Maximizing the short circuit current of organic solar cells by partial decoupling of electrical and optical properties
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The partial decoupling of electronic and optical properties of organic solar cells allows for realizing solar cells with increased short circuit current and energy conversion efficiency. The proposed device consists of an organic solar cell conformally prepared on the surface of an array of single and double textured pyramids. The device geometry allows for increasing the optical thickness of the organic solar cell, while the electrical thickness is equal to the nominal thickness of the solar cell. By increasing the optical thickness of the solar cell, the short circuit current is distinctly increased. The quantum efficiency and short circuit current are determined using finite-difference time-domain simulations of the 3D solar cell structure. The influence of different solar cell designs on the quantum efficiency and short circuit current is discussed and optimal device dimensions are proposed.
KeywordsOrganic solar cells Pyramid texture Solar cell Light trapping FDTD
In the current study, we try to decouple the electrical and optical properties of the solar cell using a 3D solar cell architecture. The solar cell is formed on an array of single or double textured pyramids. It is assumed that the films are formed conformally on the surface of the pyramids, while the dimensions of the pyramid are distinctly larger than the thickness of the organic solar cell. Due to the conformal growth of the films, the electrical thickness of the solar cell remains unchanged. The electrical thickness defines the distance between the electrical contacts of the solar cell. Hence, the electrical thickness represents the maximal charge transport distance of the photogenerated charges. Furthermore, the 3D geometry leads to a distinctly increased optical thickness of the solar cell. The optical thickness of the solar cell is defined as the thickness of the organic film calculated at each point of the substrate and averaged over the area of the substrate. The optical thickness of the solar cell on a planar or flat substrate is equal to the nominal thickness, while the optical thickness of the organic layers on a 3D surface is increased if conformal deposition is assumed. In this study, the organic solar cell is placed on conductive TiO2 pyramids. The finite-difference time-domain optical simulation method was utilized to study the wave propagation, electric field distribution, and determine the optical parameters for the organic solar cell stack placed on single and double textured pyramids. The influence of the dimensions of the pyramids on the quantum efficiency and short circuit current is investigated and optimal device dimensions are proposed.
Optical simulation modeling
Device structure of small molecule organic solar cells
Small molecule organic solar cells on smooth surface
The calculated power loss profile of a solar cell on flat substrate is shown in Fig. 2. The solar cell on the flat substrate is used as reference solar cell. The corresponding quantum efficiency and absorption are shown in Fig. 1. For short wavelengths, a significant fraction of the incident light is absorbed by the C60 electron transporting layer as shown in Fig. 2b. Furthermore, approx. 30% of the incident light is absorbed by the photoactive ZnPc:C60 absorber (Figs. 1, 2b). In the spectral range from 550 to 750 nm, most of the light is absorbed by the ZnPc:C60 layer. This is confirmed by the power loss profile for a wavelength of 640 nm as shown in Fig. 2c. The quantum efficiency reaches a maximum of approx. 85% as depicted in Fig. 1. The short circuit current of the solar cell was maximized by tuning the thickness of the conductive TiO2 layer. The TiO2 layer acts as an optical spacer (Kwanghee et al. 2006). The TiO2 spacer has the function of introducing an optical phase change. The thickness of the ZnPc:C60 absorber is smaller than half of the wavelength of the incident light, so that the maxima of the standing wave formed in the solar cell due to the constructive and destructive interference of the forward and backward (reflected) electromagnetic wave has to be matched to the position of the ZnPc:C60 absorber in the solar cell. The thickness of the TiO2 spacer was varied until a maxima of the short-circuit current of 13.8 mA/cm2 was obtained for a TiO2 spacer thickness of 10 nm.
Organic solar cells on pyramidal textured substrates
The influence of the period of the pyramid on the quantum efficiency for periods ranging from 0 (flat) to 400 nm (height 1500 nm) is provided in the supplementary material (Fig. S2). With increasing period, the quantum efficiency converges towards the solar cell on a flat substrate. The highest short circuit current is observed for a period of 200 nm. Figure 4 exhibits the cross-section and power loss maps of a solar cell with a period of 200 nm. The total thickness of the organic layer stack, 115 nm, is larger than half of the period of the surface texture and the valley between two neighboring pyramids is completely filled by organic material. Hence, the electrical thickness of the organic ZnPc:C60 absorber layer is increased and photo generated charges can not be efficiently extracted. Nevertheless, we will discus the optics of the structure to study the influence of the device geometry on the quantum efficiency and short circuit current. The power loss maps for such solar cell are depicted in Fig. 4b, c for the wavelengths of 400 and 640 nm. Most of the light is absorbed within the first 500 nm of the solar cell. The corresponding quantum efficiency is shown in Fig. 5b. The quantum efficiency is distinctly increased for short and long wavelengths. Additionally, the quantum efficiency increases continuously with increasing height of the pyramid. The enhancement is mainly caused by the increased optical thickness of the organic layer stack. The optical thickness of the ZnPc:C60 layer is calculated by the ratio of ZnPc:C60 layer volume within a unit cell to the area of the unit cell (Jovanov et al. 2013). The optical thickness increases with decreasing period and increasing height of the pyramid.
In next step, organic solar cells have to be realized on such single and double textured substrates. Excellent conformity of inorganic thin films on textured surfaces has been demonstrated for atomic layer deposition (ALD) (Ritala and Leskela 2001) and chemical vapor deposition (CVD) (Kuang et al. 2011; Adachi et al. 2013). The successful experimental realization of thin film solar cells on double or multiscale textured substrates has been demonstrated for amorphous silicon (Tamang et al. 2014), microcrystalline silicon (Hongsingthong et al. 2013; Tamang et al. 2016), and multijunction silicon solar cells (Tan et al. 2015). So far few studies have been published on the conformal deposition of organic films on 3D textured substrates. However, promising initial results have been demonstrated for the deposition of organic films by chemical vapor deposition (Loscutoff et al. 2010), molecular deposition techniques (Mullings et al. 2010; Räupke et al. 2013), and vapor phase deposition (OVPD) (Yang and Forrest 2006; Yang et al. 2005).
The short circuit current and energy conversion efficiency of organic solar cells can be increased by a partial decoupling of the electrical and optical properties. In this study, organic solar cells on 3D textured surfaces were studied. Forming conformal organic solar cells on textured arrays of single or double textured pyramids allow for increasing the optical thickness of the solar cell, while the electrical thickness is equal to the nominal thickness. By increasing the optical thickness, the quantum efficiency and short circuit current are increased. In the case of a single pyramid surface texture, the short circuit current is increased by 29%, while in the case of double or multiscale texture the short circuit current is increased by 46%. The conformal deposition of organic solar cells on multiscale textured might allow for the realization of high efficiency organic solar cells.
The work is partly supported by the Research Grants Council of Hong Kong, China (Project Number: GRF 152109/16E PolyU B-Q52T) and the Hong Kong Polytechnic University (Project Number: G-YBFR, G-UA7N).
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