H. Wiesner

CuIn1−xGaxSe2-based photovoltaic cells from electrodeposited and chemical bath deposited precursors

We have fabricated 13.7%- and 7.3%-efficient CuIn1−xGaxSe2 (CIGS)-based devices from electrodeposited and chemical bath deposited precursors. As-deposited precursors are Cu-rich films and polycrystalline (grain size is very small) in nature. Only preliminary data is presented on chemical bath deposited precursors. Additional In, Ga, and Se were added to the precursor films by physical evaporation to adjust the final composition to CuIn1−xGaxSe2. Addition of In and Ga and also selenization at high temperature are very crucial to obtain high efficiency devices. Three devices with Ga/(In+Ga) ratios of 0.16, 0.26, and 0.39 were fabricated from electrodeposited precursors. The device fabricated from the chemical bath deposited precursor had a Ga/(In+Ga) ratio of 0.19. The films/devices have been characterized by inductive-coupled plasma spectrometry, Auger electron spectroscopy, X-ray diffraction, electron-probe microanalysis, current-voltage characteristics, capacitance–voltage, and spectral response. The compositional uniformity of the electrodeposited precursor films both in the vertical and horizontal directions were studied. The electrodeposited device parameters are compared with those of a 17.7% physical vapor deposited device.

Defect Chalcopyrite Cu(In 1-x , Ga x ) 3 Se 5 Polycrystalline Thin-Film Materials

The defect chalcopyrite material CuIn 3 Se 5 has been identified as playing an essential role in efficient photovoltaic action in CuInSe 2 -based devices; it has been reported to be of n-type conductivity, forming a p-n junction with its p-type counterpart CuInSe 2 . Because the most efficient cells consist of the Cu(In 1-x Ga x )Se 2 quaternary, knowledge of some physical properties of the Ga-containing defect chalcopyrite Cu(In 1-x Ga x ) 3 Se 5 may help us better understand the junction phenomena in such devices. Polycrystalline Cu(In l-x Ga x ) 3 Se 5 (with O 2 counterparts). Micrographs of the thin films show a substantial change in morphology as the Ga content is increased—for identical conditions of growth rate and substrate temperature. X-ray diffraction patterns agree with previously publish data for the ternary case (x=0), where these materials have been referred to as ordered vacancy compounds. Pole figures confirm a high degree of texturing in the films and a change in preferred orientation as Ga content is increased.

Advances in the CIS research at NREL

This paper summarizes the research of the CIS Team at NREL in three major areas: absorber deposition; understanding the role of chemical bath deposited (CBD) CdS in CIS junctions; and in the development of devices without CdS. Low cost, scaleable processes chosen for absorber fabrication include sputtering, electrodeposition (ED), and close spaced sublimation (CSS). The interaction between the CBD and the CIS has been investigated and the results show that Cd might be instrumental in shaping the interface. We have also developed a process to fabricate a 13.5% efficiency ZnO/CulnGaSe/sub 2/ device without CdS or other buffer layers.

Issues on the chalcopyrite/defect-chalcopyrite junction model for high-efficiency Cu(In,Ga)Se2 solar cells

Abstract Considering the chalcopyrite/defect-chalcopyrite junction model for Cu(In 1− x Ga x )Se 2 -based devices and our previously reported findings for the Cu(In 1− x Ga x ) 3 Se 5 defect chalcopyrites, we have postulated that uniform high-Ga-content photovoltaic structures (with x > 0.35) do not yield acceptable device performance due to the electrical and structural differences between both types of materials (chalcopyrite and defect-chalcopyrite). In this contribution, the structural properties of the surface region of Ga containing absorber materials have been studied by grazing incidence X-ray diffraction. We find that there are significant differences between surface and bulk. A structural model is proposed for the growth of the chalcopyrite/defect-chalcopyrite junction relative to its Ga content. And we demonstrate that closely lattice matched high -Ga-content structures ( x > 0.35) can produce solar cells withv acceptable performances. The high-voltage and low-current electrical outputs from high Ga structures are very desirable in module fabrication because overall resistive losses can be substantially reduced.

Defect chalcopyrite Cu(In/sub 1-x/Ga/sub x/)/sub 3/Se/sub 5/ materials and high Ga-content Cu(In,Ga)Se/sub 2/-based solar cells

Crystallographic, optical, and electrical properties of defect chalcopyrite Cu(In/sub 1-x/Ga)/sub 3/Se/sub 5/ (0<x<1) materials in polycrystalline thin-film form are reported. Also, an energy band alignment between such materials and CdS has been calculated from X-ray photoelectron spectroscopy data. A comparison of some properties against published data on similarly prepared chalcopyrite CuIn/sub 1-x/Ga/sub x/Se/sub 2/ absorber materials is presented. Considering the chalcopyrite/defect chalcopyrite junction model, we postulate that the traditionally poor device performance of uniform high-Ga-content absorbers (x>0.3) is due to a relatively inferior character-both structural and electrical-at the very chalcopyrite/defect chalcopyrite interface. We demonstrate that this situation can be circumvented (for absorbers with x>0.3) by properly engineering such an interface by reducing Ga content in the region near the surface of the absorber.

Investigations into alternative substrate, absorber and buffer layer processing for Cu(In,Ga)Se/sub 2/-based solar cells

High-performance Cu(In,Ga)Se/sub 2/ (CIGS) based solar cells are fabricated within a narrow range of processing options. In this contribution, alternative substrate, absorber, and buffer layer processing is considered. Cell performance varies considerably when alternative substrates are employed. These variations are narrowed with the addition of Na via a Na/sub 2/S compound. Sputtered and electrodeposited CIGS precursors and completed absorbers show promise as alternatives to evaporation. A recrystallization process is required to improve their quality. (In,Ga)/sub y/Se buffer layers contribute to cell performance above 10%. Further improvements in these alternatives will lead to combined cell performance greater than 10% in the near term.

Nanoparticle colloids as spray deposition precursors to CIGS photovoltaic materials

Cu-In-Ga-Se nanoparticle colloids have been used as precursors in the spray deposition of photovoltaic films. Precursor colloid was prepared by reaction of the metal iodides in pyridine with sodium selenide in methanol at reduced temperature according to one of two routes: synthesis of each of the component binary selenides (Type I) followed by physical mixing of the isolated particles; or a one-pot synthesis with all the metal iodides reacting together in one flask to form a mixed-metal Cu-In-Ga-Se colloid (Type II). The constituent nanoparticles in these colloids were analyzed by TEM and XRD and were determined to be amorphous as-synthesized. Crystalline phase formation of these nanoparticles was observed by XRD after a thermal treatment. These precursor colloids were sprayed onto Mo-coated glass substrates at elevated temperatures. The nanoparticle-derived Cu-In-Ga-Se films were characterized by SEM and XRD prior to being finished into CIGS solar cell devices according to standard NREL protocol. I–V ch...

CuIn1-xGaxSe2-based photovoltaic cells from electrodeposited precursor

We have fabricated 13.7%-efficient CuIn1-xGaxSe2 (CIGS)-based devices from electrodeposited precursors. As- deposited electrodeposited precursors are Cu-rich films. Additional In, Ga, and Se were added to the electrodeposited precursor film by physical evaporation to adjust the final composition to CuIn1-xGaxSe2. Three devices with Ga/(In + Ga) ratio of 0.16, 0.26, and 0.39 were fabricated from electrodeposited precursors. The films/devices have been characterized by inductive-coupled plasma spectrometry, Auger electron spectroscopy, x-ray diffraction, electron-probe microanalysis, current-voltage characteristics, capacitance- voltage, and spectral response. The electrodeposited device parameters are compared with those of a 17.7% physical vapor deposited device.

Surface analytical study of CuInSe2 treated in Cd-containing partial electrolyte solution

Junction formation in CuInSe2 (CIS) has been studied by exposing thin films and single-crystal samples to solutions containing NH4OH and CdSO4. The treated samples were analyzed by secondary ion mass spectrometry to determine the amount and distribution of Cd deposited on the surface of the films. Cadmium is found to react with the surface for all the solution exposure times and temperatures studied. The reaction rapidly approaches the endpoint and remains relatively unchanged for subsequent solution exposure. Cadmium in-diffusion, as measured by secondary ion mass spectrometry, is obscured by topography effects in the thin-film samples and by ion-beam mixing and topography in the single-crystal sample.

A Study of the CdS/CuIn(Ga)Se 2 Interface in Thin Film Solar Cells

In this paper the authors describe research efforts directed towards the understanding of the CdS/CuInGaSe{sub 2} junctions and, specifically, the interaction of the chemical bath with the CuInGaSe{sub 2} (CIGS). They find that Cd and S diffuse into the absorber during the CdS growth. Heating the absorbers in Cd partial baths resulted in a significant improvement in the ZnO/CIGS device properties. Photoluminescence measurements indicate that the effect of Cd is very similar to that of chemical bath deposition (CBD) CdS.