Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p‐i‐n silicon solar cells

The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi‐quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p‐i‐n Si solar cell. The QDSL consists of a stack of four 4‐nm Si nano disks and 2‐nm SiC barrier layers embedded in a SiC matrix fabricated with a top‐down etching process. The Si nano disks were observed with bright field‐scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave‐function coupling to form a miniband in the QDSL was observed based on the solar‐cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density J_sc of 29.24 mA/cm², open circuit voltage V_oc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i‐QW solar cell with J_sc of 25.27 mA/cm², V_oc of 0.49 V, FF of 0.69, and η of 8.61% and an i‐Si solar cell with J_sc of 27.63 mA/cm², V_oc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo‐carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i‐QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.     

Graphene Intermediate Layer in Tandem Organic Photovoltaic Cells

One strategy to harvest wide spectral solar energy is to stack different bandgap materials together in a tandem solar cell. Here, it is demonstrated that CVD grown graphene film can be employed as intermediate layer (IML) in tandem solar cells. Using MoO₃‐modified graphene IML, a high open circuit voltage (V_oc) of 1 V and a high short‐circuit current density (J_sc) of 11.6 mA cm^‐2 could be obtained in series and parallel connection, respectively, in contrast to a V_oc of 0.58 V and J_sc of 7.6 mA cm^‐2 in single PV cell. The value of V_oc (J_sc) in the tandem cell is very close to the sum of V_oc (J_sc) attained from two single subcells in series (parallel), which confirms good ohmic contact at the photoactive layer/MoO₃‐modified graphene interface. Work function engineering of the graphene IML with metal oxide is essential to ensure good charge collection from both subcells.     

Simulation and fabrication of heterojunction silicon solar cells from numerical computer and hot‐wire CVD

In this paper, we will present a Pc1D numerical simulation for heterojunction (HJ) silicon solar cells, and discuss their possibilities and limitations. By means of modeling and numerical computer simulation, the influence of emitter‐layer/intrinsic‐layer/crystalline‐Si heterostructures with different thickness and crystallinity on the solar cell performance is investigated and compared with hot wire chemical vapor deposition (HWCVD) experimental results. A new technique for characterization of n‐type microcrystalline silicon (n‐µc‐Si)/intrinsic amorphous silicon (i‐a‐Si)/crystalline silicon (c‐Si) heterojunction solar cells from Pc1D is developed. Results of numerical modeling as well as experimental data obtained using HWCVD on µc‐Si (n)/a‐Si (i)/c‐Si (p) heterojunction are presented. This work improves the understanding of HJ solar cells to derive arguments for design optimization. Some simulated parameters of solar cells were obtained: the best results for J_sc = 39·4 mA/cm², V_oc = 0·64 V, FF = 83%, and η = 21% have been achieved. After optimizing the deposition parameters of the n‐layer and the H₂ pretreatment of solar cell, the single‐side HJ solar cells with J_sc = 34·6 mA/cm², V_oc = 0·615 V, FF = 71%, and an efficiency of 15·2% have been achieved. The double‐side HJ solar cell with J_sc = 34·8 mA/cm², V_oc = 0·645 V, FF = 73%, and an efficiency of 16·4% has been fabricated. Copyright © 2009 John Wiley & Sons, Ltd.     

Interface Engineering for Both Cathode and Anode Enables Low‐Cost Highly Efficient Solution‐Processed CdTe Nanocrystal Solar Cells

Low‐cost solution‐processed CdTe nanocrystal (NC) solar cells always suffer from a high interface energy barrier and unbalanced hole/electron transport as well as anisotropic atom diffusion on the CdTe surface due to the limited amount of hole/electron interface materials or the difficulty in interface processing. In this work, a novel strategy is first adopted with gradient electron transport layer (CdS/CdSe) modification in the cathode and a new crosslinkable hole transport polymer (P‐TPA) implantation in the anode. The carrier recombination at interfaces is greatly decreased and thus the carrier collection is increased. Moreover, the light harvesting is improved both in short and long wavelength regions, making J_sc and V_oc increase simultaneously. A champion solar cell shows a very high power conversion efficiency of 9.2% and an outstanding J_sc of 25.31 mA cm^?2, which are among the highest values for all solution‐processed CdTe NC solar cells with a superstrate structure, and the latter value is even higher than that of traditional thick CdTe thin‐film solar cells (2 µm) via the high temperature close space sublimation method. This work demonstrates that facile surface modifications in both the cathode and anode with stepped extraction and organic–inorganic hybridization are very promising in constructing next‐generation highly efficient NC photovoltaic devices.     

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites for highly efficient nanocrystalline silicon thin‐film solar cells

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites (p‐nc‐SiC:H) were prepared using a plasma‐enhanced chemical vapor deposition system with a mixture of CH₄, SiH₄, B₂H₆ and H₂ gases. The influence of hydrogen dilution on the material properties of the p‐nc‐SiC:H films was investigated, and their roles as window layers in hydrogenated nanocrystalline silicon (nc‐Si:H) solar cells were examined. By increasing the R_H (H₂/SiH₄) ratio from 90 to 220, the Si―C bond density in the p‐nc‐SiC:H films increased from 5.20 × 10¹⁹ to 7.07 × 10¹⁹/cm³, resulting in a significant increase of the bandgap from 2.09 to 2.23 eV in comparison with the bandgap of 1.95 eV for p‐nc‐Si:H films. For the films deposited at a high R_H ratio, the Si nanocrystallites with a size of 3–15 nm were formed in the amorphous SiC:H matrix. The Si nanocrystallites played an important role in the enhancement of vertical charge transport in the p‐nc‐SiC:H films, which was verified by conductive atomic force microscopy measurements. When the p‐nc‐SiC:H films deposited at R_H = 220 were applied in the nc‐Si:H solar cells, a high conversion efficiency of 8.26% (V_oc = 0.53 V, J_sc = 23.98 mA/cm² and FF = 0.65) was obtained compared to 6.36% (V_oc = 0.44 V, J_sc = 21.90 mA/cm² and FF = 0.66) of the solar cells with reference p‐nc‐Si:H films. Further enhancement in the cell performance was achieved using p‐nc‐SiC:H bilayers consisting of highly doped upper layers and low‐level doped bottom layers, which led to the increased conversion efficiency of 9.03%. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluation of Zn?Sn?O buffer layers for CuIn0.5Ga0.5Se2 solar cells

Thin Zn Sn O films are evaluated as new buffer layer material for Cu(In,Ga)Se₂‐based solar cell devices. A maximum conversion efficiency of 13.8% (V_oc = 691 mV, J_sc(QE) = 27.9 mA/cm², and FF = 71.6%) is reached for a solar cell using the Zn Sn O buffer layer which is comparable to the efficiency of 13.5% (V_oc = 706 mV, J_sc(QE) = 26.3 mA/cm², and FF = 72.9%) for a cell using the standard reference CdS buffer layer. The open circuit voltage (V_oc) and the fill factor (FF) are found to increase with increasing tin content until an optimum in both parameters is reached for Sn/(Zn + Sn) values around 0.3–0.4. Copyright © 2010 John Wiley & Sons, Ltd.     

High efficiency large area n‐type front junction silicon solar cells with boron emitter formed by screen printing technology

In this paper, we report on commercially viable screen printing (SP) technology to form boron emitters. A screen‐printed boron emitter and ion‐implanted phosphorus back surface field were formed simultaneously by a co‐annealing process. Front and back surfaces were passivated by chemically grown oxide capped with plasma‐enhanced chemical vapor deposition silicon nitride stack. Front and back contacts were formed by traditional SP and firing processes with silver/aluminum grid on front and local silver back contacts on the rear. This resulted in 19.6% efficient large area (239 cm²) n‐type solar cells with an open‐circuit voltage V_oc of 645 mV, short‐circuit current density J_sc of 38.6 mA/cm², and fill factor of 78.6%. This demonstrates the potential of this novel technology for production of low‐cost high‐efficiency n‐type silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Novel Conjugated Organic Dyes for Efficient Dye‐Sensitized Solar Cells

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO₂ solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm^–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (J_sc) = 12.9 mA cm^–2, open‐circuit voltage (V_oc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO₂ occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO₂ surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.     

Hybrid Inorganic–Organic Heterojunction Solar Cell

In this study, solar cells were fabricated by spin-coating polyaniline (PANI) base (EB) over an n-type Si substrate. The final heterojunction’s device structure was Al/n-type Si/EB/Au. The electrical properties of the resultant device were investigated by measuring the current density–voltage (J–V), capacitance–voltage (C–V), and impedance characteristics in the dark and under illumination. N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and tetrahydrofuran (THF) were used as solvents for EB. The effects of these solvents on the photovoltaic cell parameters were investigated, and the open-circuit voltage (V _oc), short-circuit current density (J _sc), fill factor (FF), and energy conversion efficiency (η) were determined. It was found that heterojunctions fabricated using EB dissolved in NMP, DMF, and THF produced J _sc of 10 mA/cm², 5.123 mA/cm², and 2.78 mA/cm², respectively. Rollover and crossover phenomena in the J–V curves under illumination were explained based on the back-contact barrier and surface recombination of electrons at the back contact. The linearity of Mott–Schottky plots indicated the formation of a heterojunction between EB and n-type Si, and the slope of 1/C ² versus voltage changed under illumination. The high values of shunt resistance were decreased under illumination, indicating that the efficiency of this type of heterojunction solar cell was limited by shunt resistance and the narrow absorption range of the solar spectrum by EB.     

Achievement of 17.9% efficiency in 30 × 30 cm2 Cu(In,Ga)(Se,S)2 solar cell sub‐module by sulfurization after selenization with Cd‐free buffer

We have achieved 17.9% efficiency in a 30 × 30 cm² Cu(In,Ga)(Se,S)₂ solar cell sub‐module prepared by selenization and sulfurization processes with a Cd‐free buffer. The development of an absorber layer, transparent conducting oxide window layer, and module design was the key focus. This permitted 1.8% higher efficiency than our last experimental result. The quantity and the injection time of the sodium were controlled, resulting in higher open circuit voltage (V_oc) and short circuit current (J_sc). In order to increase J_sc, we changed the thickness of the window layer. Boron‐doped zinc oxide was optimized for higher transmittance without reducing the fill factor. The uniformity of each layer was improved, and patterns were optimized for each module. Therefore, V_oc, J_sc, and FF could be theoretically improved on the reported results of, respectively, 20 mV, 2 mA/cm², and 1.4%. The module's efficiency was measured at the Korea Test Laboratory to compare with the data obtained in‐house. Various analyses were performed, including secondary ion mass spectroscopy, photoluminescence, quantum efficiency, solar simulator, and UV–vis spectrometry, to measure the cell's depth profile, carrier lifetime, external quantum efficiency, module efficiency, and transmittance, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

10.8% Efficiency Polymer Solar Cells Based on PTB7‐Th and PC71BM via Binary Solvent Additives Treatment

In this work, polymer solar cells are fabricated based on the blend of PTB7‐Th: PC₇₁BM by using a mixed solvent additive of 1,8‐diiodooctane and N‐methyl pyrrolidone to optimize the morphology of the blend. A high power conversion efficiency (PCE) of 10.8% has been achieved with a simple conventional device. In order to deeply investigate the influence of the mixed solvent additives on the morphology and device performance, the variations of the molecular packing and bulk morphology of the blend film cast from ortho‐dichlorobenzene with single or binary solvent additives are measured. Although all the blend films exhibit similar domain size and nanoscale phase separation, the blend film processed with mixed solvent additive shows the highest domain purity, resulting in the least bimolecular recombination, relatively high J_sc and FF, and hence enhanced PCE. Therefore, the best photovoltaic performance with the V_oc of 0.82 V, J_sc of 19.1 mA cm^?2, FF of 69.1%, and PCE of 10.8% are obtained for the device based on the blend with binary solvent additive treatment.     

Development of a‐SiOx:H solar cells with very high Voc × FF product

In this study, deposition conditions for making a‐SiO_x:H are investigated systematically in order to obtain a high band gap material. We found that at given optical band gap, a‐SiO_x:H with favorable opto‐electronic properties can be obtained when deposited using low CO₂ flow rates and deposition pressures. We also found that a low radio frequency power density is required in order to limit the effect of ion bombardment on the material properties of i‐a‐SiO_x:H and thereby the solar cell performance. In addition, by decreasing the heater temperature from 300 to 200°C when making the i‐a‐SiO_x:H, the V_oc can be increased. We employed optimized p‐doped and n‐doped a‐SiO_x:H films into the p‐i‐n solar cells, and as a consequence, a high V_oc of over 1 V and high fill factor (FF) are obtained. When depositing on texture‐etched ZnO:Al substrates, a high efficiency a‐SiO_x:H single junction solar cell having a high V_oc × FF product of 0.761 (V_oc: 1.042 V, J_sc: 10.3 mA/cm², FF: 0.73, efficiency: 7.83%) was obtained. The a‐SiO_x:H solar cell shows comparable light degradation characteristics to standard a‐Si:H solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Narrow‐bandgap CuIn3Te5 thin‐film solar cells

We propose CuIn₃Te₅ as a ternary semiconductor material for narrow‐bandgap thin‐film solar cells. Well‐developed CuIn₃Te₅ grains were obtained at a substrate temperature of 250 °C by single‐step co‐evaporation. The best solar cell that was fabricated using 4·0‐µm‐thick CuIn₃Te₅ layers grown at 250 °C yielded a total area efficiency of 6·92% (V_oc = 407 mV, J_sc = 33·1 mA/cm², and FF = 0·514). To clarify the loss in the device performance, the cell was compared with a standard CuInSe₂ reference cell. A band diagram of the CdS/CuIn₃Te₅ solar cell was also presented. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhanced photovoltaic performance of cross‐linked ruthenium dye with functional cross‐linkers for dye‐sensitized solar cell

Photovoltaic performance of cross‐linkable Ru(2,2′‐bipyridine‐4,4′‐bicarboxylic acid)(4,4′‐bis((4‐vinyl benzyloxy)methyl)‐2,2′‐bipyridine)(NCS)₂ (denoted as RuS dye) adsorbing on TiO₂ mesoporous film was enhanced by polymerizing with either ionic liquid monomer, 1‐(2‐acryloyloxy‐ethyl)‐3‐methyl‐imidazol‐1‐ium iodide (AMImI), to form RuS‐cross‐AMImI or di‐functional acrylic monomer with ether linkage, triethyleneglycodimethacrylate (TGDMA), to form RuS‐cross‐TGDMA. Their cross‐linking properties were investigated by UV–vis spectroscopy by rinsing with 0.1 N NaOH aqueous solution. The power conversion efficiencies (PCEs) of dye‐sensitized solar cells (DSSCs) with RuS‐cross‐AMImI and RuS‐cross‐TGDMA both reached over 8% under standard global air mass 1.5 full sunlight. The increased PCE for DSSCs with RuS‐cross‐AMImI comparing with cross‐linked RuS was attributed to the I⁻ counterion of AMImI increasing the charge regeneration rate of RuS dye, whereas that with RuS‐cross‐TGDMA was attributed to the Li⁺ coordination property of TGDMA. The photovoltaic performance of RuS‐cross‐TGDMA was also slightly better than that of RuS‐cross‐AMImI because of higher open‐circuit photovoltage (V_oc) and short‐circuit photocurrent (J_sc). Its higher V_oc was supported by the Bode plot of impedance under illumination and Nyquist plots at dark, whereas higher J_sc was supported by the incident monochromatic photon‐to‐current conversion efficiency spectra and charge extraction experiments. Copyright © 2013 John Wiley & Sons, Ltd.     

High voltage photovoltaic mini‐modules

This work describes the design, simulation, fabrication process, and characterization of high voltage photovoltaic mini‐modules using silicon on insulator (SOI) wafers. The mini‐modules are made of a number of small area photovoltaic cells (<1 mm²) monolithically connected in series. Isolation between cells is performed by means of anisotropic etching of the active layer of the SOI wafer. Measurements using standard sunlight (AM1·5 100 mW/cm²) confirm the viability of this technology to fabricate small area arrays showing open circuit voltages, V _oc, between 620 mV and 660 mV and photocurrent densities up to 22·3 mA/cm² for single cells of 0·225 mm² area and 10 µm active film thickness. Series connection scales up V _oc and the maximum power, P _m, from 625 mV and 21·2 µW, respectively, in a single cell to 103 V and 3·2 mW when 169 cells are connected in series in a 0·42 cm² module total area. Copyright © 2008 John Wiley & Sons, Ltd.     

Silicon nanowire array architecture for heterojunction electronics

Photosensitive nanostructured heterojunctions n-TiN/p-Si were fabricated by means of titanium nitride thin films deposition (n-type conductivity) by the DC reactive magnetron sputtering onto nano structured single crystal substrates of p-type Si (100). The temperature dependencies of the height of the potential barrier and series resistance of the n-TiN/p-Si heterojunctions were investigated. The dominant current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage V _oc = 0.8 V, short-circuit current I _sc = 3.72 mA/cm² and fill factor FF = 0.5 under illumination of 100 mW/cm².

     

A masked process for the industrial production of buried contact solar cells on multi‐crystalline silicon

This paper reports on the development of a masked process for the production of buried contact solar cells on multi‐crystalline silicon. The process results in high efficiencies, and only includes steps that would be feasible in an industrial environment. We report here on different mask candidates and on the importance of hydrogenation with the new process. Using the developed process, we produced 111 large area (12 × 12 cm²) cells and achieved an average cell efficiency of 16·2%. The best cell had an efficiency of 16·9%, a V_oc of 616 mV, a J_sc of 35·0 mA/cm² and a fill factor of 78·3%. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhanced performance of dye-sensitized solar cells with novel 2,6-diphenyl-4H-pyranylidene dyes

Novel 2,6-diphenyl-4H-pyranylidene derivatives were designed and synthesized as dyes for dye-sensitized solar cells (DSSC). Dyes 2a, b with a phenyl substituent showed high DSSC energy conversion efficiencies of 5.3% (J_sc = 10.3 mA/cm², V_oc = 0.72 V, FF = 0.72) and 4.7% (J_sc = 8.9 mA/cm², V_oc = 0.73 V, FF = 0.72) at 100 mW/cm² under simulated AM 1.5 G solar light conditions. These values are twice better than that of dye 1 without the phenyl substituent under the same conditions. Both the photocurrent density (J_sc) and open circuit voltage (V_oc) of DSSCs based on dyes 2a, b are increased compared with 1. It can be attributed to their twisted structures, absorption abilities and proper energy levels. This result shows that the tetraphenylpyranylidene is a promising electron-donor unit for high-efficiency DSSCs.     

20.0% Efficiency Si Nano/Microstructures Based Solar Cells with Excellent Broadband Spectral Response

Spectral response of solar cells determines the output performance of the devices. In this work, a 20.0% efficient silicon (Si) nano/microstructures (N/M‐Strus) based solar cell with a standard solar wafer size of 156 × 156 mm² (pseudo‐square) has been successfully fabricated, by employing the simultaneous stack SiO₂/SiN_x passivation for the front N/M‐Strus based n⁺‐emitter and the rear surface. The key to success lies in the excellent broadband spectral responses combining the improved short‐wavelength response of the stack SiO₂/SiN_x passivated Si N/M‐Strus based n⁺‐emitter with the extraordinary long‐wavelength response of the stack SiO₂/SiN_x passivated rear reflector. Benefiting from the broadband spectral response, the highest open‐circuit voltage (V_oc) and short‐circuit current density (J_sc) reach up to 0.653 V and 39.0 mA cm^?2, respectively. This high‐performance screen‐printed Si N/M‐Strus based solar cell has shown a very promising way to the commercial mass production of the Si based high‐efficient solar cells.     

Functionalization of Graphene Oxide Films with Au and MoOx Nanoparticles as Efficient p‐Contact Electrodes for Inverted Planar Perovskite Solar Cells

A graphene oxide (GO) film is functionalized with metal (Au) and metal‐oxide (MoO_x) nanoparticles (NPs) as a hole‐extraction layer for high‐performance inverted planar‐heterojunction perovskite solar cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X‐ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down‐shifts of work functions lead to a decreased level of potential energy and hence increased V_oc of the PSC devices. Although the GO‐AuNP film shows rapid hole extraction and increased V_oc, a J_sc improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO‐AuNP device attains 14.6%, which is comparable with that of the GO‐based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO‐MoO_x layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO‐MoO_x interface is hence significantly retarded. The GO‐MoO_x device consequently shows significantly enhanced V_oc and J_sc, for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.