Cs-treatments in Kesterite Thin-Film Solar Cells for Efficient Perovskite Tandems.

Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells are an attractive choice for a bottom cell of the low-cost and environmental tandem solar cells with perovskite. However, the progress in developing efficient perovskite/CZTSSe tandem solar cells has been hindered by the lack of high performance of the CZTSSe bottom cell. Here, an efficient CZTSSe bottom cell is demonstrated by adopting a facile and effective CsF treatment process. It is found that the CsF treatment not only facilitates grain growth and improves phase homogeneity by suppressing the detrimental deep-level defects and secondary phases, but also induces larger band bending and stronger drift force at the P-N junction. As a result, the carrier extraction/transport can be effectively accelerated, while reducing the interfacial recombination. These combined effects eventually result in a significant performance enhancement from 8.38% to 10.20%. The CsF-treated CZTSSe solar cell is finally applied to the mechanically-stacked perovskite/CZTSSe 4-terminal tandem cell by coupling a semi-transparent perovskite top cell, which exhibits the highest reported tandem efficiency of 23.01%.

Understanding the Space-Charge Layer in SnO2 for Enhanced Electron Extraction in Hybrid Perovskite Solar Cells.

Tin oxide (SnO2) has been widely used as an n-type metal oxide electron transport layer in perovskite solar cells (PSCs) owing to its superior electrical and optical properties and low-temperature synthesis process. In particular, the interfacial effect between indium tin oxide (ITO) and SnO2 is an important parameter that controls the charge transport properties and device performance of the PSCs. Therefore, understanding the interfacial effect of ITO/SnO2 and its role in PSCs is crucial, but it is not studied intensively. Herein, we investigated the space-charge effect at the interface of ITO/SnO2 using transfer length measurement and conductive atomic force microscopy as a function of SnO2 thickness. Moreover, optical, morphologic, and device measurements were performed to determine the optimal SnO2 thickness for PSCs. The space-charge effect was identified in ITO/SnO2 when the SnO2 layer was very thin due to electron depletion near the interface. Interestingly, a critical kink point was observed at approximately 10 nm SnO2 thickness, indicating the electron depletion and weak charge transfer behavior of the device. Thus, a thickness around 20 nm was favorable for the best PSC performance because charge transport behavior in the thin SnO2 layer was depressed by electron depletion. However, when the thickness of SnO2 exceeded 50 nm, the device performance deteriorated due to increased series resistance. This study provides a strategy to tune the electron transport layer and boost the charge transport behavior in PSCs, making important contributions to optimizing SnO2-based PSCs.

Photo-Effect on Ion Transport in Mixed Cation and Halide Perovskites and Implications for Photo-Demixing*.

Lead halide perovskites are considered to be most promising photovoltaic materials. Highest efficiency and improved stability of perovskite solar cells have been achieved by using cation and anion mixtures. Experimental information on electronic and ionic charge carriers is key to evaluate device performance, as well as processes of photo-decomposition and photo-demixing which are observed in these materials. Here, we measure ionic and electronic transport properties and investigate various cation and anion substitutions with a special eye on their photo-ionic effect, following our previous study on CH3 NH3 PbI3 , where we found that light enhances not only electronic but also ionic conductivities. We find that this phenomenon is very sensitive to the nature of the halide, while the cationic substitutions are less relevant. Based on the observation that the ionic conductivity enhancement found for iodide perovskites is significantly weakened by bromide substitution, we provide a chemical rationale for the photo-demixing in mixed halide compositions.

Sodium-assisted passivation of grain boundaries and defects in Cu2ZnSnSe4 thin films.

The long stagnation of the photo-conversion efficiency of kesterites below 13% is a source of frustration in the scientific community. In this study, we investigated the effects of sodium on the passivation of grain boundaries and defects in Cu2ZnSnSe4 (CZTSe) grown on a soda-lime glass (SLG) and borosilicate (BS) glass. Because BS glass does not inherently contain sodium, we placed a thin layer of NaF between CZTSe and Mo. The composition of the samples is Cu-poor and Zn-rich. The distribution of sodium and its contributions to phase formation and defects were examined by cross-sectional energy-dispersive X-ray profiling, Raman scattering spectroscopy and imaging, surface potential and photoluminescence. From the experimental results, it can be strongly claimed that sodium ions segregate predominantly near the grain boundaries and reduce CuZn-related defects. These local surface imaging analyses provided the exact locations of the secondary phases. In particular, the photo-assisted scanning probe method enabled us to observe the changes in the optoelectrical properties of the thin films and the carrier behavior within the materials. Further studies with distinct alkali ions and optimal processing conditions will pave a way to improve the performance of kesterite solar cells.

Large tunable photoeffect on ion conduction in halide perovskites and implications for photodecomposition.

In the same way as electron transport is crucial for information technology, ion transport is a key phenomenon in the context of energy research. To be able to tune ion conduction by light would open up opportunities for a wide realm of new applications, but it has been challenging to provide clear evidence for such an effect. Here we show through various techniques, such as transference-number measurements, permeation studies, stoichiometric variations, Hall effect experiments and the use of blocking electrodes, that light excitation enhances by several orders of magnitude the ionic conductivity of methylammonium lead iodide, the archetypal metal halide photovoltaic material. We provide a rationale for this unexpected phenomenon and show that it straightforwardly leads to a hitherto unconsidered photodecomposition path of the perovskite.

Trapping charges at grain boundaries and degradation of CH3NH3Pb(I1-x Br x )3 perovskite solar cells.

The electrical properties of CH3NH3Pb(I1-x Br x )3 (x = 0.13) perovskite materials were investigated under ambient conditions. The local work function and the local current were measured using Kelvin probe force microscopy and conductive atomic force microscopy, respectively. The degradation of the perovskite layers depends on their grain size. As the material degrades, an additional peak in the surface potential appears simultaneously with a sudden increase and subsequent relaxation of the local current. The potential bending at the grain boundaries and the intragrains is the most likely reason for the change of the local current surface of the perovskite layers. The improved understanding of the degradation mechanism garnered from this study helps pave the way toward an improved photo-conversion efficiency in perovskite solar cells.

The Nature of Ion Conduction in Methylammonium Lead Iodide: A Multimethod Approach.

By applying a multitude of experimental techniques including 1 H, 14 N, 207 Pb NMR and 127 I NMR/NQR, tracer diffusion, reaction cell and doping experiments, as well as stoichiometric variation, conductivity, and polarization experiments, iodine ions are unambiguously shown to be the mobile species in CH3 NH3 PbI3 , with iodine vacancies shown to represent the mechanistic centers under equilibrium conditions. Pb2+ and CH3 NH3+ ions do not significantly contribute to the long range transport (upper limits for their contributions are given), whereby the latter exhibit substantial local motion. The decisive electronic contribution to the mixed conductivity in the experimental window stems from electron holes. As holes can be associated with iodine orbitals, local variations of the iodine stoichiometry may be fast and enable light effects on ion transport.

Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic-Organic Lead Halide Solar Cells.

We fabricated a mesoporous perovskite solar cell with a ∼14% conversion efficiency, and we investigated its beneficial grain boundary properties of the perovskite solar cells through the use of scanning probe microscopy. The CH3NH3Pb(I0.88,Br0.12)3 showed a significant potential barrier bending at the grain boundary and induced passivation. The potential difference value in the x = 0.00 sample is ∼50 mV, and the distribution of the positive potential is lower than that of the x = 0.12 sample. We also investigated the polarization and hysteretic properties of the perovskite thin films by measuring the local piezoresponse. Specifically, the charged grain boundaries play a beneficial role in electron-hole depairing and in suppressing recombination in order to realize high-efficiency perovskite solar cells.

Nanostructured p-type CZTS thin films prepared by a facile solution process for 3D p-n junction solar cells.

Nanoporous p-type semiconductor thin films prepared by a simple solution-based process with appropriate thermal treatment and three-dimensional (3D) p-n junction solar cells fabricated by depositing n-type semiconductor layers onto the nanoporous p-type thin films show considerable photovoltaic performance compared with conventional thin film p-n junction solar cells. Spin-coated p-type Cu2ZnSnS4 (CZTS) thin films prepared using metal chlorides and thiourea show unique nanoporous thin film morphology, which is composed of a cluster of CZTS nanograins of 50-500 nm, and the obvious 3D p-n junction structure is fabricated by the deposition of n-type CdS on the nanoporous CZTS thin films by chemical bath deposition. The photovoltaic properties of 3D p-n junction CZTS solar cells are predominantly affected by the scale of CZTS nanograins, which is easily controlled by the sulfurization temperature of CZTS precursor films. The scale of CZTS nanograins determines the minority carrier transportation within the 3D p-n junction between CZTS and CdS, which are closely related with the photocurrent of series resistance of 3D p-n junction solar cells. 3D p-n junction CZTS solar cells with nanograins below 100 nm show power conversion efficiency of 5.02%, which is comparable with conventional CZTS thin film solar cells.

Nanoscale observation of surface potential and carrier transport in Cu2ZnSn(S,Se)4 thin films grown by sputtering-based two-step process.

Stacked precursors of Cu-Zn-Sn-S were grown by radio frequency sputtering and annealed in a furnace with Se metals to form thin-film solar cell materials of Cu2ZnSn(S,Se)4 (CZTSSe). The samples have different absorber layer thickness of 1 to 2 µm and show conversion efficiencies up to 8.06%. Conductive atomic force microscopy and Kelvin probe force microscopy were used to explore the local electrical properties of the surface of CZTSSe thin films. The high-efficiency CZTSSe thin film exhibits significantly positive bending of surface potential around the grain boundaries. Dominant current paths along the grain boundaries are also observed. The surface electrical parameters of potential and current lead to potential solar cell applications using CZTSSe thin films, which may be an alternative choice of Cu(In,Ga)Se2.PACS number: 08.37.-d; 61.72.Mm; 71.35.-y.