Physical and chemical aspects at the interface and in the bulk of CuInSe2-based thin-film photovoltaics.

Chalcopyrite CuInSe2 (CISe)-based thin-film photovoltaic solar cells have been attracting attention since the 1970s. The technologies of CISe-based thin-film growth and device fabrication processes have already been put into practical applications and today commercial products are available. Nevertheless, there are numerous poorly understood areas in the physical and chemical aspects of the underlying materials science and interfacial and bulk defect physics in CISe-based thin-films and devices for further developments. In this paper, current issues in physical and chemical studies of CISe-based materials and devices are reviewed. Correlations between Cu-deficient phases and the effects of alkali-metals, applications to lightweight and flexible solar minimodules, single-crystalline epitaxial Cu(In,Ga)Se2 films and devices, differences between Cu(In,Ga)Se2 and Ag(In,Ga)Se2 materials, wide-gap CuGaSe2 films and devices, all-dry processed CISe-based solar cells with high photovoltaic efficiencies, and also fundamental studies on open circuit voltage loss analysis and the energy band structure at the interface are among the main areas of discussion in this review.

Highly Efficient 17.6% Tin-Lead Mixed Perovskite Solar Cells Realized through Spike Structure.

Frequently observed high Voc loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

Colloidal Synthesis of Air-Stable Alloyed CsSn1-xPbxI3 Perovskite Nanocrystals for Use in Solar Cells.

Organic-inorganic hybrid perovskite solar cells have demonstrated unprecedented high power conversion efficiencies in the past few years. Now, the universal instability of the perovskites has become the main barrier for this kind of solar cells to realize commercialization. This situation can be even worse for those tin-based perovskites, especially for CsSnI3, because upon exposure to ambient atmosphere the desired black orthorhombic phase CsSnI3 would promptly lose single crystallinity and degrade to the inactive yellow phase, followed by irreversible oxidation into metallic Cs2SnI6. By alloying CsSnI3 with CsPbI3, we herein report the synthesis of alloyed perovskite quantum dot (QD), CsSn1-xPbxI3, which not only can be phase-stable for months in purified colloidal solution but also remains intact even directly exposed to ambient air, far superior to both of its parent CsSnI3 and CsPbI3 QDs. Ultrafast transient absorption spectroscopy studies reveal that the photoexcited electrons in the alloyed QDs can be injected into TiO2 nanocrystals at a fast rate of 1.12 × 1011 s-1, which enables a high photocurrent generation in solar cells.

Position Influence of Sputtered Zn1-xMgxO/Zn1-xMgxO:Al Layers in Flexible and Cd-Free Cu(In,Ga)(S,Se)2 Solar Cells.

Flexible, Cd-free, and all-dry process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells on stainless steel (SUS) substrates are fabricated, and their structure consists of SUS/glass (SiO2)/Mo/CIGSSe absorber/sputtered Zn0.84Mg0.16O/sputtered Zn1-xMgxO:Al transparent conductive oxide (TCO). The effect of the sample position during the sputtering of Zn0.84Mg0.16O buffer and Zn1-xMgxO:Al TCO layers of the solar cells is examined to avoid intense plasma exposure. The sample position plays a vital role in improving the cell performance. Namely, the sample position close to the material targets of the sputtering system causes severe exposure of the sample to the intense plasma, giving rise to low and nonuniform local external quantum efficiency (EQE) with very weak electroluminescence (EL) imaging, thereby reducing photovoltaic performance. On the other hand, the deviation of the sample position from material targets helps to avoid the intense plasma, thus resulting in high and uniform local EQE with bright EL imaging as well as reducing carrier recombination rates (or carrier lifetimes) throughout the solar cells. Ultimately, the conversion efficiency of flexible, Cd-free, and all-dry process CIGSSe solar cells is enhanced to 16.5% under the optimized sample position deviation from material targets to avoid intense plasma exposure.

Transparent Electrode and Buffer Layer Combination for Reducing Carrier Recombination and Optical Loss Realizing over a 22%-Efficient Cd-Free Alkaline-Treated Cu(In,Ga)(S,Se)2 Solar Cell by the All-Dry Process.

The structures of K or Cs alkaline-treated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are developed, and their carrier recombination rates are scrutinized. It is determined that short-circuit current density (JSC) is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and Zn0.8Mg0.2O buffers with a large band gap energy (Eg) are applied as a replacement of CdS buffer. The JSC is further increased, reducing the optical loss more, when Zn0.9Mg0.1O:B is used as the transparent conductive oxide (TCO) with a larger Eg and lower free carrier absorption than those of ZnO:Al. Furthermore, all carrier recombination rates throughout the devices with K or Cs treatment, especially at the buffer/absorber interface and in the quasi neutral region, are reduced, thereby reducing open-circuit voltage deficit (VOC,def), well consistent with the simulated ones. The carrier recombination rate at the buffer/absorber interface is further decreased, when the CdS and (Cd,Zn)S buffers, deposited by chemical bath deposition, are applied, leading to the greater reduction of the VOC,def and the high conversion efficiency (η) of about 21%. Under the trade-off between VOC,def and optical loss, the highest η of 22.6% is attained with the lowest power loss (or the highest VOC × JSC) in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:B, fabricated by the all-dry process, where the Zn0.8Mg0.2O buffer is prepared by the sputtering method. This occurs because the JSC is the highest attributable to the larger Eg of Zn0.8Mg0.2O buffer than those of the CdS and (Cd,Zn)S.

Enhanced Device Performance with Passivation of the TiO2 Surface Using a Carboxylic Acid Fullerene Monolayer for a SnPb Perovskite Solar Cell with a Normal Planar Structure.

Research on tin-lead (SnPb) perovskite solar cells (PSCs) has gained popularity in recent years because of their low band gap, which could be applied to tandem solar cells. However, most of the work is based on inverted PSCs using PEDOT:PSS as the hole-transport layer as normal-structure PSCs show lower efficiency. In this work, the reason behind the low efficiency of normal-structure SnPb PSCs is elucidated and surface passivation has been tested as a method to overcome the problem. In the case of normal PSCs, at the interface between the titania layer and SnPb perovskite, there are many carrier traps observed originating from Ti-O-Sn bonds. In order to avoid the direct contact between titania and the SnPb perovskite layer, the titania surface is passivated with carboxylic acid C60 resulting in an efficiency increase from 5.14 to 7.91%. This will provide a direction of enhancing the efficiency of the normal-structure SnPb PSCs through heterojunction engineering.

Characterization of Cd-Free Zn1- xMg xO:Al/Zn1- xMg xO/Cu(In,Ga)(S,Se)2 Solar Cells Fabricated by an All Dry Process Using Ultraviolet Light Excited Time-Resolved Photoluminescence.

Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with a structure of glass/Mo/CIGSSe/Zn1- xMg xO (buffer)/Zn1- xMg xO:Al (TCO), fabricated by an all dry process, are characterized using ultraviolet light excited time-resolved photoluminescence (UV-TRPL). The impact of bandgap energy ( Eg) values of buffer and transparent conductive oxide (TCO) layers, denoted by Eg of buffer and TCO, is examined. The Eg values of buffer and TCO layers are kept almost similar and varied from 3.30 to 3.94 eV. In this work, UV-TRPL measurement is performed to examine the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in the solar cell structure. It is revealed that the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in Cd-free solar cells are increased upon enhancing the Eg of buffer and TCO from 3.30 to 3.94 eV, thus increasing the open-circuit voltage and fill factor. Additionally, short-circuit current density is enhanced up to about 38 mA/cm2 owing to the highly transparent Zn1- xMg xO/Zn1- xMg xO:Al layers. Ultimately, an 18.5%-efficient Cd-free solar cell with the Eg of buffer and TCO of 3.94 eV, prepared by an all dry process, is fabricated, which has the same level of 18.3% for the reference solar cell (glass/Mo/CIGSSe/CdS/ZnO/ZnO:Al).

Relationship between Lattice Strain and Efficiency for Sn-Perovskite Solar Cells.

In the composition of Q0.1(FA0.75MA0.25)0.9SnI3, Q is replaced with Na+, K+, Cs+, ethylammonium+ (EA+), and butylammonium+ (BA+), respectively, and the relationship between actually measured lattice strain and photovoltaic performances is discussed. The lattice strain evaluated by the Williamson-hall plot of X-ray diffraction data decreased as the tolerance factor was close to one. The efficiency of the Sn-perovskite solar cell was enhanced as the lattice strain decreased. Among them, EA0.1(FA0.75MA0.25)0.9SnI3 having lowest lattice strain gave the best result of 5.41%. Because the carrier mobility increased with a decrease in the lattice strain, these lattice strains would disturb carrier mobility and decrease the solar cell efficiency. Finally, the results that the efficiency of the SnGe-perovskite solar cells was gradually enhanced from 6.42 to 7.60% during storage, was explained by the lattice strain relaxation during the storage.

Suppression of Charge Carrier Recombination in Lead-Free Tin Halide Perovskite via Lewis Base Post-treatment.

Lead-free tin perovskite solar cells (PSCs) show the most promise to replace the more toxic lead-based perovskite solar cells. However, the efficiency is significantly less than that of lead-based PSCs as a result of low open-circuit voltage. This is due to the tendency of Sn2+ to oxidize into Sn4+ in the presence of air together with the formation of defects and traps caused by the fast crystallization of tin perovskite materials. Here, post-treatment of the tin perovskite layer with edamine Lewis base to suppress the recombination reaction in tin halide PSCs results in efficiencies higher than 10%, which is the highest reported efficiency to date for pure tin halide PSCs. The X-ray photoelectron spectroscopy data suggest that the recombination reaction originates from the nonstoichiometric Sn:I ratio rather than the Sn4+:Sn2+ ratio. The amine group in edamine bonded the undercoordinated tin, passivating the dangling bonds and defects, resulting in suppressed charge carrier recombination.

20% Efficient Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/Cu(In,Ga)(S,Se)2 Solar Cell Prepared by All-Dry Process through a Combination of Heat-Light-Soaking and Light-Soaking Processes.

Development of Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe)-based thin-film solar cells fabricated by an all-dry process is intriguing to minimize optical loss at a wavelength shorter than 520 nm owing to absorption of the CdS buffer layer and to be easily integrated into an in-line process for cost reduction. Cd-free CIGSSe solar cells are therefore prepared by the all-dry process with a structure of Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CIGSSe/Mo/glass. It is demonstrated that Zn0.8Mg0.2O and Zn0.9Mg0.1O:Al are appropriate as buffer and transparent conductive oxide layers with large optical band gap energy values of 3.75 and 3.80 eV, respectively. The conversion efficiency (η) of the Cd-free CIGSSe solar cell without K-treatment is consequently increased to 18.1%. To further increase the η, the Cd-free CIGSSe solar cell with K-treatment is next fabricated and followed by posttreatment called the heat-light-soaking (HLS) + light-soaking (LS) process, including HLS at 110 °C followed by LS under AM 1.5G illumination. It is disclosed that the HLS + LS process gives rise to not only the enhancement of carrier density but also the decrease in the carrier recombination rate at the buffer/absorber interface. Ultimately, the η of the Cd-free CIGSSe solar cell with K-treatment prepared by the all-dry process is enhanced to the level of 20.0%.

Structural and Solar Cell Properties of a Ag-Containing Cu2ZnSnS4 Thin Film Derived from Spray Pyrolysis.

A silver (Ag)-incorporated kesterite Cu2ZnSnS4 (CZTS) thin film was fabricated by a facile spray pyrolysis method. Crystallographic analyses indicated successful incorporation of various amounts of Ag up to a Ag/(Ag + Cu) ratio of ca. 0.1 into the crystal lattice of CZTS in a homogeneous manner without formation of other impurity compounds. From the results of morphological investigations, Ag-incorporated films had larger crystal grains than the CZTS film. The sample with a relatively low Ag content (Ag/(Ag + Cu) of ca. 0.02) had a compact morphology without appreciable voids and pinholes. However, an increase in the Ag content in the CZTS film (Ag/(Ag + Cu) ca. 0.10) induced the formation of a large number of pinholes. As can be expected from these morphological properties, the best sunlight conversion efficiency was obtained by the solar cell based on the film with Ag/(Ag + Cu) of ca. 0.02. Electrostructural analyses of the devices suggested that the Ag-incorporated film in the device achieved reduction in the amounts of unfavorable copper on zinc antisite defects compared to the bare CZTS film. Moreover, the use of a Ag-incorporated film improved band alignment at the CdS(buffer)-CZTS interface. These alterations should also contribute to enhancement of device properties.

Understanding charge transfer and recombination by interface engineering for improving the efficiency of PbS quantum dot solar cells.

In quantum dot heterojunction solar cells (QDHSCs), the QD active layer absorbs sunlight and then transfers the photogenerated electrons to an electron-transport layer (ETL). It is generally believed that the conduction band minimum (CBM) of the ETL should be lower than that of the QDs to enable efficient charge transfer from the QDs to the collection electrode (here, FTO) through the ETL. However, by employing Mg-doped ZnO (Zn1-xMgxO) as a model ETL in PbS QDHSCs, we found that an ETL with a lower CBM is not necessary to realize efficient charge transfer in QDHSCs. The existence of shallow defect states in the Zn1-xMgxO ETL can serve as additional charge-transfer pathways. In addition, the conduction band offset (CBO) between the ETL and the QD absorber has been, for the first time, revealed to significantly affect interfacial recombination in QDHSCs. We demonstrate that a spike in the band structure at the ETL/QD interface is useful for suppressing interfacial recombination and improving the open-circuit voltage. By varying the Mg doping level in ZnO, we were able to tune the CBM, defect distribution and carrier concentration in the ETL, which play key roles in charge transfer and recombination and therefore the device performance. PbS QDHSCs based on the optimized Zn1-xMgxO ETL exhibited a high power conversion efficiency of 10.6%. Our findings provide important guidance for enhancing the photovoltaic performance of QD-based solar cells.

Solution-Processed Air-Stable Copper Bismuth Iodide for Photovoltaics.

Bismuth-based solar cells have been under intensive interest as an efficient non-toxic absorber in photovoltaics. Within this new family of semiconductors, we herein report a new, long-term stable copper bismuth iodide (CuBiI4 ). A solutionprocessed method under air atmosphere is used to prepare the material. The adopted HI-assisted dimethylacetamide (DMA) co-solvent can completely dissolve CuI and BiI3 powders with high concentration compared with other organic solvents. Moreover, the high vapor pressure of tributyl phosphate, selected for the solvent vapor annealing (SVA), enables complete low-temperature (≤70 °C) film preparation, resulting in a stable, uniform, dense CuBiI4 film. The average grain size increases with the precursor concentration, greatly improving the photoluminescence lifetime and hall mobility; a carrier lifetime of 3.03 ns as well as an appreciable hall mobility of 110 cm2 V-1 s-1 were obtained. XRD illustrates that the crystal structure is cubic (space group Fd3m) and favored in the [1 1 1] direction. Moreover, the photovoltaic performance of CuBiI4 was also investigated. A wide bandgap (2.67 eV) solar cell with 0.82 % power conversion efficiency is presented, which exhibits excellent long-term stability over 1008 h under ambient conditions. This air-stable material may give an application in future tandem solar cells as a stable short-wavelength light absorber.

Ultrafast Electron Injection from Photoexcited Perovskite CsPbI3 QDs into TiO2 Nanoparticles with Injection Efficiency near 99.

Photoexcited electron injection dynamics from CsPbI3 quantum dots (QDs) to wide gap metal oxides are studied by transient absorption spectroscopy. Experimental results show under a low excitation intensity that ∼99% of the photoexcited electrons in CsPbI3 QDs can be injected into TiO2 with a size-dependent rate ranging from 1.30 × 1010 to 2.10 × 1010 s-1, which is also ∼2.5 times faster than that in the case of ZnO. A demonstration QD-sensitized solar cell based on a CsPbI3/TiO2 electrode is fabricated that delivers a power conversion efficiency of 5%.

Mixed Sn-Ge Perovskite for Enhanced Perovskite Solar Cell Performance in Air.

Lead-based perovskite solar cells have gained ground in recent years, showing efficiency as high as 20%, which is on par with that of silicon solar cells. However, the toxicity of lead makes it a nonideal candidate for use in solar cells. Alternatively, tin-based perovskites have been proposed because of their nontoxic nature and abundance. Unfortunately, these solar cells suffer from low efficiency and stability. Here, we propose a new type of perovskite material based on mixed tin and germanium. The material showed a band gap around 1.4-1.5 eV as measured from photoacoustic spectroscopy, which is ideal from the perspective of solar cells. In a solar cell device with inverted planar structure, pure tin perovskite solar cell showed a moderate efficiency of 3.31%. With 5% doping of germanium into the perovskite, the efficiency improved up to 4.48% (6.90% after 72 h) when measured in air without encapsulation.

Impact of Heterointerfaces in Solar Cells Using ZnSnP2 Bulk Crystals.

We report on the optimization of interface structure in ZnSnP2 solar cells. The effects of back electrode materials and related interface on photovoltaic performance were investigated. It was clarified that a conventional structure Mo/ZnSnP2 showed a Schottky-behavior, while an ohmic-behavior was observed in the Cu/ZnSnP2 structure annealed at 300 °C. STEM-EDX analysis suggested that Cu-Sn-P ternary compound was formed at the interface. This compound is considered to play an important role to obtain the ohmic contact between ZnSnP2 and Cu. In addition, it was clarified that the aqua regia etching of ZnSnP2 bulk crystals before chemical bath deposition process for the preparation of buffer layer was effective to remove the layer including lattice defects introduced by mechanical-polishing, which was supported by TEM observations and photoluminescence measurements. This means that the carrier transport across the interface was improved because of the reduced defect at the interface. Consequently, the conversion efficiency of approximately 2% was achieved with the structure of Al/ZnO;Al/ZnO/CdS/ZnSnP2/Cu, where the values of short circuit current density, JSC, open circuit voltage, VOC, and fill factor, FF, were 8.2 mA cm-2, 0.452 V, and 0.533, respectively. However, the value of VOC was largely low considering the bandgap value of ZnSnP2. To improve the conversion efficiency, the optimization of buffer layer material is considered to be essential in the viewpoint of band alignment.

Highly Luminescent Phase-Stable CsPbI3 Perovskite Quantum Dots Achieving Near 100% Absolute Photoluminescence Quantum Yield.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems such as CdSe and PbS, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental nonradiative recombination centers and thus quenching luminescence. In this paper, we show that a high room-temperature photoluminescence quantum yield of up to 100% can be obtained in CsPbI3 perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized with our improved synthetic protocol that involves introducing organolead compound trioctylphosphine-PbI2 (TOP-PbI2) as the reactive precursor, which also leads to a significantly improved stability for the resulting CsPbI3 QD solutions. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidence the negligible electron or hole-trapping pathways in our QDs, which explains such a high quantum efficiency. We expect the successful synthesis of the "ideal" perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices.

Impact of Precursor Compositions on the Structural and Photovoltaic Properties of Spray-Deposited Cu2 ZnSnS4 Thin Films.

Pure sulfide Cu2 ZnSnS4 thin films were fabricated on Mo-coated glass substrates by facile spray deposition of aqueous precursor solutions containing Cu(NO3 )2 , Zn(NO3 )2 , Sn(CH3 SO3 )2 , and thiourea followed by annealing at 600 °C. When a precursor solution containing a stoichiometric composition of Cu, Zn, and Sn was used, the resulting Cu2 ZnSnS4 thin film contained a Cu2-x S impurity phase owing to the evaporation of Sn components during the annealing process. The Cu2-x S impurity in the Cu2 ZnSnS4 thin film was removed by reducing the concentration of Cu in the precursor solution. This resulted in an improvement of the structural features (i.e., grain sizes and compactness) as well as the electric properties such as acceptor densities, the nature of the acceptor defects, and carrier lifetimes. A solar cell based on the Cu2 ZnSnS4 film with an empirically optimal composition showed conversion efficiency of 8.1 %. The value achieved was one of the best efficiencies of Cu2 ZnSnS4 -based cells derived from a non-vacuum process.

Architecture of the Interface between the Perovskite and Hole-Transport Layers in Perovskite Solar Cells.

The interface between the perovskite (PVK, CH3 NH3 PbI3 ) and hole-transport layers in perovskite solar cells is discussed. The device architecture studied is as follows: F-doped tin oxide (FTO)-coated glass/compact TiO2 /mesoporous TiO2 /PVK/2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-MeOTAD)/Au. After a thin layer of 4,4,4-trifluorobutylammonium iodide (TFBA) was inserted at the interface between PVK and Spiro-MeOTAD, the photovoltaic efficiency increased from 11.6-14.5 % to 15.1-17.6 %. TFBA (10 ppm) was added in the PVK solution before coating. Owing to the low surface tension of TFBA, TFBA rose to the surface of the PVK layer spontaneously during spin-coating to make a thin organic layer. The PVK grain boundaries also seemed to be passivated with the addition of TFBA. However, large differences in Urbach energies and valence band energy level were not observed for the PVK layer with and without the addition of TFBA. The charge recombination time constant between the PVK and the Spiro-MeOTAD became slower (from 8.4 to 280 µsec) after 10 ppm of TFBA was added in the PVK. The experimental results using TFBA conclude that insertion of a very thin layer at the interface between PVK and Spiro-MeOTAD is effective for suppressing charge recombination and increasing photovoltaic performances.