Flexibe Carbon Nanotube/Silicon Thin Solar Cells Using Scalable Fabrication Techniques
Authors: Mariano Marina , McMillon-Brown Lyndsey , Li Xiaokai , Jung Yeonwoong , Reed Mark , Taylor André D. 
Published: February 2020

Hybrid approaches that combine silicon (Si) and single walled carbon nanotube (SWNT) thin films could lead to the development of low cost and highly efficient flexible solar cells. Their earth abundance coupled to their intrinsic mechanical flexibility open a platform for the next generation of photovoltaic devices, which could be utilized in various applications such as portable power generators for emerging wearable electronics. However, most flexible thin-Si based solar cells are fabricated by multiple patterning steps and high diffusion/annealing temperatures which lead to high production cost and low scalability(1). In this talk, we will demonstrate a new class of flexible hybrid solar cells by combining mechanically flexible p-type SWNT thin films with planar ultra-thin flexible n-type Si wafers with a power conversion efficiency (PCE) of 8.61%. We will show that the enriched semiconducting SWNTs thin films have a dual function as a charge collector and an infrared (IR) photoconvertor.

Flexible p-SWNT thin films are fabricated using a superacid sliding method developed in our lab (2, 3) and combined with flexible thin (≤50 µm) n-type Si wafers. We show flexible n-type wafers patterned to selectively expose the n-type Si windows surrounded by SiO2 insulating layers (Figure 1a). An ultrathin/flexible SWNT/Si solar cell based on 12 µm c-Si presents a Jsc
(short circuit current density) of 14.1 mA/cm2, Voc
(open circuit voltage) of 0.432 V, and FF (fill factor) of 0.571, resulting in a 3.48% PCE. By increasing the Si thickness to 50 µm, we illustrate that more photons are captured while the flexibility of the thin Si is still preserved with a 7.37% PCE (Jsc
of 23.68 mA/cm2, Voc
of 0.506 V, and FF of 0.615) (Figure 1b). We show that we can further enhance these devices to capture more photons by a simple solution coating of anti-reflective titanium oxide nanoparticles over on the SWNT/Si windows(4). This light-trapping step increases the PCE to 8.61% (Figure 1b). The added value of this new architecture provides the flexibility to tune the Si structure/thicknesses independently from the SWNT film process.

Finally, we will reveal using transient absorption data that there are significant photogenerated carriers in the SWNTs films that are injected to the Si, which contributes to the large efficiency observed for an ultrathin film silicon solar cell. This means that there is significant photogenerated current in the IR, which is due to the presence of SWNTs where Si does not absorb. Indeed, the unique capability to control the functionality and thickness of SWNT thin film coupled to ultra thin c-Si gives new implications to the use of SWNTs for carbon based electronic devices.

1. S. Wang et al., Large-Area Free-Standing Ultrathin Single-Crystal Silicon as Processable Materials. Nano Letters
13, 4393-4398 (2013).

2. X. Li et al., Improved efficiency of smooth and aligned single walled carbon nanotube/silicon hybrid solar cells. Energy & Environmental Science
6, 879-887 (2013).

3. Y. Jung, X. Li, N. K. Rajan, A. D. Taylor, M. A. Reed, Record high efficiency single-walled carbon nanotube/silicon p–n junction solar cells. Nano Letters
13, 95-99 (2012).

4. X. Li, Y. Jung, J.-S. Huang, T. Goh, A. D. Taylor, Device Area Scale-Up and Improvement of SWNT/Si Solar Cells Using Silver Nanowires. Advanced Energy Materials
4, 1614-6840 (2014).

Figure 1