Back-Contact Perovskite Solar Cells
Tai-Fu Lin, Ming-Hsien Li, Pei-Ying Lin, Itaru Raifuku, Joey Lin, and Peter Chen
National Cheng Rung University
Introduction
When the ion-based batteries are charging, batteries can convert electricity into chemical energy which is stored in batteries. While discharging, ion-based batteries undergo a reverse process to convert chemical energy into electricity for charging other devices. The working principle of solar cells is totally different to the ion- based batteries. The solar cells can convert solar light (photon energy) into electricity (chemical potential of electron) which cannot be stored in solar cells. As a result, solar cells cannot work without solar light illumination. It is worth noting that the solar cells have the potential to integrate with ion-based batteries for constructing a renewable energy system with light harvesting and energy storage. The solar cells can serve as a power supply for charging the ion-based batteries.
In conventional perovskite solar cells (PSCs), perovskite light absorber with several hundred nanometer thickness is sandwiched by tw'o selective contact electrodes, namely electron-selective layer (ESL) and hole-selective layer (HSL), as revealed in Figure 12.1a. Typically, a transparent conducting oxide (TCO) layer, such as indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), serves as the illumination window. The light enters the solar cells from the glass substrate, TCO layer, HSL, or ESL and is finally absorbed by the perovskite absorber. The electron and hole excited in perovskite active layer will diffuse in the opposite direction and be collected by the ESL and HSL, respectively. The optical transmission loss due to the refractive index mismatch, scattering from rough surface, and parasitic absorption of carrier transport material reduces the incident light intensity along with the optical flux. Moreover, deposition of carrier transport layer on the top of perovskite could require high temperature or harsh chemical process that could damage the perovskite layer.
The perovskite solar cells employing back-contact architecture, the so-called back-contact perovskite solar cells (BC-PSCs), can resolve the above issues. In the back-contact architecture, the selective contact of ESL and HSL and metal contacts are patterned and positioned on the same side below the perovskite active layer (refer to Figure 12.1b and c). Such device structure prevents the damage of top electrode deposition on the perovskite active layer and allows the light illumination directly on the perovskite absorber to reduce the optical transmission loss. The bottom TCO layer can be replaced with a metal electrode that provides a lower sheet resistance than TCO and reduces the resistive loss of device.
According to the position of patterned electrodes, the BC-PSCs can be divided into coplanar and non-coplanar types. Figure 12.1b shows the BC-PSCs with copla- nar finger electrodes, namely interdigitated electrode (IDE), in which two finger electrodes are patterned and interdigitated to separately collect carriers. The carrier diffusion length of perovskite is usually within few' micrometers [1]. The gap between electron- and hole-selective finger electrodes is highly controlled to collect carrier before recombination. Photolithographic techniques are generally applied to fabricate IDE; however, the integrity of finger electrode is critical to the charge collection. To simplify the fabrication process, Udo Bach’s group proposed BC-PSCs with non- coplanar finger electrodes, namely quasi-interdigitated electrode (QIDE), in w'hich an insulator is inserted between top patterned electrode and bottom planar electrode as shown in Figure 12.1c [2,3]. Inserting an insulator between them can effectively prevent the short-circuit contact between two selective electrodes. Furthermore, we can increase finger density, compared to the IDE structure, to further enhance charge collection capability.
FIGURE 12.1 Device structure of (a) sandwiched PSCs, (b) IDE-based BC-PSCs, and (c) QIDE-based BC-PSCs.
Coplanar Back-Contact Structure
T. Ma et al. used a numerical simulation to investigate the structural parameters in terms of finger electrode width and gap on the photovoltaic performance of the copla- nar back-contact-type perovskite solar cell [4]. The coplanar back-contact-type structure is shown in Figure 12.2a, in which the finger electrode of HSL and ESL with the same finger width are placed at the same plane and staggered to form a gap between two finger electrodes. Metal oxide semiconductors of TiO, and NiO are respectively used as ESL and HSL in the simulation. Figure 12.2b shows the calculated J-V
FIGURE 12.2 (a) Structures of the coplanar back-contact-type structure; calculated (b) J-V
curves and (c) EQE response of coplanar back-contact-type (black line) and sandwich-type (gray line) perovskite solar cell [4].
curves of PSCs with the sandwich and the coplanar back-contact structures. The short-circuit current density (Jsc) of coplanar back-contact structure exhibits an optimized value of 24.3 mA cm-2 when the width and gap are 1 and 0.1 pm, respectively, leading to a power conversion efficiency (PCE) of 22.77%. The traditional sandwich structure delivers a lower efficiency mainly due to a reduced Jsc of 21.7 m A cm-2. The increased Jsc in BC-PSCs is attributed to the reduced light loss at short wavelength region (300-380nm) as shown in the calculated external quantum efficiency (EQE) spectra (refer to Figure 12.2c). The authors also identify that the default value of finger electrode width <5 pm and gap <0.5 pm in coplanar BC-PSCs is acceptable for achieving high-efficiency BC-PSCs and simplifying the fabrication process.
It is a common technique to introduce self-assembled monolayers (SAMs) to modify the work function (WF) of metals and semiconductors in organic light-emitting diodes and organic field-effect transistors. The molecular assemblies reorganize into oriented arrange and spontaneously form on surfaces by adsorption. SAMs can affect the energy level by imparting a dipole moment on the surface [5-8]. In order to align the potential difference between two finger electrodes, U. Bach’s group apply SAM treatment at the gold-perovskite interfaces to modulate the work function of gold finger electrodes [9,10].
The interdigitated gold electrode is immersed into a 4-methoxythiophenol (OMeTP) solution, resulting in the formation of OMeTP SAMs on two finger electrodes. The OMeTP SAM on anode is subsequently desorbed electrochemically. The desorbed Au electrode is then exposed to a solution of 4-chlorothiophenol (CITP) in order to form the cathode electrode. Kelvin probe force microscopy (KPFM) is used to measure the surface potential changes of the interdigitated gold electrode after surface modification. The formation of the OMeTP SAM on electrode increases contact potential difference (CPD) of 400 mV relative to the bare electrode, and the CPD relative to the bare electrode further increases to 550-580 mV after CITP treatment. The encapsulated device delivers a Voc of 0.55 V, a Jsc of 9.73 mA cm-2 and a FF of 36.7%, leading to a PCE of 1.96%. The much lower photovoltaic performance of SAM-modified back-contact PSC is mainly attributed to the gap between interdigitated fingers. The gap of SAM-modified back-contact PSC is ~4.3 pm, which is almost tenfold the ideal value of 0.5 pm as demonstrated by simulation. The photogenerated carriers would recombine before they reach interdigitated finger electrodes. Moreover, the SAM-modified interdigitated finger electrodes only deliver less than 600 mV in work function difference, leading to a reduction in open-circuit voltage (Voc) of the resultant device [9].
The same group further optimize the device performance by controlling the perovskite grain size, since the charge-carrier diffusion length is related to perovskite grain size [10]. They grow four grain cluster sizes of perovskite which are approximately 6.0, 2.0, 0.99, and 0.23 pm and denoted as large (L), medium (M), small (S), and extra small (XS), respectively. The maximum theoretical Voc from BC-PSCs is determined by the work function difference induced by the self-assembled dipole monolayers coated on the gold electrodes. The V increases with the perovskite grain size, showing an average value of 0.46,0.4,0.31, and 0.09 V for L, M, S, and XS cluster sizes, respectively. The average Jsc values for devices with L, M, S, and XS cluster sizes are 9.44, 7.64, 5.88, and 0.69 mA cm-2, respectively. However, there is no clear trend in FF for devices with different grain size. It is noted that perovskite films with large cluster sizes exhibit longer charge recombination lifetimes, leading to a higher Voc- Bach’s group provided an approach via SAM treatment and optimized the device performance by improving the quality of the perovskite film and/or reducing the gap between the interdigitated electrodes. These pioneering works give a new' insight into further advancing the photovoltaic performance of back-contact perovskite solar cells.
