Double-junction Solar Cells Research Articles

GaInNAs/Ge (1.10/0.67 eV) double-junction solar cell grown by metalorganic chemical vapor deposition for high efficiency four-junction solar cell application

GaInNAs materials with narrow bandgaps of 1.10 eV have been grown on a Ge substrate by metalorganic chemical vapor deposition to fabricate GaInNAs/Ge (1.10/0.67 eV) double-junction solar cells. We have studied the photovoltaic characteristics and the external quantum efficiencies of the double-junction cells with various annealing conditions and different GaInNAs base layer thicknesses. The best external quantum efficiency is obtained from the double-junction cell with a 1170 nm thick GaInNAs base layer annealed at 675 °C for 30 min. Under AM1.5G illumination, the best double-junction cell has a short circuit current density (JSC) as 23.63 mA cm−2, which is dominated by the JSC of the GaInNAs subcell.

Numerical Simulation of CGS/CIGS Single and Tandem Thin-film Solar Cells using the Silvaco-Atlas Software

In this work, numerical simulations of single and double junction solar cells based copper gallium diselenide (CGS) and copper indium gallium diselenide (CIGS) electrical characteristics are performed using a physically based two-dimensional device simulator Silvaco-Atlas. The J-V characteristics and the external quantum efficiency EQE are simulated under AM1.5 illumination. In this study, where the single CGS solar cell used as the top cell and the single CIGS solar cell used as the bottom cell in the tandem configuration show conversion efficiencies of 18.92% and 20.32%, respectively. Our simulation results of the CIGS solar cell are in good agreement with the experimental high record efficiency. The simulated photovoltaic parameters of the CGS/CIGS tandem cell are: the short-circuit current density Jsc of 16.35mA/cm2, the open circuit voltage Voc of 1.8V and the fill factor FF of 85.09% and the conversion efficiency η of 25.11%. In the CGS/CIGS tandem solar cell, an improvement in the conversion efficiency to 25.11% was found. The overall investigation on copper indium gallium diselenide solar cells gives potential suggestions that may lead to the fabrication of the high efficiency CGS/CIGS single and tandem solar cells.

Fabrication of double- and triple-junction solar cells with hydrogenated amorphous silicon oxide (a-SiOx:H) top cell

In this study we investigate the potential of intrinsic a-SiOx:H, with a band gap of about 2.07eV, as absorber material in thin-film silicon solar cells. We found that single junction a-SiOx:H cells with an i-layer thickness of 100–200nm can have a high Voc and FF of up to 1.04V and 0.74, respectively. However, when the i-layer thickness is increased further to obtain higher Jsc, the Voc×FF product decreases significantly and limits the conversion efficiency to around 7.2%. Although the efficiency of a single-junction device is limited, the high Voc×FF product makes this device interesting for application as the first sub-cell in multi-junction devices. To investigate this, first double-junction (2J) solar cells were fabricated with the second sub-cell of either a-Si:H (Eg≈1.7eV) or nc-Si:H (Eg≈1.1eV), reaching efficiencies of 10.25% and 10.92%, respectively. Although these are decent efficiencies, both 2J cells are limited by the a-SiOx:H first sub-cell and so far have no advantage over the conventional a-Si:H/nc-Si:H cell. Secondly, we fabricate two types of triple-junction (3J) solar cells, which consist of the above mentioned 2J solar cells with the additional nc-Si:H third sub-cell at the bottom. In both 3J solar cells current mismatching can be reduced with a relatively thin a-SiOx:H first sub-cell (≈100nm) such that its high Voc×FF product can be fully used. An initial efficiency as high as 12.58% was obtained (Voc: 2.37V, Jsc: 7.27mA/cm2 and FF: 0.73) for the a-SiOx:H/a-Si:H/nc-Si:H 3J solar cell. This efficiency is competitive with the efficiency of other types of 3J solar cells and demonstrates that a-SiOx:H opens up new routes towards high efficiency 3J thin-film silicon solar cells.

Numerical investigation of a double-junction a:SiGe thin-film solar cell including the multi-trench region

We present a new approach based on the multi-trench technique to improve the electrical performances, which are the fill factor and the electrical efficiency. The key idea behind this approach is to introduce a new multi-trench region in the intrinsic layer, in order to modulate the total resistance of the solar cell. Based on 2-D numerical investigation and optimization of amorphous SiGe double-junction (a-Si:H/a-SiGe:H) thin film solar cells, in the present paper numerical models of electrical and optical parameters are developed to explain the impact of the multi-trench technique on the improvement of the double-junction solar cell electrical behavior for high performance photovoltaic applications. In this context, electrical characteristics of the proposed design are analyzed and compared with conventional amorphous silicon double-junction thin-film solar cells.

Three-terminal heterojunction bipolar transistor solar cell for high-efficiency photovoltaic conversion.

Here we propose, for the first time, a solar cell characterized by a semiconductor transistor structure (n/p/n or p/n/p) where the base–emitter junction is made of a high-bandgap semiconductor and the collector is made of a low-bandgap semiconductor. We calculate its detailed-balance efficiency limit and prove that it is the same one than that of a double-junction solar cell. The practical importance of this result relies on the simplicity of the structure that reduces the number of layers that are required to match the limiting efficiency of dual-junction solar cells without using tunnel junctions. The device naturally emerges as a three-terminal solar cell and can also be used as building block of multijunction solar cells with an increased number of junctions.

(Invited) Dilute-Nitride-Antimonide Materials Grown by MOVPE for Multi-Junction Solar Cell Application

We have investigated the growth by metalorganic vapor phase epitaxy (MOVPE) of multinary (four- and five- element) dilute-nitride-antimonide materials on GaAs substrates. The lowest background carbon concentration (~ 5 × 1016 cm-3) is observed in dilute-nitride materials grown at high temperature which do not contain Sb (i.e. InGaAsN). An increased depletion region width significantly improves the solar cell performance over that found from dilute-nitride cells grown at lower growth temperatures (~525oC). The device performance of the single-junction solar cells with the low carbon background InGaAsN base region (InGaAsN/Ge double-junction solar cell) exhibit short-circuit current density, open-circuit voltage, fill factor, and efficiency values of 26.05 (28.46) mA/cm2, 0.67 (0.9) V, 75.85 (72.8) %, and 13.2 (18.54) %, with anti-reflecting coating (ARC), respectively.

Structural dependences of localization and recombination of photogenerated carriers in the top GaInP Subcells of GaInP/GaAs double-junction tandem solar cells.

In high-efficiency GaInP/GaAs double-junction tandem solar cells, GaInP layers play a central role in determining the performance of the solar cells. Therefore, gaining a deeper understanding of the optoelectronic processes in GaInP layers is crucial for improving the energy conversion efficiency of GaInP-based photovoltaic devices. In this work, we firmly show strong dependences of localization and recombination of photogenerated carriers in the top GaInP subcells in the GaInP/GaAs double-junction tandem solar cells on the substrate misorientation angle with excitation intensity- and temperature-dependent photoluminescence (PL). The entire solar cell structures including GaInP layers were grown with metalorganic chemical vapor deposition on GaAs substrates with misorientation angles of 2° (denoted as Sample 2°) and 7° (Sample 7°) off (100) toward (111)B. The PL spectral features of the two top GaInP subcells, as well as their excitation-power and temperature dependences exhibit remarkable variation on the misorientation angle. In Sample 2°, the dominant localization mechanism and luminescence channels are due to the energy potential minima caused by highly ordered atomic domains; In Sample 7°, the main localization and radiative recombination of photogenerated carriers occur in the atomically disordered regions. Our results reveal a more precise picture on the localization and recombination mechanisms of photogenerated carriers in the top GaInP subcells, which could be the crucial factors in controlling the optoelectronic efficiency of the GaInP-based multijunction photovoltaic devices.

Energy level diagram around Ge-rich grain boundaries in Cu2Sn1-xGexS3 (CTGS) thin-film solar cells

Using a 3-dimensional atom probe (or atom probe tomography) method, we measured the composition ratios around the grain boundaries of Cu2Sn1-xGexS3 (CTGS) films used for the bottom cells of thin-film double-junction solar cells. We found that the Ge composition ratio at the grain boundaries in the CTGS films was higher than that inside the grains when the films were fabricated by co-sputtering depositions of Cu and Sn followed by sulfurizations with S and GeS2 vapors. We also evaluated the bandgaps of Cu2Sn1-xGexS3 with changing the x value by measuring external quantum efficiency spectra of the solar cells. From these results, we have obtained the energy level diagram around the grain boundaries, and revealed that the conduction band minimum at the grain boundaries is 0.05eV higher than that inside the grains, which suppresses recombination of photogenerated carriers.

Super transverse diffusion of minority carriers in GaxIn1−xP/GaAs double-junction tandem solar cells

In this work, remarkable transverse diffusion of minority carriers in the GaxIn1−xP top subcell of a GaxIn1−xP/GaAs double-junction tandem solar cell is revealed by the electroluminescence (EL) image surveying. As the forward bias is increased, the overall EL intensity rapidly increases, but the topographical distribution of lateral intensity becomes more uneven. By analyzing the relation between the measured EL emission intensity and diffusion parameters of electrically injected minority carriers, the transverse diffusion length of the minority carriers is determined to be ∼93μm at the forward bias of 2.75V, which is 30 times larger than that of unbiased GaxIn1−xP single layer. Possible influence of such super diffusion of charge carriers on the conversion efficiency of tandem solar cells is discussed.

1.25-eV GaAsSbN/Ge Double-Junction Solar Cell Grown by Metalorganic Vapor Phase Epitaxy for High Efficiency Multijunction Solar Cell Application

Dilute-nitride-antimonide materials grown by metalorganic vapor phase epitaxy (MOVPE) with bandgap energies of 1.25 eV have been integrated into solar cell structures employing a Ge bottom cell on Ge substrate. Single homo- and heterojunction solar cells employing narrow bandgap GaAsSbN (E g ~ 1.25 eV) are grown normally lattice-matched on a GaAs substrate, using MOVPE. Homojunction solar cell structures were realized by employing GaAsSbN material with low carbon background concentration and Si doping to form a p/n junction. External quantum efficiency measurements in the range (870 nm-1000 nm) reveal that the efficiency of the homojunction solar cell is significantly improved over that of the heterojunction structure. The GaAsSbN homojunction cell was integrated with a Ge single-junction bottom cell on Ge substrate. Under AM1.5 direct illumination, the fabricated GaAsSbN (1.24 eV)/Ge double-junction solar cell with a 600-nm-thick GaAsSbN base layer exhibits Jsc, Voc, FF, and efficiency values of 11.59 mA/cm 2, 0.83 V, 72.58%, and 7% with anti-reflection coating (ARC), respectively.

Highly transparent and conductive indium tin oxide thin films for solar cells grown by reactive thermal evaporation at low temperature

Transparent conductive tin-doped indium oxide (In2O3:Sn, ITO) thin films with various Sn-doping concentrations have been prepared using the low cost reactive thermal evaporation (RTE) technique at a low growth temperature of ~160 °C. The structural characteristics, optical and electrical properties of the ITO thin films were investigated. These polycrystalline ITO films exhibited preferential orientation along (222) plane and possessed low resistivities ranging from 3.51 to 5.71 × 10−4 Ω cm. The decreased mobility was attributed to the scattering by ionized and neutral impurities at high doping concentrations. The optimized ITO thin film deposited with 6.0 wt% Sn-doping concentration exhibited a high average transparency of 87 % in the wavelength range of 380–900 nm and a low resistivity of 3.74 × 10−4 Ω cm with a high Hall mobility of 47 cm2 V−1s−1. A hydrogenated amorphous silicon and silicon–germanium (a-Si:H/a-SiGe:H) double-junction solar cell fabricated with the RTE-grown ITO electrodes presented a conversion efficiency of 10.51 %.

Optimized structure of AlGaAs/GaAs double junction solar cells

In this paper, the sub-layers of AlGaAs/GaAs double junction (DJ) solar cell have been redesigned in order to achieve an optimum cell structure. It has been deduced with cooperation of detailed balance limit theory and structural behaviour of AlGaAs, that the Al0.45Ga0.55As is the best choice for top cell’s material in AlGaAs/GaAs DJ solar cell. Also, there is a trade-off between peak tunnelling current and transparency in tunnel junction which makes Al0.07Ga0.93As as the optimum tunnel junction of AlGaAs/GaAs cell. Finally, a smoothed reflectance senary-layer structure based on modified-DBR has been proposed to be used as anti-reflection coating of proposed structure. Also, the thickness and doping concentration level of different layers have been optimized.

InGaAsNSb/Ge double-junction solar cells grown by metalorganic chemical vapor deposition

InGaAsNSb/Ge (1.14/0.67eV) double-junction solar cells have been grown by metalorganic chemical vapor deposition (MOCVD). Ge p–n junction structures are realized by phosphorous diffusion on p-type Ge substrates. GaAs/InGaAsNSb heterojunction structures are grown on the Ge solar cells. The single-junction InGaAsNSb and Ge subcells are electrically connected by a GaAs tunnel junction. The fabricated solar cell exhibits an efficiency of 9.05% under AM 1.5G illumination with an open circuit voltage of 0.92V. The InGaAsNSb subcell possessed a low external quantum efficiency over the sub-GaAs bandgap spectral range, reflecting the short minority carrier diffusion length of the dilute-nitride-antimonide material.

Theoretical Studies of InGaN/GaN Multiple Junction Solar Cell with Enhanced Tunneling Junction Diode

Recent developments in the growth of InGaN layers made it possible to grow a heavily doped p-type layer with Indium concentration up to 40%. In this work, a tunnel junction based on these developments has been designed with the use of Silvaco TCAD. This diode introduces a low resistive path to the current carriers, effectively adds voltages and encounters the parasitic effects of the stacked sub-cells. A double-junction solar cell is designed based on our interfacing tunnel diode and simulated results are presented in this paper. Remarkable results are achieved comparing to the existing InGaN based multijunction solar cells. A highVocof ~3.1V and conversion efficiency greater than 17.5% has been achieved under AM 1.5. This paper also highlights and discusses the challenges in fabrication of such a highly efficient solar cell.

DESIGN AND SIMULATION OF INDIUM GALLIUM NITRIDE MULTIJUNCTION TANDEM SOLAR CELLS

As our global energy expenditure increases exponent ially, it is apparent that renewable energy solutio n must be utilized. Solar PV technology is the best way to utilize the unlimited solar energy. The InGaN is a recently developed no vel solar cell material for its promising tunable band gap of 0.7 eV to 3.4 eV for the realization of high efficiency tandem solar cel ls in space and terrestrial applications. In this work, various numerical simul ations were performed using Analysis of Microelectr onic and Photonic Structure (AMPS) simulator to explore the possibility of high er efficiency of InGaN based solar cells. At these aim three different types of InGaN based solar cells (single junction, double ju nction and triple junction) were designed and optim ized. Numerical simulations were done with different band gap of InGaN material and found that maximum efficiency occurs around 1. 2 eV to 1.5 eV, it has been optimized at 1.34 eV for (single junction solar cel l) with maximum conversion efficiency of 25.02%. Th e single junction solar cell were simulated and optimized for optimum thickness of p-layer and n-layer. Doping concentration and ba ck contact material of the designed cells were investigated and found that 1◊1 0 16 cm -3 of doping concentration for both p and n type material and Nical as back contact with ΦbL of 1.3 eV are best fitted for higher conversion ef ficiency. From tunable band gap of In xGa 1-xN material, selection has been done at 1.61 eV, 1.44 eV and 1.21 eV for the t op, middle and bottom cells respectively for the ta ndem triple junction and double junction (1.61 eV, 1.21 eV) solar cells. The best c onversion efficiency of the single junction, double junction and triple junction solar cells are 25.019%, 35.45% and 42.34% respectively. Effect of tunnel junction for the tandem cells also investigated and found that required thickness for tunnel junction is around 25 nm with doping concentration, N A and N D of 1◊10 19 and 1◊10 16 respectively were found in this analysis. Finally, the temperature co efficient (TC) of the above proposed cells were sim ulated to investigate the thermal stability of the proposed cells. It has been found that the TC of InGaN cells is about -0.04%/°C, whic h indicate the higher stability of the proposed cells.

Impact of sub-cell internal luminescence yields on energy conversion efficiencies of tandem solar cells: A design principle

To develop a realistic design principle, we calculated the maximum conversion efficiency ηsc and optimized sub-cell band-gap energies Eg in double-junction tandem solar cells via a detailed-balance theory, paying particular attention to their dependence on internal luminescence quantum yields yint of the top and bottom sub-cell materials. A strong drop in the maximum ηsc occurs when yint slightly drops from 1 to 0.9, where the drop in yint of the bottom cell causes a stronger effect than that of the top cell. For low values of yint, the maximum ηsc has a simple logarithmic dependence on the geometric mean of the two sub-cells'yint.

Benzothiadiazole-based polymer for single and double junction solar cells with high open circuit voltage.

Single and double junction solar cells with high open circuit voltage were fabricated using poly{thiophene-2,5-diyl-alt-[5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole]-4,7-diyl} (PBT-T1) blended with fullerene derivatives in different weight ratios. The role of fullerene loading on structural and morphological changes was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD). The XRD and AFM measurements showed that a higher fullerene mixing ratio led to breaking of inter-chain packing and hence resulted in smaller disordered polymer domains. When the PBT-T1:PC60BM weight ratio was 1 : 1, the polymer retained its structural order; however, large aggregated domains formed, leading to poor device performance due to low fill factor and short circuit current density. When the ratio was increased to 1 : 2 and then 1 : 3, smaller amorphous domains were observed, which improved photovoltaic performance. The 1 : 2 blending ratio was optimal due to adequate charge transport pathways giving rise to moderate short circuit current density and fill factor. Adding 1,8-diiodooctane (DIO) additive into the 1 : 2 blend films further improved both the short circuit current density and fill factor, leading to an increased efficiency to 4.5% with PC60BM and 5.65% with PC70BM. These single junction solar cells exhibited a high open circuit voltage at ∼ 0.9 V. Photo-charge extraction by linearly increasing voltage (Photo-CELIV) measurements showed the highest charge carrier mobility in the 1 : 2 film among the three ratios, which was further enhanced by introducing the DIO. The Photo-CELIV measurements with varying delay times showed significantly higher extracted charge carrier density for cells processed with DIO. Tandem devices using P3HT:IC60BA as bottom cell and PBT-T1:PC60BM as top cell exhibited a high open circuit voltage of 1.62 V with 5.2% power conversion efficiency.

Optimization of amorphous silicon double junction solar cells for an efficient photoelectrochemical water splitting device based on a bismuth vanadate photoanode

A photoelectrochemical water splitting device (PEC-WSD) was designed and fabricated based on cobalt-phosphate-catalysed and tungsten-gradient-doped bismuth vanadate (W:BiVO4) as the photoanode. A simple and cheap hydrogenated amorphous silicon (a-Si:H) double junction solar cell has been used to provide additional bias. The advantage of using thin film silicon (TF-Si) based solar cells is that this photovoltaic (PV) technology meets the crucial requirements for the PV component in PEC-WSDs based on W:BiVO4 photoanodes. TF-Si PV devices are stable in aqueous solutions, are manufactured by simple and cheap fabrication processes and their spectral response, voltage and current density show an excellent match with the photoanode. This paper is mainly focused on the optimization of the TF-Si solar cell with respect to the remaining solar spectrum transmitted through the W:BiVO4 photoanode. The current matching between the top and bottom cells is studied and optimized by varying the thickness of the a-Si:H top cell. We support the experimental optimization of the current balance between the two sub-cells with simulations of the PV devices. In addition, the impact of the light induced degradation of the a-Si:H double junction, the so-called Staebler-Wronski Effect (SWE), on the performance of the PEC-WSD has been studied. The light soaking experiments on the a-Si:H/a-Si:H double junctions over 1000 hours show that the efficiency of a stand-alone a-Si:H/a-Si:H double junction cell is significantly reduced due to the SWE. Nevertheless, the SWE has a significantly smaller effect on the performance of the PEC-WSD.

Surexpression de l’IL-31 dans une forme sporadique de lichen amyloïde : un lien avec le prurit ?

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

Superstrate type flexible thin-film Si solar cells using flexible glass substrates

We have investigated the flexible thin-film silicon (Si) solar cells fabricated on the flexible glass substrates. The preliminary p–i–n type flexible hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) double-junction solar cell is compared with its counterpart fabricated on the textured F-doped tin oxide-coated glass substrate. Because of the inferior light transmitting and scattering of the SnO2:F front electrode coated by a spray pyrolysis deposition in the long wavelength region, the preliminary flexible solar cell shows a severely limited short-circuit current. Without any back reflector, the initial efficiency of 8.0% and stabilized efficiency of 7.1% are achieved. The initial efficiency of 9.3% was achieved by improving the current matching. The superstrate type thin-film Si photovoltaic technology using flexible transparent substrates has a high potential to minimize the cell-to-module loss through a monolithic series integration using a laser scribe technique.