Generalized current-voltage analysis and efficiency limitations in non-ideal solar cells: Case of Cu2ZnSn(SxSe1−x)4 and Cu2Zn(SnyGe1−y)(SxSe1−x)4

Abstract

Detailed electrical characterization of nanoparticle based Cu2ZnSn(SxSe1−x)4 (CZTSSe) and Cu2Zn(SnyGe1−y)(SxSe1−x)4 (CZTGeSSe) solar cells has been conducted to understand the origin of device limitations in this material system. Specifically, temperature dependent current-voltage analysis has been considered, with particular application to the characterization of solar cells with non-ideal device behavior. Due to the presence of such non-ideal device behavior, typical analysis techniques—commonly applied to kesterite-type solar cells—are found to be insufficient to understand performance limitations, and an analysis methodology is presented to account for the non-idealities. Here, the origin of non-ideal device behavior is chiefly considered in terms of electrostatic and band gap potential fluctuations, low minority carrier lifetimes, temperature dependent band edges, high surface/bulk recombination rates, and tunneling enhanced recombination. For CZTSSe and CZTGeSSe, the main limitations to improved device performance (voltage limitations) are found to be associated with significant EA deficits (EA–EG) at 300 K, large ideality factors, and voltage-dependent carrier collection, which we associate with the bulk material properties of the absorbers. The material origin of these non-ideal electrical properties is considered. Additionally, for CZTGeSSe, the effect of Ge-incorporation on the electrical properties of the solar cells is discussed, with improvements in the electrical properties characterized for the Ge-alloyed devices.

Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials

“…In light of device simulation results and the published PL decay behavior for kesterite absorbers and devices, we believe this is a common result in TRPL analysis reported for kesterites. These results further support [20,25,39,57] that the low performance in kesterites can be explained by the bulk properties of the absorber. While bandtailing/potential fluctuations have been shown to contribute to V OC limitations in kesterite devices, [20,25,57] here we demonstrate that a low minority carrier lifetime is also responsible for significant performance limitations in these materials.…”

“…[57] These results illustrate that the minority carrier lifetime is expected to be below that of the measured PL decay time for the kesterite absorbers. This analysis is particularly relevant for kesterite absorbers where minority carrier lifetimes in the nanosecond regime-with values as high as 10-20 ns-are reported from the PL decay time, as the PL yield should be consistent with the minority carrier lifetime.…”

“…Specifically, analysis of the saturation current J 0 = J 00 × exp[−E A /kT] and V OC modeling indicate a weakly temperature-dependent saturation current prefactor J 00 for 100 K ≤ T ≤ 300 K, where J 00 ∝ τ −1 or J 00 ∝ τ −1/2 for recombination limited in the SCR or neutral bulk absorber, respectively. [57] On the other hand, an exponential dependence on temperature is exactly what is expected from thermal emission of carriers from a trap state, as detailed in Section 2.5.…”

Analysis of Failure Modes in Kesterite Solar Cells

“…VOC deficit in kesterite solar cells can originate either from the bulk material or from its interfaces with supporting layers (buffer layer and Mo back electrode). The two main reasons generally invoked to explain this high VOC deficit are a short minority carrier lifetime and the presence of band tails in the bulk of the absorber material [36][37][38] . It is worth noticing that both causes cannot be considered at the same level.…”

Spectral Responses in Quantum Efficiency of Emerging Kesterite Thin-Film Solar Cells

“…In this case, the depletion layer of n-CdS/p-CZTSSe is located in almost all the p-CZTSSe absorber layer, which is fundamentally similar to an abrupt n+/p junction diode or a Schottky diode. Following the band structure [19], the depletion width ( Figure 2) in the n-CdS/pCZTSSe is derived as follows [20].…”