Search

This contribution concerns the effect of the Ag content in wide‐gap Agw Cu1‐w In1‐x Gax Se2 (ACIGS) absorber films and its impact on solar cell performance. First‐principles calculations are conduc ...

International audience; We report on the development of highly reflective back contacts (RBCs) made of multilayer stacks for ultrathin CIGS solar cells. Two architectures are compared: they are made of a silver mirror coated either with a single layer of In2O3:Sn (ITO) or with a bilayer of ZnO:Al/ITO. Due to the improvement of CIGS rear reflectance, both back contacts result in a significant external quantum efficiency enhancement, in agreement with optical simulations. However, solar cells fabricated with Ag/ITO back contacts exhibit a strong shunting behavior. The key role of the ZnO:Al layer to control the morphology of the top ITO layer and to avoid silver diffusion through the back contact is highlighted. For a 500-nm-thick CIGS layer, this optimized RBC leads to a best cell with a short-circuit current of 27.8 mA/cm2 (+2.2 mA/cm2 as compared to a Mo back contact) and a 12.2%-efficiency (+2.5% absolute).

It is attractive to alloy Cu(In,Ga)Se2 solar-cell absorbers with Ag (ACIGSe), since they lead to similar device performances as the Ag-free absorber layers, while they can be synthesized at much lower deposition temperatures. However, a KF post-deposition treatment (PDT) of the ACIGSe absorber surface is necessary to achieve higher open-circuit voltages (Voc). The present work provides microscopic insights to the effects of this KF PDT, employing correlative scanning-electron microscope techniques on identical positions of cross-sectional specimens of the cell stacks. We found that the increase in Voc after the KF PDT can be explained by the removal of Cu-poor, Ag-poor, and Ga-rich regions near the ACIGSe/CdS interface. The KF PDT leads, when optimally doped, to a very thin K-Ag-Cu-Ga-In-Se layer between ACIGSe and CdS. If the KF dose is too large, we find that Cu-poor and K-rich regions form near the ACIGSe/CdS interface with enhanced nonradiative recombination which explains a decrease in the Voc. This effect occurs in addition to the presence of a (K,Ag,Cu)InSe2 intermediate layer, that might be responsible for limiting the short-current density of the solar cells due to a current blocking behavior. Title in Web of Science: Microscopic insight into the impact of the KF post-deposition treatment on optoelectronic properties of (Ag,Cu)(In,Ga)Se-2 solar cells

In this work, we evaluate the effect of NaF layers on the properties of Al2O3 and HfO2 rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells. The 6 nm thin passivation layers were ...

The effects of introducing a passivation layer at the rear of ultrathin Copper Indium Gallium di-Selenide Cu(In,Ga)Se2 (CIGS) solar cells is studied. Point contact structures have been created on 25 nm Al2O3 layer using e-beam lithography. Reference solar cells with ultrathin CIGS layers provide devices with average values of light to power conversion efficiency of 8.1% while for passivated cells values reached 9.5%. Electronic properties of passivated cells have been studied before, but the influence of growing the CIGS on Al2O3 with point contacts was still unknown from a structural and morphological point of view. Scanning Electron Microscopy, X-ray Diffraction and Raman spectroscopy measurements were performed. These measurements revealed no significant morphological or structural differences in the CIGS layer for the passivated samples compared with reference samples. These results are in agreement with the similar values of carrier density (~8 × 1016 cm-3) and depletion region (~160 nm) extracted using electrical measurements. A detailed comparison between both sample types in terms of current-voltage, external quantum efficiency and photoluminescence measurements show very different optoelectronic behaviour which is indicative of a successful passivation. SCAPS simulations are done to explain the observed results in view of passivation of the rear interface., P.M.P. Salomé acknowledges the funding of Fundação para a Ciência e a Tecnologia (FCT) through the project IF/00133/2015. The European Union's Horizon 2020 research and innovation programme ARCIGS-M project (grant agreement no. 720887) is acknowledged. J. M. V. Cunha acknowledges the funding of Fundação para a Ciência e a Tecnologia (FCT) through the project PD/BD/142780/2018. J. P. Teixeira and J. P. Leitão acknowledge the funding of FCT through the project UID/CTM/50025/2013.

Summary In this work, a trimetallic NiMoV catalyst is developed for the hydrogen evolution reaction and characterized with respect to structure, valence, and elemental distribution. The overpotential to drive a 10 mA cm−2 current density is lowered from 94 to 78 mV versus reversible hydrogen electrode by introducing V into NiMo. A scalable stand-alone system for solar-driven water splitting was examined for a laboratory-scale device with 1.6 cm2 photovoltaic (PV) module area to an up-scaled device with 100 cm2 area. The NiMoV cathodic catalyst is combined with a NiO anode in alkaline electrolyzer unit thermally connected to synthesized (Ag,Cu) (In,Ga)Se2 ((A)CIGS) PV modules. Performance of 3- and 4-cell interconnected PV modules, electrolyzer, and hydrogen production of the PV electrolyzer are examined between 25°C and 50°C. The PV-electrolysis device having a 4-cell (A)CIGS under 100 mW cm−2 illumination and NiMoV-NiO electrolyzer shows 9.1% maximum and 8.5% averaged efficiency for 100 h operation., Graphical Abstract, Highlights • A new catalyst NiMoV is reported for the hydrogen evolution reaction • A scalable thermally integrated PV-electrolyzer is designed for solar water splitting • Interconnected PV-electrolyzer modules provide STH efficiency between 8 and 11% • An upscaled CIGS-NiMoV-NiO device provides 8.5% STH for 100 h operation, Chemistry; Electrochemistry; Engineering; Materials Science; Energy Materials

International audience; We present a simulation strategy using ATLAS-2D to optimize the back-contact hole grid (i.e. size and pitch of openings) of the Al2O3-rear-passivation layer in ultra-thin CIGS photovoltaic cells. We first discuss and compare our simulation model with a series of experimental reference (i.e. non passivated) and passivated UT-CIGS cells to decouple the crucial parameters of the passivation. The simulation results follow the experimental trends both for the current in the dark and for the PV parameters under illumination, highlighting the beneficial effects of the passivation on the cell performances. Furthermore, it stresses the influence of the passivation quality at the Al2O3/CIGS interface and of the contact resistance at the Mo/CIGS interface within the openings. Further simulations quantify significant improvements in short-circuit current and open-circuit voltage for different sizes of openings in the Al2O3 layer, relative to an excellent passivation quality (i.e. high density of negative charges in the passivation layer). However, a degradation of performances is predicted for a poor passivation (i.e. low density of such charges) or a high contact resistance, when compared to reference cells. Consequently, we point out an optimum in efficiency when varying the opening widths at fixed hole pitch and fixed contact resistance. At equivalent contact resistance, simulations predict that the sizes of the pitch and openings can be increased without optimal performance losses when maintaining a width to pitch ratio around 0.2. This simulation trend has been confirmed by a series of experiments, indicating that it is crucial to care about the dimensions of the opening grid and the contact resistance of passivated cells apart from different material properties. These simulation results provide significant insights for optimal cell design and characterizations of passivated UT-CIGS PV cells.

We present a 2D model of an Al 2 O 3 -passivated ultrathin Cu(In, Ga)Se 2 photovoltaic cell with rear-contact pattern. Simulation results follow the experimental trends, highlighting the significant effects of the passivation quality and of the Mo/CIGS contact resistance. Improvements in Jsc and Voc are discussed for different sizes of openings, relative to an excellent passivation quality (i.e. high density of negative charges in the passivation layer). However, a degradation is predicted for a poor passivation (i.e. low density of such charges) or a high contact resistance. We point out an optimum in efficiency when varying the opening widths at fixed hole-pitch and fixed contact resistance for a width to pitch ratio around 0.2. These simulation results provide significant insights for optimal cell design and characterizations of passivated UT-CIGS PV cells.

Depth bandgap profiles via a [Ga]/([Ga]+[In]) variation in the Cu(In,Ga)Se2 (CIGS) absorber layer have been implemented as a strategy to enhance the performance of CIGS solar cells. Since the [Ga]/([Ga]+[In]) determines to a large extent the position of the conduction band minimum, different Ga-profiles lead to different electronic energy levels structures throughout the CIGS layer. In this paper, from the investigation of the dependence of the photoluminescence (PL) on excitation power and temperature, we critically analyse the impact of a notch or a linear Ga-profiles on the CIGS electronic energy levels structure and subsequent dominant recombination channels. Notwithstanding two radiative transitions involving fluctuating potentials were observed for each sample, significant differences in the luminescence resultant from the two Ga-profiles were identified. For the CIGS absorber with a notch Gaprofile, two tail-impurity radiative transitions involving equivalent donors cluster and the same deep acceptor level were ascribed to the near CIGS/CdS interface and the notch regions. The probability of radiative recombination in the two regions is discussed. For the CIGS absorber with a linear Ga-profile, two band-impurity radiative transitions involving an acceptor, with an ionization energy compatible with the VCu defect were ascribed to the region near the CIGS/CdS interface. Our results shows that the dominant acceptor defects are dependent on the Ga-profile and they also highlight the complexity of the radiative and non-radiative recombination channels revealed by a tight control of the parameters in the experiment. published

This paper focuses on the development of 2-terminal CIGS-perovskite tandem solar cells by exploring a range of stack sequences and synthetic procedures for depositing the associated layers. In the end, we converged at a stack sequence composed of SLG/Mo/CIGS/CdS/i-ZnO/ZnO:Al/NiO/PTAA/Perovskite/LiF/PCBM/SnO2/ITO. With this architecture, we reached performances only about 1% lower than the corresponding 4-terminal tandem cells, thus demonstrating functional interconnects between the two sub-cells while grown monolithically on top of each other. We go through the stack, layer-by-layer, discussing their deposition and the results, from which we can conclude what works, what does not work, and what potentially could work after additional modifications. The challenges for a successful 2-terminal tandem device include: how to deal with, or decrease, the surface roughness of the CIGS-stack, how to obtain uniform coverage of the layers between the CIGS and the perovskite while also obtaining a benign interface chemistry, and how to tune the band gaps of both the CIGS and the perovskite to obtain good optical matching. The investigation was based on CIGS with a power conversion efficiency around 14%, and perovskites with an efficiency around 12%, resulting in 2-terminal tandem cells with efficiencies of 15–16%. The results indicate that by using higher performing CIGS and perovskite sub-cells, it should be possible to manufacture highly efficient 2-terminal CIGS-perovskite tandem devices by using the protocols, principles, and procedures developed and discussed in this paper. (Less)

The Horizon 2020 project PECSYS aims to build a large area demonstrator for hydrogen production from solar energy via integrated photovoltaic (PV) and electrolysis systems of different types. In this study, Si- and CIGS-based photovoltaics are developed together with three different electrolyzer systems for use in the corresponding integrated devices. The systems are experimentally evaluated and a general model is developed to investigate the hydrogen yield under real climatic conditions for various thin film and silicon PV technologies and electrolyser combinations. PV characteristics using a Si heterojunction (SHJ), thin film CuInxGa1-xSe2, crystalline Si with passivated emitter rear totally diffused and thin film Si are used together with temperature dependent catalyst load curves from both acidic and alkaline approaches. Electrolysis data were collected from (i) a Pt-IrO2-based acidic electrolysis system, and (ii) NiMoW-NiO-based and (iii) Pt-Ni foam-based alkaline electrolysis systems. The calculations were performed for mid-European climate data from Julich, Germany, which will be the installation site. The best systems show an electricity-to-hydrogen conversion efficiency of 74% and over 12% solar-to-hydrogen (STH) efficiencies using both acidic and alkaline approaches and are validated with a smaller lab scale prototype. The results show that the lower power delivered by all the PV technologies under low irradiation is balanced by the lower demand for overpotentials for all the electrolysis approaches at these currents, with more or less retained STH efficiency over the full year if the catalyst area is the same as the PV area for the alkaline approach. The total yield of hydrogen, however, follows the irradiance, where a yearly hydrogen production of over 35 kg can be achieved for a 10 m(2) integrated PV-electrolysis system for several of the PV and electrolyser combinations that also allow a significant (100-fold) reduction in necessary electrolyser area for the acidic approach. Measuring the catalyst systems under intermittent and ramping conditions with different temperatures, a 5% lowering of the yearly hydrogen yield is extracted for some of the catalyst systems while the Pt-Ni foam-based alkaline system showed unaffected or even slightly increased yearly yield under the same conditions.

Amorphous tin-gallium oxide (a-SGO) grown with atomic layer deposition was evaluated as a buffer layer in (Ag,Cu)(In,Ga)Se2 thin-film solar cells in search for a new material that is compatible with a variety of absorber band gaps. Hard and soft X-ray photoelectron spectroscopy on absorber/a-SGO stacks combined with J–V characterization of solar cells that were fabricated, showed that the conduction band alignment at the absorber/a-SGO interface can be tuned by varying the cation composition and/or growth temperature. Here, the surface band gap was 1.1 eV for the absorber. However, optical band gap data for a-SGO indicate that a suitable conduction band alignment can most likely be achieved even for wider absorber band gaps relevant for tandem top cells. A best efficiency of 17.0% was achieved for (Ag,Cu)(In,Ga)Se2/a-SGO devices, compared to η = 18.6% for the best corresponding CdS reference. Lower fill factor and open-circuit voltage values were responsible for lower cell efficiencies. The reduced fill factor is explained by a larger series resistance, seemingly related to interface properties, which are yet to be optimized. Some layer constellations resulted in degradation in fill factor during light soaking as well. This may partly be explained by light-induced changes in the electrical properties of a-SGO, according to analysis of Al/SGO/n-Si metal-oxide-semiconductor capacitors that were fabricated and characterized with J–V and C–V. Moreover, the introduction of a 1 nm thick Ga2O3 interlayer between the absorber and a-SGO improved the open-circuit voltage, which further indicates that the absorber/a-SGO interface can be improved.

Hydrogen‐doped In2O3 (IOH) films are used as a transparent back contact in bifacial Cu(In,Ga)Se2 (CIGS) solar cells. The effect of the IOH thickness and the impact of the sodium incorporation techn ...

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se2(ACIGS) solar cells withbandgaps (Eg) > 1.40 eV is studied on a large sample set. It is conrmed thatmoving away in composition from ternary AgGaSe2by simultaneous reduction inGa and Ag content widens the chalcopyrite single-phase region and therebyreduces the amount of ordered vacancy compounds (OVCs). As a consequence, adistortion in currentvoltage characteristics, ascribed to OVCs at the back contact,can be successfully avoided. A clear anticorrelation between open-circuit voltage(VOC) and short-circuit current density (JSC) is detected with varying absorberstoichiometry, showing decreasingVOCand increasingJSCvalues for [I]/[III] > 0.9.Capacitance proling reveals that the absorber doping gradually decreases towardstoichiometric composition, eventually leading to complete depletion. It isobserved that only such fully depleted samples exhibit perfect carrier collection,evidencing a very low diffusion length in wide-gap ACIGS lms. The results indicatethat OVCs at the surface play a minor or passive role for device performance.Finally, a solar cell withVOC¼ 0.916 V atEg¼ 1.46 eV is measured, which is, to thebest of our knowledge, the highest value reported for this bandgap to date.

Perovskite solar cells, as a newly emerged solar energy conversion technology, have attracted tremendous attention in the solar cell community by demonstrating impressive enhancement in power conversion efficiencies. However, the high temperature and manually processed TiO2 underlayer prepared by spray pyrolysis has significantly limited the large-scale application and device reproducibility of the perovskite solar cells. Here, we for the first time utilize low-temperature atomic layer deposition (ALD) to prepare a compact Al2O3 underlayer for perovskite solar cells. The thickness of the Al2O3 layer can be controlled well by adjusting the deposition cycles during the ALD process. An optimal Al2O3 layer can effectively block the electron recombination at the perovskite/FTO interface as well as sufficiently transport the electrons via tunneling. Perovskite solar cells fabricated with an Al2O3 layer demonstrated the highest efficiency of 16.2% for 50 cycles of ALD (~5 nm), which is a significant improvement over the underlayer free PSCs which have an efficiency of 11.0%. Detailed characterization confirms that the thickness of the Al2O3 underlayer significantly influences the charge transfer resistance and electron recombination process in the devices. Furthermore, this work successfully shows the feasibility of using a high-bandgap semiconductor like Al2O3 as the underlayer in perovskite solar cells and opens up pathways to use ALD Al2O3 underlayers for flexible solar cells.

This contribution studies the impact of the KF-induced Cu(In,Ga)Se2 (CIGSe) absorber modification on the suitability of different transparent conductive oxide (TCO) layers in solar cells. The TCO m ...

Tandem solar cell structures require a high-performance wide band gap absorber as top cell. A possible candidate is CuGaSe2, with a fundamental band gap of 1.7 eV. However, a significant open-circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe2. In this paper, we show that the open-circuit voltage can be drastically improved in wide band gap p-Cu(In,Ga)Se2 and p-CuGaSe2 devices by improving the conduction band alignment to the n-type buffer layer. This is accomplished by using Zn1−xSnxOy, grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga)Se2 and CuGaSe2 materials. With an improved buffer band alignment for CuGaSe2 absorbers, evaporated in a 3-stage type process, we show devices exhibiting open-circuit voltages up to 1017 mV, and efficiencies up to 11.9%. This is to the best of our knowledge the highest reported open-circuit voltage and efficiency for a CuGaSe2 device. Temperature-dependent current-voltage measurements show that the high open-circuit voltage is explained by reduced interface recombination, which makes it possible to separate the influence of absorber quality from interface recombination in future studies.

Reverse breakdown is investigated in multiple Cu(In,Ga)Se2 solar cells with varying buffer layer thicknesses. A method to extract transition voltage, which marks the change of conduction mechanism that leads to electrical breakdown, is described as an alternative to the often less-meaningful breakdown voltage. Transition voltages for samples with CdS and Zn x Sn1- x O y buffers are extracted from breakdown measurements performed in darkness and under illumination. The electric field is calculated for ZTO-based samples measured in darkness, and its implications for the energy band structure are examined. Fowler–Nordheim tunneling and Poole–Frenkel conduction are considered as candidates for the main breakdown mechanism in darkness. A model combining the two conduction mechanisms is proposed, and fits for experimental data are presented and discussed. Involvement of defects is debated, and defect-and-breakdown-related phenomena are showcased.

Light-on-bias effects were investigated in multiple Cu(In, Ga)Se2 solar cells with varying absorber layer compositions. A strong link between deformations caused by red-on-bias treatments in current–voltage ( IV ) and capacitance–voltage ( CV ) characteristics was demonstrated. Similarly to red-on-bias, blue-on-bias leads to a local increase in static negative charge, but in samples with CdS buffers this increase is shifted away from the interface and has no impact on device performance. IV characteristics of samples with Cd-free buffers are not affected by any light-on-bias treatments, suggesting that CdS plays a vital role in the decreased performance after red-on-bias. A statistical approach was used to search for compositional trends in red-on-bias behavior. Deformation factors were defined for IV and CV characteristics before and after the treatment. While there is a strong relationship between the deformations observed in both types of measurements, the degree to which red-on-bias affects IV and CV curves can vary dramatically. These variations cannot be attributed to changes in composition, since no clear compositional trends were found. Rather, other factors related to sample manufacturing and to the buffer layer seem to have major impact on red-on-bias behavior.

This study reports on the beneficial effect of an absorber surface passivation by Al2O3 on the performance of Cu(In, Ga)Se-2 (CIGSe) solar cells. Here the Al2O3 layer is deposited by atomic layer d ...

In this study thin films of hydrogenated In2O3 (IOH) were fabricated by physical vapor deposition (PVD) with and without a post-annealing step, and by atomic layer deposition (ALD). The electro-opt ...

A novel architecture that comprises rear interface passivation and increased rear optical reflection is presented with the following advantages: i) an enhanced optical reflection is achieved by depositing a metallic layer over the Mo rear contact; ii) the addition of a sputtered Al2O3 layer improves the interface quality with CIGS; and, iii) the rear-openings are refilled with Mo to maintain the optimal ohmic electrical contact as generally observed from the growth of CIGS on Mo. Hence, a decoupling between the electrical function and the optical function of the substrate is achieved. We present in detail the manufacturing procedure of such type of architectures together with its benefits and caveats. A preliminary electrical analysis of resulting solar cells showing a proof-of-concept of the architecture is presented and discussed. published

Here, we specifically address device performance in ultra-thin CIGS (UT UT-CIGS) films with thickness around 500 nm by systematically implementing varying in in-depth grading of the GGI (Ga/(Ga+In)ratio). By adjusting the GGI slope, the open circuit voltage can be significantly improved, indicating a reduction of recombination in the quasiquasi-neutral region and at the back contact; the photocarrier collection efficiency over the whole absorption spectrum enhanced significantly with an aggressive GGI profile. Ultimately, a power conversion efficiency of UT-CIGS device over 12% with thickness around 500 nm by carefully applying a an appropriate GGI profile was demonstrated.

A wide range of applications benefit from transparent semiconducting oxides with tunable electronic properties, for example, electron transport layers in solar cell devices, where the electron affinity is a key parameter. Presently, a few different ternary oxides are used for this purpose, but the attainable electron affinity range is typically limited. In this study, the authors develop a low-temperature atomic layer deposition (ALD) process to grow amorphous Sn1-xGaxOy thin films from dimethylamino-metal complexes and water. This oxide is predicted to provide a wide selection of possible electron affinity values, from around 3 eV for pure Ga2O3 to 4.5 eV for pure SnO2. The ALD process is evaluated for deposition temperatures in the range of 105-195 degrees C by in situ quartz crystal microbalance and with ex situ film characterization. The growth exhibits an ideal-like behavior at 175 degrees C, where the film composition can be predicted by a simple rule of mixture. Depending on film composition, the growth per cycle varies in the range of 0.6-0.8 angstrom at this temperature. Furthermore, the film composition for a given process appears insensitive to the deposition temperature. From material characterization, it is shown that the deposited films are highly resistive, fully amorphous, and homogeneous, with moderate levels of impurities (carbon, nitrogen, and hydrogen). By tailoring the metal cation ratio in films grown at 175 degrees C, the optical bandgap can be varied in the range from 2.7 eV for SnO2 to above 4.2 eV for Ga2O3. The bandgap also varies significantly as a function of deposition temperature. This control of properties indicates that Sn1-xGaxOy is a promising candidate for an electron transport layer material in a wide electron affinity range. Published by the AVS.

Currently, one of the main limitations in ultrathin Cu(In,Ga)Se2 (CIGS) solar cells are the optical losses, since the absorber layer is thinner than the light optical path. Hence, light management, including rear optical reflection and light trapping is needed. In this work we focus on increasing the rear optical reflection. For this, a novel structure based on having a metal interlayer in between the Mo rear contact and the rear passivation layer is presented. In total, eight different metallic interlayers are compared. For the whole series, the passivation layer is aluminum oxide (Al2O3). The interlayers are used to enhance the reflectivity of the rear contact and thereby increasing the amount of light reflected back into the absorber. In order to understand the effects of the interlayer in the solar cell performance both from optical and/or electrical point of view, optical simulations were performed together with fabrication and electrical measurements. Optical simulations results are compared with current density-voltage (J-V) behavior and external quantum efficiency (EQE) measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising rear reflective layer since it provides solar cells with light to power conversion efficiency values of 9.9 %, which is 2.2 % (abs) higher than the passivated ultrathin sample and 3.7 % (abs) higher than the unpassivated ultrathin reference sample. published

In ultra-thin chalcopyrite solar cells and photovoltaic modules, efficient light management is required to increase the photocurrent and to gain in conversion efficiency. In this work we employ optical modelling to investigate different optical approaches and quantify their potential improvements in the short-circuit current density of Cu(In, Ga)Se2 (CIGS) devices. For structures with an ultra-thin (500 nm) CIGS absorber, we study the improvements related to the introduction of (i) highly reflective metal back reflectors, (ii) internal nano-textures applied to the substrate and (iii) external micro-textures by using a light management foil. In the analysis we use CIGS devices in a PV module configuration, thus, solar cell structure including encapsulation and front glass. A thin Al2O3 layer was considered in the structure at the rear side of CIGS for passivation and diffusion barrier for metal reflectors. We show that not any individual aforementioned approach is sufficient to compensate for the short circuit drop related to ultra-thin absorber, but a combination of a highly reflective back contact and textures (internal or external) is needed to obtain and also exceed the short-circuit current density of a thick (1800 nm) CIGS absorber.

Solar-driven water splitting is considered one of the promising future routes to generate fuel in a sustainable way. A carbon-free solar fuel, molecular hydrogen, can here be produced along two different but intimately related routes, photoelectrochemical (PEC) water splitting or photovoltaic electrolysis (PV-electrolysis), where the latter builds on well-established solar cell and electrolysis materials with high efficiency. The PV-electrolysis approach is also possible to construct from an integrated PEC/PV-system avoiding dc&ndash, dc converters and enabling heat exchange between the PV and electrolyzer part, to a conventionally wired PV-electrolysis system. In either case, the operating voltage at a certain current needs to be matched with the catalyst system in the electrolysis part. Here, we investigate ((Ag),Cu)(In,Ga)Se2 ((A)CIGS)-materials with varying Ga-content modules for combination with NiMo&ndash, NiO catalysts in alkaline water splitting. The use of (A)CIGS is attractive because of the low cost-to-performance ratio and the possibility to optimize the performance of the system by tuning the band gap of (A)CIGS in contrast to Si technology. The band gap tuning is possible by changing the Ga/(Ga + In) ratio. Optoelectronic properties of the (A)CIGS materials with Ga/(Ga + In) ratios between 0.23 and 0.47 and the voltage and power output from the resulting water splitting modules are reported. Electrolysis is quantified at temperatures between 25 and 60 &deg, C, an interval obtainable by varying the thermal heat exchange form a 1-sun illuminated PV module and an electrolyte system. The band gaps of the (A)CIGS thin films were between 1.08 to 1.25 eV and the three-cell module power conversion efficiencies (PCE) ranged from 16.44% with 1.08 eV band gap and 19.04% with 1.17 eV band gap. The highest solar-to-hydrogen (STH) efficiency was 13.33% for the (A)CIGS&ndash, NiMo&ndash, NiO system with 17.97% module efficiency and electrolysis at 60 &deg, C compared to a STH efficiency of 12.98% at 25 &deg, C. The increase in STH efficiency with increasing temperature was more notable for lower band gaps as these are closer to the overpotential threshold for performing efficient solar-driven catalysis, while only a modest improvement can be obtained by utilizing thermal exchange for a band gap matched PV-catalysts system. The results show that usage of cost-effective and stable thin film PV materials and earth abundant catalysts can provide STH efficiencies beyond 13% even with PV modules with modest efficiency.

This contribution presents the development of atomic layer deposited (ALD) In2O3 films for utilization as transparent conductive oxide (TCO) layers in Cu(In,Ga)Se2 (CIGSe) solar cells. The effects of ALD process parameters on the morphology and growth of In2O3 are studied and related to the electrical and optical properties of the films. Maintaining similar resistivity values compared to commonly used ZnO:Al (AZO) TCOs (ρ = (5–7) × 10−4 Ωcm), a superior mobility of μ ≈ 110 cm2/Vs could be achieved (more than five times higher than a ZnO:Al reference), which results in a significantly reduced parasitic optical absorption in the infrared region. Application of the optimized In2O3 layers in CIGSe solar cells with varying buffer layers (CdS and Zn1–xSnxOy (ZTO)) leads to a distinct improvement in short circuit current density Jsc in both cases. While for solar cells containing the ZTO/In2O3 window structure, a drop in open-circuit voltage Voc and a deterioration under illumination is observed, the TCO exchange (from AZO to In2O3) on CdS buffer layers results in an increase in Voc without detectable light bias degradation. The efficiency η of the best corresponding solar cells could be improved by about 1% absolute.

Thin film solar cells based in Cu(In,Ga)Se-2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%. P.M.P.S. acknowledges the funding of Fundacao para Ciencia e Tecnologia (FCT) through the project IF/00133/2015. B.V. has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 715027). CAPES (CAPES-INL 04/14), CNPq, and FAPEMIG funding agencies are acknowledged for financial support. The European Union's Horizon 2020 research and innovation programme (grant agreement no. 720887) ARCIGS-M project is acknowledged. M.J.M. acknowledges funding from FCT through the grant SFRH/BPD/115566/2016. This project was partially supported by FEDER funds, through the COMPETE 2020 Program, and national funds, through the FCT, under the projects POCI-01-0145-FEDER-007688 (Reference UID/CTM/50025) and ALTALUZ (Reference PTDC/CTM-ENE/5125/2014). The authors also acknowledge partial funding from the European Project BET-EU (H2020-TWINN-2015, grant 692373). The NanoFabrication department at INL, namely Joao Gaspar and Helder Fonseca, are recognized for the help in the development of the nanopatterning process.

Photo-driven catalytic (PDC) water splitting, using either photoelectrochemical cells (PEC-cells), PV-electrolyzers, or some hybrid system in-between, has attracted a lot of attention. In single-cell device architectures for solar hydrogen production, based on single band gap photoabsorbers, there is a fundamental efficiency problem originating from the energy distribution of the solar spectrum and the thermodynamic and kinetic requirements for splitting water. The minimum band gap for a single-junction device in order to withhold unbiased overall water splitting is considered to be at least 2.0 eV. This is far from the 1.35 eV which is the optimal band gap of a semiconductor for maximum power conversion of light in the solar spectrum. This discrepancy has been termed as the solar spectrum mismatch problem (the SSM-problem). The standard solution to this problem is to construct tandem devices, whereas an alternative is to interconnect several one band gap cells in series, side by side. Both approaches enable the use of low energy photons in the solar spectrum while still providing a sufficiently high photopotential for driving the full reaction, without seriously compromising with the area efficiency. In this paper, the tandem and serial architectures for handling the SSM-problem are analyzed and compared. The analysis is focused towards differences in the limits of optical absorption, the optimal number of optical absorbers, and their corresponding band gaps. Taking losses due to charge carrier separation and catalysis into account, the maximum STH-efficiency for a series interconnected solar splitting device was found to be 24.6%, compared to 32.0% for an optimum tandem device at 1 Sun (air mass 1.5, 1000 W m−2). This can be compared with the maximum efficiency of 18.0% for an ideal single band gap photoabsorber in single junction device. The analysis shows that the maximum STH efficiency limits for series interconnected architectures for unassisted solar water splitting are not particularly far behind the more commonly studied tandem devices. They could then be an interesting alternative given the simplicity and versatility of series interconnected device architectures. The analysis also compares how tandem devices and series interconnected devices can differ in terms of charge carrier separation, charge carrier transport, catalysis, overall efficiency, device architecture, and expected cost.

An optimized test structure to study rear surface passivation in Cu(In,Ga)Se-2 (CIGS) solar cells by means of photoluminescence (PL) is developed and tested. The structure - illustrated in the abst ...

The benefits of gallium (Ga) grading on Cu(In,Ga)Se2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell’s open-circuit voltage Voc. The parameter most constraining the cell’s Voc is found to be the deep-defect density close to the space charge region (SCR). In ungraded devices, high deep-defect concentrations (4.2 × 1016 cm–3) were observed near the SCR, offering a source for Shockley–Read–Hall recombination, reducing the cell’s Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 × 1015 cm–3) for back surface graded devices, and almost two orders of magnitude (8.6 × 1014 cm–3) for double surface graded devices, enhancing the cell’s Voc. In compositionally graded devices, the free-carrier density in the absorber’s bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band.

Bi-facial solar cells with ultrathin CIGS solar cells are fabricated to investigate the influence of back contact passivation. Solar cells with CIGS thicknesses of 300 and 500 nm and with an ultrathin transparent Mo layer are characterized using EQE measurements from both the front and the rear side as well as with I-V measurements. Back contact passivation consisting of Al 2 O 3 deposited by atomic layer deposition and nano-sized point contact openings is used. The results are compared to cells with only the transparent Mo layer as back contact. We find a significant effect of the passivation manifested as an increase in the current density of the solar cells with the passivation.

Reducing absorber layer thickness below 500nm in regular Cu(In,Ga)Se-2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells. (c) 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd. B. Vermang acknowledges the financial support of the European Commission via FP7 Marie Curie IEF 2011 Action No. 300998. Furthermore, this work is partly funded by the Swedish Science Foundation (VR) and the Swedish Energy Agency. Lastly, F. Henry would like to thank the European and Wallonia Region FEDER grant ECP12020011678F (MINATIS Project) for financial support.

Until this day, the most efficient Cu(In,Ga)Se2 thin film solar cells have been prepared using a rather complex growth process often referred to as three-stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu-rich, where-after the film is converted to its final Cu-poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three-stage process and a one-stage/in-line process with the same composition, thickness, and solar cell stack is made. The one-stage process is easier to be used in an industrial scale and do not have Cu-rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X-ray diffraction, current–voltage-temperature, capacitance-voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J–V behavior, quantum spectral response, and the estimated recombination losses. Copyright © 2014 John Wiley & Sons, Ltd.

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (VOC) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al2O3 is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al2O3 rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in VOC compared with unpassivated reference cells.

Materials and device concepts for renewable solar hydrogen production, and size dependent properties of ZnO quantum dots are the two main themes of this thesis.ZnO particles with diameters less than 10 nm, which are small enough for electronic quantum confinement, were synthesized by hydrolysis in alkaline zinc acetate solutions. Properties investigated include: the band gap - particle size relation, phonon quantum confinement, visible and UV-fluorescence as well as photocatalytic performance. In order to determine the absolute energetic position of the band edges and the position of trap levels involved in the visible fluorescence, methods based on combining linear sweep voltammetry and optical measurements were developed.The large band gap of ZnO prevents absorption of visible light, and in order to construct devices capable of utilizing a larger part of the solar spectrum, other materials were also investigated, like hematite , Fe2O3, and CIGS, CuIn1-xGaxSe2.The optical properties of hematite were investigated as a function of film thickness on films deposited by ALD. For films thinner than 20 nm, a blue shift was observed for both the absorption maximum, the indirect band gap as well as for the direct transitions. The probability for the indirect transition decreased substantially for thinner films due to a suppressed photon/phonon coupling. These effects decrease the visible absorption for films thin enough for effective charge transport in photocatalytic applications.CIGS was demonstrated to be a highly interesting material for solar hydrogen production. CIGS based photocathodes demonstrated high photocurrents for the hydrogen evolution half reaction. The electrode stability was problematic, but was solved by introducing a modular approach based on spatial separation of the basic functionalities in the device. To construct devices capable of driving the full reaction, the possibility to use cells interconnected in series as an alternative to tandem devices were investigated. A stable, monolithic device based on three CIGS cells interconnected in series, reaching beyond 10 % STH-efficiency, was finally demonstrated. With experimental support from the CIGS-devices, the entire process of solar hydrogen production was reviewed with respect to the underlying physical processes, with special focus on the similarities and differences between various device concepts.

This study investigates the interplay between the absorber layer of Cu(In,Ga)Se2 solar cells and the other layers of these devices. Cu(In,Ga)Se2 devices with absorbers of different thicknesses and different buffer layers are fabricated. Absorber layers and finished devices are characterized. Good efficiencies are obtained, also for devices of substandard thickness down to 0.3 μm. Best open-circuit voltages and fill factors are found for cells with half the standard absorber thickness, but the highest efficiencies are found for cells with the standard thickness of 1.6 μm due to their higher short-circuit current density. Cu(In,Ga)Se2 cells with Zn(O,S) buffer layers are more efficient than CdS reference devices for the same absorber thickness due to a higher short-circuit current. For cells with thin absorber layers, a part of the higher current is caused by higher quantum efficiency at long wavelengths. Electrical simulations indicate that the loss in the open-circuit voltage for the thinnest devices is due to recombination in the back contact region. The difference in long-wavelength quantum efficiency between the buffer layers is attributed to a difference in the CIGS band bending. Acceptors at the Cu(In,Ga)Se2-CdS interface are proposed as an explanation for this difference. A low-quality back contact region enhances the effect.

For the first time, a novel rear contacting structure for copper indium gallium (di)selenide (CIGS) thin film solar cells is discussed theoretically, developed in an industrially viable way, and demonstrated in tangible devices. The proposed cell design reduces back contacting area by combining a rear surface passivation layer and nano-sized local point contacts. Atomic layer deposition (ALD) of Al2O3 is used to passivate the CIGS surface and the formation of nano-sphere shaped precipitates in chemical bath deposition (CBD) of CdS to generate point contact openings. The Al2O3 rear surface passivated CIGS solar cells with nano-sized local rear point contacts show a significant improvement in open circuit voltage (Voc) compared to unpassivated reference cells. Comparing the passivated devices to solar cell capacitance simulator (SCAPS) modeling indicates that this increase is attributed to a decrease in rear surface recombination of a few orders. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved. B. Vermang acknowledges the financial support of the European Commission via FP7 Marie Curie IEF 2011 Action no. 300998. Furthermore, this work is partly funded by the Swedish Science Foundation (VR) and the Swedish Energy Agency. Finally, the authors would like to thank Timo Wätjen for the TEM picture, used as Graphical abstract.

The use of Na-free or low Na content glass substrates is observed to enhance the resiliency to potential-induced degradation, as compared with glass substrates with high Na content, such as soda lime glass (SLG). The results from stress tests in this study suggest that degradation caused by a combination of heat and bias across the SLG substrate is linked to increased Na concentration in the CdS and Cu(In,Ga)Se2 (CIGS) layers in CIGS-based solar cells. The degradation during the bias stress is dramatic. The efficiency drops to close to 0% after 50 h of stressing. On the other hand, cells on Na-free and low Na content substrates exhibited virtually no efficiency degradation. The degraded cells showed partial recovery by resting at room temperature without bias; thus, the degradation is nonpermanent and may be due to Na migration and accumulation rather than chemical reaction.

Partial shading of solar modules can subject shaded cells to significant reverse bias, often large enough toforce them into electrical breakdown, possibly resulting in irreversible damage. Therefor ...

The sun provides us with a surplus of energy convertible to electricity using solar cells. This thesis focuses on solar cells based on chalcopyrite (CIGSe) as well as kesterite (CZTS(e)) absorber layers. These materials yield record efficiencies of 20.4 % and 11.1 %, respectively. Especially for CZTS(e), the absorber layers often do not consist of one single desired phase but can exhibit areas with deviating material properties, referred to as secondary phases. Furthermore, several material layers are required for a working solar cell, each exhibiting interfaces. Even though secondary phases and interfaces represent a very small fraction of the solar cell they can have a profound influence on the over-all electrical solar cell characteristics. As such, it is crucial to understand how secondary phases and interfaces influence the local electrical characteristics.Characterising secondary phases and interfaces is challenging due to their small sample volume and relatively small differences in composition amongst others. This is where electronmicroscopy, especially transmission electron microscopy, offers valuable insight to material properties on the microscopic scale. The main challenge is, however, to link these material properties to the corresponding electrical characteristics of a solar cell.This thesis uses electron beam induced current imaging and introduces a new method for JV characterisation of solar cells on the micron scale. Combining microscopic structural and electrical characterisation techniques allowed identifying and characterising local defects found in the absorber layer of CIGS solar cells after thermal treatment. Furthermore, CZTSe solar cells in this thesis exhibited a low photo-current density which is traced to the formation of a current blocking ZnSe secondary phase at the front contact interface. The electron microscopy work has contributed to an understanding of the chemical stability of CZTS and has shown the need for an optimised back contact interface in order to avoid chemical decomposition reactions and formation of detrimental secondary phases. With this additional knowledge, a comprehensive picture of the material properties from the macroscopic down to the microscopic level can be attained throughout all required material layers.

Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell material Cu(2)ZnSn(S,Se)(4) (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS(2) and secondary phases at the CZTS|Mo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.

Laser micro-welding is a technique that can be used for interconnective (P2) patterning of Cu ( In , Ga ) Se 2 (CIGS) thin film solar cells after deposition of the ZnO:Al window layer. The micro-welding process transforms CIGS into a conductive compound that forms a contact between the ZnO:Al front contact and the Mo back contact. In this work we present a study on the influence of process parameters on the interconnectivity of the laser micro-weld as well as an investigation of the morphology of the micro-weld zone. We found that the current method results in an overlap of conditions where electrical connection is achieved and where Mo/glass damage is caused by laser action. Stability testing shows that interconnections processed without appreciable damage are stable. The connecting region contained metal rich zones and segregated CuxSe that we believe account for its conductivity.

We present a monitoring system for the field test of photovoltaic modules. The system is designed for the monitoring of individual modules under maximum power point (MPP) conditions that allow the extraction of the energy yield of different modules under optimum conditions. It can also be used to monitor the performance and long-term stability of modules under realistic field conditions. The monitoring system consists of an individual MPP tracker attached to each module under test. The measurement data are transmitted to a central multichannel data logger by means of analog voltages proportional to the current at MPP, i.e., Imp, and voltage at MPP, i.e., Vmp, of the modules.

A deeper understanding of Cu(In,Ga)Se2 (CIGS) solar cells is important for the further improvement of these devices. This thesis is focused on the use of electrical modelling as a tool for pursuing this aim. Finished devices and individual layers are characterized and the acquired data are used as input in the simulations. Band gap gradients are accounted for when modelling the devices. The thesis is divided into two main parts. One part that treats the influence of cadmium free buffer layers, mainly atomic layer deposited (Zn,Mg)O, on devices and another part in which the result of CIGS absorber layer modifications is studied. Recombination analysis indicates that interface recombination is limitting the open circuit voltage (Voc) in cells with ZnO buffer layers. This recombination path becomes less important when magnesium is introduced into the ZnO giving a positive conduction band offset (CBO) towards the CIGS absorber layer. Light induced persistent photoconductivity (PPC) is demonstrated in (Zn,Mg)O thin films. Device modelling shows that the measured PPC, coupled with a high density of acceptors in the buffer-absorber interface region, can explain light induced metastable efficiency improvement in CIGS solar cells with (Zn,Mg)O buffer layers. It is shown that a thin indium rich layer closest to the buffer does not give any significant impact on the performance of devices dominated by recombination in the CIGS layer. In our cells with CdS buffer the diffusion length in the CIGS layer is the main limitting factor. A thinner CIGS layer improves Voc by reducing recombination. However, for thin enough absorber layers Voc deteriorates due to recombination at the back contact. Interface recombination is a problem in thin devices with Zn(O,S) buffer layers. This recombination path is overshadowed in cells of standard thickness by recombination in the CIGS bulk. Thin cells with Zn(O,S) buffer layers have a higher efficiency than CdS cells with the same absorber thickness.

Photoconductivity and photocapacitance of Cu(In,Ga)Se2 and CuGaSe2 thin films and devices induced by sub-bandgap illumination are investigated. Both effects have been attributed to the optical transition from valence band to the same empty levels situated around 0.8–0.9 eV above the valence band. The influence of the metastable states created by illumination and voltage bias on the sub-bandgap response has been studied. The experimental results are discussed in the framework of a model based on negative-U property of a native defect in chalcopyrites, i.e. V Se –V Cu divacancy. The arguments are presented that the levels involved in the optical transition observed in photoconductivity and photocapacitance might be antibonding levels of the acceptor configuration of this defect.

A baseline parameter set for electrical modelling of Cu(In,Ga)Se2 solar cells with compositionally graded absorber and CdS buffer layer is established. The cases with and without Fermi level pinnin ...

Laser patterning of thin film solar cells has proven technically feasible for all layers but still remains a challengingtopic for research and development. We present a method where P2 laser pattern ...

We investigate Cu(In,Ga)Se2 thin films grown in multi-stage coevaporation processes and solar cells fabricated from such absorbers. Despite some interdiffusion during film growth, Ga/(Ga+In) gradients defined via evaporation-profile variations in the process are to a good part retained in the finished film. This indicates that the bandgap can be engineered in this type of process by varying the evaporation profiles, and consequently also that unintended profile variations should be noted and avoided. With front-side gradients the topmost part of many grains seems to be affected by a higher density of lattice defects due to the strong change of gallium content under copper-poor growth conditions. Electrically, both back-side gradients and moderate front-side gradients are shown to yield an improvement of device efficiency. If a front-side gradient is too wide, though, it causes strong voltage-dependent collection and the fill factor is severely reduced.