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Cu2ZnSnS4 (CZTS) has been proposed to have the potential of being an alternative absorber in thin film solar cells owing to its high absorption coefficient, earth-abundant constituents and tenability of its physical properties. In addition, it's wide band gap which can be adjusted by alloying fits well with tandem solar cells, fulfilling the increasing demand for photovoltaic (PV) development. However, the progress of the single junction CZTS solar cell is slowing down gradually in the past few years due to the multiple tough limiting issues to the device performance. This thesis aims to understand the loss mechanism in the device and find strategies to tackle these problems, enhancing the efficiency of CZTS solar cells step by step.In the first step, CZTS/Mo interface has been carefully studied and the unfavorable reaction between Mo and CZTS was suppressed by inserting an ultrathin Al2O3 between them. The formation of secondary phases and voids because of the reaction was significantly reduced, while the MoS2 layer with suitable thickness still formed due to the diffusion of sulfur during the annealing process.Secondly, the band alignment at the heterojunction has been investigated. The “cliff” like conduction band offset (CBO) at CZTS/CdS interface can be reversed to “spike” like by applying alternative ZnCdS buffer. As a result, the interface recombination can be modified dramatically and the efficiency can be boosted to over 9%.After that, the interface defects have been effectively passivated by the ZnS layer self-assembled at the interface based on the ZnCdS buffer. The relationship between the ZnS formation mechanism and the absorber composition was studied and the device performance was further improved.In the last step of this thesis, the interface chemistry and its related microstructure have been studied. The modified deposition process for ZnCdS layer with ammonia involved enabled the reduction of oxide and hydroxide impurity at the interface, thereby i

Despite remarkable progress in the performance of kesterite Cu2ZnSnS4 (CZTS)-based photovoltaic technology has been achieved, the interface recombination and associated open-circuit voltage (Voc) deficit still dominate the loss mechanism in this technology. To alleviate heterojunction interface recombination in pure sulfide thin film solar cells, passivation structure at the interface is required. In this work, we developed an ultrathin nanometer-scale ZnS dielectric passivation layer which is readily formed in situ at the CZTS/ZnCdS heterointerface during the ZnCdS buffer deposition process via Zn diffusion from the ZnCdS bulk to the interface. With this nanoscale structure, a remarkable open-circuit voltage and fill factor improvement is illustrated, and a total area efficiency of 9.25% is obtained. The formation and features of the nanoscale ZnS layer are investigated by high-resolution scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. This self-assembled ZnS layer with dielectric properties passivates defects at the interface while still enabling the electrons to transport across the buffer layer because of the ultrathin thickness, which satisfies the requirement of dielectric passivation layer but requiring no complicated regular patterning. The correlation between the effects of passivation and device performance is investigated by device simulation, presenting a reasonable understanding of the experimental results. The results open a new aspect to passivate the interface recombination and expand the potential of upscaling CZTS technology.

Heterojunction interface of the Cu2ZnSnS4 (CZTS) solar cells is one the critical factors for performance loss, especially when applying Zn-based alternative buffer replacing CdS. In this work, the CZTS/ZnCdS interface quality is optimized by applying ammonia as complexing agent in the precursor solution of the SILAR process for ZnCdS deposition. Excess oxide and hydroxide impurities formed at the interface are limited to a low level, which reduces the related electronic defects in the depletion region and alleviates the metastability behavior caused by the Zn(OH)2. The application of ammonia also enables the control of reaction mechanism, improving the epitaxial growth, thereby reducing the heterojunction interface recombination. Finally, efficiency of up to 10% is achieved by modifying the interface structure thanks to the chemistry method.

Kesterite Cu2ZnSnS4 (CZTS) solar cells have emerged as a promising candidate for scalable thin-film solar cells development, mainly due to its outstanding performance and compatibility with CuInGaSe2 and CdTe solar cells while only consisting of earth-abundant and non-toxic constituents. Here we present a method to address the relative low VOC of CZTS solar cells by applying atomic layer deposited Zn1−xSnxO (ZTO) as a Cd-free buffer layer. X-ray photoelectron spectroscopy measurement is implemented to study the band alignment at the CZTS/ ZTO hetero-interface. It shows that the favorable conduction band alignment of CZTS /ZTO resulted in increasing VOC by up to 10%. By optimizing the treatment of CZTS absorber before ZTO deposition, we finally obtained an efficiency of 9.3% which is the highest efficiency so far reported for Cd- free pure sulphide CZTS solar cells to the best of our knowledge.

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.

CuSbS2 thin films were in situ grown by reactive co-sputtering and the effects of the growth temperature on film composition, structure and morphology were investigated. It is demonstrated that orthorhombic chalcostibite CuSbS2 thin films with uniform morphology, pure phase and grain size over 2 µm can be obtained for growth temperature of 300 ℃, while higher growth temperature results in the formation of Cu3SbS4 phase and lower growth temperature leads to Sb2S3 secondary phase. The grown CuSbS2 film shows an optical absorption coefficient of higher than 104 cm−1, an optical band gap of 1.52 eV and p-type conductivity. Solar cell devices with configuration of glass/Mo/CuSbS2/CdS/i-ZnO/ITO/Ag were fabricated and yield power conversion efficiency of 0.52%. The incompatible interfaces including absorber/back contact (the absence of beneficial MoS2 interface layer) and absorber/buffer layer (unfavorable “cliff”-like conduction band offset) interfaces have been considered as the key factor limiting efficiency.

Kesterite Cu2ZnSnS4 (CZTS) photovoltaics have been comprehensively investigated in the past decades but are still hampered by a relatively large open circuit voltage (Voc) deficit, which is correlated to bulk defects in CZTS and interface recombination. Heterojunction interface management is of critical importance to tackle the interface recombination. In this work, we use atomic layer deposition (ALD) to synthesize a wide range of Zn1–xSnxO (ZTO, 0 ≤ x ≤ 1) films for application as a buffer layer in CZTS solar cells. A favorable band alignment is achieved using a 10 nm Zn0.77Sn0.23O buffer layer that enabled an impressive 10% increase in open circuit voltage of the CZTS solar cell. The microstructure and chemical nature of the CZTS/ZTO interface are carefully studied and the presence of an ultrathin Zn(S, O) tunnel layer is demonstrated. The decreased interfacial defects stemming from the minor lattice mismatch at the CZTS/Zn(S,O)/ZTO heterointerface in combination with the passivation provided by a higher sodium concentration throughout the CZTS/ZTO device explains the significant increase in open circuit voltage. Finally, we demonstrate a CZTS solar cell efficiency of 9.3%, which is the highest efficiency for Cd-free pure sulfide CZTS solar cell to date to the best of our knowledge.

In this work, a novel diode laser annealing was performed as a post-sulfurization heat treatment on pure-sulfide Cu2ZnSnS4 thin film solar cells fabricated by sputtering deposition. The effect on both film quality and device performance after annealing at various laser doses was investigated. After receiving an ultra-fast laser scan treatment, the crystallinity of the CZTS film can be improved and the Cu/Zn disordering is reduced. The power conversion efficiency of the laser annealed device was boosted to 7.33%, compared with the reference cell efficiency of 6.72%. The major increase in the short circuit current is driving force of such improvement, from 18 mA/cm2 to 19.31 mA/cm2 after laser treatment. These results indicate that the quality of CZTS thin films and the device performance can be effectively improved by the diode laser annealing.

In this research, we present thermal evaporated MoO3 layer as a primary back contact for pure sulphide Cu2ZnSnS4 solar cells. The effects of incorporating MoO3 in primary back contact and its thickness on the Cu2ZnSnS4 device performance are investigated. The MoO3 layer acts as enhancing sodium diffusion site where sodium is more effectively diffused into CZTS absorber, facilitating the formation of larger CZTS grains during sulfurization process compared to the reference absorber. The power conversion efficiency is improved from 5.2% to 6.2% on average, yielding the highest efficiency of 6.52% by the insertion of 10 nm MoO3 layer between Cu2ZnSnS4 absorber and molybdenum contact. The major efficiency enhancement arises from open circuit voltage which is improved from 530 to 615 mV.

In this work, we studied the effect of Cadmium alloying with CZTS on its optical and microstructural properties. Photothermal Deflection Spectroscopy (PDS) measurement revealed the properties of band tailing and sub-bandgap defect of the samples with different Cd concentration, which indicates the improvement in band tailing problem after Cd alloying with a reduced Urbach energy. The STEM-EDS observation suggests non-uniform elementary distribution in high Cd concentration samples which leads to a dramatically decrease in cell performance. Furthermore, the analysis of Z-contrast high-resolution HADDF STEM images provides direct evidence of Cd participating the CZTS lattice.

A hole-transporting layer is essential for effective operation of planar perovskite solar cells. Hole-transporting materials (HTMs) used so far are mainly based on organic semiconductors, which are usually expensive and relatively instable compared with inorganic materials. In this article, crystalline selenium (c-Se) prepared by thermal evaporation followed by annealing at low temperature was used as the hole-transporting layer for the planar perovskite solar cells in the device structure of fluorine-doped tin oxide (FTO)/SnO2/perovskite/selenium/Au. The device employing c-Se as HTM displayed enhanced long-term stability, keeping 80% of the original efficiency after 40 days in ambient environment. However, the power conversion efficiency of 8.3% remains low compared with the device based on spiroOMeTAD, which results from higher recombination in the c-Se device as determined by impedance spectroscopy. Thermal-evaporated selenium could be a low-cost and easy-fabrication HTM for efficient and stable perovskite solar cells after reducing the recombination in devices.

The ultrathin SnO2 intermediate layer deposited by a successive ionic layer adsorption and reaction (SILAR) method was introduced into the heterointerface between p-type Cu2ZnSnS4 (CZTS) absorber and n-type CdS buffer for interface defect passivation in kesterite thin film solar cells. CZTS solar cells with SnO2 intermediate layers show higher open circuit voltage (Voc) of 657 mV and fill factor (FF) of 62.8%, compared to its counterpart cells without the SnO2 intermediate layer, which have Voc of 638 mV and FF of 52.4%, resulting in improvement in the overall efficiency from 6.82% to 8.47%. The mitigation of the Voc deficit and the improvement of FF are believed to result from the integrated effects of CZTS/CdS heterointerface passivation, shunt blocking, and band alignments. The passivation effect is further affirmed by the improved carrier lifetime. Furthermore, external quantum efficiency profiles show a strengthened blue optical response which is contributed by the decrease of CdS thickness. This work provides a new insight into the CdS/CZTS interface passivation, shunt blocking, and optimization of band alignments, based on solution-processed SnO2.

In this work we report the Na incorporation from Na-doped Mo (Mo-Na) back contact for kesterite Cu2ZnSnS4 solar cells on flexible stainless steel substrates. It is demonstrated that Na can be effectively incorporated into CZTS by inserting Mo-Na layer at back contact. Direct contact of CZTS and Mo-Na layer leads to poor homogeneity and adhesion. The thickness of MoS2 formed at the back contact depends on the presence of Na and whether Mo contacts with CZTS directly. Back contact configuration with a Mo capping layer on Mo-Na layer is found to be helpful to maintain the advantages of Mo back contact and control the thickness of MoS2 interface. As a result, CZTS device fabricated on this configuration yields higher conversion efficiency of 6.2%. However, this efficiency is still far lower than that on traditional soda lime glass substrate which shows efficiency over 8%. The loss mechanism of device fabricated on stainless steel is investigated and analyzed according to the device performance and electrical parameters.

Cu2ZnSnS4 (CZTS) has been regarded as one of the most promising candidate for second generation solar cell and has attracted great attention recently. However, its current relative low efficiency limits its future development. In this contribution, partial Ag alloying method has been used to boost efficiency of CZTS solar cell. With Ag alloying, the microstructure has been improved and non-radiative recombination is suppressed. Consequently, the efficiency boost significantly from 7.8% to 8.8%.