With the semiconductor bulk properties reaching target values for highly efficient solar cells, efforts are applied to reduce losses at solar cell interfaces and contacts. Advances in understanding back contacts in thin‐film polycrystalline CdTe solar cells, a leading thin‐film PV technology, are reported. By using X‐Ray photoelectron spectroscopy, Kelvin probe spectroscopy, time‐ and energy‐resolved photoluminescence, defects at the back contact are analyzed. Densities of recombination centers and charged defects that induce near‐back‐contact band bending, both resulting in recombination losses, were estimated. Electro‐optical and surface analysis results are integrated into a device model, simulating the performance of CdSeTe/CdTe solar cells with 902 mV open circuit voltage.
Cu(In,Ga)Se 2 (CIGS)-based thin film solar cells hold significant promise due to the tunable, direct bandgap, high absorption coefficient, thin layers, flexible and rigid substrate applications, processing options, and consistent efficiency increases. [1][2][3] Photovoltaic conversion efficiencies have reached 23.4% in small-area CIGS solar cells, [4] and significant improvements have originated from absorber composition changes and heavy alkali postdeposition treatments (PDTs). [5][6][7][8][9][10][11][12][13] The absorber energy gap and electron affinity can be controlled by the Ga/(GaþIn) (GGI) ratio, which offers two benefits. First, increases in GGI widen the absorber bandgap which raises the maximum achievable open circuit voltage (V OC ) and efficiency of the device. [14] Second, GGI grading is used to create a "notched" graded bandgap in which the bandgap is increased at both the front and rear portions of the absorber, and a bandgap minimum is maintained in the front half of the absorber. [6] The front-side bandgap increase reduces front interface hole recombination and back-side grading reduces back interface electron recombination such that V OC improvements up to 100 mV are achievable with a change in GGI %0.5. [15,16] However, voltage losses increase for GGI > 0.4 in the minimum bandgap region such that efficiency improvements are limited. [17][18][19] This can be mitigated in part through silver-alloyed ACIGS devices ((Ag,Cu)(In,Ga)Se 2 ), which
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