The challenges of making high performance, low temperature processed, p-type transparent conductors (TCs) have been the main bottleneck for the development of flexible transparent 2 electronics. Though a few p-type transparent conducting oxides (TCOs) have shown promising results, they need high processing temperature to achieve the required conductivity which makes them unsuitable for organic and flexible electronic applications. Copper iodide is a wide band gap p-type semiconductor that can be heavily doped at low temperature (<100 o C) to achieve conductivity comparable or higher than many of the well-established p-type TCOs. However, as processed CuI loses its transparency and conductivity with time in an ambient condition which makes them unsuitable for long term applications. Herein, we propose CuI-TiO2 composite thin films as a replacement of pure CuI. We show that the introduction of TiO2 in CuI makes it more stable in ambient condition while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO2 composite thin films have been performed to understand the reasons for improved conductivity, transparency and stability of CuI-TiO2 samples in comparison to pure CuI samples. The enhanced conductivity in CuI-TiO2 stems from the spacecharge layer formation at the CuI/TiO2 interface, while the improved transparency is due to reduced CuI grain growth mobility in the presence of TiO2. The improved stability of CuI-TiO2 in comparison to pure CuI is a result of inhibited recrystallization and grain growth, reduced loss of iodine and limited oxidation of CuI phase in presence of TiO2. For optimized fraction of TiO2, average transparency of ~78% (in 450-800 nm region) and a resistivity of 14 mΩ.cm is achieved, while maintaining a relatively high mobility of ~3.5 cm 2 V-1 s-1 with hole concentration reaching as high as 1.3 x 10 20 cm-3. Most importantly, this work opens up the possibility to design a new range of p-type transparent conducting materials using CuI/insulator composite system such as CuI/SiO2, CuI/Al2O3, CuI/SiNx, etc.

According to the Shockley-Queisser limit, the maximum achievable efficiency for a single junction solar cell is ~33.2% which corresponds to a bandgap (E g) of 1.35 eV (InP). However, the maximum reported efficiency for InP solar cells remain at 24.2% ± 0.5%, that is >25% below the standard Shockley-Queisser limit. Through a wide range of simulations, we propose a new device structure, ITO/ ZnO/i-InP/p + InP (p-i-ZnO-ITO) which might be able to fill this efficiency gap. Our simulation shows that the use of a thin ZnO layer improves passivation of the underlying i-InP layer and provides electron selectivity leading to significantly higher efficiency when compared to their n + /i/p + homojunction counterpart. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p + InP substrate and achieved an open-circuit voltage (V oc) and efficiency as high as 819 mV and 18.12%, respectively, along with ~90% internal quantum efficiency. The entire device fabrication process consists of four simple steps which are highly controllable and reproducible. This work lays the foundation for a new generation of thin film InP solar cells based solely on carrier selective heterojunctions without the requirement of extrinsic doping and can be particularly useful when p-and n-doping are challenging as in the case of III-V nanostructures.

Abstract-Nanowire solar cells hold several advantages over planar solar cells such as reduced reflection, facile strain relaxation, extreme light trapping, increased defect tolerance, etc... However, due to their large surface-to-volume ratio nanowires tend to have very low effective minority carrier lifetime. To overcome this issue a radial junction solar cell was proposed. However, in experimental realization, the efficiency of a radial junction solar cell remains significantly lower than its axial counterpart. This is mainly because of the inability to simultaneously control the doping in both the core and the shell while maintaining low defect density at the interface. To overcome the above-mentioned issues, we propose and simulate a core-shell heterojunction solar cell using p-InP as a core material and ITO/ZnO as a shell material, respectively. Using FDTD simulations, we show that use of an oxide coating over InP core can significantly increase the absorption in InP nanowire arrays, and for an optimized thickness of oxide layer, InP consumption can be reduced by as much as 4 folds without sacrificing the ideal short circuit current. In addition, our device simulation results show that even for a core minority carrier lifetime of 50 ps, an efficiency of 23 % can be obtained if both core and shell can be heavily doped while maintaining an interface recombination velocity of less than 10 4 cm/s. Finally, we discuss how the proposed device structure can reduce the fabrication complexities related to epitaxial homojunction/heterojunction core-shell solar cell structure while achieving a high efficiency under optimized conditions.

Semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high-temperature requirements and substrate preparation. At the same time, plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance, and biocompatibility. We explore the direct growth of high-quality III–V nanowires on flexible plastic substrates by metal-organic vapor phase epitaxy (MOVPE). We synthesize InAs and InP nanowires on polyimide and show that the fabricated NWs are optically active with strong light emission in the mid-infrared range. We create a monolithic flexible nanowire-based p–n junction device on plastic in just two fabrication steps. Overall, we demonstrate that III–V nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowire-based flexible electronic devices.

In recent years, carrier-selective contacts have emerged as an efficient alternative to the conventional doped p–n or p–i–n homojunction for charge carrier separation in high-performance solar cells. However, so far, there has been no development in carrier-selective contacts for GaAs solar cells. This paper proposes an alternative device structure and reports an 18.5% efficient single-junction GaAs solar cell using zinc oxide (ZnO) as an electron-selective contact. A detailed X-ray and ultraviolet photoelectron spectroscopy depth profile analysis is performed to reveal the mechanism of carrier selectivity and improved efficiency compared to homojunction cells grown under similar conditions. Moreover, a detailed loss analysis shows that the fabricated solar cell has the potential to reach more than 25% efficiency with further optimization. The device structure proposed in this paper will provide a route to reduce the complexity and cost of epitaxially grown cells while also allowing for the possibility to fabricate high-efficiency III–V solar cells using low-cost growth techniques (such as closed-space vapor transport and thin-film vapor–liquid–solid) where doping can be extremely challenging.