Dieter Greiner

Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment

The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...

Hybrid solar cells with ZnO-nanorods and dry processed small molecule absorber

We demonstrate hybrid solar cells with ZnO-nanorods (ZnO-NRs) prepared by a low temperature electrochemical method and small molecule organic absorber processed by dry organic vapor phase deposition. A homogeneous coverage of ZnO-NRs by the blend absorber consisting of zinc phthalocyanine (ZnPc) as donor and of fullerene C60 as acceptor is best realized when a thin C60 layer is first inserted at the ZnO-NR/ZnPc:C60 interface. ZnO-NR/C60/ZnPc:C60/MoO3/Ag solar cell devices with efficiencies of 2.8% under an illumination of 100 mW/cm2 at 25 °C are demonstrated.

Effect of Na presence during CuInSe2 growth on stacking fault annihilation and electronic properties

While presence of Na is essential for the performance of high-efficiency Cu(In,Ga)Se2 thin film solar cells, the reasons why addition of Na by post-deposition treatment is superior to pre-deposition Na supply—particularly at low growth temperatures—are not yet fully understood. Here, we show by X-ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu-poor/Cu-rich transition of low temperature 3-stage co-evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental effect on electronic properties if Na is present during growth.

Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells

In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron–hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of – partially electronically active – planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu–Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu–Se at the surface of the Cu(In,Ga)Se2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.

The Importance of Sodium Control in CIGSe Superstrate Solar Cells

In this paper, the importance of sodium control in ZnO/Cu(In,Ga)Se2 superstrate devices is studied. The superstrate devices were fabricated by the deposition of the Cu(In,Ga)Se2 (CIGSe) absorber material directly onto intrinsic ZnO. Sodium is added to the CIGSe layer as a precursor prior to the absorber deposition or via postdeposition. Capacitance measurements combined with device simulations are presented, which indicate that sodium, if present at the heterointerface, catalyzes the interface reaction between ZnO and CIGSe and induces a high density of deep acceptor states at the heterointerface. This limits the efficiency of the photovoltaic devices. It is shown that only by a very controlled deposition of sodium after the CIGSe deposition, it is possible to achieve devices that allow efficient photocurrent transport across the interfacial GaOx layer. By employing a 10-nm-thick molybdenum buffer layer on top of the absorbers back surface, the diffusion of sodium during the posttreatment can be well controlled in order to achieve efficient and long-time stable devices.

Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces

The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.

Bifacial Cu(In,Ga)Se2 solar cells with submicron absorber thickness: back-contact passivation and light management

For bifacial Cu(In,Ga)Se₂ solar cells with submicron absorber thickness, an Al₂O₃-layer deposited by a simple, self-organized spray-pyrolysis process onto the transparent SnO₂:F back-contact is used to increase open circuit voltage and thereby increase power conversion efficiency, especially for rear-illumination, indicating reduced charge carrier back-contact recombination. On non-passivated SnO₂:F, a thin (10nm) Mo-layer improved the electrical back-contact properties, while on Al₂O₃-passivated SnO₂:F, the solar cell performance was higher without Mo-modification. However, even with Mo-modification, the solar cell performance increased for Al₂O₃-passivated compared to non-passivated back-contacts demonstrating the benefit of the Al₂O₃-layer for bifacial solar cells with submicron Cu(In,Ga)Se₂ absorber layers.

Co-evaporation of Cu(In, Ga)Se 2 at low temperatures: An In-Situ x-ray growth analysis

Cu(In, Ga)Se2 thin films have been coevaporated onto Mo coated soda-lime glass using a multi-stage approach and two different maximum growth temperatures (420°C and 530°C). In order to investigate principal differences in the growth dynamics for the two temperature regimes, the growth has been monitored by in-situ energy dispersive X-ray diffraction, performed at the EDDI beamline at the Helmholtz-Zentrum Berlins BESSY II synchrotron facility. During the in-diffusion of Cu-Se into the In-Ga-Se precursor a signature that points towards a possible Cu-deficient defect phase or a chalcopyrite phase that incorporates stacking faults in the [221] direction is observed to be visible throughout a wider compositional range for the low temperature process. It disappears once the film becomes Cu-rich and thus highlights the critical role of Cu-excess for the growth of chalcopyrite thin films, particularly at low growth temperatures.

Influence of residual gas composition and background pressure in a multi-stage co-evaporation chamber on the quality of Cu(In, Ga)Se 2 thin films and their device performance

Thin film solar cells with Cu(In, Ga)Se2 (CIGSe) absorbers prepared by co-evaporation reach efficiencies above 21%. Typical multi-stage co-evaporation chambers are MBE-like (ultra-)high vacuum systems with individual effusion sources for each element. Cleanliness of the process chamber and the background pressure during the co-evaporation process could be of importance for the chamber design and a fair comparison of production costs when comparing different PV/Chalcopyrite technologies. Here we study the influence of the background pressure quality on the electronic and structural properties of the deposited absorber layer. To achieve this, we analyzed the residual gas composition before and the background pressure during consecutive co-evaporation processes and investigate the effect of a combined cleaning (mechanical and electro-chemical) of the chamber walls together with a simple conditioning of the chamber after opening the chamber and re-filling the crucibles. Cleaning of the chamber yielded a significant reduction in carbon species and an overall lower base pressure. The background pressure during the process was reduced from ~6×10-6mbar (before cleaning with water cooling shroud) to 1*10-7 mbar (after cleaning with LN2 filled cooling shroud). The type and amounts of contaminants in the absorber layer are characterized by laser ablation inductively coupled plasma mass spectroscopy (LA ICP-MS). The impact of the process pressure on the growth of the CIGSe layer is analyzed with respect to preferential orientation (using XRD), grain-size (using SEM), in-depth elemental gradients (using GDOES) and the electronic quality (using TRPL, C-V). Analysis of completed solar cell devices shows that the absorber band-gap is hardly affected by the chamber conditions, whereas we see an improved collection of charge carriers generated by photons in the infra-red spectral range from the conditioned chamber, also resulting in slightly higher jsc. The major effect is an increase in median Voc values from 585mV (before cleaning and conditioning) to 635mV (after cleaning and condition). The overall solar cell efficiency is increased by 18% (relative).Thin film solar cells with Cu(In,Ga)Se2 (CIGSe) absorbers prepared by co-evaporation reach efficiencies above 21% [1]. Typical multi-stage co-evaporation chambers are MBE-like (ultra-)high vacuum systems with individual effusion sources for each element. Cleanliness of the process chamber and the background pressure during the co-evaporation process could be of importance for the chamber design and a fair comparison of production costs when comparing different PV/Chalcopyrite technologies. Here we study the influence of the background pressure quality on the electronic and structural properties of the deposited absorber layer. To achieve this, we analyzed the residual gas composition before and the background pressure during consecutive co-evaporation processes and investigate the effect of a combined cleaning (mechanical and electro-chemical) of the chamber walls together with a simple conditioning of the chamber after opening the chamber and re-filling the crucibles. Cleaning of the chamber yielded a significant reduction in carbon species and an overall lower base pressure. The background pressure during the process was reduced from ∼6∗10−6 mbar (before cleaning with water cooling shroud) to 1∗10−7 mbar (after cleaning with LN2 filled cooling shroud). The type and amounts of contaminants in the absorber layer are characterized by laser ablation inductively coupled plasma mass spectroscopy (LA ICP-MS). The impact of the process pressure on the growth of the CIGSe layer is analyzed with respect to preferential orientation (using XRD), grain-size (using SEM), in-depth elemental gradients (using GDOES) and the electronic quality (using TRPL, C-V). Analysis of completed solar cell devices shows that the absorber band-gap is hardly affected by the chamber conditions, whereas we see an improved collection of charge carriers generated by photons in the infra-red spectral range from the conditioned chamber, also resulting in slightly higher jsc. The major effect is an increase in median Voc values from 585mV (before cleaning and conditioning) to 635mV (after cleaning and condition). The overall solar cell efficiency is increased by 18% (relative).

Optimizing the growth path for Cu(In,Ga)Se2 thin films co-evaporated at low temperatures on flexible polyimide foil

Thin film solar cells with Cu(In,Ga)Se2 (CIGSe) absorbers prepared by co-evaporation reach efficiencies >20% even on flexible polymer substrates and when deposited at low growth temperatures. The use of flexible polyimide (PI) substrates has advantages in terms of roll-to-roll device fabrication. Also, PI-based devices are extremely light weight and have a shorter energy pay-back time compared to devices on glass substrates. Commercial PI foils, however, only tolerate temperatures below 500°C, which can lead to limitations during deposition by multi-stage co-evaporation with the result of reduced absorber quality and a pronounced Ga gradient. Furthermore PI has no intrinsic alkaline reservoir like soda-lime glass. In the historical development of the CIGSe device, major steps for an improved absorber quality were the introduction of a Cu rich growth phase during the deposition process, the incorporation of Ga to modify energy band gradients and the supply of Na by using sodalime glass substrates. In this work, we adapt these steps to our in-house low-temperature CIGSe technology on PI and show, how we could increase the solar cell efficiency from 12.6% up to 17.9%. Studying the growth dynamics during our multi-stage co-evaporation process at a substrate temperature of 450°C by in-situ real-time energy-dispersive X-ray diffraction and comparing them to the high-temperature grown CIGSe at 530°C underlined the importance of the Cu-rich growth stage for low temperature growth. Then, the Ga depth profile was adjusted by a modified deposition sequence in all stages yielding an increased minimal band-gap energy. The dominant CIGSe XRD pattern was influenced by the Se flux profile, which, hence, needed to be adjusted with respect to the cationic flux rates. Finally, the Na supply was changed from a NaF precursor to a NaF post deposition treatment. An improved performance of the device was observed when the CIGSe thin film was treated with KCN before CdS-CBD deposition. Retracing our learning curve confirms the close intercorrelation of the different device aspects to be optimized.