Extending absorption of near-infrared wavelength range for high efficiency CIGS solar cell via adjusting energy band

The efficient photon harvesting in near infrared wavelength range is still a challenging problem for high performance Cu(In_1-x, Ga_x)Se₂ (CIGS) solar cell. Herein, adjusting the energy band distribution of CIGS solar cell could provide significant academic guidance for devices with superior output electric power. To understand the role of each functional layer, the optimal 3000 nm CIGS absorber layer with 1.3 eV bandgap and 30 nm CdS buffer layer were firstly obtained via simulating the uniform band-gap structures. By introducing CIGS absorber layer with a double grading Ga/(Ga+In) profile, the power conversion efficiency of the double gradient band gap cell is superior to that of uniform band-gap cell through extending absorption of near-infrared wavelength range. Upon optimization, the best power conversion efficiency of CIGS with a double gradient band gap solar cell is improved significantly to 24.90%, among the best values reported in literatures, which is an 8.17% relative increase compared with that of the uniform band-gap cell. Our findings provide a theoretical guide toward the design of high performance solar cells and enrich the understandings of the energy band engineering for developing of novel semiconductor devices.     

Interface photoluminescence of the SnO2:F/CdS:In/CdTe thin film solar cells prepared partially by the spray pyrolysis technique

CdS/CdTe solar cells were built by depositing a 200 nm layer of SnO₂:F on glass substrates by the spray pyrolysis (SP) technique, a 500 nm CdS:In layer by the same technique and a 1–1.5 μm CdTe layer by vacuum evaporation. The cells were CdCl₂ heat-treated in nitrogen atmosphere for 30 min at 350 °C. The photoluminescence (PL) spectra were measured at the CdS/CdTe interface for two cells with different values of the CdTe layer's thickness at the temperature T=60 K. A deconvolution peak fit was performed from which it is found that the peaks are characteristic of the solid solution CdS_xTe_1?x. The parabolic relation that relates the bandgap energy with the composition was used to estimate x, where x is [S]/([Te]+[S]) and [Te], [S] are the concentrations of Te and S atoms, respectively. The results show that the interface is smooth and the change of the bandgap occurs gradually. The solar cell of the thicker CdTe layer showed more interdiffusion at the CdS/CdTe interface and better photovoltaic characteristics.     

Highly effective III-V solar cells by controlling the surface roughnesses

An advanced approach to minimize the light loss was discussed for III-V solar cells, by controlling the roughnesses of the device surface. Adhesives with different viscosities were applied to bond the III-V solar cells with the supporting substrate before the epitaxial lift-off process. The surface roughness of the III-V solar cells with epoxy adhesive (R_rms = 15.4 nm) is one order of magnitude higher than that with acrylic adhesive (R_rms = 1.6 nm), due to the differences in viscosity, resulting from the spreadability while being hardened. This roughness has increased the reflectance in the wavelength between 650 and 900 nm, implying that this reflectance is influenced by the rear surface of the solar cell. The device performance of the double-junction solar cells (Ga_0.5In_0.5P- and GaAs- based) also reflects the effect of the reflectance. The solar cell with the epoxy adhesive exhibited ~2% increase of the conversion efficiency than that with the acrylic adhesive, mainly due to the increased current density. The integrated current density from the external quantum efficiency (EQE) also exhibited ~2% increase only in the bottom (GaAs-based) cell, corresponding to the higher reflectance for red and near-infrared wavelength ranges.     

Anodic TiO<Subscript>2</Subscript> nanotubes powder and its application in dye-sensitized solar cells

An increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. TiO₂ semiconductor material is expected to play an important role in helping solve the energy crisis through effective utilization of solar energy based on photovoltaic devices. Dye-sensitized solar cells (DSSCs) are potentially lower cost alternative to inorganic silicon-based photovoltaic cells. In this study, we report on the fabrication of DSSCs from anodic TiO₂ nanotubes (NT) powder, produced by rapid breakdown potentiostatic anodization of Ti foil in 0.1 M HClO₄ electrolyte, as photoanode. TiO₂ NT powders with a typical NT outer diameter of approximately 40 nm, wall thickness of approximately 8–15 nm, and length of about 20–25 μm, have been synthesized. The counter electrode was made by electrodeposition of Pt from an aqueous solution of 5 mM H₂PtCl₆ onto fluorine-doped tin oxide (FTO) glass substrate. The above front-side illuminated DSSCs were compared with back-side illuminated DSSCs fabricated from anodic TiO₂ NTs that were grown on the top of Ti foil as photoanode. The highest cell efficiency was 3.54% under 100 mW/cm² light intensity (1 sun AM 1.5G light, Jsc = 14.3 mA/cm², Voc = 0.544 V, FF = 0.455). To the best of our knowledge, this is the first report on the fabrication of DSSC from anodic TiO₂ NTs powder. The TiO₂/FTO photoanodes were characterized by FE-SEM, XRD, and UV–Visible spectroscopy. The catalytic properties of Pt/FTO counter electrodes have been examined by cyclic voltammetry.     

Red and near infrared down-conversion in Er3+/Yb3+ co-doped YF3 performed by quantum cutting

Er³⁺/Yb³⁺ co-doped YF₃ powder is prepared by combining a nitrate decomposition method with a NH₄HF₂ fluorization process, from which efficient energy transfer induced down-conversion is achieved. An absorbed 365 nm near ultraviolet photon is split into two photons of 650 nm red and 1000 nm near infrared radiations, both falling in the responding region of Si-based solar cells. The quantum cutting mechanism has been proposed and discussed and the energy transfer efficiency for the quantum cutting is evaluated by developing an emission intensity ratio contrast method. The investigation might offer a new possible approach to achieve Si-based solar cells of high efficiency by down-converting the near ultraviolet part of the solar spectrum.     

Characterization techniques of sandwich-type TiO2/QD composites for low-cost quantum dots' solar cell

There is a high impact of the solar cells on energy manufacturing. For several years the energy efficiency was limited due to base-materials' structural and technological limits. High increase of energy harvesting of solar cells has been observed since the first solar cell based on dye-sensitized colloidal TiO₂ films occurred. One of the most promising solutions are used quantum dots (QD) for light energy conversion. In this paper, we described the use of selected characterization techniques for sandwich-type TiO₂/QD composites for a low-cost quantum dots' solar cell in the point of view of mass manufacturer of solar cells and research and development laboratory. Moreover, the increasing role of Raman spectroscopy and mapping for the TiO₂/QD was presented and compared with other necessity techniques for solar cell investigations such as ellipsometry, atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS).     

Bicontinuous network of electron donor-acceptor composites achieved by additive-free sequential deposition for efficient polymer solar cells

We report that sequential deposition of a highly crystalline polymer donor and a soluble fullerene acceptor leads to a well-defined interpenetrating network and enhanced power conversion efficiencies in bilayer polymer solar cells. Even without the use of solvent additives, layered thin films of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2’; 5′,2’’; 5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM), as electron donor and acceptor materials, respectively, showed bicontinuous networks similar to those of a PffBT4T-2OD:PC₇₁BM bulk-heterojunction (BHJ) thin film processed with 1,8-diiodooctane (DIO) as a solvent additive. Transmission electron microscopy results confirmed the BHJ-like morphology of the bilayered PffBT4T-2OD/PC₇₁BM thin films. Bilayer solar cells fabricated without the DIO additive produced a power conversion efficiency of η ≈ 7.65%, which is even higher than that of a BHJ solar cell fabricated with the DIO additive (η ≈ 7.04%). These results demonstrate that a highly crystalline polymer donor and an electron-accepting small molecule can be a good combination for efficient bilayer polymer solar cells.     

Influence of solvent on solution processed Cu2ZnSnS4 nanocrystals and annealing induced changes in the optical,structural properties of CZTS film

The well-known quaternary Cu₂ZnSnS₄ (CZTS) chalcogenide thin films are playing an important role in modern technology. The CZTS nanocrystal were successfully prepared by solution method using water, ethylene glycol and ethylenediamine as different solvent. The pure phase material was used for thin film coating by thermal evaporation method. The prepared CZTS thin films were characterized by XRD, Raman spectroscopy, FESEM, XPS and FT-IR spectroscopy. The XRD and Raman spectroscopy analysis revealed the formation of polycrystalline CZTS thin film with tetragonal crystal structure after annealing at 450 ^°C. The oxidation state of the annealed film was studied by XPS. A direct band gap about 1.36 eV was estimated for the film from FT-IR studies, which is nearly close to the optimum value of band gap energy of CZTS materials for best solar cell efficiency. The CZTS annealed thin films are more suitable for using as a p-type absorber layer in a low-cost solar cell.     

Efficiency enhancement of flexible dye-sensitized solar cell with sol–gel formed Nb2O5 blocking layer

We investigated the effect of a Nb₂O₅ blocking layer formed through the sol–gel method introduced to a titanium metal foil electrode in a flexible dye sensitized solar cell. The blocking layer formed directly on the working electrode physically separates the working electrode from the electrolyte, and prevents back transfer of electrons from the electrode to the electrolyte. The gel processing conditions (sol reaction time) and heat treatment temperature used in formation of the Nb₂O₅ blocking layer have been shown to affect the performance of the dye sensitized solar cell and optimal values of these parameters have been determined. A sol reaction time of 45 min and heat treatment temperature of 550 °C has been observed to result in optimal cell performance (η = 6.185%, J_sc = 13.233 mA/cm², V_oc = 0.672 V, ff = 0.694). Introduction of an Nb₂O₅ blocking layer enhances solar cell efficiency by 39.7%, which is much greater than the increase of 24.6% observed in a similar cell containing a TiO₂ blocking layer under standard illumination conditions. The results obtained via Nb₂O₅ have been observed to be superior to those obtained via a TiO₂ blocking layer.     

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.     

High-efficient Schottky-junction silicon solar cell using silver nanowires covering nitrogen-doped amorphous carbon

Synthesized graphene (Gr) on metal substrates that requires additional surface-to-surface transfer procedure to form Gr-on-silicon (Gr-Si) Schottky-junction configuration, which in turn results in the photovoltaic degradation caused by both mechanical damages and chemical contaminations during several wet chemical steps. This current issue has motivated us to develop alternative Schottky-junction configuration using silver nanowires (AgNWs) covering nitrogen (N)-doped amorphous carbon (a-C) films annealed in the temperature range 750–900 °C. Compared to the Schottky-junction Si solar cell based on 900 °C annealed N-doped a-C films (C_N-900-Si) with only Ag grid, all of AgNWs-C_N-900-Si solar cells exhibit the significant enhancement of photovoltaic characteristics. Consequently, the remarkable power conversion efficiency (PCE) of 6.17% is achieved on 0.2 wt% AgNWs-C_N-900-Si solar cell, which is far superior to that of the C_N-900-Si solar cell with only Ag grid (~0.13%). Furthermore, the 0.2 wt% AgNWs-C_N-900-SiNWs solar cell shows the highest short-circuit current density (J_SC) of 23.42 mA/cm² and PCE of 7.67%, which is a PCE enhancement of ~24% when compared to the 0.2 wt% AgNWs-C_N-900-Si solar cell. This study demonstrates that AgNWs network can accelerate the charge carrier extraction from Schottky-contact between C_N-900 and n-Si substrate, leading to greatly reduced series resistance that results in significantly enhanced photovoltaic characteristics.     

Improvement of solar cell efficiency by applying Si3N4 nanopattern on glass substrate

A cylindrical Si₃N₄ nanopattern whose heights was 200 nm was fabricated on a glass substrate, and an aluminum-doped zinc oxide (AZO) layer was grown on the nanopatterned glass substrate. The nanopattern was applied to an amorphous silicon solar cell in order to increase the light-scattering effect, thus enhancing the efficiency of the solar cell. The reflectance of the solar cell on the Si₃N₄ nanopattern decreased and its absorption increased. Compared to a flat substrate, the short-circuit current density (J_sc) and conversion efficiency of a solar cell on the Si₃N₄ nanopatterned substrate were improved by 17.9% and 24.2%, respectively, as determined from solar simulator measurements.     

MoO3/Ag/MoO3 top anode structure for semitransparent inverted organic solar cells

In this study, we fabricated semitransparent polymeric solar cells with an inverted structure, with the structure being indium tin oxide (ITO)/cesium carbonate (Cs₂CO₃)/poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61(PCBM)/transparent multilayer. The structure of the transparent multilayer (DMD multilayer), which acted as the anode, was MoO₃ (1–40 nm)/Ag (10 nm)/MoO₃ (0–80 nm). The inner MoO₃ layer showed a great performance changes depending on the variation of thickness, while the outer MoO₃ layer showed relatively slight changes. The best performance was observed with the of anode DMD multilayer thickness of 6/10/40 nm and with the illumination from the ITO side in organic solar cell devices. High performance result was observed in high reflectance and low transmittance of the DMD layer.     

Electroreflectance study of CuIn1−xGaxSe2 thin film solar cells

We have investigated the optical properties of CuIn_1−xGa_xSe₂ (CIGS) thin film solar cells using their electroreflectance (ER) at room temperature. The ER spectra exhibited one broad and two narrow signal regions. Using the photoluminescence (PL) and photocurrent (PC) spectra, the peaks in the low-energy region (1.02–1.35 eV) can be assigned to the CIGS thin film. The PC results implied that the peaks in the high-energy region (2.10–2.52 eV) can be assigned to the CdS band-gap energy. Using the applied bias voltage, the broad signals in the 1.35–2.09 eV region can be assigned to the Franz–Keldysh oscillation (FKO) due to the internal electric field. The ER spectra exhibited a distorted CdS signal for the CIGS thin film solar cell with low shunt resistance and efficiency.     

An approach to MgCl2 activation on CdSe thin films for solar cells

The energy demand of the world is rapidly increasing and to cater this, there is a need to explore new renewable energy resources. CdSe thin film solar cells may be promising alternative to the CdTe solar cells which are extensively studied and used in solar cell technology. The pre/post deposition chlorine based treatments (viz. CdCl₂, MgCl₂, NH₄Cl) are the important steps to enhance the performance of Cd-based thin film solar cells. Therefore, a study on MgCl₂ activation treatment to CdSe thin films for solar cell applications as absorber layer is undertaken. Different physical properties of e-beam evaporated CdSe films (thickness 550 nm) grown on glass and ITO substrates are investigated and found to be strongly dependent on the post-chlorine treatment. The films have cubic zinc-blende structure and phase transformation from cubic (111) to hexagonal (002) is achieved with the MgCl₂ treatment while the optical band gap is reduced. I-V characteristics reveal the linear relation between voltage and current as well as the surface roughness is varied with treatment and improved homogeneity. The deposition of CdSe thin films is confirmed by elemental analysis where Cd and Se were found to be rich with treatment. The investigated results suggest that CdSe thin films treated by MgCl₂ and annealed at 320 °C may be a viable alternative absorber layer to the Cd-based solar cells.     

2,7-Carbazole and thieno[3,4-c]pyrrole-4,6-dione based copolymers with deep highest occupied molecular orbital for photovoltaic cells

Three kinds of donor–acceptor (D–A) type photovoltaic polymers were synthesized based on 2,7-carbazole and thieno[3,4-c]pyrrole-4,6-dione (TPD). The conjugation of weakly electron (e)-donating 2,7-carbazole and strongly e-accepting TPD moieties yielded a deep highest occupied molecular orbital (HOMO) and its energy level was fine-controlled to be −5.72, −5.67 and −5.57 eV through the incorporation of thiophene (T), thieno[3,2-b]thiophene (TT) and bithiophene (BT) as a π-bridge. Polymer:[6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) based bulk heterojunction solar cells exhibited a high open-circuit voltage (V_OC) in the range, 0.86–0.94 V, suggesting good agreement with the measured HOMO levels. Despite the high V_OC, the thiophene (or thienothiophene)-containing PCTTPD (or PCTTTPD) showed poor power conversion efficiency (PCE, 1.14 and 1.25%) because of the very low short-circuit current density (J_SC). The voltage-dependent photocurrent and photoluminescence quenching measurements suggested that hole transfer from PC₇₁BM to polymer depends strongly on the HOMO level of the polymer. The PCTTPD and PCTTTPD devices suffered from electron–hole recombination at the polymer/PC₇₁BM interfaces because of the insufficient energy offset between the HOMOs of the polymer and PC₇₁BM. The PCBTTPD:PC₇₁BM device showed the best PCE of 3.42% with a V_OC and J_SC of 0.86 V and 7.79 mA cm⁻², respectively. These results show that photovoltaic polymers should be designed carefully to have a deep HOMO level for a high V_OC and sufficient energy offset for ensuring efficient hole transfer from PC₇₁BM to the polymer.     

Comparative study on Cu2ZnSn(S,Se)4 based thin film solar cell performances by adding various back surface field (BSF) layers

In this research work, SCAPS-1D (Solar Cell Capacitance Simulator in one Dimension) is used to simulate the CZTSSe (Cu2ZnSn(S,Se)4) solar cell with Al/ZnO:Al/ZnO(i)/CdS/CZTSSe/Mo structure. The simulation results have been compared and validated with real experimental results. After that, an effective receipt is proposed with the aim of improving the efficiency of the CZTSSe solar cell, in which a BSF layer is inserted using various materials (SnS, CZTSSe and CZTSe). The obtained results show that the efficiencies of CZTSSe solar cells are increased from 12.3% to 15.7%, 15.3% and 15% by the insertion of SnS, CZTSSe and CZTSe materials as BSF layers, respectively. This enhancement corresponds with a BSF layer thickness of 30 nm and doping concentration of 1E18 cm⁻³. Next, an optimization of BSF layers thickness has been conducted. The optimum value of thickness is considered at 40 nm with an enhancement ratio in efficiency of 36.70%, 26.21% and 21.53% for SnS, CZTSSe and CZTSe, respectively. Better performances have been noted for SnS material. The optimized CZTSSe solar cell with SnS as a BSF layer achieves an efficiency of 16.95% with J_SC = 36.34 mA/cm², V_OC = 0.69 V, and FF = 67% under Standard Test Conditions (AM1.5 G and cell temperature of 25 °C).     

Effects of Ga contents on properties of CIGS thin films and solar cells fabricated by co-evaporation technique

This study examined the effects of Ga content in the CIGS absorber layer on the properties of the corresponding thin films and solar cells fabricated using a co-evaporation technique. The grain size of CIGS films decreased with increasing Ga content presumably because Ga diffusion during the 2nd stage of the co-evaporation process is more difficult than In diffusion. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. Band gap energy and the net carrier concentration of CIGS films increased with Ga/(In + Ga) ratios. Regarding the solar cell parameters, the short circuit current density (J_SC) decreased linearly with Ga/(In + Ga) ratios due to the lack of absorption in the long-wavelength portion of the spectrum, while the open circuit voltage (V_OC) increase with those. However, V_OC values at high Ga/(In + Ga) regions (>0.35) was far below than those extrapolated from the low Ga contents regions, finally resulting in an optimum Ga/(In + Ga) ratio of 0.28 where the solar cell showed the highest efficiency of 15.56% with V_OC, J_SC and FF of 0.625 V, 35.03 mA cm⁻² and 0.71, respectively.     

Determination of electron-diffusion length from photocurrent characteristics of the structure ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd

In this study the electron diffusion length L _n is determined from the relative spectral response of the photocurrent characteristics of the p/i/n sandwich structure ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd. The techniques used for the preparation of the a-Sic:H and a-Si:H amorphous films were glow-discharge and rf magnetron sputtering, respectively. The thickness of the p-type, intrinsic and n-type layer were 400 Å, 7000 Å and 600 Å, respectively. The response of the short-circuit current density J _sc was measured versus the photon energy hv at both constant light intensity and constant temperature. The electron diffusion length was found to be 0.31 m by means of the method of Agarwala and Tewary. Although, in the case of single crystals many diffusion length measurements have been made, there are only few papers for amorphous silicon this films [1]. As it is well-known, the diffusion length of the charge carriers is the most important parameter from the point of view of solar cell applications [2]. In order to obtain a high efficiency in a solar cell all carriers created under illumination in the intrinsic layer should reach the electrodes [3]. In the case that the thickness of the intrinsic layer is much larger than the diffusion length, not all carriers can reach the electrodes and, accordingly, a low efficiency results [4]. On the other hand, carriers which reach the electrodes without thermalizing do not contribute to the photocurrent and finally the efficiency of the solar cell is negatively affected. In order to avoid such an effect to a large extent, the thickness of the amorphous layers in a p/i/n solar cell must be conveniently chosen compared to the diffusion length of the carriers.Here it is aimed to determine the electron diffusion length. In order to achieve this goal, the photocurrent characteristics of an ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd structure was measured versus the photon energy at constant light intensity and constant temperature. In order to determine the electron diffusion length, the method of Agarwala and Tewary [5] was utilized.     

Optical properties of xenon implanted CuInSe2 by photoacoustic spectroscopy

A theoretical relation is derived for the normalized photoacoustic amplitude signal of a gas-coupled cell for the case of double-layer solid samples with particular application given to ion implanted semiconductors. Numerical estimates for a solar cell of the type CdS/CuInSe₂ based on experimental measured data of these compounds are given to illustrate the photoacoustic effect originating from double-layer samples. In application to ion implanted semiconductors, we show that the absorption coefficient of the implanted layer can be very easily extracted by photoacoustic spectroscopy if the absorption coefficient of the untreated substrate is known. We also present the optical properties results obtained from the analysis of the effect of xenon implantation into CuInSe₂ single crystals with the energy of 40 keV and a dose of 5×10¹⁶ ions/cm².