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Technical issues which remain in CuInSe2-based solar cells are reviewed. A study of single-crystalline Cu(In,Ga)Se2film devices, carrier recombination analysis, and effects of alkali-metal doping and silver-alloying are particularly focused on.

In this study, we present an effective method of improving the performance of pure sulfide Cu(InGa)S2 (CIGS) solar cells via injection annealing system. The injection annealing system can perform annealing at desired temperatures, and therefore, the CIGS thin film passed over the temperature range in which secondary phases occurs. Via the injection annealing system, secondary phase InSx was effectively removed from the surface of the CIGS thin films at the temperatures over 550°C. This resulted in the formation of good-quality P–N junction CIGS devices, thereby improving significantly the performance of the CIGS solar cell. In addition, the open-circuit-voltage (VOC) and fill factor (FF) of the CIGS devices increased gradually with increasing annealing temperature in the range of 550–640°C. It is speculated that the bulk defects were decreased as the annealing temperature increased. Finally, via injection annealing system, a pure sulfide CIGS solar cell with an efficiency of 12.16% was achieved.

Enhancement of photovoltaic performance due to alkali-metal doping in various thin-film compound solar cells such as chalcogenide Cu(In,Ga)Se2 (CIGS), Cu2ZnSnS4 (CZTS), and organic–inorganic hybrid perovskite, is significant in the development of photovoltaic technologies. In this work, the effects of light Li- and heavy Cs-doping in ternary CuGaSe2 (CGS) and quaternary CIGS thin-films and devices are compared. The new findings are: (i) heavy alkali-metal halide CsF postdeposition treatment (PDT) modifies the CGS film surface morphology and increases the carrier density; however, the beneficial effects of Cs on the CGS photovoltaic performance are low when compared to the results observed from quaternary CIGS devices; (ii) LiF-PDT leads to no significant CGS and CIGS film surface (hetero p-n junction interface) modifications; thus, the use of a thin (approximately 30 nm) CdS buffer layer is not effective in enhancing photovoltaic performance; and (iii) the beneficial effects of either light Li or heavy Cs on ternary CGS devices are modest; nonetheless, Li may be more effective than Cs, unlike quaternary CIGS devices.

In this study, the influences of bromine-based etching (Br etching) of narrow band gap CuInSe2 (CIS) absorbers and Cu(In,Ga)Se2 absorbers with various single Ga gradings (CIS:Ga) on the properties of solar cells were investigated. Absorbers with narrow absorption edge energies (Eabs) of 1.0-1.02 eV, ideal for the application as a bottom cell in a tandem device, were fabricated using a modified three-stage process and subjected to Br etching. The evolution of surface flatness and their optical and electrical properties upon Br etching were investigated. Br etching typically reduced the root-mean-square deviation of the surface roughness height (Rq) for a CIS:Ga absorber from several hundreds to several tens of nanometers, whereas for some CIS absorbers, Rq reduction was limited by the remaining voids. Moreover, Br etching reduced the leakage current across the pn junction. The high shunt resistances (Rsh) typically up to >10 kΩ·cm2 were obtained by introduction of Br etching. However, etching sometimes adversely increased the VOC deficit. The investigation of the minority carrier lifetime and diode parameters revealed that back-surface recombination in CIS and low-Ga CIS:Ga solar cells increased as the absorber layer thickness decreased. A higher Ga grading significantly reduced back-surface recombination. Narrow band gap CIGS solar cells with improved surface flatness and high VOC were achieved by introducing Br etching and proper Ga grading.

The beneficial effects of heavy alkali metals such as K, Rb, and Cs in enhancing Cu(In,Ga)Se2 (CIGS) photovoltaic efficiencies are widely known, though the detailed mechanism is still open for disc...

Radiation tolerance of Cu(In,Ga)Se2 (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environments. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 1016 MeVneq cm−2, however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10 μm thick CIGS solar cells were recovered by dark annealing in the same manner as for 2 μm thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects on passivate the recombination centers, even using thicker CIGS layers.

Chalcopyrite CuInSe

We report a unique bonding technology using Pd nanoparticle array and adhesive material for multijunction (MJ) solar cells. The technology, which is modified with our previous “smart stack technology”, expands the possibility of MJ solar cell. In GaAs//CuInGaSe 3-junction solar cell, a record-efficiency of 28.06% was attained.

Both elemental Rb and Cs are known as effective dopants in enhancing Cu(In,Ga)Se2 (CIGS) thin-film photovoltaic device performance. It is, however, found that there are critical differences in the ...

$\mathrm{Cu}$-deficient phases of chalcopyrite $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS), such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}{)}_{3}{\mathrm{Se}}_{5}$ and $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}{)}_{5}{\mathrm{Se}}_{8}$, which are occasionally referred to as ordered vacancy compounds and often unintentionally form on CIGS film surfaces and at grain boundaries, are among the most important subjects in developing chalcopyrite thin-film energy-conversion materials and devices. Here, we revisit the role of the $\mathrm{Cu}$-deficient surface layer (CDL) present at the p-CIGS/n-$\mathrm{Cd}\mathrm{S}$ interface in photovoltaic devices with the effects of alkali-metal doping. The device structure used is alkali-containing soda-lime glass or alkali-free zirconia substrate/$\mathrm{Mo}$/CIGS/${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_{2}$-based CDL/$\mathrm{Cd}\mathrm{S}$/intrinsic and $\mathrm{Al}$-doped $\mathrm{Zn}\mathrm{O}$. The photovoltaic device performance deteriorates with increasing CDL thickness when no alkali metal is added. However, the CDL thickness significantly affects the results of alkali-halide $\mathrm{Rb}\mathrm{F}$ postdeposition treatment (PDT) and the use of a thick CDL enhances the beneficial effects of $\mathrm{Rb}\mathrm{F}$ PDT, resulting in photovoltaic performance enhancements manifested by improvements in the open-circuit voltage and fill-factor values. On the other hand, $\mathrm{Rb}\mathrm{F}$ PDT leads to degradation of the device performance when the CDL is thin. High $\mathrm{Rb}$ concentration and significant $\mathrm{Ga}$ diffusion from the CIGS layer to the surface CDL with $\mathrm{Rb}\mathrm{F}$ PDT are observed when the CDL is thick. The formation of metastable acceptors, namely, an increase in the nominal carrier density in CIGS, is observed with light-soaking (LS) treatments, regardless of the thickness of CDL. Nevertheless, improvements in photovoltaic efficiency with LS treatments are observed only from devices grown with a thick CDL.

In this study, the influences of bromine-based etching (Br etching) of narrow band gap CuInSe

Over the last several decades, champion photovoltaic (PV) devices using CuInGaSe2 (CIGS) as the absorber material have been achieved using polycrystalline films exclusively. This has led to the assumption that polycrystalline CIGS generally outperform single-crystal CIGS in PV devices. However, recently, very high-quality epitaxial CIGS has been grown on GaAs substrates producing PV device efficiencies of 20.0%. These results have revived the debate on what effects grain boundaries have on PV device efficiencies. In this contribution, we compare the optoelectronic properties of polycrystalline CIGS films with those of high-efficiency epitaxial CIGS films. This comparison reveals that grain boundaries are associated with properties that negatively impact PV device efficiency. Additionally, we find that the grain interiors in polycrystalline films exhibit properties that are similar to the high-performance epitaxial films. Our results suggest that it may be possible to achieve higher device efficiencies with epitaxial CIGS than with polycrystalline films.

Heat light soaking (HLS) has been known to impact the photovoltaic parameters of Cu(In,Ga)Se2 (CIGS) solar cells for a long time. Recently, the focus shifted to the effect of the procedure on alkali fluoride-treated CIGS. Here, we investigate the impact of long-term HLS on the open-circuit (V OC) loss in high-efficiency CIGS with potassium fluoride (KF) and sodium fluoride (NaF) post-deposition treatment (PDT). HLS is shown to increase the net doping density, however, the subsequent improvement of the V OC is lower than expected. Using an analysis based on the SQ theory, we show that HLS reduces the nonradiative recombination rate in the bulk but increases the one at the interface. We present a model to explain the increase of interface recombination. We further demonstrate that a combination of HLS and KF/NaF-PDT is necessary to enhance the positive impacts of HLS and mitigate the detrimental ones leading to high-efficiency CIGS devices (22%).

For copper indium gallium selenide [Cu(In1−x,Gax)Se2, CIGS]-based solar cells, defect states or impurity phase always form due to both the multinary compositions of CIGS film and the difficulty of controlling the growth process, especially for high Ga concentration. To further improve device performance, it is important to understand such formation of impurity phase or defect states during fabrication. In the work presented herein, the formation mechanism of impurity phase Cu2−δSe and its depth profile in CIGS film with high Ga content, in particular CuGaSe2 (i.e., CGS), were investigated by applying different growth conditions (i.e., normal three-stage process and two-cycle three-stage process). The results suggest that impurity phase Cu2−δSe is distributed nonuniformly in the film because of lack of Ga diffusion. The formed Cu2−δSe can be removed by etching the as-deposited CGS film with bromine-methanol solution, resulting in improved device performance.

We found that elemental Si-doped Cu(In,Ga)Se2 (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition...

The effects of post p-n junction formation processes, namely transparent conducting oxide (TCO) layer deposition conditions on Cu(In,Ga)Se2 (CIGS) and CuGaSe2 (CGS) solar cell device properties were comparatively studied. It was found that CIGS devices were relatively insensitive to the presence of oxygen and heat during TCO layer deposition, whereas CGS devices were extremely sensitive to oxygen and heat, resulting in a degradation of device performance in the presence of them. The use of the relatively low TCO deposition temperature (Ts) of 80 °C led to significant improvements in the fill factor values of CGS devices compared to the case for Ts ~ 180 °C, though the open circuit voltage was reduced. These results suggest that TCO deposition conditions can be an important parameter which critically affects ternary CGS solar cell device properties. It is, therefore, suggested that TCO layer deposition conditions for CGS devices should be optimized independently of that for conventional narrow-gap CIGS devices.

The impact of back-contact recombination on Cu(In,Ga)Se 2 (CIGS) cells is discussed based on device simulations in which the typical Ga/III grading is considered. When the diffusion length (L n ) is large relative to the absorber thickness, there is room to improve open circuit voltage (V OC ) by preventing back-contact recombination. Whereas with small L n , the impact of back-contact recombination is negligible. Efficiency gains can be realized by employing contact passivation techniques and/or larger conduction band grading which keep electrons away from the back contact.

Photoelectrochemical activity for water reduction (H2 liberation) over a co-evaporated CuGaSe2 compact thin film modified with a CdS layer and Pt deposits under simulated sunlight (AM 1.5G) radiation was evaluated, specifically focusing on the impact of a Cu-deficient layer (CDL) loaded on the top part of the CuGaSe2 film. It was found that the intentional loading of the CDL with an appropriate thickness was effective for achieving a large current flow and relatively positive photocurrent onset. The half-cell solar-to-hydrogen efficiency reached 6.6% over the best photocathode used. Moreover, the highest photocurrent onset potential of more than 0.9 V vs reversible hydrogen electrode was achieved over the photocathode based on the CuGaSe2 film having an extremely thick CDL (200 nm) with a relatively thick CdS layer (90 nm) due to efficient spatial separation of photogenerated carriers.

In this article, we look, from the carrier perspective, at the effect of the double grading in a CIGS absorber, in order to understand its benefit compared to a single grading. We focus on the double grading effects on the conduction band edge, generation rate and carrier collection. We first show that, when the minimum bandgap position (notch position) is outside of the absorber depletion region, a local maximum of the conduction band edge appears at the back of the depletion region, creating a hump, which hinders the collection of electrons. Interpreting the effect of the grading as an effective electric field, we show that the hump is caused by the competition between the absorber internal electric field and the grading created effective electric field. From this, we deduce that the notch should be positioned in the depletion region. As expected, the generation rate increases significantly at the notch position. However, studying carrier collection and the recombination rate, we show that, even if in the ideal case we should observe a total collection of carriers generated in the depletion region, the recombination rate in the SCR increases significantly when the notch is in it. We propose that this is caused by the increased carrier generation close to recombination centres. The ideal notch position is, therefore, a compromise between being close enough to the front to have a maximal carrier collection and avoiding the creation of a hump, while minimizing the carrier recombination.

It is expected that Cu-deficient phases of chalcopyrite CuGaSe2 (CGS), such as CuGa3Se5, will be used widely as functional materials in energy conversion devices. In this study, the effect of a Cu-deficient CGS layer (CDL) at the p-CGS/n-CdS interface was investigated. The presence of the CDL was found to have a significant effect on increasing the open-circuit voltage (Voc) of CGS photovoltaic devices. This result is contrary to the results obtained for CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) devices, which showed a decrease in Voc with the increasing CDL thickness. It was also found that the use of a thicker n-CdS layer is effective in increasing the short-circuit current density (Jsc) and Voc for CGS devices fabricated with a thick CDL. These trends are unique to CGS devices and are, thus, expected to offer unique and specific routes for improving wide-gap chalcopyrite photovoltaic device performance, in addition to offering a conventional strategy for adjusting the conduction band alignment of the p-CGS/n-buffer interface with proper n-buffer materials.

Alkali metal treatment is an indispensable technology for obtaining highly efficient CuInSe2 (CIS) solar cells. This study evaluated the structural change of Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)(S,Se)2 (CIGSSe) films with and without the CsF post-deposition treatment (CsF-PDT) (for CIGSe) and Cs treatment (for CIGSSe) by the Se K-edge depth-resolved X-ray absorption fine structure. The CIGSe films were deposited by a three-stage process, and the CIGSSe films were deposited by the sulfurization after the selenization method. Although CsF-PDT can re-form the surface of CIGSe film, the Cs treatment cannot modify the surface of the CIGSSe film. Our result suggests that the improvement is due to not only the surface reforming but also an effect on the inside of the CIGSe and CIGSSe films. This study provides useful information for the preparation of highly efficient CIS solar cells.

*1 Research Institute for Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan E-mail: narazaki-aiko@aist.go.jp *2 Electronics and Photonics Research Institute, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Research Center for Photovoltaics, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Renewable Energy Research Center, AIST, 2-2-9 Machiikedai, Koriyama, Fukushima 963-0298, Japan

Cu (In, Ga)Se2 (CIGS) solar cells were irradiated with 100 keV electrons to reveal the characteristics of created radiation defects. 100 keV electrons cannot produce any displacement defects in CIGS. Low-fluence electrons improve the electrical performance of the CIGS solar cells due to the change in the conductive type of donor to acceptor in a metastable defect, which is equivalent to the light-soaking effect. However, high fluence electrons cause the cell performance to decline. From analysis based on changes in carrier density and electroluminescence, defects causing the decline in performance include donor- and non-radiative types. In addition, red-on-bias experiments showed an increase in IIICu defects due to electron irradiation. Based on these results, the degradation in the electrical performance of the CIGS solar cells irradiated with high electron fluence would be attributable to a change in the conductive type of IIICu defects.

An alkali-metal containing (AC) layer, specifically sodium (Na), is necessary for improving Cu(In x Ga 1- x )Se 2 (CIGS) solar cell performance on an alkali-free flexible substrate. However, we found that Na from the AC precursor layer diffused not only into the CIGS absorber layer but also into our flexible metal foil substrate, the surface of which was covered with an Al 2 O 3 layer, and thus creating Na depletion in the CIGS layer. In this work, newly developed alkali-metal block (AB) layers are introduced between the AC layers and the metal foil substrates in order to suppress Na diffusion into the substrates and enhance Na diffusion into the CIGS layers. Using different AB layer materials—Ti, TiO 2 , and TiN—and AC layers with systematically-varied thickness, Na concentrations in the CIGS layers and the solar cell efficiencies are investigated. Ti and TiN AB layers efficiently enhance the Na diffusion into the CIGS absorber layers. As a result, a high efficiency of 17.6% is obtained on the metal foil substrate using optimally prepared AC and AB layer of TiN. The AB layer controls the Na diffusion from the AC layer in one direction, and thus contributes to the precise control of Na concentrations in the CIGS layer. Hence, high efficiency solar cells on a variety of flexible substrates will be achieved comparable to those on soda-lime glass substrates.

CIGS thin films were irradiated with 100 or 250 keV electrons to reveal the radiation defect by analyzing PL measurement. The PL intensity decreased due to non-radiative recombination defects induced by electron irradiation. Furthermore, the intensity 0.8 eV peak of the PL spectrum was observed from CIGS films irradiated with 250 eV electrons and is said to correspond to In-antisite defects in CIGS materials. The defects can usually change into InCu-VCu complex defects combined with VCu, since the formation energy of the complex defect is lower than that of each defect. Cu interstitial defects induced by 250 keV electron irradiation would diffuse to VCu of the complex defect, whereupon the complex defect might become an In-antisite defect due to 250 keV electron irradiation.

We found that elemental Si-doped Cu(In,Ga)Se

Defect study of the CuGaSe2 thin-films deposited with three-stage evaporation process under different Se-flux (PSe) conditions were performed using admittance spectroscopy (AS) technique. A dominant defect, A2 has been identified around 230~350 meV above the valance band (Ev) of the CuGaSe2. In general, density of defects was found to decrease with an increase in the PSe condition during deposition of the CuGaSe2 layer. Effect of the Se-flux on the performance of the fabricated solar-cells were discussed in relation to the defect-properties of the corresponding CuGaSe2 absorber-layer identified by admittance spectroscopy.

Polycrystalline thin films of ternary CuGaSe2 (CGS), which are one of the potential materials for wide-gap top cells in tandem structure photovoltaic devices, are grown on alkali-free and alkali-containing substrates. The effects of alkali-metals on CGS film morphology, grain size, and photovoltaic device properties are studied. High temperature (>600 °C) growth of CGS films is also investigated with a focus upon variations in alkali-metal distribution profiles in CGS films and the formation of a surface Cu-deficient layer (CDL), which plays an important role in p-n junction formation. The use of a high growth temperature is effective in realizing large grain sizes due to enhanced elemental migration during growth, whereas the formation of a surface CDL is diminished and the alkali-distribution profiles in CGS films are modified resulting in a decrease in carrier density. It is suggested that large CGS grain sizes contribute to obtaining high photocurrent values, but coping with modifications in surface/interface and carrier density with alkali-metals is important to enhance overall photovoltaic performance.

In this study, interfacial recombination observed by activation energy (Ea) is reduced with an improvement in the built-in potential (Vbi) by raising the conduction band minimum (EC) in Al-doped (Zn, Mg)O (AZMO) layer for pure-sulfide Cu(In, Ga)S2 (CIGS) solar cells. It is observed that the optical band gap in AZMO films can be widened from 3.56 to 3.97 eV with increasing Mg/(Mg + Zn) ratio from 0 to 0.23, suggesting the shift of EC toward the vacuum level. AZMO layers with Mg/(Mg + Zn) ratio of 0–0.23 are applied as transparent conductive oxide (TCO) for the pure-sulfide CIGS solar cells. The open-circuit voltage is clearly enhanced from 0.641 to 0.713 V with increasing Mg/(Mg + Zn) ratio from 0 to 0.09 and then decreased to 0.651 V at Mg/(Mg + Zn) ratio of 0.23 in the AZMO layer. Reverse saturation current density (J0) was minimized to 9.4 × 10−7 A/cm2 at Mg/(Mg + Zn) of 0.09, although J0 was 4.7 × 10−6 A/cm2 in Al-doped ZnO (Mg/(Mg + Zn) of 0). From Mott-Schottky plot, it is observed that Vbi for the pure-sulfide CIGS solar cells gradually enhanced with an increase in Mg/(Mg + Zn) from 0 to 0.23 in the AZMO layer. These results suggest that Vbi improves by controlling EC in the TCO layer, which ultimately reduces the recombination at the hetero interface owing to strengthened electric field.

Simultaneous realization of high values of open circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η) in wide-gap CuGaSe2 (CGS) solar cells has long been one of the most challenging issues in the realm of chalcopyrite photovoltaics. In this communication, structural tuning of CGS thin films by means of controlling the amount of Se flux used during CGS film growth and improvements in solar cell performance (Voc > 0.9 V, FF > 0.7, and η > 10%) are demonstrated. Systematic variations in CGS film properties with the Se flux and correlation with device properties are shown. The unique CGS thin-film growth kinetics, which are different from narrow-gap Cu(In,Ga)Se2, are also presented and discussed. This development of double digit efficiency for CGS solar cells opens a new frontier for the broad application of a new class of chalcopyrite-based devices. Copyright © 2014 John Wiley & Sons, Ltd.