Se2 Solar Cells Research Articles

On the Origin of Tail States and Open Circuit Voltage Losses in Cu(In,Ga)Se2

The detrimental effect of tail states on the radiative and non‐radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage (VOC) in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single‐crystal films. The results demonstrate that sodium and potassium decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of sodium or potassium incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high‐efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.

Development of n-type Zn(O, S) buffer layer deposited by open-air CVD method for Cu(In, Ga)Se2 solar cells

A Zn(O, S) thin-film is deposited utilizing an open-air CVD method by evaporating zinc-diethyldithiocarbamate, which is a non-vacuum and dry process. In an X-ray diffraction measurement, it is revealed that the films have a wurtzite structure and an [O]/([O] + [S]) ratio of 10%. A bandgap energy of 3.1 eV is estimated from the transmittance and reflectance spectra. By applying the Zn(O, S) as an n-type buffer layer, Cu(In, Ga)Se2 solar cells are fabricated. In the current density–voltage characteristics, distortion is observed at the bias voltages above the open-circuit voltage. It is implied that a large conduction band offset exists at a Zn(O, S)/CIGS interface. A quantum efficiency spectrum in the wavelength region of 380–512 nm is improved compared to a traditional CdS buffer layer. Finally, a 9.2%-efficient CIGS solar cell is demonstrated utilizing the Zn(O, S) buffer layer through an all-dry process.

Chemical phases in the solution-grown Zn(O,S) buffer of post-annealed Cu(In,Ga)Se2 solar cells investigated by transmission electron microscopy and electroreflectance

Thin-film solar cells with Cu(In,Ga)Se2 (CIGS) absorber layers have been intensively studied due to their high power conversion efficiencies. CIGS solar cells with Zn(O,S) buffer layers achieved record efficiencies due to their reduced parasitic absorption compared with the more commonly used CdS buffer. Accordingly, we have studied solution-grown Zn(O,S) buffer layers on polycrystalline CIGS absorber layers by complementary techniques. A bandgap energy Eg of 2.9 eV is detected by means of angle-resolved electroreflectance spectroscopy corresponding to Zn(O,S), whereas an additional Eg of 2.3 eV clearly appeared for a post-annealed CIGS solar cell (250 °C in air) compared with the as-grown state. To identify the chemical phase that contributes to the Eg of 2.3 eV, the microstructure and microchemistry of the Zn(O,S) buffer layers in the as-grown state and after annealing were analyzed by different transmission electron microscopic techniques on the submicrometer scale and energy-dispersive x-ray spectroscopy. We demonstrate that the combination of these methods facilitates a comprehensive analysis of the complex phase constitution of nanoscaled buffer layers. The results show that after annealing, the Cu concentration in Zn(O,S) is increased. This observation indicates the existence of an additional Cu-containing phase with Eg close to 2.3 eV, such as Cu2Se (2.23 eV) or CuS (2.36 eV), which could be one possible origin of the low power conversion efficiency and low fill factor of the solar cell under investigation.

Analysis of EBIC time-variation using 2D simulation including charge states in V Se–V Cu divacancy complex

Electron beam induced current (EBIC) measurements have been widely used to investigate charge carrier collection in Cu(In,Ga)Se2 solar cells. However, we found that this electron beam irradiation could significantly change the EBIC signal intensity during the measurement. In this study, the charge state variation of the V Se–V Cu divacancy proposed by Lany et al. was introduced into the device simulator to explain the phenomenon. In the simulation, the defects take on three different charged states, i.e. positive, neutral, and negative states, where their transitions are affected by the quasi-Fermi level position in the bandgap. The transient response of the EBIC signal was successfully explained by incorporating these complex state defects.

Nonradiative Recombination Dominates Voltage Losses in Cu(In,Ga)Se2 Solar Cells Fabricated using Different Methods

Voltage losses reduce the photovoltaic conversion efficiency of thin‐film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se2. Herein, voltage loss analysis of Cu(In,Ga)Se2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute >75% of the total compared to <25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition‐driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells.

Efficiency Enhancement by BSF Optimization on Cu (In1−x, Gax) Se2 Solar Cells with Tin (IV) Sulfide Buffer Layer

Efficiency Enhancement by BSF Optimization on Cu (In1−x, Gax) Se2 Solar Cells with Tin (IV) Sulfide Buffer Layer

Annealing effects on Cu(In,Ga)Se2 solar cells irradiated by high-fluence proton beam

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.

Radio-frequency magnetron sputtering deposition process for In2O3:H transparent conductive oxide films for application in Cu(In,Ga)Se2 solar cells

Typical Cu(In,Ga)Se2 solar cells are deposited on an opaque molybdenum back contact. However, for applications such as bifacial, semi-transparent or tandem solar cells a transparent back contact is required, for which various transparent conductive oxides have been tested, such as indium- or fluorine-doped tin oxide, and hydrogen-doped indium oxide. Here, a radio-frequency magnetron sputtering deposition process for In2O3:H (IOH) is investigated, where H is supplied from a Ar/H2 (5%) mixed gas and oxygen is pulsed during the entire deposition at room temperature. After deposition, the films are post-annealed in vacuum to optimize their optoelectronic properties. The oxygen plays an important role for the optoelectronic properties, where a high content of oxygen allows higher transparency but also increases the film sheet resistance. Optimum oxygen and Ar/H2 partial pressures of 3.2 × 10−2 Pa and 13×10−2 Pa, respectively, were found, producing IOH films with average visible transparency of 87% and sheet resistance after annealing of 19 Ohm/sq.

Reverse-Bias Defect Creation in Cu(In,Ga)Se2 Solar Cells and Impact of Encapsulation

Reverse breakdown in Cu(In,Ga)Se2 (CIGS) solar cells can lead to defect creation and performance degradation. We present pulsed reverse-bias experiments, where we stress CIGS solar cells with a short reverse voltage pulse of ten milliseconds and detect the electrical and thermal response of the cell. This way, we limit the duration of the reverse stress, allowing us to study the initial stages of reverse-bias defect creation in CIGS solar cells and modules. Our results show that permanent damage can develop very fast in under milliseconds. Furthermore, we find the location of defect creation as well as the susceptibility to defect creation under reverse bias depends strongly on whether the cell is encapsulated or not, where encapsulated cells are generally more robust against reverse bias.

Anisotropic Charge Transport in Cu(In,Ga)Se2 by Heavy Alkali Postdeposition Treatment for Reducing Cell‐to‐Module Efficiency Loss in Monolithically Integrated Photovoltaic Modules

The recent efficiency boosting of Cu(In,Ga)Se2 (CIGS) solar cells is undoubtedly triggered by heavy alkali postdeposition treatments (PDTs). However, the effects are not obvious under monolithically integrated CIGS modules where various current‐shunting sources can deteriorate the device performance. Herein, It is reported that KF PDT can effectively suppress the major shunting sources caused by P1 and P3 laser scribing for monolithic interconnection, reducing the cell‐to‐module (CTM) efficiency gap in CIGS photovoltaics. CIGS with NaF PDT exhibits nearly isotropic and high hole mobilities, causing a large CTM efficiency loss. CIGS with additional KF PDT, on the other hand, reveals much lower in‐plane hole mobility than the out‐of‐plane component, significantly increasing the P1 shunt resistance without exacerbating the photocarrier extraction in the active area. It is suggested that such anisotropic charge transport is due to carrier scattering by low‐conductivity phases at the CIGS grain boundaries. Furthermore, passivation of the front junction by KF PDT raises the tolerance to P3 scribing‐induced damage, increasing the P3 shunt resistance while preserving the junction property unlike the NaF PDT case. The work implies that the recent trend of employing heavy alkali PDTs for a high‐efficiency cell is also crucial for designing a high‐efficiency CIGS module.

Toward lossless photovoltaic efficiency of Laser-shaped flexible Cu(In,Ga)Se2 solar cells on stainless steel substrates

Flexible Cu(In,Ga)Se2 (CIGS) solar cells on metal substrates are susceptible to thermal effects during laser shaping, resulting in their failure. In this study, a two-step scribing–cutting approach for shaping CIGS cells on stainless steel foil substrates was developed. Electrical–thermal dual isolations were performed by laser scribing to guard against damage to the CIGS cells during the laser shaping process. The efficiency loss ratio after an ultraviolet nanosecond laser shaping decreased from an average of 80% for the one-step method to an average of < 10% for the two-step method. The efficiency loss could be further decreased with a larger cell area of > 1 cm2, which is closer to lossless laser shaping. This method was also demonstrated successfully on curved-shaped cells. This work opens up the prospect of using laser shaping in the mass manufacture of flexible CIGS solar cells on metal substrates.

Effect of thiourea concentration on chemical bath deposited CdxZn1 − xS thin films properties for Cu(In,Ga)Se2 solar cells

Effect of thiourea concentration on chemical bath deposited CdxZn1 − xS thin films properties for Cu(In,Ga)Se2 solar cells

Back interface passivation for ultrathin Cu(In,Ga)Se2 solar cells with Schottky back contact: A trade-off of electrical effects

Back interface passivation reduces the back recombination of photogenerated electrons, whereas aggravates the blocking of hole transport towards back contact, which complicate the back interface engineering for ultrathin CIGSe solar cells with a Schottky back contact. In this work, theoretical explorations were conducted to study how the two contradictory electrical effects impact cell performance. For ultrathin CIGSe solar cells with a pronounced Schottky potential barrier (E h > 0.2 eV), back interface passivation produces diverse performance evolution trends, which are highly dependent on cell structures and properties. Since a back Ga grading can screen the effect of reduced recombination of photogenerated electrons from back interface passivation, the hole blocking effect predominates and back interface passivation is not desirable. However, when the back Schottky diode merges with the main pn junction due to a reduced absorber thickness, the back potential barrier and the hole blocking effect is much reduced on this occasion. Consequently, cells exhibit the same efficiency evolution trend as ones with an Ohmic contact, where back interface passivation is always advantageous. The discoveries imply the complexity of back interface passivation and provide guidance to manipulate back interface for ultrathin CIGSe solar on TCOs with a pronounced Schottky back contact.

Fabrication of semi-transparent Cu(In,Ga)Se2 solar cells aided by Bromine etching

This study introduces a straightforward and reproducible fabrication method to achieve micro-structured semi-transparent Cu(In,Ga)Se2 (CIGSe) solar cells by lithography and wet etch techniques. We fabricated a segmented micro-structured solar cell by selective etching of an opaque CIGSe solar cell with bromine and bleach solutions and obtained an array of micro lines from 200 µm to 1000 µm in width separated by transparent spaces, resulting in an overall semi-transparent structure. The CIGSe solar cell lines exhibit a maximum power conversion efficiency of 12.4 % for 1000 µm width, which reduces to 6.8 % for 200 µm wide lines. The obtained values are among the highest reported in the literature for similarly architected devices. Our results are a promising step towards the implementation of building integrated CIGSe photovoltaics as windows.

Effects of ITO based back contacts on Cu(In,Ga)Se2 thin films, solar cells, and mini-modules relevant for semi-transparent building integrated photovoltaics

Effects of ITO based back contacts on Cu(In,Ga)Se2 thin films, solar cells, and mini-modules relevant for semi-transparent building integrated photovoltaics

Modeling and Investigation of Rear-Passivated Ultrathin CIGS Solar Cell

In this paper, we use numerical simulations to investigate ultrathin Cu (In1−xGax) Se2 solar cells. In the first part, we focus on the cell configuration in which the PV parameters fit and match the fabricated cell characteristics. Our goal is to investigate the impact of different loss mechanisms, such as interface trap density (Dit) and absorber trap density (Nt), in different cell pitch sizes on cell performances. Dit defines the number of carrier traps at CIGS/Al2O3 interfaces to recombine with photogenerated carriers. Nt defines the number of carrier traps in the absorber layer. Recombination through traps has been found to be the primary loss process in the investigated cell. Additional numerical simulations reveal appreciable gains in cell performance for various cell pitch sizes, absorber doping densities, Ga content, and graded bandgap under AM1.5 illumination. Research during the recent decade has clarified that the most promising strategy to achieve maximum efficiency consists of the so-called tandem configuration. Therefore, we here propose a u-CIGS/PERT silicon device employing, as a top cell, a u-CIGS cell optimized to take into account the above procedure. The results of these simulations provide insights into the optimization of ultrathin-film CIGS solar cells.

Revealing surface modifications of caesium indium selenium treated Cu-rich Cu(In,Ga)Se2: A study of the suppression of the interfacial recombination

Cu(In,Ga)Se2 (CIGS) solar cell is a kind of high efficiency thin film photovoltaic device. These CIGS solar cells with high efficiency tend to be Cu-poor. Actually, the Cu-rich CIGS solar cells which have the superior electrical characteristics should have higher efficiency compared with the Cu-poor ones. However, the performance (especially the VOC) of Cu-rich devices is limited by the serious interfacial recombination. To suppress the interfacial recombination and improve the efficiency of Cu-rich devices, we co-evaporate CsF, In and Se at the surface of the Cu-rich absorber (Cs-In-Se-PDT). By this way, the compound of Cs-In-Se is formed at the absorber surface, which leads to the valence band energy, work function and the grain boundary recombination at the surface of the Cu-rich absorber decrease. This contributes to inhibiting the interfacial recombination. Especially, the tunneling assistant interfacial recombination can be suppressed significantly. Besides, the Cs-In-Se-PDT mainly changes the Cu content of the absorber surface, the advantage of Cu-rich in the interior of the absorber can still be maintained. After Cs-In-Se-PDT, the VOC of the Cu-rich device (CGI = 1.03 %) increases by about 90 mV, and the device efficiency also boosts from 12.7 % to 16.04 %.

Graded Bandgap Ultrathin CIGS Solar Cells

In this paper, we physically modeled passivated ultrathin Cu (In1−xGax) Se2 solar cells with different bandgap grading configurations. Firstly, we have designed the cell architecture according to the fabricated model. The novelty in this work is the modeling of passivated u-CIGS solar cells with different bandgap grading profile configurations in order to achieve high efficiency with a thickness of 500 nm. A significant influence on device performance has been observed while changing absorber doping density, electron affinity, and operating temperature (range of 10–70 °C) for the investigated samples. ZnS has been used as a buffer layer to replace the conventional CdS material in order to improve cell efficiency. The impact of the buffer doping density and electron affinity on u-CIGS cell performance is explored. The simulation results show that a high bandgap at the front and rear sides with an acceptor density of 2 × 1016 provide the best electrical cell parameters: Jsc of 31.53 mA/cm2, Voc of 742.78 mV, FF of 77.50%, η of 18.15%. Our findings can be considered guidelines for new single and/or tandem cell optimization to achieve high efficiency.

Improvement of Quantitative STEM/EDXS Analyses for Chemical Analysis of Cu(In,Ga)Se2 Solar Cells with Zn(O,S) Buffer Layers

Abstract Energy-dispersive X-ray spectroscopy (EDXS) in a transmission electron microscope is frequently used for the chemical analysis of Cu(In,Ga)Se2 (CIGS) solar cells with high spatial resolution. However, the quantification of EDXS data is complicated due to quantification errors and artifacts. This work shows how quantitative EDXS analyses of CIGS-based solar cells with Zn(O,S) buffer and ZnO-based window layers can be significantly improved. For this purpose, CIGS-based solar cells and a reference sample with a stack of Zn(O,S) layers with different [O]/[S] ratios were analyzed. For Zn(O,S), the correction of sample-thickness-dependent absorption of low-energy O–Kα X-rays significantly improves the results of quantitative EDXS. Absorption of characteristic X-rays in CIGS is less relevant. However, for small transmission electron microscopy (TEM) sample thicknesses, artifacts can occur due to material changes by focused-ion-beam (FIB)-based preparation of TEM samples, electron-beam-induced damage, and oxidation of the sample surface. We also show that a Pt-protection layer, deposited on the sample surface before FIB preparation of TEM lamellae, can induce artifacts that can be avoided by first depositing a carbon layer.

Indoor photovoltaics awaken the world's first solar cells.

Selenium (Se) solar cells were the world's first solid-state photovoltaics reported in 1883, opening the modern photovoltaics. However, its wide bandgap (~1.9 eV) limits sunlight harvesting. Here, we revisit the world's oldest but long-ignored photovoltaic material with the emergence of indoor photovoltaics (IPVs); the absorption spectrum of Se perfectly matches the emission spectra of commonly used indoor light sources in the 400 to 700 nm range. We find that the widely used Te adhesion layer also passivates defects at the nonbonded Se/TiO2 interface. By optimizing the Te coverage from 6.9 to 70.4%, the resulting Se cells exhibit an efficiency of 15.1% under 1000 lux indoor illumination and show no efficiency loss after 1000 hours of continuous indoor illumination without encapsulation, outperforming the present IPV industry standard of amorphous silicon cells in both efficiency and stability. We further fabricate Se modules (6.75 cm2) that produce 232.6 μW output power under indoor illumination, powering a radio-frequency identification-based localization tag.