Flexible photovoltaics have been and will be increasingly demanded in various applications in todays and future society. The search for an ideal flexible photovoltaic technology that can perfectly meet these expanding demands has long been an active branch of photovoltaic research. Monograin layer technology (MGL) is one possible path to develop the lightweight, flexible, and semi-transparent solar cells. The major innovations in the MGL solar cell technology are the light absorbing layer made of high quality micro-crystalline semiconductor powder enabling theoretically higher efficiencies and the low cost and easily up-scalable roll-to-roll PV module production process. In recent years, the main research focus of the monograin technology has been on the understanding of the synthesis and optoelectronic properties of Cu2ZnSnS4 absorber materials. The highest power conversion efficiency of this type of devices is 12.06% with output parameters as follows: VOC =0.745 V, JSC=28.36 mA/cm2 and FF = 57.10 %. In this study temperature dependence (T = 20- 320K) of current-voltage (J-V) characteristics of record efficiency Cu2ZnSnS4 MGL solar cell were investigated to clarify the main losses in CZTS, which are still not fully understood. The light J−V curve analysis was used to evaluate the quality of the p−n junction and losses related to resistive components of the device. In this study, the single exponential diode equation was employed to analyze the light J−V data. It turned out that at lower temperatures (T < 180 K) a second blocking diode appears and it is related to back contact barrier. We believe that this back contact barrier has an effect also at higher temperatures causing relative low values of FF. At T=300 K the diode ideality factor n has a value 2.58 and the series resistance Rs =2.6 Ωcm2.
We show that trapping impacts charge carrier dynamics in undoped CdSeTe, undoped CdSeTe/CdTe bilayers, and in As-doped CdSeTe/CdTe solar cells. Trapping has much smaller influence in CdTe-only (no Se, no As) films. Electrostatic potential fluctuation model (amplitude γ > 30 meV) applies to As-doped CdSeTe/CdTe, and defect model (defect activation energy Ea = 0.2 eV) applies to undoped CdSeTe. Unusually, undoped CdSeTe with trap states can have high radiative efficiencies and TRPL lifetimes (10-15 μs). As a result, thermodynamic CdTe solar cell analysis (radiative voltage, implied voltage, external radiative efficiency metrics) needs to be used with caution. As a metric for trap states, PL emission spectroscopy and charge carrier lifetimes at low temperatures provide distinct trap signatures. Trapping impact on devices can be evaluated in modeling and assessed in measurements outlined in this paper, perhaps providing a verifiable hypothesis when we search how to overcome voltage bottleneck in CdTe solar cells.
The high cell efficiency of ~ 22% and cost-effective commercially available modules make thin-film polycrystalline CdTe-based solar cells promising alternatives to conventional energy sources. The photoconversion efficiency of CdTe-based photovoltaics can be further improved by Se doping in the absorber layer. The superior performance of CdSe1-xTex (CdSeTe)-based solar cells is attributed to a lower band gap of the CdSeTe layer in the absorber, which leads to increased carrier lifetime, and simultaneous passivation of grain interiors and grain boundaries. Se doping in the absorber layer also suppresses non-radiative recombination due to the reduction of mid-gap defect states. Despite conclusive evidence of enhanced device performance upon Se doping in the absorber layer, direct atomic-scale observations of changes in the bulk, defect, and electronic structures for CdSeTe are still lacking. Moreover, the diffusion of Se through the absorber layer, especially at low temperatures, is not well understood. Here, we have used atomic-resolution scanning transmission electron microscopy and first-principles density functional theory calculations to understand the role of Se doping on the atomic and electronic structure of CdSeTe-based photovoltaic devices. We observe that the smaller grains (< 1μm) in the CdSeTe layer are accompanied by an increased density of dislocations and dislocation cores. We find that the Se concentration profile changes across the absorber layer upon aging for 100' of hours at operational temperatures. We show that the presence of planar defects in the absorber layer leads to heterogeneities in the chemical distribution of Se. Overall, our findings provide atomic-scale insights into the role of Se doping, which can be further leveraged to improve the photovoltaic efficiency of CdSeTe-based solar cells.
Estimating glare is becoming an increasingly important step in the planning stage of PV installations. This is especially true for building-integrated photovoltaics (BIPV), where atypical orientations and tilt angles can cause reflections in unusual locations. Currently, simulation tools perform simplified ray-tracing calculations to determine specular reflections from PV installations in their environment, often based on reflectance fit functions. In this work, gonioreflectometric measurements are used to determine the bi-directional reflectance distribution function (BRDF) of PV mini-modules with different surface glasses. They show significant differences between smooth and structured glass surfaces, with satinated glass showing overall lowest reflectance. At medium to high incidence angles, however, reflections are not only higher than at normal incidence, but may also peak at different view angles than the specular reflection angles. This indicates, that the common methodology of simulating only specular reflections may underestimate glare. It furthermore suggests, that a low number of reflectance fit functions may be insufficient to describe possible glare from structured surfaces, as they can exhibit vastly different BRDFs, despite possibly similar specular reflectance fit functions. In addition, calculations based on retinal irradiance threshold values for eye damage show that even for PV modules with flat glass, temporary retinal damage in form of flash blindness is highly unlikely. In any case, retinal irradiance values are far below values required for retinal burn damage, limiting reflections from PV installations to disability or discomfort glare levels. Even for the lowest reflecting surfaces, i.e. satinated glass, reflectances at incidence angles beyond 50° are high enough to possibly cause uncomfortable reflections, requiring ray-tracing simulations for accurate estimations of glare.
PV modules of the same make and model are often assembled with different bills of materials (BOMs). We describe two case studies of utility-scale silicon PV systems in which these differing BOMs were associated with faster-than-expected degradation. In one of the sites we found that different metallization paste had been used for grid lines in some cells leading to loss of contact to the cell and severe series resistance degradation. In another site, we found that two different types of cell had been used, one of which suffered from light and elevated temperature induced degradation (LeTID). Our results from both sites underscore that variations in BOM, even among modules of the same make and model can lead to reliability challenges.
