power conversion efficiency of ≈0.37% with an open circuit voltage of 0.25 V and a short circuit current density of 4.16 mA cm^?2. The prime strategies, on how to improve solar cell efficiency concerning band offsets and band alignment engineering are also discussed.     

Photovoltaic characteristics of Au/PVA (Bi-doped)/n-Si Schottky barrier diodes (SBDs) at various temperatures

The charge conduction properties of the Au/PVA (Bi-doped)/n-Si Schottky barrier diodes (SBDs) were investigated using current–voltage–temperature (I–V–T) measurements in dark and under various illumination levels. For this purpose, the main diode parameters such as reverse-saturation current (I_o), zero-bias barrier height (Φ_Bo), ideality factor (n), series resistance (R_s) and shunt resistance (R_sh) of diode were obtained as function of temperature and illumination level. Experimental results show that all of these electrical parameters are strong functions of illumination and temperature. The change in all electrical parameters becomes more important at low temperatures and illumination levels. While the n value decreases with increasing temperature and illumination level, Φ_Bo value increases. The fill factor (FF = V_m·I_m/V_oc·I_sc) values were obtained as 0.34 at 80 K and 0.40 at 320 K under 50 W and these values are near to a photodiode. Therefore, the fabricated diode can be used as a photodiode in optoelectronic applications. The forward bias I–V characteristics of the diode have also been explained by the space charge limited current (SCLC) model.