Optical absorption and photoelectron collection properties of silicon wafers with conical quantum nanocrystals structure

A conical form of nano-sized quantum cluster was formed on the surface of p-type crystalline silicon [111] wafer by anode electrochemical etching in HF-based solution. The conical surface is highly effective in absorbing sunlight and transporting photoelectrons to semiconductor material. These are because each cone has a graded band gap with the energy level in the range from 1.1 to 3 eV which can be considered as consisting of quantum dots in different sizes. Since the boron concentration on the surface of each cone gradually decreases from top to bottom, a continuously varying electrical field is created along the cone height. This electric field is forcing photoelectrons generated in the cone to move rapidly to the direction perpendicular to wafer surface. Hence the drift time of photoelectrons can be less than their recombination time within the thin layer close to the bottom of the cone.

Searching new materials and methods which can increase the efficiency of the solar cell is important in both theoretical and practical application point of view [1]. Now, solar cells with the base of polycrystalline Cu(InGa)Se 2 (CIGS or CIS with no Ga) and of cadmium telluride thin films [2], with triple-junction on the base of GaInP-GaAs-Ge layers [3], with polymer mixtures combined with InP nanowires thin-film [4] are very promising.
To further increase the efficiency, silicon solar cells with a tandem structure of silicon quantum dot/crystalline silicon is proposed [5]. However, the solar cells on the crystalline silicon base are currently reached to have efficiency of 24% [6], and are the most popular technique because there is well-developed production equipment, and the material is cheap and nontoxic.
A method of multilayered semiconductors, where the top layer absorbs short-wave light and the subsequent layers absorb long-wave sun light, is proposed [7]. Semiconductors with band gap energy matching to quantum energy of sun light can be used most effectively for solar cells. However, to cover all solar spectrums, the number of layers should be large. It has shown theoretically that in case of multilayer (n=20 to 25) thin film semiconductors with different band gaps in the range from E g1 =1.1 eV to E g2 =3 eV with a step (E g2 −E g1 )/n. It is possible to achieve solar cells with efficiency near 60% [8]. It is known recently that a semiconductor band gap increases as crystal's geometrical size decreases (quantum nano-size effect) [9]. For example, crystalline silicon quantum dots in different geometrical size from 2 nm to 6 nm have a magnitude of band gap energy from 3 to 1.3 eV [10]. Band-gap energy of crystalline silicon is around 1.1 eV [11]. Thus, this idea can be realized today with the help of nanotechnology when a serially connected quantum dots system with size in the range from 1 nm to 10 nm or nano-sized cone crystal on a crystalline silicon surface is fabricated.
In this short communication, a new method of forming conical nanocrystals as anti-reflection layer, on the silicon surface and the preliminary analysis of its light collection efficiency are presented.
A nano-sized crystal having a cone shape has been produced on the surface of p-type crystalline silicon wafer in is shown in Fig. 1. Figure 1 shows that a large number of cones are almost uniformly disposed on the surface. The surface morphology of the silicon wafer is clearly different from that of the porous silicon which is fabricated through the similar electrochemical etching procedure [10]. Apparently, the difference is brought by the thermal diffusion of boron into silicon wafer. The silicon wafer absorbs practically all visible light spectral range, i.e., from violet to near infrared. The measurement shows that it reflects less than 2% of the sun light in the wavelength range from 300 nm to 1120 nm. This means that it works almost like a black body, i.e., the cone layer forms an anti-reflection layer on the p-type silicon layer. The small reflectivity values are probably induced by two physical reasons such as evanescing of wave along the surface of the cones and graded band gap semiconductor properties of the cones. It is possible to consider that the coned surface is working as a nanoscale grating (see Fig. 2). In this case, the lights will propagate along the coned surface. Hence they are mostly absorbed on the surface of each cone as the evanescent wave does [13]. The graded band gap can only be explained by quantum-size effect, i.e., the cone diameters in nano size increase as getting closer to its bottom. Hence it forms a quantum dot system. This system has a continuously varying band gaps within 1.1 to 3 eV. Hence each cone works like a serially connected quantum dot system with different band gap energies. The maximum energy appears at the top of the cones as shown in Fig. 3. The diameter of the top part is approximately 2 nm.
The number of photons from sun light in the waveband within  1 to  2 that get on square meter of semiconductor surface per second, can be calculated by Plank's radiation law [14]: in Kelvin of the sun in sea level [15], h is Planck's constant, c is speed of light, and k is Boltzmann's constant [14].   [11]. For this reason, the efficiency of solar cell will be increased compared with that of the solar cells based on crystalline silicon by reducing the recombination rate 4 times.
It is considered that the current losses in the solar cells based on crystalline silicon are attributed to different physical effects: The thermalization of photoelectrons causes 29.2% loss, the incomplete collection of photoelectrons 4.5%, added shunting resistor 4.7%, and the recombination in junction area 19.2% [17]. With the anti-reflection layer, the losses due to the photoelectron thermalization and the incomplete collecting of photoelectrons will be eliminated by the graded band gap presence. Ballistics electrons have a higher probability of passing the cones. Therefore the efficiency of solar cell with conical nano-sized cluster layer can be characterized with the photocurrent I ph , the recombination current I rec , and current through added shunting resistor, I