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See detailThe cause of interface recombination in Cu-rich CIS thin film solar cells
Elanzeery, Hossam UL

Doctoral thesis (2019)

Cu(In,Ga)Se2 (CIGS) thin film solar cells are considered one of the most promising thin film technologies reaching efficiencies beyond 22 %. The record efficiencies for CIGS thin film solar cells are ... [more ▼]

Cu(In,Ga)Se2 (CIGS) thin film solar cells are considered one of the most promising thin film technologies reaching efficiencies beyond 22 %. The record efficiencies for CIGS thin film solar cells are based on CIGS absorbers grown under Cu-deficiency conditions. CIGS absorbers grown under Cu-excess (Cu-rich) show larger grains and better transport properties compared to CIGS absorbers grown under Cu-deficiency (Cu-poor) conditions. However, solar cells based on Cu-rich CIGS absorbers suffer from significantly lower efficiencies compared to Cu-poor CIGS solar cells. The lower efficiency of Cu-rich CIGS solar cells compared to Cu-poor CIGS cells is attributed to lower open circuit voltage (VOC) in Cu-rich CIGS cells compared to Cu-poor CIGS cells. The reason behind the lower VOC values was investigated and was attributed to recombination losses at the absorber/buffer interface and higher doping of Cu-rich CIGS cells compared to Cu-poor CIGS cells but the complete picture behind the origin of these interface recombination losses and high doping in Cu-rich CIGS cells was not fully understood. The work of this thesis explains why Cu-rich CIGS cells suffer from interface recombination losses, higher doping and lower efficiencies. This explanation is divided into three parts: The first part characterizes Cu-rich and Cu-poor solar cells of the ternary CIS and the quaternary CIGS. This part confirms that Cu-rich CI(G)S solar cells suffer from lower efficiencies, lower VOC, interface recombination losses and higher doping compared to Cu-poor CI(G)S solar cells. Moreover, a 200±20 meV defect was observed for Cu-rich CIS cells. The second part introduces different post-deposition treatments (PDTs) to Cu-rich CI(G)S cells. An ex-situ KF, in-situ KF and a Se-only PDT were introduced to Cu-rich CIS cells. All the three treatments succeeded in improving the VOC, improving the interface recombination losses, decreasing the doping and passivating the 200±20 meV defect that has been identified as a Se-related defect in Cu-rich CIS solar cells. A Ga-Se PDT was introduced to Cu-rich CIGS solar cells and successfully improved the VOC, improved the interface recombination losses and decreased the doping of Cu-rich CIGS solar cells. The third part analyses the changes observed on Cu-rich CI(G)S cells before and after the PDTs. Based on these observations, it was concluded that the origin behind both the interface recombination losses and the high doping of Cu-rich CI(G)S cells is a Se-related acceptor defect (detected by admittance measurements for Cu-rich CIS and speculated for Cu-rich CIGS). The passivation of this defect reduces the recombination losses at the absorber/buffer interface, decreases the doping, improves the VOC and consequently leads to an increase in the efficiency of Cu-rich CI(G)S solar cells. Moreover, this part shows that the Se-related defect is formed as a result of the strong etching step that is mandatory for Cu-rich CI(G)S absorbers to remove conductive copper selenide secondary phases. Applying the same strong etching conditions to Cu-poor CIS absorbers leads to the formation of the Se-related defect. After understanding that the Se-related defect is formed as a result of the strong etching conditions and that the Se-related defect can be passivated with PDTs that are rich in Se, an alternative mean of passivating this defect without PDTs was proposed. The Se-related defect was shown to be passivated using buffer layers of high enough thiourea (source of Sulphur) and without any PDTs leading to the reduction of interface recombination losses, decrease of the doping, increase of the VOC and increase of the efficiency of Cu-rich CIS cells. To conclude, the reason behind the interface recombination losses and high doping in Cu-rich CI(G)S solar cells is a Se-related acceptor defect originating after etching the absorbers with strong etching conditions. This defect can be passivated with high enough chalcogen either with PDTs (high enough Selenium) or buffer layers (high enough Sulphur). [less ▲]

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