Transistor Solar Cell Research Articles

Progress in three‐terminal heterojunction bipolar transistor solar cells

AbstractConventional solar cells, including multijunction solar cells, are based on pn junctions as building blocks. In contrast, the three‐terminal heterojunction bipolar transistor solar cell (3T‐HBTSC) explores the use of a bipolar transistor structure to build a solar cell. The limiting efficiency of this transistor structure equals that of a double‐junction solar cell. However, since the 3T‐HBTSC does not require tunnel junctions, its minimal structure has only three semiconductor layers, while the minimal structure of a double junction solar cell has six. This work reviews the operation principles of this solar cell and the steps carried out towards its practical implementation. Experimental results on a GaInP/GaAs HBTSC prototype with bottom interdigitated contacts are presented.

Efficiency Enhancement of Perovskite Solar Cells Using Heterojunction Bipolar Transistor Configuration

Recently, perovskites have shown immense potential as solar cell active layer materials, achieving photo-conversion efficiency over 20%. In the future, a novel device architecture has the ability to further stimulate the device performance. In this article, we propose a three-terminal heterojunction bipolar transistor (HBT) architecture to enhance the photo-conversion efficiency of perovskite solar cells. We performed an optoelectronic simulation to characterize the proposed heterojunction bipolar transistor solar cell and analyze its performance metrics by varying different design parameters. In doing so, we vary the thickness of active layers in the structure and change the doping concentration of three regions to maximize the device efficiency. We estimate an absolute improvement in efficiency of 10.84% compared to the current record efficiency of single-junction perovskite solar cells (34.54% versus 23.7%), for a 400 nm/450 nm perovskite/perovskite heterostructure. The analyses and results presented in this article would provide a promising direction to the design of solar cell architecture utilizing HBT configuration.

Considerations for the Design of a Heterojunction Bipolar Transistor Solar Cell

Independent current extraction in multi-junction solar cells has gained attention in recent years because it can deliver higher annual energy yield and can work for more semiconductor material combinations than the more established series-connected multi-junction technology. The heterojunction bipolar transistor solar cell concept (HBTSC) was recently proposed as a simple, compact and cost-effective multi-terminal device structure that allows independent current extraction. It consists of only three main layers: emitter, base and collector. In this work we use a drift-diffusion model to analyze important aspects in the design of an HBTSC structure based on typical III-V semiconductor materials. We find that carrier injection from the emitter into the collector (transistor effect) degrades the open-circuit voltage of the top sub-cell, but this risk can be eliminated by optimizing the base design. We find requirements for the base layer which are, in principle, achievable in the context of current III-V semiconductor technology.

Three-terminal heterojunction bipolar transistor solar cells with non-ideal effects: Efficiency limit and parametric optimum selection

Without fabricating intermediate tunnel junctions or wafer bonding schemes for interconnecting the subcells, heterojunction bipolar transistor solar cells offer a promising new route in solar energy conversion. In this work, an improved theory for the three-terminal heterojunction bipolar transistor solar cell is presented with inclusion of non-ideal effects missing from the previous treatment, namely the non-radiative recombination and the thermal conduction losses that are inevitably present in realistic devices. Following detailed balance theory, the revised analytical formula for the cell conversion efficiency is derived, and the maximum efficiencies under different conditions are further calculated. Under the condition of 100 sun irradiance and 50% injection efficiency, a Gallium arsenide/Gallium antimonide-based solar cell operating at 465 K yields a maximum efficiency of 46.4%. Moreover, the effects of solar concentration, injection efficiency, and other key parameters on the cell performance are analyzed, and, consequently, optimal operating conditions and limiting factors on the conversion efficiency are determined. Simulation results show that such a solar cell operating with low injection efficiency under moderate concentration factor and low cell temperature can significantly boost its conversion performance. This work provides new physical insights for optimal designs, thus paving a route towards the development of low-cost high-performance solar cells.

Novel heterojunction bipolar transistor architectures for the practical implementation of high-efficiency three-terminal solar cells

Practical device architectures are proposed here for the implementation of three-terminal heterojunction bipolar transistor solar cells (3T-HBTSCs). These photovoltaic devices, which have a potential efficiency similar to that of multijunction cells, exhibit reduced spectral sensitivity compared with monolithically and series-connected tandem solar cells. In addition, the simplified n-p-n (or p-n-p) structure does not require the use of tunnel junctions. In this framework, four architectures are proposed and discussed in this paper: 1) one in which the top cell is based on silicon and the bottom cell is based on a heterojunction between silicon and III-V nanomaterials; 2) one in which the top cell is made of amorphous silicon and the bottom cell is made of an amorphous silicon-silicon heterojunction; 3) one based on the use of III-V semiconductors aimed at space applications; and 4) one in which the top cell is based on a perovskite material and the bottom cell is made of a perovskite-silicon heterostructure.

Layer characterization and photovoltaic properties of CdS/multi-wall carbon nanotube/n-Si device with an n-p-n transistor structure

A new type of n-p-n transistor photovoltaic device based on CdS/multi-wall carbon nanotube (MWNT)/n-Si configuration was fabricated in a facile process. CdS quantum dots were deposited on fluorine-doped tin-oxide glass using a chemical bath deposition method, and MWNT film was coated on n-type Si substrate by airbrushing. The materials used for the n-p-n transistor solar cells were characterized by multiple techniques including X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman, Ultraviolet visible (UV-vis) spectrophotometer, and I-V characteristic measurements. The CdS layer acts as a good n-type material for the transistor solar cells. The thickness of the CdS layer can be controlled by the chemical bath deposition time to achieve different photovoltaic responses. I-V characteristic measurements show that the efficiency increases with increasing the thickness of the CdS thin layer. Compared with the tandem solar cells based on (p/n)–(p/n) semiconductor junctions, our n-p-n transistor solar cell has a simple structure without using tunnel junctions or wafer bonding schemes for interconnecting the cells.