Evaluation of Kelvin probe force microscopy for imaging grain boundaries in chalcopyrite thin films

In view of the outstanding performance of polycrystalline thin film solar cells on the basis of Cu In,Ga Se2, the electrical activity at grain boundaries currently receives considerable attention. Recently, Kelvin probe force microscopy KPFM has been applied to characterize of the properties of individual grain boundaries, observing a drop in the surface potential in many cases. We present finite element simulations of the electrostatic forces to assess the experimental resolution of KPFM. Depending on the tip sample distance, the observed drop in the work function amounts to only a fraction of the real surface potential drop. The simulations are considered for different grain boundary models and consequences for the quantitative evaluation of experimental results are discusse

Raman and photoreflectance study of Cu In,Ga S2 films and solar cells

The structural and optical properties of CuIn1 xGaxS2 CIGS , CdS CuIn1 xGaxS2, and ZnO CdS CuIn1 xGaxS2 polycrystalline films, with applications in photovoltaics, were studied by Raman and Photoreflectance PR spectroscopy for two different compositions, [Ga] [In] [Ga] 0.04 and 0.12, of the CuIn1 xGaxS2 absorber. The energy band gap of the absorber film was determined by fitting the PR spectra with a third derivative functional form. Moreover, the thickness of the film was calculated from the interference fringes observed in the PR spectra below band gap energy. Raman scattering was excited by the 514.5nm line of Ar laser and the 647.1nm line of Kr laser. The Raman spectra of the absorber films consist of phononmodes assigned to CuInS2, CuGaS2 and CuS. The results of the present study are discussed together with the results of SEM and XRD studies of the films and the results of electrical measurements performed on solar cells based on the CIGS absorber

Evidence for a neutral grain boundary barrier in chalcopyrites

Single grain boundaries in CuGaSe2 have been grown epitaxially. Hall measurements indicate a barrier of 30 40 meV to majority carrier transport. Nevertheless, local surface potential measurements show the absence of space charge around the grain boundary, i.e. it is neutral. Theoretical calculations [Persson and Zunger, Phys. Rev. Lett. 91, 266401 2003 ] have predicted a neutral barrier for the present S3 grain boundary. Thus, we have experimentally shown the existence of a neutral grain boundary barrier, however, smaller than theoretically predicte

Charge Generation and Selective Separation at PbS Quantum Dot Metal Oxide Interfaces

Charge separation and transfer at the interface between layers of oleic acid capped PbS quantum dots QDs and Titanium and Indium Tin oxide TiO2 and ITO films were investigated by surface photovoltage SPV measurements. Photoluminescence PL measurements were performed in order to check for excitonic transitions and determine the QD band gaps. The QDs diameter of 4.2 nm and 5.0 nm were estimated by using the PL band gaps and the theoretical equation derived by Wang et al. [J. Chem. Phys. 87 1987 7315]. The SPV spectra of the PbS QDs TiO2 system reveal a positive charge on the PbS film surface and show three distinguished regions which demonstrate i the charge separation across QDs, ii the electron injection from QDs into TiO2 and iii the fundamental absorption in TiO2. The on set of the electron injection depends on the QD size QD band gap it shifts to lower photon energies for lower QD dimensions for higher QD band gaps . Thus, a better conduction band alignment is achieved in the latter case. In contrast to PbS QDs TiO2, the SPV spectra of the PbS QDs ITO structure reveal the negative charge on PbS surface. Moreover, the charge transfer at this interface is not observed. Instead, the SPV peculiarities in the photon energy range 1.4 3.0 eV point out to trapped holes on the ITO surface state

Study of Zn O,S Films grown by Aerosol Assisted Chemical Vapour Deposition and their Application as Buffer Layers in Cu In,Ga S,Se 2 Solar Cells

To reduce the use of toxic and expensive elements in chalcopyrite thin film solar cells, materials such as cadmium or indium used in buffer layers need to be substituted. Zn O,S is considered to be a potential buffer layer material when deposited with a fast and inexpensive method. Zn O,S layers have been prepared by aerosol assisted chemical vapour deposition AACVD technique. AACVD technique is a simple non vacuum process where the thin film deposition temperatures do not exceed 250 C. 10 mM spray solution was made by dissolving zinc II acetylacetonate monohydrate in ethanol. The films were grown on Mo substrate at 225 C film growth temperature . The effect of deposition parameters spray solution concentration, N2 flow rate, H2S flow rate on Zn O,S thin film properties were studied with SEM and XRD. Thereupon optimizing the deposition parameters, homogeneous and compact Zn O,S thin films were obtained and the films were employed in the chalcopyrite thin film solar cell structure by growing films on Cu In,Ga S,Se 2 substrates industrially produced by BOSCH Solar CISTech GmbH. The resulting cells were studied using current voltage and quantum efficiency analysis and compared with solar cell references that include In2S3 and CdS as buffer layer deposited by ion layer gas reaction and chemical bath deposition, respectively. The best output of the solar cell containing Zn O,S as buffer layer and without intrinsic ZnO under standard test conditions AM 1.5G, 100 mW cm2, 25 C is Voc 573 mV, Jsc 39.2 mA cm2, FF 68.4 and efficiency of 15.4 being slightly better than the In2S3 or CdS containing solar cell reference

Concentrating light in Cu(In,Ga)Se 2

Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here we investigate light concentration for cost efficient thin film solar cells which show nano or microtextured absorbers. Our absorber material of choice is Cu In,Ga Se2 CIGSe which has a proven stabilized record efficiency of 22.6 and which despite being a polycrystalline thin film material is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscale approach mm sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared to the large scale concentrators and promise compact high efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically ultra thin absorbers for dielectric nanostructures or horizontally micro absorbers for concentrating lenses and have significant potential for efficiency enhancemen

Transport properties of CuGaSe(2)-based thin-film solar cells as a function of absorber composition

The transport properties of thin-film solar cells based on wide-gap CuGaSe(2) absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E(a)) of approx. 0.95 eV in the dark and 1.36 eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E(a) values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe(2) absorber independently from the absorber composition. The increase of E(a) with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells