Ultrafast pump-probe reflectance spectroscopy: Why sodium makes Cu(In,Ga)Se2 solar cells better

Abstract

(O.F.M.) 2 | P a g e TOC graphicCIGS thin-film samples were investigated for the first time using femtosecond pump-probe differential reflection spectroscopy with broadband capabilities and 120-fs temporal resolution.The pump-and-probe beams were focused on ~1.6 m-thick CIGS films. The reflected probe light from the samples was collected and focused on the broadband infrared detector (D) for recording the carrier dynamics of CIGS in real time.3 | P a g e ABSTRACT Although Cu(In,Ga)Se 2 (CIGS) solar cells have the highest efficiency of any thin-film solar cell, especially when sodium is incorporated, the fundamental device properties of ultrafast carrier transport and recombination in such cells remain not fully understood. Here, we explore the dynamics of charge carriers in CIGS absorber layers with varying concentrations of Na by femtosecond (fs) broadband pump-probe reflectance spectroscopy with 120-fs time resolution.By analyzing the time-resolved transient spectra in a different time domain, we show that a small amount of Na integrated by NaF deposition on top of sputtered Cu(In,Ga) prior to selenization forms CIGS, which induces slower recombination of the excited carriers. Here, we provide direct evidence for the elongation of carrier lifetimes by incorporating Na into CIGS. KEYWORDSCIGS solar cells, ultrafast photophysics, pump-probe reflectance spectroscopy, carrier recombination, dielectric function model 4 | P a g e

Enhanced Charge Carrier Separation and Improved Biexciton Yield at the p–n Junction of SnSe/CdSe Heterostructures: A Detailed Electrochemical and Ultrafast Spectroscopic Investigation

“…The morphology of particles is still spherical as that of CdSe QDs (used as a template) after cation exchange, which indicates that the shape and morphology of CdSe particles in the SnSe/CdSe HS system Understanding the carrier cooling and transfer dynamics across the junction of the SnSe/CdSe HS plays a pivotal role in improving the performance of the devices such as solar cells, photodetectors etc. 36,37 The optical absorption spectrum of SnSe NSs presented in Figure 3A,a shows a broad absorption band in the region from 400 to 1600 nm peaking around 700 nm. The band gap of SnSe was calculated using a tauc plot, which is shown in Figure 3A inset.…”

Impact of RbF and NaF Postdeposition Treatments on Charge Carrier Transport and Recombination in Ga‐Graded Cu(In,Ga)Se2 Solar Cells

“…[ 7,9,11,12 ] Other studies showed that the increase in V OC and FF can result from the passivation of non‐radiative recombination centers at grain boundaries (GBs), evidenced by several atom probe tomography studies showing Na accumulation predominantly at GBs compared to grain interiors (GIs). [ 13–20 ] Transient optical studies correlated increased charge carrier lifetime with optimal Na atom incorporation [ 21–23 ] Eid et al utilized ultrafast pump‐probe reflectance spectroscopy to study Ga‐graded CIGSe devices with different thicknesses of NaF (incorporated prior to selenization) and found a 1.5‐fold increase in minority carrier lifetime in samples with higher Na concentrations which was correlated with higher device performance [ 10,21 ] However, owing to the variation in the composition gradient with different Na concentrations in this experiment design, the role of Na atoms could not be clearly identified. Lee et al detected a ≈fivefold prolonged charge carrier lifetime on CIGSe (GGI = 0.14) grown on SLG compared to reference films grown on borosilicate glass, concluding that the addition of Na atoms prohibits the formation of defect states at the CdS/CIGSe junction and hence increases carrier lifetime.…”

Imaging the Reduction of Electron Trap States in Shelled Copper Indium Gallium Selenide Nanocrystals Using Ultrafast Electron Microscopy

“…This surface treatment improves the open circuit voltage ( V OC ) and fill factor (FF) of the device. ,− Depending on the deposition technique, several mechanisms have been proposed for the efficiency enhancement, which can be broadly classified into two categories(a) influence on growth kinetics that leads to different grain sizes, smoother texture, and suppression of defect formation − and (b) improvement of electronic properties by removal of deleterious donor states and introduction of acceptor states, which in turn increases the net hole concentration and hence p-type conductivity. ,, In addition, Na is reported to passivate grain boundary defects and reduce minority carrier traps. , However, the exact reason for such efficiency enhancement remains under debate. One of the reasons behind this is the lack of suitable surface-sensitive characterization techniques that can observe charge carrier dynamics in real time, precisely on the surface, as it is well-known that Na is mostly localized onto the surface of CIGSe, rather than in the bulk of the film. ,,,, The structural changes on the surface post Na incorporation can be verified following several microscopic and spectroscopic characterization techniques; however, its effects on modifying the electronic properties of CIGSe have only been inferred from the device measurements or predicted from theoretical calculations. − Spectroscopic studies have also been reported to understand the effect of Na on charge carrier dynamics; however, because of the large penetration depth of the laser beam, these studies provide the dynamical information mainly from the bulk of the material rather than the surface. , …”

“…12−28 Spectroscopic studies have also been reported to understand the effect of Na on charge carrier dynamics; however, because of the large penetration depth of the laser beam, these studies provide the dynamical information mainly from the bulk of the material rather than the surface. 29,30 Herein, we explore and decipher the impact of Na incorporation on CIGSe NCs thin film by four-dimensional scanning ultrafast electron microscopy (4D S-UEM), which has the unique capability to visualize charge carrier dynamics selectively on material surfaces in real time and space with unprecedented spatial and temporal resolution. 11,31−39 The concept of the experimental technique is depicted in Scheme 1.…”