Interconnection between Trait,Structure, and Composition of Grain Boundaries in Cu(In,Ga)Se2 Thin‐Film Solar Cells

Grain boundaries (GBs) are crucial for solar cells incorporating polycrystalline absorbers and particularly for those characterized by small grain sizes (≈2 µm). For example, random GBs in Si solar cells are found to have a detrimental effect on the cell performance being characterized by an increased recombination activity relative to grains. Yet, their role in Cu(In,Ga)Se₂ (CIGS) solar cells still remains controversial. The recent electron‐beam‐induced current (EBIC) study shows that 58% of the GBs in CIGS exhibit enhanced electrical properties considered to be benign (for the device performance). Yet, they coexist with 16% detrimental GBs (reduced electrical properties) and 27% neutral ones (no change in electrical property when compared with the bulk). In the present study, these different GBs are investigated by combining EBIC with electron backscattered diffraction and atom probe tomography techniques on identical GBs. For the first time, a successful correlation is shown (for any device) that interconnects the GB characteristics to its composition. Sufficient statistics demonstrate that the collective fluctuations of all elements at GBs determine its trait. In general, benign (detrimental) GBs are characterized by Cu depletion (enrichment) that favored the formation of donor (acceptor) defects.     

Direct evidence of enhanced chlorine segregation at grain boundaries in polycrystalline CdTe thin films via three‐dimensional TOF‐SIMS imaging

For more than 25 years, the CdTe photovoltaic research and manufacturing communities have been subjecting CdTe materials to a CdCl₂ treatment or activation step to improve performance. However, little work has been carried out using imaging to elucidate the spatial distribution of chlorine in the CdTe devices after this treatment. This work addresses fundamental questions about the spatial distribution of chlorine in the CdTe absorber material after a CdCl₂ treatment comparable to industrial practices. We used a state‐of‐the‐art, time‐of‐flight secondary ion mass spectrometer (ION‐TOF GmbH) (Muenster, Germany) with a lateral resolution of about 80 nm to complete three‐dimensional depth‐profiling and imaging of two CdTe devices. The results clearly demonstrate enhanced chlorine concentration along grain boundaries, supporting the hypothesis that chlorine plays an important role in passivating grain boundaries in CdTe solar cells. The results are discussed in terms of possible passivation mechanisms, and the effect of chlorine on grain interiors and grain boundaries. The data are also used to estimate the free energy of segregation of chlorine to grain boundaries in CdTe. Copyright © 2014 John Wiley & Sons, Ltd.     

Beam injection methods for characterizing thin‐film solar cells

II–VI and I–III–VI solar cells are promising for future thin‐film photovoltaics. In this paper, the roles of electron‐beam‐induced current (EBIC) and cathodoluminescence in evaluating the influence of interfaces on those solar cells are reviewed. CdTe and Cu(In,Ga)Se₂ (CIGS) are the absorbers of the cells investigated. For CdTe/CdS solar cells, a detailed study has been conducted of the effects of grain boundaries and the Te/CdTe or ZnTe:Cu/CdTe interfaces for back‐contacting. For CIGS solar cells, we have investigated different buffer layer schemes, showing that these interfaces are critical in the definition of the mechanisms for carrier collection. Copyright © 2002 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.     

An investigation of band profile around the grain boundary of Cu(InGa)Se2 solar cell material by scanning probe microscopy

We performed scanning tunneling spectroscopy on an as‐grown Cu(InGa)Se₂ (CIGS) thin film and photo‐assisted Kelvin probe force microscopy on a CIGS solar cell. From these measurements, we estimated the band profile around the grain boundaries (GBs). The results indicate both downward bending of the conduction band edge and broadening of the band gap near GBs. We can therefore conclude that photo‐generated electrons and holes are easily separated by the built‐in field near GBs, and consequently their recombination at the GBs should be suppressed. Copyright © 2011 John Wiley & Sons, Ltd.     

Copper migration in cdte heterojunction solar cells

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150Å on polycrystalline CdTe/CdS/SnO₂/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R_s), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R_sh) and cell performance. Light I-V and secondary ion mass spectros-copy (SIMS) measurements were performed to understand the correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell performance. The CdTe/CdS/SnO₂/glass, CdTe/ CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore, grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation can mitigate the adverse effect of Cu and improve the cell performance.     

Influence of defect type on hydrogen passivation efficacy in multicrystalline silicon solar cells

We examine the effectiveness of hydrogen passivation as a function of defect type and microstructure at grain boundaries (GBs) in multicrystalline silicon. We analyze a solar cell with alternating mm‐wide bare and SiN_x‐coated stripes using laser‐beam‐induced current, electron backscatter diffraction, X‐ray fluorescence microscopy, and defect etching to correlate pre‐ and post‐hydrogenation recombination activity with GB character, density of iron‐silicide nanoprecipitates, and dislocations. A strong correlation was found between GB recombination activity and the nature/density of etch pits along the boundaries, while iron silicide precipitates above detection limits were found to play a less significant role. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of P‐induced gettering on extended defects in n‐type multicrystalline silicon

The electrical properties and the minority charge carrier recombination behaviour of grain boundaries (GBs) and intragrain dislocations in different n‐type multicrystalline silicon (mc‐Si) ingots were systematically studied through microwave‐detected PhotoConductance Decay (µW‐PCD), Electron Beam Induced Current (EBIC) and PhotoLuminescence (PL) spectroscopy on as‐grown samples and on samples submitted to P‐diffusion step. It was confirmed that the overall quality of n‐type mc‐Si is high, indicating that n‐type‐Si is a valid source for photovoltaic applications. As expected, the average lifetime increases after the P‐diffusion process, which induces impurity gettering effects at the external surfaces, like in the case of p‐type samples, but an evident local increase of electrical activity of some GBs after that process was also observed using the EBIC mapping technique. Apparently, a redistribution of impurities occurs at the processing temperature and impurities are captured at the deepest sinks. In fact, while all GBs act as heterogeneous segregation/precipitation sites, some of them will compete with the external surfaces sinks, partly vanishing the effect of P‐gettering. Last but not least, it was experimentally demonstrated that the average lifetime values measured with the µW‐PCD technique well correlate with the recombination activity of GBs measured with the EBIC technique, showing the extreme importance of GBs on the effective lifetime of this material. Copyright © 2007 John Wiley & Sons, Ltd.     

Defect Engineering of Grain Boundaries in Lead‐Free Halide Double Perovskites for Better Optoelectronic Performance

Halide double perovskites (HDPs) are promising lead‐free perovskites for various optoelectronic applications. However, the device performances of HDPs are far below the optimized values, which open a critical question regarding the origin of low performance in these HDPs. In this article, using first‐principles calculations, it is found that some types of grain boundaries (GBs) are easy to form in polycrystalline HDPs. Importantly, the existence of low‐energy Σ5(310) GBs can induce harmful deep‐level defect states within the bandgaps of type‐I (e.g., Cs₂AgInCl₆) and type‐II (e.g., Cs₂AgBiCl₆) HDPs, which may dramatically reduce the device performances. Interestingly, it is found that the formation of some intrinsic defects and defect complexes could effectively eliminate these deep‐levels in type‐II and type‐I HDPs, respectively. Under some exactly predesigned growth conditions identified by utilizing thousands of chemicals through a potential screening process, these defects or defect complexes can spontaneously incorporate into the GB cores, meanwhile the harmful deep‐level defects in the bulk can also be effectively eliminated. In addition, the self‐passivated GBs could generate band bending, which may be beneficial for charge separation. The understanding of GB formation as well as the self‐passivation mechanism in HDPs can provide a new viewpoint and guidance for designing polycrystalline perovskites with improved optoelectronic performance.     

Overcoming the poor short wavelength spectral response of CdS/CdTe photovoltaic modules via luminescence down‐shifting: ray‐tracing simulations

The short‐wavelength response of cadmium sulfide/cadmium telluride (CdS/CdTe) photovoltaic (PV) modules can be improved by the application of a luminescent down‐shifting (LDS) layer to the PV module. The LDS layer contains a mixture of fluorescent organic dyes that are able to absorb short‐wavelength light of λ < 540 nm, for which the PV module exhibited low external quantum efficiency (EQE), and re‐emit it at a longer wavelength (λ > 540 nm), where the solar cell EQE is high. Ray‐tracing simulations indicate that a mixed LDS layer containing three dyes could lead to an increase in the short‐circuit current density from J_sc = 19.8 mA/cm² to J_sc = 22.9 mA/cm² for a CdS/CdTe PV module. This corresponds to an increase in conversion efficiency from 9.6% to 11.2%. This indicates that a relative increase in the performance of a production CdS/CdTe PV module of nearly 17% can be expected via the application of LDS layers, possibly without any making any alterations to the solar cell itself. Copyright © 2006 John Wiley & Sons, Ltd.     

Factors Affecting the Stability of CdTe/CdS Solar Cells Deduced from Stress Tests at Elevated Temperature

CdTe/CdS solar cells were subjected to heat stress at 200 °C in the dark under different environments (in N₂ and in air), and under illumination (in N₂). We postulate that two independent mechanisms can explain degradation phenomena in these cells: i) Excessive Cu doping of CdS: Accumulation of Cu in the CdS with stress, in the presence of Cl, will increase the photoconductivity of CdS. With limited amounts of Cu in CdS, this does NOT affect the photovoltaic behavior, but explains the crossover of light/dark current–voltage (J–V) curves. Overdoping of CdS with Cu can be detrimental to cell performance by creating deep acceptor states, acting as recombination centers, and compensating donor states. Under illumination, the barrier to Cu cations at the cell junction is reduced, and, therefore, Cu accumulation in the CdS is enhanced. Recovery of light‐stress induced degradation in CdTe/CdS cells in the dark is explained by dissociation of the acceptor defects. ii) Back contact barrier: Oxidation of the CdTe back surface in O₂/H₂O‐containing environment to form an insulating oxide results in a back‐contact barrier. This barrier is expressed by a rollover in the J–V curve. Humidity is an important factor in air‐induced degradation, as it accelerates the oxide formation. Heat treatment in the dark in inert atmosphere can stabilize the cells against certain causes of degradation, by completing the back contact anneal.     

Focussed ion beam and field emission gun–scanning electron microscopy for the investigation of voiding and interface phenomena in thin‐film solar cells

The use of focussed ion beam milling combined with high resolution scanning electron microscopy analysis as a characterisation tool for thin‐film photovoltaics is reported. CdTe solar cell cross sections are examined in high detail with as‐grown and CdCl₂‐treated devices being compared. Observed changes in microstructure of the thin‐film layers are related to the device performance. The CdCl₂ treatment is shown to cause a reduction in the CdTe defect density at regions close to the interface and induce recrystallization of the CdS layer. Furthermore, the focussed ion beam technique is shown to reveal voids formed within the device's thin‐film layers at various processing stages that have not been previously observed in working cell structures. The back‐contacting Te‐rich layer resulting from nitric–phosphoric acid etching is also observed, with the etched layer being seen to propagate down the CdTe grain boundaries. Copyright © 2011 John Wiley & Sons, Ltd.     

Simulation of Current Transport in Polycrystalline CdTe Solar Cells

Polycrystalline thin-film CdTe solar cells have demonstrated laboratory efficiency exceeding 17% and are nowadays a commercial technology (albeit with somewhat lower efficiencies). The standard process features a poorly understood recrystallization step, obtained by annealing with a source of chlorine. This study uses two-dimensional numerical modeling to investigate current transport inside the polycrystalline CdTe absorber with and without recrystallization effects [increase of grain size and donor Cl_Te states at grain boundaries (GBs)]. Solving the Poisson equation and the drift–diffusion model for transport with Fermi statistics, while treating the optical problem by the one-dimensional transfer matrix method and complex refractive indexes, this study shows that: (i) in a columnar absorber (i.e., one where only vertical GBs exist), the presence of Cl_Te donor traps at GBs results in a dip in the band profiles that effectively serves as an electron collector, significantly increasing the short-circuit current and efficiency compared with nondecorated GBs; (ii) while the same dip acts as a hole barrier and thus can be expected to block holes from flowing when horizontal GBs are present, under illuminated conditions electron collection at GBs reduces the dip enough to allow substantial hole flow, and the cell performance is only moderately affected.     

High Performance PbS Colloidal Quantum Dot Solar Cells by Employing Solution‐Processed CdS Thin Films from a Single‐Source Precursor as the Electron Transport Layer

CdS thin films are a promising electron transport layer in PbS colloidal quantum dot (CQD) photovoltaic devices. Some traditional deposition techniques, such as chemical bath deposition and RF (radio frequency) magnetron sputtering, have been employed to fabricate CdS films and CdS/PbS CQD heterojunction photovoltaic devices. However, their power conversion efficiencies (PCEs) are moderate compared with ZnO/PbS and TiO₂/PbS heterojunction CQD solar cells. Here, efficiencies have been improved substantially by employing solution‐processed CdS thin films from a single‐source precursor. The CdS film is deposited by a straightforward spin‐coating and annealing process, which is a simple, low‐cost, and high‐material‐usage fabrication process compared to chemical bath deposition and RF magnetron sputtering. The best CdS/PbS CQD heterojunction solar cell is fabricated using an optimized deposition and air‐annealing process achieved over 8% PCE, demonstrating the great potential of CdS thin films fabricated by the single‐source precursor for PbS CQDs solar cells.     

Ionic Additive Engineering Toward High‐Efficiency Perovskite Solar Cells with Reduced Grain Boundaries and Trap Density

Organic–inorganic hybrid perovskite solar cells have emerged as one of the promising photovoltaic candidates to generate renewable energy. However, the large amounts of grain boundaries and trap states that exist in the bulk or interfacial regions of perovskite films limit further enhancement of device efficiency. Herein, an additive engineering strategy is introduced employing trimethylammonium chloride in the methylammonium iodide precursor solution to prepare methylammonium lead iodide perovskite films with reduced grain boundaries and trap densities. This leads to an increased charge carrier diffusion coefficient and diffusion length, as evaluated by impedance and voltage decay measurements, intensity‐modulated photovoltage, and photocurrent spectroscopies. The proportion of nonradiative recombination processes is significantly reduced, consequently increasing device efficiency from 19.1% to 20.9% in these perovskite solar cells.     

Analysis of governing factors for photovoltaic loss mechanism of n‐CdS/p‐CdTe heterojunction via multi‐way data decomposition

An analytical framework for identifying key factors of the degradation of photovoltaic efficiency over time is presented. We demonstrate that, in many photovoltaic experimental settings, reliability data sets are easily cast in a multi‐ or N‐way format. We adopt a statistical technique, N‐way partial least squares, that generates a multi‐linear model using all of the data simultaneously. With this approach, we are able to model variables of interest such as cell efficiency while representing the data in a lower‐dimensional space in which salient features are more easily identified. We illustrate our approach with reliability data for CdS/CdTe heterojunction solar cell devices. Even with the inclusion of a noisy parameter such as the net acceptor density, and with a relatively small number of devices, we automatically identify key factors that are highly related to performance degradation. In particular, the conductance at the back contact is related to short stress‐time degradation (0–300 h), whereas the net acceptor density near the junction (at +0.08 V DC bias) is correlated with more gradual, long stress‐time degradation (300–1000 h). These notable degradation modes are explained with respect to our processing conditions and Cu‐diffusion in the cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Improving the photovoltaic performance of co‐evaporated Cu2ZnSnS4 thin‐film solar cells by incorporation of sodium from NaF layers

We have investigated the influence of sodium (Na) on the properties of co‐evaporated Cu₂ZnSnS₄ (CZTS) layer microstructures and solar cells. The photovoltaic performance and diode properties were improved by incorporating Na from NaF layers into the CZTS layers, while Na had a negligible effect on the microstructural properties of the layer. The best cell fabricated by using an optimal CZTS layer (Cu/(Zn + Sn) = 0.70, Zn/Sn = 1.8) yielded an active area efficiency of 5.23%. The analysis of device properties suggests that charge‐carrier recombination at CZTS/CdS interface is suppressed by intentional Na incorporation from NaF layers. Copyright © 2016 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.     

Influence of surface composition on back‐contact performance in CdTe/CdS PV devices

The atomic composition of the surface of the CdTe layer in a CdTe/CdS photovoltaic (PV) device has a significant influence on the quality of the electrical contact to this layer. This paper reports the results of a systematic study that correlates the composition of the back surface as measured with X‐ray photoelectron spectroscopy (XPS) with pre‐contact processing and device performance. We found that certain processing steps produce an oxide layer that degrades device performance by producing a metal – oxide – semiconductor (MOS) contact, rather than the intended metal – semiconductor, Schottky barrier contact. We also found that the as‐deposited CdTe film is cadmium‐rich for several hundred angstroms at the back surface. This n‐type layer may impede current flow for majority holes, degrading device performance. Published in 2000 by John Wiley & Sons, Ltd.     

Radiative efficiency of state‐of‐the‐art photovoltaic cells

Maximum possible photovoltaic performance is reached when solar cells are 100% radiatively efficient, with different photovoltaic technologies at different stages in their evolution towards this ideal. An external radiative efficiency is defined, which can be unambiguously determined from standard cell efficiency measurements. Comparisons between state‐of‐the‐art devices from the representative cell technologies produce some interesting conclusions. Copyright © 2011 John Wiley & Sons, Ltd.