Impact of compositional grading and overall Cu deficiency on the near-infrared response in Cu(In, Ga)Se2 solar cells

Abstract

Highly efficient thin film solar cells based on co-evaporated Cu(In,Ga)Se 2 (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu-poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near-infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection-transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu-dependent modifications of the NIR-absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re-adjustments of the coevaporation process and of the alkali-fluorides post-deposition treatments allow maintaining V oc and FF values, yielding an overall increase of efficiency as compared to a reference baseline.

Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance

“…CIGS layers were deposited by multistage evaporation in an in-house built unit on soda-lime glass substrate, as described elsewhere [11]. A silicon oxide barrier layer was deposited between the glass and the sputtered molybdenum back contact in order to prevent diffusion of alkali elements into the CIGS.…”

“…Sample 3 corresponds to a full solar cell stack including a complete CIGS deposition, NaF and RbF postdeposition treatment (PDT), CdS chemical bath deposition, sputtered unintentionally doped ZnO and ZnO:Al (2 wt.% Al 2 O 3 ) window layers, Ni/Al grids, and MgF 2 antireflective coating (ARC). All processes are described in detail in a previous publication [11]. …”

“…A good understanding has been achieved regarding the formation of beneficial GGI gradings across the depth of the CIGS absorbers [11,12], and also the existence of voids in the CIGS layer has previously been reported [13,14]. However, the formation mechanism of lateral inhomogeneities of the CIGS composition has not been investigated yet.…”

Advanced Alkali Treatments for High‐Efficiency Cu(In,Ga)Se2 Solar Cells on Flexible Substrates

“…[8] and references therein). [12,13] CIGS layers presenting a degree of Cu excess also exhibit improved crystallinity [14] and sharper photoluminescence (PL) spectra. [9,10] Multiple reasons motivate an increase of the Cu content toward stoichiometry: lower Urbach energies indicating lesser amplitude of potential fluctuations, lower defect density, and improved transport [11] or increased optical absorption.…”

A short review on the advancements in electroplating of CuInGaSe2 thin films

“…The experimental and simulation studies have shown that double grading in the CIGS absorber layer has greatly improved the performance of single-junction CIGS cells [40,41]. For detailed information on grading schemes and their benefits in device performance, the readers can refer the cited literatures [42][43][44].…”