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.
The strong growth of the PV market is accompanied by an increasing number of ''new'' PV technologies and concepts now mature for commercialization. A correct calibration of these devices is in some cases very difficult, because indoor and outdoor performance measurements often lead to different results. In this paper we compare the indoor and outdoor performance measurements of a set of recent commercially available PV modules (conventional and high-efficiency c-Si, single-, double-, and triple-junction thin film (TF) technologies) and we observe that the maximum power P max of some devices measured indoors using our large area pulsed solar simulator is usually lower than the power measured outdoors under natural sunlight. The major effects which lead to these discrepancies are identified, as follows: (a) spectral mismatch errors, very significant for CdTe, and all a-Si TF technologies; (b) measurement-related sweep-time effects, which seem to strongly influence the performance of high efficiency c-Si devices and to a lesser extend of all a-Si TF technologies; and (c) short-time light-soaking effects, which influence the performance of CIS and to a lesser extent CdTe.
Several works report on seasonal fluctuations of power production of amorphous silicon (a-Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler-Wronski effects. It is hence difficult to assess-for a given location and climatic conditions-which one has the largest impact. By means of a straightforward approach based on two sets of singlejunction a-Si photovoltaic modules (stored indoors/exposed outdoors) and on two different I-V measurement set-ups (indoor and outdoor), we were able to separate the different contributions to this phenomenon. For the test-site of Lugano, seasonal oscillations account for performance variations of a-Si of~10% (AE5% around an annual average value with a minimum around the mid of January and a maximum around mid-July). The time-phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a-Si with an amplitude corresponding to 10.5% (AE~5.2%). Moreover, the influence of spectral variations on the outdoor performance of a-Si (and for comparison of c-Si) was modeled, and the experimental data were found to be in excellent agreement with the theoretical simulation; (ii) the Staebler-Wronski effect has a slightly lower influence with an amplitude of~8% (AE4% with a minimum at the middle of February and a maximum around mid-August). Because of the position (46 N) and average climatic conditions (southern Alpine climate) of Lugano, these observations are possibly representative of a large part of continental Europe.
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