## 1 Introduction  
Organic solar cells have become one of the substitutes for traditional silicon solar cells owing to its merits (clean, environmentally friendly, no pollution, etc.) [1]. As an important photodevice, Dye-Sensitized Solar Cells (DSSCs) display many advantages and have been paid considerable attention [2; 3], and their working processes can be briefly described as below [4]. During absorption of sunlight, sensitive dye is excited, forming an excited state, and electrons are subsequently shifted into TiO\({}_{2}\). This process is designated as the electron injection process. Subsequently, electrons reach the external circuit and generate an electric current. Dyes with a loss of electrons can be recovered from electrolytes. In these working cycles, one important process is the efficiency of strong molecular absorption and efficient charge separation [4].  
The electronic transition with strong absorption results in more electrons and hole accumulation in TiO\({}_{2}\) nanoparticles and dyes, respectively, which corresponds to a Charge Transfer (CT) state. This means that a CT excited state with strong absorption is a photoinduced CT complex [5]. A weak absorption was also found in intermolecular CT for donor-acceptor dyad, and intramolecular CT leads to strong absorption in the dye with a donor-\(\pi\)-acceptor structure. After photoexcitation, the excitons on the molecule are usually dissociated to electron-hole pairs, intramolecular CT occurs, and the electrons are shifted from molecule to TiO\({}_{2}\) nanoparticles. Apart from CT, a matching of absorption spectra is hoped for full utility of sunlight. Because it is difficult for a single dye to cover the ultraviolet and near-infrared region, desensitization provides a way with a variety of complementary dyes in order to broaden the absorption spectra range and strength [6; 7; 8; 9; 10; 11]. A series of porphyrin dyes (XW1-XW4) desensitized with Dye C1, in which highest cell efficiency has reached 10.45% [11], and the absorption band from dye C1 makes up for poor absorption in 500 nm for porphyrin dye. [PERSON] et al. used the coabsorption/cossenitization based on a phenothiazze-based electron donor to improve conversion for nonruthenium solar cells (reported to be 11.5%) [6]. PTZ-2 and N719 in phenothiazze-based desensitization possess efficiency of 8.12% compared with PTZ-2 (5.81%) and N719 (6.97%) [7]. The desensitization (D35 and XY1) displays a higher VOC and improves conversion efficiency [12]. As an auxiliary means for experiment, quantum chemistry methods were universally used to deal with the structure, spectra, and excited state properties of multibranched dyes [13; 14], metal-free dyes with multidonor moiety [15], natural dyes [16; 17], and metal-containing dyes [18; 19; 20].  
In this work, we performed a quantum chemistry calculation to research the geometries, spectra, CT processes, and photoelectric properties for the DS-35, XY1, and desensitization systems. Important characteristics for intramolecular CT in a single dye attached to TiO\({}_{2}\) were estimated, which were compared with intermolecular CT for desensitization systems between DS-35 and XY1, owing to the fact that there is competition for CT and charge loss. Furthermore, photoelectric properties of single dyes and desensitization systems were studied to reveal the different performance in solar cells. The current investigation provides a clue for understanding the micromechanism and deduces the possible reason why a desensitization system has good efficiency in the field of solar cells.