Impact of Nanoscale Elemental Distribution in High‐Performance Kesterite Solar Cells

Abstract

The effort to develop earth‐abundant kesterite solar cells has led to an approximate doubling of the power conversion efficiency over the past five years to 12.6%, primarily due to increases in short‐circuit current and fill factor; open‐circuit voltage has resisted similar change, limiting further efficiency improvement. In the present investigation, Auger nanoprobe spectroscopy, X‐ray/ultraviolet photoelectron spectroscopy, and device characterization are used to provide a comprehensive understanding of the role of grain boundaries and interfaces in limiting performance in kesterite‐based devices. High photovoltaic performance is found to correlate with grain boundaries that are Cu‐depleted and enriched with SnOx. The formation of this bulk‐like oxide at grain boundaries with type I band offset provides a unique effective passivation that limits electron‐hole recombination. Building on these new insights, photovoltaic device simulations are performed that show optimized electrostatic designs can compensate for bulk defects, allowing efficiencies closer to the theoretical limit.

Analysis of Failure Modes in Kesterite Solar Cells

“…Shockley-Read-Hall (SRH) recombination on point defects in the bulk of the absorber is generally said to be responsible for this short lifetime 42 but the influence of band tails, grain boundaries or even interfaces on lifetime is rarely discussed 13 . SRH recombination and tunneling-enhanced recombination 38 , grain boundary recombination 43 , low carrier mobility due to defect scattering 46 or carrier localization 50 and insufficient quasi-Fermi level splitting because of too low carrier density 51 or Fermi level pinning 52 are mentioned as well as responsible for this VOC deficit. Moreover, the absence of internal electric field cannot counterbalance these poor electronic properties 43 .…”

Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers

“…In studies where the annealing treatment is performed in air, the focus is mainly directed to oxidation of grain boundaries [5,8]. It is suggested that SnO x formation in grain boundaries effectively passivate grain boundaries [5]. Another study find that oxygen substitutes for Se in the lattice near grain boundaries [8].…”

“…It is argued that the annealing treatment can be used to obtain an appropriate level of Na [7]. In studies where the annealing treatment is performed in air, the focus is mainly directed to oxidation of grain boundaries [5,8]. It is suggested that SnO x formation in grain boundaries effectively passivate grain boundaries [5].…”

Oxide-induced grain growth in CZTS nanoparticle coatings

“…39 The absence of copper at the particle surfaces may be attributed to the formation of CuSO 4 , which is volatile at temperatures much lower than the air annealing temperature.…”

“…Nevertheless, the conclusion from a recent study by Sardashti et al is that the presence of tin-oxide phases and copper-depletion at the surface of the grains in a CZTS thin lm act as a passivating layer, inhibiting charge recombination at grain boundaries. 39 Air annealing CZTS/CZTSSe-based PVs aer production is now commonly applied to enhance device efficiency by this passivation mechanism. 40 Structurally, both of the annealed lms in Fig.…”