Assembly And Properties Of Cu₂O/ZnO Three-dimensional Heterojunction Solar Cells

With the increase of serious energy and environmental issues, solar cell is facing twoproblems which are improving conversion efficiency and reducing of the cost. Looking fornew solar cell materials with low-cost, high environmental stability, good photovoltaic effecthas become an effective way for developing low-cost solar cells. Cuprous oxide （Cu₂O）which as an ancient semiconductor materials, has been paid attention again. Cu₂O is a p-typedirect band-gap semiconductor material and its band gap is2.0-2.2eV. It has much advantage,such as non-toxic and low-cost preparation process, absorption coefficient of102-106cm-1, thetheoretical efficiency of20%. The Cu₂O has important application in electrode materials,catalysis, electronic devices and sensors for its advantage. In our work, we prepared the Cu₂Ofilms and assembled the three-dimensional heterojunction of Cu₂O/ZnO by electrochemicaldeposition method. We analyzed the influence of deposition parameters on the structure andperformance of thin film heterojunction and found the principal cause restricting theperformance of Cu₂O/ZnO three-dimensional heterojunction cell.We prepared the Cu₂O thin film of （100）,（110）,（111） preferred orientation, respectivelywhen the pH of solution was9.0,11.5,12.0using the electrochemical deposition method andstudied the effects of pH, temperature, voltage on the film structure and properties. Wecharacterized the phase composition, surface morphology, transmittance and the forbiddenband width of the thin film by XRD, SEM, and Uv-Vis, respectively. The flat-band potentialand carrier concentration of Cu₂O film were characterized by testing capacitance-voltage. Theorientation of Cu₂O was toward to the face which has more Cu atoms with the pH increasing,due to the different atom arrangement of Cu₂O faces and the different stabilization ofCH3（OH）COO-to the different crystal surface. The grain size of Cu₂O growth with thedeposition temperature for the growth of Cu₂O films is a thermally activated process. Whenthe deposition temperature is60We know that the film has high absorbance and high carrierconcentration7.7×1018cm-3when the deposition temperature is60°C, pH=11.50.Three-dimensional （3D） heterojunction consisting of Cu₂O and ZnO nanorods wereassembled via the electrochemical approach at the pH=11.50. The ZnO nanorods are about4 μm in length. We studied the influence of deposition temperature and voltage forheterojunction structure and properties. A seeding process deposited at low temperaturewhile the high overpotential prior to the electrochemical growth not only help guaranteefilling density when the condition is high temperature while low overpotential but alsomaintain the integrity of ZnO nanorods by preventing the etching in the high-alkalinitysolution. High deposition potential clog the top opening of the ZnO nanorod arrays that thefilling can not be dense. The dense structure had good electrical contact and large forwardcurrent but the enlarged junction area and grain boundaries produced great interface statedensity, which contributed to the relative lower conversion efficiency of0.08%. In future, theenhancement of seeding layer crystallization quality, grain size of Cu₂O, photoelectricefficiency of heterojunction will depend on heat treatment, changing the solution parameters,reducing the interface state density, respectively.