Solar heterojunction

Four different types of junctions can be used to separate the charge carriers in solar cebs (/) a homojunction joins semiconductor materials of the same substance, eg, the homojunction of a p—n sibcon solar ceb separates two oppositely doped layers of sibcon 2) a heterojunction is formed between two dissimbar semiconductor substances, eg, copper sulfide, Cu S, and cadmium sulfide, CdS, in Cu S—CdS solar cebs (J) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal—insulator—semiconductor junction (MIS), a thin insulator layer, generaby less than 0.003-p.m thick, is sandwiched between a metal and semiconductor material. 

Copper Sulfide—Cadmium Sulfide. This thin-film solar cell was used in early aerospace experiments dating back to 1955. The Cu S band gap is ca 1.2 eV. Various methods of fabricating thin-film solar cells from Cu S/CdS materials exist. The most common method is based on a simple process of serially overcoating a metal substrate, eg, copper (16). The substrate first is coated with zinc which serves as an ohmic contact between the copper and a 30-p.m thick, vapor-deposited layer of polycrystaUine CdS. A layer is then formed on the CdS base by dipping the unit into hot cuprous chloride, followed by heat-treating it in air. A heterojunction then exists between the CdS and Cu S layers. 

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

Anodization generally results in the formation of films with limited thickness, uncertain composition, defects, and small crystallite size. Thus, the barrier nature of the n-type semiconducting CdS film obtained in the previous manner makes it too thin to form the basis of Cu2S/CdS or CdTe/CdS solar cells by the normal dipping process. Heterojunction cells of low efficiency have, however, been made by anodization followed by vacuum deposition of the added layer (CU2S). 

Basol BM (1984) High-efficiency electroplated heterojunction solar cell. J Appl Phys 55 601-603... 

Duffy NW, Peter LM, Wang RL, Lane DW, Rogers KD (2000) Electrodeposition and characterisation of CdTe films for solar ceU applications. Electrochim Acta 45 3355-3365 Duffy NW, Peter LM, Wang RL (2002) Characterisation of CdS/CdTe heterojunctions by photocurrent spectroscopy and electrolyte electroreflectance/absorbance spectroscopy (EEA/EER). J Electroanal Chem 532 207-214 (see also references therein). 

Heske C, Eich D, Fink R, Umbach E, van Buuren T, Bostedt C, Terminello LJ, Kakar S, Grush MM, Callcott TA, Himpsel FJ, Ederer DL, Perera RCC, Riedl W, Karg F (1999) Observation of intermixing at the buried Cds/Cu(In, Ga)Se2 thin film solar cell heterojunction. Appl Phys Lett 74 1451-1453... 

For capture and conversion of solar energy, semiconductors possess several key advantages, including relatively high absorption coefficients and the possibility to separate the photogenerated carriers at homo and heterojunctions. The energy... 

The second type of solar cell is based on a /m-heterojunction in analogy to semiconductor devices [274]. Excitons generated by light, diffuse and dissociate at the interface between a hole and an electron-conducting material. The optimum layer thickness was calculated to be 1.5 times the exciton diffusion length [275]. 

Dennler, G. Scharber, M. C. Brabec, C. ]., Polymer-fullerene bulk-heterojunction solar cells. Adv. Mater. 2009, 21,1323-1338. 

Australia, and scaled up by BP Solar in Spain, the heterojunction with intrinsic thin layer (HIT) cells developed by Sanyo by replacing the diffused P-doped emitter with an amorphous silicon layer and the back contact cells developed by Stanford University for use in concentrator technology and now converted to a large area for flat plate use. All three use single-crystalline silicon, while the majority of screen-printed cells use multicrystalline silicon wafers. 

Schubert M, Yin CH, Castellani M, Bange S, Tam TL, Sellinger A, Horhold HH, Kietzke T, Neher D (2009) Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells. J Chem Phys 130 094703... 

---