Locating the electrical junctions in Cu(In,Ga)Se2 and Cu2ZnSnSe4 solar cells by scanning capacitance spectroscopy

We determined the electrical junction (EJ) locations in Cu(In,Ga)Se₂ (CIGS) and Cu₂ZnSnSe₄ (CZTS) solar cells with ~20‐nm accuracy by developing scanning capacitance spectroscopy (SCS) applicable to the thin‐film devices. Cross‐sectional sample preparation for the SCS measurement was developed by high‐energy ion milling at room temperature for polishing the cross section to make it flat, followed by low‐energy ion milling at liquid nitrogen temperature for removing the damaged layer and subsequent annealing for growing a native oxide layer. The SCS shows distinct p‐type, transitional, and n‐type spectra across the devices, and the spectral features change rapidly with location in the depletion region, which results in determining the EJ with ~20‐nm resolution. We found an n‐type CIGS in the region next to the CIGS/CdS interface; thus, the cell is a homojunction. The EJ is ~40 nm from the interface on the CIGS side. In contrast, such an n‐type CZTS was not found in the CZTS/CdS cells. The EJ is ~20 nm from the CZTS/CdS interface, which is consistent with asymmetrical carrier concentrations of the p‐CZTS and n‐CdS in a heterojunction cell. Our results of unambiguously determination of the junction locations contribute significantly to understanding the large open‐circuit voltage difference between CIGS and CZTS. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Bandgap optimization of submicron‐thick Cu(In,Ga)Se2 solar cells

Reducing Cu(In,Ga)Se₂ (CIGS) absorber thickness into submicron regime provides an opportunity for reducing CIGS solar cell manufacturing time and cost. However, CIGS with submicron‐thick absorber would suffer strong absorption loss in the long‐wavelength region. In this paper, we report a new fabrication route for CIGS solar cells on soda‐lime glass substrates with different Ga content (0.3 < [Ga]/([Ga] + [In]) < 0.6), all with absorber thicknesses around 0.9 µm. Efficiency of 17.52% has been achieved for cells with high Ga content of [Ga]/([Ga] + [In]) = 41%, which is currently the best reported efficiency for submicron‐thick CIGS solar cells. Unlike the normal‐thickness absorber (2–3 µm) that has an optimal [Ga]/([Ga] + [In]) of ~32%, the increased value of optimal [Ga]/([Ga] + [In]) in submicron‐thick absorber greatly enhances the open‐circuit voltage, by nearly 15% compared with that of samples with Ga content optimized at normal absorber thickness. This large gain in V_OC well compensates the absorption loss in the long‐wavelength region and contributes to the enhancement of final solar cell efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

Cu(In,Ga)Se2 solar cell grown on flexible polymer substrate with efficiency exceeding 17%

Development of a Cu(In,Ga)Se₂ thin film solar cell on a polyimide film with a conversion efficiency of 17.1%, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process. Copyright © 2011 John Wiley & Sons, Ltd.     

Failure analysis of Cu(In,Ga)Se2 photovoltaic modules: degradation mechanism of Cu(In,Ga)Se2 solar cells under harsh environmental conditions

High‐temperature‐induced and humidity‐induced degradation behaviors were investigated through the failure analysis of encapsulated Cu(In,Ga)Se₂ (CIGS) modules and non‐encapsulated CIGS cells. After being exposed to high temperature (85 °C) for 1000 h, the efficiency loss of CIGS modules and the resistivities of the aluminum‐doped zinc oxide (AZO) layer, CIGS layer, and Mo layer were slightly increased. After damp heat (DH) testing (85 °C/85% RH), the efficiency of some modules decreased significantly accompanied by discoloration, and in these areas, the resistivity of the AZO layers increased markedly. The causes of degradation of CIGS cells after high temperature and DH tests were suggested through X‐ray photoelectron spectroscopy analysis. The high‐temperature‐induced degradation behaviors were revealed to be increases in series resistance of the CIGS cells, due to the adsorption of oxygen on the AZO, CIGS, and Mo layers. The degradation behavior after DH (85 °C/85% RH) exposure was caused by the adsorption of oxygen, as well as the generation of Zn(OH)₂ due to water molecules. In particular, the humidity‐induced degradation behavior in discolored CIGS modules was ascribed to the generation of Zn(OH)₂ and carboxylic acids in the AZO layer, due to a chemical reaction between the AZO, ethylene‐vinyl acetate copolymer, and water. Copyright © 2014 John Wiley & Sons, Ltd.     

The structural evolution of Cu(In,Ga)Se2 thin film and device performance prepared through a three-stage process

Cu(In,Ga)Se₂ (CIGS) absorber layer is grown on Mo-coated soda-lime glass (SLG) substrates using co-evaporation deposition technique. The growth characteristics of the CIGS films deposited through a three-stage process are examined by interrupting the deposition along the reaction pathway. In the three-stage process, the absorber layer undergoes several phase transformations with Cu content. The γ-(In,Ga)₂Se₃ layer is formed first and is then converted to α-Cu(In,Ga)Se₂ via β-Cu(In,Ga)₃Se₅. When α-Cu(In,Ga)Se₂ stoichiometry is reached, Cu_2−xSe segregation at the surface and at grain boundaries begins to occur. The Cu_2−xSe improved the densification and grain growth of the absorber layer. Then, as the absorber layer reverts to substoichiometric composition, the Cu_2−xSe phase disappears and the depleted server Cu near the surface instead. This paper reports several types of defects found in absorber layers that act as non-radiative recombination centers, such as impurity phases (Cu_2−xSe and Cu(In,Ga)₃Se₅), deep point defects (In_Cu), grain boundaries, and voids. The highest efficiency at 10.97% was achieved when the bulk [Cu]/([In]+[Ga]) ratio was 0.98 at the third stage of the process. This result is attributed to the low-concentration deep-level defects that act as recombination centers and to the denser structure with larger grain size.     

Influence of the Cu(In,Ga)Se2 thickness and Ga grading on solar cell performance

Thin‐film solar cells with Cu(In,Ga)Se₂ (CIGS) absorber layers ranging from 1.8 to 0.15 μm in thickness were fabricated by co‐evaporation, with both homogeneous and Ga/(Ga + In) graded composition. The absorption of the CIGS layers was determined and compared with corresponding QE measurements in order to obtain the optical related losses. The material characterization included XRD as well as cross‐sectional SEM analysis. Devices with CIGS layers of all thicknesses were fabricated, and down to 0.8–1 μm they showed a maintained high performance (η ∼ 15%). When the CIGS layer was further reduced in thickness the loss in performance increased. The main loss was observed for the short‐circuit current, although the loss was not only due to a reduced absorbance. The open‐circuit voltage was essentially not affected by the reduction of the CIGS thickness, while the fill factor showed a slight decrease. The fill factor loss was eliminated by introducing a Ga/(Ga+In) graded CIGS, which also resulted in an increased open‐circuit voltage of 20–30 mV for all CIGS thicknesses. Device results of 16.1% efficiency at 1.8 μm CIGS thickness, 15.0% at 1.0 μm and 12.1% at 0.6 μm (total area without anti‐reflective coating) were achieved. Copyright © 2002 John Wiley & Sons, Ltd.     

Electronic properties of Cu(In,Ga)Se2 solar cells on stainless steel foils without diffusion barrier

Compared with rigid glass, manufacturing of Cu(In,Ga)Se₂ (CIGS) solar cells on flexible stainless steel (SS) substrates has potential to reduce production cost because of the application of roll‐to‐roll processing. Up to now, high‐efficiency cells on SS could only be achieved when the substrate is coated with a barrier layer (e.g. SiO_x or Si₃N₄) for hindering the diffusion of impurities, especially Fe, into the CIGS layer. In this paper, the effect of these impurities on the electronic transport properties of the device is investigated. Using admittance spectroscopy, the presence of a deep defect level at around 320 meV is observed, which deteriorates the efficiency of the solar cells. Furthermore, it is shown that reducing substrate temperature during CIGS deposition is an effective alternative to a barrier layer for reducing diffusion of detrimental Fe impurities into the absorber layer. By applying a CIGS growth process for deposition at low substrate temperatures, an efficiency of 17.7%, certified by Fraunhofer Institute ISE, Freiburg, was achieved on Mo/Ti‐coated SS substrate without an additional metal‐oxide or metal‐nitride impurity diffusion barrier layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Determination of the optimum material parameters for intermediate band solar cells using diffusion model

In this study, the optimum material parameters capable of providing high efficiencies close to the detailed balance limit are determined for intermediate band solar cells. A diffusion model, including the overlap effect between absorption coefficients, is used during the calculations for the first time. It is obtained that to achieve high efficiencies close to the detailed balance limit; the effective density of state value, N_CV, should be higher than 10¹⁷ cm⁻³ and the carrier mobility should be larger than 200 cm²/Vs, where the light concentration should not be higher than nearly 1000 sun. Besides, it is found out that the optimum intermediate band level and the base width depend on the mobility and effective density of state values. So they need to be optimized according to the material parameters. The effect of overlap between absorption coefficients on the performance of intermediate band solar cells is also investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

Compositional engineering of chemical bath deposited (Zn,Cd)S buffer layers for electrodeposited CuIn(S,Se)2 and coevaporated Cu(In,Ga)Se2 solar cells

A comparative study of chemical bath deposition (CBD) of ZnS, CdS, and a mixture of (Cd,Zn)S buffer layers has been carried out on electrodeposited CuIn(S,Se)₂ (CISSe) and coevaporated Cu(In,Ga)Se₂ (CIGS) absorbers. For an optimal bath composition with the ratio of [Zn]/[Cd] = 25, efficiencies higher than those obtained with CdS and ZnS recipes, both on co‐evaporated CIGS and electrodeposited CISSe, have been obtained independent of the absorber used. In order to better understand the (Cd,Zn)S system and its impact on the increased efficiency of cells, predictions from the solubility diagrams of CdS and ZnS in aqueous medium were made. This analysis was completed by in situ growth studies with varying bath composition by quartz crystal microbalance (QCM). The morphology and composition of the films were studied using scanning electron microscopy (SEM) and X‐ray photoelectron spectra (XPS) techniques. Preliminary XPS studies showed that films are composed of a mixture of CdS and Zn(O,OH) phases and not a pure ternary Cd_{1 − x}Zn_xS compound. The effect of the [Zn]/[Cd] molar ratio on properties of the corresponding CISSe and CIGS solar cells was investigated by current voltage [J(V)] and capacitance voltage [C(V)] characterizations. The origin of optimal results is discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Improved fill factor and open circuit voltage by crystalline selenium at the Cu(In,Ga)Se2/buffer layer interface in thin film solar cells

A surface treatment by evaporated selenium on Cu(In,Ga)Se₂ (CIGS) is shown to improve open circuit voltage, V_oc, and in some cases fill factor, FF, in solar cells with CdS, (Zn,Mg)O or Zn(O,S) buffer layers. V_oc increases with increasing amount of crystalline Se, while FF improves only for small amounts. The improvements are counteracted by a decreasing short circuit current assigned to absorption in hexagonal Se. Improved efficiency is shown for device structures with (Zn,Mg)O and Zn(O,S) buffer layers by atomic layer deposition. Analysis by grazing incidence X‐ray diffraction and photoelectron spectroscopy show partial coverage of the CIGS surface by hexagonal selenium. The effects on device performance from replacing part of the CIGS/buffer interface area by a Se/buffer junction are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Impact of compositional grading and overall Cu deficiency on the near‐infrared response in Cu(In,Ga)Se2 solar cells

Highly efficient thin film solar cells based on co‐evaporated Cu(In,Ga)Se₂ (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu‐poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near‐infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection‐transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu‐dependent modifications of the NIR‐absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re‐adjustments of the co‐evaporation process and of the alkali‐fluorides post‐deposition treatments allow maintaining V_oc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. © 2016 The Authors. Progress in Photovoltaics : Research and Applications published by 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.     

Composition and bandgap control in Cu(In,Ga)Se2‐based absorbers formed by reaction of metal precursors

The control of composition and bandgap in chalcopyrite thin‐film absorber layers formed by a metal precursor reaction is addressed. Two processes using reaction with either H₂Se or H₂S as the final step of a three‐step reaction process were compared as follows: a three‐step H₂Se/Ar/H₂S reaction and a three‐step H₂Se/Ar/H₂Se reaction. In both processes, significant Ga homogenization was obtained during the second‐step Ar anneal, but the third‐step selenization resulted in Ga depletion near the Cu(InGa)Se₂ surface, whereas the third‐step sulfization did not. Solar cells were fabricated using absorbers formed using each method, and the surface Ga depletion significantly affected device performances. The solar cell incorporating the sulfization yielded a better device performance, with an efficiency of 14.4% (without an anti‐reflection layer) and an open‐circuit voltage of 609 mV. The bandgap control in the metal precursor reaction is discussed in conjunction with the device behavior. Copyright © 2014 John Wiley & Sons, Ltd.     

Non‐destructive optical analysis of band gap profile,crystalline phase,and grain size for Cu(In,Ga)Se2 solar cells deposited by 1‐stage, 2‐stage,and 3‐stage co‐evaporation

Cu(In,Ga)Se₂ (CIGS) thin films co‐evaporated by 1‐stage, 2‐stage, and 3‐stage processes have been studied by spectroscopic ellipsometry (SE). The disappearance of a Cu_2‐xSe optical signature, detected by real time SE during multistage CIGS, has enabled precise endpoint control. Band gap energies determined by SE as depth averages show little process variation for fixed [Ga]/([In] + [Ga]) atomic ratio, whereas their broadening parameters decrease with increasing number of stages, identifying successive grain size enhancements. Refined SE analysis has revealed band gap profiling only for 3‐stage CIGS. Solar cells incorporating these absorbers have yielded increased efficiencies in correlation with phase control, grain size, and band gap profiling. Copyright © 2013 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.     

Cu(In1-xGax)Se2薄膜太阳电池的J-V特性

对Cu（In1-xGax）Se2（CIGS）太阳电池J-V特性曲线进行了测试和分析,采用Matlab软件进行计算,得到电池的二极管品质因子、反向饱和电流密度、串联电阻、并联电阻等特性参数，采用数值逼近法,将得到的参数回归J-V方程,与测试结果符合较好，对不同光照强度下电池的特性参数进行计算,发现并联电阻随光照强度增加而降低,并分析了原因。     

Cu(In1-xGax)Se2薄膜太阳电池的J-V特性

对Cu(In1-xGax)Se2(CIGS)太阳电池J-V特性曲线进行了测试和分析,采用Matlab软件进行计算,得到电池的二极管品质因子、反向饱和电流密度、串联电阻、并联电阻等特性参数.采用数值逼近法,将得到的参数回归J-V方程,与测试结果符合较好.对不同光照强度下电池的特性参数进行计算,发现并联电阻随光照强度增加而降低,并分析了原因.     

Direct observation of Cu,Zn cation disorder in Cu2ZnSnS4 solar cell absorber material using aberration corrected scanning transmission electron microscopy

Chemical analysis of individual atom columns was carried out to determine the crystal structure and local point defect chemistry of Cu₂ZnSnS₄. Direct evidence for a nanoscale composition inhomogeneity, in the form of Zn enrichment and Cu depletion, was obtained. The lateral size of the composition inhomogeneity was estimated to be between ~1.5 and 5 nm. Photoluminescence confirmed the presence of a broad donor–acceptor transition consistent with the observed cation disorder. Areas of relatively high concentration of Zn_Cu⁺ antisite atom donors locally increases the electrostatic potential and gives rise to band bending. Troughs in the conduction band and peaks in the valence band are ‘potential wells’ for electrons and holes, respectively. For a solar cell, these prevent minority carrier electrons from diffusing towards the edge of the space charge region, thereby reducing the carrier separation efficiency as well as reducing the carrier collection efficiency of majority carrier holes. Furthermore, electrons and holes ‘trapped’ within potential wells in close proximity have a high probability of recombining, so that the carrier lifetime is also reduced. High quality Cu₂ZnSnS₄ crystals free from composition inhomogeneities are therefore required for achieving high efficiency solar cell devices. Copyright © 2012 John Wiley & Sons, Ltd.