The sentence 'have contributed to improve' is correct and usable in written English.
You can use this sentence when you want to talk about the positive effects of what one or more people have done. For example: "The team of volunteers have contributed to improve the local park's environment."
UNICEF Representative in Bangladesh called Mr. Yunus's work "an outstanding contribution to humanity," adding that "microcredit programmes have contributed to improve sustainably the status of the poorest women – in Bangladesh and many other countries".
Furthermore, Velazquez and Buffaria ask themselves whether such instruments have contributed to improve efficiency, farmers' income, and consumers' welfare.
Satellite observations over the last two decades have contributed to improve the understanding of climate variations and the physical and biological relations of the environment.
The transient hot-wire method was first suggested in 1931 to measure the absolute thermal conductivity, and ever since many authors have contributed to improve the method, making it more accurate.
As a result, the EU's efforts to implement enhanced food safety controls may have contributed to improve system-wide efficiency and reduced costs of meeting the requirements of other export markets.
Several scientists have contributed to improve and automate the original pinch method over the last decades, increasing its capability to deal with a number of specific issues; the expertise of the analyst, however, remains determinant in achieving optimal results.
The changes of WER values, before and after improvements, testify to the fact that implemented modifications have contributed to improving the effectiveness of G2P conversion.
A number of process improvements, notably performing an on-bead posthybridization PCR, may have contributed to improving the PCR duplicate rate for phase 2 (mean = 54.3%, range = 37.8%69.9%%).
Efforts made to help disadvantaged employees have contributed to improving work relations.
In 1986 Borlaug created the World Food Prize to honour individuals who have contributed to improving the availability and quality of food worldwide.
Urban clinics and rural health centres provide basic medical care, and the U.S. Medicare and Medicaid programs have contributed to improving health among lower-income residents, as have various other social programs.
