20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

Abstract

Bandgap gradient is a proven approach for improving the open-circuit voltages (VOCs) in Cu(In,Ga)Se2 and Cu(Zn,Sn)Se2 thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers. We show that the introduction of the bandgap gradient reduces the hole density in the front junction region and introduces a small spike in the band alignment between this and the absorber regions, effectively suppressing the nonradiative recombination therein and leading to improved VOCs in Cd(Se,Te) solar cells using commercial SnO2 buffers. A champion device achieves an efficiency of 20.03% with a VOC of 0.863 V.

Water-Assisted Lift-Off Process for Flexible CdTe Solar Cells

“…Figure a,b shows the PL and TRPL spectra measured from the glass side with a 633 nm laser excitation. The PL for the device with CdCl 2 -treated CdS has a significantly higher PL peak intensity than that from the device with untreated CdS, suggesting less nonradiative recombination at the front interface and higher front junction quality. ,, The TRPL shows an improved tau2 from 15.3 to 69.2 ns, further confirming this conclusion. Admittance spectroscopy measurement shows that the apparent hole density is also improved from 1.66 × 10 13 to 2.45 × 10 13 cm –3 through conducting the CdCl 2 treatment for the CdS film (Figure c).…”

Oxygen Management to Avoid Photo-Inactive Cd(S,Se) for Efficient Cd(Se,Te) Solar Cells

“…12−14 As an alternative to ZMO, stable and conductive commercial tin oxide (SnO 2 ) can be used as the front buffer layer to improve the device's performance and the reproducibility. 5 The construction of the Cd(O,S,Se,Te) gradient region via intermixing of multiple layers (Figure S1a) is achieved through appropriate oxygen management. Oxygen management has been demonstrated to be a facile and crucial strategy to improve traditional solar cell performance, such as converting the surface dead layer into an insulating layer for defect passivation in crystalline silicon solar cells 15 and regulating the intermixing at the CdS/Cu(In,Ga)Se 2 interface to form a buried p−n homojunction.…”

“…This phenomenon has also been demonstrated in previous publications. 5,20 The reduced crystal size and crystallinity lead to higher surface energies, which facilitate intermixing with the CdTe layer and the formation of the Cd(O,S,Se,Te) gradient region at the front interface.…”

“…Recently, we demonstrated the formation of a photoactive penternary cadmium chalcogenide Cd(O,S,Se,Te) region at the front junction by utilizing CdS/CdSe/CdTe stacks. 5 The Cd(O,S,Se,Te) material has a zinc blend crystal structure, in contrast to the photoinactive wurtzite structure of Cd(S,Se), which is the same crystal structure as that of the remaining absorber components, i.e., the Cd(Se,Te) and CdTe. Thus, there is no detrimental heterointerface between the Cd(O,S,Se,Te) and Cd(Se,Te), as shown in Figure S1.…”

“…Cadmium sulfide (CdS) has been excluded from the fabrication of Cd­(Se,Te) thin-film solar cells due to the formation of a photoinactive Cd­(S,Se) region at the front interface. − The formation of the photoinactive layer generally happens during the CdCl 2 treatment of the CdS/CdSe/CdTe stacks, which introduces significant nonradiative recombination at the front interface and thereby lowers the short-circuited current density ( J SC ) and open-circuited voltage ( V OC ). Recently, we demonstrated the formation of a photoactive penternary cadmium chalcogenide Cd­(O,S,Se,Te) region at the front junction by utilizing CdS/CdSe/CdTe stacks . The Cd­(O,S,Se,Te) material has a zinc blend crystal structure, in contrast to the photoinactive wurtzite structure of Cd­(S,Se), which is the same crystal structure as that of the remaining absorber components, i.e., the Cd­(Se,Te) and CdTe.…”