Investigation of surface passivation effects on thin crystalline silicon solar cell performance

Two major losses in silicon solar cells become important as the cell thickness is decreased: a lower percentage of photons are absorbed on the initial pass of light through the devices, and the back surface recombination has more influence on the electrical performance due to the proximity of the back surface to the front collecting junction. The former primarily affects short-circuit current and can be reduced by effective light-trapping designs. The latter mainly limits the open-circuit voltage and must be minimized through effective surface passivation. In this work, the major limiting factors that affect the open-circuit voltage of silicon solar cells are studied. In particular, the designs that minimize the surface recombination losses of thin silicon solar cells and the measurement techniques used to characterize these losses are investigated. A new device structure is developed that achieves a high open-circuit voltage through the minimization of internal recombination losses, especially surface recombination losses. A new and effective measurement technique, Bifacial Spectral Response Measurement (BFSR), is developed to characterize these losses. 15.4% efficiency and an open-circuit voltage of 660 to 670mV are demonstrated. This structure allows, for the first time, the modulation of the silicon surface recombination velocity and hence the solar cell performance by an externally-controlled bias voltage. This enables the in-depth investigation of injection level effects and surface electric field effects on surface recombination behavior and their impact on solar cell performance. The new solar cell structure itself has important implications for silicon solar cells that require a one-sided contact scheme. In summary, this work provides the fundamental understanding, the design opportunities, and the device processing and characterization needed to make efficient thin silicon solar cells.