Ch.-H. Fischer

The eta-solar cell with CuInS2: A photovoltaic cell concept using an extremely thin absorber (eta)

Diffusion length of charge carriers within the absorbing material is one of the important restricting properties for the efficiency of solar cell devices. A new cell design using an extremely thin absorber (eta-solar cell) is prepared to obtain an effective separation of charge carriers within the depletion layer. It could be figured out that the properties of CuInS2 (CIS) strongly depend on the porosity of the base layer. Multiple scattering within the porous structure is evident. Moreover, it can be demonstrated that there is a maximum in short-circuit current density for a medium thickness of the absorbing layer.

Determination of the band gap depth profile of the penternary Cu(In(1-X)GaX)(SYSe(1-Y))2 chalcopyrite from its composition gradient

A simple model is introduced which determines the optical band-gap energy Eg for penternary Cu(In(1−X)GaX)(SYSe(1−Y))2 (CIGSSe) alloys from its Ga∕(Ga+In) ratio as well as from its S∕(S+Se) ratio. In order to verify the model the depth dependent composition of a CIGSSe sample was revealed by elastic recoil detection analysis. Applying the model, the concentration profiles were transferred in an Eg profile. Finally, these values were compared with optical band-gap energies, which were obtained directly by independent characterization methods.

CdS and Cd(OH)2 formation during Cd treatments of Cu(In,Ga)(S,Se)2 thin-film solar cell absorbers

The surface modifications induced by treating Cu(In,Ga)(S,Se)2 films in an aqueous ammonia hydroxide-based solution of Cd2+ ions—as used in record Cu(In,Ga)(S,Se)2 solar cells without a CdS buffer layer—have been investigated for different Cd2+ concentrations. Employing a combination of x-ray photoelectron spectroscopy, Auger electron spectroscopy, and x-ray emission spectroscopy, it is possible to distinguish two different surface modifications. For Cd2+ concentrations below 4.5 mM in the solution we observe the formation of a CdS monolayer, while higher Cd2+ concentrations lead to the additional deposition of a cadmium hydroxide film on the CdS/Cu(In,Ga)(S,Se)2 surface.

A novel deposition technique for compound semiconductors on highly porous substrates: ILGAR

Abstract ILGAR (ion layer gas reaction), a novel low-cost technology for the preparation of sulfidic thin layers is described, which can be analogously applied for other chalcogenides. The process consists of three steps: (1) application of a precursor solution on a substrate by dipping or spraying, (2) drying in an inert gas stream, (3) sulfurization of the solid precursor (e.g. a metal halide) by hydrogen sulfide gas. This cycle is repeated until the desired layer thickness is obtained. Not only on smooth, but also on structured and porous substrates the method allows the deposition of homogenous thin films following the microscopic structure, where other methods often have problems with shading. Once the film is closed, the growth per dip cycle is constant and reproducible during the process. The binary compounds CdS, Cu 2 S, In 2 S 3 and also the ternary CuInS 2 have been prepared by ILGAR on glass and on porous TiO 2 or SiO 2 . The layers were characterised by XRD, SEM and EDX.

Optical properties of ZnO thin films: Ion layer gas reaction compared to sputter deposition

Zinc oxide films prepared by the recently developed ion layer gas reaction (ILGAR) technique and by rf magnetron sputtering are compared with respect to their linear optical properties. Spectral ellipsometry as well as reflectance and transmittance measurements in the UV-visible-near infrared range were employed to deduce the complex refractive index. The band gap energy, Urbach energy, and packing density were determined. The results are compared to literature data of single crystals and thin films.

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.

ILGAR technology IV:: ILGAR thin film technology extended to metal oxides☆

An ion layer gas reaction (ILGAR) technique for the deposition of thin metal oxide films such as zinc oxide has been developed. In a cyclic process a solid precursor layer was applied on a substrate by dipping in a Zn(ClO 4 ) 2 solution and subsequent drying. Reaction with gaseous NH 3 /H 2 O led to a hydroxide layer which is thermally dehydrated to ZnO. The steps were repeated until the desired layer thickness was obtained. Under optimized conditions the chlorine remainder lay below 0.3 at%. X-ray-diffraction revealed a preferred orientation concerning the (002) plane. The band gap was determined to E gap = 3.38 eV. First ZnO/CIGSSe solar cells showed efficiencies of 10.7%.

Processes for chalcopyrite-based solar cells

Abstract This contribution deals with the investigations of chalcopyrite solar cells. Main attention is paid to absorber materials with band gaps larger than 1.5 eV. Besides the different efforts to modify and optimise stoichiometric CuInS 2 films, novel deposition technologies for CuGaSe 2 films and buffer layers as well as alternative buffer layers were studied and compared. With ZnSe as alternative buffer layer on Cu(InGa)(S,Se) 2 absorbers developed by SSI Camarillo and Siemens Solar, Munich, total area efficiencies up to 13.7% and active area efficiencies up to 15.7% could be reached, respectively. For CuInS 2 two important results were achieved. The efficiency of Cu-poor CuInS 2 cells could be increased to 8.3%. Standard Cu-rich prepared devices led to a new record efficiency of 12.5%.

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 ...

Monitoring chemical reactions at a liquid–solid interface: Water on CuIn(S,Se)2 thin film solar cell absorbers

The chemical and electronic structure of the interface between liquid water and a CuIn(S,Se)2 thin film surface was studied with synchrotron-based, high energy-resolution soft x-ray emission spectroscopy (XES). By probing the local environment around the sulfur atoms, an x-ray-induced sulfate formation at the CuIn(S,Se)2 surface can be monitored, correlated with a substantial enhancement of sodium impurity atoms from the CuIn(S,Se)2 film and its glass substrate. The results demonstrate that, with XES, an experimental probe is available to in situ study chemical reactions at liquid–solid interfaces or at surfaces in a high-pressure gas environment in a chemically sensitive and atom-specific way.