Iver Lauermann

Influence of Na on Cu(In,Ga)Se2 solar cells grown on polyimide substrates at low temperature: Impact on the Cu(In,Ga)Se2/Mo interface

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated.

Formation of a ZnS∕Zn(S,O) bilayer buffer on CuInS2 thin film solar cell absorbers by chemical bath deposition

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and Zn(S,O) with a ZnS∕(ZnS+ZnO) ratio of ∼80% is deposited. Thus, a ZnS∕Zn(S,O) bilayer buffer is deposited on the CIS thin film solar cell absorbers by the alternative chemical deposition route used in this investigation. No major changes of these findings after a postannealing of the buffer/CIS sample series and recharacterization could be identified.The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and ...

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

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...

Deposition of In2S3 on Cu(In,Ga)(S,Se)2 thin film solar cell absorbers by spray ion layer gas reaction: Evidence of strong interfacial diffusion

Recently, Cd-free Cu(In,Ga)(S,Se)2-based “CIGSSe” thin film solar cells with a nominal In2S3 buffer layer deposited by the spray ion layer gas reaction technique resulted in photovoltaic performances comparable to that of CdS buffered references. In the past it was argued that diffusion processes across the In2S3∕CIGSSe interface play a significant role for the device quality. Investigating the interface formation by using x-ray photoelectron spectroscopy, the authors were able to confirm a strong interfacial diffusion involving Cu and Na from the CIGSSe.

Zn(O,OH) layers in chalcopyrite thin-film solar cells: Valence-band maximum versus composition

Zn(O,OH) layers deposited by the ion layer gas reaction (ILGAR) technique have the potential to replace the conventionally used CdS buffer layer in Cu(In(1−X)GaX)(SYSe(1−Y))2-based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn(OH)2 should be reduced in the final buffer layer. However, hydroxide-poor or -free ZnO “buffers” result in noncompetitive devices. We have therefore investigated the impact of different oxide/hydroxide ratios on the electronic band alignment at the absorber/buffer heterointerface. The surface composition as well as the position of the valence-band maximum (VBM) of respective ILGAR-Zn(O,OH) samples was determined by photoelectron spectroscopy. The position of the conduction-band minimum (CBM) was estimated using optical band gaps determined from optical reflection/transmission measurements. From the comparison of these VBM and CBM values with the respective values of the absorber surface, predictions are made in terms of valence- and conduction-band ...

Intermixing at the heterointerface between ZnS∕Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin-film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. After identifying the deposited Zn compound, as ZnS∕Zn(S,O) bilayer buffer in former investigations [M. Bar et al., J. Appl. Phys. 99, 123503 (2006)], this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy (PES) measurements were also conducted using tunable monochromat...

Impact of Na Dynamics at the Cu2ZnSn(S,Se)4/CdS Interface During Post Low Temperature Treatment of Absorbers.

Cu2SnZn(S,Se)4 (CZTSSe) solar cells based on earth abundant and nontoxic elements currently achieve efficiencies exceeding 12%. It has been reported that, to obtain high efficiency devices, a post thermal treatment of absorbers or devices at temperatures ranging between 150 and 400 °C (post low temperature treatment, PLTT) is advisable. Recent findings point toward a beneficial passivation of grain boundaries with SnOx or Cu-depleted surface and grain boundaries during the PLTT process, but no investigation regarding alkali doping is available, even though alkali dynamics, especially Na, are systematically reported to be crucial within the field. In this work, CZTSSe absorbers were subjected to the PLTT process under different temperatures, and solar cells were completed. We found surprisingly behavior in which efficiency decreased to nearly 0% at 200 °C during the PLTT process, being recovered or even improved at temperatures above 300 °C. This unusual behavior correlates well with the Na dynamics in the devices, especially with the in-depth distribution of Na in the active CZTSSe/CdS interface region, indicating the key importance of Na spatial distribution on device properties. We present an innovative model for Na dynamics supported by theoretical calculations and additional specially designed experiments to explain this behavior. After optimization of the PLTT process, a Se-rich CZTSSe solar cell with 8.3% efficiency was achieved.

Three-dimensional structure of the buffer/absorber interface in CdS/CuGaSe2 based thin film solar cells

The chemical structure of the CdS/CuGaSe2 chalcopyrite solar cell buffer/absorber interface is investigated by combining element depth profiling using elastic recoil detection analysis and surface-near bulk sensitive x-ray emission spectroscopy. Significant Cd and S concentrations (≥0.1 at. %) are found deep in the absorber bulk. The determined high Cd and S diffusion coefficient values at 333 K of 3.6 and 3.4×10−12 cm2/s, respectively, are attributed to diffusion along CuGaSe2 grain boundaries. As a result, a three-dimensional buffer/absorber interface geometry is proposed.

Inducing and monitoring photoelectrochemical reactions at surfaces and buried interfaces in Cu(In,Ga)(S,Se)2 thin-film solar cells

We report the direct observation of a photoinduced oxidation process at the buried buffer/absorber interface in high-efficiency Zn(O,OH)∕Cu(In,Ga)(S,Se)2 thin-film solar cell structures by means of x-ray emission and photoelectron spectroscopy. We propose a reaction mechanism that involves the decomposition of a hydroxide compound in the buffer layer into water and an oxide and present evidence that this process also occurs with visible light excitation and after accelerated lifetime tests of nonencapsulated devices. This suggests a possible photoinduced aging effect in solar cell devices with other hydroxide containing buffer layers or under humid conditions.