Polycrystalline Photovoltaic Devices Research Articles

MXene-Enhanced Nanoscale Photoconduction in Perovskite Solar Cells Revealed by Conductive Atomic Force Microscopy.

The use of MXene materials in perovskite solar cells (PSCs) has received significant interest due to their distinct features that result from the termination of functional groups and the oxidation of MXene. Herein, we have used photoconductive atomic force microscopy (pcAFM) to map the local (nanoscale) photovoltaic performances of the Ti3C2Tx MXene nanosheet-integrated TiO2 (MXene@TiO2) electron transport layer-based PSCs to determine the influence of the treatment on the microscopic charge flow inside the devices. At different applied voltages, the morphology and current have been simultaneously measured with nanoscale resolution from the top surfaces of the solar cells without back contacts. The PSCs based on MXene@TiO2 exhibit more enhanced current flow across the grains than the only TiO2-based PSCs. At zero applied bias, the average local photocurrent for MXene-integrated PSCs is several times higher than the reference PSCs and decreases gradually when the positive bias is increased until the open circuit voltage. Considerable differences were also observed in the short circuit current among different locations that appear identical in AFM topography. Our findings reveal the potential of MXene-integrated ETLs to enhance the nanoscale photoconduction and inherent characteristics of the active layers, thereby improving the performance of the polycrystalline photovoltaic devices.

Light-induced current mapping in oxide based solar cells with nanoscale resolution

Transport properties of photo-induced charge carriers through different grains in the polycrystalline photovoltaic devices strongly depend on the microstructural pattern of the active layers. Therefore, photocurrent mapping with nanoscale resolution is important to know about the electrical responses of the different grains in the polycrystalline photovoltaic devices. Here, we have used photoconductive atomic force microscopy for mapping the photocurrent with nanoscale resolution of two types of ZnO nanorods/Cu2O based solar cells. The morphology and current have been measured simultaneously with nanoscale resolution from the top surfaces of the devices at different applied voltages. It is demonstrated that the nanostructure of the active layers is one of the most important variables determining device performances. Different local photovoltaic performances have been observed from these two devices due to various microstructural and electrical phenomena of their seed layers. On the other hand, significant variations in short-circuit current have been observed from different grains of the devices which appeared more alike in the micrograph owing to various transport properties of photocarriers. It is observed that the grain boundaries are more preferable for charge collection over the grain interiors. It shows a higher short circuit current close to the boundary than the grain inside. This study illustrates an important area for future fundamental research to enhance the performances of the polycrystalline photovoltaic devices through better control of morphology and improving the inherent properties of the active layers.

Nanoscale imaging and spectroscopy of band gap and defects in polycrystalline photovoltaic devices.

Improving the power conversion efficiency of photovoltaic (PV) devices is challenging because the generation, separation and collection of electron-hole pairs are strongly dependent on details of the nanoscale chemical composition and defects which are often poorly known. In this work, two novel scanning probe nano-spectroscopy techniques, direct-transmission near-field scanning optical microscopy (dt-NSOM) and photothermal induced resonance (PTIR), are implemented to probe the distribution of defects and the bandgap variation in thin lamellae extracted from polycrystalline CdTe PV devices. dt-NSOM provides high-contrast spatially-resolved maps of light transmitted through the sample at selected wavelengths. PTIR provides absorption maps and spectra over a broad spectral range, from visible to mid-infrared. Results show variation of the bandgap through the CdTe thickness and from grain to grain that is spatially uncorrelated with the distributions of shallow and deep defects.

Characterization of Poly-Crystalline CdTe Solar Cells Using Aberration-Corrected Transmission Electron Microscope

Single CdS/CdTe hetero-junction poly-crystalline photo-voltaic devices are present leaders in thin film solar technology. Commercial success of CdTe-based devices stems from a combination of a nearly ideal direct band gap of the absorber material (~1.5eV), as well as established manufacturing methods and low cost of the modules, which yield fastest return of investment among currently available technologies. Despite substantial progress in the device engineering and fabrication, the conversion efficiency has seen only minor improvements over the last ~20 years with the most recent laboratory record held at ~19.6%. Going beyond 20% efficiency will require breakthrough advances in increasing open circuit voltage, Voc, and minority carrier lifetime. Grain boundaries and intra-grain defects are likely to provide carrier recombination centers but also attract dopants and their influence on a CdTe device performance still needs to be elucidated.

Hall effect measurements on Bridgman-grownCuInSe2 with sodium

The presence of sodium, either in the substrate or as a co-evaporant during absorber deposition,has been shown to improve the performance of polycrystalline photovoltaic devices made withCu(In, Ga)Se2, as well asthe ternary CuInSe2. Investigations have shown Na or Na compounds deposited on the grainboundaries, but none have been found within intact crystal grains, leading tosuggestions that grain boundaries may play a role in the improved performance ofthe cells. Therefore, in this study, ingots containing large monocrystals ofCuInSe2 have been grown, using a vertical-Bridgman method, from melts that also include a varyingquantity of sodium. In order to simulate the conditions under which cells are constructed, aproportion of Se above stoichiometry has been added to some of the melts. Resistivity andHall effect measurements were then performed on the material after growth. The resultsshow no large change in either resistivity or majority hole concentration in either set ofsamples, although a slight decrease in the latter value was apparent in the excess Sesamples with 0.2 and 0.3 at.% Na additions. No clear trend in hole mobility could bediscerned, although an increase was seen with 0.2 at.% Na addition for both samples.

Microstructural properties of Cu(In,Ga)Se 2 thin films used in high-efficiency devices

Thin-film polycrystalline photovoltaic devices based on Cu(In,Ga)Se 2 have a demonstrated efficiency approaching 19%. The best performance was achieved when the Ga/In+Ga ratio was in the 25–30% range. The short-circuit current density exhibited for the device containing CdS was almost at its expected maximum. The open-circuit voltage was relatively low considering the optical bandgap ( Eg) of the above absorber (∼1.15 eV); at best, it is 0.6× Eg. In this work, we examined the microstructural properties, e.g. defects due to misorientation, micro-twinning, stacking faults, and dislocations, for films prepared by our ‘ three-stage’ process, including the CIGS and Mo back-contact. We also attempted to make a correlation between the above observations and device performance.