• Energy Research

Des cellules solaires à film mince à base de Cu(In,Ga)Se2 (CIGS), où seule la couche tampon est modifiée, ont été fabriquées et étudiées. Les effets de deux couches tampons différentes, CdS et ZnxSn1 −xOy (ZnSnO), sont comparés en utilisant plusieurs techniques de caractérisation. Nous avons comparé les deux dispositifs et observé que les cellules solaires à base de ZnSnO ont des valeurs similaires d'efficacité de conversion d'énergie par rapport aux cellules avec des couches tampons CdS. Les dispositifs à base de ZnSnO ont des valeurs plus élevées dans le courant de court-circuit (Jsc) qui compensent des valeurs plus faibles dans le facteur de remplissage (FF) et la tension en circuit ouvert (Voc) que les dispositifs à base de CdS. Les résultats de la microscopie à force de sonde de Kelvin (KPFM) indiquent que le CdS fournit des jonctions avec une photovoltaïque de surface (SPV) légèrement plus élevée que le ZnSnO, expliquant ainsi le potentiel Voc plus faible pour l'échantillon de ZnSnO. L'analyse met montre une couche de ZnSnO poly-cristallin et nous n'avons détecté aucune preuve forte de diffusion de Zn ou Sn dans le CIGS. À partir des mesures de photoluminescence, nous avons conclu que les deux échantillons sont affectés par des potentiels fluctuants, bien que cet effet soit plus élevé pour l'échantillon CdS. Se fabricaron y estudiaron células solares de película delgada basadas en Cu(In,Ga)Se2 (CIGS), donde solo se cambia la capa amortiguadora. Los efectos de dos capas amortiguadoras diferentes, CdS y ZnxSn1−xOy (ZnSnO), se comparan utilizando varias técnicas de caracterización. Comparamos ambos dispositivos y observamos que las células solares basadas en ZnSnO tienen valores similares de eficiencia de conversión de energía en comparación con las células con capas amortiguadoras de CdS. Los dispositivos basados en ZnSnO tienen valores más altos en la corriente de cortocircuito (Jsc) que compensan los valores más bajos en el factor de llenado (FF) y la tensión de circuito abierto (Voc) que los dispositivos basados en CdS. Los resultados de la microscopía de fuerza de sonda Kelvin (KPFM) indican que el CdS proporciona uniones con un fotovoltaje de superficie (SPV) ligeramente más alto que el ZnSnO, lo que explica el menor potencial de Voc para la muestra de ZnSnO. El análisis TEM muestra una capa policristalina de ZnSnO y no hemos detectado ninguna evidencia fuerte de difusión de Zn o Sn en el CIGS. A partir de las mediciones de fotoluminiscencia, concluimos que ambas muestras están siendo afectadas por potenciales fluctuantes, aunque este efecto es mayor para la muestra de CdS. Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two different buffer layers, CdS and ZnxSn1−xOy (ZnSnO), are compared using several characterization techniques. We compared both devices and observe that the ZnSnO-based solar cells have similar values of power conversion efficiency as compared to the cells with CdS buffer layers. The ZnSnO-based devices have higher values in the short-circuit current (Jsc) that compensate for lower values in fill factor (FF) and open circuit voltage (Voc) than CdS based devices. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO, thus explaining the lower Voc potential for the ZnSnO sample. The TEM analysis shows a poly-crystalline ZnSnO layer and we have not detected any strong evidence of diffusion of Zn or Sn into the CIGS. From the photoluminescence measurements, we concluded that both samples are being affected by fluctuating potentials, although this effect is higher for the CdS sample. تم تصنيع ودراسة الخلايا الشمسية ذات الأغشية الرقيقة القائمة على Cu(In،Ga) Se2 (CIGS)، حيث يتم تغيير الطبقة العازلة فقط. تتم مقارنة تأثيرات طبقتين عازلتين مختلفتين، CdS و ZnxSn1-xOy (ZnSnO)، باستخدام العديد من تقنيات التوصيف. قارنا كلا الجهازين ولاحظنا أن الخلايا الشمسية القائمة على ZnSnO لها قيم مماثلة لكفاءة تحويل الطاقة مقارنة بالخلايا ذات الطبقات العازلة CdS. تحتوي الأجهزة المستندة إلى ZnSnO على قيم أعلى في تيار الدائرة القصيرة (Jsc) التي تعوض عن القيم الأقل في عامل التعبئة (FF) وجهد الدائرة المفتوحة (Voc) من الأجهزة المستندة إلى CdS. تشير نتائج الفحص المجهري لقوة مسبار كلفن (KPFM) إلى أن CdS يوفر تقاطعات بجهد ضوئي سطحي أعلى قليلاً (SPV) من ZnSnO، مما يفسر انخفاض جهد Voc لعينة ZnSnO. يُظهر تحليل TEM طبقة ZnSnO متعددة البلورات ولم نكتشف أي دليل قوي على انتشار الزنك أو SN في CIGS. من قياسات اللمعان الضوئي، خلصنا إلى أن كلتا العينتين تتأثران بجهد التذبذب، على الرغم من أن هذا التأثير أعلى بالنسبة لعينة CdS.

AbstractIn this work, we used K‐rich glass substrates to provide potassium during the coevaporation of Cu(In,Ga)Se2 (CIGS) absorber layers. Subsequently, we applied a postdeposition treatment (PDT) using KF or RbF to some of the grown absorbers. It was found that the presence of K during the growth of the CIGS layer led to cell efficiencies beyond 17%, and the addition of a PDT pushed it beyond 18%. The major finding of this work is the observation of discontinuous 100‐ to 200‐nm‐deep Cu‐depleted patches in the vicinity of the CdS buffer layer, correlated with the presence of K during the growth of the absorber layer. The PDT had no influence on the formation of these patches. A second finding concerns the composition of the Cu‐depleted areas, where an anticorrelation between Cu and both In and K was measured using scanning transmission electron microscopy. Furthermore, a steeper Ga/(In+Ga) ratio gradient was measured for the absorbers grown with the presence of K, suggesting that K hinders the group III element interdiffusion. Finally, no Cd in‐diffusion to the CIGS layer could be detected. This indicates that if CdCu substitution occurs, either their concentration is below our instrumental detection limit or its presence is contained within the first 6 nm from the CdS/CIGS interface.

To improve the conduction band alignment and explore the influence of the buffer-absorber interface, we here investigate an alternative buffer for Cu2ZnSnS4 (CZTS) solar cells. The Zn(O, S) system was chosen since the optimum conduction band alignment with CZTS is predicted to be achievable, by varying oxygen to sulfur ratio. Several sulfur to oxygen ratios were evaluated to find an appropriate conduction band offset. There is a clear trend in open-circuit voltage (Voc), with the highest values for the most sulfur rich buffer, before going to the blocking ZnS, whereas the fill factor peaks at a lower S content. The best alternative buffer cell in this series had an efficiency of 4.6% and the best CdS reference gave 7.3%. Extrapolating Voc values to 0 K gave activation energies well below the expected bandgap of 1.5 eV for CZTS, which indicate that recombination at the interface is dominating. However, it is clear that the values are affected by the change of buffer composition and that increasing sulfur content of the Zn(O, S) increases the activation energy for recombination. A series with varying CdS buffer thickness showed the expected behavior for short wavelengths in quantum efficiency measurements but the final variation in efficiency was small.

In this paper, co-evaporation of Cu(In,Ga)Se2 (CIGS) in an inline single-stage process is used to fabricate solar cell devices with up to 18.6% conversion efficiency using a CdS buffer layer and 18.2% using a Zn1-xSnxOy Cd-free buffer layer. Furthermore, a 15.6-cm2 mini-module, with 16.8% conversion efficiency, has been made with the same layer structure as the CdS baseline cells, showing that the uniformity is excellent. The cell results have been externally verified. The CIGS process is described in detail, and material characterization methods show that the CIGS layer exhibits a linear grading in the [Ga]/([Ga]+[In]) ratio, with an average [Ga]/([Ga]+[In]) value of 0.45. Standard processes for CdS as well as Cd-free alternative buffer layers are evaluated, and descriptions of the baseline process for the preparation of all other steps in the Angstrom Solar Center standard solar cell are given.

AbstractTandem 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.

ABSTRACTA new atomic layer deposition process was developed for deposition of Zn–Sn–O buffer layers for Cu(In,Ga)Se2 solar cells with tetrakis(dimethylamino) tin, Sn(N(CH3)2)4, diethyl zinc, Zn(C2H5)2, and water, H2O. The new process gives good control of thickness and [Sn]/([Sn] + [Zn]) content of the films. The Zn–Sn–O films are amorphous as found by grazing incidence X‐ray diffraction, have a high resistivity, show a lower density compared with ZnO and SnOx, and have a transmittance loss that is smeared out over a wide wavelength interval. Good solar cell performance was achieved for a [Sn]/([Sn] + [Zn]) content determined to be 0.15–0.21 by Rutherford backscattering. The champion solar cell with a Zn–Sn–O buffer layer had an efficiency of 15.3% (Voc = 653 mV, Jsc(QE) = 31.8 mA/cm2, and FF = 73.8%) compared with 15.1% (Voc = 663 mV, Jsc(QE) = 30.1 mA/cm2, and FF = 75.8%) of the best reference solar cell with a CdS buffer layer. There is a strong light‐soaking effect that saturates after a few minutes for solar cells with Zn–Sn–O buffer layers after storage in the dark. Stability was tested by 1000 h of dry heat storage in darkness at 85 °C, where Zn–Sn–O buffer layers with a thickness of 76 nm retained their initial value after a few minutes of light soaking. Copyright © 2011 John Wiley & Sons, Ltd.

ABSTRACTThe influence of the thickness of atomic layer deposited Zn1−xSnxOy buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se2 solar cells are investigated. The amorphous Zn1−xSnxOy layer, with a [Sn]/([Sn] + [Zn]) composition of approximately 0.18, forms a conformal and in‐depth uniform layer with an optical band gap of 3.3 eV. The short circuit current for cells with a Zn1−xSnxOy layer are found to be higher than the short circuit current for CdS buffer reference cells and thickness independent. On the contrary, both the open circuit voltage and the fill factor values obtained are lower than the references and are thickness dependent. A high conversion efficiency of 18.0%, which is comparable with CdS references, is attained for a cell with a Zn1−xSnxOy layer thickness of approximately 13 nm and with an i‐ZnO layer. Copyright © 2012 John Wiley & Sons, Ltd.

The search for alternatives to the CdS buffer layer in Cu(In,Ga)Se $_2$ (CIGS) solar cells has turned out to be quite promising in terms of power conversion efficiency. In this paper, the typically used chemical-bath-deposited CdS layer is compared with an atomic-layer-deposited Zn $_{1-x}$ Sn $_{x}$ O $_{y}$ (ZnSnO). An optical study by external quantum efficiency and photoluminescence on the influence of different buffer layers on the defect properties of CIGS is presented. For both buffer layers, the CIGS bulk and CIGS/buffer interface are strongly influenced by electrostatic fluctuating potentials, which are less pronounced for the sample with the ZnSnO buffer layer. This is associated with a lower concentration of donor defects at the CIGS near-interface layer. A change in the bandgap of the CIGS as a consequence of the buffer layer deposition is observed. This study expands the knowledge of defects in the complex quaternary semiconductor CIGS, which, as discussed, can be affected even by the choice of buffer layer and its deposition process.