CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide/Au back contacts

Superstrate CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide (TMO)/Au and Au‐only back contacts have been fabricated. The TMOs include MoO_3‐x, V₂O_5‐x, and WO_3‐x. The incorporation of the TMO buffer layers at the back contacts resulted in significant improvement on open‐circuit voltage (V_OC) as compared with the cells with Cu‐free Au‐only back contacts. Among the cells using TMO buffer layers, the ones with MoO_3‐x buffer layers exhibited the best performance, yielding an efficiency of 14.1% under AM1.5 illumination with V_OC of 815 mV and a fill factor of 67.9%. Though the performance is slightly behind the best reference cell with a Cu/Au back contact fabricated in our lab with V_OC of 844 mV, fill factor of 76.3%, and efficiency of 15.7%, the use of Cu‐free back contacts may lead to improved long‐term cell stability. Copyright © 2013 John Wiley & Sons, Ltd.     

Electronic structure of the Zn(O,S)/Cu(In,Ga)Se2 thin‐film solar cell interface

The electronic band alignment of the Zn(O,S)/Cu(In,Ga)Se₂ interface in high‐efficiency thin‐film solar cells was derived using X‐ray photoelectron spectroscopy, ultra‐violet photoelectron spectroscopy, and inverse photoemission spectroscopy. Similar to the CdS/Cu(In,Ga)Se₂ system, we find an essentially flat (small‐spike) conduction band alignment (here: a conduction band offset of (0.09 ± 0.20) eV), allowing for largely unimpeded electron transfer and forming a likely basis for the success of high‐efficiency Zn(O,S)‐based chalcopyrite devices. Furthermore, we find evidence for multiple bonding environments of Zn and O in the Zn(O,S) film, including ZnO, ZnS, Zn(OH)₂, and possibly ZnSe. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of Mo back contact ageing on Cu(In,Ga)Se2 thin‐film solar cells

In this work, we investigate the effect of ageing Mo‐coated substrates in a dry and N₂ flooded cabinet. The influence was studied by preparing Cu(In,Ga)Se₂ solar cells and by comparing the electrical performance with devices where the Mo layer was not aged. The measurements used for this study were current–voltage (J‐V), external quantum efficiency (EQE), secondary ion mass spectroscopy (SIMS) and capacitance–voltage (C‐V). It was concluded that devices prepared with the aged Mo layer have, in average, an increase of 0.8% in efficiency compared with devices that had a fresh Mo layer. Devices with aged Mo exhibited a nominal increase of 12.5 mV of open circuit voltage, a decrease of 1.1 mA/cm⁻² of short circuit current and a fill factor increase of 2.4%. Heat treatment of fresh Mo layers in oxygen atmosphere was also studied as an alternative to ageing and was shown to provide a similar effect to the aged device's performance. Copyright © 2013 John Wiley & Sons, Ltd.     

CuxS背接触层制备及在碲化镉薄膜太阳电池中应用研究

本文采用化学水浴法沉积CuxS薄膜,通过改变Cu元素比例研究其对碲化镉电池效率的影响。研究表明化学水浴法沉积的CuxS是非晶的,采用适当退火条件可以使其晶化,随着退火温度的提高,薄膜变得致密且结晶明显。CuxS薄膜厚度对电池性能有很大的影响,结果表明,随着CuxS薄膜厚度增加,电池性能先增加后减少。薄膜厚度为75nm时,CdS/CdTe电池性能最佳,达到了最高转化效率(η)为12.19%,填充因子(FF)为68.82%,开路电压(Voc)为820mV。     

Application of copper thiocyanate for high open‐circuit voltages of CdTe solar cells

Copper thiocyanate (CuSCN) has proven to be a low‐cost, efficient hole‐transporting material for the emerging organic–inorganic perovskite solar cells. Herein, we report that CuSCN can also be applied to CdTe thin‐film solar cells to achieve high open‐circuit voltages (V_OCs). By optimizing the thickness of the thermally evaporated CuSCN films, CdTe cells fabricated by close space sublimation in the superstrate configuration have achieved V_OCs as high as 872 mV, which is about 20–25 mV higher than the highest V_OC for the reference cells using the standard Cu/Au back contacts. CuSCN is a wide bandgap p‐type conductor with a conduction band higher than that of CdTe, leading to a conduction band offset that reflects electrons in CdTe, partially explaining the improved V_OCs. However, due to the low conductivity of CuSCN, CdTe cells using CuSCN/Au back contacts exhibited slightly lower fill factors than the cells using Cu/Au back contacts. With optimized CdS:O window layers, the power conversion efficiency of the best CdTe cell, using CuSCN/Au back contact, is 14.7%: slightly lower than that of the best cell (15.2%) using Cu/Au back contact. Copyright © 2015 John Wiley & Sons, Ltd.     

New explicit current/voltage equation for p‐i‐n solar cells including interface potential drops and drift/diffusion transport

Analytical modeling of p‐i‐n solar cells constitutes a practical tool to extract material and device parameters from fits to experimental data, and to establish optimization criteria. This paper proposes a model for p‐i‐n solar cells based on a new approximation, which estimates the electric field taking into account interface potential drops at the intrinsic‐to‐doped interfaces. This leads to a closed‐form current/voltage equation that shows very good agreement with device simulations, revealing that the inclusion of the interface potential drops constitutes a major correction to the classical uniform‐field approach. Furthermore, the model is able to fit experimental current/voltage curves of efficient nanocrystalline Si and microcrystalline Si p‐i‐n solar cells under illumination and in the dark, obtaining material parameters such as mobility‐lifetime product, built‐in voltage, or surface recombination velocity. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Impacts of surface sulfurization on Cu(In1−x,Gax)Se2 thin‐film solar cells

In this work, the impacts of surface sulfurization of high‐quality Cu(In_1−x,Ga_x)Se₂ (CIGS) thin films deposited by three‐stage process on the film properties and the cell performance were investigated. The CIGS thin films were sulfurized at 550 °C for 30 min using H₂S gas. The X‐ray photoelectron spectroscopy analysis revealed that sulfur atoms diffused into the CIGS surface layer and that the valence band minimum was lowered by the film sulfurization. The open circuit voltage (V_oc) drastically increased from 0.590 to 0.674 V as a result of the sulfurization process. Temperature‐dependent current–voltage and capacitance–frequency measurements also revealed that interface recombination was drastically decreased by the lowering of the defect's activation energy level at the vicinity of the buffer/CIGS interface after the sulfurization. Copyright © 2014 John Wiley & Sons, Ltd.     

Laser‐fired contact for n‐type crystalline Si solar cells

Laser‐fired contacts to n‐type crystalline silicon were developed by investigating novel metal stacks containing Antimony (Sb). Lasing conditions and the structure of metals stacks were optimized for lowest contact resistance and minimum surface damage. Specific contact resistance for firing different metal stacks through either silicon nitride or p‐type amorphous silicon was determined using two different models and test structures. Specific contact resistance values of 2–7 mΩcm² have been achieved. Recombination loss due to laser damage was consistent with an extracted local surface recombination velocity of ~20 000 cm/s, which is similar to values for laser‐fired base contact for p‐type crystalline silicon. Interdigitated back contact silicon heterojunction cells were fabricated with laser‐fired base contact and proof‐of‐concept efficiencies of 16.9% were achieved. This localized base contact technique will enable low cost back contact patterning and innovative designs for n‐type crystalline solar cell. Copyright © 2014 John Wiley & Sons, Ltd.     

Control of the preferred orientations of Cu(In,Ga)Se2 films and the photovoltaic conversion efficiency using a surface‐functionalized molybdenum back contact

The surface microstructures of molybdenum (Mo) back contacts were shown to play a crucial role in the preferred orientations of Cu(In,Ga)Se₂ (CIGS) films. The lower surface density of Mo tends to drive the growth of CIGS films toward favoring a (220)/(204) orientation, attributed to the higher likelihood of a MoSe₂ reaction. This work showed that the presence of a very thin layer on a Mo bilayer facilitated the tuning of the CIGS grain orientations from strongly favoring (112) to strongly favoring (220)/(204) without sacrificing the electrode conductivity. The efficiency of Na‐doped CIGS cells was increased toward decreasing Mo surface density, that is, increasing (220)/(204) CIGS orientation. Although slight changes in Na doping found between different Mo surface properties could contribute in part, the comparison with Na‐reduced CIGS cells showed that it was more likely due to the (220)/(204) orientation‐related enhancement of CdS/CIGS junction characteristics, which were possibly attributed to a favorable CdS reaction and a reduction in the defect metastabilities. Copyright © 2012 John Wiley & Sons, Ltd.     

Through‐the‐glass spectroscopic ellipsometry for analysis of CdTe thin‐film solar cells in the superstrate configuration

Polycrystalline CdS/CdTe thin‐film solar cells in the superstrate configuration have been studied by spectroscopic ellipsometry (SE) using glass side illumination. In this measurement method, the first reflection from the ambient/glass interface is rejected, whereas the second reflection from the glass/film‐stack interface is collected; higher order reflections are also rejected. The SE analysis incorporates parameterized dielectric functions ε for solar cell component materials obtained by in situ and variable‐angle SE. In the SE analysis of the complete cells, a step‐wise procedure ranks the fitting parameters, including thicknesses and those defining the spectra in ε, according to their ability to reduce the root‐mean‐square deviation between the simulated and measured SE spectra. The best fit thicknesses from this analysis are found to be consistent with electron microscopy. Based on the SE results, the solar cell quantum efficiency (QE) can be simulated without any free parameters, and comparisons with measured QE enable optical model refinements as well as identification of optical and electronic losses. These capabilities have wide applications in photovoltaic module mapping and in‐line monitoring. Copyright © 2016 John Wiley & Sons, Ltd.     

Improving the open‐circuit voltage of Cu2ZnSnSe4 thin film solar cells via interface passivation

This study reports on substantial improvement of the open‐circuit voltage (V _oc) of Cu₂ZnSnSe₄ (CZTSe) thin film solar cells by applying a passivation strategy to both the top and bottom interfaces of the CZTSe absorber, which involves insertion of a thin dielectric layer between the CZTSe and the surrounding layers. The study also presents in‐depth material characterizations using transmission electron microscopy, energy dispersive X‐ray spectroscopy, low‐temperature photoluminescence, and secondary ion mass spectrometry, to reveal the effects of the interface passivation. To passivate the bottom Mo/CZTSe interface, a dielectric layer with patterned local contacts of dimensions down to ~100 nm was prepared using nanosphere lithography. With this, the V _oc, short‐circuit current, and fill factor (FF) were significantly enhanced due to reduction in carrier recombination at the bottom Mo/CZTSe interface. The top CZTSe/CdS interface was passivated by a thin dielectric layer which prevented inter‐diffusion of Cd and Cu at the top interface, thereby improving the junction quality. Application of the top passivation layers resulted in substantial improvement in V _oc and FF, thereby achieving the V _oc deficit of 0.542 V which is the record value among reported CZTSe solar cells. Copyright © 2017 John Wiley & Sons, Ltd.     

Efficient Field Emission from Vertically Aligned Cu2O1‐δ(111) Nanostructure Influenced by Oxygen Vacancy

In the architecture described, cuprous oxide (Cu₂O) is tamed to be highly (111) plane oriented nanostructure through adjusting the deposition postulate by glancing angle deposition technique. In the controlled atmosphere oxygen vacancy is introduced into the Cu₂O crystal subsequently fostering an impurity energy state (E_im) close to the conduction band. Our model of Cu₂O electronic structure using density functional theory suggests that oxygen vacancies enhance the electron donating ability because of unshared d‐electrons of Cu atoms (nearest to the vacancy site), allowing to pin the work function energy level around 0.28 eV compared to the bulk. This result is also complemented by Kelvin probe force microscopy analysis and X‐ray photoelectron spectroscopy method. Oxygen vacancy in Cu₂O (Cu₂O_1‐δ) exhibits promising field emission properties with interesting field electron tunneling behavior at different applied fields. The films show very low turn‐on and threshold voltages of value 0.8 and 2.4 V μm^?1 respectively which is influenced by the oxygen vacancy. Here, a correlation between the work function modulation due to the oxygen vacancy and enhancement of field emission of Cu₂O_1–δ nanostructure is demonstrated. This work reveals a promising new vision for Cu₂O as a low power field emitter device.     

Interconnection of busbar‐free back contacted solar cells by laser welding

We are presenting the module integration of busbar‐free back‐junction back‐contact (BJBC) solar cells. Our proof‐of‐concept module has a fill factor of 80.5% and a conversion efficiency on the designated area of 22.1% prior to lamination. A pulsed laser welds the Al metallization of the solar cells to an Al foil carried by a transparent substrate. The weld spots electrically contact each individual finger to the Al foil, which serves as interconnect between different cells. We produce a proof‐of‐concept module using busbar‐free cell strips of 25 × 125 mm². These are obtained by laser‐dicing of a 125 × 125 mm² BJBC solar cell. The fill factor of this module is increased by 3.5% absolute compared with the initial cell before laser‐dicing. This is achieved mainly by omitting the busbars and reduction of the finger length. The improvement of the module fill factor results in an increase in the module performance of 0.9% absolute before lamination in comparison with the efficiency of the initial 125 × 125 mm² BJBC solar cell. Hence, this interconnection scheme enables the transfer of high cell efficiencies to the module. Copyright © 2014 John Wiley & Sons, Ltd.     

UV‐nano‐imprint lithography technique for the replication of back reflectors for n‐i‐p thin film silicon solar cells

Texturing of interfaces in thin film silicon solar cells is essential to enhance the produced photocurrent and thus the efficiencies. A UV nano‐imprint‐lithography (UV‐NIL) replication process was developed to prepare substrates with textures that are suitable for the growth of n‐i‐p thin film silicon solar cells. Morphological and optical analyses were performed to assess the quality of the replicas. A comparison of single junction amorphous solar cells on the original structures and on their replicas on glass revealed good light trapping and excellent electrical properties on the replicated structures. A tandem amorphous silicon/amorphous silicon (a‐Si/a‐Si) cell deposited on a replica on plastic exhibits a stabilized efficiency of 8.1% and a high yield of 90% of good cells in laboratory conditions. It demonstrates the possibility to obtain appropriate structure on low cost plastic substrate. Copyright © 2010 John Wiley & Sons, Ltd.     

A 3‐in‐1 doping process for interdigitated back contact solar cells exploiting the understanding of co‐diffused dopant profiles by use of PECVD borosilicate glass in a phosphorus diffusion

Boron and phosphorus doping of crystalline silicon using a borosilicate glass (BSG) layer from plasma‐enhanced chemical vapor deposition (PECVD) and phosphorus oxychloride diffusion, respectively, is investigated. More specifically, the simultaneous and interacting diffusion of both elements through the BSG layer into the silicon substrate is characterized in depth. We show that an overlying BSG layer does not prevent the formation of a phosphorus emitter in silicon substrates during phosphorus diffusion. In fact, a BSG layer can even enhance the uptake of phosphorus into a silicon substrate compared with a bare substrate. From the understanding of the joint diffusion of boron and phosphorus through a BSG layer into a silicon substrate, a model is developed to illustrate the correlation of the concentration‐dependent diffusivities and the emerging diffusion profiles of boron and phosphorus. Here, the in‐diffusion of the dopants during diverse doping processes is reproduced by the use of known concentration dependences of the diffusivities in an integrated model. The simulated processes include a BSG drive‐in step in an inert and in a phosphorus‐containing atmosphere. Based on these findings, a PECVD BSG/capping layer structure is developed, which forms three different n⁺⁺−, n⁺− and p⁺−doped regions during one single high temperature process. Such engineered structure can be used to produce back contact solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

High‐efficiency black silicon interdigitated back contacted solar cells on p‐type and n‐type c‐Si substrates

This work demonstrates the high potential of Al₂O₃ passivated black silicon in high‐efficiency interdigitated back contacted (IBC) solar cells by reducing surface reflectance without jeopardizing surface passivation. Very low reflectance values, below 0.7% in the 300–1000 nm wavelength range, together with striking surface recombination velocities values of 17 and 5 cm/s on p‐type and n‐type crystalline silicon substrates, respectively, are reached. The simultaneous fulfillment of requirements, low reflectance and low surface recombination, paves the way for the fabrication of high‐efficiency IBC Si solar cells using black silicon at their front surface. Outstanding photovoltaic efficiencies over 22% have been achieved both in p‐type and n‐type 9‐cm² cells. 3D simulations suggest that efficiencies of up to 24% can be obtained in the future with minor modifications in the baseline fabrication process. Copyright © 2015 John Wiley & Sons, Ltd.     

CO2‐Laser‐Induced Growth of Epitaxial Graphene on 6H‐SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large‐area, low‐strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single‐step epitaxial growth of large‐area homogeneous graphene film on the surface of SiC(0001) using an infrared CO₂ laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost‐effective in that it does not necessitate SiC pre‐treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X‐ray photoelectron spectroscopy, secondary ion‐mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO₂‐laser‐induced graphene growth and the lack of strict sample–environment conditions.     

Effects of combined additional indium deposition and potassium fluoride post‐deposition treatments on Cu(In,Ga)Se2 thin film solar cells

The effect of additional indium on copper indium gallium selenide (CIGS) thin films and solar cells was investigated with respect to potassium fluoride post‐deposition treatment (KF‐PDT) using current‐voltage, external quantum efficiency, scanning electron microscopy, X‐ray photoelectron spectroscopy, time‐resolved photoluminescence and capacitance‐voltage measurements. The cell performance, particularly open‐circuit voltage (V _oc) improved drastically by the combined treatments of additional indium deposition after CIGS growth and subsequent KF‐PDT. A Cu deficient layer at the CIGS surface increased after both treatments rather than only KF‐PDT. Photoluminescence intensity, lifetime and net carrier concentration of KF‐untreated CIGS solar cells did not change significantly by only additional indium deposition. However, they improved because of the combined treatments. Copyright © 2017 John Wiley & Sons, Ltd.     

Evidence for Chemicals Intermingling at Silicon/Titanium Oxide (TiOx) Interface and Existence of Multiple Bonding States in Monolithic TiOx

Metal oxides (MOs) are used in photovoltaics and microelectronics as surface passivating layers and gate dielectrics, respectively. The effectiveness of MOs predominantly depends on their structure and the nature of the semiconductor/MO (S/MO) interface. While some efforts are made to analyze interface behavior of a few MOs, greater fundamental understanding on the interface and structural behaviors of emerging MOs is yet to be established for enhanced scientific and technological developments. Here, the structure of atomic layer deposited titanium oxide (TiO_x) and the nature of the c‐Si/TiO_x interface on the atomic‐ to nanoscale are probed. A new breed of mixed oxide (SiO_x+TiO_x) interfacial layer with a thickness of ≈1.3 nm at the c‐Si/TiO_x interface is discovered, and its thickness further increases to ≈1.5 nm after postdeposition annealing. It is observed that both as‐deposited and annealed monolithic TiO_x films comprise multiple bonding states at varying film thickness, with an oxygen‐deficient TiO_x layer located close to the mixed oxide/TiO_x interface. The stoichiometry of this layer improves when reaching the middle and near surface regions of the TiO_x layer, respectively. This work uncovers several critical structural and interface aspects of TiO_x, and thus creates opportunities to control and design improved photovoltaic and electronic devices for future development.