El viernes, 1 de junio de 2012, a las 12 horas, en el sala de seminarios del Instituto de Energía solar, de la Escuela Técnica Superior de Ingenieros de Telecomunicación de la Universidad Politécnica de Madrid, se va a impartir el seminario titulado Electronic and structural grain boundary properties of chalcopyrite solar cell materials por S. Sadewasser.

Polycrystalline p-type Cu(In,Ga)Se2 semiconductors represent the absorber material in thin film solar cells currently reaching the highest power conversion efficiency. Efficiencies above 20% are surprising considering the high density of grain boundaries in these thin films. Their electronic structure as well as their role in the solar cell are largely investigated and discussed. To improve understanding of the grain boundary properties detailed studies are required providing information on materials characteristics on the nanometer scale. We have applied Kelvin probe force microscopy (KPFM), scanning tunneling microscopy (STM) and electron backscatter diffraction (EBSD) to investigate the electronic grain boundary
properties and relate them to their structural properties.
We present KPFM and STM studies on individual grain boundaries in polycrystalline solar cell grade Cu(In,Ga)Se2 materials. STM reveals a reduced density of states in the band gap directly at the grain boundary [1]. For a series of samples with different Ga-contents between 0% and 100% we studied the work function variation across grain boundaries using KPFM [2]. Evaluation of many grain boundaries shows the presence of upward, downward and no band bending, which we attribute to negative, positive or no charges present at the grain boundary, respectively.
To enhance the understanding, model samples were prepared consisting of large bicrystals that allow, in addition to KPFM characterization, also the application of macroscopic electrical characterization techniques. Structural information about the symmetry of the grain boundary was obtained by EBSD. In the case of a Σ3 grain boundary a charge neutral barrier (ΔΦ~30meV) to majority carrier transport could be identified, while a higher disorder Σ9 grain boundary showed the presence of charges and a significantly higher transport barrier [3]. We present a model for the electronic grain boundary structure, which requires an electronic barrier of 1-2nm in width and several 100meV in depth to fully describe electrical transport.
For a polycrystalline CuInSe2 thin film, we succeeded in combining EBSD and KPFM on the same position [4]. We find that the probability of Σ3 grain boundaries to be charge neutral is very high, while non-Σ3 grain boundaries exhibit predominantly positive or negative band bending.
Combining our results, we conclude that the abundantly present Σ3 grain boundaries are predominantly charge neutral and therefore most likely harmless to the solar cell device.
Higher disorder grain boundaries exhibit a very thin and high barrier to charge transport, through which tunneling dominates.
[1] H. Mönig et al., Phys. Rev. Lett. 105, 116802 (2010).
[2] R. Baier et al., Sol. Energy Mat. Sol. Cells, in print (2012).
[3] M. Hafemeister et al., Phys. Rev. Lett. 104, 196602 (2010).
[4] R. Baier et al., Appl. Phys. Lett. 99, 172102 (2011).