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The properties of electronic defects and their relation to structural defects are of high relevance for $\mathrm{CuInS}{\mathrm{e}}_{2}$ photovoltaic absorbers. Here, we use Raman scattering and steady-state photoluminescence to study the intrinsic optoelectronic properties of near-stoichiometric $\mathrm{CuInS}{\mathrm{e}}_{2}$ samples with a lateral composition gradient around the Cu saturation point. Apart from a well-known shallow defect band at 0.97 eV, we also observe a deep defect band at 0.8 eV, which is not discernable in photoluminescence spectra at lower temperatures. The preparation of a laterally graded sample with a very precise relative composition range by in situ process control allows for a measurement of a significant decrease of the photoluminescence emission yield at the Cu-poor/Cu-rich transition on a very narrow composition scale. Possible assignments of the bands to structural point defects are discussed.

Perovskite based tandem solar cells can increase the power conversion efficiency PCE of conventional single junction photovoltaic devices. Here, we present monolithic perovskite CIGSe tandem solar cells with a perovskite top cell fabricated directly on an as grown, rough CIGSe bottom cell. To prevent potential shunting due to the rough CIGSe surface, a thin NiOx layer is conformally deposited via atomic layer deposition on the front contact of the CIGSe bottom cell. The performance is further improved by an additional layer of the polymer PTAA at the NiOx perovskite interface. This hole transport bilayer enables a 21.6 stabilized PCE of the tandem device at amp; 8764;0.8 cm2 active area. We use TEM EDX measurements to investigate the deposition uniformity and conformality of the NiOx and PTAA layers. By absolute photoluminescence measurements, the contribution of the individual subcells to the tandem VOC is determined, revealing that further fine tuning of the recombination layers might improve the tandem VOC. Finally, on the basis of the obtained results, we give guidelines to improve monolithic perovskite CIGSe tandems toward predicted PCE estimates above 30

Its high electron mobility makes hydrogen doped indium oxide (IOH) a promising transparent conductive oxide for photovoltaic and other applications. We report on a deposition process for IOH that is attractive for industrial application, using large area pulsed direct current (DC) magnetron sputtering and annealing in air. Structural, optical and electrical properties as well as stability of films annealed in air were assessed and compared to vacuum annealed films. Critical factors were found to be water and oxygen partial pressures during the sputter process, which, when chosen inappropriately, can cause a low density film structure, enhancing diffusion of atmospheric species into the indium oxide layer during crystallization in air. This leads to a lower charge carrier density of air annealed films compared to vacuum annealed films. We introduce a bi-layer stack to reduce diffusion processes during air annealing and thus improve the charge carrier density without adversely affecting the electron mobility or optical absorption.

Significant structural evolution occurs during the deposition of ${\mathrm{CuInSe}}_{2}$ solar materials when the Cu content increases. We use in situ heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu-Se phase topotactic with the ${\mathrm{CuInSe}}_{2}$ grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu-Se phase makes the grain boundaries highly mobile.

Because of its high mobility, hydrogen-doped indium oxide (In2O3:H) has a high potential as front contact in thin film or Si heterojunction solar cells. Certain growth conditions are required to process high-mobility In2O3:H. In Cu(In,Ga)Se2 (CIGS) devices, the growth of In2O3:H is influenced by the existing sublayers. Therefore, the intention of the present study is to investigate these influences on In2O3:H, deposited by pulsed direct current magnetron sputtering onto bare glass substrates or Zn(O,S) and ZnO layers, as they are commonly used in CIGS solar cells. The amorphous Zn(O,S) and the crystalline ZnO films were deposited by radio frequency sputtering onto planar glass as well as rough CIGS samples. On the basis of X-ray diffraction and transmission electron microscopy measurements, the structure of the as-grown In2O3:H films was evaluated. X-ray diffraction patterns show amorphous growth on glass and Zn(O,S) and a higher crystallinity of In2O3:H films grown on ZnO layers. The preferred orientatio...

Significant structural evolution occurs during the deposition of CuInSe2 solar materials when the Cu content increases. We use in situ heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu-Se phase topotactic with the CuInSe2 grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu-Se phase makes the grain boundaries highly mobile.

The superstrate configuration of Cu(In,Ga)Se2 (CIGS) photovoltaic devices exhibits the potential for improved light harvesting compared to the standard substrate configuration. Due to the limited power conversion efficiency of superstrate devices caused by the poor hetero-interface between the p-type CIGS and the n-type component, these advantages have not been fully considered so far. This work shows that the superstrate configuration can significantly improve the light management within CIGS devices. Experimentally, it is shown that the electric and optical properties of ZnO window layer strongly improve during the CIGS deposition, allowing the use of structured ZnO layers for efficient light-trapping. A highly reflective and scattering back contact is realized by using the high work function material MoO3−x for the hole-transport layer in combination with a low work function Ag metal layer. Combined with the unique Ga gradient achievable in the superstrate configuration, simulation results show that a strong reduction of the CIGS layer thickness is possible while increasing the power conversion efficiency. This proof of concept highlights the appeal of the superstrate configuration for the future of cost-effective CIGS solar cell production.

A new industrial approach for the production of CuInSe2 (CISe) absorber layers in a roll-to-roll process is described. This process is used by Sunplugged GmbH and combines magnetron sputtering with thermal coevaporation. A CISe-based device with a conversion efficiency of more than 10% has been fabricated from absorbers grown on polyimide at low temperature. The characteristic properties of a copper-poor ([Cu]/[In] = 0.73) CISe layer, deposited by Sunplugged's industrial process, are compared to those of a state-of-the-art, more copper-rich ([Cu]/[In] = 0.93) layer, grown in a molecular beam epitaxy system at Helmholtz-Zentrum Berlin. The presence of a so called vacancy-compound in low [Cu]/[In] CISe devices exhibiting an increased bandgap energy is visible by external quantum efficiency measurements, Raman scattering, and scaps simulations.

Buried wurtzite structures composed by stacking faults of the 111 planes in zinc blende and 112 planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of stacking fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co evaporation of Cu In,Ga Se2 a photovoltaic absorber material showing record efficiencies of up to 22.9 for thin film solar cells a reduction of stacking faults occurs at the transition from a Cu poor to a Cu rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X ray diffraction during annealing of Cu poor CuInSe2 thin films that stacking faults can be strongly reduced through annealing, without passing through a Cu rich film composition. We simulate the evolution of the X ray diffraction stacking fault signal with a simple numerical model of grain growth driven by stacking fault energy and grain boundary curvature. The results support the hypothesis that the stacking fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for stacking fault reduction and could be adapted for polycrystalline thin films with similar morphology

The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.

Indium oxide based transparent conductive oxides (TCOs) are promising contact layers in solar cells due to their outstanding electrical and optical properties. However, when applied in Cu(In,Ga)Se2 or Si-hetero-junction solar cells the specific roughness of the material beneath can affect the growth and the properties of the TCO. We investigated the electrical properties of hydrogen doped and hydrogen-tungsten co-doped indium oxides grown on rough Cu(In,Ga)Se2 samples as well as on textured and planar glass. At sharp ridges and V-shaped valleys crack-shaped voids form inside the indium oxide films, which limit the effective electron mobility of the In 2 O 3 :H and In 2 O 3 :H,W thin films. This was found for films deposited by magnetron sputtering and reactive plasma deposition at several deposition parameters, before as well as after annealing and solid phase crystallization. This suggests universal behavior that will have a wide impact on solar cell devices.

Alkali metals play a crucial role in the performance and growth of polycrystalline CIGSe solar cells. Here solar cells are investigated, that were grown in a high substrate temperature co-evaporation process with a focus on the depth distribution of Na at very high process temperatures. Devices with and without RbF post deposition treatment are characterized with the goal of understanding how the two alkali metals interact. It can be shown that at high process temperatures only very little Na is present in the films and located only at the absorber surface. After a RbF-PDT even the superficial Na disappears in all finished devices. The strong influence of the temperature on the Na content in the films needs to be accounted for and optimized for high efficiency solar cells grown above 600°C.

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses.

The continuous improvement of Cu(In,Ga)Se2 (CIGSe) solar cells relies considerably on advanced characterization of individual layers in the solar-cell stacks as well as of completed CIGSe devices. The present contribution provides an overview of corresponding efforts performed by various research groups at Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH. In-situ growth monitoring of CIGSe absorber layers by means of energy-dispersive X-ray spectrometry and light scattering is described, as well as structural analyses by means of X-ray and neutron diffraction. In addition, the characterization of surfaces and interfaces by soft X-ray and electron spectroscopy, the microscopic analysis by means of correlative electron microscopy, and optoelectronic characterization by optical spectroscopy are highlighted. The present contribution shows which substantial efforts in a research network are necessary in order to obtain deeper insight into materials properties and potentially limiting factors for the device performance, as well as to be able to control these factors during the solar-cell production.

In this paper, the importance of sodium control in ZnO/Cu(In,Ga)Se 2 superstrate devices is studied. The superstrate devices were fabricated by the deposition of the Cu(In,Ga)Se 2 (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 GaO x layer. By employing a 10-nm-thick molybdenum buffer layer on top of the absorber's back surface, the diffusion of sodium during the posttreatment can be well controlled in order to achieve efficient and long-time stable devices.

Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo-conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero-interface with electron microscopy and X-ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaOx, which forms at the CIGSe/ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero-interface presumably because of a high density of negatively charged acceptor states like CuGa. The addition of sodium enhances the p-type doping of the absorber but also increases the net doping within the GaOx. Hence, a trade-off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field-induced diffusion of sodium cations out from the GaOx layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices. Copyright © 2014 John Wiley & Sons, Ltd.

ZnO nanorod arrays placed within the architecture of the Cu(In,Ga)Se2 solar cell present promising concepts to maximize photoconversion by optical and electrical enhancement. Three different Cu(In,Ga)Se2 solar cell architectures utilizing ZnO nanorods are introduced. New technological insights are given for electrodeposited ZnO nanorod anti-reflection coatings, examining the reflectance of devices before and after encapsulation. For the first time, device results from superstrate and bifacial CIGSe solar cells incorporating ZnO nanorods are presented. The potential benefits as well as technological aspects of fabricating these new architectures are discussed. Scanning electron microscope picture of the cross-section of an industrially produced CIGSe solar cell with a ZnO nanorod anti-reflection coating.

Intensity-modulated photocurrent spectroscopy (IMPS) characterizes the charge carrier transport in a solar cell by subjecting it to a sinusoidally varying illumination and measuring the resultant photocurrent. The IMPS spectra of certain P3HT:PCBM bulk heterojunction solar cells exhibit a poorly understood feature: a quadrant I semicircle, which implies that the sinusoidal variation of the photocurrent leads that of the illumination in a certain frequency range. To understand the mechanisms underlying this feature, we have adopted a mathematical framework based on drift-diffusion modeling instead of the conventional equivalent circuit approach. By incorporating the effects of traps into the mathematical model and conducting targeted experiments, we have demonstrated that the quadrant I semicircle arises from degradation-related trap states behaving as recombination centers. Our analysis of the device’s intrinsic mechanisms indicates that the trap-assisted recombination can have a time scale that is at lea...

Optical and electrical properties of complex semiconducting alloys like Cu(In,Ga)Se2 (CIGS) are strongly influenced by the reaction pathways occurring during their deposition process. This makes it desirable to observe and control these properties in real-time during the deposition. Here we show for the first time the evolution of the band gap and the sub-band-gap defect absorption of CIGS thin film as well as surface roughness during a three-stage co-evaporation process by means of an optical analysis technique, based on white light reflectometry (WLR). By simultaneously recording structural information with in-situ energy dispersive X-ray diffraction and X-ray fluorescence we can directly correlate the evolution of opto-electronic material parameters with the structural properties of the film during growth. We find that the surface roughness and the sub-gap light absorption can be correlated with the phase evolution during the transformation from (In,Ga)2Se3 to Cu(In,Ga)Se2 by the incorporation of Cu into the film. Sub-bandgap light absorption is found to be influenced by the Cu-saturated growth phase and is lowered close to the points of stoichiometry, allowing for an advanced process design.

Thin film solar cells with Cu(In, Ga)Se 2 (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 ∼6×10−6mbar (before cleaning with water cooling shroud) to 1∗10−7 mbar (after cleaning with LN 2 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 j sc . The major effect is an increase in median V oc values from 585mV (before cleaning and conditioning) to 635mV (after cleaning and condition). The overall solar cell efficiency is increased by 18% (relative).

Metal oxide semiconductors are promising interfacial materials for organic photovoltaics (OPVs) because of their electrical properties and solution processability. In this article, we report the fabrication of poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) OPV devices incorporating solution-based NiO interfacial layers that show promising enhancements of the device photocurrent and stability. We discuss the impact of parasitic shunt and series resistances on device performance as well as the ambient degradation of these devices, studied with intensity modulated photocurrent spectroscopy (IMPS). The results showed that charge extraction was predominantly affected by degradation via decrease in carrier mobility and increased trapping/recombination, revealing the physical mechanism behind the degradation observed. 2012 Elsevier Ltd. All rights reserved.

The photo-induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3-hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc-CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are studied as well. The light-induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo-induced absorption (PIA) and light-induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo-excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long-term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc-CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed.

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich., This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively., 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)Se-2 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)Se-2 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)Se-2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.

In polycrystalline compound semiconductor thin films, structural defects such as grain boundaries as well as lateral stress can form during film growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ or ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$, temporary Cu excess during film growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-efficiency chalcopyrite thin film solar cells. However, the mechanisms responsible for the beneficial effect of Cu excess are yet not fully clarified. Here, we investigate the evolution of lateral stress, grain growth, and Cu-Se segregation during Cu-Se deposition onto Cu-poor ${\mathrm{CuInSe}}_{2}$. Real-time x-ray diffraction and fluorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diffusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the film can be already reached without the necessity of an overall Cu-rich film composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber films.

Photoluminescence (PL) is commonly used for investigations of Cu2ZnSnS(e)4 [CZTS(e)] and Cu(In,Ga)Se2 (CIGS) thin film solar cells. The influence of interference effects on these measurements is, however, largely overlooked in the community. Here, it is demonstrated that PL spectra of typical CZTS absorbers on Mo/glass substrates can be heavily distorted by interference effects. One reason for the pronounced interference in CZTS is the low reabsorption of the PL emission that typically occurs below the band gap. A similar situation occurs in band gap graded CIGS where the PL emission originates predominantly from the band gap minimum located at the notch region. Based on an optical model for interference effects of PL emitted from a thin film, several approaches to reduce the fringing are identified and tested experimentally. These approaches include the use of measured reflectance data, a calculated interference function, use of high angles of incidence during PL measurements as well as the measurement of polarized light near the Brewster angle.

Cu(In,Ga)Se2 (CIGS) solar cells in superstrate configuration promise improved light management and higher stability compared to substrate devices, but they have yet to deliver comparable power conversion efficiencies (PCEs). Chemical reactions between the CIGS layer and the front contact were shown in the past to deteriorate the p-n junction in superstrate devices, which led to lower efficiencies compared to the substrate-type devices. This work aims to solve this problem by identifying a buffer layer between the CIGS layer and the front contact, acting as the electron transport layer, with an optimized electron affinity, doping density and chemical stability. Using combinatorial material exploration we identified amorphous gallium oxide (a-GaOx) as a potentially suitable buffer layer material. The best results were obtained for a-GaOx with an electron affinity that was found to be comparable to that of CIGS. Based on the results of device simulations, it is assumed that detrimental interfacial acceptor states are present at the interface between CIGS and a-GaOx. However, these initial experiments indicate the potential of a-GaOx in this application, and how to reach performance parity with substrate devices, by further increase of its n-type doping density.

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 Berlin's 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.

The current state of the art solution processed organic photovoltaic (OPV) cells are based on the bulk heterojunction (BHJ) architecture, consisting of an active layer, in which the donor and the acceptor material are dissolved in a common solvent. In this report, we show that by sequential processing of the active components and by subsequent annealing a “pseudo-bilayer” can be formed which results in an enhanced nanomorphology for efficient charge generation and extraction. The enhanced nanomorphology arises from the inter-diffusion of the PCBM nanoparticles into a layer of pre-organised polymer P3HT instead of a phase separation that happens at the time of spin-coating and annealing in the case of conventional BHJ devices. To get a further insight into the inter-diffusion process, TOF-SIMS depth profiling and XPS measurements were carried out for different thermal annealing treatments of these pseudo-bilayer devices, which show full inter-diffusion after only seconds of annealing at 140°C. The optimised photovoltaic devices that were fabricated showed an efficiency of 4.1%.

The potential of effectively n-type doping Ga2O3 considering its large band gap has made it an attractive target for integration into transistors and solar cells. As a result amorphous GaOx is now attracting interest as an electron transport layer in solar cells despite little information on its opto-electrical properties. Here we present the opto-electronic properties, including optical band gap,electron affinity, and charge carrier density, for amorphous GaOx thin films deposited by pulsed laser deposition. These properties are strongly dependent on the deposition temperature during the deposition process. The deposition temperature has no significant influence on the general structural properties but produces significant changes in the oxygen stoichiometry of the films. The density of the oxygen vacancies is found to be related to the optical band gap of the GaOx layer. It is proposed that the oxygen deficiency leads to defect band below the conduction band minimum that increases the electron affinity. These properties facilitate the use of amorphous GaOx as an electron transport layer in Cu(In,Ga)Se2 and in Cu2O solar cells. Further it is shown that at low deposition temperatures, extrinsic doping with Sn is effective at low Sn concentrations.

Doping activity in both beta-phase (β-) and amorphous (a-) Sn-doped gallium oxide (Ga2O3:Sn) is investigated by X-ray absorption spectroscopy (XAS). A single crystal of β-Ga2O3:Sn grown using edge-defined film-fed growth at 1725 °C is compared with amorphous Ga2O3:Sn films deposited at low temperature (

We present a new strategy for highly efficient perovskite/CIGSe monolithic tandem solar cells, fabricated directly on top of a rough CIGSe surface. This was enabled by conformally depositing the metal oxide NiOx by atomic layer deposition, as confirmed by TEM/EDX measurements. The resulting limiting NiOx/perovskite interface was improved by spin-coating additional thin layer of p-type polymer PTAA, thus forming a NiOx/PTAA hole selective bilayer. The fabricated tandem device has a 21.6% power conversion efficiency on 0.778 cm2, stable over 10 minutes of maximum power point tracking. The device performance is further analyzed by absolute photoluminescence measurements from which a VOC contribution of each subcell to the tandem VOC can be estimated.