Absorber composition: A critical parameter for the effectiveness of heat treatments in chalcopyrite solar cells

Abstract

Post‐device heat treatment (HT) in chalcopyrite [Cu (In,Ga)(S,Se)2] solar cells is known to improve the performance of the devices. However, this HT is only beneficial for devices made with absorbers grown under Cu‐poor conditions but not under Cu excess. We present a systematic study to understand the effects of HT on CuInSe2 and CuInS2 solar cells. The study is performed for CuInSe2 solar cells grown under Cu‐rich and Cu‐poor chemical potential prepared with both CdS and Zn(O,S) buffer layers. In addition, we also study Cu‐rich CuInS2 solar cells prepared with the suitable Zn(O,S) buffer layer. For Cu‐poor selenide device, low‐temperature HT leads to passivation of bulk, whereas in Cu‐rich devices, no such passivation was observed. The Cu‐rich devices are hampered by a large shunt. The HT decreases shunt resistance in Cu‐rich selenides, whereas it increases shunt resistance in Cu‐rich sulfides. The origin of these changes in device performance was investigated with capacitance–voltage measurement, which shows the considerable decrease in carrier concentration with HT in Cu‐poor CuInSe2, and temperature‐dependent current–voltage measurements show the presence of barrier for minority carriers. Together with numerical simulations, these findings support a highly doped interfacial p+ layer device model in Cu‐rich selenide absorbers and explain the discrepancy between Cu‐poor and Cu‐rich device performance. Our findings provide insights into how the same treatment can have a completely different effect on the device depending on the composition of the absorber.

Near surface defects: Cause of deficit between internal and external open‐circuit voltage in solar cells

“…There are no deep defects at the interface in this model (Figure 4A). Recently we have presented a similar model of a defective layer with an acceptor defect 220 meV away from the valence band 36 . All the features of the temperature dependence of the JV characteristics, as discussed in the following, are the same, independent of the energetic position of the defect (see also Figure S9).…”

“…The etching results in high concentration >10 16 cm -3 of deep defects (~200 meV) in Cu-rich CISe absorbers. 32,33 However, it is unknown whether the defect originates specifically from the KCN etching or from the etching process of secondary phase independent of the etchant used. To investigate this, Cu-rich CISe solar cells are prepared using two different etching solutions: 10% aqueous KCN solution (for reference) and 0.16 % mM aqueous Br solution.…”

“…This is due to the necessary KCN etching step required to remove the secondary Cu 2‐x Se phase. The etching results in high concentration >10 16 cm −3 of deep near‐interface defects (~200 meV) in Cu‐rich CISe absorbers 35,36 . The defects are termed as near‐interface as AS performed at different DC applied voltage does not yield a voltage‐dependent defect activation energy, which would be typical for interface defects (see Figure S1B,C).…”

“…A device model is designed in SCAPS‐1D emulating the Cu‐rich CISe devices (back contact/CISe/CdS/ZnO/Al:ZnO/front contact). Table S1 records the electrical and optical parameters used in the simulations, which were set constant, taking values from previous measurements, 46–48 and are the same as in our earlier simulations 36 . Further, no conduction band offset at the absorber/buffer (A/B) interface and flat band conditions at the absorber back contact were assumed to keep the model as simple as possible and to avoid convergence problems in SCAPS.…”

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

“…They are related to a photo-dependent barrier for electron flow due to several reasons-band offsets at the absorber-buffer and buffer-window interface, defective chalcopyrite layer near the absorber surface, defects at the buffer/window interface. 43,60 Figure 4A shows crossover in the J-V curves for Cu-poor Cu(In,Ga)S 2 solar cells with CdS and Zn(O,S) buffer layers. Also, a similar crossover is observed in the J-V curve for Curich Cu(In,Ga)S 2 solar cells with Zn(O,S) buffer layer, as shown in Figure 4B.…”