Controlled activation of ZnTe:Cu contacted CdTe solar cells using rapid thermal processing

Abstract

Back contacts can significantly limit CdTe solar cell performance, reducing both open circuit voltage (V oc) and fill factor (FF). Copper is an essential component of effective back contacts, but its presence in the CdTe absorber creates detrimental recombination centers. Rapid thermal processing (RTP) is demonstrated as a highly effective approach for reducing back contact barriers in CdTe solar cells contacted with ZnTe:Cu buffer layers, substantially improving both FF (>73%) and V oc (>850 mV). Current density and quantum efficiency remain essentially unchanged, but a five-fold increase in minority carrier lifetime is observed which is attributed to passivation of recombination sites in the back contact region. Quantitative analysis of secondary ion mass spectrometry shows that the majority of Cu segregates to the Au metallization layer and that the ZnTe buffer appears to inhibit the Cu diffusion into CdTe. 3D imaging of the back contact region using atom probe tomography shows that optimized devices are characterized by preferential segregation of copper to both the Au|ZnTe and CdTe|ZnTe interfaces, perhaps in the form of Cu x Te. With its low thermal budget the RTP process has been successfully applied to multiple device architectures. including devices with certified efficiencies in excess of 16%.

Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells

“…Second, it is observed that there is significant accumulation of Cu at the Au|ZnTe interface. This observation is consistent with previous analysis of quantified SIMS results [8], which suggested that up to 80% of the Cu deposited in the ZnTe:Cu layer segregates into the gold during RTP processing. This is perhaps unsurprising as these two metals are completely miscible.…”

“…The RTP treatment results in significant improvement in both Voc and FF [8,9]. It is postulated that the Cu migration and ZnCd interdiffusion observed in this work are responsible for these improvements.…”

“…There appears to be substantial grain growth, in particular one observes twins not unlike those seen in CdTe, particularly in the region adjacent to the gold metallization layer. In contrast the region adjacent to the CdTe layer has a much more amorphous appearance, and it is suggested that the cause may be Zn-Cd interdiffusion, as previously observed in these contacts by atom probe tomography (APT) [8]. To further assess this the composition was evaluated by performing vertical EDAX line scans through this region, with representative results being shown in Figure 5.…”

“…Deposition of the FTO/TO/CdS:O/CdTe layers occurred at the National Renewable Energy Lab (NREL) [10]. Application of the back contact and RTP activation was conducted at the Colorado School of Mines (CSM) [8]. In this work we compare the composition and structure of the back contact region in nominally identical devices, one in the asdeposited state and a second that was provided with an optimal RTP treatment.…”

“…With copper having both positive and deleterious effects it is critical to precisely control both its amount and spatial distribution in order to obtain high efficiency. We have recently introduced a back contact procedure in which ZnTe:Cu is coevaporated at low temperature followed by activation using rapid thermal processing (RTP) [8,9]. In this work, high resolution transmission electron microscopy (HR-TEM) coupled with elemental analysis is applied to understand the microstructure and composition of the back contact region in optimally processed devices.…”