Development and characterization of (Ag,Cu)(In,Ga)Se2 thin films deposited by three-stage co-evaporation
Date
2016
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
A novel thin-film semiconductor material (Ag,Cu)(In,Ga)Se2 (ACIGS) has kindled interest on new chalcopyrite and even kesterite system alloyed with silver (Ag) with its achievement in photovoltaic application. Solar cells based on ACIGS absorbers have reached several efficiency milestones at a variety of bandgaps (Eg), including a champion device built on a soda-lime glass substrate at Eg=1.2 eV achieving 19.9%, a device on glass substrate at Eg=1.5 eV with 13.0% efficiency, and a device on polyimide substrate at Eg=1.35 eV with 17.9% efficiency. Compared with the more mature thin-film photovoltaic absorbers Cu(In,Ga)Se2 (CIGS), ACIGS features lower melting temperature (Tm), wider Eg and even presumably different defect formations. The lowered melting temperature could lead to larger grain size and possibly reduced defects, if films are deposited at the same growth temperature (Tss). The adjustability of the Eg via Ag incorporation offers a new pathway to wide-bandgap CIGS absorber which historically has been obstructed by the defects introduced from widening Eg via Ga alloying only. Different synthesis methods have been adopted to ACIGS deposition from CIGS growth including element co-evaporation and sputtered metal precursor reaction. A particular co-evaporation method call three-stage co-evaporation is the focus of the study, and this dissertation investigates and discusses how the material evolves throughout this deposition process and explores unique properties arising from the incorporation of Ag. ☐ The three-stage co-evaporation process has been widely used and successfully produced high performance CIGS absorbers. The suitability of depositing ACIGS films using this process and growth at higher substrate temperature were evaluated with the use of specialty high temperature glass as substrates. Thin-film ACIGS was deposited by the three-stage process at both Tss=580°C and 650°C. It was determined by secondary ion mass spectroscopy (SIMS) that composition ratios Ag/(Ag+Cu) and Ga/(In+Ga) formed a hump and notch shape gradient profiles, respectively, and the gradients were flattened by the 650°C growth temperature. Using an intercept counting method, the grain size of ACIGS films was found almost twice as large as that of corresponding CIGS films grown under the same conditions, and also twice as large at higher growth temperature. These observations are consistent with a zone growth model with grain growth affected by Tm/Tss. ACIGS films exhibited the structure of a single chalcopyrite phase and their film texture wasn't affected by different growth temperature. ☐ Secondary phases, besides the primary chalcopyrite phase, have been shown in literature to be one of the key features in CIGS three-stage deposition influencing films' surface morphology, Ga gradient, distribution of sodium diffused from glass substrates and eventually device performance. To understand and optimize the ACIGS three-stage growth, a systematic study was conducted on possible secondary phases formed during the group-I rich [(Ag+Cu)/(In+Ga)>1] ACIGS growth, by depositing films with Ag/(Ag+Cu)=0, 0.1 and 0.5 until the end of the second stage of the three-stage process. SIMS depth profiling located secondary phases containing Ag-Cu-Se compounds near the film surfaces with composition ratio (Ag+Cu)/(In+Ga)>1 and higher Ag/(Ag+Cu) than in the bulk films. Scrutiny of this surface region by transmission electron microscopy discovered an intermixture of the secondary and chalcopyrite phases when Ag was added, distinct from a separate layer of secondary phases when there was no Ag. X-ray diffraction identified the secondary phases as HT--Cu2-xSe, RT--Cu2Se, HT--(Ag,Cu)2-xSe, RT--AgCuSe and RT--Ag2Se structures. Their existence in the films depended on Ag/(Ag+Cu), which is consistent with the phases along the Cu2Se -- Ag2Se pseudo-binary tie-line. Through changes in the secondary phases, the increase in Ag/(Ag+Cu) smoothed the film surfaces, reduced facets and voids, suppressed sodium accumulation, and decreased the Ga gradient. ☐ Possible preferential reaction involving Ag and other constituent elements urge an examination of the composition variation in ACIGS films at microscopic scales. Such variation has been suspected to be a source of non-uniformity in CIGS films that were detected by electron beam-induced current, surface electric potential and photoluminescence (PL) etc. Using energy dispersive x-ray spectroscopy in a scanning transmission electron microscope, the composition gradients along ACIGS films' growth direction were found independent of grain distribution and exist even within a single grain. The gradients matched those measured by SIMS at sub-millimeter scales, indicating small composition variation along films' planar direction. The planar composition variation was not correlated with Ga/(In+Ga) or presence of Ag. However, even small, this variation can generate Eg fluctuation that is comparable with the non-uniformity derived from absorptance, PL and electroluminescence measurements. As extracted from microscopy images, grain size clearly increased along the film growth direction, and the grain growth seemed affected by both Ag and Ga composition. ☐ The findings in this dissertation can aid the optimization of the ACIGS deposition process, and provide reference for studies on junction formation with buffer layers and device non-uniformity etc.
Description
Keywords
Silver, Copper, Photovoltaics, Three-stage co-evaporation, Indium, Gallium, Selenium