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The performance of five hole‐transporting layers (HTLs) is investigated in both single‐junction perovskite and Cu(In, Ga)Se2 (CIGSe)‐perovskite tandem solar cells: nickel oxide (NiOx,), copper‐doped nickel oxide (NiOx:Cu), NiOx+SAM, NiOx:Cu+SAM, and SAM, where SAM is the [2‐(3,‐6Dimethoxy‐9H‐carbazol‐9yl)ethyl]phosphonic acid (MeO‐2PACz) self‐assembled monolayer. The performance of the devices is correlated to the charge‐carrier dynamics at the HTL/perovskite interface and the limiting factors of these HTLs are analyzed by performing time‐resolved and absolute photoluminescence ((Tr)PL), transient surface photovoltage (tr‐SPV), and X‐ray/UV photoemission spectroscopy (XPS/UPS) measurements on indium tin oxide (ITO)/HTL/perovskite and CIGSe/HTL/perovskite stacks. A high quasi‐Fermi level splitting to open‐circuit (QFLS‐Voc) deficit is detected for the NiOx‐based devices, attributed to electron trapping and poor hole extraction at the NiOx‐perovskite interface and a low carrier effective lifetime in the bulk of the perovskite. Simultaneously, doping the NiOx with 2% Cu and passivating its surface with MeO‐2PACz suppresses the electron trapping, enhances the holes extraction, reduces the non‐radiative interfacial recombination, and improves the band alignment. Due to this superior interfacial charge‐carrier dynamics, NiOx:Cu+SAM is found to be the most suitable HTL for the monolithic CIGSe‐perovskite tandem devices, enabling a power‐conversion efficiency (PCE) of 23.4%, Voc of 1.72V, and a fill factor (FF) of 71%, while the remaining four HTLs suffer from prominent Voc and FF losses. [ABSTRACT FROM AUTHOR]

Polycrystalline Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells exhibit gradual onset in their external quantum efficiency (EQE) spectra whose shape can be affected by various CIGSe material properties. Apart from influences on the charge‐carrier collection, a broadening of the EQE onset leads to enhanced radiative losses in open‐circuit voltage (Voc). In the present work, Gaussian broadening of parameters describing the EQE onset of thin‐film solar cells, represented by the standard deviation, 휎total, was evaluated to study the impacts of the effective band‐gap energy, the electron diffusion length, and the Ga/In gradient in the CIGSe absorber. It is shown that 휎total can be disentangled into contributions of these material properties, in addition to a residual component 휎residual. Effectively, 휎total depends only on a contribution related to the Ga/In gradient as well as on 휎residual. The present work highlights the connection of this compositional gradient, the microstructure in the polycrystalline CIGSe absorber, and the luminescence emission with the residual component 휎residual. It is demonstrated that a flat band‐gap with no compositional gradient in the bulk of the CIGSe absorber is essential to obtain the lowest 휎total values and thus result in lower recombination losses and gains in Voc. [ABSTRACT FROM AUTHOR]

Due to their tunable bandgap energy, Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells are an attractive option for use as bottom devices in tandem configurations. In monolithic tandem devices, the thermal stability of the bottom device is paramount for reliable application. Ideally, it will permit the processing of a top device at the required optimum process temperature. Here, we investigate the degradation behavior of chemical bath deposited (CBD) CdS‐buffered CIGSe thin‐film solar cells with and without Na incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of their degradation mechanisms. For the devices studied, we observe severe degradation after annealing at 300°C independent of the atmosphere. The electrical and compositional properties of the samples before and after a defined application of thermal stress are studied. In good agreement with literature reports, we find pronounced Cd diffusion into the CIGS absorber layer. In addition, for Na‐containing samples, the observed degradation can be mainly explained by the formation of Na‐induced acceptor states in the TCO front contact and a back contact barrier formation due to the out‐diffusion of Na. Supported by numerical device simulation using SCAPS‐1D, various possible degradation models are discussed and correlated with our findings. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se2 (CIGS) solar cells based on Cu-rich instead of Cu-poor absorbers are investigated. Despite superior bulk transport properties and lower defect density, Cu-rich CIGS performances are strongly reduced due to unfavorable features at the grain boundaries and CIGS/CdS interface. A very thin (<10 nm) gallium post-deposition treatment followed by an annealing step is used to solve these issues. Such a treatment is found to efficiently passivate the grain boundaries when performed under low-selenium flux, resulting in an improved short-circuit current compared to Cu-poor references. Under higher selenium flux, the grain boundaries are no more passivated, but the CIGS/CdS interface can be strongly improved, leading to an open-circuit voltage approaching that of Cu-poor. Succession of both treatments if finally performed, improves efficiency from 7.5 to 14.4%, with higher short-circuit current (+6%) compared to Cu-poor reference. These results show that the short-circuit current of CIGS solar cells can be improved by an adequate metal-chalcogen treatment of Cu-rich absorbers, and paves the way for efficiency enhancement. [ABSTRACT FROM AUTHOR]

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n-type window of low band gap solar cells. We demonstrate that low-voltage radio frequency (RF) biasing of growing ZnO films during their deposition by non-reactive sputtering makes them as conductive as when doped by aluminium ( ρ≤1·10−3Ω cm). The films prepared with additional RF biasing possess lower free-carrier concentration and higher free-carrier mobility than Al-doped ZnO (AZO) films of the same resistivity, which results in a substantially higher transparency in the near infrared region (NIR). Furthermore, these films exhibit good ambient stability and lower high-temperature stability than the AZO films of the same thickness. We also present the characteristics of Cu(InGa)Se2, CuInSe2 and Cu2ZnSnSe4-based solar cells prepared with the transparent window bilayer formed of the isolating and conductive ZnO films and compare them to their counterparts with a standard ZnO/AZO bilayer. We show that the solar cells with nominally undoped ZnO as their transparent conductive oxide layer exhibit an improved quantum efficiency for λ > 900 nm, which leads to a higher short circuit current density JSC. This aspect is specifically beneficial in preparation of the Cu2ZnSnSe4 solar cells with band gap down to 0.85 eV; our champion device reached a JSC of nearly 39 mAcm−2, an open circuit voltage of 378mV, and a power conversion efficiency of 8.4 %. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Besides their better electronic properties, Cu-rich CuInSe2 solar cells performed worse than the Cu-poor ones. Dominated by interface recombination which lowers their open circuit voltage, they also exhibit lower current. They are indeed limited by a high native doping which leads to tunneling enhanced recombination. In order to decrease this doping, we investigate the effect of the selenium environment during the absorber growth. We demonstrate that the chemical activity of the Se during the growth strongly influences both the film microstructure and the solar cell performance via various structural and opto-electronic characterization on both the absorber and the resulting solar cells: scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction in addition to classical current–voltage and quantum efficiency measurements on the solar cell devices. We show that low Se environment is beneficial to obtain better Cu-rich solar cells. [Copyright &y& Elsevier]

Modulated photocurrent experiments have been widely used to study defects in semiconductors. Previous studies have found a number of defects in CuInSe2, which is used as an absorber in solar cells. We apply a method of analysis, which has previously not been used for Cu(In,Ga)Se2 semiconductors and which allows the determination of defect concentrations in addition to defect energies. We found that at least one of the previously discovered defects shows a metastable behaviour, increasing in concentration, and can be related to the efficiency loss in corresponding solar cells. [ABSTRACT FROM AUTHOR]

In conventional Cu(In,Ga)Se 2 (CIGSe) solar cells a chemical bath deposited CdS thin film is used as a buffer layer. However, it is desired to replace CdS due to the toxicity of cadmium and the rather narrow bandgap energy of CdS. Zn(O,S) is considered to be one of the most attractive candidates as an alternative, non-toxic buffer layer with a larger bandgap. This paper aims to compare the properties of the CdS/CIGSe and the Zn(O,S)/CIGSe interfaces depending on the absorber composition and the application of an RbF post deposition treatment (PDT). Synchrotron-based hard X-ray photoelectron spectroscopy revealed a strong correlation of Cd diffusion and concentration of V Cu in CIGSe before the PDT. Additionally, the RbF-PDT enhanced the Cd diffusion into the CIGSe. On the other hand, it was found that Zn atoms are not as easily incorporated into the CIGSe as Cd atoms. As a result, we consider the formation of donor-like Zn Cu + defects at the interface to be less likely than the formation of Cd Cu + defects. Moreover, we observed that the Zn(O,S)/CIGSe interface is less sensitive to changes of the CIGSe composition and to the RbF-PDT compared to the CdS/CIGSe interface. • Cd diffusion into CIGSe surface depends on the Cu concentration before RbF-PDT. •RbF-PDT enhances the Cd diffusion into CIGSe surface. •Zn atoms are not as easily incorporated into the CIGSe as Cd atoms. •Formation of Zn Cu + defects at the interface is less likely than the Cd Cu + defects. •Zn(O,S)/CIGSe interface is less affected by the RbF-PDT compared to the CdS/CIGSe. [ABSTRACT FROM AUTHOR]

We investigate the performance of monolithic copper-indium-gallium-selenide (CIGS)/perovskite tandem solar cells with two different CIGS bottom device absorbers: Cu(In,Ga)Se 2 or Cu(In,Ga)(S,Se) 2 and with three different hole-transporting layers (HTLs): NiO x + SAM, NiO x :Cu + SAM and SAM alone. NiO x (:Cu) is (2 wt% copper-doped) nickel oxide and SAM is the MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) self-assembled monolayer. The CIGSe is fabricated by physical-vapor deposition (PVD), has a E g ∼ 1.06 e V, and a σ RMS,PVD ∼ 65 nm, while the CIGSSe is fabricated by rapid-thermal processing (RTP), has a E g ∼ 1.01 e V, and a σ RMS,RTP ∼ 120 nm. While the current certified, 24.2%-efficient, world-record monolithic CIGSe-perovskite tandem solar cell has previously been achieved with SAM as a stand-alone HTL, this work investigates whether SAM can yield similarly high efficiencies also on industrially compatible, very rough RTP CIGSSe absorbers. We find that the devices with SAM as stand-alone HTL suffer from severe FF and V oc losses and that NiO x :Cu is needed to act as a shunt-quenching layer below that SAM, ensuring conformal coverage of the rough bottom sub-cell surface. Within this work the highest-achieved (in-house measured) PCE s for the RTP and PVD CIGS-based tandems are 21.6% and 23.2% respectively, on a cell area of 1.08 cm2, both of which are obtained with NiO x :Cu + SAM as an HTL. • First >20%-efficient monolithic RTP CIGSSe-perovskite tandem on a cell area >1c m2. • RTP CIGSSe is industrially-compatible, but has a very high surface roughness >120 nm. • First integration of low-temperature sputtered NiO x :Cu in CIGSe-perovskite tandems. • NiO x :Cu acts as a shunt-quenching layer below a self-assembled monolayer (SAM). • NiO x :Cu + SAM enables 270 mV and 21.8% higher V oc and FF than the tandems with SAM. [ABSTRACT FROM AUTHOR]

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)Se 2 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)Se 2 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. • Electron mobility of In 2 O 3 :H-based TCOs decreases with increased CIGS roughness. • Crystalline fraction after growth of In 2 O 3 :H-based TCOs affects electron mobility. • Crack-shaped void formation at sharp ridges and V-shaped valleys. • Introduction of a new parameter to describe CIGS topography more suitable. [ABSTRACT FROM AUTHOR]