Small atom doping: a synergistic strategy to reduce SnZn recombination center concentration in Cu2ZnSnSe4

Kesterite Cu2ZnSnS x Se4-x (CZTSSe) is among the most promising inorganic Earth-abundant thin-film photovoltaic technologies, although currently, the larger voltage deficit compared with more mature chalcogenide technologies is hampering solar-to-electricity conversion efficiency progress in these materials. Most of the latest reports agree on the CZTSSe defect structure as the main limitation for the open-circuit voltage. Small atom doping is suggested as an interesting strategy to reduce the concentration of defects without affecting secondary phase formation. Herein, an innovative approach based on the introduction of LiAlH4 and its further decomposition during the selenization process of CZTSe precursors, as a pathway for hydrogen and lithium/alkali transient doping, is explored. This process shows a strong beneficial influence on the crystal growth and solar cell device performance, especially with a significant improvement in V oc and fill factor. A reduction of nonradiative recombination and a remarkable fourfold increase in the carrier lifetime correlating with the reduction of the open-circuit voltage (V oc) deficit below 330¿mV is demonstrated. A mechanism on how small atoms (Li and H) interact to reduce the concentration of SnZn recombination centers while keeping doping relatively unchanged is proposed, opening fundamental perspectives for the simple and universal transient doping of thin-film chalcogenide compounds.Peer ReviewedPostprint (published version

Chemical bath deposition route for the synthesis of ultra-thin CuIn(S,Se)2 based solar cells.

CuIn(S,Se)2 (CISSe) photovoltaic grade thin films are usually grown by expensive vacuum based methods or chemical routes that require highly toxic precursors. In this work, we present the synthesis of CISSe absorbers by a simple chemical bath deposition (CBD) route. In the first step, In2S3/Cu2 − xS stack was deposited as a precursor by CBD on Mo-coated soda lime glass substrates, using respectively thioacetamide and N,N′ dimethylthiourea as S source. Then the CISSe thin films were synthesized by the precursor's selenization at 450 °C. The obtained films were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The tetragonal chalcopyrite structure of CISSe was identified by XRD and Raman, confirming that the major part of S was replaced by Se. SEM images show a compact and homogeneous film and by cross-section the thickness was estimated to be around 700 nm. Solar cells prepared with these absorbers exhibit an open circuit voltage of 369 mV, a short circuit current density of 13.7 mA/cm2 , a fill factor of 45% and an efficiency of 2.3%

A new approach for alkali incorporation in Cu2ZnSnS4 solar cells

The addition of alkali elements has become mandatory for boosting solar cell performance in chalcogenide thin films based on kesterites (Cu2ZnSnS4, CZTS). A novel doping process is presented here, that consists in the incorporation of sodium or lithium during the deposition of the CdS buffer layer, followed by a post-deposition annealing (PDA). As the doping route leads to more efficient devices in comparison with the undoped reference sample, the influence of PDA temperature was also investigated. Compositional profiling techniques, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and glow discharge optical mission spectroscopy (GDOES), revealed a dependence of the alkaline distribution in kesterites with the PDA temperature. Although the doping process is effective in that it increases the alkaline concentration compared to the undoped sample, the compositional profiles indicate that a significant proportion of Li and Na remains 'trapped' within the CdS layer. In the 200 °C–300 °C range the alkali profiles registered the higher concentration inside the kesterite. Despite this, an additional alkali accumulation close to the molybdenum/fluorine doped tin oxide substrate was found for all the samples, which is frequently related to alkali segregation at interfaces. The addition of both, lithium and sodium, improves the photovoltaic response compared to the undoped reference device. This is mainly explained by a substantial improvement in the open-circuit potential (Voc) of the cells, with best devices achieving efficiencies of 4.5% and 3% for lithium and sodium, respectively. Scanning-electron microscopy images depicted a 'bilayer structure' with larger grains at the top and small grains at the bottom in all samples. Moreover, the calculated bandgap energies of the CZTS films account for changes in the crystallographic order-disorder of the kesterites, more related to the PDA treatment rather than alkali incorporation. Even if further optimization of the absorber synthesis and doping process will be required, this investigation allowed the evaluation of a novel strategy for alkali incorporation in kesterite based solar cells.Peer ReviewedPostprint (published version

Does Sb2Se3 admit nonstoichiometric conditions? How modifying the overall se content affects the structural, optical, and optoelectronic properties of Sb2Se3 thin films

Sb2Se3 is a quasi-one-dimensional (1D) semiconductor, which has shown great promise in photovoltaics. However, its performance is currently limited by a high Voc deficit. Therefore, it is necessary to explore new strategies to minimize the formation of intrinsic defects and thus unlock the absorber’s whole potential. It has been reported that tuning the Se/Sb relative content could enable a selective control of the defects. Furthermore, recent experimental evidence has shown that moderate Se excess enhances the photovoltaic performance; however, it is not yet clear whether this excess has been incorporated into the structure. In this work, a series of Sb2Se3 thin films have been prepared imposing different nominal compositions (from Sb-rich to Se-rich) and then have been thoroughly characterized using compositional, structural, and optical analysis techniques. Hence, it is shown that Sb2Se3 does not allow an extended range of nonstoichiometric conditions. Instead, any Sb or Se excesses are compensated in the form of secondary phases. Also, a correlation has been found between operating under Se-rich conditions and an improvement in the crystalline orientation, which is likely related to the formation of a MoSe2 phase in the back interface. Finally, this study shows new utilities of Raman, X-ray diffraction, and photothermal deflection spectroscopy combination techniques to examine the structural properties of Sb2Se3, especially how well-oriented the material is.Postprint (published version

The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

Cu2ZnSn(SxSe1−x)4 (CZTSSe) photovoltaic absorbers could be the earth-abundant and low toxicity replacement for the already commercialized CuIn1−xGaxSe2 (CIGS) thin film technology. In order to make this possible, specific research efforts applied to the bulk, front and back interfaces must be performed with the aim of improving CZTSSe performance. In this paper the importance of back contact modification to obtain high efficiency Cu2ZnSnSe4 (CZTSe) solar cells and to increase a paramount and limiting parameter such as VOC is highlighted. Several Mo configurations (monolayer, bi-layer and tri-layer) with different electrical and morphological properties are investigated in CZTSe solar cells. An optimum tri-layer configuration in order to minimize overselenization of the back contact during thermal annealing while keeping reasonable electrical features is defined. Additionally, a thin intermediate MoO2 layer that results in a very effective barrier against selenization and innovative way to efficiently assist in the CZTSe absorber sintering is introduced. The use of this layer enhances grain growth and subsequently the efficiency of solar cells increases via major VOC and FF improvement. An efficiency increase from 7.2% to 9.5% is obtained using a Mo tri-layer with a 20nm intermediate MoO2 layer

Over 10% efficient wide bandgap CIGSe solar cells on transparent substrate with Na predeposition treatment

With the recent rise of new photovoltaic applications, it has become necessary to develop specific optoelectronic properties for thin‐film technologies such as Cu(In,Ga)Se2 and to take advantage of their high degree of tunability. The feasibility of efficient wide bandgap absorbers on transparent conductive oxide substrates is, in that context, of critical importance. Using an original approach based on a predeposition sodium treatment, Cu(In,Ga)Se2 absorbers fabricated by sputtering and reactive annealing with a Ga to (Ga + In) content over 0.7 and an optical bandgap above 1.4 eV are deposited on transparent fluorine‐doped tin oxide films, with the insertion of an ultrathin MoSe2 layer preserving the contact's ohmicity. Different material characterizations are carried out, and a thorough Raman analysis of the absorber reveals that the sodium pretreatment significantly enhances the Ga incorporation into the chalcopyrite matrix, along with markedly improving the film's morphology and crystalline quality. This translates to a spectacular boost of the photovoltaic performance for the resulting solar cell as compared with a reference device without Na, specifically in the voltage and fill factor. Eventually, an efficiency exceeding 10% is obtained without antireflection coating, a record value bridging the gap with the state of the art on nontransparent substrates

Síntesis química de capas buffer para nuevas tecnologías de calcogenuros con aplicaciones fotovoltaicas

[spa] En éste trabajo de Tesis se presentan los resultados más relevantes obtenidos en el desarrollo de capas buffer de CdS y de capas buffer libres de cadmio (sulfuro de indio y sulfuro de zinc) preparadas mediante la técnica de baño químico, más conocida por sus siglas en inglés CBD (Chemical Bath Deposition). La finalidad de estas capas buffer es el procesamiento de celdas solares de bajo coste basadas en calcogenuros de cobre, fundamentalmente CZTSSe. La producción de celdas solares de bajo coste, alta eficiencia mediante una tecnología respetuosa con el medioambiente, es uno de los mayores retos científicos en el campo de las energías renovables. La obtención de dispositivos que cumplan estas condiciones ha incentivado el desarrollo de nuevos materiales. Las celdas solares de CIGS ostentan el récord absoluto de eficiencia con valores cercanos al 22%. Sin embargo, este compuesto presenta diversas limitaciones que pueden detienen su desarrollo a gran escala, en particular la escasez de dos de sus elementos constituyentes: el In y el Ga. Estas circunstancias han motivado a la comunidad científica internacional a buscar propuestas de nuevos compuestos. En esta búsqueda se han destacado especialmente una familia de compuestos llamados genéricamente Kesterita al ser el nombre de su estructura cristalina. El material por excelencia utilizado como capa buffer en la kesterita y en general en toda la tecnología de celdas solares de segunda generación es el CdS depositado por baño químico (CBD). Pero el elemento Cd, se considera como un compuesto no adecuado para la producción masiva de módulos fotovoltaicos. Es por ello que usar capas buffer libres de Cd es un aspecto que está siendo investigado alrededor del mundo sin embargo, no se ha encontrado un substituto que permita obtener las altas eficiencias que se han logrado con el CdS. El uso de las capas buffer libres de Cd por lo general va acompañado de pasos de activación, tales como un recocido térmico con la combinación del llamado "light soaking", lo cual al final, incrementa el costo de producción. El objetivo fundamental de esta tesis se centra en el estudio sistemático de películas delgadas como capas buffer (CdS y materiales libres de Cd) para tecnologías fotovoltaicas basadas en kesteritas. Se hace especial hincapié en la optimización de los parámetros de crecimiento que influyen en las propiedades de las celdas solares procesadas mediante la realización de diversos estudios de las propiedades ópticas, estructurales, morfológicas y de composición química de las películas depositadas, cuyos resultados se encuentran publicados en diferentes revistas científicas arbitradas. Los aspectos más destacados e innovadores en este trabajo son: estudiar el dopado con Cu del CdS para reducir su espesor, estudiar la influencia del uso de diversas sales precursoras de Cd, el estudio de tratamientos térmicos al CdS para sus propiedades físicas y con ello mejorar las características de los dispositivos procesados y finalmente el estudio de multicapas (incluidas las capas buffer libres de Cd) para entender el comportamiento de estas capas con el material absorbedor.[eng] This thesis presents the most relevant results of the synthesis of n-type buffer layers, deposited by chemical bath deposition technique (CBD), for the processing of solar cells based on two semiconductors of direct band gap: CZTSSe and CIGSe. The motivation of this thesis is to produce non-expensive and high efficiency solar cells by environmentally friendly technologies; it represents one of the biggest challenges of the photovoltaic technology. Because of that, the objective of this thesis is to deposit CdS thin films by CBD to study the influence of the growth parameters on the optoelectronic properties of the processed solar cells. These results were obtained through optical, structural, morphological and chemical composition studies. Nowadays, the best material used as buffer layer in almost all the thin films solar cell technologies is CdS obtained by CBD. The option of not using Cd compounds is an aspect that is been investigated around the world to only use environmentally friendly processes; in this thesis Cd-free buffer layers are also studied. However, no other substitute compound has achieved such high efficiencies as CdS. Highlights of the thesis include: adding a Cu salt to the chemical bath to obtain ultra-thin CdS layers, studying the influence of the use of various salts of Cd precursors, investigating the post thermal treatments of the CdS/absorber junction and the study of Cd-free buffer layers such as indium sulfide and zinc sulfide and multilayers of them all. This work was performed in the Solar Energy Materials and Systems (SEMS) group at Catalonia Institute for Energy Research (IREC)

Partial substitution of the CdS buffer layer with interplay of fullerenes in kesterite solar cells

ver the last few decades, significant progress has been made with inorganic materials to enable them as next generation photovoltaic materials that can fulfil the demands of green energy. Cu2ZnSn(S,Se)4stands out as a p-type absorber material due to exemption from scarce and strategic elements and its similarities with Cu2InGa(S,Se)4. Organic materials such as fullerenes and its derivatives are effective n-type semiconductors. We report the usage of n-type fullerene materials with kesterite-based absorbers in a thin film polycrystalline solar cell for the partial substitution of the CdS buffer layer with C60or C70fullerenes. Impedance measurements reveal that using C60as an interlayer increases the built-in potential, suggesting reduction in the interfacial recombination. This promotes charge conduction, resulting in an increased open circuit voltage and thus device performance.Peer ReviewedPostprint (author's final draft

Perfil en profundidad de defectos en absorbedores de kesterita a través de un ataque químico y análisis superficial

A method to probe the depth morphology, defect profile and possible secondary phases in a thin film semiconductor is presented, taking a standard Kesterite film as an example. Using a top-down approach based on a previously reported controlled Methanol-Br2 chemical etching, well-defined slabs of a state of the art Kesterite absorber are fabricated. The analysis of their morphology both by Scanning Electron Microscopy and 3D optical Profilometry reveals the extent of a previously reported poor film morphology toward the back interface, and we are able to determine that more than 50% of a standard absorber is disconnected from the substrate. More importantly, these etched films are subsequently analyzed by surface sensitive techniques such as X-ray Photoelectron Spectroscopy and UV-Raman analysis. An accurate composition profile is established, and for the first time, a direct observation of the defects’ nature and their depth profiling in Kesterite is made possible. While VCu are found with a constant amount throughout the absorber, indicating a homogenous carrier concentration, a prevalence of the ZnSn defect is observed with a steep gradient toward the back interface, associated with an increase in the SnSe2 secondary phase. With bulk defects being often pointed out as the intrinsic limitation of this material, this result highlights what possibly is the main impediment of Kesterite solar cells, and a critical point to address in the design of future devices. Beyond the case of Kesterite absorbers, the method presented here offers a combination of simplicity, tunability and versatility making a straightforward transfer to other emerging thin film absorbers feasible, and it could possibly be an important tool in their future performance assessment and comparison.Peer ReviewedPostprint (author's final draft

Investigation on limiting factors affecting Cu2ZnGeSe4 efficiency: Effect of annealing conditions and surface treatment

This work aims to unveil the optimal annealing conditions and surface treatments of CZGeSe absorbers, synthesized using vacuum-based deposition technique, with an eye to optimizing the main parameters allowing better control of secondary phases formation and improving crystalline quality of this absorber. Firstly, a comparative study is given of one and two-step annealing profiles, where, for each thermal treatment, the optimal temperature is probed. The second section of this study underlines the evaluation of the surface treatment effect on the as-annealed absorber using different etching agents. Finally, the effect of different post-annealing treatment temperatures on the overall performance of the fabricated devices is evaluated. For the studied optimizations, a deep understanding of the cell behavior is provided through structural, morphological and electrical characterizations. Preliminary results have given an efficiency up to 5.6% with higher Voc = 572 mV and FF = 65% compared to the reported record cell using similar absorber (Voc = 558 mV, FF = 59%). This performance is linked to the implementation of a two-step annealing process with lower temperatures (330 °C/480 °C) as it showed the best crystallinity-efficiency trade-off along with the smallest amount of ZnSe secondary phase among all the thermal routines studied. In addition, after the evaluation of several etching agents, the implementation of a KCN etching has shown to be the most effective leading to a remarkable improvement of the PN junction through a surface passivation.Peer ReviewedPostprint (author's final draft