Optoelectronic properties of thin film Cu2ZnGeSe4 solar cells

Abstract

The fabrication and properties of a Ge-based Kesterite Cu2ZnGeSe4 (CZGSe)solar cell are discussed. The existence of the quaternary compound has been verified by physical methods such as X Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The Cu2ZnGeSe4 solar cell has a power conversion efficiency (PCE) of 5.5% under AM1.5G illumination which is among the highest reported for pure Ge substitution. Detailed low temperature current-voltage and time-resolved photoluminescence measurements show that the Cu2ZnGeSe4 absorber has less bulk defects and less band tailing in contrast to the typical characteristics of Cu2ZnSnSe4 devices. These beneficial opto-electronic properties also resulted in a high open circuit voltage (Voc) of 744 mV which is amongst the highest values reported for Kesterite materials.

Band Gap Tuning of Cu2ZnGeSxSe4-x Absorbers for Thin-Film Solar Cells

“…In this series, the lowest V OC deficit of 868 mV could be obtained with 5 mg of GeS. However, this value is still much higher than for the most efficient CZTSSe absorber (617 mV [27]) and higher than the deficit of 656 mV that can be obtained for a CZGSe absorber with a band gap of 1.4 eV [4]. As band gap increases, the V OC deficit increases to values larger than 1 V, thereby drastically limiting the photovoltaic performance.…”

“…Its similarity to the extensively studied Cu 2 ZnSnS x Se 4-x (CZTSSe) offers potential for fast progress in the improvement of material properties and solar cell efficiency, with a record value of 6.0% so far [3]. However, the band gaps in previous publications are in a range between 1.4 eV [4] and 1.5 eV [5] and are therefore still too low for application as top cells in multijunction solar cells.…”

Doping and alloying of kesterites

“…Thus, the full range of Ge substitution has been investigated to some extent, including the pure Ge compound in recent reports, demonstrating that the CZGSe exhibits also kesterite structure [123]. Among the most relevant pure Ge kesterite results, Schnabel et al reported efficiencies exceeding 5% for a sulfo-selenide CZGSSe (E g ∼1.5 eV) solar cell [124], while for the selenide CZGSe (Eg∼1.4 eV) compound, Sahayaraj et al published a 5.5% efficiency device with a remarkable V OC of 744 mV [125]. Intriguingly, the Ge based kesterite showed less electrical losses from band tailing/electrostatic potential fluctuations compared to Ge free devices [125,126].…”

Analysis of Failure Modes in Kesterite Solar Cells

“…The origin of these low efficiencies can be mainly attributed to a limited VOC, which never surpasses 65% of SQ limit whereas FF and JSC can reach up to 80% of this limit. Introduction of Ge in CZTGSe absorbers has been claimed to improve devices properties and particularly VOC [22][23][24][25][26] but Ge containing devices currently do not outperform their Ge-free counterparts in literature. At low Ge content, FF and VOC are similar to Ge-free CZTSSe solar cells but with lower JSC while at higher content, an additional drop in FF is noticed.…”

“…12 . PV data and related bandgaps from References [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] . Bandgaps are extracted from EQE spectra.…”