Interrelation of the CdTe Grain Size, Postgrowth Processing, and Window Layer Selection on Solar Cell Performance.

Recent improvements to the CdTe solar cell device structure have focused on replacing the CdS window layer with a more transparent material to reduce parasitic absorption and increase Jsc, as well as incorporating selenium into the absorber layer to achieve a graded band gap. However, altering the CdTe device structure is nontrivial due to the interdependent nature of device processing steps. The choice of the window layer influences the grain structure of the CdTe layer, which in turn can affect the chloride treatment, which itself may contribute to intermixing between the window and absorber layers. This work studies three different device architectures in parallel, allowing for an in-depth comparison of processing conditions for CdTe solar cells grown on CdS, SnO2, and CdSe. Direct replacement of the CdS window layer with a wider band gap SnO2 layer is hindered by poor growth of the absorber, producing highly strained CdTe films and a weak junction. This is alleviated by inserting a CdSe layer between the SnO2 and CdTe, which improves the growth of CdTe and results in a graded CdSexTe1-x absorber layer. For each substrate, the CdTe deposition rate and postgrowth chloride treatment are systematically varied, highlighting the distinct processing requirements of each device structure

Impedance spectroscopy of Sb₂Se₃ photovoltaics consisting of (Sb₄Se₆)_<i>n</i> nanoribbons under light illumination.

Sb2Se3, consisting of one-dimensional (Sb4Se6)n nanoribbons has drawn attention as an intriguing light absorber from the photovoltaics (PVs) research community. However, further research is required on the performance-limiting factors in Sb2Se3 PVs. In this study, we investigated the charge carrier behavior in Sb2Se3 PVs by impedance spectroscopy (IS) under light illumination. (Sb4Se6)n nanoribbons with two different orientations were used to investigate the effect of crystal orientation on the device performance. Regardless of the (Sb4Se6)n orientation, negative capacitance was observed at forward bias, representing a recombination pathway at the TiO2/Sb2Se3 interface. A comparison of the recombination resistances and lifetimes of two different Sb2Se3 PVs showed that a better interface could be formed by placing the (Sb4Se6)n ribbons parallel to the TiO2 layer. Based on these observations, an ideal structure of the Sb2Se3/TiO2 interface is proposed, which will enhance the performance of Sb2Se3 PVs toward its theoretical limit

Analysis of charge trapping and long lived hole generation in SrTiO₃ photoanodes

Long lived hole generation in SrTiO3 is observed herein using transient absorption spectroscopy, even in the absence of applied bias to drive charge separation.</jats:p

Understanding the Role of Organic Hole Transport Layers on Pinhole Blocking and Performance Improvement in Sb₂Se₃ Solar Cells

AbstractSb2Se3 is an emerging semiconductor which has shown promise for low‐cost photovoltaic applications. After successive record‐efficiencies using a range of device structures, spiro‐OMeTAD has emerged as the default hole transport material (HTM), however, the function of HTM layers remains poorly understood. Here, thin‐film Sb2Se3 solar cells are fabricated with which three organic HTM layers ‐ namely P3HT, PCDTBT, and spiro‐OMeTAD are investigated. By comparing these against one another, and to a reference device, their role in the device stack are clarified. These organic HTM layers are found to serve a dual purpose, increasing both the average and peak efficiency by simultaneously blocking pinholes and improving the band alignment at the back contact, with marginal differences in performance between the different HTMs. This produced a champion device of 7.44% using P3HT, resulting from an improvement in all performance parameters. A more complex processing route, run‐to‐run variability, and lower overall device performance compared to the other organics challenge the assumption that spiro‐OMeTAD is the optimal HTM for Sb2Se3 devices. A Schottky barrier at the Au‐Sb2Se3 contact despite the deep work function of gold implies Fermi level pinning due to a defective interface, which each of the organic HTMs are equally capable of alleviating.</jats:p

Impedance spectroscopy of Sb2_2Se3_3 photovoltaics consisting of (Sb4_4Se6_6)n_n nanoribbons under light illumination

Sb$_2$Se$_3$, consisting of one-dimensional (Sb$_4$Se$_6$)$_n$ nanoribbons has drawn attention as an intriguing light absorber from the photovoltaics (PVs) research community. However, further research is required on the performance-limiting factors in Sb$_2$Se$_3$ PVs. In this study, we investigated the charge carrier behavior in Sb$_2$Se$_3$ PVs by impedance spectroscopy (IS) under light illumination. (Sb$_4$Se$_6$)$_n$ nanoribbons with two different orientations were used to investigate the effect of crystal orientation on the device performance. Regardless of the (Sb$_4$Se$_6$)$_n$ orientation, negative capacitance was observed at forward bias, representing a recombination pathway at the TiO$_2$/Sb$_2$Se$_3$ interface. A comparison of the recombination resistances and lifetimes of two different Sb$_2$Se$_3$ PVs showed that a better interface could be formed by placing the (Sb$_4$Se$_6$)$_n$ ribbons parallel to the TiO$_2$ layer. Based on these observations, an ideal structure of the Sb$_2$Se$_3$/TiO$_2$ interface is proposed, which will enhance the performance of Sb$_2$Se$_3$ PVs toward its theoretical limit

Incorporation of CdSe layers into CdTe thin film solar cells

Incorporation of CdSe layers into CdTe thin film solar cells has recently emerged as a route to improve cell performance. It has been suggested that the formation of lower band gap CdTe(1-x)Se(x) phases following Se diffusion induces bandgap grading which may increase the carrier lifetime and thereby open circuit voltage. In this study we investigate the impact of CdSe incorporation on CdTe solar cell performance. We demonstrate that the standard CdS/CdTe device architecture is incompatible with Se incorporation, owing to large optical losses. An alternative cell structure with an oxide partner layer replacing the CdS with SnO2/CdSe/CdTe is developed, leading to cell efficiencies of > 13.5%. The differences in processing required for effective selenium incorporation are investigated with performance improvements resulting from additional post-growth annealing. Finally, other oxides such as TiO2, ZnO and FTO are demonstrated to be unsuitable partner layers but highlight that the choice of partner layer is key to further improving the performance

Reactive DC Sputtered TiO 2 Electron Transport Layers for Cadmium‐Free Sb 2 Se 3 Solar Cells

The evolution of Sb2Se3 heterojunction devices away from CdS electron transport layers (ETL) to wide bandgap metal oxide alternatives is a critical target in the development of this emerging photovoltaic material. Metal oxide ETL/Sb2Se3 device performance has historically been limited by relatively low fill factors, despite offering clear advantages with regards to photocurrent collection. In this study, TiO2 ETLs are fabricated via direct current reactive sputtering and tested in complete Sb2Se3 devices. A strong correlation between TiO2 ETL processing conditions and the Sb2Se3 solar cell device response under forward bias conditions is observed and optimized. Numerical device models support experimental evidence of a spike‐like conduction band offset, which can be mediated, provided a sufficiently high conductivity and low interfacial defect density can be achieved in the TiO2 ETL. Ultimately, a SnO2:F/TiO2/Sb2Se3/P3HT/Au device with the reactively sputtered TiO2 ETL delivers an 8.12% power conversion efficiency (η), the highest TiO2/Sb2Se3 device reported to‐date. This is achieved by a substantial reduction in series resistance, driven by improved crystallinity of the reactively sputtered anatase‐TiO2 ETL, whilst maintaining almost maximum current collection for this device architecture

Reactive DC Sputtered TiO₂ Electron Transport Layers for Cadmium‐Free Sb₂Se₃ Solar Cells

AbstractThe evolution of Sb2Se3 heterojunction devices away from CdS electron transport layers (ETL) to wide bandgap metal oxide alternatives is a critical target in the development of this emerging photovoltaic material. Metal oxide ETL/Sb2Se3 device performance has historically been limited by relatively low fill factors, despite offering clear advantages with regards to photocurrent collection. In this study, TiO2 ETLs are fabricated via direct current reactive sputtering and tested in complete Sb2Se3 devices. A strong correlation between TiO2 ETL processing conditions and the Sb2Se3 solar cell device response under forward bias conditions is observed and optimized. Numerical device models support experimental evidence of a spike‐like conduction band offset, which can be mediated, provided a sufficiently high conductivity and low interfacial defect density can be achieved in the TiO2 ETL. Ultimately, a SnO2:F/TiO2/Sb2Se3/P3HT/Au device with the reactively sputtered TiO2 ETL delivers an 8.12% power conversion efficiency (η), the highest TiO2/Sb2Se3 device reported to‐date. This is achieved by a substantial reduction in series resistance, driven by improved crystallinity of the reactively sputtered anatase‐TiO2 ETL, whilst maintaining almost maximum current collection for this device architecture.</jats:p