Influence of Cu content on electronic transport and shunting behavior of Cu(In,Ga)Se2 solar cells

Abstract

The paper presents the structural and electrical characterizations of Cu(In,Ga)Se2 thin films and thin-film solar cells realized with different Cu contents in the absorber material. It is shown that the bulk resistivity of the Cu(In,Ga)Se2 thin films (measured in coplanar geometry) dramatically increases with decreasing Cu content. Simultaneously, the shunt resistance Rp of the Cu(In,Ga)Se2 solar cells increases with decreasing Cu content in the absorber material. For a wide range of Cu contents, the resistivity of the thin films is directly proportional to Rp of the solar cell made from the same absorber material. We propose that Rp in Cu(In,Ga)Se2 solar cells originates from highly localized shunt regions in the absorber material. The higher Rp of cells made from Cu-poor material is then due to the high resistivity of the embedding Cu-poor material resistively suppressing extensive current flow towards these shunt regions. Further, we observe an increase of the film resistivity by two orders of magnitude if the Cu(In,Ga)Se2 absorbers are produced using a Na blocking layer on the glass substrate. However, the high resistivity of these Na-free samples does not go along with an increase of Rp in the corresponding solar cells, indicating that the high resistivity of Na-free Cu(In,Ga)Se2 results from electrostatic barriers at the grain boundaries and is not a bulk property.

Universality of non-Ohmic shunt leakage in thin-film solar cells

“…13 Experimental work showing the involvement of Al diffusion into the n layers has also been reported. 23 In the case of CIGS cells, one also finds disparate explanations in the literature for this shunt leakage phenomenon, including excess Cu content leading to conduction at grain boundaries or nanoscale phase segregation, 7 and pinholes requiring the use of a i-ZnO buffer layer. 24 It is apparent from the discussion above that while the shunt leakage problem has been observed in all thin film PV technologies, it has been only discussed in isolated contexts.…”

“…This would typically not result in a SCL current. However, due to nonuniformity of electronic properties in CIGS layer ͑e.g., presence of crystal nanodomains, 57 percolating dislocations or grain boundaries 7 etc.͒, certain regions may behave as intrinsic material, which can result in SCL shunt current.…”

“…Also, the magnitude of this leakage current is known to vary significantly and unpredictably from one cell to the other, even when the cells are fabricated under nominally identical conditions. 7,8,12 This variation in shunt leakage magnitude introduces another problem at the module level, where many identical cells must be connected in series to increase the output voltage. However, the variation in shunt current magnitude from cell to cell makes it difficult to predict the final panel output characteristics.…”

“…5 One such key issue affecting performance consistency for large area thin-film solar cells is an excess variable dark leakage current at low biases, commonly referred to as shunt leakage current ͑I sh ͒. [7][8][9][10][11] As shown in the schematic of Fig. 1͑a͒, when this shunt leakage is sufficiently high, it can reduce the fill factor significantly, in turn adversely affecting the cell efficiency.…”

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

“…[34][35][36] It is therefore reasonable to conclude that a Cu-poor region at the GBs is more likely to be present toward the surface of the absorber than toward the back contact. Second, measurements of the majority-carrier transport properties of CIGS films in coplanar geometry, i.e., perpendicular to many GBs, exhibit activation energies in a range between 60 and 120 meV, 41 i.e., much smaller than an effective internal band offset. Therefore, the extra barrier cannot be present along the full length of all grains in the CIGS films.…”