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Solar & Alternative Energy

Hybrid photovoltaics for eco-friendly solar cells

Low-cost hybrid photovoltaics based on organic conjugated molecules and silicon nanowire arrays with a power conversion efficiency of ∼10% enable high-performance solar cells.
15 August 2012, SPIE Newsroom. DOI: 10.1117/2.1201208.004313

Solar cells are widely believed to be one of the most promising emission-free, eco-friendly energy sources. These cells can directly convert radiation from the sun into electricity using photovoltaic (PV) effects with minimal safety hazards and detrimental impacts on the environment. At present, silicon (Si) PVs dominate more than 90% of the market. Unfortunately, the total share of photogenerated electricity is still limited because Si PVs are expensive to manufacture. Therefore, there is a need for cheaper photoactive materials or structures with which to construct efficient PVs.

Numerous new materials and device structures have been explored in an attempt to cut the cost of PV manufacture. In particular, low-cost organic PVs are undergoing rapid development. A certified power conversion efficiency (PCE) of 8.37% was achieved in bulk heterojunction PV devices using a low-bandgap conjugated polymer as a donor and the fullerene derivative PC71BM as an acceptor.1 In addition, new developments in nanotechnology allow us to observe and manipulate materials on a subnanometer scale. Organic-inorganic hybrid solar cells based on nanostructured semiconductors have gained popularity in recent years, merging the advantages of both the organic and inorganic components. However, the performance of these devices is not as good as purely inorganic PV devices as a result of numerous surface defects and improper organic-inorganic phase segregation. Here, we demonstrate hybrid PVs based on organic conjugated molecules and Si nanowire (SiNW) arrays that can achieve a high PCE (∼10%) by controlling both phase separation and surface passivation.2, 3

Conjugated organic materials exhibit low-cost solution processability. PVs employing organic and SiNW hybrid materials as photoactive layers benefit from the advantages of both the organic and Si components. The organic molecules make hybrid cells superior to those made with conventional Si molecules in terms of both their cost and scalable solution processing.

Other advantages of hybrid devices include the excellent light-harvesting capability of SiNWs and a relatively simple fabrication process. The antireflection property of SiNW arrays made using chemical etching is significantly enhanced over a wide spectrum range (see Figure 1). In addition, we can passivate the surface defects by methyl group termination, which significantly suppresses the surface recombination velocity. Furthermore, we can control phase separation by tuning the density of the SiNW arrays and the shell thickness of the polymer.

Figure 1. (a) Reflectance spectra. (b) Scanning electron microscopy image of silicon (Si) nanowire arrays prepared via silver nitrate/hydrogen fluoride etching of n-type Si layers. As a reference, a single-side polished crystalline-Si wafer is used for comparison of light reflection.

The device structure is not complicated. We make the SiNW arrays on a planar Si substrate by metal ion-assisted aqueous electroless etching at room temperature: see Figure 1(b). Wafer-sized SiNW substrates can be fabricated by dipping Si wafers into a hydrogen fluoride and silver nitrate aqueous solution without the need for complicated facilities. We controlled the length and diameter of the SiNW arrays by tuning the fabrication conditions. We then deposited the conjugated molecule poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) onto the arrays using a wet spin-coating process. The device is constructed by depositing the grid electrode onto the organic layer and the rear side of the metal contact: see Figure 2(a).

Figure 2. (a) Device structure. (b) Electrical output curves of hybrid photovoltaic device. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). Voc: Open-circuit voltage. Jsc: Short-circuit current density. FF: Fill factor. PCE: Power conversion efficiency.

SiNW arrays achieve an impressive performance, with an open-circuit voltage of 0.527V, a short-circuit current density of 31.3mA/cm2, and a fill factor of 0.588. The result was a PCE of 9.70%: see Figure 2(b). Recently, a simulation of a hybrid device based on this structure predicted a PCE of more than 20%,4 which is comparable to crystalline-Si PV devices.

In summary, we demonstrated an inorganic-organic hybrid PV based on SiNW arrays and conjugated molecules. The PV was able to achieve a PCE of up to ∼10%, which is high for a hybrid device. We believe this model is appropriate for fabricating heterojunction hybrid devices in conjunction with alternative organic semiconductors, or even inorganic semiconducting nanocrystals. The ability to realize high-performance hybrid solar cells means that these PVs can potentially reduce the material and processing costs of such cells. Solution-processed conjugated molecules, for example, as found in inkjet printing, further enhance the versatility of the proposed solar modules. SiNWs may potentially achieve more efficient light absorption and carrier collection than reported here. However, further optimization of the top and bottom contacts, in terms of both optical and electrical transparency, is necessary to enable better performance. By rationally designing the electrical and optical characteristics of the hybrid system, we believe it is possible to construct high-performance, low-cost solar cells with a PCE of up to 20%. We are currently investigating the optimization of Si surface passivation and top conjugated molecules to further improve PV performance.

Baoquan Sun
Institute of Functional Nano & Soft Materials
Soochow University
Suzhou, China

Baoquan Sun obtained his PhD from Tsinghua University (2002). His interests include elaborating nanostructured solar cells optimized for low production costs.

1. Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells, Adv. Mater. 23, p. 4636-4643, 2011.
2. F. Zhang, B. Sun, T. Song, X. Zhu, S. Lee, Air stable, efficient hybrid photovoltaic devices based on poly(3-hexylthiophene) and silicon nanostructures, Chem. Mater. 23, p. 2084-2090, 2011.
3. X. Shen, B. Sun, D. Liu, S.-T. Lee, Hybrid heterojunction solar cell based on organic-inorganic silicon nanowire array architecture, J. Am. Chem. Soc. 133, p. 19408-19415, 2011.
4. T. Chen, B. Huang, E. Chen, P Yu, H. Meng, Micro-textured conductive polymer/silicon heterojunction photovoltaic devices with high efficiency, Appl. Phys. Lett., 2012. doi:10.1063/1.4734240