J. Ohland

Influence of particle size in hybrid solar cells composed of CdSe nanocrystals and poly(3-hexylthiophene)

Inorganic semiconductor nanoparticles, such as CdSe quantum dots, are considered to be a promising alternative to fullerene derivates for application as electron acceptors in polymer-based bulk heterojunction solar cells. The main potential advantage is the strong light absorption of CdSe nanoparticles with a spectral bandwidth, which can even be tuned, due to the quantum size effect. However, the impact of the particle size on the performance of polymer/CdSe solar cells has remained largely unexplored so far. Therefore, the influence of particle size in hybrid solar cells using a blend of poly(3-hexylthiophene) (P3HT) and quasi-spherical CdSe nanoparticles on relevant cell parameters and the overall solar cell performance is systematically studied in the present work. As the most important result, an increase of the open-circuit voltage (VOC) can be found for smaller nanoparticles and can be explained by an “effective bandgap” model. In contrast, no significant changes of the short-circuit current densit...

Depth-resolved and temperature dependent analysis of phase formation processes in Cu–Zn–Sn–Se films on ZnO substrates

The constitution of secondary phases in kesterite Cu2ZnSnSe4 (CZTSe) thin films is still a limiting factor for their application in solar cells. Therefore an enhanced understanding of phase formation processes during the fabrication of CZTSe films is required. In this study we present a temperature and film-depth dependent phase analysis of Zn/Sn/Cu precursors on ZnO substrates selenized at different temperatures. A special sample preparation step using a focused ion beam was applied to prepare shallow angle cross sections for depth-resolved Raman profiling of the thin films. At low selenization temperatures multiphase structures are demonstrated and a first formation of CZTSe besides secondary phases at only 250 °C is detected. At high selenization temperatures an accumulation of ZnSe at the interface of CZTSe and ZnO substrates is observed. Furthermore indications for the formation of a thin SnO2 interface layer were found by X-ray diffraction, secondary electron microscopy and energy dispersive X-ray spectrometry.

CuIn(Ga)Se 2 based thin film solar cells with electrodeposited absorber on flexible steel foils

In this work we studied the electrical properties of CuIn(Ga)Se2 solar cells electrodeposited on steel substrate with chromium as diffusion barrier and molybdenum back electrode. The open circuit voltage (VOC=435 mV) of the samples lies approx. 80 mV below the value commonly observed for devices based on neat CuInSe2 [1]. From current-voltage measurements (I-V) under variable temperature and light intensity we found that the activation energy Ea of the recombination current J0 equals the absorber band gap Eg. Hence, interface recombination via defects at the heterojunction seems to be less likely to explain the VOC loss. Another explanation for this performance limit could be the presence of deep recombination centers in the volume of the absorber layer caused, e.g., by iron diffusion from the substrate. Admittance (AS) and deep level transient spectroscopy (DLTS) revealed the presence of such deep states close to mid-gap position which may facilitate Shockley-Read-Hall (SRH) recombination. To verify or falsify that the observed recombination centers originate from iron diffusion we performed the same experiments on CuIn(Ga)Se2 solar cells prepared on Ti substrates. In contrast to our expectations we found the same mid-gap states in both sample configurations which point towards an origin being independent of the substrate.

Accessing the band alignment in high efficiency Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells with an InxSy:Na buffer based on temperature dependent measurements and simulations

We present a one-dimensional simulation model for high efficiency Cu(In,Ga)(Se,S)2 solar cells with a novel band alignment at the hetero-junction. The simulation study is based on new findings about the doping concentration of the InxSy:Na buffer and i-ZnO layers as well as comprehensive solar cell characterization by means of capacitance, current voltage, and external quantum efficiency measurements. The simulation results show good agreement with the experimental data over a broad temperature range, suggesting the simulation model with an interface-near region (INR) of approximately 100 nm around the buffer/absorber interface that is of great importance for the solar cell performance. The INR exhibits an inhomogeneous doping and defect density profile as well as interface traps at the i-layer/buffer and buffer/absorber interfaces. These crucial parameters could be accessed via their opposing behavior on the simulative reconstruction of different measurement characteristics. In this work, we emphasize th...

Depth-resolved and temperature-dependent analysis of phase formation mechanisms in selenized Cu-Zn-Sn precursors by Raman spectroscopy

We present a study on the temperature and film depth dependent phase formation in Cu-Zn-Sn-Se thin films. Zn/Sn/Cu precursors were selenized at different temperatures, followed by depth-resolved Raman profiling. A high depth resolution for Raman analysis was achieved by a special sample preparation step using a focused ion beam to prepare shallow angle cross sections (SACS) of the absorber. Multi-phase structures were observed at low selenization temperatures with a first formation of the quaternary Cu2ZnSnSe4 at only 250 °C and the existence of SnxOy in films annealed at 330 °C. At high selenization temperatures up to 560 °C Cu2ZnSnSe4 was the main phase with some traces of ZnSe and a MoSe2 interface layer at the back contact. Furthermore, compositional gradients were investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy measurements on sample cross sections. The combination of these results allows for observation of both elemental composition and phase composition of the films and their dynamics during the annealing procedure.