Status of selective emitter technology

Abstract

Current industrial monocrystalline Cz Si solar cells based on screen-printing technology for contact formation and homogeneous emitter have an efficiency potential of around 18.4%. Apart from limitations at the rear side by the fully covering Al-BSF the front side is limiting e.g. by relatively high j0E values. This can be changed by selective emitter designs allowing a decoupling and separate optimization of the metalized and non-metalized areas. Several selective emitter concepts that are already in industrial mass production or close to it are presented, and their specialties and status concerning cell performance are demonstrated. Key issues to be considered are costeffectiveness, added complexity, additional benefits, reliability, and efficiency potential. The efficiency increase for best cells is around 0.5-0.6%abs and the current efficiency potential already demonstrated for all technologies is around 19.0%. Average efficiencies in industrial mass production for selected technologies are 18.5-18.6% for Cz and 17.1% for mc Si. The efficiency increase by selective emitter formation is higher for inline emitters, but selective emitters based on POCl3 show the highest absolute efficiency. By decreasing the phosphorous surface concentration, selective emitters are more sensitive to surface passivation and the use of a SiNx:H layer with a higher refractive index increases implied Voc values even further. Encapsulation under module glass and EVA reduces part of the gained jsc caused by a better blue response on cell level, but a calculation reveals that this extra loss for selective emitter solar cells is <0.1 mA/cm2 and therefore negligible. The full benefit of the improved front side in terms of a selective emitter structure will be achieved when local rear contacts will be used.