Sonia R. Raga

Analysis of the Origin of Open Circuit Voltage in Dye Solar Cells

Changes in the composition of the electrolyte are known to affect the parameters that determine the performance of dye solar cells. This paper describes a robust method for the analysis of the photovoltage in dye solar cells. The method focuses on the study of recombination resistance and chemical capacitance of TiO2 obtained from impedance spectroscopy. Four dye solar cells with electrolytes producing known effects on photovoltage behavior have been studied. Effects of conduction band shifts and changes in recombination rate in the photovoltage have been evaluated quantitatively.

High performance perovskite solar cells by hybrid chemical vapor deposition

Organometal halide based perovskites are promising materials for solar cell applications and are rapidly developing with current devices reaching ∼19% efficiency. In this work we introduce a new method of perovskite synthesis by hybrid chemical vapor deposition (HCVD), and demonstrate efficiencies as high as 11.8%. These cells were found to be stable with time, and retained almost the same efficiency after approximately 1100 h storage in dry N2 gas. This method is particularly attractive because of its ability to scale up to industrial levels and the ability to precisely control gas flow rate, temperature, and pressure with high reproducibility. This is the first demonstration of a perovskite solar cell using chemical vapor deposition and there is likely still room for significant optimization in efficiency.

Fabrication of semi-transparent perovskite films with centimeter-scale superior uniformity by the hybrid deposition method

We report the development of instrumentation and methodology for fabricating large area semi-transparent organo-lead-halide perovskite films. In our method, the growth of perovskite films relies on the control of CH3NH3I flow and vapor pressure inside a vacuum chamber. Solar cell devices based on the prepared semi-transparent perovskite films as thin as ∼135 nm achieved an efficiency of 9.9% and a high open circuit voltage of 1.09 V.

Thermal degradation of CH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermogravimetry–mass spectrometry analysis

Thermal gravimetric and differential thermal analysis (TG-DTA) coupled with quadrupole mass spectrometry (MS) and first principles calculations were employed to elucidate the chemical nature of released gases during the thermal decomposition of CH3NH3PbI3. In contrast to the common wisdom that CH3NH3PbI3 is decomposed into CH3NH2 and HI, the major gases were methyliodide (CH3I) and ammonia (NH3). We anticipate that our findings will provide new insights into further formulations of the perovskite active material and device design that can prevent methylammonium decomposition and thus increase the long-term stability of perovskite-based optoelectronic devices.

Design and characterization of alkoxy-wrapped push-pull porphyrins for dye-sensitized solar cells

Three alkoxy-wrapped push-pull porphyrins were designed and synthesized for dye-sensitized solar cell (DSSC) applications. Spectral, electrochemical, photovoltaic and electrochemical impedance spectroscopy properties of these porphyrin sensitizers were well investigated to provide evidence for the molecular design.

Large formamidinium lead trihalide perovskite solar cells using chemical vapor deposition with high reproducibility and tunable chlorine concentrations

Chemical vapor deposition is an inexpensive way to batch-process solar cells with good uniformity and facilitates low-cost production. Formamidinium lead iodide perovskite has a smaller energy band gap and greater potential efficiency than the widely studied methylammonium lead iodide perovskite and better temperature stability. This work is the first demonstration of vapor deposition of formamidinium-based perovskite. A self-limiting perovskite formation process is recommended, with efficiencies as high as 14.2% and stability up to 155 days after fabrication. Using this process, a batch of semi-transparent solar cells with a large area of 1 cm2 was fabricated. We monitored the growth of perovskite in real time and provide insight that may not be accessible for a solution based process. We directly measured chlorine in perovskite films and correlated the concentration of chlorine with efficiency and stability.

Temperature-dependent hysteresis effects in perovskite-based solar cells

Staircase voltage sweep measurements were performed on a perovskite solar cell at 250 K, 300 K, and 360 K. Time-dependent photocurrent data reveal the complexity of the signal that cannot be described by a simple mono-exponential function, suggesting that multiple charging–discharging processes are responsible for the complex hysteresis behavior.

Pinhole-free hole transport layers significantly improve the stability of MAPbI3-based perovskite solar cells under operating conditions

Pinhole-free 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) hole transport layers (HTLs) were deposited on perovskite films. MAPbI3-based perovskite solar cells employing the pinhole-free HTL showed a prolonged lifetime under one sun and was operated at the maximum power point. The solar cell architecture (planar versus mesoporous-layers) was also observed to strongly influence the cells stability.

Substantial improvement of perovskite solar cells stability by pinhole-free hole transport layer with doping engineering

We fabricated perovskite solar cells using a triple-layer of n-type doped, intrinsic, and p-type doped 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) (n-i-p) as hole transport layer (HTL) by vacuum evaporation. The doping concentration for n-type doped spiro-OMeTAD was optimized to adjust the highest occupied molecular orbital of spiro-OMeTAD to match the valence band maximum of perovskite for efficient hole extraction while maintaining a high open circuit voltage. Time-dependent solar cell performance measurements revealed significantly improved air stability for perovskite solar cells with the n-i-p structured spiro-OMeTAD HTL showing sustained efficiencies even after 840 h of air exposure.

Smooth perovskite thin films and efficient perovskite solar cells prepared by the hybrid deposition method

We provide details on the development of instrumentation and methodology to overcome the common difficulties that the vacuum-related techniques face for fabrication of perovskite thin films and perovskite solar cells (PSCs). Our methodology relies on precisely controlling the flow of methylammonium iodide (CH3NH3I, MAI), which has a high-vapor pressure nature, and the deposition rate of metal halides (PbCl2 or PbI2). This hybrid deposition method allows the growth of perovskite films with smooth surface, good crystallinity, high surface coverage, uniform chemical composition and semi-transparency. We also systematically investigated the effects of the evaporation source materials (PbCl2 : MAI versus PbI2 : MAI), substrate temperatures, and post-annealing on the properties of perovskite films, as well as device performances based on this method. By employing a thin perovskite film (<200 nm), the power conversion efficiency of PSC can be as high as 11.5%.