Numerical Modeling of Tin-based Absorber Devices for Cost-effective Solar Photovoltaics

Open Access
Author:
Chandrasekharan, Ramprasad
Graduate Program:
Energy and Geo-Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 11, 2012
Committee Members:
  • Jeffrey Brownson, Dissertation Advisor
  • Semih Eser, Committee Member
  • Sarma V Pisupati, Committee Member
  • Joan Marie Redwing, Committee Member
Keywords:
  • tin sulfide
  • numerical modeling
  • thin film
  • photovoltaics
  • solar cells
Abstract:
Due to the pressures of decreasing the electricity generation costs from solar photovoltaic (PV) modules, there is a need for novel light absorbing materials that can promise comparable conversion efficiencies at lower manufacturing costs than the incumbent technologies based on crystalline Si or thin films (CdTe or Cu-In-Ga-S). This research evaluates the tin-suite of absorber materials (based on tin monosulfide; SnS, and Cu-Zn-Sn-S; CZTS) as the next generation of PV cells that can yield the desired performance in the long term. Numerical models have been developed using Analysis of Microelectronic and Photonic Structures (AMPS-1D) to reveal efficiencies of cells under AM1.5 illumination based on three n-p heterojunctions: CdS|CdTe, CdS|SnS and ZnO|SnS, and identify avenues for further efficiency improvements in SnS PV. It has been predicted that PV devices based on the SnS (absorber)-ZnO (oxide) configuration yield higher conversion efficiencies (η=20.3%) compared to the inverted configuration oxide-absorber, with respect to the incidence of light. The difference has been attributed to variations in open-circuit voltages for the two situations, and suggests the adoption of the absorber-oxide design for further device development. The other approach to increasing open-circuit voltages in tin-based cells by using the wider band gap CZTS revealed upto 17.6% efficiency limits in FTO (F-doped SnO2)-oxide (ZnO)-sulfide(CdS)-absorber(CZTS) baseline cells, lower than the absorber-oxide configuration in SnS-based PV. Modeling of the CZTS cells under inverted configuration showed remarkably poor efficiencies (~3%) due to high hole affinities in the absorber that promoted surface recombination at the absorber-metal ohmic contact. This observation suggests the requirement of smaller hole affinities and indicates optimal absorber band gaps of 1.2-1.3 eV for high-efficiency PV conversion. The optimal value should be achieved by cationic substitution of the tin sublattice with copper and/or zinc atoms rather than substituting sulfur with selenium in CZTS. It is concluded that future research efforts in device development should utilize the absorber-oxide design wherein the absorbers have the optimal band gaps as stated. The role of simulation tools such as AMPS will be crucial in aiding critical decision making on tin-based materials as solar PV continues to become an affordable source of electricity.