Chemical Consequences of Alkali Inhomogeneity in Cu2ZnSnS4 Thin‐Film Solar Cells

This study offers new insight into the role of Na in Cu₂ZnSnS₄ (CZTS) thin film solar cells by studying samples with a spatially varying alkali distribution. This is achieved by omitting a diffusion barrier between the soda‐lime glass substrate and the Mo back contact, where compositional variations of the glass inherently result in non‐uniform alkali distributions in the CZTS. By correlating light beam induced current (LBIC) maps with secondary ion mass spectrometry composition maps, it is shown that samples containing regions of higher Na concentration (“hot spots”) have corresponding LBIC hot spots on comparable length scales. Samples containing an alkali diffusion barrier have lower LBIC dispersion; thus, LBIC can be used to evaluate non‐uniformity in CZTS devices, where a common cause is Na inhomogeneity. Moreover, it is shown that the Na hot spots are strongly correlated with other compositional variations in the device, including increased Cu in‐diffusion with the underlying MoS₂ layer and decreased diffusion of Cd to the back contact. Neither of these effects are well understood in CZTS devices, and neither have previously been correlated with the presence or absence of Na.     

Cadmium Free Cu2ZnSnS4 Solar Cells with 9.7% Efficiency

Cu₂ZnSnS₄(CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn_1?xSn_xO_y (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.     

Kesterite Thin‐Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4

Quaternary semiconducting materials based on the kesterite (A₂BCX₄) mineral structure are the most promising candidates to overtake the current generation of light‐absorbing materials for thin‐film solar cells. Cu₂ZnSnS₄ (CZTS), Cu₂ZnSnSe₄ (CZTSe) and their alloy Cu₂ZnSn(Se,S)₄ consist of abundant, low‐cost and non‐toxic elements, unlike current CdTe and Cu(In,Ga)Se₂ based technologies. Zinc‐blende related structures are formed by quaternary compounds, but the complexity associated with the multi‐component system introduces difficulties in material growth, characterization, and application. First‐principles electronic structure simulations, performed over the past five years, that address the structural, electronic, and defect properties of this family of compounds are reviewed. Initial predictions of the bandgaps and crystal structures have recently been verified experimentally. The calculations highlight the role of atomic disorder on the cation sub‐lattice, as well as phase separation of Cu₂ZnSnS₄ into ZnS and CuSnS₃, on the material performance for light‐to‐electricity conversion in photovoltaic devices. Finally, the current grand challenges for materials modeling of thin‐film solar cells are highlighted.     

A High Efficiency Electrodeposited Cu2ZnSnS4 Solar Cell

High‐performance Cu₂ZnSnS₄ photovoltaic devices are demonstrated using electrodeposition of metal stacks and annealing of a CuZnSn precursor in a sulfur atmosphere. A champion electroplated Cu₂ZnSnS₄ solar cell achieves a power conversion efficiency of 7.3%, which is a record efficiency for electrodeposited Cu₂ZnSnS₄ solar devices. The device performance points to electrodeposition and annealing as a low‐cost and viable approach to earth‐abundant solar cell fabrication.     

Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

Earth‐abundant Cu₂BaSnS₄ (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 10⁴ cm^?1, and a p‐type conductivity, suitable as a top‐cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open‐circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine‐doped SnO₂ (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum‐doped ZnO (front contact).     

A Hierarchically Organized Photoelectrode Architecture for Highly Efficient CdS/CdSe‐Sensitized Solar Cells

A form of photoelectrode architecture suitable for inorganic semiconductor solar cells is reported. The developed architecture consists of hierarchically organized TiO₂ nanostructures with several tens of nanometer‐sized particles that have a large surface area and open channels with several hundred‐nanometer‐gaps perpendicular to the substrate. These are tailored by controlling the kinetic energy of the ablated species during pulsed laser deposition (PLD). To fabricate the solar cells, CdS and CdSe inorganic sensitizers are assembled onto the architecture by successive ionic layer adsorption and reaction and polysulfide solution is used as an electrolyte with lead sulfide counter‐electrodes. The inorganic semiconductor solar cells using the developed architecture (PLD‐TiO₂) show high energy conversion efficiencies of 5.57% compared to a conventional mesoporous TiO₂ film(NP‐TiO₂) (3.84%) with an optical mask at 1 sun of illumination. The improved cell performance of PLD‐TiO₂ is attributed to greater light‐harvesting ability, which results in the enhancement of the J_sc value. PLD‐TiO₂ absorbs more CdS/CdSe because of its larger surface area and excellent adhesion properties with fluorine‐doped tin oxide (FTO) substrates. Additionally, due to its unique channel‐shaped architecture, PLD‐TiO₂ has a longer electron lifetime compared to NP‐TiO₂.     

The Role of Hydrogen from ALD‐Al2O3 in Kesterite Cu2ZnSnS4 Solar Cells: Grain Surface Passivation

In this research, a new route of surface passivation is reported by introducing hydrogen from the atomic layer deposited (ALD) Al₂O₃ layer into pure sulfide Cu₂ZnSnS₄ (CZTS) solar cells. Different amounts of hydrogen are incorporated into the Cu₂ZnSnS₄/CdS interface through controlling the thickness of the ALD‐Al₂O₃ layer. The device with three cycles of ALD‐Al₂O₃ yields the highest efficiency of 8.08% (without antireflection coating) with improved open‐circuit voltage of up to 70 mV. With closer examination on the passivation route of ALD‐Al₂O₃, it is revealed by the surface chemisty study that the Al₂O₃ can be etched away by ammonium hydroxide in the CdS buffer deposition process. Instead, the hydrogen is detected within a shallow depth from the CZTS surface, and makes a significant difference in the measured distribution of contact potential difference and device performance. This may be interpreted by the effect of hydrogen passivation of the CZTS surface by curing dangling bonds at the surface of CZTS grains. This work may provide a new direction of further improving the performance of kesterite solar cells.     

Cation Substitution of Solution‐Processed Cu2ZnSnS4 Thin Film Solar Cell with over 9% Efficiency

To alleviate the limitations of pure sulfide Cu₂ZnSnS₄ (CZTS) thin film, such as band gaps adjustment, antisite defects, secondary phase and microstructure, Cadmium is introduced into CZTS thin film to replace Zn partially to form Cu₂Zn_1?xCd_xSnS₄ (CZCTS) thin film by low‐cost sol–gel method. It is demonstrated that the band gaps and crystal structure of CZCTS thin films are affected by the change in Zn/Cd ratio. In addition, the ZnS secondary phase can be decreased and the grain sizes can be improved to some degree by partial replacement of Zn with Cd in CZCTS thin film. The power conversion efficiency of CZTS solar cell device is enhanced significantly from 5.30% to 9.24% (active area efficiency 9.82%) with appropriate ratio of Zn/Cd. The variation of device parameter as a function of Zn/Cd ratio may be attributed to the change in electronic structure of the bulk CZCTS thin film (i.e., phase change from kesterite to stannite), which in turn affects the band alignment at the CZCTS/buffer interface and the charge separation at this interface.     

Impact of Nanoscale Elemental Distribution in High‐Performance Kesterite Solar Cells

The effort to develop earth‐abundant kesterite solar cells has led to an approximate doubling of the power conversion efficiency over the past five years to 12.6%, primarily due to increases in short‐circuit current and fill factor; open‐circuit voltage has resisted similar change, limiting further efficiency improvement. In the present investigation, Auger nanoprobe spectroscopy, X‐ray/ultraviolet photoelectron spectroscopy, and device characterization are used to provide a comprehensive understanding of the role of grain boundaries and interfaces in limiting performance in kesterite‐based devices. High photovoltaic performance is found to correlate with grain boundaries that are Cu‐depleted and enriched with SnOx. The formation of this bulk‐like oxide at grain boundaries with type I band offset provides a unique effective passivation that limits electron‐hole recombination. Building on these new insights, photovoltaic device simulations are performed that show optimized electrostatic designs can compensate for bulk defects, allowing efficiencies closer to the theoretical limit.     

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the row Li–Na–K–Rb–Cs the nominal Sn content should be reduced by more than 20% (relative) to achieve the highest conversion efficiency. The alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). Utilization of a wide range of characterization techniques helps to unveil the complex interplay between absorber composition and alkali doping. A ranking of alkali for best device performances, when employing alkali treatment, resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. Finally, a champion device with 11.5% efficiency (12.3% active area) is achieved using a high Li concentration with an optimized Sn content.     

Thin Film Co3O4/TiO2 Heterojunction Solar Cells

Co₃O₄ is investigated as a light absorber for all‐oxide thin‐film photovoltaic cells because of its nearly ideal optical bandgap of around 1.5 eV. Thin film TiO₂/Co₃O₄ heterojunctions are produced by spray pyrolysis of TiO₂ as a window layer, followed by pulsed laser deposition of Co₃O₄ as a light absorbing layer. The photovoltaic performance is investigated as a function of the Co₃O₄ deposition temperature and a direct correlation is found. The deposition temperature seems to affect both the crystallinity and the morphology of the absorber, which affects device performance. A maximum power of 22.7 μW cm^?2 is obtained at the highest deposition temperature (600 °C) with an open circuit photovoltage of 430 mV and a short circuit photocurrent density of 0.2 mA cm^?2. Performing deposition at 600 °C instead of room temperature improves power by an order of magnitude and reduces the tail states (Urbach edge energy). These phenomena can be explained by larger grains that grows at high temperature, as opposed to many nucleation events that occur at lower temperature.