Reduced recombination through CZTS/CdS interface engineering in monograin layer solar cells

Abstract

The power conversion efficiency of CZTS solar cells is still limited by deep defects, low minority carrier lifetime and high recombination rates at the CZTS/CdS interface. The objective of this study was to find an effective method to reduce interface recombination of CZTS monograin layer solar cells. Two-step heterojunction formation process was applied by controlling the intermixing of Cd and Cu in the CZTS/CdS interface, which resulted in improved device efficiency up to 11.7%. Surface analysis by X-ray photoelectron spectroscopy confirmed the Cd diffusion into surface of CZTS after CdS air-annealing by forming an ultra-thin Cu2ZnxCd1-xSnS4 layer. Also, external quantum efficiency measurements showed that the absorption edge shifts to longer wavelengths with addition of Cd into CZTS surface layer. This surface modification and replacement of CdS:Cu buffer layer by fresh CdS greatly reduced the interface recombination and improved the junction quality, contributing to an enhancement of JSC ∼3 mA/cm2 (from 20.5 to 23.6 mA/cm2), and fill factor ~14% (from 59.4 to 67.7%). Serial resistance of the CZTS monograin layer solar cells was significantly reduced from 2.4 Ω cm2 to 0.67 Ω cm2. To understand in more detail the electrical behavior of highest efficiency CZTS monograin layer solar cell, the temperature dependent current–voltage characteristics were analyzed.

Sb2S3 Thin-Film Solar Cells Fabricated from an Antimony Ethyl Xanthate Based Precursor in Air

“…The series resistance of both devices under illumination and in the dark (see Figure c) is fitted with a simple exponential dependence R normalS ( T ) = R normalS 0 / ( 1 + β exp false( prefix− E normala / k T false) ) where R S is the series resistance, R S0 a temperature-independent series resistance, β a prefactor, E a the activation energy, k Boltzmann’s constant, and T temperature.…”

“…The series resistance of both devices under illumination and in the dark (see Figure 5c) is fitted with a simple exponential dependence 46…”

“…Therefore, interface recombination dominates in these solar cells, apparently increasing when the absorber is grown at a higher temperature, meaning that the absorber interfaces deteriorate. Curiously, temperature-dependent V OC of fully inorganic Sb 2 S 3 solar cells has only ever been reported down to 210 and 140 K, with a built-in voltage of 1.08 or 0.97 V. , However, whenever tunneling is present the ideality factor becomes temperature dependent, turning V OC vs T nonlinear . Therefore, the activation energy calculated by extrapolating V OC might be inaccurate and should be supplemented with Φ B calculated from n and J 0 .…”

“…The 0.1 eV smaller activation energy under illumination reveals that interface recombination is the dominant mechanism, which is ascribed to Fermi level pinning or narrowing of the band gap at the interface. 46 For the 260 °C Sb 2 S 3 device, the activation energy is 1.18 ± 0.11 eV at the "high" temperature, 1.55 ± 0.02 eV at the "low" temperature in the dark, and 1.84 ± 0.09 eV under illumination (Figure 5e). As Φ B > E g , bulk recombination within the absorber is likely in balance with or dominates interface recombination.…”

Emergence of flexible kesterite solar cells: progress and perspectives

“…After removing the defective surface layer by chemical etching, and ordering treatment and surface passivation by air annealing, the efficiency of the monograin CZTSSe unit cell is estimated to be ~12% 79 . While, due to the non-active area between the monograins, the overall efficiency of monograin layer CZTSSe solar cells is slightly lower than the unit cells 18,78,80,81 .…”