Damp Heat Treatment of Cu(In,Ga)Se2 Solar Cells with Different Sodium Content

Long term stability is crucial to maturing any photovoltaic technology. We have studied the influence of sodium, which plays a key role in optimizing the performance of Cu(In,Ga)Se2 (CIGSe) solar cells, on the long-term stability of flexible CIGSe solar cells on polyimide foil. The standardized procedure of damp heat exposure (85% relative humidity at 85 °C) was used to simulate aging of the unencapsulated cells in multiple time steps while they were characterized by current-voltage analysis, capacitance-voltage profiling, as well as electroluminescence imaging. By comparing the aging process to cells that were exposed to heat only, it could be confirmed that moisture plays the key role in the degradation process. We found that cells with higher sodium content suffer from a more pronounced degradation. Furthermore, the experimental results indicate the superposition of an enhancing and a deteriorating mechanism during the aging process. We propose an explanation based on the corrosion of the planar contacts of the solar cell

Impact of sodium on the device characteristics of low temperature deposited Cu In,Ga Se2 solar cells

The variation of the sodium content in low temperature grown Cu In,Ga Se2 CIGSe absorbers has a strong influence on the doping level and the photoelectrical properties of the bare CIGSe films and corresponding solar cells. The negative impact of excessive sodium contents is studied for samples from CIGSe processes with coevaporation of NaF during absorber deposition. By a combined analysis of temperature dependent current voltage, capacitance voltage and quantumefficiencymeasurementswe conclude that the dominating recombination takes place in the bulk of the absorber for all samples independent of the sodium content. The optimum sodium contentwith respect to solar cell performance results froma tradeoff between the beneficial influence of sodium on open circuit voltage VOC and the negative impact of high sodium contents on short circuit current ISC and fill factor FF . The change of VOC, ISC and FF can be explained by a concurrent increase of net doping and deep defect density for increasing sodium content and the presence of barriers at the CIGSe CdS and the CIGSe Mo interfac

Potential of CuS cap to prevent decomposition of Cu2ZnSnS4 during annealing

One of the challenges associated with processing of Cu2ZnSnS4 (CZTS) is the thermal decomposition reaction that causes loss of S and SnS from the absorber surface. To reduce the decomposition a sufficiently high SnS and S partial pressure must be supplied during annealing. The absorber surface can alternatively be protected with a thin cap. Aiming to obtain a more flexible process, CZTS precursors were capped with a thin CuS layer before annealing. The cap was subsequently removed with a KCN etch before device finishing. It was found that the cap coverage decreased during annealing, exposing a part of the absorber surface. At the same time, the initially Cu poor absorber took up Cu from the cap, ending up with a stoichiometric Cu content. Devices made from capped precursors or precursors annealed without sulfur had poor device characteristics. An increased doping density of almost one order of magnitude could be the reason for the very poor performance. CuS is therefore not a suitable cap material for CZTS. Other cap materials could be investigated to protect the CZTS absorber surface during annealing

Fabrication and characterisation of Cu(In,Ga)Se-2 solar cells on polyimide

Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se-2/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se-2 (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E-g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed