• Energy Research

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 paper, 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 measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as a rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising a 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.

Previously, an innovative way to reduce rear interface recombination of Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2(Zn,Sn)(S,Se)4 (CZTSSe) cells, to demonstrate its potential for other thin-film technologies. Therefore, ultra-thin CZTS cells with an Al2O3 rear surface passivation layer having nano-sized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open circuit voltage (Voc; +49%rel.) or short circuit current (Jsc; +17%rel.), compared to corresponding unpassivated cells. Hence, a promising efficiency improvement of 52%rel. is obtained for the rear passivated cells.

AbstractFor 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.

Interface recombination in sub-μm optoelectronics has a major detrimental impact on devices’ performance, showing the need for tailored passivation strategies to reach a technological boost. In this article, SiOx passivation based substrates were de- veloped and integrated into ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. This article aims to understand the impact of a passivation strategy, which uses several SiOx layer thicknesses (3, 8, and 25 nm) integrated into high-performance substrates (HPS). The experimental study is complemented with 3-D lumerical finite-difference time-domain and 2-D Silvaco ATLAS optical and electrical simulations, respectively, to perform a decoupling of optical and electronic gains, allowing for a deep discussion on the impact of the SiOx layer thickness in the CIGS solar cell performance. This article shows that as the passivation layer thickness increases, a rise in parasitic losses is observed. Hence, a balance between beneficial passivation and optical effects with harmful architectural constraints defines a threshold thickness to attain the best solar cell performance. Analyzing their electrical parameters, the 8-nm novel SiOx based substrate achieved a light to power conversion efficiency value of 13.2%, a 1.3% absolute improvement over the conventional Mo substrate (without SiOx).

Dans ce travail, nous présentons une méthode de croissance de couches minces de Sb2Se3 avec une utilisation potentielle comme couches absorbantes dans les structures de cellules solaires. Les films ont été cultivés sur plusieurs substrats : verre sodo-calcique, verre sodo-calcique revêtu de Mo et Si. Les films du précurseur Sb-Se ont été déposés par pulvérisation cathodique magnétron RF puis sélénisés sous flux gazeux H2Se. Différentes températures de sélénisation ont été testées et analysées. Les analyses compositionnelles et morphologiques ont été effectuées par spectroscopie dispersive d'énergie et par microscopie électronique à balayage, respectivement. L'identification de la phase et la caractérisation structurelle ont été effectuées par diffraction des rayons X et spectroscopie de diffusion Raman montrant que Sb2Se3 est la phase dominante avec une structure cristalline orthorhombique. Des traces de phases secondaires Se rhomboédriques et amorphes ont également été observées, soutenues par leurs compositions riches en Se. Les mesures de réflectance visible-NIR ont permis d'extraire une bande interdite directe avec une valeur proche de 1,06 eV. La spectroscopie de photoluminescence montre une émission avec une large bande à 0,85 eV pour les échantillons séléniés à des températures plus basses et un pic intense à 0,75 eV pour l'échantillon sélénié à des températures plus élevées. La caractérisation électrique montre de faibles concentrations et mobilités de trous libres. À basse température, le saut du voisin le plus proche est le mécanisme dominant pour le transport électronique des échantillons analysés. Les propriétés électriques et optiques sont influencées par le type de défauts présents sur les échantillons. Une discussion est faite sur les propriétés qui doivent être améliorées afin que ces films puissent être intégrés dans des cellules solaires à couches minces. En este trabajo presentamos un método para hacer crecer películas delgadas de Sb2Se3 con un uso potencial como capas absorbentes en estructuras de células solares. Las películas se cultivaron sobre varios sustratos: vidrio de sosa-cal, vidrio de sosa-cal recubierto de Mo y Si. Las películas del precursor de Sb-Se se depositaron mediante pulverización catódica con magnetrón de RF y luego se selenizaron bajo un flujo de gas H2Se. Se probaron y analizaron diferentes temperaturas de selenización. Los análisis de composición y morfológicos se realizaron mediante espectroscopía dispersiva de energía y microscopía electrónica de barrido, respectivamente. La identificación de fases y la caracterización estructural se realizaron mediante difracción de rayos X y espectroscopía de dispersión Raman que muestran que Sb2Se3 es la fase dominante con una estructura cristalina ortorrómbica. También se observaron trazas de fases secundarias romboédricas y amorfas de Se respaldadas por sus composiciones ricas en Se. Las mediciones de reflectancia visible-NIR permitieron extraer una banda prohibida directa con un valor cercano a 1,06 eV. La espectroscopia de fotoluminiscencia muestra una emisión con una banda ancha a 0.85 eV para muestras selenizadas a temperaturas más bajas y un pico intenso a 0.75 eV para la muestra selenizada a temperaturas más altas. La caracterización eléctrica muestra bajas concentraciones y movilidades de agujeros libres. A bajas temperaturas, el salto del vecino más cercano es el mecanismo dominante para el transporte electrónico de las muestras analizadas. Tanto las propiedades eléctricas como las ópticas están influenciadas por el tipo de defectos presentes en las muestras. Se discute sobre las propiedades que deben mejorarse para que estas películas puedan integrarse en células solares de película delgada. In this work we present a method to grow Sb2Se3 thin films with a potential use as absorber layers in solar cell structures. The films were grown on several substrates: soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursor's films were deposited by RF magnetron sputtering and then selenized under a H2Se gas flow. Different selenization temperatures were tested and analysed. Compositional and morphological analyses were performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Phase identification and structural characterization were done by X-ray Diffraction and Raman scattering spectroscopy showing that Sb2Se3 is the dominant phase with an orthorhombic crystalline structure. Traces of rhombohedral and amorphous Se secondary phases were also observed supported by their Se-rich compositions. Visible-NIR reflectance measurements allowed to extract a direct bandgap with a value close to 1.06 eV. Photoluminescence spectroscopy shows an emission with a broad band at 0.85 eV for samples selenized at lower temperatures and an intense peak at 0.75 eV for the sample selenized at higher temperatures. Electrical characterization shows low free hole concentrations and mobilities. At low temperatures, the nearest neighbour hopping is the dominant mechanism for the electronic transport for the analysed samples. Both electrical and optical properties are influenced by the type of defects present on samples. A discussion is made on the properties that need to be improved in order that these films can be integrated into thin film solar cells. في هذا العمل نقدم طريقة لزراعة الأغشية الرقيقة Sb2Se3 مع احتمال استخدامها كطبقات امتصاص في هياكل الخلايا الشمسية. نمت الأغشية على عدة ركائز: زجاج الصودا والجير وزجاج الصودا والجير المطلي بالألياف الزجاجية. تم ترسيب أفلام سلائف Sb - Se بواسطة رش مغنطرون الترددات اللاسلكية ثم تم سيلينها تحت تدفق غاز H2Se. تم اختبار وتحليل درجات حرارة السيلينات المختلفة. تم إجراء التحليلات التركيبية والمورفولوجية بواسطة التحليل الطيفي لتشتت الطاقة والمسح المجهري الإلكتروني، على التوالي. تم تحديد الطور والتوصيف الهيكلي بواسطة حيود الأشعة السينية والتحليل الطيفي لتشتت رامان الذي يوضح أن Sb2Se3 هي المرحلة المهيمنة ذات البنية البلورية المعينية المستقيمة. كما لوحظت آثار للمراحل الثانوية المعينية وغير المتبلورة مدعومة بتركيباتها الغنية بالسي. يسمح لقياسات انعكاس NIR المرئي باستخراج فجوة نطاق ترددي مباشرة بقيمة قريبة من 1.06 فولت. يُظهر التحليل الطيفي للتلألؤ الضوئي انبعاثًا بنطاق عريض عند 0.85 إلكترون فولت للعينات التي تم سيلينها عند درجات حرارة منخفضة وذروة شديدة عند 0.75 إلكترون فولت للعينة التي تم سيلينها عند درجات حرارة أعلى. يُظهر التوصيف الكهربائي تركيزات منخفضة من الثقوب الحرة والتنقلات. في درجات الحرارة المنخفضة، أقرب جار هو الآلية المهيمنة للنقل الإلكتروني للعينات التي تم تحليلها. تتأثر كل من الخصائص الكهربائية والبصرية بنوع العيوب الموجودة في العينات. يتم إجراء مناقشة حول الخصائص التي تحتاج إلى تحسين حتى يمكن دمج هذه الأغشية في الخلايا الشمسية ذات الأغشية الرقيقة.

Cu(In,Ga)Se2 thin-film solar cells with Ga-graded absorber layers and a [Cu]/([In] + [Ga]) ratio varying between 0.5 and 1.0 were prepared by coevaporation and investigated. Except for the sample with a final [Cu]/([In] + [Ga]) ratio of 1.0, the samples were Cu-poor at all times during the evaporation. The variation in copper was found to influence the material properties in several ways: 1) Changing the Cu content had a strong impact on In and Ga interdiffusion, resulting in decreased Ga gradients in samples with large Cu deficiency; 2) the Cu-poor Cu(In,Ga)3Se5 phase was detected in absorbers with [Cu]/([In] + [Ga]) ratios of 0.65 and below; and 3) the grain size changed significantly with the Cu variation. We observe a trend of reduced solar cell efficiencies for [Cu]/([In] + [Ga]) ratios of 0.65 and below, with an efficiency of 13.4% for the sample with a [Cu]/([In] + [Ga]) ratio of only 0.5, i.e., far from stoichiometry. We tentatively attribute the efficiency loss to a high concentration of point defects caused by the Cu deficiency.

A novel architecture that comprises rear interface passivation and increased rear optical reflection is presented with the following advantages: i) enhanced optical reflection is achieved by the deposition of a metallic layer over the Mo rear contact; ii) improved interface quality with CIGS by adding a sputtered Al2O3 layer over the metallic layer; and, iii) optimal ohmic electrical contact ensured by rear-openings refilling with a second layer of Mo as generally observed from the growth of CIGS on Mo. Hence, a decoupling between the electrical function and the optical purpose of the rear substrate is achieved. We present in detail the manufacturing procedure of such type of architecture together with its benefits and caveats. A preliminary analysis showing an architecture proof-of-concept is presented and discussed.