Ag Incorporation with Controlled Grain Growth Enables 12.5% Efficient Kesterite Solar Cell with Open Circuit Voltage Reached 64.2% Shockley–Queisser Limit

The large open-circuit voltage deficit (V_oc,def) is the key issue that limits kesterite (Cu₂ZnSn(S,Se)₄, [CZTSSe]) solar cell performance. Substitution of Cu⁺ by larger ionic Ag⁺ ((Ag,Cu)₂ZnSn(S,Se)₄, [ACZTSSe]) is one strategy to reduce Cu–Zn disorder and improve kesterite V_oc. However, the so far reported ACZTSSe solar cell has not demonstrated lower V_oc,def than the world record device, indicating that some intrinsic defect properties cannot be mitigated using current approaches. Here, incorporation of Ag into kesterite through a dimethyl sulfoxide (DMSO) solution that can facilitate direct phase transformation grain growth and produce a uniform and less defective kesterite absorber is reported. The same coordination chemistry of Ag⁺ and Cu⁺ in the DMSO solution results in the same reaction path of ACZTSSe to CZTSSe, resulting in significant suppression of Cu_Zn defects, its defect cluster [2Cu_Zn + Sn_Zn], and deep level defect Cu_Sn. A champion device with an efficiency of 12.5% (active area efficiency 13.5% without antireflection coating) and a record low V_oc,def (64.2% Shockley–Queisser limit) is achieved from ACZTSSe with 5% Ag content.     

Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

The development of kesterite Cu₂ZnSn(S,Se)₄ thin‐film solar cells is currently hindered by the large deficit of open‐circuit voltage (V_oc), which results from the easy formation of Cu_Zn antisite acceptor defects. Suppressing the formation of Cu_Zn defects, especially near the absorber/buffer interface, is thus critical for the further improvement of kesterite solar cells. In this paper, it is shown that there is a large disparity between the defects in Cu‐ and Ag‐based kesterite semiconductors, i.e., the Cu_Zn or Cu_Cd acceptor defects have high concentration and are the dominant defects in Cu₂ZnSn(S,Se)₄ or Cu₂CdSnS₄, but the Ag_Zn acceptor has only a low concentration and the dominant defects are donors in Ag₂ZnSnS₄. Therefore, the Cu‐based kesterites always show p‐type conductivity, while the Ag‐based kesterites show either intrinsic or weak n‐type conductivity. Based on this defect disparity and calculated band alignment, it is proposed that the V_oc limit of the kesterite solar cells can be overcome by alloying Cu₂ZnSn(S,Se)₄ with Ag₂ZnSn(S,Se)₄, and the composition‐graded (Cu,Ag)₂ZnSn(S,Se)₄ alloys should be ideal light‐absorber materials for achieving higher efficiency kesterite solar cells.     

Growth mechanisms of co‐evaporated kesterite: a comparison of Cu‐rich and Zn‐rich composition paths

Earth abundant kesterite solar cells have achieved 7–10% cell efficiency mostly by processes that separate the film deposition and the annealing into two sequential steps. In contrast, co‐evaporation onto a high‐temperature substrate, demonstrating previous success in chalcopyrite (Cu(In,Ga)Se₂) solar cells, allows real‐time composition control. Chalcopyrite research widely supports the model that Cu‐rich growth conditions assist grain growth, and subsequently, the endpoint composition can be adjusted back to Cu‐poor via monitoring the surface emissivity of the film. On the basis of the same intentions, the recent development of co‐evaporated kesterite (Cu₂ZnSnSe₄) adapts the concept and achieves 9.2% efficiency. To understand the effect of growth strategies, this study examines the phase evolution, grain morphology, and device performance in Cu‐rich growth and other strategies (Zn‐rich and close‐to‐stoichiometric). By characterizing films obtained from interrupted depositions and also interpreting the variation in surface emission during growths, this study found a subtle hindrance in the reaction of Cu_xSe_y and ZnSe possibly caused by the volatile nature of SnSe_x. The hindrance explains why, distinctive from chalcopyrite, little difference in grain size is observed between kesterite films made by Cu‐rich versus Zn‐rich growth at these deposition rates. At last, a Zn‐rich growth 9.1% device, certified by the National Renewable Energy Laboratory, is presented, which equals the performance of the previously‐reported Cu‐rich growth device. At the present stage, we believe the Cu‐rich and Zn‐rich growth share equal promise for the optimization of kesterite solar cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Structural tuning of wide‐gap chalcopyrite CuGaSe2 thin films and highly efficient solar cells: differences from narrow‐gap Cu(In,Ga)Se2

Simultaneous realization of high values of open circuit voltage (V_oc), 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 (V_oc > 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.     

19·9%‐efficient ZnO/CdS/CuInGaSe2 solar cell with 81·2% fill factor

We report a new record total‐area efficiency of 19·9% for CuInGaSe₂‐based thin‐film solar cells. Improved performance is due to higher fill factor. The device was made by three‐stage co‐evaporation with a modified surface termination. Growth conditions, device analysis, and basic film characterization are presented. Published in 2008 by John Wiley & Sons, Ltd.     

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.     

Hybrid Cd‐free CIGS solar cell/TEG device with ZnO nanowires

A green energy device with a CuInGaSe₂ (CIGS) photovoltaic (PV) cell covered with a passive light‐trapping structure (ZnO nanowires (NWs)) and connected to an active energy‐harvesting device (thermoelectric generator (TEG)) is presented. The efficiency of the ZnO NWs/CIGS PV device obtained using a deposition temperature of 550 °C and Cd‐free processes reaches 16.5%. The series‐connected CIGS PV cell with a TEG had a record‐high efficiency of 22% at a cool‐side temperature (T_c) below 5 °C. The open‐circuit voltage (V_oc) of the hybrid CIGS PV/TEG device was increased from 0.64 to 0.85 V. This technology has potential for high‐efficiency energy‐harvesting applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Electrodeposited Cu2ZnSnSe4 thin film solar cell with 7% power conversion efficiency

High performance Cu₂ZnSnSe₄ (CZTSe) photovoltaic materials were synthesized by electrodeposition of metal stack precursors followed by selenization. A champion solar cell with 7.0% efficiency is demonstrated. This is the highest efficiency among all of the CZTSe solar cells prepared from electrodeposited metallic precursors reported to‐date. Device parameters are discussed from the perspective of material microstructure and composition in order to improve performance. In addition, a high performance electrodeposited CZTS (S only) solar cell was demonstrated and its device characteristics were compared against the CZTSe (Se only) cell. Using secondary ion mass spectrometry for the analysis of the chemical composition of the absorber layer, a higher concentration of oxygen in the electrodeposited absorber is thought to be the root cause of the lower performance of the electrodeposited CZTS or CZTSe solar cells with respect to a solar cell fabricated by evaporation. The grain boundary areas of Sn‐rich composition are thought to be responsible for the lower shunt resistance commonly observed in CZTSe devices. We measured the longest minority carrier lifetime of 18 ns among all reported kesterite devices. This work builds a good baseline for obtaining higher efficiency earth‐abundant solar cells, while it highlights electrodepositon as a low cost and feasible method for earth‐abundant thin film solar cell fabrication. Copyright © 2013 John Wiley & Sons, Ltd.     

High photo‐conversion efficiency in double‐graded Cu(In,Ga)(S,Se)2 thin film solar cells with two‐step sulfurization post‐treatment

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells and to improve the open circuit voltage (V_OC ) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double‐graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two‐step sulfurization process provides an increase of V_OC of 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm² monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm²), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two‐step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two‐step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two‐step‐sulfurization appeared to have less of Cu_2‐xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.     

High efficiency low temperature grown Cu(In,Ga)Se2 thin film solar cells on flexible substrates using NaF precursor layers

We report a total‐area efficiency of 15.9% for flexible Cu(In,Ga)Se₂ thin film solar cells on polyimide foil (cell area 0.95 cm²). The absorber layer was grown by a multi‐stage deposition process at a maximum nominal process temperature of 420°C. The Na was added via evaporation of a NaF layer prior to the absorber deposition leading to an enhanced V_oc and FF. Growth conditions and device characterization are described. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis of Ultrathin,Homogeneous Copolymer Dielectrics to Control the Threshold Voltage of Organic Thin‐Film Transistors

This work demonstrates that threshold voltage (V_T) of organic thin‐film transistors (OTFTs) can be controlled systematically by introducing new copolymer dielectrics with electropositive functionality. A series of homogeneous copolymer dielectrics are polymerized from two monomers, 1,3,5‐trimethyl‐1,3,5‐trivinyl cyclotrisiloxane (V3D3) and 1‐vinylimidazole (VI), via initiated chemical vapor deposition. The chemical composition of the copolymer dielectrics is exquisitely controlled to tune the V_T of C₆₀ OTFTs. In particular, all the copolymer dielectrics demonstrated in this work exhibit extremely low leakage current densities (lower than 2.5 × 10^?8 A cm^?2 at ±3 MV cm^?1) even with a thickness less than 23 nm. Furthermore, by introducing an ultrathin pV3D3 interfacial layer (about 3 nm) between the copolymer dielectrics and C₆₀ semiconductor, the high mobility of the C₆₀ OTFTs (about 1 cm² V^?1 s^?1) remains unperturbed, showing that V_T can be controlled independently by tuning the composition of the copolymer dielectrics. Coupled with the ultralow dielectric thickness, the independent V_T controllability allows the V_T to be aligned near 0 V with sub‐3 V operating voltage, which enables a substantial decrease of device power consumption. The suggested method can be employed widely to enhance device performance and reduce power consumption in various organic integrated circuit applications.     

Cu2ZnSnSe4 solar cells with 10.6% efficiency through innovative absorber engineering with Ge superficial nanolayer

In our recently published work, the positive effect of a Ge nanolayer introduced into the processing of Cu₂ZnSnSe₄ absorbers (CZTSe) was demonstrated. In this contribution, the complete optimization of this new approach is presented for the first time. Hence, the optimum Ge nanolayer thickness range is defined in order to achieve an improved performance of the devices, obtaining a record efficiency of 10.6%. By employing this optimized approach, the open‐circuit voltage (V_OC) is boosted for our pure selenide CZTSe up to 489 mV, leading to V_OC deficit among the lowest reported so far in kesterite technology. Additionally, two important effects related to the Ge are unambiguously demonstrated that might be the origin of the V_OC boost: the improvement of the grain size and the corresponding crystalline quality, and the interaction between Ge and Na that allows for dynamic control over the CZTSe doping. Finally, evidences pointing to the origin of the deterioration of devices properties for large Ge concentrations are presented. Copyright © 2016 John Wiley & Sons, Ltd.     

Dye‐sensitized TiO2 solar cells based on nanocomposite photoanode containing plasma‐modified multi‐walled carbon nanotubes

A series of anatase TiO₂‐based nanocomposite incorporated with plasma‐modified multi‐walled carbon nanotubes (MWNTs) was prepared by physical blending and shows its capability for efficient electron transport when used as photoanode in dye‐sensitized solar cells (DSSCs). These MWNTs characterized with good dispersal performance were obtained by functionalization technique via in situ plasma treatment and subsequent grafting with maleic anhydride (MA) onto the external walls reported previously. Compared with the conventional DSSCs, the TiO₂ film with 1D carbon nanotubes possesses more outstanding ability to transport electrons injected from the excited dye within the device under illumination. As a result, at an optimum addition of 0.3 wt% MWNTs‐MA in TiO₂ matrix, the photocurrent–voltage (J–V) characteristics showed a significant increase in the short‐circuit photocurrent (J_sc) of 50%, leading to an increase in overall solar conversion efficiency by a factor of 1.5. Electrochemical impedance spectroscopy analyses reveal that the MWNTs‐MA/TiO₂ incur smaller resistances at the photoanode in assembled DSSCs when compared with those in the anatase titania DSSCs. These features suggest that the conducting properties of the MWNTs‐MA within the anodes are crucial for achieving a higher transport rate for photo‐induced electrons in TiO₂ layer by exhibiting lower resistance in the porous network and hence retard charge recombination that could result in poor conversion efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Functionalization of Graphene Oxide Films with Au and MoOx Nanoparticles as Efficient p‐Contact Electrodes for Inverted Planar Perovskite Solar Cells

A graphene oxide (GO) film is functionalized with metal (Au) and metal‐oxide (MoO_x) nanoparticles (NPs) as a hole‐extraction layer for high‐performance inverted planar‐heterojunction perovskite solar cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X‐ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down‐shifts of work functions lead to a decreased level of potential energy and hence increased V_oc of the PSC devices. Although the GO‐AuNP film shows rapid hole extraction and increased V_oc, a J_sc improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO‐AuNP device attains 14.6%, which is comparable with that of the GO‐based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO‐MoO_x layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO‐MoO_x interface is hence significantly retarded. The GO‐MoO_x device consequently shows significantly enhanced V_oc and J_sc, for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.     

Accelerated development of CuSbS2 thin film photovoltaic device prototypes

Development of alternative thin film photovoltaic technologies is an important research topic because of the potential of low‐cost, high‐efficiency solar cells to produce terawatt levels of clean power. However, this development of unexplored yet promising absorbers can be hindered by complications that arise during solar cell fabrication. Here, a high‐throughput combinatorial method is applied to accelerate development of photovoltaic devices, in this case, using the novel CuSbS₂ absorber via a newly developed three‐stage self‐regulated growth process to control absorber purity and orientation. Photovoltaic performance of the absorber, using the typical substrate CuIn_xGa_{1 − x}Se₂ (CIGS) device architecture, is explored as a function of absorber quality and thickness using a variety of back contacts. This study yields CuSbS₂ device prototypes with ~1% conversion efficiency, suggesting that the optimal CuSbS₂ device fabrication parameters and contact selection criteria are quite different than for CIGS, despite the similarity of these two absorbers. The CuSbS₂ device efficiency is at present limited by low short‐circuit current because of bulk recombination related to defects, and a small open‐circuit voltage because of a theoretically predicted cliff‐type conduction band offset between CuSbS₂ and CdS. Overall, these results illustrate both the potential and limits of combinatorial methods to accelerate the development of thin film photovoltaic devices using novel absorbers. Copyright © 2016 John Wiley & Sons, Ltd.     

Improved photovoltaic response of a near-infrared sensitive solar cell by a morphology-controlling seed layer

We demonstrate that a thin seed layer of indium phthalocyanine chloride (ClInPc) annealed under mild conditions effectively controls the morphology of both post-annealing deposited ClInPc films and ClInPc:C₆₀ mixed films, introducing the triclinic phase into the commonly monoclinic phase dominating film. ClInPc/C₆₀ planar solar cells and ClInPc:C₆₀ (1:1) planar-mixed solar cells with and without the triclinic phase were studied. Increased short circuit current (J_sc), fill factor (FF), external quantum efficiency (EQE) and internal quantum efficiency (IQE) of the devices containing triclinic phase is attributed to the enhanced absorption in the near infrared (NIR) region and decreased series resistance. The correlation between open circuit voltage (V_oc) and dark saturation pre-exponential factor (J_so) was analyzed to investigate V_oc loss upon annealing. The overall performance of device is considerably improved by introducing the triclinic phase of ClInPc.     

CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide/Au back contacts

Superstrate CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide (TMO)/Au and Au‐only back contacts have been fabricated. The TMOs include MoO_3‐x, V₂O_5‐x, and WO_3‐x. The incorporation of the TMO buffer layers at the back contacts resulted in significant improvement on open‐circuit voltage (V_OC) as compared with the cells with Cu‐free Au‐only back contacts. Among the cells using TMO buffer layers, the ones with MoO_3‐x buffer layers exhibited the best performance, yielding an efficiency of 14.1% under AM1.5 illumination with V_OC of 815 mV and a fill factor of 67.9%. Though the performance is slightly behind the best reference cell with a Cu/Au back contact fabricated in our lab with V_OC of 844 mV, fill factor of 76.3%, and efficiency of 15.7%, the use of Cu‐free back contacts may lead to improved long‐term cell stability. Copyright © 2013 John Wiley & Sons, Ltd.     

p‐CuO/n‐Si heterojunction solar cells with high open circuit voltage and photocurrent through interfacial engineering

Heterojunction solar cells of p‐type cupric oxide (CuO) and n‐type silicon (Si), p‐CuO/n‐Si, have been fabricated using conventional sputter and rapid thermal annealing techniques. Photovoltaic properties with an open‐circuit voltage (V_oc) of 380 mV, short circuit current (J_sc) of 1.2 mA/cm², and a photocurrent of 2.9 mA/cm² were observed for the solar cell annealed at 300 °C for 1 min. When the annealing duration was increased, the photocurrent increased, but the V_oc was found to reduce because of the degradation of interface quality. An improvement in the V_oc resulting to a record value of 509 mV and J_sc of 4 mA/cm² with a high photocurrent of ~12 mA/cm² was achieved through interface engineering and controlling the phase transformation of CuO film. X‐ray diffraction, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy analysis have been used to investigate the interface properties and crystal quality of sputter‐deposited CuO thin film. The improvement in V_oc is mainly due to the enhancement of crystal quality of CuO thin film and interface properties between p‐CuO and n‐Si substrate. The enhancement of photocurrent is found to be due to the reduction of carrier recombination rate as revealed by transient photovoltage spectroscopy analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Low‐Temperature‐Grown Transition Metal Oxide Based Storage Materials and Oxide Transistors for High‐Density Non‐volatile Memory

An effective stacked memory concept utilizing all‐oxide‐based device components for future high‐density nonvolatile stacked structure data storage is developed. GaInZnO (GIZO) thin‐film transistors, grown at room temperature, are integrated with one‐diode (CuO/InZnO)–one‐resistor (NiO) (1D–1R) structure oxide storage node elements, fabricated at room temperature. The low growth temperatures and fabrication methods introduced in this paper allow the demonstration of a stackable memory array as well as integrated device characteristics. Benefits provided by low‐temperature processes are demonstrated by fabrication of working devices over glass substrates. Here, the device characteristics of each individual component as well as the characteristics of a combined select transistor with a 1D–1R cell are reported. X‐ray photoelectron spectroscopy analysis of a NiO resistance layer deposited by sputter and atomic layer deposition confirms the importance of metallic Ni content in NiO for bi‐stable resistance switching. The GIZO transistor shows a field‐effect mobility of 30 cm² V⁻¹ s⁻¹, a V_th of +1.2 V, and a drain current on/off ratio of up to 10⁸, while the CuO/InZnO heterojunction oxide diode has forward current densities of 2 × 10⁴ A cm⁻². Both of these materials show the performance of state‐of‐the‐art oxide devices.     

Improved performance in ZnO/CdS/CuGaSe2 thin‐film solar cells

We report the growth and characterization of improved efficiency wide‐bandgap ZnO/CdS/CuGaSe₂ thin‐film solar cells. The CuGaSe₂ absorber thickness was intentionally decreased to better match depletion widths indicated by drive‐level capacitance profiling data. A total‐area efficiency of 9·5% was achieved with a fill factor of 70·8% and a V_oc of 910 mV. Published in 2003 by John Wiley & Sons, Ltd.