Architecture Optimization Dramatically Improves Reverse Bias Stability in Perovskite Solar Cells: A Role of Polymer Hole Transport Layers

We report that device architecture engineering has a substantial impact on the reverse bias instability that has been reported as a critical issue in commercializing perovskite solar cells. We demonstrate breakdown voltages exceeding -15 V in typical pin structured perovskite solar cells via two steps: i) using polymer hole transporting materials; ii) using a more electrochemically stable gold electrode. While device degradation can be exacerbated by higher reverse bias and prolonged exposure, our as-fabricated perovskite solar cells completely recover their performance even after stressing at -7 V for 9 hours both in the dark and under partial illumination. Following these observations, we systematically discuss and compare the reverse bias driven degradation pathways in perovskite solar cells with different device architectures. Our model highlights the role of electrochemical reaction rates and species in dictating the reverse bias stability of perovskite solar cells

Impact on Structural and Optical Properties of CZTS Thin Films with Solvents and Ge Incorporation

This report demonstrates nontoxic colloidal nitrate route CZTS (Cu2ZnSnS4) synthesis at room temperature, along with their band grading due to incorporation of Ge as cost of Sn (3%). The parent CZTS, CZGTS (Cu2ZnGeSnS4), and their polyvinyl alcohol (PVA), dimethyl sulfoxide (DMSO) solvent containing solutions doctor blade-coated thin film structural and optical properties are discussed. Their sulfurized thin films thickness have been achieved ±2 μ. It has noticed that addition of Ge in CZTS alloy affects the grain sizes, crystallographic structure, Raman spectral peak shift toward the higher wave number side. The addition of PVA and DMSO is also substantially contributed in their physical property modification by demonstrating the gradual improvement in grain sizes and compactness. Moreover, the gradual changes have also appeared in their X-ray diffractometer (XRD) and Raman spectroscopic results. The optical energy band gaps of the CZTS, CZGTS, and their PVA, DMSO mixed alloyed thin films are obtained in between 1.27 eV to 1.57 eV and 1.58 eV to 1.83 eV

Limiting effects of conduction band offset and defect states on high efficiency CZTSSe solar cell

We investigated limitation factors of high efficiency Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, where the CZTSSe absorbers were made by using sulfo-selenization process. CZTSSe absorbers with two S/(S+Se) ratios, ~ 0.12 (Se-rich) and ~ 0.22 (S-increased), were prepared by varying the sulfo-selenization temperature. The Se-rich CZTSSe solar cells were found to have larger conduction band offset (CBO) between the absorber and the buffer, which was reflected in the kinked J-V curves at low temperatures. Considering that the larger CBO prevents electron transport from absorber to buffer and resultantly reduces short circuit current and fill factor, it could be possible limitation factor of the high efficiency solar cell. Contrary to Se-rich solar cells, S-increased solar cells showed reduced CBO and no kinked J-V curve. However, deep defects were found to be generated, which induced defect centers of charge recombination both at interface and in bulk of the absorber. The larger CBO in Se-rich CZTSSe solar cell and deep defects in S-increased CZTSSe solar cell are observed even in ~ 12% efficiency solar cells. Thus, we believe that these possible limitation factors should be resolved to achieve high efficiency kesterite CZTSSe solar cell above 12%. © 2017 Elsevier Ltd1

Silver Nanowires Binding with Sputtered ZnO to Fabricate Highly Conductive and Thermally Stable Transparent Electrode for Solar Cell Applications

Silver nanowire (AgNW) film has been demonstrated as excellent and low cost transparent electrode in organic solar cells as an alternative to replace scarce and expensive indium tin oxide (ITO). However, the low contact area and weak adhesion with low-lying surface as well as junction resistance between nanowires have limited the applications of AgNW film to thin film solar cells. To resolve this problem, we fabricated AgNW film as transparent conductive electrode (TCE) by binding with a thin layer of sputtered ZnO (40 nm) which not only increased contact area with low-lying surface in thin film solar cell but also improved conductivity by connecting AgNWs at the junction. The TCE thus fabricated exhibited transparency and sheet resistance of 92% and 20Ω/□, respectively. Conductive atomic force microscopy (C-AFM) study revealed the enhancement of current collection vertically and laterally through AgNWs after coating with ZnO thin film. The CuInGaSe₂ solar cell with TCE of our AgNW­(ZnO) demonstrated the maximum power conversion efficiency of 13.5% with improved parameters in comparison to solar cell fabricated with conventional ITO as TCE