Suchergebnisse

On the way to achieving high efficiencies for CuIn0.7Ga0.3Se2 (CIGS) thin film solar cells, alkali doping has become a necessary step. However, it is expected that a migration of the various alkali atoms over the lifetime of the cell will lead to efficiency losses [1]. Logically, ensuring the reliability of a solar cell material is a paramount step towards the commercialization of a product. Indeed, it is necessary for companies to able to guarantee the efficiencies of the "as-sold" panels over periods of time that expand well over 20 or even 25 years. Experiments studying the reliability of the CIGS solar cells and their different components have already been performed in the past [1-4], nevertheless, the exact effects of alkali, and more generally, the precise reasons for, and effect of, the degradation, are still widely unknown. In order to evaluate the viability of some of the most common techniques used to produce highly efficient thin-film solar cells, ultra-thin coevaporated CIGS solar cell absorber material is produced and doped with various alkali atoms (Na, K, .). The absorber material is deposited in ultra-thin (400-500nm) layers on soda-lime glass (SLG) equipped with a Si(O,N) diffusion barrier. By reducing the amount of bulk material, the aim is to reduce the concentration of defects in the material and limit their effects as much as possible. The barrier prevents the diffusion of alkali atoms from the SLG into the CIGS and allows for a better control of the concentration of alkali atoms in the layer. Additionally, it allows for a better isolation of the effects of the different alkalis by preventing an uncontrolled mix of alkali from being present in the absorber material. All alkalis are deposited in their fluoride form (NaF, KF, etc.) either by evaporation, prior to CIGS deposition, or by a post deposition treatment. We submitted the produced cells to highly degrading conditions in a damp heat environment. This aggressive setting is expected to simulate standard outdoor condition equivalent to 20 years in an experimentally reasonable timespan of a 1000 hours. We monitored the effects of the degradation by regular current-voltage (IV) measurements all along the 1000h experiment. These measurements showed a clear, and alkali concentration dependent, degradation of the solar cells as time increases. Using Atom Probe Tomography (APT) on degraded as well as on undegraded (reference) cells, it was possible to show that in the absence of a potential, the alkali atom migration is minimal, and that K can be mostly found in the grain boundary region of the absorber material. Given the lack of the expected alkali migration, the main reason for the degradation of the cells seems to be the presence of water that seeps into the grain boundaries of the solar cell material, which could also be observed using APT. The possibility to reverse that degradation mechanisms involving water inclusion is currently under investigation. ; This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027

Producing the green energy of tomorrow will require highly efficient as well as energy -, and cost-effective solar cells in addition to having reasonable lifetimes To determine if CIGS can be made to submit to these constraints, we produced ultra thin (500nm ) single stage coevaporated CIGS solar cells. We doped these cells with varying amounts and t ypes of a lkali atoms a nd submitted them to accelerated lifetime testing Results showed definite effect of the alkali concentration on the degradation of the cells but showed limited migration. Instead, the seeping of water into the grain boundaries was identified as the main culprit for performance degradation. ; This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027

Reducing the thickness of Cu(In,Ga)Se2 (CIGS) absorber layers has potential to decrease its cost significantly, but has drawbacks like incomplete absorption and increased back-contact recombination, both resulting in power conversion efficiency losses. One solution is to implement a rear surface passivation layer, which has potential to reduce rear surface recombination velocity and increase rear internal reflection [1]. Alumina (Al2O3) is such an ideal passivation layer, but unfortunately also acts as an electron and diffusion barrier layer, and thus prevents current flow and sodium (Na) diffusion. As is discussed below, our novel approach to generate point contact openings in this passivation layer overcomes both problems at once. The proposed method is to use sodium fluoride (NaF) on top of the Al2O3 passivation layer, which will generate contact openings during selenization. This applied approach is industrially viable, as compared to proven methods – e.g. using nanoparticles, e-beam or nano-imprint lithography – which are too expensive, time-consuming or not applicable for larger areas. D. Ledinek et al. previously proposed using a very thin layer of Al2O3 rear surface passivation in combination with NaF deposition to enhance the electrical characteristics of CIGS solar cells. In their study this surface passivation is claimed to allow tunneling [2]. In our study, we prove that point contacts have been generated in this Al2O3 surface passivation layer. Atomic layer deposition (ALD) was used to deposit very thin Al2O3 layers on molybdenum (Mo) rear contact, and NaF was deposited on these layers by spin coating. Thereafter CIGS layers (±500 nm thick) were grown by single stage co-evaporation at 550°C, followed by standard solar cell process described in [3]. To detect the contact openings in the passivation layer, scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy was applied. Glass/Mo/Al2O3/NaF characterization samples were used for this analysis, where the co-evaporation process was mimicked by using a selenization step. After the selenization, point openings in the thin Al2O3 passivation layers were determined by SEM, and supported by EDX measurement. To investigate the impact of Al2O3 thickness on passivation of the CIGS rear surface, time resolved photo-luminescence (TR-PL) measurements on finished absorber layers (capped with CdS) were used. According to these TR-PL results, a 6 nm thick passivation layer gave the highest free charge carrier lifetime and PL response in all sets of samples. This was confirmed in full solar cell devices, where a significant increase in power conversion efficiency, and gain in open-circuit voltage, current density and fill-factor values was measured for the 6 nm Al2O3 rear passivated CIGS cells, as compared to unpassivated reference cells. Hence, it can be concluded that by using a simple, cost-effective and fast way, i.e. ALD for Al2O3 and spin coating for NaF, we succeeded to make point contact openings to passivate the back surface of ultra-thin CIGS solar cells. The ongoing work focuses on different aspects: (i) At present the point openings are still submicron size, now we try to make them nano-size. (ii) We also concentrate on controlling the density of the contact openings, e.g. by changing the temperature of ALD process or the molarity of NaF solution. (iii) This approach also enables us to investigate different metal oxides as passivation layers, such as HfO2 and TiO2, in combination with the same contact opening approach. (iv) And finally, we are investigating this rear surface passivation approach for other absorber layer deposition/growth techniques, e.g. a 2-step sputtering + selenization method. ; This work received funding from the European Union's H 2020 research and innovation program under grant agreement No 715027.

Cu2ZnGeSe4 (CZGSe) is a promising earth-abundant and non-toxic semiconductor material for large-scale thin-film solar cell applications. Herein, we have employed a joint computational and experimental approach to characterize and assess the structural, optoelectronic, and heterojunction band offset and alignment properties of a CZGSe solar absorber. The CZGSe films were successfully prepared using DC-sputtering and e-beam evaporation systems and confirmed by XRD and Raman spectroscopy analyses. The CZGSe films exhibit a bandgap of 1.35 eV, as estimated from electrochemical cyclic voltammetry (CV) measurements and validated by first-principles density functional theory (DFT) calculations, which predicts a bandgap of 1.38 eV. A fabricated device based on the CZGSe as a light absorber and CdS as a buffer layer yields power conversion efficiency (PCE) of 4.4% with V-OC of 0.69 V, FF of 37.15, and J(sc) of 17.12 mA cm(-2). Therefore, we suggest that interface and band offset engineering represent promising approaches to improve the performance of CZGSe devices by predicting a type-II staggered band alignment with a small conduction band offset of 0.18 eV at the CZGSe/CdS interface. ; SRR and NYD acknowledge the UK Engineering and Physical Sciences Research Council (EPSRC) for funding (Grant No. EP/S001395/1). This work has also used the Advanced Research Computing computational facilities at Cardiff (ARCCA) Division, Cardiff University, and HPC Wales. This work also made use of ARCHER facilities (http://www.archer.ac.uk), the UK's national supercomputing service via the membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202). YAJ is thankful to Savitribai Phule Pune University Post Doctorate Fellowship (SPPU-PDF) program (Grant No. SPPU PDF/ST/CH/2019/0004) and School of Energy Studies for financial support and laboratory facilities. DGB, GB, and BV acknowledge the European Union's Horizon 2020 Research and Innovation Program (Grant agreement No. 640868). SRR and BV acknowledge UHasselt-BOF for the mobility grant project R-8751 (https://www.uhasselt.be/UH/Research-groups/en-projecten_DOC/en-project_details.html?pid=16044&t=en). ; Rondiya, SR; Dzade, NY (corresponding author), Cardiff Univ, Sch Chem, Main Bldg,Pk Pl, Cardiff CF10 3AT, S Glam, Wales. Vermang, B (corresponding author), Hasselt Univ Partner Solliance, Inst Mat Res IMO, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; Imec Div IMOMEC Partner Solliance, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; EneryVille, Thorpk,Poort Genk 8310 & 8320, B-3600 Genk, Belgium. rondiyas@cardiff.ac.uk; dzadeny@cardiff.ac.uk; bart.vermang@uhasselt.be

In this work, metal-insulator-semiconductor structures were fabricated in order to study different types of insulators, namely, aluminum oxide (Al2O3), silicon nitride, and silicon oxide (SiOx) to be used as passivation layers in Cu(In,Ga)Se-2 (CIGS) thin-film solar cells. The investigated stacks consisted of SLG/Mo/CIGS/insulator/Al. Raman scattering and photoluminescence measurements were done to verify the insulator deposition influence on the CIGS surface. In order to study the electrical properties of the CIGS-insulator interface, capacitance versus conductance and voltage (C-G-V) measurements were done to estimate the number and polarity of fixed insulator charges (Q(f)). The density of interface defects (D-it) was estimated from capacitance versus conductance and frequency (C-G-f) measurements. This study evidences that the deposition of the insulators at high temperatures (300 degrees C) and the use of a sputtering technique cause surface modification on the CIGS surface. We found that, by varying the SiOx deposition parameters, it is possible to have opposite charges inside the insulator, which would allow its use in different device architectures. The material with lower Dit values was Al2O3 when deposited by sputtering. ; This work was supported by European Union's Horizon 2020 research and innovation programme ARCIGS-M project under Grant 720887. The work of J. M. V. Cunha and P. M. P. Salome was supported by the Fundacao para a Ciencia e a Tecnologia (FCT) through the project IF/00133/2015. The work of J. P. Teixeira and J. P. Leitao was supported by the FCT through the project UID/CTM/50025/2013. The work of B. Vermang was supported by the European Research Council under the European Union's Horizon 2020 research and innovation programme under Grant 715027.