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While presence of Na is essential for the performance of high-efficiency Cu(In,Ga)Se2 thin film solar cells, the reasons why addition of Na by post-deposition treatment is superior to pre-deposition Na supply--particularly at low growth temperatures--are not yet fully understood. Here, we show by X-ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu-poor/Cu-rich transition of low temperature 3-stage co-evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental effect on electronic properties if Na is present during growth. [ABSTRACT FROM AUTHOR]

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated. [ABSTRACT FROM AUTHOR]

The variation of the sodium content in low-temperature grown Cu(In,Ga)Se 2 (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 co-evaporation of NaF during absorber deposition. By a combined analysis of temperature dependent current–voltage, capacitance-voltage and quantum efficiency measurements we conclude that the dominating recombination takes place in the bulk of the absorber for all samples independent of the sodium content. The optimum sodium content with respect to solar cell performance results from a tradeoff between the beneficial influence of sodium on open circuit voltage ( V OC ) and the negative impact of high sodium contents on short circuit current ( I SC ) and fill factor ( FF ). The change of V OC , I SC 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 interface. [ABSTRACT FROM AUTHOR]

Abstract: Sodium is known to play an important role for the performance optimization of Cu(In,Ga)Se2 (CIGSe) thin film solar cells, i.e. it is found to significantly increase the open-circuit voltage (V OC). For this work CIGSe absorbers were produced in a low temperature deposition process on polyimide substrates with varying sodium content. In agreement with previous studies we find an increase of V OC and overall solar cell performance with increasing sodium content and a decrease of the overall performance for very high sodium content. At the same time time-resolved photoluminescence measurements indicate that the minority carrier lifetime decreases from 50ns to about 5ns for increasing sodium content in the absorber. This correlation was found to be valid for three different types of low-temperature CIGSe deposition processes. The results can be explained by an increase in deep defect density with increasing Na content, proportional to the increased net charge carrier density as measured by capacitance–voltage profiling. For very high sodium content this leads to an actual decrease in the overall performance of the solar cells. [Copyright &y& Elsevier]

Abstract: The objective of this work is to study the influence of Na on the properties of Cu(In,Ga)Se2 (CIGS) absorber layers and finished solar cell devices on polyimide substrates. For this study Na is added to 3-stage grown CIGS thin films by evaporation of a NaF precursor layer prior to the absorber deposition. The precursor layer modifies the CIGS growth kinetics. A stronger Ga-gradient and a decrease of grain size are observed when the Na content increases. An increase in V oc for a higher Na concentration at a nominal growth temperature of T sub,max =500 °C during CIGS deposition is explained by a higher carrier density, as obtained by DLCP measurements. The higher carrier concentration for the higher Na content could be attributed to the reduction of a compensating donor. However, a low J sc does not allow for an enhanced efficiency possibly due to a shorter depletion region, as observed by admittance spectroscopy, and effective diffusion length. [Copyright &y& Elsevier]