Brian Egaas

Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin‐film solar cells

This short communication reports on achieving 18·8% total-area conversion efficiency for a ZnO/CdS/Cu(In,Ga)Se2/Mo polycrystalline thin-film solar cell. We also report a 15%-efficient, Cd-free device fabricated via physical vapor deposition methods. The Cd-free cell includes no buffer layer, and it is fabricated by direct deposition of ZnO on the Cu(In,Ga)Se2 thin-film absorber. Both results have been measured at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m2, 25°C, ASTM E 892 Global). The 18·8% conversion efficiency represents a new record for such devices (Notable Exceptions) and makes the 20% performance level by thin-film polycrystalline materials much closer to reality. We allude to the enhancement in performance of such cells as compared to previous record cells, and we discuss possible and realistic routes to enhance the performance toward the 20% efficiency level. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.

On the role of Na and modifications to Cu(In,Ga)Se/sub 2/ absorber materials using thin-MF (M=Na, K, Cs) precursor layers [solar cells]

The growth and characterization of Cu(In,Ga)Se/sub 2/ polycrystalline thin film solar cells under the presence of thin-MF (M=Na, K, Cs) precursor layers is presented. Some electrical, structural and electronic absorber properties due to the presence of such Group Ia impurities are quantified along with their influence in device performance. The authors present a growth model for the role of Na in Cu(In,Ga)Se/sub 2/ that attributes the enhancements in electrical conductivity and photovoltaic device performance to the extinction of a finite number of donor states (i.e., In/sub Cu/) at the bulk and grain-boundary regions.

Texture manipulation of CuInSe2 thin films

We present a growth method that allows tailoring of preferred orientation in CuInSe2 thin-films grown on Mo-coated soda-lime glass substrates. In particular, films exhibiting a (204) preferred orientation are demonstrated, in addition to already reported (112) and randomly oriented films. Effects of substrate composition, growth temperature, and final film composition on texture phenomena are presented. In general, we find that texture is highly dependent on growth temperature, substrate material and, in the case of Mo-coated soda-lime glass substrates, the structural properties of the Mo layer. We provide evidence for the attainment of such structural orientation, and we present a growth model to explain the mechanism allowing such modifications.

High efficiency thin-film CuIn1−xGaxSe2 photovoltaic cells using a Cd1−xZnxS buffer layer

The authors have fabricated 19.52% thin-film CuIn1−xGaxSe2 (CIGS)-based photovoltaic devices using single layer chemical bath deposited Cd1−xZnxS (CdZnS) buffer layer. The efficiency equals the world record for any thin-film solar cell and is achieved with reduced optical absorption in the window layer. Using current-voltage, quantum efficiency, and capacitance-voltage measurements, the CIGS/CdZnS device parameters are directly compared with those of CIGS/CdS devices fabricated with equivalent absorbers.The authors have fabricated 19.52% thin-film CuIn1−xGaxSe2 (CIGS)-based photovoltaic devices using single layer chemical bath deposited Cd1−xZnxS (CdZnS) buffer layer. The efficiency equals the world record for any thin-film solar cell and is achieved with reduced optical absorption in the window layer. Using current-voltage, quantum efficiency, and capacitance-voltage measurements, the CIGS/CdZnS device parameters are directly compared with those of CIGS/CdS devices fabricated with equivalent absorbers.

Characterization of 19.9%-efficient CIGS absorbers

We recently reported a new record total-area efficiency, 19.9%, for CuInGaSe2 (CIGS)-based thin-film solar cells [1]. Current-voltage analysis indicates that improved performance in the record device is due to reduced recombination. The reduced recombination was achieved by terminating the processing with a Ga-poor (In-rich) layer, which has led to a number of devices exceeding the prior (19.5%) efficiency record. This paper documents the properties of the high-efficiency CIGS by a variety of characterization techniques, with an emphasis on identifying near-surface properties associated with the modified processing.

Improved energy conversion efficiency in wide bandgap Cu(In, Ga)Se 2 solar cells

This report outlines improvements to the energy conversion efficiency in wide bandgap (Eg>1.2 eV) solar cells based on CuIn1−xGaxSe2. Using (a) alkaline containing high temperature glass substrates, (b) elevated substrate temperatures 600°C-650°C and (c) high vacuum evaporation from elemental sources following NRELs three-stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2<Eg<1.45 eV. Initial results of this work have led to efficiencies >18% for absorber bandgaps ∼1.30 eV and efficiencies ∼16% for bandgaps up to ∼1.45 eV. In comparing J-V parameters in similar materials, we establish gains in the open-circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures may be due to reduced recombination at the grain boundary of such absorber films. Solar cell results, absorber materials characterization, and experimental details are reported.

Fiber-fed time-resolved photoluminescence for reduced process feedback time on thin-film photovoltaics

Fiber-fed time-resolved photoluminescence is demonstrated as a tool for immediate process feedback after deposition of the absorber layer for CuInxGa(1-x)Se2 and Cu2ZnSnSe4 photovoltaic devices. The technique uses a simplified configuration compared to typical laboratory time-resolved photoluminescence in the delivery of the exciting beam, signal collection, and electronic components. Correlation of instrument output with completed device efficiency is demonstrated over a large sample set. The extraction of the instrument figure of merit, depending on both the initial luminescence intensity and its time decay, is explained and justified. Limitations in the prediction of device efficiency by this method, including surface effect, are demonstrated and discussed.

A new real-time quantum efficiency measurement system

We have developed a new technique for measuring the quantum efficiency (QE) in solar cells in real-time using a unique, electronically controlled, full-spectrum light source. Full-spectrum QE graphs can be obtained in less than one second (as opposed to 20 minutes using traditional QE instruments). The high measurement speed is achieved by parallel processing of information from a multitude of spectral channels encoded in modulation frequency bands. The reduction in time scale makes this QE measurement technique compatible with inline production diagnostics, high-fidelity, spectral-matching cell binning, and thin-film module spatial spectral response uniformity tests. The instrument is completely solid-state with no moving parts, is robust enough for manufacturing environments, and is significantly less expensive than a traditional QE instrument.

Electron drift-mobility measurements in polycrystalline CuIn1−xGaxSe2 solar cells

We report photocarrier time-of-flight measurements of electron drift mobilities for the p-type CuIn1−xGaxSe2 films incorporated in solar cells. The electron mobilities range from 0.02 to 0.05 cm2/Vs and are weakly temperature-dependent from 100–300 K. These values are lower than the range of electron Hall mobilities (2-1100 cm2/Vs) reported for n-type polycrystalline thin films and single crystals. We propose that the electron drift mobilities are properties of disorder-induced mobility edges and discuss how this disorder could increase cell efficiencies.

A study on the humidity susceptibility of thin-film CIGS absorber

The susceptibility of a thermally co-evaporated CuInGaSe2 (CIGS) thin-film absorber to humidity and its consequence on composition, morphology, electrical and electronic properties, and device efficiency was investigated. CIGS films on Mo-coated soda lime glass were degraded either in the ambient at ∼21°C and ∼21% relative humidity (RH) for a period of several months or in damp heat (DH) at 85°C and 85% RH briefly for 15–30 min; then the films were processed simultaneously into devices in a batch that included an unexposed control. In addition to severe delamination on some samples of the absorber films, prolonged ambient exposure resulted in numerous “spot” formations that lost CIGS with scale-like disintegration rippling around the spots and showed a significant presence of Na. Exposure in DH for 5 h was able to reproduce the spot formations on the CIGS films. A significant to large decrease of cell efficiency was observed from 14%–16% for the unexposed control to 8%–11% for the CIGS absorber exposed in DH for 15 and 30 min and 1%–4%% for the ambient-degraded CIGS with high series resistance and very low shunt resistance.