Emitter/absorber interface of CdTe solar cells

Abstract

The performance of CdTe solar cells can be very sensitive to the emitter/absorber interface, especially for high-efficiency cells with high bulk lifetime. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. Numerical simulations show that the emitter/absorber band alignment, the emitter doping and thickness, and the defect properties of the interface (i.e., defect density, defect type, and defect energy) can all play significant roles in the interface recombination. In particular, a type I heterojunction with small conduction-band offset (0.1 eV ≤ ΔEC ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se2 (CIGS) cells. The basic principle is that positive ΔEC, often referred to as a “spike,” creates an absorber inversion and hence a large hole barrier adjacent to the interface. As a result, the electron-hole recombination is suppressed due to an insufficient hole supply at the interface. A large spike (ΔEC ≥ 0.4 eV), however, can impede electron transport and lead to a reduction of photocurrent and fill-factor. In contrast to the spike, a “cliff” (ΔEC < 0 eV) allows high hole concentration in the vicinity of the interface, which will assist interface recombination and result in a reduced open-circuit voltage. Another way to mitigate performance losses due to interface defects is to use a thin and highly doped emitter, which can invert the absorber and form a large hole barrier at the interface. CdS is the most common emitter material used in CdTe solar cells, but the CdS/CdTe interface is in the cliff category and is not favorable from the band-offset perspective. The ΔEC of other n-type emitter choices, such as (Mg,Zn)O, Cd(S,O), or (Cd,Mg)Te, can be tuned by varying the elemental ratio for an optimal positive value of ΔEC. These materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.

Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

“…This observation suggests that the dominant recombination for CdS buffered CBTS solar devices takes place at the heterojunction interface where holes from CBTS recombine with electrons from CdS, largely because of the conduction band misalignment and large lattice mismatch between CBTS and CdS. [13,30] As oxygenation increases, more CdSO 4 passivators form and the CBM of CdS:O shifts to higher energy as well. [9] In addition, the large lattice mismatch between CBTS and CdS may cause a high density of interface defects, which can act as effective paths for carrier recombination at the heterojunction interface.…”

“…[18] Therefore, the electron transport paths from CBTS are gradually reduced in CdS:O with the increase of oxygenation due to the formation of insulating CdSO 4 as illustrated in Figure 4f. This large conduction band offset, CBO = +0.56 V, can act as a robust barrier for the electron transport from the CBTS to the front contact, [13,30,31] even with the advantage of reducing interface recombination. Eventually, the increased R S 's dramatically degrade the FF's.…”

Influences of buffer material and fabrication atmosphere on the electrical properties of CdTe solar cells

“…A recent advance in CdTe solar cells is the replacement of the traditional CdS buffer (emitter) by a wide bandgap metal oxide, such as ZnMgO (ZMO) . This replacement not only avoids the current loss caused by CdS but also introduces a spike to the conduction band offset (CBO) between the buffer and CdTe absorber . Device modeling has shown that an appropriate CBO spike can reduce interface recombination and increase the open‐circuit voltages (V OC ) of CdTe solar cells .…”

Solution-Processed CdTe Thin-Film Solar Cells Using ZnSe Nanocrystal as a Buffer Layer

“…It is noted that the hole barrier levels for CdTe/ZnSe heterojunction depends on both the valence band levels and the Fermi levels of the CdTe and ZnSe. A small positive conduction-band offset may help maintain good cell efficiency as it created a large hole barrier adjacent to the interface and reduced interface recombination [32][33][34]. Efficiency up to 11% has been attained in the case of Cu(In,Ga)Se 2 thin-film solar cells with ZnSe buffer layer [35] while >21% efficiency has been achieved for the ZnS buffer layer [36].…”